Phase I trial of parenteral 6-thioguanine given on 5 consecutive days

For almost 30 years, 6-thioguanine (6-TG) has been administered p.o. for treatment of various human cancers, especially leukemias, even though the systemic availability of the drug given p.o. is known to be low and highly variable. Parenterally administered 6-TG has been studied in detail in humans only on a single-day intermittent schedule, although multiple-day intermittent schedules are known to produce maximal cytotoxic effects in several animal species. To develop a multiple-day regimen for parenteral 6-TG therapy, we carried out a dose-seeking and pharmacokinetic study of the drug given i.v. daily for 5 days in patients with various refractory advanced solid tumors. Dose-limiting myelosuppression without other significant toxicity occurred at 55-65 mg/m2 daily for 5 days. After i.v. administration at 65 mg/m2, the mean peak plasma concentration of 6-TG ranged from 6-10 microM. These concentrations are 8-300 times greater than peak plasma concentrations of 6-TG in plasma reported to occur after p.o. administration at 100 mg/m2. We suggest that the antitumor activity of 6-TG be reassessed against human cancers in regimens of i.v. administration on multiple-day intermittent schedules.