• Open Access

    Role of PI3K/Akt/mTOR pathway in mediating endocrine resistance: concept to clinic

    Aglaia Skolariki 1†*
    Jamie D’Costa 1†*
    Martin Little 2
    Simon Lord 1

    Explor Target Antitumor Ther. 2022;3:172–199 DOI: https://doi.org/10.37349/etat.2022.00078

    Received: October 30, 2021 Accepted: February 11, 2022 Published: April 24, 2022

    Academic Editor: Valerie Speirs, University of Aberdeen, UK

    This article belongs to the special issue Endocrine Resistant Breast Cancer


    The majority of breast cancers express the estrogen receptor (ER) and for this group of patients, endocrine therapy is the cornerstone of systemic treatment. However, drug resistance is common and a focus for breast cancer preclinical and clinical research. Over the past 2 decades, the PI3K/Akt/mTOR axis has emerged as an important driver of treatment failure, and inhibitors of mTOR and PI3K are now licensed for the treatment of women with advanced ER-positive breast cancer who have relapsed on first-line hormonal therapy. This review presents the preclinical and clinical data that led to this new treatment paradigm and discusses future directions.


    Breast cancer, endocrine therapy, PI3K/Akt/mTOR pathway


    Endocrine therapy (ET) remains a key systemic treatment for both early and advanced breast cancer. Over the past 40 years, 3 main classes of ET have demonstrated clinical benefit and been licensed by the Food and Drug Administration (FDA) for the treatment of breast cancer; selective estrogen receptor modulators (SERMs; tamoxifen and toremifene); selective estrogen receptor degraders (SERDs; fulvestrant); and aromatase inhibitors (AI, letrozole, anastrozole, and exemestane) [1, 2]. However, resistance to ET in estrogen receptor-positive (ER+) breast cancer is common and remains a significant clinical challenge. A number of endocrine resistance mechanisms have been evaluated in the laboratory and clinic but to date only targeting the cyclin-dependent kinases 4 and 6 (CDK4/6) and PI3K/Akt/mTOR signaling axes has successfully translated to licensed drugs. In this review, we describe the multiple agents that target the PI3K/Akt/mTOR pathway which has and is being evaluated in the clinic for the treatment of breast cancer but with variable success. In particular, the selection of patients and drug toxicity have both proved to be challenged in the clinical development of this treatment class.

    Discovery of the PI3K/Akt/mTOR pathway

    The PI3K/Akt/mTOR pathway is a key intracellular signal transduction pathway that has provoked great interest as a therapeutic target in cancer [3]. This signaling cascade is implicated in tumorigenesis via activation of downstream signaling that regulates cellular proliferation, survival, metabolism, angiogenesis, and increasing motility [4]. In 1988 the Cantley group identified the agent that catalyzed the phosphorylation of phosphatidylinositol 4,5-bisphosphate (PIP2) into phosphatidylinositol-3,4,5-trisphosphate (PIP3), naming this enzyme PI3K [5], and subsequently, insulin signaling was found to regulate PI3K activity [6]. The design of oligonucleotide probes enabled the isolation of the first complementary DNA (cDNA) of a PI3K catalytic subunit, named p110α [7]. The discovery that activation of the PI3K pathway could be promoted by rat sarcoma (RAS) and insulin signaling, offered insight into how the PI3K pathway was involved in cell growth and proliferation [3, 810]. The serendipitous discovery that wortmannin, a mold metabolite inhibited PI3K signaling and the creation of a first synthetic inhibitor of PI3K, led to further research into the role that PI3K has in regulating cellular metabolism, and in particular glucose uptake and chemotaxis [3, 1113]. Subsequently, the PI3K pathway was implicated in tumorigenesis with somatic mutations in PIK3CA genes found in several tumor types [14].

    PI3K/Akt/mTOR signaling

    In nature, there are three different classes of PI3K, yet only class I PI3K isoforms are able to produce PIP3 from PIP2, which subsequently works as a secondary cellular messenger [5, 15]. The PI3K pathway can be initiated by binding of ligands, either to transmembrane tyrosine kinase linked receptors [insulin receptor (IR) and Erb-B2 receptor tyrosine kinase 3 (ERBB3) receptor human epidermal growth factor receptor 3 (HER3)] or to G-protein-coupled receptors and subsequent RAS GTPases (Figure 1) [6, 1619]. The heterodimeric complex of p85–p110 determines PI3K activity, with p85 having no intrinsic PI3K activity and thus acting to stabilize the complex and inhibit PI3K activity [2023]. Upon binding of the ligand to the receptor, cellular phosphoproteins bind to the Src homology 2 (SH2) domain of p85, inducing a conformational change and leading to the release of p85 from p110 [2325]. Activated p85 (p85a) can have downstream effects via the mitogen-activated protein kinase (MAPK)-extracellular signal-regulated kinase (ERK) pathways to cause cellular proliferation and increase cellular motility [26]. ERα is methylated by the arginine methyltransferase protein arginine methyltransferase 1 (PRMT1) and this has been shown to be a prerequisite for the formation of the methylated ERα/Src/PI3K complex and for the activation of downstream signaling [27].

    p110a/PI3K catalyzes the phosphorylation of PIP2 into PIP3 [5] whilst the reverse of this reaction, the dephosphorylation of PIP3 to PIP2, is catalyzed via PTEN [28]. PIP3 translocates to the plasma membrane and binds to the pleckstrin homology domain (PHD) of Akt/protein kinase B (PKB) [29] and this allows partial activation of Akt by facilitating the phosphorylation of Akt on threonine 308 (T308) by PDK1 [30]. Full activation of Akt occurs upon binding of mTOR to the serine 473 (S473) carboxy-terminal hydrophobic motif of Akt [31]. Fully activated Akt regulates and enhances angiogenesis, anabolic metabolism, proliferation, and inhibition of apoptosis [32].

    The mTOR comprises 2 structurally different catalytic complexes, mTOR complex 1 (mTORC1) and mTORC2 [33]. mTOR regulates several cell processes including proliferation, autophagy, metabolism, survival, stress response, angiogenesis, and survival. mTORC1 phosphorylates T389 on P70-S6kI with full activation provided by the phosphorylation on T229 by PDK1 leading to transcription of pro-growth factors [34]. mTORC1 negatively inactivates autophagy by the actions of unc-51-like kinase 1/2 (ULK1/2) and autophagy-related 12 (ATG12) [3537]. mTORC2 is involved in the action of Akt by phosphorylating S473, thus mTORC2 could be considered upstream of mTORC1 [38]. Constitutive upregulated mTOR activity leads to cancer cells with unregulated growth and inhibition to autophagy thus conferring them a survival advantage and as such targeting mTOR activity has been a therapeutic approach of great interest in a number of tumor types [38].

    PI3K/Akt/mTOR pathway and interaction with ER signaling. The binding of IGF to IGFR leads to autophosphorylation of the receptor and IRS-1. IRS-1 leads to splitting of heterodimeric complex (p85, p110) and activation of p110 which via its PI3K activity converts PIP2 to PIP3. PIP3 translocates to the plasma membrane and binds to Akt, allowing phosphorylation at T308 by PDK1 which achieves partial activation of Akt. Full activation of Akt is achieved by phosphorylation at S473 by mTOR. Active Akt phosphorylates S167 facilitate activation of downstream ER nuclear transcriptional activity [39]. Upregulation of IGFR, constitutively active PI3K or loss of PTEN leads to activation of PI3K/Akt/mTOR pathway, increased ligand-independent activation of ER, and resistance to ET. IGF: insulin growth factor; IGFR: IGF receptor; IRS-1: IR substrate-1; ERE: estrogen response element; PTEN: phosphatase and tensin homolog; PDK1: 3-phosphoinositide-dependent protein kinase-1; ER: estrogen receptor

    Interaction between ER signaling and the PI3K/Akt/mTOR signaling

    The identification of the ER as a therapeutic target led to a paradigm shift in the treatment of breast cancer and the development of a number of different classes of hormonal therapy, in particular, selective SERM, SERD, and AI [40].

    The two ERs (ERα and ERβ) are derived from the receptor family of ligand activating transcription factors. ERα and ERβ are encoded by distinct genes; ERα is encoded by estrogen receptor 1 (ESR1), on 6q25.1 and ERβ is encoded by ESR2, on 14q22-24 [41, 42]. Circulating estrogen has differential downstream effects on signaling networks depending on the proportion of expression of the two isoforms and the ligands to which they bind. ERα is more widely expressed than ERβ and while ERβ is thought to have antiproliferative actions, it is believed ERα activation leads to proliferation in breast cancers and is the main focus of the review article. In the absence of estrogen, ERα is held in an inactive ER-chaperon complex [43]. Activation of downstream signaling by ER may be ligand-dependent or ligand-independent. Upon ligand binding by estrogens, ER dissociates from its chaperone proteins facilitating homodimerization or heterodimerization between the isoform occurs, and a conformational change results in binding of response elements to the regulatory region of ER regulated genes. When either isoform of ER is bound to DNA, protein complexes form to either activate or repress target genes [43, 44].

    Ligand independent activation of the ER can be mediated by intracellular/extracellular signals such as the epidermal growth factor receptor (EGFR) and IGF-1 [45]. Here, downstream activation of the MAPK by epidermal growth factor (EGF) binding to its receptor, leads to subsequent phosphorylation of S118 on the activation function-1 (AF1) domain of ER and downstream signaling via binding of ER to ER regulatory regions on genes [4648]. Furthermore, ligand-independent activation may occur through phosphorylation of S167 on the AF1 of ER via the actions of Akt [49].

    Whole-exome sequencing by The Cancer Genome Atlas (TCGA) program has confirmed earlier work, showing that gain of function mutations in PIK3CA and AKT1, and inactivating mutations in PTEN are common in ER+ breast cancer (32–49%, 13–24%, and approximately 7% respectively) [5052]. The common mutational hotspots of PIK3CA are in the kinase and helical domain which are understood to increase kinase activity thus driving cell proliferation [51, 53]. A meta-analysis of 26 studies encompassing 4,754 patients demonstrated that PIK3CA mutations are strongly associated with ER expression [odds ratio (OR) 1.92, 95% confidence interval (CI): 1.65–2.23; P < 0.00001] [54]. Overall clinical data suggest that PIK3CA mutations in ER+ tumors, may be a favorable prognostic marker [55, 56]. Murine models have demonstrated that a constitutively active Akt can lead to breast cancer tumorigenesis, and in human samples, 60% of ductal carcinoma in situ demonstrated Akt overexpression [57, 58]. Furthermore constitutively activated Akt was found to confer resistance to both SERM and SERD therapy by causing estrogen-independent activation of ER [59]. PTEN mutations causing constitutive activation of the PI3K pathway have also been implicated in tumorigenesis in murine breast cancer models and also in conferring resistance to ET [60, 61].

    Preclinical studies combining ET and inhibition of the PI3K/Akt/mTOR pathway

    In the context of ER+ breast cancer, the rationale for targeting the PI3K/Akt/mTOR pathway is to overcome resistance to ET by reducing proliferation driven by ligand-dependent activation and inhibiting ligand-independent activation of the ER. Both pan-PI3K inhibitors target all four PI3K class I isoforms and isoform-specific PI3K inhibitors have been developed [62]. Buparlisib is a pan-class I PI3K inhibitor that demonstrated inhibition of tumor growth in a murine PIK3CA-mutant xenograft model [63]. Similar preclinical anti-tumor activity was observed for other class I pan PI3K inhibitors. Copanlisib (BAY 80-6946) and pictilisib (GDC-0941) induced tumor regression in a rat HER2-amplified PIK3CA-mutated and murine HER2 amplified PIK3CA-mutated breast cancer models, respectively [64, 65]. In vitro, fulvestrant sensitized ER+ breast cancer cells to PI3K inhibition and subsequently induced apoptosis [66].

    The potent, allosteric Akt inhibitor, MK2206, has demonstrated in vitro activity against both thyroid and breast cancer cell lines with PI3KCA mutations [6771] and, in a breast cancer PTEN mutated xenograft model, MK2206 inhibited tumor growth [68]. Capivasertib (AZD5363), an ATP-competitive Akt inhibitor has demonstrated anti-tumor activity in a xenograft model of PIK3CA-mutated breast cancer [72]. High Akt activity was found to be a predictive biomarker of sensitivity to ipatasertib (GDC-0068) in PTEN/PIK3CA-mutant MCF breast cancer cells [73].

    In a screen of breast cancer cell lines, PTEN, Akt, and phosphorylated ribosomal S6 kinase 1 (pS6K1) levels were associated with sensitivity to the mTOR inhibitor, rapamycin [74]. Insulin and IGF signaling are implicated in breast cancer tumorigenesis [75] and IGF binding to its receptor and subsequent autophosphorylation leads to phosphorylation of IR substrate-1 (IRS-1) and activation of the PI3K/Akt/mTOR pathway [75]. Upregulation of IGF-1 receptor (IGF-1R) by cancer cells has been observed to associate with resistance to hormonal therapy [76]. Monoclonal antibodies and ligand neutralizing strategies have been developed to target IGF-1R [77]. NT157 which targets the IRS-1 for destruction via the proteasome has demonstrated a pro-apoptotic effect in PLX4032 melanoma cells and prostate cancer cells [78, 79]. MEDI-573 is a monoclonal antibody that neutralizes IGF-1 and IGF-2 with activity in IGF-1/IGF-2 driven tumors in a murine model [80]. BMS-536924, a dual IGF-1R/IR tyrosine kinase inhibitor inhibited tumor growth in tamoxifen-resistant MCF-7 cells [81]. Finally, in a resistant ER+ xenograft model, there was a synergistic interaction between another IGF-1R/IR inhibitor (OSI-906) and fulvestrant [82].

    In summary, the ER has complex interactions with the IGF/PI3K/Akt/mTOR signaling cascade and substantial preclinical data have demonstrated the potential for synergistic co-targeting of these pathways.

    Targeting the PI3K/Akt/mTOR pathway in the clinic

    Several drugs targeting the PI3K/Akt/mTOR pathway have received regulatory approval in the treatment of a number of solid and hematological malignancies. Two of these agents, the mTOR inhibitor, everolimus, and α-specific PI3K inhibitor, alpelisib, are now established and licensed drugs for the treatment of advanced ER+ breast cancer when used in combination with ET [8385]. See Table 1 for a summary of clinical trials combining ET with drugs targeting the PI3K/Akt/mTOR pathway.

    Published clinical trials combining ET and inhibitors of the PI3K/Akt/mTOR pathway

    Name of trial Study design Comparators Primary endpoint
    mTOR inhibitors plus ET
    BOLERO-2 [86] Phase III RCT
    2 arms
    724 patients
    Everolimus/exemestane vs. placebo/exemestane PFS 7.8 vs. 3.2 months
    HR 0.45; 95% CI: 0.38–0.54; P < 0.0001
    BOLERO-6 [87] Phase II RCT
    3 arms
    309 patients
    Everolimus/exemestane vs. everolimus alone vs. capecitabine alone PFS 8.4 vs. 6.8 vs. 9.6 months
    Everolimus/exemestane vs. everolimus: HR 0.74 (90% CI: 0.57–0.97)
    Everolimus/exemestane vs. capecitabine: HR 1.26 (90% CI: 0.96–1.66)
    HORIZON [88] Phase III RCT
    2 arms
    1,112 patients
    Letrozole/temsirolimus vs. letrozole/placebo PFS 8.9 vs. 9.0 months
    HR 0.90; 95% CI: 0.76–1.07; P = 0.25
    TAMRAD [89] Phase II open-label
    2 arms
    111 patients
    Tamoxifen/everolimus vs. tamoxifen alone 6-month CBR
    61% (95% CI: 47–74) vs. 42% (95% CI: 29–56); P = 0.045
    PrE0102 [90] Phase II RCT
    2 arms
    131 patients
    Fulvestrant/everolimus vs. fulvestrant/placebo PFS 10.3 vs. 5.1 months
    HR 0.61; 95% CI: 0.40–0.92; P = 0.02
    NCT02049957 [91] Phase Ib/II open-label
    2 cohorts
    118 patients
    Everolimus-sensitive group sapanisertib/exemestane or fulvestrant vs. everolimus-resistant group sapanisertib/exemestane or fulvestrant 16-week CBR
    45% (95% CI: 31.1–59.7) vs. 23% (95% CI: 11.8–38.6)
    MANTA [92] Phase II open-label
    3 arms
    333 patients
    Fulvestrant/vistusertib (continuous or intermittent dosing) vs. fulvestrant/everolimus vs. fulvestrant alone PFS 7.6 (daily vistusertib) and 8.0 (intermittent vistusertib) vs. 12.3 vs. 5.4 months
    Fulvestrant/daily vistusertib vs. fulvestrant: HR 0.88; 95% CI: 0.63–1.24; P = 0.46
    Fulvestrant/intermittent vistusertib vs. fulvestrant: HR 0.79; 95% CI: 0.55–1.12; P = 0.16
    Fulvestrant/daily vistusertib vs. fulvestrant/everolimus: HR 0.63; 95% CI: 0.45–0.90; P = 0.01
    Fulvestrant/intermittent vistusertib vs. fulvestrant/everolimus: HR 0.71; 95% CI: 0.49–1.01; P = 0.06
    TRINITI-1 [93] Phase I/II open-label
    95 patients
    Ribociclib/everolimus/exemestane CBR at week 24; 41.1% (95% CI: 31.1–51.6)
    NCT02123823 [94] Phase Ib/II open-label
    2 arms
    140 patients
    Xentuzumab/everolimus/exemestane vs. everolimus/exemestane PFS 7.3 vs. 5.6 months
    HR 0.97; 95% CI 0.57–1.65; P = 0.9057
    PI3K inhibitors plus ET
    BELLE-2 [95] Phase III RCT
    2 arms
    1,147 patients
    Buparlisib/fulvestrant vs. placebo/fulvestrant PFS total population 6.9 vs. 5.0 months
    HR 0.78; 95% CI: 0.67–0.89; P = 0.00021
    PFS PI3K pathway-activated patients 6.8 vs. 4.0 months
    HR 0.76; 95% CI: 0.60–0.97; P = 0.014
    BELLE-3 [96] Phase III RCT
    2 arms
    432 patients
    Buparlisib/fulvestrant vs. placebo/fulvestrant PFS 3.9 vs. 1.8 months
    HR 0.67; 95% CI: 0.53–0.84; P = 0.00030
    SOLAR-1 [97] Phase III RCT
    2 arms
    572 patients
    Alpelisib/fulvestrant vs. placebo/fulvestrant PFS 11.0 vs. 5.7 months
    HR 0.65; 95% CI: 0.50–0.85; P < 0.001 in PIK3CA-mutant patients
    NCT01870505 [98] Phase I dose-escalation
    2 arms
    14 patients
    Arm A: alpelisib/letrozole
    Arm B: alpelisib/exemestane
    DLTs were maculopapular rash, hyperglycemia, and abdominal pain
    8-week best response SD in 5 patients and PR in 1 patient
    NCT01791478 [99] Phase Ib
    26 patients
    Letrozole/alpelisib MTD of alpelisib plus letrozole at 300 mg/day
    CBR 35%; 95% CI: 17–56% (44% for PIK3CA-mutant vs. 20% for PIK3CA wild-type patients)
    NCT01219699 [100] Phase Ib open-label
    87 patients
    Alpelisib/fulvestrant MTD of alpelisib combined with fulvestrant 400 mg once daily, and the RP2D 300 mg
    PFS at the MTD 5.4 months (95% CI: 4.6–9.0)
    FERGI [101] Phase II RCT
    2 arms, part 1 and 2 (only patients with PIK3CA mutations)
    229 patients
    Pictilisib/fulvestrant vs. placebo/fulvestrant Part 1 PFS 6.6 vs. 5.1 months
    HR 0.74; 95% CI: 0.52–1.06; P = 0.096
    Part 2 PFS 5.4 vs. 10.0 months
    HR 1.07; 95% CI: 0.53–2.18; P = 0.84
    NCT01082068 [102] Phase I/II open-label
    2 arms
    21 patients in phase I and 51 patients in phase II
    Arm A: pilaralisib/letrozole
    Arm B: voxtalisib/letrozole
    Arm A: ORR 4% (90% CI: 0.2–18.3)
    PFS 8 weeks (90% CI: 7.7–16.1)
    Arm B: no patient achieved ORR
    PFS 7.9 weeks (90% CI: 7.1–15.7)
    BYlieve [103] Phase II open-label
    3 cohorts
    127 patients
    Alpelisib/fulvestrant 50.4% (95% CI: 41.2–59.6) alive without disease progression at 6 months
    NCT02077933 [104] Phase Ib open-label
    Breast cancer expansion cohort
    11 patients
    Alpelisib/exemestane with or without everolimus Triplet escalation phase: MTD was alpelisib 200 mg, everolimus 2.5 mg, exemestane 25 mg
    Triplet cohort: ORR of 25.0% and DCR of 62.5% (90% CI: 28.9–88.9)
    NCT02058381 [105] Phase Ib open-label
    2 arms
    29 patients
    Arm A: tamoxifen/goserelin/alpelisib
    Arm B: tamoxifen/goserelin/buparlisib
    Arm A: treatment discontinuation 18.8%, PFS 25.2 months (95% CI: 2.7–36.3)
    Arm B: treatment discontinuation 53.8%, PFS 20.6 months (95% CI: 2.9 to not reached)
    NEO-ORB [106] Phase II RCT
    2 arms
    257 patients
    Letrozole/alpelisib vs. letrozole/placebo ORR 43% vs. 45% for PIK3CA-mutant patients and 63 vs. 61% for PIK3CA wild-type patients
    pCR 1.7% vs. 3% for PIK3CA-mutant patients and 2.8% vs. 1.7% for PIK3CA wild-type patients
    NCT02734615 [107] Phase I open-label, dose-escalation
    3 arms
    198 patients
    Arm A: LSZ102 alone
    Arm B: LSZ102/ribociclib
    Arm C: LSZ102/alpelisib
    Arm A: DLTs 5%, ORR 1.3% (95% CI: 0.0–7.0)
    Arm B: DLTs 3%, ORR 16.9% (95% CI: 9.3–27.1%)
    Arm C: DLTs 19%, ORR 7% (95% CI: 1.5–19.1)
    SANDPIPER [108] Phase III RCT
    2 arms
    516 patients
    Taselisib/fulvestrant vs. placebo/fulvestrant PFS 7.4 vs. 5.4 months
    HR 0.70; 95% CI: 0.56–0.89; P = 0.0037
    NCT01296555 [109] Phase II open-label single arm
    60 patients
    Taselisib/fulvestrant CBR total population 29.5% (95% CI: 16.8–45.2)
    CBR PIK3CA-mutant 38.5% (95% CI: 13.9–68.4)
    ORR total population 22.7% (95% CI: 11.5–37.8)
    ORR PIK3CA-mutant 38.5% (95% CI: 13.9–68.4)
    LORELEI [110] Phase II RCT
    2 arms
    334 patients
    Taselisib/letrozole vs. placebo/letrozole ORR 38% for placebo vs. 50% for taselisib
    OR 1.55; 95% CI: 1.00–2.38; P = 0.049

    pCR 2% for taselisib vs. 1% for placebo
    OR 3.07; 95% CI: 0.32–29.85; P = 0.37
    PIPA [111] Phase Ib expansion
    25 patients
    Palbociclib/taselisib/fulvestrant ORR 37.5% (95% CI: 18.8–59.4)
    CBR 58.3% (95% CI: 36.6–77.9)
    PFS 7.2 months (95% CI: 3.9–9.9)
    NCT03006172 [112] Phase I open-label, dose-escalation
    2 arms
    70 patients
    Arm A: galone (GDC-0077)/letrozole
    Arm B: galone (GDC-0077)/palbociclib/letrozole
    Arm A: no DLTs, confirmed PR 8%, CBR 35%
    Arm B: no DLTs, confirmed PR 36%, CBR 76%
    Akt inhibitors plus ET
    FAKTION [113] Phase II RCT
    2 arms
    140 patients
    Capivasertib/fulvestrant vs. placebo/fulvestrant PFS 10.3 vs. 4.8 months
    HR 0.58; 95% CI: 0.39–0.84; P = 0.0044
    NCT01776008 [71] Phase II open-label
    16 patients
    MK-2206/anastrozole plus goserelin for premenopausal patients pCR rate 0% (90% CI: 0–17.1)
    TAKTIC [114] Phase Ib open-label
    3 arms
    25 patients
    Arm A: ipatasertib/AI
    Arm B: ipatasertib/fulvestrant
    Arm C: ipatasertib/fulvestrant/palbociclib
    Arm C (12 patients): no DLTs/discontinuations
    PR 2/12 patients
    SD 3/12 patients
    Dual PI3K/mTOR inhibitors plus ET
    NCT02684032 [115] Phase Ib dose-escalation/expansion
    3 arms
    35 patients
    Arm A: gedatolisib/palbociclib/letrozole first-line
    Arm B: gedatolisib/palbociclib/fulvestrant second-line, CDKi 4/6 naive
    Arm C: gedatolisib/palbociclib/fulvestrant prior CDKi 4/6
    Gedatolisib/palbociclib/letrozole DLTs 4/15 patients, SD/PR 53%/33%
    Gedatolisib/palbociclib/fulvestrant DLTs 4/20 patients, SD/PR 55%/20%
    Display full size

    RCT: randomized controlled trial; PFS: progression-free survival; HR: hazard ratio; CBR: clinical benefit rate; DLT: dose-limiting toxicity; SD: stable disease; PR: partial response; MTD: maximum tolerated dose; ORR: objective response rate; pCR: pathologic complete response; CDKi: CDK inhibitor; DCR: disease control rate; RP2D: recommended phase 2 dose

    mTOR inhibitors

    Since rapamycin (sirolimus) and the first generation of mTOR inhibitors were used in clinical practice, a wide range of agents have been developed, demonstrating more potent specificity [116]. Three generations of mTOR inhibitors have been studied for their effectiveness in different types of solid cancers. Rapamycin and its analogs (the rapalogs), temsirolimus, everolimus, and ridaforolimus, are allosteric inhibitors of mTOR, targeting the activity of the mTORC1 complex and inhibiting phosphorylation of downstream substrates [117]. Selective small-molecule mTORC2 inhibitors have been difficult to develop due to the intricate protein-protein interactions of the mTORC2 complex. Rapalogs lack sufficient mTORC2 inhibition and are known to stimulate the IGF-1 and Akt pathways through a feedback mechanism, ultimately leading to treatment resistance [118]. ATP-competitive mTOR inhibitors such as vistusertib and sapanisertib, and rapalink-1, are second and third-generation agents respectively, designed to overcome these issues by showing a higher affinity for both mTORC1 and 2 complexes [117, 119, 120]. The mTOR inhibitors temsirolimus and everolimus were the first to enter clinical practice in the treatment of advanced renal cell cancer, with indications later expanding to include ER+ HER2-negative (HER2) breast cancer, pancreatic neuroendocrine tumors, and astrocytomas [121124].

    The combination of mTOR inhibition with ET has been extensively studied in the metastatic breast cancer setting. Following the positive results of the BOLERO-2 trial, everolimus was the first mTOR inhibitor to be approved in the treatment of breast cancer, when used in combination with exemestane [86]. This multicentre, phase III trial randomized 724 patients to receive either exemestane plus everolimus or exemestane plus placebo, with visceral disease and ET sensitivity as stratification factors. Clinical benefit was demonstrated for the combination, with a more than twofold increase in PFS compared to placebo, across all predefined subgroups although statistical significance was not achieved for its secondary endpoint, overall survival [125, 126]. BOLERO-6, a three-arm phase II study, compared everolimus plus exemestane vs. either everolimus or capecitabine alone. The study was powered to provide estimates of treatment effect, but no formal statistical analysis was preplanned. After a median follow-up of 37.6 months, PFS for the combination arm was estimated at 8.4 months vs. 6.8 months for exemestane monotherapy. Capecitabine outperformed everolimus plus exemestane, although the authors noted that this might be due to imbalances in the baseline characteristics [87]. Further supportive evidence of the combination of everolimus with ET was provided by the single-arm, phase II BOLERO-4 trial in which 202 patients were treated with letrozole plus everolimus in the first-line setting, and a median PFS of 22 months was observed [127]. The combination of everolimus plus exemestane in both the BOLERO-2 and BOLERO-4 trials demonstrated an acceptable safety profile. The main reported grade 3 and 4 adverse events (AE) were stomatitis, anemia, and fatigue, while hyperglycemia and pneumonitis were less frequent [86, 87].

    Everolimus was further investigated in combination with tamoxifen, in the TAMRAD study, a phase II trial of metastatic, breast cancer resistant to AI therapy [89]. At 6 months, the CBR was shown to be significantly higher, compared to those treated with tamoxifen alone. The risk of progression and risk of death was also significantly reduced for patients receiving the combination, demonstrating a 4-month improvement in time to progression compared to monotherapy. In a subsequent predefined exploratory subgroup analysis, the response was found to be associated with acquired rather than primary hormone resistance, while further translational analysis of 55 tumor samples indicated mTORC1 activation as a potential predictive biomarker for treatment efficacy [89, 128]. The efficacy of another combination was assessed in the metastatic setting of AI-resistant ER+ breast cancer, comparing fulvestrant plus everolimus vs. fulvestrant plus placebo. PrE0102, a phase II randomized trial, reported a doubling of the PFS from 5.1 months to 10.3 months in favor of the combination. The objective response and CBRs showed a similar trend, whilst relatively few grade 3 and 4 AEs were observed [90].

    In comparison, results from a phase III study of temsirolimus were disappointing. HORIZON compared first-line letrozole plus temsirolimus vs. letrozole plus placebo, for postmenopausal patients with advanced ER+ breast cancer [88]. The trial enrolled 1,112 patients, 40% of whom had received prior adjuvant ET. The primary endpoint, PFS, was not reached at the time of the second predefined interim analysis, and the study was consequently discontinued. Nevertheless, an interaction between age and treatment response was observed and further investigated in an exploratory analysis using subpopulation treatment effect pattern plot (STEPP) methodology, showing consistently improved outcomes in women aged ≤ 65 years (P = 0.003 for interaction). The authors concluded that the lack of PFS benefit could have been attributed to the difference in AI exposure between the populations recruited in HORIZON and BOLERO-2 trials, as well as the suspected alterations developed in endocrine-resistant tumors. In addition, the different drug formulations and dosing schedules between everolimus and temsirolimus, might have contributed to the contrasting findings [129].

    Sapanisertib is a potent ATP-competitive inhibitor of mTORC1 and 2 and has been investigated in a non-randomized phase Ib/II study in which a total of 118 patients with metastatic, ER+ breast cancer that had previously progressed on everolimus with either exemestane or fulvestrant were recruited to receive sapanisertib in combination with exemestane or fulvestrant [91]. CBR was reported at 48% compared to 23% in the exemestane-sensitive and exemestane-resistant cohorts, respectively, with an overall response rate of 8% vs. 2%, respectively. Only a few patients exhibited dose-limiting toxicities, with nausea, diarrhea, fatigue, and hyperglycemia the most common AEs. Vistusertib, another dual mTORC1/2 inhibitor, has also been investigated in combination with fulvestrant in phase II randomized trial that recruited 333 post-menopausal women that had progressed on an AI [92]. The MANTA trial failed to meet its primary objective of PFS against both fulvestrant/everolimus combination and fulvestrant monotherapy.

    PI3K inhibitors

    Buparlisib is an oral, potent, pan-PI3K inhibitor that has been extensively studied in both solid cancers and haematologic malignancies [84]. Its activity in ER+ breast cancer was investigated in two large, randomized, double-blind, multicentre phase III trials, BELLE-2 and BELLE-3. In the BELLE-2 trial, 1,147 postmenopausal women with advanced, AI-resistant breast cancer were randomized to receive daily doses of either buparlisib or placebo with monthly intramuscular fulvestrant, in 28-day cycles. The study met its primary endpoint, with a modest PFS benefit reported at 6.9 months for buparlisib vs. 5 months for the control arm. However, the authors concluded that in view of the increased toxicities observed in the buparlisib group, including liver toxicity, rash, and hyperglycemia, no further study of this combination should be pursued [95]. Results of the BELLE-3 trial, published soon after, led to a similar conclusion. In this placebo-controlled trial, the combination of buparlisib plus fulvestrant was investigated in patients who had progressed on or after ET and mTOR treatment. Again, a two-month PFS benefit was demonstrated in favor of buparlisib (3.9 months vs. 1.8 months). However, the toxicity profile of buparlisib proved unacceptable and two treatment-related deaths were attributed to the combination [96].

    Pictilisib, a pan-PI3K inhibitor with higher affinity for the α and δ isoforms, was studied in combination with fulvestrant in the FERGI trial a randomized, placebo-controlled phase II trial that recruited patients with metastatic breast cancer resistant to ET. Patients were stratified according to PIK3CA mutation status and previous exposure to AI, leading to primary or secondary resistance. However, no difference in PFS was achieved in any of the subgroups treated with the combination, and tolerability was challenging [101]. Pilaralisib, another pan-PI3K inhibitor, as well as the dual PI3K/mTOR inhibitor voxtalisib were each tested in combination with letrozole in phase I/II trial of AI-refractory breast cancer. However clinical activity was disappointing and among the 25 and 26 patients enrolled in phase II, in the pilaralisib and voxtalisib arms respectively, only one treated with pilaralisib achieved a PR with a median PFS of 8 weeks [102].

    The need for improved efficacy and tolerability has led to a shift in interest toward isoform-specific PI3K-inhibitors. Alpelisib selectively inhibits the PI3Kα isoform and its effectiveness against tumors harboring PIK3CA mutations was initially established in several early phase studies, testing the combination of alpelisib with ET in patients with ER+ breast cancer [98, 100]. The publication of results from the pivotal phase III SOLAR-1 trial eventually led to the approval of alpelisib in combination with fulvestrant for patients with PIK3CA-mutant, ER+ advanced breast cancer, and prior exposure to ET [85]. In this pivotal study, the cohort of 341 patients with PIK3CA mutations showed a significantly improved PFS of 11 months in the fulvestrant/alpelisib group vs. 5.7 months in the fulvestrant/placebo group meeting its primary endpoint. The most common grade 3/4 toxicities involved hyperglycemia, rash, and diarrhea, which attributed to an increased treatment discontinuation rate in patients receiving the combination. Final overall survival results were recently published, showing a difference of 7.9 months in favor of alpelisib, but failing to reach statistical significance. Nonetheless, a stronger treatment effect with borderline significance was observed in patients with lung and/or liver disease for alpelisib [97].

    Although prior treatment with CDK4/6 inhibitor (CDK4/6i) was one of the stratification factors in SOLAR-1, only 20 patients were identified in this subgroup, with the HR for progression or death not reaching statistical significance (HR 0.48; 95% CI: 0.17–1.36). However, the BYlieve study, a phase II cohort study of 127 patients with PIK3CA mutations pretreated with a CDK4/6i, was recently published showing 50.4% of patients were without progression or death at 6 months. Although there was no control arm, these data provide some support for the use of alpelisib/fulvestrant as a therapeutic option in patients with PIK3CA-mutated disease following progression on first-line CDK4/6i [103]. In addition, alpelisib has been investigated as part of a triplet regimen in combination with everolimus and exemestane in a phase Ib study of postmenopausal women with ER+ breast cancer, with an acceptable toxicity profile [130]. Another phase Ib study tested the combination of tamoxifen and goserelin acetate with either alpelisib or buparlisib in premenopausal women with advanced breast cancer [131]. Poor tolerability was observed with buparlisib and here most patients were discontinued due to AEs. A randomized phase II study of alpelisib in combination with letrozole vs. letrozole alone showed no improvement in response in the neoadjuvant setting [106, 132].

    Lastly, the effectiveness of taselisib, a potent PI3Kα-inhibitor, was assessed in a single-arm, phase II study, in combination with fulvestrant in patients with advanced ER+ breast cancer with encouraging activity [109]. Further to this exploratory data, the SANDPIPER study recruited 516 patients with endocrine-resistant, PIK3CA-mutant breast cancer to fulvestrant/taselisib vs. fulvestrant/placebo and met its primary endpoints of a statistically significant improvement in PFS (7.4 months vs. 5.4 months in favor of taselisib). However, given the high rate of serious AEs in the taselisib arm and the modest clinical benefit, taselisib has not become an established therapeutic option [108]. The combination of taselisib/letrozole vs. placebo/letrozole has also been explored in the neoadjuvant setting, in the randomized phase II LORELEI trial with a higher ORR observed for the taselisib combination [110].

    Akt inhibitors

    In the clinic the most extensively studied Akt inhibitors are capivasertib and ipatasertib, both ATP-selective pan-Akt inhibitors with activity against Akt 1, 2, and 3. The FAKTION randomized phase II study examined the addition of capivasertib to fulvestrant vs. placebo in 140 women with ER+ metastatic breast cancer resistant to an AI. PFS was significantly prolonged in the combination arm (10.3 months vs. 4.8 months in favor of capivasertib) and frequent grade 3/4 AEs in the capivasertib arm were hypertension, diarrhea, rash, and infection [113]. An ongoing phase III trial, CAPItello-291, designed to further evaluate this combination, is currently recruiting [133]. Ipatasertib is currently under investigation in combination with fulvestrant in the ongoing, phase III FINER trial, in patients who progressed after first-line treatment with a CDK4/6i and an AI (NCT04650581) [134]. The allosteric pan-Akt inhibitor, MK-2206, was assessed in combination with anastrozole in the neoadjuvant setting in a phase single-arm II study of patients with PIK3CA-mutant disease but clinical activity was not encouraging and toxicity significant [71].

    Agents targeting IGF-1 axis

    Several clinical trials assessing agents that target IGF-1 signaling in combination with ET are ongoing in the setting of both endocrine-sensitive and resistant diseases [77, 135].

    Xentuzumab is a humanized, IGF-1 and IGF-2 neutralizing antibody that has been tested in combination with everolimus and exemestane in a phase Ib/II trial of advanced, ER+ breast cancer. The triplet regimen was administered to 24 post-menopausal women with the endocrine-resistant disease in phase Ib of the study and was well-tolerated, with disease control observed in 57% of patients, and PRs reported in 19% [136]. In a phase II study, patients were randomized to receive either the triplet regimen or the exemestane/everolimus doublet but no PFS benefit was observed in the overall population, although in a pre-specified subgroup analysis there was a suggestion of benefit in patients without the visceral disease [94]. XENERA-1, a phase II trial also assessing this combination, is ongoing in patients with the non-visceral disease. The addition of xentuzumab to abemaciclib, a CDK4/6i, in combination with fulvestrant, has also been investigated in a phase Ib study of women with the endocrine-resistant disease and no prior treatment with CDK4/6i or chemotherapy. Early data suggest encouraging clinical activity and tolerability [137]. A phase II study of another IGF-1/2 neutralizing monoclonal antibody, dusigitumab, in combination with an AI in ER+ breast cancer is yet to report [80].

    A different approach involves the employment of monoclonal antibodies directed toward the IGF-1R. Three agents, ganitumab, cixutumumab, and dalotuzumab have been tested in phase II trials in combination with mTOR inhibitors and ET, with no supportive evidence of their treatment benefit to date [138141]. In addition to their limited efficacy, the AE profile, in particular hyperglycemia and hyperinsulinemia, have complicated clinical development [77].

    In summary, the mTOR inhibitor, everolimus, and PI3Kα inhibitor, alpelisib, are now licensed therapies that have been shown to improve treatment outcomes for metastatic breast cancer when used in combination with hormonal therapy. Other avenues of investigation are ongoing, in particular combining either Akt inhibitors or agents targeting IGF-1 signaling with hormonal therapy.

    Molecular markers and resistance pathways to PI3K/Akt/mTOR inhibition

    The clinical development of therapeutics that target the PI3K/Akt/mTOR pathway in ER+ breast cancer has met with significant challenges. PI3K/Akt/mTOR activation is not ubiquitous in ER+ breast cancer and can happen at several different nodes in the pathway as described above. PI3K/Akt/mTOR pathway blockade is associated with a variety of significant toxicities, including hyperglycemia, which, as described below, may be a mechanism of resistance. Additionally, multiple other resistance mechanisms exist, both intrinsic to the PI3K/Akt/mTOR pathway and in related pathways. Identification of suitable predictive biomarkers is therefore paramount for providing a personalized approach to treatment and to amplifying the chances of demonstrating efficacy in late-phase clinical trials.

    Biomarkers that predict response to inhibition of the PI3K/Akt/mTOR inhibition

    The utility of predictive biomarkers is, in part, dependent on the target of interest within the PI3K/Akt/mTOR pathway. mTOR inhibitors were the first treatment class to be licensed and, as described above, everolimus has shown clinical benefit when given in combination with exemestane for metastatic ER+ breast cancer [142]. However, many patients do not benefit, and significant toxicities are a major factor in discontinuing therapy. Despite the importance of patient selection, there is still no established predictive biomarker in the clinic, though ER+/HER2 positive (HER2+) and ER+ basal-like subtypes appeared to fare worse in retrospective analyses [126, 143145]. A retrospective study of patients receiving everolimus/exemestane suggested AKT1E17K mutations may predict longer PFS, although this needs prospective investigation [146].

    Preclinical work identified mutations within PIK3CA, the gene encoding the PI3Kα isoform p110α catalytic subunit, as an early potential predictive biomarker for PI3K inhibitors [65, 147]. Multiple PI3K inhibitors have been developed for solid tumors, though as previously described, pan-PI3K inhibition (PI3K isoforms α, β, γ, and δ) in ER+ breast cancer has yielded negative trial results [95, 96, 101, 148, 149]. There is also some evidence for the predictive value of PIK3CA mutations for the pan-class I PI3K inhibitor, buparlisib, in metastatic ER+ breast cancer [95, 96]. Assaying for PIK3CA as a predictive biomarker for this therapeutic class has been inconsistent, both in approach and results. The pan-PI3K inhibitor trials used varied biomarkers including assessing PIK3CA hotspot mutations within exons 1, 4, 7, 9, and 20, specific point mutations within these exons such as PIK3CAE545K, or composite “PI3K pathway activation” that included either PIK3CA mutation or PTEN loss of function. Additionally, the PIK3CA mutational analysis included both tumor biopsy samples and circulating tumor DNA (ctDNA), further complicating matters.

    The development of isoform-specific inhibitors followed the early failures of pan-PI3K inhibitors in ER+ breast cancer, with improved efficacy and toxicity profiles [85, 108]. As discussed above, the SOLAR-1 trial showed that PIK3CA mutations, detected either using ctDNA or in tumor tissue, were predictive of improved PFS for alpelisib plus fulvestrant [85].

    Results for taselisib plus fulvestrant in the SANDPIPER-3 trial in metastatic ER+ breast cancer were less clear. Although PIK3CA mutations were predictive for a statistically significant improvement in PFS, the HR in the PIK3CA wild-type group was essentially the same as the mutant group, albeit lacking statistical significance [108]. In both trials, the predictive value of activating PIK3CA mutations was independent of both site and type of mutation. Despite their predictive value in metastatic disease, PIK3CA mutations were not predictive in neoadjuvant trials of either alpelisib or taselisib [106, 110].

    Potential predictive biomarkers, other than PIK3CA, have also been investigated for PI3K inhibitors. Preclinical evidence suggests that PTEN loss should sensitize cells to PI3K/Akt inhibition, although this has not been demonstrated unambiguously in the clinic for either pan-PI3K or PI3Kα inhibitors [65, 95, 149, 150]. Alterations in the p85 regulatory subunit of PI3K due to mutations can lead to constitutive hyperactive PI3K/Akt signaling and this genetic alteration is relatively uncommon in breast cancer (2.8% in the TCGA database) [151]. Luminal B subtype and progesterone receptor-negative disease both appeared to be predictive of Ki67 suppression for pictilisib in the neoadjuvant setting [150]. However, this was only in a small, negative, phase II study, and not seen in trials of other PI3K inhibitors.

    Preclinical evidence suggests mutations in PIK3CA, PIK3R1, Akt, or mutation/loss of PTEN could predict treatment response for Akt inhibitors [152154]. However, although predictive in vitro and in vivo, PIK3CA and PTEN alterations have not been predictive of treatment response in clinical trials of these agents [71, 113, 155157]. In an early phase single cohort study, fulvestrant combined with the Akt inhibitor, capivasertib, has shown encouraging clinical activity in heavily pretreated patients with ER+ breast cancer and AKT1E17K mutations [158]. There are ongoing phase III trials investigating biomarker-driven response and survival for capivasertib and ipatasertib, with stratification according to PI3K/PTEN/Akt alterations, which will hopefully provide clear evidence for a biomarker-driven approach [133, 159].

    Mechanisms of resistance to PI3K/Akt/mTOR inhibition

    Understanding primary or secondary resistance to targeted therapies can inform the patient selection and novel combinations. Resistance mechanisms may be intrinsic to the pathway or arise through extrinsic, related signaling. For example, mTOR’s auto-regulatory feedback loop attenuates its own response, with inhibition of mTORC1 leading to increased activity of PI3K [160]. For PI3K inhibitors, constitutive activity of any of the regulatory or downstream effector proteins has the potential to mediate resistance. PTEN loss has been proposed as a key mechanism of resistance to inhibitors directed at PI3K pathway but not pan-PI3K inhibitors [149, 161]. PTEN loss leads to cells becoming dependent on PI3Kβ, which may explain this differential response to therapy [162]. Activating Akt mutations may also define resistance to PI3K inhibitors, with some early evidence of re-sensitization to PI3K inhibitors with the addition of an Akt inhibitor in a translational study [163].

    The EGFR/Ras/Raf/MAPK kinase (MEK)/ERK and PI3K/Akt/mTOR signaling cascades co-regulate many downstream effectors in parallel. The presence of Kirsten RAS 2 viral oncogene homolog (KRAS) mutations associates with PIK3CA mutations in cancer. KRAS mutations appear to confer resistance to both alpelisib and ipatasertib in early phase trials [164, 165]. While combination inhibition with alpelisib, cetuximab, encorafenib, and binimetinib has been trialed in B-Raf proto-oncogene, serine/threonine kinase (BRAF)-mutant colorectal cancer, evidence for ER+ breast cancer is lacking [166]. For alpelisib, tumor protein p53 (TP53) mutation and fibroblast growth factor receptors (FGFR) 1 and 2 amplification have also been proposed as markers of resistance [99].

    Insulin and resistance to PI3K inhibitors

    Hyperglycaemia is a common on-target toxicity of PI3K inhibitors. PI3K is a necessary downstream effector of the IR and hence inhibition leads to insulin resistance [167]. The resultant increase in circulating glucose further increases insulin secretion. A detailed preclinical study showed that higher levels of circulating insulin as a result of PI3K inhibition may act as a resistance mechanism [168]. The excess insulin secreted in response to hyperglycemia increases signal transduction from the IR to PI3K, abrogating the PI3K inhibitor’s antagonism and restoring pro-oncogenic downstream signaling. Using in vivo patient-derived xenograft mouse models, this study showed that metformin, sodium-glucose cotransporter-2 (SGLT-2) inhibitors, and a ketogenic diet were all capable of restoring inhibition of PI3K/Akt/mTOR signaling. Exogenous insulin nullified the benefit of a ketogenic diet in these mice, suggesting insulin should be avoided in managing this toxicity. To date, in the clinic, standard practice for managing hyperglycemia has been to use metformin [85, 108, 169]. However, it was notable in this study that SGLT-2 inhibition led to a greater restoration of PI3K-inhibitor sensitivity than metformin [168]. A case report of the use of SGLT-2 inhibitors with alpelisib suggests this is well tolerated and can prevent the discontinuation of alpelisib [170]. Of note SGLT-2 inhibitors are associated with urinary tract infections and very rare, life-threatening toxicities including euglycaemic diabetic ketoacidosis, hence safety studies in combination with PI3K inhibitors would help inform practice [171, 172]. See Figure 2 for an overview of hyperglycemia-mediated PI3K inhibitor resistance.

    Mechanisms of hyperglycemia-mediated PI3K inhibitor resistance. Constitutively active PI3Kα leads to the transport of GLUT4 vesicles to the cell membrane, causing glucose uptake into cancer cells. PI3K inhibition limits downstream pathway activation in tumor and non-tumor cells. GLUT4 vesicles are no longer transferred to the cell membrane as an on-target negative consequence, leading to extracellular hyperglycemia. This stimulates excess insulin secretion in pancreatic beta cells, binding to IRs on cancer cells. Overactivation of the IR overcomes the PI3Ki effect, partially reactivating the PI3K/Akt/mTOR pathway. This effect can also be mediated by exogenous insulin. Minimizing hyperglycemia through fasting, metformin or SGLT-2 inhibition reduces insulin secretion, restoring the effectiveness of PI3K inhibition. GLUT4: glucose transporter type 4; PI3Ki: PI3K inhibition

    Calorie restriction and fasting have long been recognized as limiting tumor growth in animal studies [173] and it has been proposed that this is due to reduced insulin-mediated PI3K/Akt/mTOR signaling [174, 175]. Different methods of calorie restriction have been assessed in both animal and human studies [176]. One approach is the fasting-mimicking diet (FMD) which is a low-calorie plant-based diet, high in fat, and low in protein and carbohydrate with micronutrient supplementation [177]. FMD lasts for 4 days, followed by a recovery diet for 26 days. Preclinical work in ER+ breast cancer showed that FMD could induce cancer cell regression, both in vitro and in vivo [178]. Subsequent early phase studies have demonstrated that FMD is tolerable and safe in patients with a variety of cancers, including breast cancer [177]. FMD alone would not be sufficient for restoring endocrine sensitivity in all breast cancers, as tumors with constitutive activation of PI3K/Akt/mTOR signaling are likely to be resistant to fasting approaches [179]. However, fasting approaches in combination with FMD with ET warrant further study in clinical trials.

    In summary, multiple mechanisms of resistance and several putative biomarkers have been identified for PI3K and mTOR inhibitors in breast cancer, mostly focusing on specific genetic alterations. However, new data suggests that the feedback hyperglycemia from targeting this pathway may also play a key role in resistance to treatment and hence therapeutic opportunity.

    Future perspectives

    As new drugs for the treatment of ER+ breast cancer continue to emerge, there may be further opportunities for efficacious novel combinations with mTOR and PI3K inhibitors in appropriately selected populations. To date, published data from clinical trials have used fulvestrant, AI, or tamoxifen as the ET backbone to mTOR, Akt, or PI3K inhibition. Fulvestrant is the only SERD that is licensed for clinical practice but novel, oral SERDs are now in clinical development with improved pharmacokinetics and bioavailability [180]. Four agents have already entered phase III testing; amcenestrant, camizestrant, and giredestrant are examined in combination with palbociclib in the AMEERA-5, SERENA-4, and persevERA breast cancer trials respectively, while elacestrant monotherapy is tested vs. fulvestrant or AI in patients who progressed on CDK4/6i (EMERALD trial) [181]. Early phase studies are assessing combinations of novel SERDs with inhibitors of the PI3K/Akt/mTOR pathway. AMEERA-1, a phase Ia/b trial of amcenestrant in combination with alpelisib and everolimus, is actively recruiting. Similarly, the ongoing SERENA-1 trial is testing camizestrant alongside everolimus or capivasertib (NCT03616587). The only currently available evidence comes from another phase I study, investigating the oral SERD LSZ102. In one of the experimental arms, 43 patients with endocrine-resistant breast cancer were treated with LSZ102 plus alpelisib. Clinical activity was encouraging, with ORR and CBR measured at 7.0% (95% CI: 1.5–19.1) and 20.9% (95% CI: 10.0–36.0) respectively, with an estimated modest median PFS of 3.5 months, irrespective of PIK3CA mutation status [107].

    Co-targeting of the ER and CDK4/6 has become the standard of care for patients with advanced, ER+ breast cancer, albeit acquired resistance to CDK4/6i still almost inevitably develops [182]. Preclinical data have suggested potential synergy in targeting PI3K/Akt/mTOR and CDK4/6 signaling [183]. Preliminary results were published from a phase Ib trial assessing the addition of gedatolisib, a dual PI3K-mTOR inhibitor, to palbociclib plus either letrozole or fulvestrant. Early data suggests promising clinical activity for this combination alongside a manageable toxicity profile, with nausea, neutropenia, and stomatitis as the most frequent AEs [115]. Similarly, recent results of an early phase trial, investigating the combination of inavolisib, a PI3Kα inhibitor, and letrozole with or without palbociclib in patients with PIK3CA-mutant disease, suggested augmented anti-tumor activity with the triplet regimen. Confirmed PR and CBR were measured at 36% and 76%, respectively, for the inavolisib combination, while hyperglycemia, stomatitis, gastrointestinal, and hematological toxicities were among the most common AEs. Further insight into the potential of this triplet combination is expected to be provided by a phase III trial that is currently recruiting (NCT04191499) [112].

    Taselisib has also been tested as a triplet therapy along palbociclib and fulvestrant in PIK3CA-mutant, ER+ breast cancer in the single-arm PIPA trial. The authors concluded that a response rate of 37.5% was promising for superiority to the palbociclib/fulvestrant doublet and warrants further investigation [111]. The Akt inhibitor ipatasertib combined with palbociclib/fulvestrant is under investigation in the phase Ib/III trial, IPATunity150 (NCT04060862) [158]. This placebo-controlled study in the first-line setting of endocrine-resistant breast cancer will add to the existing evidence of the phase Ib TAKTIC trial, whose interim results of 12 patients treated with this triplet combination displayed manageable tolerability and some promise of clinical benefit [114]. Finally, the phase I/II TRINITI-1 trial has just reported results of the combination everolimus/exemestane/ribociclib in 104 patients who progressed on CDK4/6i [93]. The favorable toxicity profile and the observed clinical benefit, with CBR at 41.1%, warrant further study.

    In the clinic, prospective future directions for targeting PI3K/Akt/mTOR in ER+ breast cancer also include further combinations with IGF-targeted agents, antagonists of the androgen receptor (AR), FGFR inhibitors, and checkpoint immunotherapy. As outlined above, data from clinical studies of xentuzumab and other anti-IGF-1R antibodies have, however, been disappointing to date. The role of AR expression in breast cancer and the efficacy of its inhibition has been better understood and established in the ER-negative disease [184]. Preclinical breast cancer models have associated increased AR levels with the presence of PIK3CA mutations and have suggested prognostic value in breast cancer, irrespective of the hormonal status [185, 186]. Further evidence of increased sensitivity of AR-positive (AR+) breast cancer to PI3K pathway inhibition and its potential as a predictive biomarker has provided a rationale for the combined use of anti-androgens with drugs targeting PI3K/Akt/mTOR [187, 188]. An early-phase trial exploring the safety and efficacy of alpelisib in combination with enzalutamide is ongoing for patients with metastatic breast cancer that is AR+ with PTEN loss (NCT03207529) [189].

    Another approach takes advantage of the interplay between the fibroblast growth factor (FGF)/FGFR and PI3K/Akt/mTOR pathways, as FGFR overexpression leads to activation of PI3K/Akt signaling [190]. Aberrant expression of FGFR has been identified as a mediator of endocrine resistance and hence targeting both pathways, especially in the presence of concurrent genetic alterations, is an attractive strategy to augment the efficacy of the PI3K/Akt/mTOR inhibitors [191, 192]. In light of this evidence, a phase I trial studied the combination of alpelisib plus infigratinib in patients with PIK3CA-mutant solid cancers. Unfortunately, at this point results were disappointing without indication of significant clinical activity [193].


    In summary, targeting the PI3K/Akt/mTOR pathway to subvert resistance to ET in breast cancer is now proven to be of clinical benefit with everolimus and alpelisib already used routinely in clinical practice. A new generation of therapeutics awaits full evaluation in this setting, including drugs targeting Akt and IGF-1 signaling. However, challenges remain, in particular, the need for the development of improved biomarkers for patient selection and clinical evaluation of strategies to abrogate mechanisms resistance, such as feedback hyperglycemia.



    adverse events


    aromatase inhibitors


    androgen receptor


    clinical benefit rate


    cyclin-dependent kinases 4 and 6


    cyclin-dependent kinases 4 and 6 inhibitor


    confidence interval


    dose-limiting toxicity


    estrogen receptor


    estrogen receptor-positive


    endocrine therapy


    fibroblast growth factor receptors


    fasting-mimicking diet


    human epidermal growth factor receptor 3


    hazard ratio


    insulin growth factor


    insulin growth factor-1 receptor


    insulin growth factor receptor


    insulin receptor


    insulin receptor substrate-1


    mitogen-activated protein kinase


    mechanistic target of rapamycin complex 1


    odds ratio


    objective response rate


    3-phosphoinositide-dependent protein kinase-1


    progression-free survival


    phosphatidylinositol 4,5-bisphosphate




    partial response


    phosphatase and tensin homolog


    rat sarcoma


    serine 473


    selective estrogen receptor degraders


    selective estrogen receptor modulators


    sodium-glucose cotransporter-2


    threonine 308


    Author contributions

    SL designed, identified key literature, and edited the review. AS, JDC, and ML contributed to the literature search, designed the figures and table. All authors wrote sections of the manuscript, contributed to manuscript revision, read and approved the submitted version.

    Conflicts of interest

    The authors declare that they have no conflicts of interest.

    Ethical approval

    Not applicable.

    Consent to participate

    Not applicable.

    Consent to publication

    Not applicable.

    Availability of data and materials

    Not applicable.


    JDC is funded by the National Institute for Health Research Academic Clinical Fellowship Award (ACF-2020-13-009) and the Oxford Hospital’s Charity (1415). SL is funded by Against Breast Cancer. The funders had no role in design, data collection and analysis, preparation of the manuscript, or decision to publish.


    © The Author(s) 2022.


    Turner NC, Neven P, Loibl S, Andre F. Advances in the treatment of advanced oestrogen-receptor-positive breast cancer. Lancet. 2017;389:240314. [DOI] [PubMed]
    Walsh EM, Smith KL, Stearns V. Management of hormone receptor-positive, HER2-negative early breast cancer. Semin Oncol. 2020;47:187200. [DOI] [PubMed] [PMC]
    Vanhaesebroeck B, Stephens L, Hawkins P. PI3K signalling: the path to discovery and understanding. Nat Rev Mol Cell Biol. 2012;13:195203. [DOI] [PubMed]
    Fruman DA, Chiu H, Hopkins BD, Bagrodia S, Cantley LC, Abraham RT. The PI3K pathway in human disease. Cell. 2017;170:60535. [DOI] [PubMed] [PMC]
    Whitman M, Downes CP, Keeler M, Keller T, Cantley L. Type I phosphatidylinositol kinase makes a novel inositol phospholipid, phosphatidylinositol-3-phosphate. Nature. 1988;332:6446. [DOI] [PubMed]
    Ruderman NB, Kapeller R, White MF, Cantley LC. Activation of phosphatidylinositol 3-kinase by insulin. Proc Natl Acad Sci U S A. 1990;87:14115. [DOI] [PubMed] [PMC]
    Hiles ID, Otsu M, Volinia S, Fry MJ, Gout I, Dhand R, et al. Phosphatidylinositol 3-kinase: structure and expression of the 110 kd catalytic subunit. Cell. 1992;70:41929. [DOI]
    Sjölander A, Yamamoto K, Huber BE, Lapetina EG. Association of p21ras with phosphatidylinositol 3-kinase. Proc Natl Acad Sci U S A. 1991;88:790812. [DOI] [PubMed] [PMC]
    Rodriguez-Viciana P, Warne PH, Dhandt R, Vanhaesebroeckt B, Goutt I, Fry MJ, et al. Phosphatidylinositoi-3-OH kinase as a direct target of Ras. Nature. 1994;370:52732. [DOI] [PubMed]
    Kodaki T, Woscholski R, Hallberg B, Rodriguez-Viciana Julian Downward P, Parker PJ. The activation of phosphatidylinositol 3-kinase by Ras. Curr Biol. 1994;4:798806. [DOI] [PubMed]
    Hara K, Yonezawa K, Sakaue H, Ando A, Kotani K, Kitamura T, et al. 1-phosphatidylinositol 3-kinase activity is required for insulin-stimulated glucose transport but not for RAS activation in CHO cells. Proc Natl Acad Sci U S A. 1994;91:74159. [DOI] [PubMed] [PMC]
    Okada T, Kawano Y, Sakakibara T, Hazeki O, Ui M. Essential role of phosphatidylinositol 3-kinase in insulin-induced glucose transport and antilipolysis in rat adipocytes. Studies with a selective inhibitor wortmannin. J Biol Chem. 1994;269:356873. [DOI] [PubMed]
    Wennström S, Siegbahn A, Yokote K, Arvidsson AK, Heldin CH, Mori S, et al. Membrane ruffling and chemotaxis transduced by the PDGF beta-receptor require the binding site for phosphatidylinositol 3’ kinase. Oncogene. 1994;9:65160. [PubMed]
    Samuels Y, Wang Z, Bardelli A, Silliman N, Ptak J, Szabo S, et al. High frequency of mutations of the PIK3CA gene in human cancers. Science. 2004;304:554. [DOI] [PubMed]
    Jean S, Kiger AA. Classes of phosphoinositide 3-kinases at a glance. J Cell Sci. 2014;127:9238. [DOI] [PubMed] [PMC]
    Guo RX, Wei LH, Tu Z, Sun PM, Wang JL, Zhao D, et al. 17 beta-estradiol activates PI3K/Akt signaling pathway by estrogen receptor (ER)-dependent and ER-independent mechanisms in endometrial cancer cells. J Steroid Biochem Mol Biol. 2006;99:918. [DOI] [PubMed]
    Soltoff SP, Carraway KL 3rd, Prigent SA, Gullick WG, Cantley LC. ErbB3 is involved in activation of phosphatidylinositol 3-kinase by epidermal growth factor. Mol Cell Biol. 1994;14:35508. [DOI] [PubMed] [PMC]
    Prigent SA, Gullick WJ. Identification of c-erbB-3 binding sites for phosphatidylinositol 3’-kinase and SHC using an EGF receptor/c-erbB-3 chimera. EMBO J. 1994;13:283141. [DOI] [PubMed] [PMC]
    Nakano N, Matsuda S, Ichimura M, Minami A, Ogino M, Murai T, et al. PI3K/AKT signaling mediated by G protein-coupled receptors is involved in neurodegenerative Parkinson’s disease (review). Int J Mol Med. 2017;39:25360. [DOI] [PubMed]
    Otsu M, Hiles I, Gout I, Fry MJ, Ruiz-Larrea F, Panayotou G, et al. Characterization of two 85 kd proteins that associate with receptor tyrosine kinases, middle-T/pp60c-src complexes, and PI3-kinase. Cell. 1991;65:91104. [DOI] [PubMed]
    Skolnik EY, Margolis B, Mohammadi M, Lowenstein E, Fischer R, Drepps A, et al. Cloning of PI3 kinase-associated p85 utilizing a novel method for expression/cloning of target proteins for receptor tyrosine kinases. Cell. 1991;65:8390. [DOI] [PubMed]
    Escobedo JA, Navankasattusas S, Kavanaugh WM, Milfay D, Fried VA, Williams LT. cDNA cloning of a novel 85 kd protein that has SH2 domains and regulates binding of PI3-kinase to the PDGF beta-receptor. Cell. 1991;65:7582. [DOI] [PubMed]
    Yu J, Zhang Y, McIlroy J, Rordorf-Nikolic T, Orr GA, Backer JM. Regulation of the p85/p110 phosphatidylinositol 3’-kinase: stabilization and inhibition of the p110alpha catalytic subunit by the p85 regulatory subunit. Mol Cell Biol. 1998;18:137987. [DOI] [PubMed] [PMC]
    Rordorf-Nikolic T, Van Horn DJ, Chen D, White MF, Backer JM. Regulation of phosphatidylinositol 3’-kinase by tyrosyl phosphoproteins. Full activation requires occupancy of both SH2 domains in the 85-kDa regulatory subunit. J Biol Chem. 1995;270:36626. [DOI] [PubMed]
    Carpenter CL, Auger KR, Chanudhuri M, Yoakim M, Schaffhausen B, Shoelson S, et al. Phosphoinositide 3-kinase is activated by phosphopeptides that bind to the SH2 domains of the 85-kDa subunit. J Biol Chem. 1993;268:947883. [DOI] [PubMed]
    Di Zazzo E, Feola A, Zuchegna C, Romano A, Donini CF, Bartollino S, et al. The p85 regulatory subunit of PI3K mediates cAMP-PKA and insulin biological effects on MCF-7 cell growth and motility. ScientificWorldJournal. 2014;2014:565839. [DOI] [PubMed] [PMC]
    Le Romancer M, Treilleux I, Leconte N, Robin-Lespinasse Y, Sentis S, Bouchekioua-Bouzaghou K, et al. Regulation of estrogen rapid signaling through arginine methylation by PRMT1. Mol Cell. 2008;31:21221. [DOI] [PubMed]
    Myers MP, Pass I, Batty IH, Van der Kaay J, Stolarov JP, Hemmings BA, et al. The lipid phosphatase activity of PTEN is critical for its tumor supressor function. Proc Natl Acad Sci U S A. 1998;95:135138. [DOI] [PubMed] [PMC]
    Andjelković M, Alessi DR, Meier R, Fernandez A, Lamb NJ, Frech M, et al. Role of translocation in the activation and function of protein kinase B. J Biol Chem. 1997;272:3151524. [DOI] [PubMed]
    Vanhaesebroeck B, Alessi DR. The PI3K-PDK1 connection: more than just a road to PKB. Biochem J. 2000;346 Pt 3:56176. [DOI] [PMC]
    Sarbassov DD, Guertin DA, Ali SM, Sabatini DM. Phosphorylation and regulation of Akt/PKB by the rictor-mTOR complex. Science. 2005;307:1098101. [DOI] [PubMed]
    Hemmings BA, Restuccia DF. PI3K-PKB/Akt pathway. Cold Spring Harb Perspect Biol. 2012;4:a011189. Erratum in: Cold Spring Harb Perspect Biol. 2015;7:a026609. [DOI] [PubMed] [PMC]
    Hua H, Kong Q, Zhang H, Wang J, Luo T, Jiang Y. Targeting mTOR for cancer therapy. J Hematol Oncol. 2019;12:71. [DOI] [PubMed] [PMC]
    Ma XM, Blenis J. Molecular mechanisms of mTOR-mediated translational control. Nat Rev Mol Cell Biol. 2009;10:30718. [DOI] [PubMed]
    Dunlop EA, Tee AR. mTOR and autophagy: a dynamic relationship governed by nutrients and energy. Semin Cell Dev Biol. 2014;36:1219. [DOI] [PubMed]
    Kim YC, Guan KL. mTOR: a pharmacologic target for autophagy regulation. J Clin Invest. 2015;125:2532. [DOI] [PubMed] [PMC]
    Jung CH, Jun CB, Ro SH, Kim YM, Otto NM, Cao J, et al. ULK-Atg13-FIP200 complexes mediate mTOR signaling to the autophagy machinery. Mol Biol Cell. 2009;20:19922003. [DOI] [PubMed] [PMC]
    Hare SH, Harvey AJ. mTOR function and therapeutic targeting in breast cancer. Am J Cancer Res. 2017;7:383404. [PubMed] [PMC]
    Le Goff P, Montano MM, Schodin DJ, Katzenellenbogen BS. Phosphorylation of the human estrogen receptor. Identification of hormone-regulated sites and examination of their influence on transcriptional activity. J Biol Chem. 1994;269:445866. [DOI] [PubMed]
    Lumachi F, Brunello A, Maruzzo M, Basso U, Basso SM. Treatment of estrogen receptor-positive breast cancer. Curr Med Chem. 2013;20:596604. [DOI] [PubMed]
    Enmark E, Pelto-Huikko M, Grandien K, Lagercrantz S, Lagercrantz J, Fried G, et al. Human estrogen receptor beta-gene structure, chromosomal localization, and expression pattern. J Clin Endocrinol Metab. 1997;82:425865. [DOI] [PubMed]
    Gosden JR, Middleton PG, Rout D. Localization of the human oestrogen receptor gene to chromosome 6q24—q27 by in situ hybridization. Cytogenet Cell Genet. 1986;43:21820. [DOI] [PubMed]
    Maggi A. Liganded and unliganded activation of estrogen receptor and hormone replacement therapies. Biochim Biophys Acta. 2011;1812:105460. [DOI] [PubMed] [PMC]
    Nelson ER, Wardell SE, McDonnell DP. The molecular mechanisms underlying the pharmacological actions of estrogens, SERMs and oxysterols: implications for the treatment and prevention of osteoporosis. Bone. 2013;53:4250. [DOI] [PubMed] [PMC]
    Smith CL. Cross-talk between peptide growth factor and estrogen receptor signaling pathways. Biol Reprod. 1998;58:62732. [DOI] [PubMed]
    Hall JM, Couse JF, Korach KS. The multifaceted mechanisms of estradiol and estrogen receptor signaling. J Biol Chem. 2001;276:3686972. [DOI] [PubMed]
    Shrivastav A, Murphy L. Interactions of PI3K/Akt/mTOR and estrogen receptor signaling in breast cancer. Breast Cancer Manage. 2012;1:23549. [DOI]
    Kato S. Estrogen receptor-mediated cross-talk with growth factor signaling pathways. Breast Cancer. 2001;8:39. [DOI] [PubMed]
    Campbell RA, Bhat-Nakshatri P, Patel NM, Constantinidou D, Ali S, Nakshatri H. Phosphatidylinositol 3-kinase/AKT-mediated activation of estrogen receptor alpha: a new model for anti-estrogen resistance. J Biol Chem. 2001;276:981724. [DOI] [PubMed]
    Cancer Genome Atlas Network. Comprehensive molecular portraits of human breast tumours. Nature. 2012;490:6170. [DOI] [PubMed] [PMC]
    Bachman KE, Argani P, Samuels Y, Silliman N, Ptak J, Szabo S, et al. The PIK3CA gene is mutated with high frequency in human breast cancers. Cancer Biol Ther. 2004;3:7725. [DOI] [PubMed]
    Razavi P, Chang MT, Xu G, Bandlamudi C, Ross DS, Vasan N, et al. The genomic landscape of endocrine-resistant advanced breast cancers. Cancer Cell. 2018;34:42738.e6. [DOI] [PubMed] [PMC]
    Mukohara T. PI3K mutations in breast cancer: prognostic and therapeutic implications. Breast Cancer (Dove Med Press). 2015;7:11123. [DOI] [PubMed] [PMC]
    Pang B, Cheng S, Sun SP, An C, Liu ZY, Feng X, et al. Prognostic role of PIK3CA mutations and their association with hormone receptor expression in breast cancer: a meta-analysis. Sci Rep. 2014;4:6255. [DOI] [PubMed] [PMC]
    Ellis MJ, Lin L, Crowder R, Tao Y, Hoog J, Snider J, et al. Phosphatidyl-inositol-3-kinase alpha catalytic subunit mutation and response to neoadjuvant endocrine therapy for estrogen receptor positive breast cancer. Breast Cancer Res Treat. 2010;119:37990. [DOI] [PubMed] [PMC]
    Di Cosimo S, Baselga J. Phosphoinositide 3-kinase mutations in breast cancer: a “good” activating mutation? Clin Cancer Res. 2009;15:50179. [DOI] [PubMed]
    Bose S, Chandran S, Mirocha JM, Bose N. The Akt pathway in human breast cancer: a tissue-array-based analysis. Mod Pathol. 2006;19:23845. [DOI] [PubMed]
    Riggio M, Polo ML, Blaustein M, Colman-Lerner A, Lüthy I, Lanari C, et al. PI3K/AKT pathway regulates phosphorylation of steroid receptors, hormone independence and tumor differentiation in breast cancer. Carcinogenesis. 2012;33:50918. [DOI] [PubMed]
    Faridi J, Wang L, Endemann G, Roth RA. Expression of constitutively active Akt-3 in MCF-7 breast cancer cells reverses the estrogen and tamoxifen responsivity of these cells in vivo. Clin Cancer Res. 2003;9:29339. [PubMed]
    Shoman N, Klassen S, McFadden A, Bickis MG, Torlakovic E, Chibbar R. Reduced PTEN expression predicts relapse in patients with breast carcinoma treated by tamoxifen. Mod Pathol. 2005;18:2509. [DOI] [PubMed]
    Alimonti A, Carracedo A, Clohessy JG, Trotman LC, Nardella C, Egia A, et al. Subtle variations in Pten dose determine cancer susceptibility. Nat Genet. 2010;42:4548. [DOI] [PubMed] [PMC]
    Yang J, Nie J, Ma X, Wei Y, Peng Y, Wei X. Targeting PI3K in cancer: mechanisms and advances in clinical trials. Mol Cancer. 2019;18:26. [DOI] [PubMed] [PMC]
    Maira SM, Pecchi S, Huang A, Burger M, Knapp M, Sterker D, et al. Identification and characterization of NVP-BKM120, an orally available pan-class I PI3-kinase inhibitor. Mol Cancer Ther. 2012;11:31728. [DOI] [PubMed]
    Liu N, Rowley BR, Bull CO, Schneider C, Haegebarth A, Schatz CA, et al. BAY 80-6946 is a highly selective intravenous PI3K inhibitor with potent p110α and p110δ activities in tumor cell lines and xenograft models. Mol Cancer Ther. 2013;12:231930. [DOI] [PubMed]
    O’Brien C, Wallin JJ, Sampath D, GuhaThakurta D, Savage H, Punnoose EA, et al. Predictive biomarkers of sensitivity to the phosphatidylinositol 3’ kinase inhibitor GDC-0941 in breast cancer preclinical models. Clin Cancer Res. 2010;16:367083. [DOI] [PubMed]
    Sanchez CG, Ma CX, Crowder RJ, Guintoli T, Phommaly C, Gao F, et al. Preclinical modeling of combined phosphatidylinositol-3-kinase inhibition with endocrine therapy for estrogen receptor-positive breast cancer. Breast Cancer Res. 2011;13:R21. [DOI] [PubMed] [PMC]
    Liu R, Liu D, Trink E, Bojdani E, Ning G, Xing M. The Akt-specific inhibitor MK2206 selectively inhibits thyroid cancer cells harboring mutations that can activate the PI3K/Akt pathway. J Clin Endocrinol Metab. 2011;96:E57785. [DOI] [PubMed] [PMC]
    Sangai T, Akcakanat A, Chen H, Tarco E, Wu Y, Do KA, et al. Biomarkers of response to Akt inhibitor MK-2206 in breast cancer. Clin Cancer Res. 2012;18:581628. [DOI] [PubMed] [PMC]
    Schneeweiss A, Hess D, Joerger M, Varga A, Moulder S, Tsimberidou AM, et al. Phase 1 dose escalation study of the allosteric AKT inhibitor BAY 1125976 in advanced solid cancer-lack of association between activating AKT mutation and AKT inhibition-derived efficacy. Cancers (Basel). 2019;11:1987. [DOI] [PubMed] [PMC]
    Hyman DM, Smyth LM, Donoghue MTA, Westin SN, Bedard PL, Dean EJ, et al. AKT inhibition in solid tumors with AKT1 mutations. J Clin Oncol. 2017;35:22519. [DOI] [PubMed] [PMC]
    Ma CX, Suman V, Goetz MP, Northfelt D, Burkard ME, Ademuyiwa F, et al. A phase II trial of neoadjuvant MK-2206, an AKT inhibitor, with anastrozole in clinical stage II or III PIK3CA-mutant ER-positive and HER2-negative breast cancer. Clin Cancer Res. 2017;23:682332. [DOI] [PubMed] [PMC]
    Davies BR, Greenwood H, Dudley P, Crafter C, Yu DH, Zhang J, et al. Preclinical pharmacology of AZD5363, an inhibitor of AKT: pharmacodynamics, antitumor activity, and correlation of monotherapy activity with genetic background. Mol Cancer Ther. 2012;11:87387. [DOI] [PubMed]
    Lin J, Sampath D, Nannini MA, Lee BB, Degtyarev M, Oeh J, et al. Targeting activated Akt with GDC-0068, a novel selective Akt inhibitor that is efficacious in multiple tumor models. Clin Cancer Res. 2013;19:176072. [DOI] [PubMed]
    O’Reilly T, McSheehy PM. Biomarker development for the clinical activity of the mTOR inhibitor everolimus (RAD001): processes, limitations, and further proposals. Transl Oncol. 2010;3:6579. [DOI] [PubMed] [PMC]
    Rocha RL, Hilsenbeck SG, Jackson JG, VanDenBerg CL, Weng Cn, Lee AV, et al. Insulin-like growth factor binding protein-3 and insulin receptor substrate-1 in breast cancer: correlation with clinical parameters and disease-free survival. Clin Cancer Res. 1997;3:1039. [PubMed]
    Rechoum Y, Rovito D, Iacopetta D, Barone I, Andò S, Weigel NL, et al. AR collaborates with ERα in aromatase inhibitor-resistant breast cancer. Breast Cancer Res Treat. 2014;147:47385. [DOI] [PubMed] [PMC]
    Ekyalongo RC, Yee D. Revisiting the IGF-1R as a breast cancer target. NPJ Precis Oncol. 2017;1:14. [DOI] [PubMed] [PMC]
    Reuveni H, Flashner-Abramson E, Steiner L, Makedonski K, Song R, Shir A, et al. Therapeutic destruction of insulin receptor substrates for cancer treatment. Cancer Res. 2013;73:438394. [DOI] [PubMed] [PMC]
    Ibuki N, Ghaffari M, Reuveni H, Pandey M, Fazli L, Azuma H, et al. The tyrphostin NT157 suppresses insulin receptor substrates and augments therapeutic response of prostate cancer. Mol Cancer Ther. 2014;13:282739. [DOI] [PubMed]
    Gao J, Chesebrough JW, Cartlidge SA, Ricketts SA, Incognito L, Veldman-Jones M, et al. Dual IGF-I/II-neutralizing antibody MEDI-573 potently inhibits IGF signaling and tumor growth. Cancer Res. 2011;71:102940. [DOI] [PubMed]
    Law JH, Habibi G, Hu K, Masoudi H, Wang MY, Stratford AL, et al. Phosphorylated insulin-like growth factor-i/insulin receptor is present in all breast cancer subtypes and is related to poor survival. Cancer Res. 2008;68:1023846. [DOI] [PubMed]
    Fox EM, Miller TW, Balko JM, Kuba MG, Sánchez V, Smith RA, et al. A kinome-wide screen identifies the insulin/IGF-I receptor pathway as a mechanism of escape from hormone dependence in breast cancer. Cancer Res. 2011;71:677384. [DOI] [PubMed] [PMC]
    Pal I, Mandal M. PI3K and Akt as molecular targets for cancer therapy: current clinical outcomes. Acta Pharmacol Sin. 2012;33:144158. [DOI] [PubMed] [PMC]
    Janku F, Yap TA, Meric-Bernstam F. Targeting the PI3K pathway in cancer: are we making headway? Nat Rev Clin Oncol. 2018;15:27391. [DOI] [PubMed]
    André F, Ciruelos E, Rubovszky G, Campone M, Loibl S, Rugo HS, et al.; SOLAR-1 Study Group. Alpelisib for PIK3CA-mutated, hormone receptor-positive advanced breast cancer. N Engl J Med.. 2019;380:192940. [DOI] [PubMed]
    Baselga J, Campone M, Piccart M, Burris HA 3rd, Rugo HS, Sahmoud T, et al. Everolimus in postmenopausal hormone-receptor-positive advanced breast cancer. N Engl J Med. 2012;366:5209. [DOI] [PubMed] [PMC]
    Jerusalem G, de Boer RH, Hurvitz S, Yardley DA, Kovalenko E, Ejlertsen B, et al. Everolimus plus exemestane vs everolimus or capecitabine monotherapy for estrogen receptor-positive, HER2-negative advanced breast cancer: the BOLERO-6 randomized clinical trial. JAMA Oncol. 2018;4:136774. [DOI] [PubMed] [PMC]
    Wolff AC, Lazar AA, Bondarenko I, Garin AM, Brincat S, Chow L, et al. Randomized phase III placebo-controlled trial of letrozole plus oral temsirolimus as first-line endocrine therapy in postmenopausal women with locally advanced or metastatic breast cancer. J Clin Oncol. 2013;31:195202. [DOI] [PubMed] [PMC]
    Bachelot T, Bourgier C, Cropet C, Ray-Coquard I, Ferrero JM, Freyer G, et al. Randomized phase II trial of everolimus in combination with tamoxifen in patients with hormone receptor-positive, human epidermal growth factor receptor 2-negative metastatic breast cancer with prior exposure to aromatase inhibitors: a GINECO study. J Clin Oncol. 2012;30:271824. [DOI] [PubMed]
    Kornblum N, Zhao F, Manola J, Klein P, Ramaswamy B, Brufsky A, et al. Randomized phase II trial of fulvestrant plus everolimus or placebo in postmenopausal women with hormone receptor-positive, human epidermal growth factor receptor 2-negative metastatic breast cancer resistant to aromatase inhibitor therapy: results of PrE0102. J Clin Oncol. 2018;36:155663. [DOI] [PubMed] [PMC]
    Lim B, Potter DA, Salkeni MA, Silverman P, Haddad TC, Forget F, et al. Sapanisertib plus exemestane or fulvestrant in women with hormone receptor-positive/HER2-negative advanced or metastatic breast cancer. Clin Cancer Res. 2021;27:332938. [DOI] [PubMed]
    Schmid P, Zaiss M, Harper-Wynne C, Ferreira M, Dubey S, Chan S, et al. Fulvestrant plus vistusertib vs fulvestrant plus everolimus vs fulvestrant alone for women with hormone receptor-positive metastatic breast cancer: the MANTA phase 2 randomized clinical trial. JAMA Oncol. 2019;5:155664. [DOI] [PubMed] [PMC]
    Bardia A, Hurvitz SA, DeMichele A, Clark AS, Zelnak A, Yardley DA, et al. Phase I/II trial of exemestane, ribociclib, and everolimus in women with HR+/HER2 advanced breast cancer after progression on CDK4/6 inhibitors (TRINITI-1). Clin Cancer Res. 2021;27:417785. [DOI] [PubMed] [PMC]
    Schmid P, Sablin MP, Bergh J, Im SA, Lu YS, Martínez N, et al. A phase Ib/II study of xentuzumab, an IGF-neutralising antibody, combined with exemestane and everolimus in hormone receptor-positive, HER2-negative locally advanced/metastatic breast cancer. Breast Cancer Res. 2021;23:8. [DOI] [PubMed] [PMC]
    Baselga J, Im SA, Iwata H, Cortés J, De Laurentiis M, Jiang Z, et al. Buparlisib plus fulvestrant versus placebo plus fulvestrant in postmenopausal, hormone receptor-positive, HER2-negative, advanced breast cancer (BELLE-2): a randomised, double-blind, placebo-controlled, phase 3 trial. Lancet Oncol. 2017;18:90416. [DOI] [PubMed] [PMC]
    Di Leo A, Johnston S, Lee KS, Ciruelos E, Lønning PE, Janni W, et al. Buparlisib plus fulvestrant in postmenopausal women with hormone-receptor-positive, HER2-negative, advanced breast cancer progressing on or after mTOR inhibition (BELLE-3): a randomised, double-blind, placebo-controlled, phase 3 trial. Lancet Oncol. 2018;19:87100. [DOI] [PubMed]
    André F, Ciruelos EM, Juric D, Loibl S, Campone M, Mayer IA, et al. Alpelisib plus fulvestrant for PIK3CA-mutated, hormone receptor-positive, human epidermal growth factor receptor-2-negative advanced breast cancer: final overall survival results from SOLAR-1. Ann Oncol. 2021;32:20817. [DOI] [PubMed]
    Shah PD, Modi S, Datko FM, Moynahan ME, Zamora S, D’Andrea G, et al. Phase I trial of daily PI3Kα inhibitor BYL719 plus letrozole (L) or exemestane (E) for patients (pts) with hormone receptor-positive (HR+) metastatic breast cancer (MBC). J Clin Oncol. 2014;32:2605. [DOI]
    Mayer IA, Abramson VG, Formisano L, Balko JM, Estrada MV, Sanders ME, et al. A phase Ib study of alpelisib (BYL719), a PI3Kα-specific inhibitor, with letrozole in ER+/HER2 metastatic breast cancer. Clin Cancer Res. 2017;23:2634. [DOI] [PubMed] [PMC]
    Juric D, Janku F, Rodón J, Burris HA, Mayer IA, Schuler M, et al. Alpelisib plus fulvestrant in PIK3CA-altered and PIK3CA-wild-type estrogen receptor-positive advanced breast cancer: a phase 1b clinical trial. JAMA Oncol. 2019;5:e184475. [DOI] [PubMed] [PMC]
    Krop IE, Mayer IA, Ganju V, Dickler M, Johnston S, Morales S, et al. Pictilisib for oestrogen receptor-positive, aromatase inhibitor-resistant, advanced or metastatic breast cancer (FERGI): a randomised, double-blind, placebo-controlled, phase 2 trial. Lancet Oncol. 2016;17:81121. [DOI] [PubMed] [PMC]
    Blackwell K, Burris H, Gomez P, Lynn Henry N, Isakoff S, Campana F, et al. Phase I/II dose-escalation study of PI3K inhibitors pilaralisib or voxtalisib in combination with letrozole in patients with hormone-receptor-positive and HER2-negative metastatic breast cancer refractory to a non-steroidal aromatase inhibitor. Breast Cancer Res Treat. 2015;154:28797. [DOI] [PubMed]
    Rugo HS, Lerebours F, Ciruelos E, Drullinsky P, Ruiz-Borrego M, Neven P, et al. Alpelisib plus fulvestrant in PIK3CA-mutated, hormone receptor-positive advanced breast cancer after a CDK4/6 inhibitor (BYLieve): one cohort of a phase 2, multicentre, open-label, non-comparative study. Lancet Oncol. 2021;22:48998. [DOI] [PubMed]
    Curigliano G, Martin M, Jhaveri K, Beck JT, Tortora G, Fazio N, et al. Alpelisib in combination with everolimus ± exemestane in solid tumours: phase Ib randomised, open-label, multicentre study. Eur J Cancer. 2021;151:4962. [DOI] [PubMed]
    Lu YS, Lee KS, Chao TY, Tseng LM, Chitapanarux I, Chen SC, et al. A phase Ib study of alpelisib or buparlisib combined with tamoxifen plus goserelin in premenopausal women with HR-positive HER2-negative advanced breast cancer. Clin Cancer Res. 2021;27:40817. [DOI] [PubMed]
    Mayer IA, Prat A, Egle D, Blau S, Fidalgo JAP, Gnant M, et al. A phase II randomized study of neoadjuvant letrozole plus alpelisib for hormone receptor-positive, human epidermal growth factor receptor 2-negative breast cancer (NEO-ORB). Clin Cancer Res. 2019;25:297587. [DOI] [PubMed] [PMC]
    Jhaveri K, Juric D, Yap YS, Cresta S, Layman RM, Duhoux FP, et al. A phase I study of LSZ102, an oral selective estrogen receptor degrader, with or without ribociclib or alpelisib, in patients with estrogen receptor-positive breast cancer. Clin Cancer Res. 2021;27:576070. [DOI] [PubMed]
    Dent S, Cortés J, Im YH, Diéras V, Harbeck N, Krop IE, et al. Phase III randomized study of taselisib or placebo with fulvestrant in estrogen receptor-positive, PIK3CA-mutant, HER2-negative, advanced breast cancer: the SANDPIPER trial. Ann Oncol. 2021;32:197207. [DOI] [PubMed] [PMC]
    Dickler MN, Saura C, Richards DA, Krop IE, Cervantes A, Bedard PL, et al. Phase II study of taselisib (GDC-0032) in combination with fulvestrant in patients with HER2-negative, hormone receptor-positive advanced breast cancer. Clin Cancer Res. 2018;24:43807. [DOI] [PubMed] [PMC]
    Saura C, Hlauschek D, Oliveira M, Zardavas D, Jallitsch-Halper A, de la Peña L, et al. Neoadjuvant letrozole plus taselisib versus letrozole plus placebo in postmenopausal women with oestrogen receptor-positive, HER2-negative, early-stage breast cancer (LORELEI): a multicentre, randomised, double-blind, placebo-controlled, phase 2 trial. Lancet Oncol. 2019;20:122638. [DOI] [PubMed]
    Pascual J, Lim JSJ, Macpherson IR, Armstrong AC, Ring A, Okines AFC, et al. Triplet therapy with palbociclib, taselisib, and fulvestrant in PIK3CA-mutant breast cancer and doublet palbociclib and taselisib in pathway-mutant solid cancers. Cancer Discov. 2021;11:92107. [DOI] [PubMed]
    Jhaveri K, Kalinsky K, Bedard P, Cervantes A, Saura C, Krop I, et al. Abstract P1-19-46: a phase Ib dose escalation study evaluating the mutant selective PI3K-alpha inhibitor GDC-0077 (G) in combination with letrozole (L) with and without palbociclib (P) in patients with PIK3CA-mutant HR+/HER2 breast cancer. Cancer Res. 2020;80:P119–46. [DOI]
    Jones RH, Casbard A, Carucci M, Cox C, Butler R, Alchami F, et al. Fulvestrant plus capivasertib versus placebo after relapse or progression on an aromatase inhibitor in metastatic, oestrogen receptor-positive breast cancer (FAKTION): a multicentre, randomised, controlled, phase 2 trial. Lancet Oncol. 2020;21:34557. [DOI] [PubMed] [PMC]
    Wander SA, Juric D, Supko JG, Micalizzi DS, Spring L, Vidula N, et al. Phase Ib trial to evaluate safety and anti-tumor activity of the AKT inhibitor, ipatasertib, in combination with endocrine therapy and a CDK4/6 inhibitor for patients with hormone receptor positive (HR+)/HER2 negative metastatic breast cancer (MBC) (TAKTIC). J Clin Oncol. 2020;38:1066. [DOI]
    Forero-Torres A, Han H, Dees EC, Wesolowski R, Bardia A, Kabos P, et al. Phase Ib study of gedatolisib in combination with palbociclib and endocrine therapy (ET) in women with estrogen receptor (ER) positive (+) metastatic breast cancer (MBC) (B2151009). J Clin Oncol. 2018;36:1040. [DOI]
    Li H, Prever L, Hirsch E, Gulluni F. Targeting PI3K/AKT/mTOR signaling pathway in breast cancer. Cancers (Basel). 2021;13:3517. [DOI] [PubMed] [PMC]
    Popova NV, Jücker M. The role of mTOR signaling as a therapeutic target in cancer. Int J Mol Sci. 2021;22:1743. [DOI] [PubMed] [PMC]
    Yin Y, Hua H, Li M, Liu S, Kong Q, Shao T, et al. mTORC2 promotes type I insulin-like growth factor receptor and insulin receptor activation through the tyrosine kinase activity of mTOR. Cell Res. 2016;26:4665. [DOI] [PubMed] [PMC]
    Rodrik-Outmezguine VS, Okaniwa M, Yao Z, Novotny CJ, McWhirter C, Banaji A, et al. Overcoming mTOR resistance mutations with a new-generation mTOR inhibitor. Nature. 2016;534:2726. [DOI] [PubMed] [PMC]
    Liu Y, Wan WZ, Li Y, Zhou GL, Liu XG. Recent development of ATP-competitive small molecule phosphatidylinostitol-3-kinase inhibitors as anticancer agents. Oncotarget. 2017;8:7181200. [DOI] [PubMed] [PMC]
    Hudes G, Carducci M, Tomczak P, Dutcher J, Figlin R, Kapoor A, et al.; Global ARCC Trial. Temsirolimus, interferon alfa, or both for advanced renal-cell carcinoma. N Engl J Med. 2007;356:227181. [DOI] [PubMed]
    Motzer RJ, Escudier B, Oudard S, Hutson TE, Porta C, Bracarda S, et al.; RECORD-1 Study Group. Efficacy of everolimus in advanced renal cell carcinoma: a double-blind, randomised, placebo-controlled phase III trial. Lancet. 2008;372:44956. [DOI] [PubMed]
    Krueger DA, Care MM, Holland K, Agricola K, Tudor C, Mangeshkar P, et al. Everolimus for subependymal giant-cell astrocytomas in tuberous sclerosis. N Engl J Med. 2010;363:180111. [DOI] [PubMed]
    Yao JC, Shah MH, Ito T, Bohas CL, Wolin EM, Van Cutsem E, et al.; RAD001 in Advanced Neuroendocrine Tumors, Third Trial (RADIANT-3) Study Group. Everolimus for advanced pancreatic neuroendocrine tumors. N Engl J Med. 2011;364:51423. [DOI] [PubMed] [PMC]
    Yardley DA, Noguchi S, Pritchard KI, Burris HA 3rd, Baselga J, Gnant M, et al. Everolimus plus exemestane in postmenopausal patients with HR+ breast cancer: BOLERO-2 final progression-free survival analysis. Adv Ther. 2013;30:87084. [DOI] [PubMed] [PMC]
    Piccart M, Hortobagyi GN, Campone M, Pritchard KI, Lebrun F, Ito Y, et al. Everolimus plus exemestane for hormone-receptor-positive, human epidermal growth factor receptor-2-negative advanced breast cancer: overall survival results from BOLERO-2. Ann Oncol. 2014;25:235762. [DOI] [PubMed] [PMC]
    Royce M, Bachelot T, Villanueva C, Özgüroglu M, Azevedo SJ, Cruz FM, et al. Everolimus plus endocrine therapy for postmenopausal women with estrogen receptor-positive, human epidermal growth factor receptor 2-negative advanced breast cancer: a clinical trial. JAMA Oncol. 2018;4:97784. [DOI] [PubMed] [PMC]
    Treilleux I, Arnedos M, Cropet C, Wang Q, Ferrero JM, Abadie-Lacourtoisie S, et al. Translational studies within the TAMRAD randomized GINECO trial: evidence for mTORC1 activation marker as a predictive factor for everolimus efficacy in advanced breast cancer. Ann Oncol. 2015;26:1205. [DOI] [PubMed]
    Dees EC, Carey LA. Improving endocrine therapy for breast cancer: it’s not that simple. J Clin Oncol. 2013;31:1713. [DOI] [PubMed]
    Baselga J, Curigliano G, Martín M, André F, Beck JT, Tortora G, et al. Abstract CT061: a phase Ib study of alpelisib (BYL719) + everolimus ± exemestane in patients with advanced solid tumors or HR+/HER2-breast cancer. Cancer Res. 2016;76:CT061. [DOI]
    Lu YS, Ro J, Tseng LM, Chao TY, Chitapanarux I, Valenti R, et al. Abstract P4-13-27: a phase Ib dose de-escalation study of combined tamoxifen and goserelin acetate with alpelisib (BYL719) or buparlisib (BKM120) in premenopausal patients with HR+/HER2 locally advanced or metastatic breast cancer. Cancer Res. 2016;76:P413–27. [DOI]
    Sharma P, Abramson VG, O’Dea A, Pathak HB, Pessetto ZY, Wang YY, et al. Clinical and biomarker results from phase I/II study of PI3K inhibitor BYL 719 (alpelisib) plus nab-paclitaxel in HER2-negative metastatic breast cancer. J Clin Oncol. 2018;36:1018. [DOI] [PubMed] [PMC]
    Turner N, Howell S, Jhaveri K, Gomez H, Toi M, Hu X, et al. 350TiP a phase III trial of capivasertib and fulvestrant versus placebo and fulvestrant in patients with HR+/HER2 cancer (CAPItello-291). Ann Oncol. 2020;31:S3889. [DOI]
    Fulvestrant and Ipatasertib for advanced HER-2 negative and estrogen receptor positive (ER+) breast cancer following progression on first line CDK 4/6 inhibitor and aromatase inhibitor (FINER) [Internet]. U.S. National Library of Medicine; [cited 2022 Jan 08]. Available from: https://clinicaltrials.gov/ct2/show/NCT04650581
    Ianza A, Sirico M, Bernocchi O, Generali D. Role of the IGF-1 axis in overcoming resistance in breast cancer. Front Cell Dev Biol. 2021;9:641449. [DOI] [PubMed] [PMC]
    Cortes J, Janez NM, Sablin MP, Perez-Fidalgo JA, Neven P, Hedayati E, et al. Phase 1b/2 trial of BI 836845, an insulin-like growth factor (IGF) ligand-neutralizing antibody, combined with exemestane (Ex) and everolimus (Ev) in hormone receptor-positive (HR+) locally advanced or metastatic breast cancer (BC): primary phase 1b results. J Clin Oncol. 2016;34:530. [DOI]
    Yee D, LoRusso P, Sablin MP, Prat A, Stradella A, Utriainen M, et al. A phase Ib study of xentuzumab plus abemaciclib and fulvestrant in patients (pts) with advanced hormone receptor-positive (HR+), HER2-negative breast cancer (BC) with visceral or non-visceral disease. J Clin Oncol. 2021;39:1057. [DOI]
    Robertson JF, Ferrero JM, Bourgeois H, Kennecke H, de Boer RH, Jacot W, et al. Ganitumab with either exemestane or fulvestrant for postmenopausal women with advanced, hormone-receptor-positive breast cancer: a randomised, controlled, double-blind, phase 2 trial. Lancet Oncol. 2013;14:22835. [DOI] [PubMed]
    Ma CX, Suman VJ, Goetz M, Haluska P, Moynihan T, Nanda R, et al. A phase I trial of the IGF-1R antibody cixutumumab in combination with temsirolimus in patients with metastatic breast cancer. Breast Cancer Res Treat. 2013;139:14553. [DOI] [PubMed] [PMC]
    Gradishar WJ, Yardley DA, Layman R, Sparano JA, Chuang E, Northfelt DW, et al. Clinical and translational results of a phase II, randomized trial of an anti-IGF-1R (cixutumumab) in women with breast cancer that progressed on endocrine therapy. Clin Cancer Res. 2016;22:3019. [DOI] [PubMed] [PMC]
    Rugo HS, Trédan O, Ro J, Morales SM, Campone M, Musolino A, et al. A randomized phase II trial of ridaforolimus, dalotuzumab, and exemestane compared with ridaforolimus and exemestane in patients with advanced breast cancer. Breast Cancer Res Treat. 2017;165:6019. [DOI] [PubMed] [PMC]
    Beaver JA, Park BH. The BOLERO-2 trial: the addition of everolimus to exemestane in the treatment of postmenopausal hormone receptor-positive advanced breast cancer. Future Oncol. 2012;8:6517. [DOI] [PubMed] [PMC]
    Prat A, Brase JC, Cheng Y, Nuciforo P, Paré L, Pascual T, et al. Everolimus plus exemestane for hormone receptor-positive advanced breast cancer: a PAM50 intrinsic subtype analysis of BOLERO-2. Oncologist. 2019;24:893900. [DOI] [PubMed] [PMC]
    Hortobagyi GN, Chen D, Piccart M, Rugo HS, Burris HA 3rd, Pritchard KI, et al. Correlative analysis of genetic alterations and everolimus benefit in hormone receptor-positive, human epidermal growth factor receptor 2-negative advanced breast cancer: resultsfrom BOLERO-2. JClin Oncol. 2016;34:41926. [DOI] [PubMed] [PMC]
    Chandarlapaty S, Chen D, He W, Sung P, Samoila A, You D, et al. Prevalence of ESR1 mutations in cell-free DNA and outcomes in metastatic breast cancer: a secondary analysis of the BOLERO-2 clinical trial. JAMA Oncol. 2016;2:131015. [DOI] [PubMed] [PMC]
    Smyth LM, Zhou Q, Nguyen B, Yu C, Lepisto EM, Arnedos M, et al.; AACR Project GENIE Consortium. Characteristics and Outcome of AKT1E17K-mutant breast cancer defined through AACR project GENIE, a clinicogenomic registry. Cancer Discov. 2020;10:52635. [DOI] [PubMed] [PMC]
    Beaver JA, Gustin JP, Yi KH, Rajpurohit A, Thomas M, Gilbert SF, et al. PIK3CA and AKT1 mutations have distinct effects on sensitivity to targeted pathway inhibitors in an isogenic luminal breast cancer model system. Clin Cancer Res. 2013;19:541322. [DOI] [PubMed] [PMC]
    Vuylsteke P, Huizing M, Petrakova K, Roylance R, Laing R, Chan S, et al. Pictilisib PI3Kinase inhibitor (a phosphatidylinositol 3-kinase [PI3K] inhibitor) plus paclitaxel for the treatment of hormone receptor-positive, HER2-negative, locally recurrent, or metastatic breast cancer: interim analysis of the multicentre, placebo-controlled, phase II randomised PEGGY study. Ann Oncol. 2016;27:205966. [DOI] [PubMed]
    Martín M, Chan A, Dirix L, O’Shaughnessy J, Hegg R, Manikhas A, et al. A randomized adaptive phase II/III study of buparlisib, a pan-class I PI3K inhibitor, combined with paclitaxel for the treatment of HER2 advanced breast cancer (BELLE-4). Ann Oncol. 2017;28:31320. [DOI] [PubMed]
    Schmid P, Pinder SE, Wheatley D, Macaskill J, Zammit C, Hu J, et al. Phase II randomized preoperative window-of-opportunity study of the PI3K inhibitor pictilisib plus anastrozole compared with anastrozole alone in patients with estrogen receptor-positive breast cancer. J Clin Oncol. 2016;34:198794. [DOI] [PubMed] [PMC]
    Cizkova M, Vacher S, Meseure D, Trassard M, Susini A, Mlcuchova D, et al. PIK3R1 underexpression is an independent prognostic marker in breast cancer. BMC Cancer. 2013;13:545. [DOI] [PubMed] [PMC]
    Hirai H, Sootome H, Nakatsuru Y, Miyama K, Taguchi S, Tsujioka K, et al. MK-2206, an allosteric Akt inhibitor, enhances antitumor efficacy by standard chemotherapeutic agents or molecular targeted drugs in vitro and in vivo. Mol Cancer Ther. 2010;9:195667. [DOI] [PubMed]
    Yu Y, Savage RE, Eathiraj S, Meade J, Wick MJ, Hall T, et al. Targeting AKT1-E17K and the PI3K/AKT pathway with an allosteric AKT inhibitor, ARQ 092. PLoS One. 2015;10:e0140479. [DOI] [PubMed] [PMC]
    Politz O, Siegel F, Bärfacker L, Bömer U, Hägebarth A, Scott WJ, et al. BAY 1125976, a selective allosteric AKT1/2 inhibitor, exhibits high efficacy on AKT signaling-dependent tumor growth in mouse models. Int J Cancer. 2017;140:44959. [DOI] [PubMed]
    Yap TA, Yan L, Patnaik A, Tunariu N, Biondo A, Fearen I, et al. Interrogating two schedules of the AKT inhibitor MK-2206 in patients with advanced solid tumors incorporating novel pharmacodynamic and functional imaging biomarkers. Clin Cancer Res. 2014;20:567285. [DOI] [PubMed] [PMC]
    Ma CX, Sanchez C, Gao F, Crowder R, Naughton M, Pluard T, et al. A phase I study of the AKT inhibitor MK-2206 in combination with hormonal therapy in postmenopausal women with estrogen receptor-positive metastatic breast cancer. Clin Cancer Res. 2016;22:26508. [DOI] [PubMed] [PMC]
    Turner NC, Alarcón E, Armstrong AC, Philco M, López Chuken YA, Sablin MP, et al. BEECH: a dose-finding run-in followed by a randomised phase II study assessing the efficacy of AKT inhibitor capivasertib (AZD5363) combined with paclitaxel in patients with estrogen receptor-positive advanced or metastatic breast cancer, and in a PIK3CA mutant sub-population. Ann Oncol. 2019;30:77480. [DOI] [PubMed] [PMC]
    Smyth LM, Tamura K, Oliveira M, Ciruelos EM, Mayer IA, Sablin MP, et al. Capivasertib, an AKT kinase inhibitor, as monotherapy or in combination with fulvestrant in patients with AKT1E17K-mutant, ER-positive metastatic breast cancer. Clin Cancer Res. 2020;26:394757. [DOI] [PubMed] [PMC]
    A study of ipatasertib plus palbociclib and fulvestrant versus placebo plus palbociclib and fulvestrant in hormone receptor positive and HER2 negative locally advanced unresectable or metastatic breast cancer (IPATunity150) [Internet]. U.S. National Library of Medicine; [cited 2022 Jan 08]. Available from: https://clinicaltrials.gov/ct2/show/NCT04060862
    Harrington LS, Findlay GM, Gray A, Tolkacheva T, Wigfield S, Rebholz H, et al. The TSC1-2 tumor suppressor controls insulin-PI3K signaling via regulation of IRS proteins. J Cell Biol. 2004;166:21323. [DOI] [PubMed] [PMC]
    Juric D, Castel P, Griffith M, Griffith OL, Won HH, Ellis H, et al. Convergent loss of PTEN leads to clinical resistance to a PI3Kα inhibitor. Nature. 2015;518:2404. [DOI] [PubMed] [PMC]
    Jia S, Liu Z, Zhang S, Liu P, Zhang L, Lee SH, et al. Essential roles of PI(3)K-p110beta in cell growth, metabolism and tumorigenesis. Nature. 2008;454:7769. Erratum in: Nature. 2016;533:278. [DOI] [PubMed] [PMC]
    Vora SR, Juric D, Kim N, Mino-Kenudson M, Huynh T, Costa C, et al. CDK 4/6 inhibitors sensitize PIK3CA mutant breast cancer to PI3K inhibitors. Cancer Cell. 2014;26:13649. [DOI] [PubMed] [PMC]
    Janku F, Hong DS, Fu S, Piha-Paul SA, Naing A, Falchook GS, et al. Assessing PIK3CA and PTEN in early-phase trials with PI3K/AKT/mTOR inhibitors. Cell Rep. 2014;6:37787. [DOI] [PubMed] [PMC]
    Saura C, Roda D, Roselló S, Oliveira M, Macarulla T, Pérez-Fidalgo JA, et al. A first-in-human phase I study of the ATP-competitive AKT inhibitor ipatasertib demonstrates robust and safe targeting of AKT in patients with solid tumors. Cancer Discov. 2017;7:10213. [DOI] [PubMed] [PMC]
    van Geel RMJM, Tabernero J, Elez E, Bendell JC, Spreafico A, Schuler M, et al. A phase Ib dose-escalation study of encorafenib and cetuximab with or without alpelisib in metastatic BRAF-mutant colorectal cancer. Cancer Discov. 2017;7:6109. [DOI] [PubMed] [PMC]
    Bendell JC, Rodon J, Burris HA, de Jonge M, Verweij J, Birle D, et al. Phase I, dose-escalation study of BKM120, an oral pan-class I PI3K inhibitor, in patients with advanced solid tumors. J Clin Oncol. 2012;30:28290. [DOI] [PubMed]
    Hopkins BD, Pauli C, Du X, Wang DG, Li X, Wu D, et al. Suppression of insulin feedback enhances the efficacy of PI3K inhibitors. Nature. 2018;560:499503. [DOI] [PubMed] [PMC]
    Khan KH, Wong M, Rihawi K, Bodla S, Morganstein D, Banerji U, et al. Hyperglycemia and phosphatidylinositol 3-kinase/protein kinase B/mammalian target of rapamycin (PI3K/AKT/mTOR) inhibitors in phase I trials: incidence, predictive factors, and management. Oncologist. 2016;21:85560. [DOI] [PubMed] [PMC]
    Sahakian N, Cattieuw L, Ramillon-Cury C, Corroller AB, Silvestre-Aillaud P, Béliard S, et al. SGLT2 inhibitors as potentially helpful drugs in PI3K inhibitor-induced diabetes: a case report. Clin Diabetes Endocrinol. 2021;7:17. [DOI] [PubMed] [PMC]
    Storgaard H, Gluud LL, Bennett C, Grøndahl MF, Christensen MB, Knop FK, et al. Benefits and harms of sodium-glucose co-transporter 2 inhibitors in patients with type 2 diabetes: a systematic review and meta-analysis. PLoS One. 2016;11:e0166125. [DOI] [PubMed] [PMC]
    Storgaard H, Bagger JI, Knop FK, Vilsbøll T, Rungby J. Diabetic ketoacidosis in a patient with type 2 diabetes after initiation of sodium-glucose cotransporter 2 inhibitor treatment. Basic Clin Pharmacol Toxicol. 2016;118:16870. [DOI] [PubMed]
    Deligiorgi MV, Liapi C, Trafalis DT. How far are we from prescribing fasting as anticancer medicine? Int J Mol Sci. 2020;21:9175. [DOI] [PubMed] [PMC]
    Lee C, Raffaghello L, Brandhorst S, Safdie FM, Bianchi G, Martin-Montalvo A, et al. Fasting cycles retard growth of tumors and sensitize a range of cancer cell types to chemotherapy. Sci Transl Med. 2012;4:124ra27. [DOI] [PubMed] [PMC]
    Belardi V, Gallagher EJ, Novosyadlyy R, LeRoith D. Insulin and IGFs in obesity-related breast cancer. J Mammary Gland Biol Neoplasia. 2013;18:27789. [DOI] [PubMed]
    Brandhorst S. Fasting and fasting-mimicking diets for chemotherapy augmentation. Geroscience. 2021;43:120116. [DOI] [PubMed] [PMC]
    Valdemarin F, Caffa I, Persia A, Cremonini AL, Ferrando L, Tagliafico L, et al. Safety and feasibility of fasting-mimicking diet and effects on nutritional status and circulating metabolic and inflammatory factors in cancer patients undergoing active treatment. Cancers (Basel). 2021;13:4013. [DOI] [PubMed] [PMC]
    Caffa I, Spagnolo V, Vernieri C, Valdemarin F, Becherini P, Wei M, et al. Fasting-mimicking diet and hormone therapy induce breast cancer regression. Nature. 2020;583:6204. [DOI] [PubMed] [PMC]
    Kalaany NY, Sabatini DM. Tumours with PI3K activation are resistant to dietary restriction. Nature. 2009;458:72531. [DOI] [PubMed] [PMC]
    Hernando C, Ortega-Morillo B, Tapia M, Moragón S, Martínez MT, Eroles P, et al. Oral selective estrogen receptor degraders (SERDs) as a novel breast cancer therapy: present and future from a clinical perspective. Int J Mol Sci. 2021;22:7812. [DOI] [PubMed] [PMC]
    Im SA, Hamilton EP, Cussac AL, Baird RD, Ettl J, Goetz MP, et al. SERENA-4: a phase 3 comparison of AZD9833 (camizestrant) plus palbociclib, versus anastrozole plus palbociclib, for patients with ER-positive, HER2-negative advanced breast cancer who have not previously received systemic treatment for advanced disease. J Clin Oncol. 2021;39:TPS1101. [DOI]
    McCartney A, Migliaccio I, Bonechi M, Biagioni C, Romagnoli D, De Luca F, et al. Mechanisms of resistance to CDK4/6 inhibitors: potential implications and biomarkers for clinical practice. Front Oncol. 2019;9:666. [DOI] [PubMed] [PMC]
    O’Brien NA, McDermott MSJ, Conklin D, Luo T, Ayala R, Salgar S, et al. Targeting activated PI3K/mTOR signaling overcomes acquired resistance to CDK4/6-based therapies in preclinical models of hormone receptor-positive breast cancer. Breast Cancer Res. 2020;22:89. [DOI] [PubMed] [PMC]
    Brumec M, Sobočan M, Takač I, Arko D. Clinical implications of androgen-positive triple-negative breast cancer. Cancers (Basel). 2021;13:1642. [DOI] [PubMed] [PMC]
    Gonzalez-Angulo AM, Stemke-Hale K, Palla SL, Carey M, Agarwal R, Meric-Berstam F, et al. Androgen receptor levels and association with PIK3CA mutations and prognosis in breast cancer. Clin Cancer Res. 2009;15:24728. [DOI] [PubMed]
    Lehmann BD, Bauer JA, Schafer JM, Pendleton CS, Tang L, Johnson KC, et al. PIK3CA mutations in androgen receptor-positive triple negative breast cancer confer sensitivity to the combination of PI3K and androgen receptor inhibitors. Breast Cancer Res. 2014;16:406. [DOI] [PubMed] [PMC]
    Cochrane DR, Bernales S, Jacobsen BM, Cittelly DM, Howe EN, D’Amato NC, et al. Role of the androgen receptor in breast cancer and preclinical analysis of enzalutamide. Breast Cancer Res. 2014;16:R7. [DOI] [PubMed] [PMC]
    Wang Y, Yu Q, He X, Romigh T, Altemus J, Eng C. Activation of AR sensitizes breast carcinomas to NVP-BEZ235’s therapeutic effect mediated by PTEN and KLLN upregulation. Mol Cancer Ther. 2014;13:51727. [DOI] [PubMed]
    Alpelisib and enzalutamide in treating patients with androgen receptor and PTEN positive metastatic breast cancer [Internet]. U.S. National Library of Medicine; [cited 2022 Jan 08]. Available from: https://clinicaltrials.gov/ct2/show/NCT03207529
    Zhou Y, Wu C, Lu G, Hu Z, Chen Q, Du X. FGF/FGFR signaling pathway involved resistance in various cancer types. J Cancer. 2020;11:20007. [DOI] [PubMed] [PMC]
    Wheler JJ, Atkins JT, Janku F, Moulder SL, Stephens PJ, Yelensky R, et al. Presence of both alterations in FGFR/FGF and PI3K/AKT/mTOR confer improved outcomes for patients with metastatic breast cancer treated with PI3K/AKT/mTOR inhibitors. Oncoscience. 2016;3:16472. [DOI] [PubMed] [PMC]
    Formisano L, Lu Y, Servetto A, Hanker AB, Jansen VM, Bauer JA, et al. Aberrant FGFR signaling mediates resistance to CDK4/6 inhibitors in ER+ breast cancer. Nat Commun. 2019;10:1373. [DOI] [PubMed] [PMC]
    Hyman DM, Tran B, Paz-Ares L, Machiels JP, Schellens JH, Bedard PL, et al. Combined PIK3CA and FGFR inhibition with alpelisib and infigratinib in patients with PIK3CA-mutant solid tumors, with or without FGFR alterations. JCO Precis Oncol. 2019;3:113. [DOI] [PubMed]