Abstract Synthetic nucleobase and nucleoside analogs (NNA) represent an important class of therapeutic agents with a variety range of indications, with 41 NNA drugs approved by the US Food and Drug Administration (FDA) for anti-cancer and/or anti-viral therapy. However, most NNA drugs are associated severe adverse effects that can be life-threatening, and there is a wide inter-patient variability in their pharmacologic effects. Therefore, there is a pressing need to understand the biological basis of the variance in NNA drug toxicity and response, to develop ways to more safely and more effectively use this important class of drugs. To address these challenges, our NIGMS-funded work in the past 5 years focused on pharmacogenetics of thiopurines, an NNA drug widely used to treat leukemia and autoimmune diseases. We discovered genetic variants in a novel drug metabolism gene NUDT15 associated with severe myelosuppression during thiopurine treatment in children with leukemia, elucidated the pharmacological mechanism by which NUDT15 modulates thiopurine cytotoxicity, systematically identified pharmacogenetic variants in NUDT15, and also led the development of clinical guides for NUDT15-guided thiopurine dosing. More recently, we have identified two anti- viral drugs, namely acyclovir and ganciclovir, as novel substrates of NUDT15, indicating that NUDT15 can promiscuously metabolize a variety of NNA drugs. In fact, NUDT15 belongs to a large class of human NUDIX enzymes that dephosphorylate native or chemically modified nucleotides. Based on these findings, I hypothesize that NUDIX enzymes (including NUDT15) are potentially important metabolizing enzymes for NNA drugs, primarily by regulating dephosphorylation of nucleotide drug metabolite. To this end, I plan to rationally expand my research program to study pharmacogenetics of NNA drugs broadly, with the overarching objectives to identify novel pharmacogenetic markers for NNA drug response and then use this information to develop approaches for genetics-guided treatment individualization. In the next funding cycle, I will pursue research in two distinctive but related directions: 1) to identify NUDT15 variants associated with ganciclovir drug inactivation and develop NUDT15 genotype-guided approaches to individualize anti-viral therapy, and 2) to broadly examine human NUDIX enzymes for their role in the activity of NNA drugs, focusing on 14 FDA-approved anti-cancer agents. Given the large number of NNA drugs used clinically and the pressing need to use NNA drugs more safely and effectively, my research program addresses a significant scientific challenge and is substantive in scope and appropriate for long-term pursuit. I am confident that sustained progress in the next funding cycle is likely with continuation of the NIGMS support.

Project narrative Synthetic analogs of nucleobase and nucleoside (NNA) are one of the most widely used classes of therapeutic agents with broad activities as anti-cancer and/or anti-viral agents. NNA drugs are frequently associated with dose-limiting toxicities and their pharmacologic effects vary significantly across patients, and there is a pressing need to understand the biological basis of inter-patient variability in NNA drug response and to develop genetics- based treatment individualization for safe and more effective therapy. Building upon our NIGMS-funded work in the past 5 years focusing on pharmacogenetics of thiopurines, we now propose to expand this program to characterize pharmacogenetic factors related to efficacy and side effects of NNA drugs broadly, e.g., to identify genetic variants linked to antiviral effects of ganciclovir and to characterize the role of human NUDIX enzymes in NNA drug metabolism.