The p53 pathway as a target in cancer therapeutics: obstacles and promise

Abstract

A large fraction of human tumors carry p53 mutations, which allow tumor initiation and progression; furthermore, it is now clear that restoration or reactivation of wild-type p53 function prompts rapid elimination of tumors. The discovery and design of compounds that reactivate or enhance the p53 pathway has resulted in the identification of promising drug candidates that have now entered clinical trials for anticancer strategies. However, some of these agents appear to elicit undesirable toxic effects on normal cells and tissues and therefore are restricted in the dose that can be applied in tumors. In this Review, we discuss the concerns about and promise of these p53 activators and propose ways to expand and optimize screening strategies to identify such molecules.

Fig 1. Modulating the p53 pathway with small molecules

Wild-type p53 is activated by a variety of stressors, including DNA damage, oncogene expression, nutrient starvation, oxidative stress, and depletion of ribonucleotide triphosphates (used in RNA synthesis). (A) Small molecules that target MDM2 and block p53 binding stabilize p53. (B) SJ172550 targets the p53 binding pocket of MDMX, also promoting p53 stabilization. (C) Tenovin-6 inhibits the protein deacetylase activity of SIRT. Acetylation results in the stabilization of p53 and interferes with MDM2-mediated degradation. (D) RITA binds to p53 and interferes with the interaction of MDM2 and p53, activating p53 function. (E) Small molecules designed to bind transcriptionally inert mutant p53 proteins stabilize the core domain, restore the native state, and eventually enable binding to DNA. CREDIT: C. BICKEL/SCIENCE TRANSLATIONAL MEDICINE

Fig 2. Strategies for identifying activators of the p53 response

(A) Cell-based screening. In this approach, small molecules are tested for their ability to activate p53 pathways in cultured cells. Activation can be detected by measuring endogenous levels of transcripts from p53 target genes or by using various reporter systems. In this second method, the promoter of a p53 target gene is placed upstream of a gene encoding a product that can be easily monitored, such as the enzyme luciferase (luc). Cell-based screening will identify compounds with certain biological activity but with an unknown direct target molecule. (B) Target-based screening. In this approach, p53 protein activators are sought in vitro. Specific methods include the following: (i) Direct protein binding assays, such as the SMM assay. In this example, a library of small molecules is assembled on a microarray and the ability of p53 to bind the molecules is tested. Here, p53 binding is detected by a set of antibodies. (ii) Forward structural design, in which the structures of mutant versions of p53 are analyzed and molecules are designed to interact with relevant regions of these structures. (iii) Direct DNA binding assays, in which molecules that allow mutant p53 to bind DNA are sought. Such methods will enable the identification of compounds with a defined molecular mechanism. CREDIT: C. BICKEL/SCIENCE TRANSLATIONAL MEDICINE