PROteolysis TArgeting Chimeras (PROTACs) are heterobifunctional molecules consisting of two ligands; an “anchor” to bind to an E3 ubiquitin ligase and a “warhead” to bind to a protein of interest, connected by a chemical linker. Targeted protein degradation by PROTACs has emerged as a new modality for the knock down of a range of proteins, with the first agents now reaching clinical evaluation. It has become increasingly clear that the length and composition of the linker play critical roles on the physicochemical properties and bioactivity of PROTACs. While linker design has historically received limited attention, the PROTAC field is evolving rapidly and currently undergoing an important shift from synthetically tractable alkyl and polyethylene glycol to more sophisticated functional linkers. This promises to unlock a wealth of novel PROTAC agents with enhanced bioactivity for therapeutic intervention. Here, the authors provide a timely overview of the diverse linker classes in the published literature, along with their underlying design principles and overall influence on the properties and bioactivity of the associated PROTACs. Finally, the authors provide a critical analysis of current strategies for PROTAC assembly. The authors highlight important limitations associated with the traditional “trial and error” approach around linker design and selection, and suggest potential future avenues to further inform rational linker design and accelerate the identification of optimised PROTACs. In particular, the authors believe that advances in computational and structural methods will play an essential role to gain a better understanding of the structure and dynamics of PROTAC ternary complexes, and will be essential to address the current gaps in knowledge associated with PROTAC design.

Dysregulation of cellular pH is frequent in solid tumours and provides potential opportunities for therapeutic intervention. The acidic microenvironment within a tumour can promote migration, invasion and metastasis of cancer cells through a variety of mechanisms. Pathways associated with the control of intracellular pH that are under consideration for intervention include carbonic anhydrase IX, the monocarboxylate transporters (MCT, MCT1 and MCT4), the vacuolar-type H⁺-ATPase proton pump, and the sodium-hydrogen exchanger 1. This review will describe progress in the development of inhibitors to these targets.

Triple negative breast cancer (TNBC) represents an aggressive disease associated with a high risk of recurrence after curative treatment and a poor prognosis in the metastatic setting. Chemotherapy was for years the only treatment available in the early and metastatic setting, due to the lack of actionable targets. Clinical practice has changed following the results obtained with the addition of immunotherapy to standard chemotherapy, the development of novel drugs [i.e. antibody-drug conjugates (ADCs)], and the use of targeted treatments for patients carrying germline pathogenic breast cancer susceptibility genes (BRCA) 1 or BRCA 2 variants. The treatment of early-stage disease has had a shift in clinical practice since July 2021, after the Food and Drug Administration (FDA) approval of pembrolizumab in association with chemotherapy as neoadjuvant treatment for TNBC and as a single agent in the subsequent adjuvant setting. This intensive treatment based on the combination of a poly-chemotherapy and an immune checkpoint inhibitor (ICI) led to the improvement of short- and long-term outcomes, but it has highlighted some new unmet clinical needs in the treatment of early-stage TNBC: the selection of the most effective adjuvant therapy and the integration of pembrolizumab with other therapeutic strategies [capecitabine, poly(ADP-ribose) polymerase (PARP) inhibitors] based on the achievement of pathologic complete response (pCR); the identification of predictive biomarkers to select patients who could most benefit from the addition of ICI, to minimize toxicities and to maximize outcomes; the possibility of de-escalating chemotherapy in favor of immune-combo or novel agents, such as ADCs; the role of immunotherapy in estrogen receptor (ER)-low patients. The advent of immunotherapy not only addresses current challenges in TNBC treatment but also holds the promise of a radical transformation in its therapeutic paradigm, enhancing significantly clinical outcomes and offering new perspectives for patients grappling with this aggressive form of breast cancer.

The Fanconi anaemia (FA) pathway is an important mechanism for cellular DNA damage repair, which functions to remove toxic DNA interstrand crosslinks. This is particularly relevant in the context of ovarian and other cancers which rely extensively on interstrand cross-link generating platinum chemotherapy as standard of care treatment. These cancers often respond well to initial treatment, but reoccur with resistant disease and upregulation of DNA damage repair pathways. The FA pathway is therefore of great interest as a target for therapies that aim to improve the efficacy of platinum chemotherapies, and reverse tumour resistance to these. In this review, we discuss recent advances in understanding the mechanism of interstrand cross-link repair by the FA pathway, and the potential of the component parts as targets for therapeutic agents. We then focus on the current state of play of inhibitor development, covering both the characterisation of broad spectrum inhibitors and high throughput screening approaches to identify novel small molecule inhibitors. We also consider synthetic lethality between the FA pathway and other DNA damage repair pathways as a therapeutic approach.

Proteolysis targeting chimeras (PROTACs) represent a promising class of hetero-bivalent molecules that facilitate ubiquitination of a target protein by simultaneously binding and bringing together both the E3 enzyme and the target. These compounds consist of three structural components: two ligands one of which binds the protein of interest (POI) while the other binds an E3 ubiquitin ligase to promote POI ubiquitination, and a linker connecting both moieties. Recent developments in the field highlight the fact that linker composition and length play a crucial role in achieving optimal PROTAC properties, modulate binding kinetics and substantially impacts the potency and selectivity. In this review, the authors briefly discuss the recent findings in PROTAC design approaches with focus on the linker. For each PROTAC such linker parameters as chemical nature, length, hydrophilicity and rigidity have to be optimized to achieve improved stability, bioavailability cell membrane permeability and suitable spatial orientation between the target POI and the E3 ubiquitin ligase. Thus rational linker design with respect to composition, length and attachment sites is essential for the development of potent PROTAC compounds. Computer-aided design and novel innovative linker strategies, such as PROTAC shortening, photo-switchable PROTACs, in-cell click-formed CLIPTACs, “click chemistry” approaches are also discussed in the review.

Chronic lymphocytic leukemia is a common form of leukemia and is dependent on growth-promoting signaling via the B-cell receptor. The Bruton tyrosine kinase (BTK) is an important mediator of B-cell receptor signaling and the irreversible BTK inhibitor ibrutinib can trigger dramatic clinical responses in treated patients. However, emergence of resistance and toxicity are major limitations which lead to treatment discontinuation. There remains, therefore, a clear need for new therapeutic options. In this review, we discuss recent progress in the development of BTK-targeted proteolysis targeting chimeras (PROTACs) describing how such agents may provide advantages over ibrutinib and highlighting features of PROTACs that are important for the development of effective BTK degrading agents. Overall, PROTACs appear to be an exciting new approach to target BTK. However, development is at a very early stage and considerable progress is required to refine these agents and optimize their drug-like properties before progression to clinical testing.