APRIL Is Significantly Elevated at All Stages of Multiple Myeloma (MM) and Interferes with Anti-Bcma Monoclonal Antibody-Mediated Cytolysis, Supporting the Clinical Evaluation of Bion-1301 As a Novel Therapeutic Approach in MM

A proliferation inducing ligand (APRIL) is a natural ligand with higher affinity than BAFF for both B cell maturation antigen (BCMA) and transmembrane activator and CAML interactor (TACI), which are overexpressed on multiple myeloma (MM) cells. APRIL, which is abundantly secreted by myeloma-supporting osteoclasts and macrophages, promotes MM cell progression in vivo and further induces regulatory T cells (Treg) via TACI, but not BCMA, to promote an immunosuppressive MM bone marrow (BM) microenvironment (Blood 2017;130:3066). In preclinical studies, an antagonistic APRIL monoclonal antibody (mAb) significantly inhibited human MM cell growth in SCID-hu mice and abrogated APRIL-induced immunosuppression mediated by Tregs. Here we characterized the ability of BION-1301, a neutralizing APRIL mAb, currently under clinical development in MM (NCT03340883) to overcome protection conferred by APRIL against MM cell lysis induced by the anti-BCMA J6M0 mAb (Blood 2017 130:499). Our studies show that APRIL protects against MM cell lysis induced by J6M0 in the presence of FcR-expressing effector cells (PBMC, monocytes or NK), an effect which is blocked by neutralizing anti-APRIL mAb. APRIL also downregulates J6M0-induced cell membrane CD107a expression on NK cells co-cultured with BCMA-expressing MM cells, which is similarly abrogated by anti-APRIL mAb. Importantly, anti-APRIL mAb also significantly decreases protection conferred by osteoclasts against MM cell lysis induced by J6M0. These data indicate that blocking APRIL with anti-APRIL mAb may enhance BCMA mAb targeted (J6M0)-induced MM cell lysis. Next, using anti-human IgG1 to detect J6M0 binding to MM cell surface BCMA, we found that APRIL in a dose-dependent manner directly competes with J6M0 for binding to BCMA, which was confirmed by ELISA. In addition, APRIL effectively inhibits J6M0 binding to BCMA at 4°C, which argues against APRIL-induced BCMA receptor shedding and/or internalization. In contrast, BAFF affects J6M0 binding to BCMA only at higher concentrations (>1 µg/mL), consistent with > 2-log higher affinity of APRIL vs. BAFF for BCMA. We further assessed APRIL, BAFF, BCMA, and TACI levels in the serum of patients with MM at various stages of disease. Specifically, we used ELISA to measure free APRIL and the ELLA automated immunoassay platform to determine the levels of soluble BAFF, soluble BCMA, and soluble TACI in serum samples from patients with MM (n=193) as well as serum samples from healthy volunteers (HV, n=100). Patient samples included monoclonal gammopathy of undetermined significance (MGUS, n=12), smoldering MM (SMM, n=20), newly diagnosed MM (ND, n=39), post induction pre-autologous stem cell transplant (ASCT, n=55), post-ASCT (n=6), and relapsed refractory MM (RR, n=61). We found that free APRIL levels are significantly increased in serum samples from patients with MM at all stages of disease, when change from baseline levels were compared to those from HVs (post-ASCT: p=0.0003; other groups (MGUS, SMM, ND, pre-ASCT, and RR): p Our studies therefore indicate that therapies directed at the APRIL/BCMA and APRIL/TACI axes may simultaneously target MM cells and counteract APRIL-induced immunosuppression, and that combination strategies targeting APRIL with BCMA directed therapy may augment anti-MM activity. Moreover, elevated free APRIL serum levels in MGUS and all stages of MM suggest a role for APRIL in mediating immunosuppression during the development and in the pathogenesis of MM. Disclosures Dulos: Aduro Biotech: Employment. Guelen: Aduro Biotech Europe: Employment. van Zandvoort: Aduro Biotech Europe: Employment. van de Wiel: Aduro Biotech Europe: Employment. van de Crommert: Aduro Biotech Europe: Employment. Lejeune: Aduro Biotech Europe: Employment. Namini: Aduro Biotech: Employment. Eenennaam: Aduro Biotech Europe: Employment, Equity Ownership. van Elsas: Aduro Biotech Europe: Employment. Richardson: Oncopeptides: Membership on an entity9s Board of Directors or advisory committees; Celgene: Membership on an entity9s Board of Directors or advisory committees, Research Funding; Janssen: Membership on an entity9s Board of Directors or advisory committees; Takeda: Membership on an entity9s Board of Directors or advisory committees, Research Funding; Karyopharm: Membership on an entity9s Board of Directors or advisory committees; BMS: Research Funding; Jazz Pharmaceuticals: Membership on an entity9s Board of Directors or advisory committees, Research Funding; Amgen: Membership on an entity9s Board of Directors or advisory committees. Munshi: OncoPep: Other: Board of director. Anderson: Takeda Millennium: Consultancy; C4 Therapeutics: Equity Ownership; Bristol Myers Squibb: Consultancy; Gilead: Membership on an entity9s Board of Directors or advisory committees; Celgene: Consultancy; Oncopep: Equity Ownership.