Sumoylation Inhibition Potentiates Daratumumab Activity Against Multiple Myeloma

Introduction Multiple myeloma (MM), the second most common blood cancer, is characterized by abnormal plasma cell growth in the bone marrow. Daratumumab (Dara) is the first-in-class human monoclonal antibody targeting CD38, a receptor highly expressed on MM cell surface. Despite the remarkably improvement of Dara in the treatment of MM in recent years, MM remains incurable, as patients inevitably relapse upon treatment. There is an urgent need for new therapies to enhance Dara efficacy. We propose inhibition of SUMOylation as a novel therapy with the potential to address this need. SUMOylation regulates protein function by covalently attaching Small Ubiquitin-like MOdifier (SUMO) proteins to target proteins via an enzymatic cascade. SUMOylation has been reported to play an important role in tumor progression and immune response. Our study aims to evaluate the effects of SUMOylation inhibition on Dara activity against MM and dissert the underlying mechanism.

Methods Using MM cell lines and primary samples from MM patients, we evaluated the effects of knockdown of SUMO E1 (SAE2) or using TAK-981, a novel and specific SUMO E1 inhibitor, on CD38 level by flow cytometry, qPCR and western blot. We determined the effects of TAK-981 on Dara cytotoxicity in ADCC assay. We generated humanized NOD-SCID mouse model with MM1S-Luc cells engraftment and weekly transplantation of healthy donor PBMCs to determine the in vivo anti-MM effects of TAK-981 as a single agent and in combination with Dara.

Results We observed increased CD38 expression in TAK-981-treated MM cell lines MM1S, H929, and KMS11 by measuring MFI in flow cytometry. Knockdown of SAE2 caused similar induction of CD38 level. Primary MM cells presented variable CD38 expression and TAK-981 treatment increased CD38 expression in all 3 primary MM. TAK-981 pre-treated MM1S cells showed higher sensitivity to Dara-mediated killing in PBMC co-culture ADCC assay, indicating SUMOylation inhibition enhanced Dara activity via upregulating CD38. We also observed TAK-981 treatment induced T, NK cell and monocytes activation. In line with this observation, TAK-981 pre-treated PBMCs presented significantly higher cytotoxicity with addition of Dara in ADCC assay. We conducted ADCC assay using primary MM cells and PBMCs isolated from relapsing MM patients and observed addition of TAK-981 remarkably potentiated the efficacy of Dara. Inhibition of Type I IFN pathway by IFNAR1 antibody blockage decreased TAK-981 induced CD38 level in MM cells and prevented TAK-981 induced activation of T, NK cell and monocytes, indicating the effects of SUMOylation inhibition on Dara efficacy are Type I IFN pathway dependent. In humanized MM mouse model, we found combination of TAK-981 to Dara significantly suppressed MM tumor growth and prolonged survival, while addition of IFNAR1 antibody reversed the anti-MM efficacy. Flow cytometry analysis of bone marrow cells showed TAK-981 treated mice showed higher CD38 level on MM cells, combination with Dara treatment further increased CD38 level, while addition of IFNAR1 antibody reduced CD38 to same level as vehicle group. The results validated that TAK-981 potentiated anti-MM activity of Dara via Type I IFN pathway in vivo.

Conclusion Our study proves SUMOylation inhibition potentiates Dara efficacy against multiple myeloma by increasing CD38 level in MM cells and activating immune cells including T, NK cells and monocytes in Type I IFN pathway dependent manner. Our work presents TAK-981 as an efficient combination to Dara therapy in MM treatment, with relevance to other mono antibody therapies for blood cancers and solid tumors.

Rosen:January Therapeutics: Current holder of stock options in a privately-held company.