Targeting the Wnt/Beta-Catenin Pathway in Multiple Myeloma with a Novel Orally Bioavailable Beta-Catenin Inhibitor
Authors: Savvidou Ioanna , Khong Tiffany T. , Horrigan Stephen K. , Spencer Andrew 
Published: October 2019

Abstract
Abstract 1842
The introduction of novel chemotherapeutics and stem cell transplantation has resulted in an improved outlook for patients with multiple myeloma (MM) but the disease remains incurable emphasising the need for alternative therapeutic approaches. Beta-catenin, the central molecule of the Wnt canonical pathway is dysregulated in multiple solid tumours and haematological malignancies, including MM, providing a rationale to evaluate the potential of beta-catenin inhibitors as a therapeutic paradigm for MM.
We have demonstrated that the orally bioavailable beta-catenin inhibitor BC2059 (BetaCat Pharmaceuticals, Gaithersburg, USA) induced apoptosis in a range of genetically heterogenous human myeloma cell lines (HMCL) (n=10). Dose responsiveness to BC2059 was initially determined by MTS assays with concentrations ranging from 50nM to 10000nM showing a dose and time dependent BC2059-induced inhibition of HMCL viability. Further analyses demonstrated the accumulation of cells in the G0/G1 phase suggesting G1 cell cycle arrest with IC50 concentrations for the 10 HMCL ranging from 53nM (KMS-18) to 247nM (NCI H929). The expression of the active nuclear non-phosphorylated form of beta-catenin was confirmed by immunoblotting and intranuclear flow cytometry in all 10 HMCL but the levels did not correlate with BC2059 IC50. Mimicking the bone marrow microenvironment utilising HS5, an immortalised human stromal cell line, by co-culturing with HMCL interestingly showed the capacity of BC2059 to overcome the protective effect of the stromal layer. At the IC80 of BC2059 for 5 HMCL, the level of apoptosis in the absence or presence of HS5 was comparable. For example, forANBL-6 (IC50 =115nM) BC2059-induced cell death at 100nM was 65% when cultured alone and 35% in the context of co-culture, whereas at 220nM cell death was 83% and 70% respectively. Beta-catenin undergoes proteasome mediated destruction and has been demonstrated to be elevated subsequent to exposure to bortezomib, we therefore evaluated whether BC2059 could mitigate this pro-survival stress response. 5 HMCL were tested and the combination of BC2059 and bortezomib was found to be synergistic in all instances with synergy quotients [SQ] ranging from 1.5 to 2.25 (where a SQ>1 confirms synergy). Furthermore, optimal killing was induced with either concomitant exposure to both drugs or by drug sequencing with bortezomib exposure 4 hours prior to BC2059 but not the reverse sequence. Finally, as a single agent BC2059 effectively killed primary MM tumour cells from relapsed and/or refractory MM patients (n=11) in an autologous bone marrow (BM) co-culture assay with a median cell death of 46±7,5% and 59±5.6% at 1μM and 5μM, respectively. In conclusion BC2059 effectively kills both HMCL and primary MM tumour cells at therapeutically feasible concentrations and can overcome the pro-survival effect of stromal cell and BM co-culture. BC2059 warrants further evaluation as a potential anti-MM therapeutic.

Disclosures:
Horrigan: BetaCat Pharmaceuticals: Employment.