Phase I and pharmacodynamic study of Triapine®, a novel ribonucleotide reductase inhibitor, in patients with advanced leukemia

In a phase I study, 24 patients with refractory leukemia received Triapine®, a novel ribonucleotide reductase (RR) inhibitor, as a continuous intravenous infusion over 96h beginning on days 1 and 15 or days 1 and 8. On the days 1 and 15 regimen, the starting dose was 120mg/m² per day, and the maximum tolerated dose (MTD) was 160mg/m² per day. Three of eight patients receiving 160mg/m² per day in the first course, and one patient escalated to this dose in a second course, developed hepatic dose-limiting toxicity (DLT). For the days 1 and 8 regimen, the first 96h infusion was administered at a fixed dose of 140mg/m² per day. The dose of the second infusion beginning on day 8 was escalated from 120 to 160mg/m² per day without observing DLT. No objective responses occurred. Over 70% of patients had a >50% reduction in white blood cell counts. The steady-state levels of Triapine were between 0.6 and 1μM. As expected from the in vitro studies, at these plasma concentrations there was a decline in dATP and dGTP pools and a decrease in DNA synthetic capacity of the circulating leukemia cells. Based on these clinical, pharmacokinetic, and pharmacodynamic data, Triapine warrants further study in patients with hematologic malignancies.