Pharmacokinetic interactions of cyclophosphamide and 5-fluorouracil with methotrexate in an animal model.

In tumor-bearing WAG/Rij rats the interaction of cyclophosphamide and/or 5-fluorouracil (5-FU) with methotrexate as manifested at the pharmacokinetic level was studied. Four groups were formed of at least ten animals. The control group, which received single-agent methotrexate, was compared with groups that received methotrexate plus cyclophosphamide, methotrexate plus 5-FU, and methotrexate plus cyclophosphamide plus 5-FU. There appeared to be an increase of 40% in the clearance of methotrexate by the triple combination. Cyclophosphamide especially diminished the terminal part of the concentration-time curve of methotrexate.