Carfilzomib (CFZ) is the first in a new class of proteasome inhibitors that produce specific and sustained inhibition. The three proteolytic activities of the 20S proteasome are the chymotrypsin-like (CT-L), trypsin-like (T-L) and caspase-like (C-L) associated with the \#946;5, \#946;2 and \#946;1 subunits, respectively in the constitutive proteasome and the LMP7, MECL1 and LMP2 subunits in the immunoproteasome. Of these activities, the one that makes the greatest contribution to cellular protein turnover is the CT-L (Kisselev et al., J. Biol. Chem. 281:8582, 2006). CFZ binds selectively and irreversibly to the CT-L active sites. Phase 1 studies have demonstrated that CFZ is well tolerated on schedules of QDx2 weekly every 3 weeks or QDx5 every other week. Objective responses have been observed in patients with multiple myeloma (MM), non Hodgkin\#8217;s lymphoma (NHL) and solid tumors. Pharmacodynamic testing was performed to determine the extent and duration of proteasome inhibition in CFZ treated patients. Relationships between proteasome inhibition and efficacy and safety endpoints were also investigated. Proteasome activity in blood and peripheral blood mononuclear cells (PBMCs) was measured at baseline and at multiple time points during the first cycles of dosing using a fluorogenic substrate specific for the CT-L activity of the proteasome. In Phase 1 hematologic tumor trials (dose range 1.2 mg/m2 to 27 mg/m2), dose-dependent inhibition following the first dose was observed. Inhibition was comparable in blood and PBMCs and exceeded 70% at doses > 11 mg/m2. Consecutive dosing, either QDx2 or QDx5, resulted in cumulative proteasome inhibition (up to 90%) in blood and PBMCs. Proteasome activity in PBMCs recovered to >50% of baseline between cycles. Minimal recovery was observed in blood, likely due to the inability of red blood cells to synthesize new proteasomes, a prerequisite for recovery from irreversible inhibition by CFZ. Subjects with MM or NHL that achieved a clinical response (MR or better) had >70% proteasome inhibition in blood following the first dose. In solid tumor patients (n=12), CFZ was dosed at 20 mg/m2 on the QDx2 schedule with dose escalation, starting in the second week, up to 36 mg/m2. In addition to the fluorogenic CT-L assay, inhibition of all catalytic sites of the constitutive and immunoproteasome were monitored using an active site ELISA. The CT-L activity was inhibited by >70% in blood (\#946;5) and in PBMCs (LMP7) in all subjects. A modest inhibition of \#946;2 (15±7%), LMP2 (23±14%), and MECL-1 (40±19%) was noted in subjects receiving 36 mg/m2. Proteasome activity in PBMCs recovered to >80% of baseline between cycles. These studies support that inhibition of the CT-L subunit is the predominant mechanism of anti-tumor activity in both hematologic and solid tumors. Further studies on the relationship between proteasome inhibition and clinical endpoints are underway.

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 5439.

100th AACR Annual Meeting-- Apr 18-22, 2009; Denver, CO