Combination of Melphalan Flufenamide and Anti-PD-L1 or Anti-CD38 Antibodies Enhances Anti-Myeloma Immunity

Introduction Melphalan flufenamide (Melflufen; Oncopeptides AB) is a novel enzyme-activated analogue of melphalan that enables a more rapid and higher intracellular accumulation of melphalan in tumor cells than is achievable by direct exposure to equimolar doses of melphalan. Our preclinical study showed that melflufen is a more potent anti-myeloma (MM) agent than melphalan, overcomes drug-resistance, and induces synergistic anti-MM activity in combination with bortezomib, lenalidomide, or dexamethasone (Chauhan et al, Clinical Cancer Res 2013;19:3019). However, the effect of melflufen on the immunosuppressive and tumor-promoting MM-host bone marrow (BM) accessory cells such as immunologically dysfunctional plasmacytoid dendritic cells (pDCs; CD123/IL-3Rα) remains unclear. Here, we utilized our coculture models of pDCs, T-, and NK cells with autologous patient MM cells to examine whether a combination of melflufen and immune checkpoint inhibitor anti-PD-L1 Ab, or daratumumab (anti-CD38 Ab), restores anti-MM immunity.

Methods MM patient BM and PB samples (N=10; obtained after informed consent), and cell lines were used for the study. Minimally cytotoxic concentration of melflufen (0.1 µM) was used to assess immune functions. CTL/NK activity assays MM CD8 ⁺ T- or NK-cells were cultured with autologous pDCs (1:10 pDC:T/NK ratio) with melflufen (0.1 μM) alone, and with anti-PD-L1 (5 μg/ml) or anti-CD38 (0.5 μg/ml) Abs for 3-5 days; cells were washed to remove the drugs, and then cultured for another 24h with pre-stained target MM cells (10:1 E/T ratio; T/NK:MM), followed by quantification of viable MM cells by flow.

Results 1) Both MM tumor cells and pDCs showed higher PD-L1 and CD38 levels vs normal plasma cells; 2) Treatment of MM patient total BM mononuclear cells or purified MM cells with melflufen (0.1 µM) increased PD-L1 expression on MM cells (1.84-fold, treated vs untreated; p<0.05). Importantly, treatment of MM cells with melflufen and anti-PD-L1 Abs enhanced anti-MM cytotoxicity; 3) Combination of melflufen and anti-PD-L1 Ab triggers activation of CD3 ⁺T cells, evidenced by an increase in CD69 expression on CD3 ⁺T cells (1.15-fold, treated vs untreated, p<0.05); 4) Combination of melflufen and anti-PD-L1 Ab induced a more robust autologous MM-specific CD8 ⁺ cytotoxic T lymphocyte (CTL) activity than melflufen alone (% MM lysis: melflufen: 20%; melflufen plus anti-PD-L1 Ab: 60%; n=5; p=0.013); 5) Meflufen and anti-PD-L1 also triggered pDC-induced NK cell-mediated MM-specific cytolytic activity (p<0.05); and finally, 6) Low doses of melflufen and anti-CD38 Abs enhanced pDC-induced NK cell-mediated MM-specific cytolytic activity (%Viability: melflufen: 75%; melflufen + anti-CD38 Ab: 12.5%; n=4; p=0.001).

Conclusions The combination of melflufen and anti-PD-L1 increases pDC-induced T- and NK cell-mediated cytolytic activities against MM. Moreover, combined melflufen and anti-CD38 Abs modestly enhance pDC-induced NK cell-mediated MM-specific cytolytic activity. Our preclinical data suggest targeting PD-L1 in combination with melflufen as well as support an ongoing clinical trial of melflufen with anti-CD38 Abs to enhance anti-MM immunity.