Synthesis and Potent Antifolate Activity and Cytotoxicity of B-Ring Deaza Analogues of the Nonpolyglutamatable Dihydrofolate Reductase Inhibitor Nα-(4-Amino-4-deoxypteroyl)-Nδ-hemiphthaloyl-l-ornithine (PT523)

Six new B-ring analogues of the nonpolyglutamatable antifolate Nα-(4-amino-4-deoxypteroyl)-Nδ-hemiphthaloyl-l-ornithine (PT523, 3) were synthesized with a view to determining the effect of modifications at the 5- and/or 8-position on dihydrofolate reductase (DHFR) binding and tumor cell growth inhibition. The 5- and 8-deaza analogues were prepared from methyl 2-l-amino-5-phthalimidopentanoate and 4-amino-4-deoxy-N10-formyl-5-deaza- and -8-deazapteroic acid, respectively. The 5,8-dideaza analogues were prepared from methyl 2-l-[(4-aminobenzoyl)amino]-5-phthalimidopentanoate and 2,4-diaminoquinazoline-6-carbonitriles. The Ki for inhibition of human DHFR by the 5-deaza and 5-methyl-5-deaza analogues was about the same as that of 3 (0.35 pM), 11-fold lower than that of aminopterin (AMT, 1), and 15-fold lower than that of methotrexate (MTX, 2). However the Ki of the 8-deaza analogue was 27-fold lower than that of 1, and that of the 5,8-dideaza, 5-methyl-5,8-dideaza, and 5-chloro-5,8-dideaza analogues was appro...