LAUSR

Precision medicine targeting gene mutations holds the promise of changing the landscape of cancer care and prognosis, but currently approved drugs in this category are efficacious in only a very small percentage of all cancer patients (Tannock and Hickman, 2016). TP53, encoding the tumor suppressor and transcription factor p53, is the most frequently mutated gene in human cancers (Joerger and Fersht, 2016; Sabapathy and Lane, 2018; Levine, 2019). Pharmacologically rescuing mutant p53 by restoring wild-type function could therefore potentially be widely applicable in cancer treatment and is considered to be a holy grail of cancer research (Joerger and Fersht, 2010). Indeed, at least 17 compounds that can rescue mutant p53 variants were reported by 2018 (Sabapathy and Lane, 2018). Unfortunately, p53 mutations still remain therapeutically nonactionable due to challenges such as heterogeneous mechanisms of inactivation by different mutations and the absence of obvious targetable drug-binding pockets (except Y220C mutant). In a recent publication (Chen et al., 2021), we reported the identification of smallmolecule compounds that rescue a broad class of p53 mutations. Notably, these include arsenic trioxide (ATO), which is used to treat acute promyelocytic leukemia (de Thé et al., 2017). The study differentiates itself from previous reports in: (i) rescuing mutant p53 at striking levels when benchmarked against previously reported rescue compounds; (ii) providing a structural mechanism, wherein the arsenic atom binds to a cryptic allosteric site connecting the loop–sheet–helix (LSH) motif with the bsandwich skeleton to increase the thermostability of mutant p53; (iii) offering a largely defined spectrum of applicable p53 mutations—the structural mutations that compromise the wild-type structure of p53 and collectively account for more than half of all clinically relevant p53 alterations. In this essay, we further focus on the broad-spectrum rescue compounds for structural p53 mutants, rather than ATO itself. We will discuss our opinions on future prospects in drug discovery and the challenges faced by both basic and translational research on this type of p53-rescuing compounds. The Y220Ctargeting compounds that specifically rescue the individual Y220C mutation and the DNA contacting mutationrescuing compounds that remains elusive for the rescuing mechanism at the atomic level are not discussed here. At the drug discovery stage, we normally apply three easily accessible and highly sensitive assays to validate a hit achieved in library screens—the differential scanning fluorimetry assay, PAb1620 immunoprecipitation assay, and luciferase reporter assay—which are respectively used to assess the thermostability, folding status, and transcriptional activity of p53. However, we noticed that mutants such as V272M, R282W, and E285K are relatively inert in PAb1620 epitope promotion despite substantial increases of thermostability, while mutants such as R175H, R249S, and H179Y are relatively inert in terms of transcriptional activity promotion. After validation, we normally rely on three goto criteria to decide whether to push the rescue compounds into animal and preclinical studies—the availability of a cocrystal structure, a structure–activity relationship consistent with crystal structure, and target specificity in cells. We frequently encounter compounds that generate reproducible positive readouts in rescuing some aspects of p53 activity in the library screens, but <1% of the compounds meet all three criteria, particularly the criterion of target specificity. For example, compounds preferentially inhibiting isogenic cell lines with mutant p53 are frequently found to actually target p53 mutation-associated downstream or compensatory events, rather than the mutant p53 itself.