Despite advancements in therapy, multiple myeloma (MM) remains an incurable blood cancer. Our mission is to maximize the efficacy of a primary treatment for myeloma, proteasome inhibitors (PIs) which cause intracellular waste buildup, leading to ER stress and cell death. p62(sequestosome-1) provides an alternate pathway when the proteasome is inhibited, by breaking down cytotoxic material via autophagy. Upregulation of p62 is associated with PI resistance. We identified a small molecule, XRK3F2, that binds to the ZZ domain of p62 and inhibits its autophagic function. We hypothesized that XRK3F2 would improve MM killing when combined with PIs.
We tested XRK3F2 and PI combinations in vitro, in ex vivo co-cultures, and in a human MM xenograft model. We tested XRK3F2 plus bortezomib in vitro and in ex vivo myeloma: bone cocultures and analyzed effects on tumor burden in a prior mouse xenograft experiment. Results: XRK3F2 induced cell death in various human MM cell lines, with a IC50s of 3-6 M. When combined with carfilzomib, the most potent approved PI, at physiologically relevant doses, there was strong synergy (Combinatorial index of 0.4 to 0.6, by Chou-Talalay analysis). The combination of the two agents significantly increased tumor killing in a tumor: bone co-culture model, where the microenvironment of the tumor provides MM survival signals and potential drug resistance. Enhanced tumor killing was further confirmed in a plasmacytoma model of the human MM cell line RPMI-8226 in NSG mice. We also identified soluble BCMA (B-cell maturation antigen, sBCMA) as a sensitive biomarker for tumor burden, which allowed for serial tumor measurements in all tested models.
Conclusion and Potential Impact:
Combining the p62-ZZ domain inhibitor XRK3F2 with PIs shows great promise in improving the killing of MM. Work is ongoing to validate the combination in xenograft models, where tumor cells colonize bones, and in immunocompetent models. Further mechanistic studies using primary MM cells from patients are also ongoing. sBCMA is a cheap, specific, and sensitive tool for serial tumor measurement and should be further validated for preclinical and clinical usage.
Trautman, Ellen; Crean, Colin; Halladay, David; and Suvannasankha, Attaya, "XRK3F2 inhibits p62 signaling and augments myeloma killing by proteasome inhibitors" (2023). Annual Student Research Poster Session. 114, Scholarly and Creative Work from DePauw University.