BIOCHEMICAL AND PHARMACOKINETIC EFFECTS OF LEUCOVORIN AFTER HIGH-DOSE METHOTREXATE IN A MURINE LEUKEMIA MODEL

  • 1 January 1976
    • journal article
    • research article
    • Vol. 36  (12) , 4679-4686
Abstract
The administration of calcium leucovorin, concurrently or after high doses of methotrexate in L1210 leukemic mice, has pharmacokinetic and biochemical effects in tumor cells and drug limiting proliferative normal tissue in small intestine. A reduction in the maximal level of accumulation and retention of exchangeable drug (unbound to dihydrofolate reductase) in tissue could be demonstrated when calcium leucovorin was given simultaneously with methotrexate at equal or greater doses than the latter. The dose dependence for calcium leucovorin induced drug loss is similar in both tissues and showed the expected variation when the time interval between methotrexate and calcium leucovorin doses was increased. With 400 mg methotrexate/kg, > 96 mg calcium leucovorin/kg were required maximally to affect overall drug retention in tissue 2 h after drug, whereas only 24 mg calcium leucovorin/kg were required 16 h after drug. Calcium leucovorin, given after methotrexate, induced synchronous recovery of DNA synthesis (measured by labeled deoxyuridine incorporation) in small intestine and L1210 cells. An initial cycle of synthesis was induced in the presence of exchangeable levels of drug. Two hours after methotrexate, 12 mg/kg, calcium leucovorin induced an immediate but only partial (20-25% of control rate) recovery of synthesis with dose dependence from 3-12 mg calcium leucovorin/kg. Less synthesis was induced after 96 mg/kg and almost none after methotrexate, 400 mg/kg. With calcium leucovorin, 24 mg/kg, given 2 h after methotrexate, 12 or 96 mg/kg, a major cycle of synthesis occurred when total drug levels approached the equivalence of the dihydrofolate reductase content. The magnitude of this cycle of synthesis in L1210 cells and small intestine was the same as that seen in mice recovering from methotrexate alone. This is based on the assumption that an approximately equivalent relationship exists between DNA synthesis and labeled deoxyuridine incorporation in each tissue during the period of maximal incorporation within the cycle. The major effect of calcium leucovorin was to induce an earlier resumption of DNA synthesis as a consequence of the pharmacokinetic effect in each tissue. With calcium leucovorin, 24 or 400 mg/kg, given 16 h after methotrexate, an identical effect on drug retention was observed in L1210 cells and small intestine. Although there was a difference in the time course for recovery in small intestine at each dosage of calcium leucovorin, the recovery of DNA synthesis as drug levels approached the dihydrofolate reductase content was similar in magnitude. In L1210 cells, substantial recovery of synthesis to a comparable level and with a similar time course occurred only after leucovorin, 400 mg/kg. Little or no recovery of DNA synthesis was observed after calcium leucovorin, 24 mg/kg, during the same time period. This dosage schedule (methotrexate, 400 mg/kg, s.c. followed 16 h later by calcium leucovorin, 24 mg/kg s.c.) administered once gave a 2 log leukemia cell kill and prevented lethal toxicity in most of animals.