Dual Inhibition of Canonical and Non-Canonical NF-KB Pathway Demonstrates Significant Anti-Tumor Activities in Multiple Myeloma (MM)

Abstract 2902

NF-kB plays a crucial role in the pathogenesis of multiple myeloma (MM). In MM cells, NF-kB pathway is constitutively activated and regulates transcription of genes whose protein products mediate proliferation, survival and drug resistance. In the context of the bone marrow (BM) microenvironment, NF-kB modulates the expression of cytokines (ie, IL6, TNFalpha) and adhesion molecules (ie, ICAM-1). Importantly, these cytokines and adhesion to BM stromal cells (BMSCs) further activate NF-kB pathway. Previous studies have shown that both canonical and non-canonical pathways contribute to total NF-kB activity in MM cells. Therefore inhibition of both pathways is necessary to target NF-kB. However, current therapeutic strategies can only inhibit the canonical, but not the non-canonical pathway. In this study, we examined the biologic impact of dual inhibition of both canonical and non-canonical pathways in MM cells using a novel small molecule inhibitor PBS-1086 (Profectus BioSciences) which selectively inhibits binding of Rel family member proteins to DNA. Importantly, the binding activity of all Rel family member proteins (RelA, RelB, NF-kB1, NF-kB2, cRel) to DNA was inhibited by PBS-1086, confirming that PBS-1086 blocks both canonical and non-canonical pathways in MM cell lines. We first investigated growth inhibitory effect of PBS-1086 in vitro. PBS-1086 potently inhibited the growth of MM cell lines (MM1S, MM1R, INA6, LR5, Dox40, KMS18, RPMI-8226 and U266) in a dose-dependent fashion with IC50 ranges of 0.15–5 μM. In contrast, PBS-1086 showed modest cytotoxicity on normal peripheral blood mononuclear cells from healthy volunteers. Similar growth inhibitory effect were observed in CD138+ primary tumor cells derived from MM patients. PBS-1086 induced apoptosis in MM1S cell line in a time-dependent manner, evidenced by annexin V-PI staining by flow cytometry and cleaved caspase 8, 9, 3 and PARP, suggesting that PBS-1086 activates both extrinsic and intrinsic apoptotic pathways. Importantly, PBS-1086 can overcome the proliferative and anti-apoptotic effects of BMSCs, associated with inhibition of NF-kB activity. We next examined the combination effect of PBS-1086 with other agents. PBS-1086 with bortezomib synergistically enhanced anti-MM activity even in bortezomib-resistant cell lines (Dox40, ANBL6-VR5) and primary tumor cells from MM patients. Finally, we investigated the effect of PBS-1086 in vivo in a murine xenograft model of human MM cells. Tumor-bearing mice were divided into 6 groups: non-treated, vehicle control, PBS-1086 (7.5 mg/kg ip daily), bortezomib (0.5 mg/kg IV, twice weekly) and the combination of PBS-1086 (either at 2.5 mg/kg or 7.5 mg/kg) with bortezomib. PBS-1086 showed significant anti-MM activity in combination (2.5 and 7.5 mg/kg) groups versus control group (p =0.00039 and p =0.00084, respectively). Combination groups also had significantly (p < 0.05) prolonged survival compared to single agent treatment group (PBS-1086 or bortezomib). In conclusion, our preclinical studies show that PBS-1086 is a promising novel therapeutic agent and our data supports further clinical evaluation of this agent in combination with bortezomib for the treatment of MM.