Studies on the antitumor effects of analogues of 5,8-dideazaisofolic acid and 5,8-dideazaisoaminopterin.

Six new analogues of 5,8-dideazaisofolic acid and 5,8-dideazaisoaminopterin were synthesized in an effort to obtain enhanced antitumor activity. The modifications included the replacement of the 2-amino group by hydrogen or methyl as well as the inclusion of a methyl substituent at position 9. Based upon activity against L1210 leukemia cells in culture, three of the new analogues together with one compound described previously were evaluated for cytotoxicity in vitro using three human tumor cell lines (Colo 320 DM, Hep G2 and HL-60). The most effective compound was 2-desamino-N9-methyl-5,8-dideazaisoaminopterin (2c) with the HL-60 cells being the most sensitive to its cytotoxic effects. These analogues were evaluated in vitro as inhibitors of dihydrofolate reductase (DHFR) and thymidylate synthase (TS) from human as well as bacterial (Lactobacillus casei) sources. All four of the 4-amino analogues were most effective toward L. casei DHFR compared with human DHFR, with 2-desamino-2-methyl-5,8-dideazaisoaminopterin (2d) and its 9-methyl derivative (2e) having 818- and 430-fold greater selectivity (L. casei/human). Most of the compounds studied were found to be only modest inhibitors of human TS (I50 values = 1.5 to 20 microM) and were therefore at least 40-fold less inhibitory than 10-propargyl-5,8-dideazafolic acid. Nevertheless, reversal of cytotoxicity studies with thymidine, hypoxanthine and folinic acid using the HL-60 cell line suggested that TS is the primary target for these analogues.