Bioavailability of cyclophosphamide in the CMF regimen.

Cyclophosphamide (CY) was administered to 22 breast cancer patients treated routinely according to the CMF regimen: 75 mg/m3/d x 14 d p.o. CY, 30 mg/m2 days 1 and 8 i.v. methotrexate (MTX) and 500 mg/m2 days 1 and 8 i.v. 5-Fluorouracil (5-FU). The sequence of drug administration was always the same: 1) CY, 2) MTX; and 3) 5-FU. Capillary gas chromatography was performed for determination of CY in blood. Bioavailability (F) could be determined in 14 patients since CY was also administered intravenously in the same dose. The data of systemic exposure of oral CY in the other 8 patients were matched to those of the first 14 in whom bioavailability could be determined. Mean F was 0.85 +/- 0.22 (85% +/- 22%); in 1 patient F was 0.43 (43%). Furthermore, 3 patients treated with only p.o. CY had low estimated F values: 0.45, 0.49 and 0.50. In comparing patient characteristics with pharmacokinetic data, it was concluded that age might have a predictive value for elimination half-life t 1/2 z of i.v. CY. The youngest patients showed shortest t1/2 z and were also amongst those with the lowest F. This indication requires an extension of the study as well as monitoring of CY metabolism as a function of age. For the premenopausal patients this might be of particular importance, since this group is known to be prone to benefit from chemotherapeutic treatment according to the CMF regimen.