Mapatumumab, A TRAIL Receptor 1 Agonist Antibody, Induces Apoptosis in Bortezomib Resistant Multiple Myeloma.
Authors: Ghoshal Pushpankur , Chitta Kasyapa , Vujcic Slavoljub , Gaddy Jacquelyne , Miles Kiersten M. , Stein Leighton , Sher Taimur , Lee Kelvin P. , Chanan-Khan Asher A. 
Published: October 2019

Abstract
Abstract 2832
Poster Board II-808
Bortezomib, a novel proteasomal inhibitor, is an important treatment for patients with multiple myeloma (MM). Despite its clinical success, acquired resistance to bortezomib is an unresolved challenge experienced in the clinic. Limited therapeutic options are available for patients with bortezomib resistant disease and novel therapies are urgently required to target bortezomib resistant MM. In this report, we evaluated the efficacy of Mapatumumab, a fully human agonistic antibody that targets and activates the tumor necrosis factor–related apoptosis-inducing ligand (TRAIL) receptor-1, on MM cell lines. Myeloma cell lines OPM2, KMS11 and U266 wild type (WT) and their respective bortezomib resistant (BR) derivatives were utilized to investigate the efficacy of mapatumumab to overcome bortezomib resistance. Biochemical analyses showed BR cells to have a higher proteasomal activity with cross-resistance to many of the conventional therapeutic agents. Treatment of both WT and BR cells with mapatumumab resulted in a significant (p<0.005) inhibition in their viability by 24h, compared to cells treated with the control antibody. Inhibition of tumor cell viability varied between 20-70%, depending on the cell line. The inhibitory response was higher in WT cells, as compared to the BR cells. Annexin-V staining of mapatumumab treated cells showed the decrease in viability was due to the induction of apoptosis. Immunoblot analyses revealed that mapatumumab treatment resulted in the activation of caspase-8 and caspase-3, suggesting activation of the extrinsic apoptosis pathway in the presence of the antibody. A pan caspase inhibitor, z-vad-fmk (at 25mM), blocked apoptosis induced by mapatumumab. Investigation into the effect of mapatumumab on intracellular signaling showed that treatment with the antibody inhibited the phosphorylation of AKT, and ERK1/2, as well as the expression of Bcl-2 in these cells as early as 4h post-treatment. This suggests mapatumumab treatment affects cellular proliferation and survival pathways. Furthermore, combined treatment of MM cells with mapatumumab and bortezomib resulted in enhanced cell killing compared to mapatumumab or bortezomib alone in both WT and BR cells, thus indicating that there a co-operation in apoptosis signaling induced by these agents. Results presented in this report suggest that TRAIL-R1 mediated apoptosis can be exploited as a therapeutic option in bortezomib resistant MM and warrant clinical evaluation of mapatumumab in bortezomib resistant MM.

Disclosures:
Chanan-Khan: Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Millennium: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Immunogen: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau.