SNS-032 has potent antitumor activity in vivo against human leukemia and multiple myeloma xenografts

A258 SNS-032 (formerly known as BMS-387032) is a potent, selective inhibitor of cyclin dependent kinase (CDK)s 2, 7 and 9 that inhibits both cell cycle progression and transcription and is currently in Phase I for the treatment of hematological malignancies. We hypothesize that SNS-032 inhibits transcriptional initiation and elongation by blocking the phosphorylation of Ser5 and Ser2 of the C-terminal domain (CTD) of RNA pol II by CDK7 and CDK9, respectively. Short half-life (T1/2) transcripts and proteins are maximally affected by transient exposure to SNS-032. Down-regulation of short T1/2 survival signaling proteins by SNS-032 was recently demonstrated in a MM cell line (RPMI-8226) (Conroy et al AACR 2007). Clinical evidence of SNS-032 target modulation was demonstrated ex vivo in peripheral blood mononuclear cells from treated cancer patients (Hawtin et al, Haematologica 2007;92,suppl 1). In this study, we investigate the in vivo anti-tumor activity of SNS-032 and correlation with potential pharmacodynamic (PD) markers of activity in athymic mouse subcutaneous xenograft models of human acute leukemia (MV4-11 and HL-60) and multiple myeloma (RPMI-8226). Dosing was initiated when tumor volumes were on average at least 200 mm3. Tumor growth was monitored throughout the study and PD endpoints were analyzed by Western blot. Single IP administration of 30 or 15 mg/kg of SNS-032 induces decreased phosphorylation of both Ser2 and Ser5 of RNA Pol II CTD pS2 and pS5 in tumor lysates from mice with MV4-11, HL-60 and RPMI-8226 xenografts, consistent with inhibition of CDKs 7 and 9. This occurs as early as 2 hours post dose and is sustained for at least 6 hours. In HL-60, the effects of SNS-032 on RNA Pol II phosphorylation correlate with down-regulation of the pro-survival protein MCL-1 at 30 mg/kg. PARP cleavage, indicative of apoptosis, is observed at both 15 and 30mg/kg dose levels. Administration of SNS-032 qdx5 at the MTD (30 mg/kg) results in 121% and 130 % tumor growth inhibition (TGI) in HL-60 and RPMI-8226 xenografts, respectively. Moreover, SNS-032 has potent anti-tumor activity against HL-60 xenografts using an intermittent q4d schedule at MTD and ½ MTD, with 119% and 86% TGI, respectively. When 30 mg/kg SNS-032 is administered qdx5 in the HL-60 xenograft model, complete responses are observed in 7 out of 10 animals on day 78; q4d administration of SNS-032 for 3 doses results in complete responses in 6 out of 10 animals on day 78. Animals remain tumor free for at least 78 days post-treatment. Similarly, 5/9 RPMI-8226 engrafted mice have complete responses and do not relapse 78 days following 5 daily doses of 30 mg/kg SNS-032. These results show that in models of hematologic malignancy, the in vivo activity of SNS-032 is associated with modulation of CDK7 and CDK9, modulation of short T1/2 survival signaling proteins such as MCL-1, induction of apoptosis and sustained tumor regression. Taken together, these studies support the therapeutic hypothesis being tested in an ongoing Phase I clinical study of SNS-032 in patients with chronic lymphocytic leukemia and multiple myeloma.