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.

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.

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.

BCL-X_L is an anti-apoptotic protein that plays an important role in tumorigenesis, metastasis, and intrinsic or therapy-induced cancer drug resistance. More recently, BCL-X_L has also been identified as a key survival factor in senescent cells. Accumulation of senescent cells has been indicated as a causal factor of aging and many age-related diseases and contributes to tumor relapse and metastasis. Thus, inhibition of BCL-X_L is an attractive strategy for the treatment of cancer and extension of healthspan. However, development of BCL-X_L inhibitors such as navitoclax for clinical use has been challenging because human platelets depend on BCL-X_L for survival. In this review, the authors discuss how BCL-X_L-targeted proteolysis targeting chimeras (PROTACs) afford a novel approach to mitigate the on-target thrombocytopenia associated with BCL-X_L inhibition. The authors summarize the progress in the development of BCL-X_L PROTACs. The authors highlight the in vitro and in vivo data supporting that by hijacking the ubiquitin protein ligase (E3) that are poorly expressed in human platelets, BCL-X_L PROTACs can significantly improve the therapeutic window compared to conventional BCL-X_L inhibitors. These findings demonstrated the potentially broad utility of PROTAC technology to achieve tissue selectivity through recruiting differentially expressed E3 ligases and to reduce on-target toxicity.

About 15–20% of all breast cancers (BCs) are defined human epidermal growth factor receptor 2 (HER2)-positive, based on the overexpression of HER2 protein and/or amplification of ERBB2 gene. Such alterations lead to a more aggressive behavior of the disease, but also predict response to treatments targeting HER2. Indeed, several anti-HER2 compounds have been developed and approved in the last two decades, significantly improving our ability to cure patients in the early setting, and greatly extending their survival in the advanced setting. However, recent evolutions in this field promise to improve outcomes even further, through advancements in established HER2-targeting strategies, as well as the exploration of novel strategies. In particular, the engineering of new antibody-drug conjugates, with higher drug-to-antibody ratios (DARs) and cleavable linkers, has already led to the development of a highly effective drug, namely trastuzumab deruxtecan, recently approved by the Food and Drug Administration (FDA) and European Medicines Agency (EMA) for the treatment of advanced HER2-positive (HER2⁺) BC, and currently in study in the early setting. Moreover, the novel tyrosine kinase inhibitor tucatinib was recently approved by FDA and EMA, showing to improve survival of HER2⁺ advanced BC patients, particularly in those with brain metastasis. Immunotherapy is also being investigated in the HER2⁺ subtype, through immune-checkpoint inhibition, cancer vaccines and adoptive-cell therapies. Overall, the enlarging arsenal of promising anti-HER2 compounds is expected to deliver significant improvements in the prognosis of both early and advanced HER2⁺ BC in the years to come. Moreover, some of such agents are showing encouraging activity in the much wider population of HER2-low advanced BC patients, challenging current BC classifications. If confirmed, this new paradigm would potentially expand the population deriving benefit from HER2-targeted treatments to up to 70% of all advanced BC patients, leading to a revolution in current treatment algorithms, and possibly to a redefinition of HER2 classification.