Overcoming Cancer Drug Resistance with NUC-3373: A Promising Pyrimidine Nucleotide Analogue

The research paper presents an innovative nucleotide analogue, NUC-3373, which is designed to surpass the drug resistance issues linked with 5-fluorouracil (5-FU) and 5-fluorodeoxyuridine (FUDR). These resistances are often due to inadequate levels of activating enzymes and transporters, as well as the overproduction of enzymes that break down the drug. The effectiveness of 5-FU is primarily due to its metabolite, 5-fluorodeoxyuridine monophosphate (FdUMP), which inhibits the enzyme thymidylate synthase. NUC-3373 is a pre-activated form of FdUMP and has shown significant biological activity both in vitro and in vivo.

The study compared the cytotoxicity of 5-FU and NUC-3373 using viability assays on various human cancer cell lines, including colorectal, lung, ovarian, leukemia, and cervical. The assays were conducted under conditions that simulated drug resistance, with mutations in the activating enzyme thymidine kinase (TK) and the nucleoside transporter, hENT1. Additionally, the sensitivity of both drugs to degradation by dihydropyrimidine dehydrogenase (DPD) was evaluated using spectroscopy and mass spectrometry.

The results indicated that NUC-3373 was significantly more cytotoxic than 5-FU, with lower EC50 values across most cancer cell lines. TK inhibition had a lesser impact on NUC-3373's cytotoxicity compared to FUDR, indicating a higher level of independence from TK. Furthermore, the cytotoxic activity of NUC-3373 was only mildly affected by the inhibition of nucleoside transport, contrasting with a substantial reduction in FUDR's cytotoxicity. The DPD degradation assessment revealed that NUC-3373 concentrations were not affected by the presence of the DPD inhibitor, gimeracil, unlike 5-FU, which showed a significant increase in concentration with gimeracil treatment. In terms of anti-cancer activity, NUC-3373 demonstrated superior tumor growth inhibition in colorectal cancer xenografts compared to 5-FU.

A toxicology study in beagle dogs suggested that NUC-3373, when administered at doses higher than 4 mg/kg/day for five days a week over four weeks, had a favorable safety profile compared to a single intravenous dose of 5-FU.

The conclusion of the study highlights NUC-3373's ability to overcome key resistance mechanisms associated with 5-FU, showing efficacy and resistance to DPD-mediated degradation. The results support the initiation of human clinical studies, and a Phase I/II clinical trial has been launched at Oxford University to explore the safety, pharmacokinetics, and clinical activity of NUC-3373 in patients with advanced solid tumors.