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.

Epigenetics: It's all about the individual: The bromodomain and extraterminal domain (BET) proteins are a family of epigenetic regulators that affect transcription through interactions with acetylated chromatin. These proteins have attracted interest as drug targets because of their roles in human diseases, including cancer. Parsing out the biological functions of individual family members is important for drug development but has been challenging because of the proteins' shared structural domains. In a proof-of-concept study, Baud et al. used a chemical genetics strategy to design a highly selective small-molecule probe that inhibits the chromatin binding activity of one family member. Extension of this approach may help elucidate the function of individual BET proteins and prioritize them as drug targets. Science , this issue p. 638

The tumor suppressor p53 has the most frequently mutated gene in human cancers. Many of p53’s oncogenic mutants are just destabilized and rapidly aggregate, and are targets for stabilization by drugs. We found certain 2-sulfonylpyrimidines, including one named PK11007, to be mild thiol alkylators with anticancer activity in several cell lines, especially those with mutationally compromised p53. PK11007 acted by two routes: p53 dependent and p53 independent. PK11007 stabilized p53 in vitro via selective alkylation of two surface-exposed cysteines without compromising its DNA binding activity. Unstable p53 was reactivated by PK11007 in some cancer cell lines, leading to up-regulation of p53 target genes such as p21 and PUMA. More generally, there was cell death that was independent of p53 but dependent on glutathione depletion and associated with highly elevated levels of reactive oxygen species and induction of endoplasmic reticulum (ER) stress, as also found for the anticancer agent PRIMA-1^MET(APR-246). PK11007 may be a lead for anticancer drugs that target cells with nonfunctional p53 or impaired reactive oxygen species (ROS) detoxification in a wide variety of mutant p53 cells. Significance Cancers with mutant p53 often show increased metastasis, genomic instability, and higher chemoresistance. The development of drugs targeting tumors with mutant p53 background is a current strategy for anticancer therapy. We found that certain activated electrophilic 2-sulfonylpyrimidines are a new class of thiol-reactive anticancer agents. These agents are especially effective in killing cancer cells with mutant or inactivated p53 or impaired reactive oxygen species detoxification and have relatively low cytotoxicity toward normal cells; they are mild electrophiles, some of which will, for example, stabilize mutant p53 by selective targeting of its thiol groups and have little general alkylating reactivity.