A Phase 1 Trial of SNS-032, a Potent and Specific CDK 2, 7 and 9 Inhibitor, in Chronic Lymphocytic Leukemia and Multiple Myeloma

SNS-032, formerly BMS-387032, is a highly selective and potent inhibitor of cyclin-dependent kinases (CDK) 2, 7 and 9. CDK2 and CDK7 are involved in cell cycle regulation. CDK7, along with CDK9, regulate RNA polymerase (Pol) II-dependent transcription. Temporary inhibition of RNA Pol II-dependent transcription by SNS-032 has significant effects on short half-life transcripts and proteins, particularly survival factors, cell cycle regulatory proteins, and cytokines that are critical for the survival of malignant B-cells in chronic lymphocytic leukemia (CLL) and multiple myeloma (MM). A phase 1 dose-escalation study in patients with MM and CLL is ongoing with separate dose escalations for each indication. The study is designed to evaluate safety, pharmacokinetics (PK) and preliminary evidence of activity of a loading dose (LD) followed by a 6 hour infusion of SNS-032 given weekly for 3 consecutive weeks of each 28-day cycle. Dose and schedule aim to maintain for 6 hours threshold plasma concentrations of 115 ng/mL (in vitro IC90) and higher. The study incorporates an exploratory analysis of potential pharmacodynamic (PD) biomarkers such as decreased phosphorylation of RNA Pol II C-terminal domain to demonstrate inhibition of CDK7 and CDK9, and decreased expression of survival factors to indicate transcriptional inhibition.

Methods: Previously treated patients with advanced CLL or MM, measurable disease, and ECOG status 0–1 were eligible. Increasing doses of SNS-032 given as an LD followed by a 6 hr infusion were evaluated. The total starting dose was 15 mg/m² comprised of a LD of 5 mg/m² followed by 10 mg/m² over 6 hr with dose escalation by modified Fibonacci. PD studies of target modulation were performed on peripheral blood mononuclear cells (PBMC) obtained pre- and post-dose. Direct target modulation or downstream effects of target inhibition were evaluated.

Results: 35 patients have been treated to date, 18 MM patients and 17 CLL patients. Median age was 61 (range 45–82), with 13 females and 24 males. Median number of prior therapies was 5 (range: 1–11). MM patients have received total doses of 15 – 75 mg/m². No drug-related dose limiting toxicities (DLTs) or objective responses have been reported thus far in MM. CLL patients have received total doses of 15 –100 mg/m². No drug-related DLTs were observed through the 50 mg/m² dose cohort. At 75 mg/m², concentrations of SNS- 032 exceeded IC90 (mean maximum concentration during the 6 hr infusion was 261 ± 45 ng/mL). Evidence of biochemical tumor lysis syndrome (TLS) was observed in all CLL patients treated at this dose level. One patient experienced a DLT of vascular leak syndrome and failure to receive all 3 cycle 1 doses. One CLL patient has been treated thus far at 100 mg/m². This patient experienced TLS with a DLT of elevated liver function enzymes for >48 hr and received only 2 of 3 doses in cycle 1. No objective responses have been observed. PD analyses showed evidence of decreased Mcl-1 or XIAP in several patients.

Conclusions: The mechanism of action of SNS-032 supports testing this agent in B-cell malignancies such as MM and CLL. A pharmacologically-derived dose regimen that sustains IC90 SNS-032 concentrations or higher for 6 hr is being studied; target levels were achieved and exceeded in cohort 5 (75 mg/m²) for both MM and CLL. No DLTs or objective responses have been observed thus far in MM. AEs and DLTs related to mild to moderate TLS were observed in CLL patients at 75 mg/m² and higher. No objective responses have been observed. Preliminary evidence of target-specific PD modulation has been demonstrated. Enrollment in this trial is continuing.