Potent Inhibition of Multiple Proteasome Subunits by Carfilzomib in Multiple Myeloma and Solid Tumor Patients

Abstract 5068

Proteasome inhibition has been validated as a therapeutic approach for multiple myeloma (MM) and mantle cell lymphoma, however the contributions of inhibiting individual active-site subunits of the constitutive proteasome (c20S) and the immunoproteasome (i20S) has not been fully explored due the lack of effective tools. A novel assay was developed, validated, and used to quantitatively measure the levels of individual proteasome active site subunits in vitro and in tissue samples from patients exposed to the proteasome inhibitor (PI) carfilzomib from 5 clinical trials, PX-171-003, PX-171-004, PX-171-005, PX-171-006 & PX-171-007. This assay, called ProCISE (proteasome constitutive/immune subunit ELISA), was shown to have good analytical recovery without interfering matrix effects and was used to measure the activity of c20S subunits (b5, b2 and b1) and i20S subunits (LMP7, MECL1 and LMP2) using subunit-specific antibodies in whole blood, PBMC, and bone marrow derived CD138⁺ MM cells. Following an initial dose of carfilzomib across doses of 15 – 45 mg/m², ≥80% inhibition of the chymotrypsin-like (CT-L) active sites b5 and LMP7 as well as dose-dependent inhibition of MECL1 and LMP2 was observed. Additionally, neither renal function nor co-administration of the commonly used MM agents, lenalidomide or dexamethasone, had an effect on the pharmacodynamics of carfilzomib. Carfilzomib inhibited 63% of all active sites of the immunoproteasome at 45 mg/m² and 78% at 56mg/m². In tumor cells, which express a mixture of both proteasome types, inhibition of CT-L activity correlated with levels of inhibition in whole blood. Proteasome inhibition with carfilzomib was prolonged in both whole blood and PBMC. Cumulative and sustained proteasome inhibition was seen in whole blood while complete or near complete recovery was noted in PBMC by the start of a second cycle of administration. The depth and duration of proteasome inhibition with carfilzomib is greater than what has been reported with the reversible inhibitor bortezomib. In a limited analysis of MM patients, stratified by best response to carfilzomib, we did not detect a difference in the full proteasome inhibition profile in patients achieving an objective response and those that did not achieve clinical benefit. While these data do not currently demonstrate that proteasome inhibition alone is a predictive marker of clinical response, with larger sample sizes and further investigation regarding subunit-specific inhibition of the proteasome, we hope to be able to demonstrate a correlation between proteasome inhibition and patient response.