Selective induction of apoptosis in tumors by small molecules reactivating p53

Abstract

Lesions in the p53 pathway that inactivate p53 tumor suppression function occur with extremely high frequency in cancer, targeting almost all tumors regardless of patient age or tumor type. This study is aimed at developing approaches for the selective elimination of tumors carrying defects in the p53 pathway, namely protection of p53 from degradation by its own destructor HDM2 in wild type p53-carrying tumors and refolding of p53 by small molecules in mutant p53-containing tumors.

Among genes known to be involved in tumor development, the p53 gene has an exclusive status. Mutations of p53 gene occur in around 50% of human tumors. The great majority of p53 mutaitons are point missense mutations resulting in a substitution of only one amino acid residue that causes unfolding of the DNA binding domain. These particular features of p53 alterations in tumors, taken together with the observation that mutant p53 proteins accumulate at high levels in tumor cells, makes the strategy aimed at reactivation of p53 quite feasible.

We identified and characterized a small molecule, PRIMA-1, which restored the sequencespecific DNA binding and active conformation to mutant p53 proteins. In vivo studies in mice revealed an antitumor effect with no apparent toxicity.

We also showed that rationally designed nine-residue peptide CDB3 that bound to the tumor suppressor p53 and stabilized it against denaturation in vitro, binds p53 in cells, restores the active conformation and transcription transactivation function to mutant p53. These molecules,PRIMA-1 and CDB3, may serve as a lead compounds for the development of anticancer drugs targeting mutant p53.

In wt p53-carrying human tumors, p53 pathway is disrupted via several mechanisms, including overexpression of p53's negative regulator, HDM2, or loss of expression of p14ARF via deletion of INK4a/ARF locus or methylation of p14ARF promoter. Given the central role of HDM2/ p 14ARF in control of p53 function, the potential for non-genotoxic cancer therapies based on the inhibition of HDM2 in tumors expressing wt p53 is of considerable interest.

Restoration of p53 function by blocking p53/HDM2 interaction would result in induction of apoptosis in tumors while inducing transient growth arrest in normal cells, as it was shown that normal cells are less sensitive to p53 activation. This provides a powerful new approach to target tumor cells selectively.

We have screened the library of low molecular weight compounds for molecules that can suppress the growth of human tumor cells in a wtp53-dependent manner, and identified the compound, named RITA, that binds p53 protein, prevents p53/HDM2 interaction in vitro and in vivo and affects p53's interaction with several negative regulators. RITA induces expression of p53 target genes and massive apoptosis in various tumor cells lines carrying wtp53. RITA demonstrates substantial p53-dependent anti-tumor effect in vivo. The strong anti-tumor activity of RITA exceeds the standard criteria of National Cancer Institute for candidate drugs.