N -[4-[2-(2-AMINO-3,4-dihydro-4-oxo-7H- pyrrolo[2,3 -d]-pyrimidin-5-yl) ethyl]- benzo-yl]-L-glutamic acid, LY231514 (MTA), was discovered through structure activity relationship studies based on the novel antipurine antifolate lometrexol.1 MTA contains a pyrrole moiety in the place of the tetrahydropyridine ring of lome- trexol, which results in a major loss of activity in the inhibition of purine biosynthesis and a shift to the inhibition of pyrimidine biosynthesis (thymi- dylate cycle).2"4 MTA is a substrate for mamma- lian folylpolyglutamate synthase5 and is a potent inhibitor, especially as the triglutamate, of the enzymes thymidylate synthase, dihydrofolate reductase, glycinamide ribonucleotide formyltransferase (GARFF), and aminoimidazole carboxamide ribo- nucleotide formyltransferase.6 In 1984, Jackman et al7 reported that relatively high concentrations of circulating thymidine have been found in human plasma.
Drugs MTA and LY329201 (a GARFT inhibitor) were prepared according to published methods and procedures.2,11,12 Cispla- tin, methotrexate, 5-flt~orouracil, paclitaxel, and doxorubicin were purchased from Sigma Chemical Co (St Louis, MO).
Treatment of HCT 116 tumor-bear- ing animals with MTA and irinotecan resulted in greater than additive tumor growth for the two drugs, reaching 27 days when the iri~otecan dose was 30 mg/kg.
Fractionated radiation therapy was administered locally to the tumor-bearing limb of the nude mice carrying human HCT 116 colon carcinoma xeno- grafts twice daily for 5 days.
Combination treatment regimens including MTA were also used in nude mice bearing subcu- taneously implanted human H460 non-small cell lung carcinoma (Table 1).