Safety and Antitumor Efficacy of the Proteasome Inhibitor Carfilzomib (PR-171) Dosed for Five Consecutive Days in Hematologic Malignancies: Phase 1 Results.

Introduction: Carfilzomib (CFZ) is a structurally- and mechanistically-novel proteasome inhibitor (PI) of the peptide epoxyketone class that exhibits a high level of selectivity for the unique N-terminal threonine active sites within the proteasome. CFZ is similar to bortezomib (BTZ; VELCADE®), in that it is a potent inhibitor of the chymotrypsin-like activity, but unlike BTZ, CFZ has shown minimal cross-reactivity with the other catalytic sites within the proteasome or across other protease classes. Preclinical human tumor xenograft models indicate that consecutive day (D1, D2) dosing resulting in 48 hours of proteasome suppression was superior to split dosing (D1, D4).

Methods: This phase 1 sequential dose escalation trial tested an intensive proteasome inhibition schedule with CFZ administered as an intravenous push (IVP) daily (QD) for 5 consecutive days on a 14 day cycle. The doses administered ranged from 1.2 mg/m² to 20 mg/m². Multiple myeloma (MM), non-Hodgkin’s lymphoma (NHL) that included mantle cell lymphoma (MCL), Hodgkin’s disease (HD), or Waldenström’s macroglobulinemia (WM) were eligible if they had relapsed after 2 or more prior therapies.

Results: A total of 29 subjects have been treated. The minimal effective dose (MED) was 11 mg/m². A maximum of 80% proteasome inhibition was achieved in peripheral blood and mononuclear cells at ≥15 mg/m². Five responses have occurred in 14 patients (6 MM, 3 MCL, 1 WM, 4 NHL) enrolled at ≥ the MED: 1 MM partial response (PR), 2 MM minimal responses (MR), 1 WM MR and one macroscopic CR in a patient with gastrointestinal MCL. Responses occurred in patients whose disease had failed therapy with BTZ, immunomodulatory agents, and stem cell transplant. Four NHL patients have had stable disease and remained on treatment for up to 7 months. Dose limiting toxicity reported at 20 mg/m² included febrile neutropenia (reversible) and grade 4 thrombocytopenia. Grade 3 (25%) or grade 4 events (8%) were primarily hematologic events. Most withdrawals occurred due to the inconvenience of frequent visits, or progressive disease.

Conclusions: This study demonstrates that intensive CFZ dosing leading to sustained proteasome inhibition is well tolerated. The selectivity of the epoxyketone class of PI may promote a favorable safety profile. Furthermore, antitumor responses in heavily pretreated patients suggest non-cross resistant activity. Phase 2 trials in MM and solid tumors are proceeding with an alternate IV schedule. An oral agent suitable for frequent dosing schedules is under development.