EQUIVALENCE OF METHOTREXATE CONCENTRATION IN ERYTHROCYTES BETWEEN IV AND ORAL DOSING REGIMENS

Methotrexate, a folate analog, is stored in red blood cells (rbc). Mon itoring rbc methotrexate (MTX) during chemotherapy may be useful to co nfirm patient compliance and/or drug metabolism. It may also be a mean s of comparing the pharmacodynamics of different schedules and dose. W e measured the rbc MTX from patients receiving 1000 mg/M2 as an IV inf usion over 24 hrs or 100 mg/M2 as 25 mg/M2 q6h x 4 doses and contraste d this data to our previous studies of 20 mg/M2/dose. There is no diff erence in the rbc accumulation between the two larger dose schedules ( 270 and 287 pmol/ml rbc), which are 3 x that found for the standard do se. This implies a saturable process for MTX uptake and suggests that HDMTX may not offer significant advantages (increase therapeutic effic acy) over moderate doses not requiring leucovorin rescue or hospitaliz ation.