Phase I study of the antipurine antifolate lometrexol (DDATHF) with folinic acid rescue

Lometrexol (5,10-dideazatetrahydrofolic acid) is a new antifolate that is highly selective in inhibiting the key enzyme of purine synthesis glycinamide ribonucleotide formyltransferase. The most promising preclinical features of lometrexol in animal models were its significant activity against a broad panel of solid tumors, the schedule dependency of its antitumor activity, and the availability of a rescue regimen with folinic or folic acid. In the present study, lometrexol was first given daily for 3 consecutive days, repeated every 4 weeks (part I). The occurrence of delayed myelotoxicity prompted the development of a rescue regimen with lometrexol given in a single dose on day 1, followed by oral folinic acid, 15 mg four times a day, from day 3 to day 5 (part II). Longer time intervals between administration of lometrexol and start of rescue were then evaluated (part III), and in the last part of the study (part IV), the maximum tolerated dose of single intermittent doses of lometrexol with folinic acid given from day 7 to day 9 was established. Sixty adult patients entered the study. In part I, the highest daily dose that could be safely given was 4 mg/m2, for a total dose of 12 mg/m2. Cumulative early stomatitis and delayed thrombocytopenia were dose limiting. The use of oral folinic acid made it possible to escalate the dose up to 60 mg/m2, and the maximum tolerated dose was reached at this dose when folinic was given from day 7 to day 9, with anemia being the dose-limiting toxicity. A shorter time interval between lometrexol and folinic acid administrations (from day 5 to day 7) is recommended for Phase II evaluations to optimize the antitumor effect. Anemia was normochromic and macrocytic, possibly due to a deficiency of folic acid. One partial response of 8 months' duration was reported in a patient with epithelial cancer of the ovary, relapsing after cisplatin and alkylating agents. The use of folic acid as rescue, proposed on the basis of experimental data and pharmacological considerations, has also allowed the repeated administration of lometrexol at doses higher than in the previous studies. The advantages of rescue with folinic acid over supplementation with folic acid, however, are difficult to define.