Inhibition of DNA synthesis is a keystone of cancer therapy. The purine and pyrimidine precursors of DNA are obtained either by the de novo pathway (DNP) or the nucleotide salvage pathway (NSP). The DNP generates nucleotides that are reduced to the corresponding deoxynucleotide by ribonucleotide reductase (RNR). Thymidine, a potent inhibitor of de novo synthesis, acts by specific inhibition of RNR. The NSP salvages circulating deoxynucleosides released from dead cells or from the liver and converts them to deoxynucleotides, a process that is catalyzed by deoxycytidine kinase (dCK) and thymidine kinase 1 (TK1).
In this issue, Nathanson et al. provide an elegant dissection of the roles of the DNP and NSP in leukemic cells and propose that the combined inhibition of both pathways offers a new therapeutic approach. Using multiple tumor models, including human-to-mouse xenografts and mouse models, they showed that the NSP is required to generate deoxycytidine triphosphate (dCTP) to compensate when de novo dCTP synthesis is blocked using thymidine, which may explain why using thymidine alone to target the DNP has shown limited efficacy in clinical trials. The availability of a fluorinated analogue of deoxycytidine allowed the authors, in prior work, to track the effect of DI-39—a potent small molecule inhibitor of dCK activity—in vivo using positron emission tomography. Crystallographic studies of DI-39 bound to dCK showed that it inhibits dCK activity by binding to the nucleoside-binding site. Using DI-39 in combination with thymidine (to inhibit de novo synthesis) profoundly decreased dCTP levels, leading to regression of T and B cell acute lymphoblastic leukemia in mice, with no significant toxicity to normal hematopoiesis. Although thymidine was the chosen inhibitor for de novo synthesis in this study, there may be better compounds to inhibit RNR, specifically hydroxyurea.
Could this strategy work in humans? This study offers important insights into how cells generate dCTP and convincingly demonstrates the clinical potential of a dual inhibition strategy for cancer therapy that targets both de novo dCTP synthesis and the salvage pathway. Further work will establish the basic pharmacology (kinetics, bioavailability, and toxicology) of dCK inhibitors and identify which human tumors are most sensitive to this strategy of combined inhibition.