(6R,6S)-5,8,10-Trideaza-5,6,7,8-tetrahydropteroic acid was synthesized in several steps from 4,4-(ethylenedioxy)-cyclohexanone and [4-(tert-butyloxycarbonyl)benzyl]triphenylphosphonium bromide and was elaborated to (6R,6S)-5,8,10-trideaza-5,6,7,8-tetrahydropteroyl-L-glutamic acid and (6R,6S)-5,8,10-trideaza-5,6,7,8-tetrahydropteroyl-L-ornithin e. Compound 1 was found to be a good substrate for partially purified mouse liver folypolyglutamate synthetase (FPGS), with a Michaelis constant (Km = 15 microM) comparable to that reported for the reduced folate substrate (6S)-5,6,7,8-tetrahydropteroyl-L-glutamic acid and for (6R,6S)-5,10-dideaza-5,6,7,8-tetrahydropteroyl-L-glutamic acid (DDATHF). However, in striking contrast to DDATHF, which is potently cytotoxic, 1 failed to inhibit tumor cell growth in culture at concentrations of up to 100 microM. These results suggested that the NH at position 8 of DDATHF is important for cytotoxic activity but not for polyglutamylation. Just as 1 was a good substrate for FPGS, the ornithine analogue 2 proved to be among the more potent competitive inhibitors of this enzyme discovered to date, with a Ki,s of 10 microM. While the binding affinity of 2 was lower than that reported for 5,6,7,8-tetrahydropteroyl-L-ornithine (H4PteOrn), very substantial FPGS inhibition was observed even though N5,N8, and N10 in H4PteOrn were replaced by carbon. Binding to FPGS thus appears to be tolerant of bioisosteric replacements made simultaneously in ring B and the bridge region. Neither 1 nor 2 was active in preventing cell growth in culture at concentrations of up 100 microM. The N delta-hemiphthaloyl derivative of 2, synthesized as a potential prodrug, was also inactive.