Doctor of Philosophy (PhD)
Michael A. Hollingsworth, PhD
Pankaj K. Singh, PhD
Amarnath Natarajan, PhD
Kurt Fisher, MD/PhD
Cancer cells require supraphysiologic levels of nucleotide triphosphates to fuel their malignant behaviors, including uncontrolled proliferation, immune evasion, metastasis, and therapy resistance. This requirement is met by hyperactive flux through the de novo pyrimidine and de novo purine pathways. Dihydroorotate dehydrogenase (DHODH) is an essential enzyme in the de novo pyrimidine pathway that can be targeted by FDA approved and experimental drug compounds. A robust body of preclinical evidence, encompassing hundreds of independent studies over several decades, demonstrates impressive activity of DHODH inhibitors in animal models of cancer. Paradoxically, however, all clinical trials to date have failed to demonstrate efficacy of DHODH inhibitors against a wide variety of human malignancies.
We set out to uncover clinically actionable mechanisms of DHODH inhibitor resistance and/or therapeutic vulnerabilities that are imposed by pyrimidine nucleotide starvation in cancer cells. Our preliminary observations gave rise to two major lines of investigation. First, we discovered that the preclinical drug compound CNX-774 overcomes DHODH inhibitor resistance by inhibiting equilibrative nucleoside transporter 1 (ENT1). Second, we found that DHODH inhibition causes increased cancer cell antigen presentation via major histocompatibility complex class 1 (MHC-I), which has been shown to render cancer cells more vulnerable to T-cell-mediated immune destruction.
We validated these findings using immunocompetent mouse models of cancer. In the KPC orthotopic implantation model of human pancreatic adenocarcinoma, cancer cell specific ENT1 knockout profoundly sensitized tumors to DHODH inhibitor treatment, resulting is severely diminished tumor growth and dramatically prolonged survival of tumor bearing mice. In the B16F10 model of poorly immunogenic human melanoma, we showed that DHODH inhibitor treatment enhanced the efficacy of immune checkpoint blockade therapy and prolonged mouse survival compared to either treatment alone.
Thus, we provide compelling proof-of-concept evidence for two different combination therapy strategies to treat cancer using DHODH inhibition. Our work paves the way for future studies that will unlock the potential of DHODH inhibitors as a backbone of combination cancer therapy.
Mullen, Nicholas, "Development of Combination Therapy Strategies to Treat Cancer Using Dihydroorotate Dehydrogenase Inhibitors" (2023). Theses & Dissertations. 757.
Available for download on Sunday, July 20, 2025