Panobinostat (LBH589) in Combination with the β-Catenin Inhibitor Tegavivint (BC2059) Exerts Significant Anti-Myeloma Activity Both in Vitro and In Vivo

Introduction: Panobinostat (LBH589) is approved for the treatment of relapsed multiple myeloma (MM). The Wnt canonical pathway and its key player, β-catenin are dysregulated in advanced MM supporting the evaluation of β-catenin inhibition as a potential therapy. We evaluated the anti-MM effect of Tegavivint in combination with Panobinostat in vitro and in vivo.

Results and Methods: In vitro combination of low doses (BC2059≤100nM; LBH589≤10nM) for 48h was synergistic against OCI-My1 and U266 MM cells, with combination indices from 0.569 to 0.883 (CI<1: synergism). The combination demonstrated synergistic killing of primary MM tumour cells in an autologous co-culture assay with synergism quotients from 1.2 to 2 (SQ>1: synergism). The combination rapidly (<24h) decreased the expression of downstream β-catenin targets myc, cyclin D1 and cyclin D2 as shown by immunoblotting. Furthermore, by 20h, the combination decreased both oxidative phosphorylation and aerobic glycolysis as measured by oxygen consumption rate (OCR) and extracellular acidification rate (ECAR), respectively (Seahorse XFe96 analyser). Basal, maximal and ATP coupled OCR were significantly reduced by the combination when compared to vehicle (for OCI-My1 from 203.93 to 56.20, 105.81 to 33.29 and 156.29 to 34.67 pmol/min/50K cells OCR respectively). Similarly, basal glycolysis and glycolytic capacity were both reduced by the combination (from 48.88 to 9.77 and 61.68 to 8.58 mpH/min/50K cells ECAR, respectively).

In vivo, the combination was superior to either single drug treatment in a murine xenograft of U266-luciferase cells. Disease burden was reduced in the combination arm compared to single drug and vehicle arms as early as day 14 after inoculation (p=0.02). This difference increased further until the last bioluminescence imaging (day 49, p<0.001) and translated into significantly prolonged OS for the combination (p=0.006). Potential on-target toxicities with BC2059 are a concern as the Wnt canonical pathway is essential for stem cell maintenance in several organs and has a role in bone homeostasis, but the combination did not result in cytopenias nor body weight loss. After euthanasia, µCT demonstrated that neither BC2059 nor the combination negatively affected bone morphometric indices (bone volume fraction, trabecular thickness, connectivity density). Likewise, osteoblastic activity as measured by serum osteocalcin was unaffected, whereas osteoclastic activity (as indicated by serum CTX1) was reduced when compared to healthy mice (p=0.008 and p=0.013, respectively).

In conclusion Tegavivint in combination with Panobinostat may be a useful therapeutic modality for MM patients with advanced/refractory disease and warrants further evaluation.