Studies of mutant p53-targeting small molecules

Abstract

Multiple cellular stresses, such as DNA damage, oncogene activation, hypoxia, and telomere erosion induce p53 protein levels leading to an array of biological responses, including cell cycle arrest and apoptosis. p53 exerts its function mainly through transcriptional regulation of specific target genes, but is also able to induce transcriptional-independent apoptosis.

The high frequency of p53 mutations in human tumors, the expression of high levels of mutant p53 protein, and the fact that the mutant p53-harbouring tumors frequently show increased resistance to conventional chemotherapy makes p53 an attractive target for cancer therapy. In the past years, several small molecules that restore mutant p53 function have been identified, among them PRIMA-1 and MIRA-1. Both compounds induce mutant p53-dependent apoptosis and restore native conformation, DNA binding, and transcriptional transactivation to mutant p53. PRIMA-1 and its more potent analog PRIMA-1MET inhibit tumor growth in SCID mice. The maleimide analog MIRA-3 shows anti-tumor activity in SCID mice, however, the therapeutic window is narrow. Moreover, we show that PRIMA-1MET not only induces apoptosis in human mutant p53-carrying tumor cells, but also has potent growth inhibitory effects in mouse tumors containing mutant p53 in syngeneic mice.

Another approach for a novel strategy of cancer therapy is based on already existing cancer drugs in combination with p53-reactivating molecules. This may reduce the side-effects of the currently used anti-cancer therapy. Therefore, we treated human tumor cells with the commonly used chemotherapeutic drug cisplatin in combination with the mutant p53-rescuing molecule PRIMA-1MET. We observed a synergistic apoptotic effect in vitro and in vivo. This effect was dependent on mutant p53. This synergistic effect may be due to the ability of cisplatin to promote elevated levels of mutant p53 in tumor cells, presumably enhancing their sensitivity to PRIMA-1MET, thus suggesting that any agent inducing mutant p53 levels may synergize with PRIMA-1MET.

STIMA-1, another small mutant p53-reactivating molecule has structural resemblance to the already identified CP-31398 compound. We show that both CP-31398 and STIMA-1 have similar chemical activity as traditional Michael acceptors and this activity is related to the observed mutant p53-dependent growth suppression. However, mutant p53-dependent growth suppression of tumor cells was more pronounced for STIMA-1 than that for CP-31398.

Although several mutant p53-reactivating drugs have been successfully identified, it is important to continue searching for new molecules by applying diverse screening techniques. Identification of different structural types of mutant p53-rescue molecules may provide a better understanding of the molecular mechanisms of mutant p53 reactivation. In addition, already identified lead molecules may not be suitable for clinical use due to non-specific toxicity or undesirable pharmacodynamic properties.

This thesis characterizes three structurally different small molecules that target mutant p53. Comparing all three different scaffolds will shed light on the possible molecular mechanisms of mutant p53 rescue. This should facilitate the design of more potent and selective mutant p53-targeting anti-cancer drugs.