Abstract

Studies on the mode of action of PT523 [N alpha-(4-amino-4-deoxypteroyl)-N delta-hemiphthaloyl-L-ornithine], a potent nonpolyglutamatable antifolate, were carried out in sensitive and resistant H35 rat hepatoma cell lines in culture, to compare it with other antifolates, including three dihydrofolate reductase (DHFR) inhibitors, i.e., methotrexate (MTX), gamma-fluoro-MTX, and trimetrexate (TMQ), two thymidylate synthase inhibitors, i.e., N10-propargyl-5,8- dideazafolate (PDDF) and 2-desamino-2-methyl-N10-propargyl-5,8-dideazafolate (dmPDDF), and the glycinamide ribonucleotide formyltransferase inhibitor 5,10-dideaza-5,6,7,8-tetrahydrofolate. PT523 was the most active compound in this group against the parental H35 cells, with an IC50 ranging from 2.5 nM for 72 hr of treatment to 0.21 microM for 2 hr of treatment. Sublines resistant to MTX by virtue of a transport defect or a combination of defective transport and increased DHFR activity were resistant to PT523 and MTX but not to PDDF, whereas sublines resistant to fluoropyrimidines by virtue of increased thymidylate synthase activity were resistant to PDDF but not to PT523, TMQ, or MTX. Inhibition of H35 cell growth by PT523 was associated with a concentration- and time-related decrease in de novo dTMP and purine biosynthesis. Growth inhibition by PT523, MTX, and TMQ was prevented by leucovorin or a combination of thymidine (dThd) and hypoxanthine but not by dThd or hypoxanthine alone; in contrast, growth inhibition by dmPDDF was prevented by dThd alone. Intracellular reduced folate polyglutamate pools were markedly altered by PT523 treatment, with the most pronounced effect being an increase in 7,8-dihydrofolate mono- and polyglutamates and a decrease in 5,10-methylene-5,6,7,8-tetrahydrofolate mono- and polyglutamates, 5,6,7,8-tetrahydrofolate mono- and polyglutamates, and 10-formyl-5,6,7,8-tetrahydrofolate mono- and polyglutamates. This pattern was qualitatively similar to that observed with MTX and TMQ but different from that observed with dmPDDF or 5,10-dideaza-5,6,7,8-tetrahydrofolate, which resulted in little or no change in the folate species. Uptake of [3H]MTX and [3H]folinic acid, but not [3H]folic acid, by H35 cells was inhibited in a dose-related manner by PT523, suggesting that penetration of the cell probably involves, at least in part, active transport by the MTX/reduced folate carrier. To determine whether the potent cellular effects of PT523 might be due to chemical or enzymic clevage to N'-(4-amino-4-deoxypteroyl)-L-ornithine, a potent inhibitor of folylpolyglutamate synthetase, the formation of [3H]MTX polyglutamates in CCRF-CEM lymphoblasts pulsed with [3H]MTX after preincubation with PT523 was examined.(ABSTRACT TRUNCATED AT 400 WORDS)