The Potent Dihydroorotate Dehydrogenase Inhibitor, Hosu-53, Exhibits Compelling Monotherapy Efficacy in Multiple Myeloma and Augments CD47 Targeted Therapy

Worldwide, multiple myeloma (MM) is the second most common hematological malignancy characterized by the expansion of aberrant mature plasma cells in the bone marrow. Despite the FDA approval of several drugs with different mechanisms of action such as immunomodulatory drugs (IMiDs) and proteosome inhibitors (PIs), MM remains a challenging incurable disease where most patients ultimately relapse and become refractory to available . The resistant nature of MM underscores the need for novel therapeutic strategies that can directly affect MM cells while enhancing the outcome with MM current standard of care (SOC). Among the most promising MM SOC agents is monoclonal antibodies (mAb) such as the CD38 antibody daratumumab, which significantly improved the management of newly diagnosed and relapsed/refractory MM. Therefore, we aimed to develop novel therapies that can enhance mAb therapy in MM.

Dihydroorotate dehydrogenase (DHODH) is an enzyme which mediates the fourth and rate-limiting step in the de novo pyrimidine synthesis pathway converting dihydroorotate to orotate, the precursor of uridine. Pyrimidine starvation using DHODH inhibitors has been shown to have potent antiproliferative activity in several malignancies including acute myeloid leukemia (AML) and MM. Thus, we developed a novel potent DHODH inhibitor, HOSU-53 for the treatment of hematological malignancies. We previously discovered the ability of HOSU-53 to modulate surface CD38 expression and found that HOSU-53 had potent monotherapy activity in MM and provided impressive synergy in combination with daratumumab using the NCI-H929 MM cell line derived xenograft (CDX) model (2022 AACR annual meeting abstracts). Herein, we further expanded our studies to pre-clinically develop HOSU-53 as a new potential MM therapy and explore additional synergistic immunotherapy combinations that could maintain efficacy in the case of anti-CD38 therapy resistance.

We evaluated the in vitro antiproliferative efficacy of HOSU-53 against a panel of MM cell lines and found nanomolar potency validating that HOSU-53 would have monotherapy efficacy. Indeed, we conducted in vivo studies using two additional MM CDX subcutaneous (s.c) models, OPM-2 and RPMI-8226, and found significant tumor delay and survival advantage. In the OPM-2 model, HOSU-53 has a median survival of 54-days compared to vehicle at 28-days, while the median survival for HOSU-53 was 60-days compared to vehicle at 26-days in the RPMI-8226 model. We further verified HOSU-53 efficacy using the disseminated MM1.S luciferase CDX model and found a significant prolonged survival in the HOSU-53 cohort (median survival53-days) compared to vehicle (median survival28-days) that was further enhanced with isatuximab combination resulting in superior survival benefit (median survival 69-days).

Given the continuous challenge in maintaining a durable response with anti-CD38 therapies due to resistance, we sought to explore additional combination regimens to maximize HOSU-53 efficacy. Currently there is significant clinical interest in CD47 antibody therapy such as magrolimab for both solid tumors and hematological malignancies. Our previous work suggested a strong synergy between CD47 antibodies and HOSU-53 in AML with curative potential (2022 ASH annual meeting abstracts), and hypothesized that we would observe similar synergy with HOSU-53 in combination with anti-CD47 in MM. Indeed, we found complete tumor regression in all mice treated with HOSU-53 + B6.H12 anti-CD47 therapy in the NCI-H929 s.c CDX model and significant prolonged survival and reduced bioluminescence in the NCI-H929 luciferase disseminated CDX model. Together, these two studies validate a potential clinical benefit to explore HOSU-53 + anti-CD47 regimen in MM patients. Furthermore, we observed that calreticulin (pro-phagocytosis signal) was modulated post HOSU-53 in vitro treatment suggesting its role in the observed synergy between HOSU-53 and CD47 blockade therapy.

In summary, we show compelling survival benefit for HOSU-53 as a monotherapy which is further enhanced when combined with anti-CD38 or anti-CD47 therapies. HOSU-53 is expected to enter phase 1 clinical trials in 2024 and our data is supportive for its expansion into MM.