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Enhancement of Folate Analogue Transport Inward in L1210 Cells during Methotrexate Therapy of Leukemic Mice: Evidence of the Nature of the Effect, Possible Host Mediation, and Pharmacokinetic Significance¹

Memorial Sloan-Kettering Cancer Center, New York, New York 10021

Studies are described that sought the basis for a discrepancy in values for a key kinetic parameter of methotrexate transport (influx V_max) in L1210 cells derived alternately from biochemical or pharmacokinetic measurements. Our results show that, within a short period of time following administration of a therapeutic dose of methotrexate to leukemic mice, influx of this folate analogue measured in L1210 cells removed from these mice was markedly stimulated. Enhancement of [³H]methotrexate influx in these cells was observed within 15 min of drug administration, was maximum (up to 3-fold) within 2 to 3 h, then decreased with time until 24 h when influx was at the control level. Measurements of [³H]methotrexate influx in cells removed from drugtreated mice were made after a period of incubation in drug-free medium to allow for efflux of exchangeable drug. Enhanced influx of [³H]methotrexate was accounted for by an increase in influx V_max (influx K_m was unchanged) and was further enhanced (to a total of 5-fold) by coadministration of leucovorin. Also, enhancement of influx of [³H]methotrexate in L1210 cells did not occur following administration of 1-ß-D-arabinofuranosylcytidine at a therapeutically equivalent dose to leukemic mice or following exposure of these cells to methotrexate or methotrexate with leucovorin during growth in culture. Methotrexate therapy did not affect all transport systems, since the same therapy of leukemic mice had no effect on influx of the purine nucleoside analogue, 9-ß-D-arabinofuranosyl-2-fluoroadenine, in these same L1210 cells. These findings suggest that stimulation of [³H]methotrexate influx in L1210 cells during therapy with this folate analogue was not due to transstimulation during exchange between folate compounds and was not related to the antiproliferative effect of methotrexate on these tumor cells. The coadministration of cycloheximide with methotrexate to leukemic mice at a dose which markedly inhibited ³H-leucine incorporation into L1210 cell protein severely diminished the stimulation of [³H]methotrexate influx. However, in L1210 cells removed from leukemic mice treated with methotrexate, there was no increase compared to control cells in affinity labeling with the N-hydroxysuccinimide ester of [³]methotrexate. This suggested that the effect of cycloheximide was not on increased synthesis of folate transporter and that increased rate of translocation of folate transporter, rather than increased amount of transporter, accounted for the increase in [³H]methotrexate influx.

Although the stimulation of [³H]methotrexate influx observed in L1210 cells removed from mice pretreated with this antifolate did not seem to result from the increased content of folate transporter, the nullification of this effect by cycloheximide does suggest that there is a requirement for new protein synthesis elsewhere. Since the stimulation of [³H]methotrexate influx could not be demonstrated by preexposure of growing L1210 cells in culture to methotrexate with or without leucovorin, a role for the host is indicated in the mediation of this effect. Pharmacokinetic implications of these results are discussed.

¹ Supported in part by Grants CA 08748, CA 18856, and CA 22764 from the National Cancer Institute; CH-26 from the American Cancer Society; and the Else U. Pardee Foundation.

² To whom requests for reprints should be addressed, at Laboratory for Molecular Therapeutics, Memorial Sloan-Kettering Cancer Center, 1275 York Avenue, New York, NY 10021.

³ Present address: A. H. Robbins Corp., 1211 Sherwood Ave., Richmond, VA 23220.

Received 3/26/87. Revised 6/29/87. Accepted 7/23/87.