Abstract A260: BP-4-018, a novel inhibitor of STATs demonstrates anti-myeloma activity in preclinical models: Therapeutic implications of targeting aberrant STAT3 signaling in multiple myeloma.

Abstract

Abstract As a master regulator of the malignant process, the oncogenic transcription factor STAT3 has become the focus of therapeutic targeting in human tumors that harbor constitutive STAT3 activity, including Multiple Myeloma (MM). In the present studies, we employ genetic and chemical tools to further validate STAT3 as a therapeutic target in MM and investigate a novel small molecule STAT3 inhibitor, BP-4-018, with promising in vitro and in vivo anti-MM activity. Using three molecularly heterogeneous human myeloma cell lines (HMCLs), we show that shRNA-mediated knockdown (KD) of STAT3 induces apoptosis, as evidenced by increased Annexin V+ staining and PARP cleavage. Consistent with this biological readout, we observed a decrease in protein expression of STAT3-regulated survival genes including c-Myc, Mcl-1, Bcl-2 and Bcl-xL in STAT3 KD cells. These findings are therapeutically relevant as treatment of HMCLs with BP-4-018 displays broad anti-myeloma activity with IC50 values in the low μM range (1.91-6.48 μM). This rationally designed STAT3 SH2 domain-binding compound induces apoptotic responses in HMCLs and primary MM cells. Mechanistic studies reveal that BP-4-018 dose-dependently inhibits constitutive STAT3 phosphorylation (pSTAT3). Moreover, in selected HMCLs the dose of BP-4-018 required to inhibit pSTAT3 correlated precisely with their respective sensitivity to BP-4-018. HMCLs expressing a STAT3-driven luciferase reporter construct confirmed inhibition of STAT3 transcriptional activity following BP-4-018 treatment, which was associated with a decrease in protein expression of STAT3 target genes. In cell-based assays, BP-4-018 did not demonstrate selectivity for inhibiting pSTAT3 over that of STATs 1 or 5. Importantly however, BP-4-018 did not inhibit phosphorylation of other SH2-dependent proteins including AKT, ERK, SRC and LYN. In vitro combinatorial studies reveal that BP-4-018 is highly synergistic with bortezomib (Btz; CI=0.37). Preliminary data suggests that this interaction may be related to Btz-induced STAT3 activation and induction of Mcl-1 accumulation as concomitant treatment with BP-4-018 inhibited Btz-induced Mcl-1 accumulation. Supporting these findings, cells expressing a dominant negative STAT3 mutant demonstrate enhanced sensitivity to Btz. Finally, we report findings from in vivo efficacy studies using a novel STAT3-driven luciferase xenograft model of MM. We show that BP-4-018 (15 mg/kg/day, PO) reduces STAT3-driven luciferase expression in vivo, significantly delays tumor growth when administered as a single agent, and in combination with Btz, promotes tumor regression without significant toxicities. Taken together, our results highlight the validity of STAT3 as a therapeutic target in MM and support continued study of BP-4-018 and its analogs for the treatment of MM. Citation Information: Mol Cancer Ther 2013;12(11 Suppl):A260. Citation Format: Danielle C. Croucher, Zhi Hua Li, Brent DG Page, Ellen Wei, Patrick T. Gunning, Suzanne Trudel. BP-4-018, a novel inhibitor of STATs demonstrates anti-myeloma activity in preclinical models: Therapeutic implications of targeting aberrant STAT3 signaling in multiple myeloma. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr A260.