Abstract C94: Deubiquitylases as novel anticancer targets: Discovery and development of novel USP7 inhibitors

Abstract

Abstract The ubiquitin-proteasome pathway is a complex system consisting of enzymes that conjugate or deconjugate ubiquitin to/from target proteins upstream of the 26S proteasome. In particular, ubiquitin deconjugation is performed by families of proteases known as deubiquitylating enzymes (DUBs). The therapeutic validation of the ubiquitin-proteasome system was provided by the marketing approval of bortezomib (PS-341) for the treatment of multiple myeloma (MM). In our search for alternative approaches for the treatment of MM, we investigated the therapeutic potential of inhibiting another component of the ubiquitin-proteasome pathway, the DUB USP7. Our rationale for targeting USP7 is as follows: 1) Blockade of USP7 using siRNA or homologous recombination results in elevated levels of the tumor suppressor p53 and cell cycle arrest in cancer cells. In contrast to the situation in the majority of human tumors, mutations or deletions of p53 in MM are relatively rare and thus the activation of p53 is likely to offer a therapeutic benefit in MM. 2) Knockdown of USP7 destabilizes the adaptor protein Claspin, inhibiting activation of Chk1, an effector kinase in the DNA damage response, abrogating the G2M cell cycle checkpoint and providing a mechanism for inducing tumor cell death in a p53 independent manner. Thus inhibition of USP7 would be predicted to activate pro-apoptotic and inactivate pro-survival pathways in multiple myeloma, resulting in tumor suppression. Recently, using its proprietary screening technology, Progenra discovered and characterized a small molecule inhibitor of USP7, P5091. Specifically, P5091 selectively inhibits USP7 relative to other proteases and exhibits cytotoxic activity against both p53 wild type and p53 mutant tumor cell lines. Furthermore, P5091 destabilizes the pharmacodynamic markers HDM2 and Claspin in tumor cells and induces cell death synergistically in combination with genotoxic agents. Medicinal chemistry has identified a series of potent and selective novel chemical entities. Initial preclinical studies at the Dana-Farber Cancer Institute demonstrate that P5091 induces apoptosis and is cytotoxic against human MM cells but not normal plasma cells. In conclusion, these data demonstrate that 1) USP7 is a viable target for the treatment of MM and other cancers; and 2) P5091 is an efficacious inhibitor of USP7. Preclinical development of the P5091 series continues, augmented by a comprehensive medicinal chemistry program designed to identify a clinical candidate from this series in 2010. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):C94.