PHARMACOKINETICS AND BIOLOGICAL RESPONSES AFTER TREATMENT OF THE RAT R-1 RHABDOMYOSARCOMA WITH METHOTREXATE

Time relationships of drug concentrations in tissue of a transplantabl e rat rhabdomyosarcoma and of tumour responses up to 120 hr after trea tment with methotrexate (MTX) were analysed and compared. MTX was show n to be retained within the tumour in a substantial concentration for several days, although no evidence of MTX polyglutamation was obtained . The response data confirm that MTX is active in the tumour for up to at least 3 days after injection. Within the first day after MTX treat ment the nucleotide pools are only partly depleted. This indicates tha t the inhibition of DNA synthesis is still incomplete at the time when salvage precursors in increasing amounts are becoming available from decaying cells. From flow cytometric analysis of cell-cycle progressio n it is concluded that subsequent cohorts arriving in early S-phase we re retarded, but not inhibited, in their progression through the S pha se. At 3 days after MTX treatment the mean rate of cell-cycle progress ion as well as the relative clonogenic capacity were maximally reduced to 30% and 1% of control values, respectively. From 3 to 5 days the r ate of cell-cycle progression was gradually restored, whereas from day 5 onwards the clonogenic capacity increased at a high rate correspond ing to the proliferation rate of exponentially growing rhabdomyosarcom a cells in culture. However, a continuous reduction of cell recovery l asting for at least 12 days after treatment contributed to an 8-day de lay in tumour volume growth. (C) 1993 Wiley Liss, Inc.