Hosu-53, a Novel Potent Dihydroorotate Dehydrogenase Inhibitor Accentuates CD47 Blockade Therapy Resulting in Long Term Survival in Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is a complex disease characterized by leukemia stem cells in a state of differentiation arrest leading to suboptimal innate immune function and immunosuppression. The effectiveness of allogeneic hematopoietic stem cell transplantation, NK cell directed therapies, and recently CD47 blockade highlights the importance of immune surveillance as potentially curative therapies for AML. Dihydroorotate dehydrogenase inhibitors (DHODHi) have been reported to have both cytotoxic and differentiation properties which propelled their clinical development for AML. We developed HOSU-53, a novel DHODHi which is described in detail in another abstract at this meeting. HOSU-53 has (i) nanomolar potency against human DHODH biochemical activity, (ii) in vitro anti-proliferative activity toward AML with a potent tumor DNA damage response, (iii) favorable oral bioavailability, (iv) a monitorable on-target pharmacodynamic biomarker predictive of efficacy and toxicity, and (v) superior in vivo survival outcome in disseminated AML cell line derived xenograft models.

Prior anti-viral studies using DHODHi have shown that deprivation of pyrimidine synthesis and associated DNA damage response can enhance innate immune activation. Therefore, we hypothesized that HOSU-53 could enhance phagocytosis and antibody dependent cellular phagocytosis (ADCP) and thereby synergize with myeloid immune checkpoint blockade therapies. Herein, we report compelling preclinical data demonstrating curative potential in AML xenograft models.

Treating AML cells with HOSU-53 resulted in restored ability to phagocytose E.coli suggesting enhanced innate immunity. Given our previous reports that DHODHi could induce cell surface markers such as CD38, we next validated that HOSU-53 can increase expression of calreticulin (CALR) and CD47. In vitro treatment with HOSU-53 demonstrated increased tumor surface expression of the pro-phagocytic signal, CALR and a dose dependent increase in CD47 expression. We performed ADCP assays using murine bone marrow derived macrophages and observed enhanced phagocytosis when AML cells were pretreated with HOSU-53, prompting us to conduct in vivo validation using the MOLM-13 disseminated model. Mice were enrolled four days post-engraftment to receive single agent anti-CD47 monoclonal antibody at 0.5mg daily for 21 days and either daily 4 or 10 mg/kg HOSU-53, based on our prior dose optimization studies. We also investigated the combination of CD47 antibody with each dose level of HOSU-53. All monotherapy cohorts resulted in significant prolonged survival, but still eventually succumb to AML disease. In contrast, the combination cohorts resulted in disease-free survival as determined by the absence of measurable human CD45+ cells in spleen or bone marrow cells harvested from mice at the end of treatment on day 80, or on day 106 after remaining on study for 25 days without continued treatment.

The curative potential in this model prompted us to repeat our study in a model with higher tumor burden. In this follow-up study, mice were enrolled ten days post engraftment to receive daily HOSU-53 (4 or 10 mg/kg) or B6H12 anti-CD47 (0.5mg daily or every three days for 21 days) or combinations of these treatments. We observed that anti-CD47 monotherapy was ineffective with this more established tumor model, but HOSU-53 monotherapy was comparable to our previous study and both 10 mg/kg combination cohorts significantly prolonged survival (Fig. 1, ongoing study). Further mechanistic studies of HOSU-53-mediated synergy of CD47 therapy are ongoing.

Herein, using our DHODHi HOSU-53, we describe that pyrimidine depletion can enhance innate immunity to potentiate CD47 blockade therapy, representing a novel treatment approach. We show HOSU-53 increased tumor innate immune function, CALR expression and subsequently increased in vitro phagocytosis with CD47 blockade. Furthermore, in vivo combination of HOSU-53 with anti-CD47 antibody promoted dramatic synergy exhibiting a potentially curative approach in an otherwise very aggressive tumor model. HOSU-53 is currently in the IND-enabling studies phase with plans for filing an IND application in 2023 for phase 1 clinical trials in AML and MM. This data and our previously reported combination of HOSU-53 with anti-CD38 antibodies warrants the clinical translation of DHODHi in combination with immunotherapy.

Byrd:Zencor: Research Funding; Janssen: Consultancy; Vincerx: Consultancy, Current equity holder in private company, Current equity holder in publicly-traded company, Current holder of stock options in a privately-held company; Novartis: Consultancy; Ohio State University: Patents & Royalties; Syndax: Consultancy; AbbVie: Consultancy; Kura: Consultancy; Trillium: Consultancy; AstraZeneca: Consultancy; Newave: Consultancy; Pharmacyclics: Research Funding.