Development of the proteasome inhibitor PS-341

Over the last decade, the critical role of the proteasome in cell-cycle regulation has become increasingly apparent. The proteasome, a multicatalytic protease present in all eukaryotic cells, is the primary component of the protein degradation pathway of the cell. By degrading regulatory proteins (or their inhibitors), the proteasome serves as a central conduit for many cellular regulatory signals and, thus, is a novel target for therapeutic drugs. PS-341 is a small molecule that is a potent and selective inhibitor of the proteasome. In vitro and mouse xenograft studies of PS-341 have shown antitumor activity in a variety of tumor types, including myeloma, chronic lymphocytic leukemia, prostate cancer, pancreatic cancer, and colon cancer, among others. Although PS-341 rapidly leaves the vascular compartment, a novel pharmacodynamic assay has shown that inhibition of proteasome-the biologic target-is dose dependent and reversible. These studies provided the rationale for a twice-weekly dosing schedule employed in ongoing clinical studies. Phase I trials in a variety of tumor types have shown PS-341 to be well tolerated, and phase II trials in several hematologic malignancies and solid tumor types are now in progress. Efficacy and safety data from the most advanced of these, a phase II multicenter trial in myeloma, will be available in early 2002.