BIOCHEMISTRY AND PHARMACOLOGY OF GLYCINAMIDE RIBONUCLEOTIDE FORMYLTRANSFERASE INHIBITORS - LY309887 AND LOMETREXOL

Lometrexol, a tight-binding antifolate inhibitor of the purine de novo enzyme glycinamide ribonucleotide formyl-transferase (GARFT), was the first GARFT inhibitor to be investigated clinically. Unexpected obser vations of delayed cumulative toxicity prompted a search for a second generation antimetabolite with a more favorable biochemical, pharmacol ogical and toxicological profile. LY309887, 6R-2',5'-thienyl-5,10-dide azatetrahydrofolic acid, had 9-fold greater potency to inhibit GARFT ( Ki = 6.5 nM) compared to lometrexol. Like lometrexol, LY309887 was act ivated by folypolyglutamate synthetase, however, it had a lower first order rate constant. In vitro and in vivo data were consistent with th ese observations: polyglutamation of LY309887 was less extensive compa red to lometrexol and livers of mice accumulated fewer polyglutamates of LY309887 than polyglutamates of lometrexol. The affinities of these two compounds for isoforms of human folate receptors (FR) were compar ed. Lometrexol had a 6-fold higher affinity for FR alpha than LY309887 and both compounds had higher affinity for the alpha isoform compared to the beta isoform. The selectivity of LY309887 for FR alpha (beta(K i)/alpha(Ki) = 10.5) was twice that of lometrexol's (beta/alpha = 5.0) . Lometrexol and LY309887 were potent cytotoxic compounds against the human leukemia cell line CCRF-CEM with IC50's of 2.9 nM and 9.9 nM, re spectively. In vivo, LY309887 was more potent than lometrexol at inhib iting tumor growth in the C3H mammary murine tumor model and several t umor xenografts. Excellent efficacy was achieved by both compounds in several colon xenografts. In two pancreatic human xenografts, LY309887 achieved greater efficacy than lometrexol. In summary, the biochemica l and pharmacological properties of lometrexol and LY309887 support th e hypothesis that these antifolates will have clinical activity agains t human solid tumors. LY309887 is a second generation GARFT inhibitor with biochemical and pharmacological properties which distinguish it f rom lometrexol and suggest that it will have broad antitumor activity, a different pharmacokinetic profile and produce less toxicity than lo metrexol in cancer patients.