Cyclophosphamide-pomalidomide combination alters the tumour cell secretome and enhances the anti-myeloma activity of elotuzumab through NK cell-mediated cytotoxicity

Multiple myeloma (MM), a malignancy of plasma cells, remains largely incurable despite recent advances in treatment options. Elotuzumab (ELO) is a humanised IgG1 monoclonal antibody that targets the extracellular domain of signalling lymphocytic activation molecule F7 (SLAMF7). While ELO alone has shown modest single agent activity, combination with immunomodulatory drugs (IMIDs), such as lenalidomide and pomalidomide (POM), has proven beneficial in treating MM. However, further research is needed to find optimal combination strategies to improve efficacy. We assessed the effects of the combination of cyclophosphamide (CTX) with POM and ELO alone or in combination with novel immune checkpoint inhibitors on NK cell-mediated anti-MM effects. Tumour cell secretome (TCS) was collected from cyclophosphamide-treated (^CTX) or pomalidomide-treated MM cells (^POM) or a combination of both (^CTX+POM). Primary human NK cells were conditioned with TCS^CTX, TCS^POM or TCS^CTX+POM and NK phenotype, migration and cytotoxicity were assessed. We show that CTX alone or in combination with POM alters the tumour secretome of MM cells and promotes NK cell recruitment and anti-tumour cytotoxicity. CTX and POM treatment maintained SLAMF7 expression on MM cells and NK cells exposed to TCS^CTX or ^CTX+POM from MM cells potentiated the effects of ELO in enhancing NK-mediated cytotoxicity. NK cells exposed to the TCS^CTX or ^CTX+POM from MM cells potentiated NK cell cytotoxicity of MM cells and induced expression of PD-L1 and CD47 on MM cells. Dual targeting of PD-L1 and CD47 using anti-PD-1 and an anti-CD47 antibody significantly enhanced NK cytotoxicity and secretion of anti-tumour effector molecules, TNF-α and granzyme B. These findings support the addition of CTX to ELO-containing MM regimens and provide a rationale for combining POM with immune checkpoint blockade to maximise NK cytotoxicity against MM.