Inhibitory action of 10-deazaaminopterins and their polyglutamates on human thymidylate synthase

The action of 10-deazaaminopterin, its 10-alkyl derivatives, and their polyglutamates against thymidylate synthase (TMPS) from human acute myeloblastic leukemia was examined. Comparison of aminopterin with methotrexate showed that the methylation of the N10-position (methotrexate) increased the inhibitory effect of aminopterin on TMPS. In contrast, alkylation of the 10-position of 10-deazaaminopterin decreased inhibition of TMPS, and the 50% inhibitory concentration values were progressively higher, in the order 10,10-dimethyl-, 10-methyl-, and 10-ethyl-derivatives. The addition of gamma-glutamyl moieties to both 10-deazaaminopterin, and one of its alkylated analogs, 10-ethyl-10-deazaaminopterin, enhanced inhibition. The maximum inhibition was achieved with the addition of three glutamyl moieties to 10-deazaaminopterin and two glutamyl moieties to 10-ethyl-10-deazaaminopterin, respectively. Thus, 10-deazaaminopterin-tetraglutamate was 138-fold and 10-ethyl-10-deazaaminopterin-triglutamate was greater than 51-fold more active than their respective parental compound. The compounds 10-deazaaminopterin and its polyglutamates, 10-methyl- and 10,10-dimethyl-analogs, inhibited TMPS in a noncompetitive fashion with respect to 5,10-methylene-tetrahydropteroylglutamate. Ki values for the monoglutamates were 220 microM, 310 microM, and 225 microM, respectively. In contrast, 10-ethyl-10-deazaaminopterin and its polyglutamates inhibited TMPS in a competitive fashion with a Ki value of 410 microM for the monoglutamate. With 5,10-methylene-tetrahydropteroylpentaglutamate as a substrate, 10-deazaaminopterin and its polyglutamates behaved as mixed type inhibitors, and 10-ethyl-10-deazaaminopterin, monoglutamate and diglutamate, behaved as noncompetitive inhibitors, whereas its pentaglutamate behaved as a mixed-type inhibitor. These results suggest that the addition of gamma-glutamyl moieties to the substrate also caused the change in the mode of inhibitory action of these compounds. These findings also show that both replacement of the N10-position of the 4-aminopteroyl structure with a methylene group and its alkylation caused interesting and unexpected changes in the structure-activity relationships and the mode of action for these 4-aminopteroyl antifolates as inhibitors of TMPS, which may be therapeutically relevant.