Preferential selection during therapy in vivo by edatrexate compared to methotrexate of resistant L1210 cell variants with decreased folylpolyglutamate synthetase activity.

Acquired resistance of the L1210 leukemia in mice developed with less rapidity during therapy with edatrexate (10-ethyl-10-deazaaminopterin, EDX) than with MTX. Since this was explained only partially by the somewhat greater antitumor activity of EDX, this result may also reflect a difference in biochemical phenotypes selected in each case. Among 20 sublines selected for resistance to MTX, a reduction in influx, an elevation of DHFR, and a reduction of DHFR inhibition by MTX were all delineated. Among 14 sublines selected for resistance to EDX, both a reduction in influx and an elevation in level of DHFR were also commonly found. In addition, however, 7 of 14 EDX-resistant sublines exhibited a reduction in the level of folylpolyglutamate synthetase (FPGS) activity. Clonal derivatives of these 7 EDX-resistant cell lines exhibited 2- to 28-fold reductions in FPGS activity and a commensurate reduction in [3H]-MTX polyglutamate formation in situ following exposure to [3H]-MTX during growth in mice. An analysis of the kinetics and relative substrate preferences for FPGS from variant and parental L1210 cells revealed that the various changes in FPGS activity were at the level of the Vmax rather than Km. These results derived from an in vivo tumor model provide further evidence for a role of FPGS as a determinant of cytotoxicity and acquired resistance to classical folate analogs. They also provide evidence in the same pharmacologic model for a manifestation of resistance to 4-aminofolates in vivo that involves all of the alterations of its primary target, transport, and metabolism that have ever been associated with acquired resistance in cell culture systems.