Novel 6-5 fused ring heterocycle antifolates with potent antitumor activity: bridge modifications and heterocyclic benzoyl isosters of 2,4-diamino-6,7-dihydro-5H-cyclopenta[d]pyrimidine antifolate.

Structural modifications of an extremely potent inhibitor of dihydrofolate reductase (DHFR) activity and tumor cell growth, N-[4-[3-(2,4-diamino-6,7-dihydro-5H-cyclopenta[d]pyrimidin-5- yl)propyl]benzoyl]-L-glutamic acid (1), have led to the synthesis of new cyclopenta[d]pyrimidine-based antifolates, including those with low alkyl substituted trimethylene bridges (2a, b) and isosterically modified bridges (ethyleneoxa, 2c; ethyleneamino, 2d; the N-methyl- and N-ethyl derivatives of 2d, 2e, f) and those in which the benzene ring of 1 has been replaced by heterocyclic isosters (indole, 2g; indoline, 2h; thiophene, 2i). These new analogs are highly potent as DHFR and cell growth inhibitors, and most of them are more potent than methotrexate (MTX) and 10-ethyl-10-deazapterin (10-EDAM) in inhibiting tumor cell growth (P388 MTX-sensitive and MTX-resistant, colon 26 and KB) on 72 h drug exposure. Among them, 2a (the 10-methyl derivative of 1) and 2i were most potent, being 2- to 3-fold more potent than 10-EDAM. On 4 h drug exposure, the growth-inhibitory activity of these analogs was radically influenced by even minor structural changes. Compounds 1, 2a--e, g--i were much more cytotoxic in colon 26 cell line than were MTX and 10-EDAM, with 2d and 2i being most potent, followed by 2a. Structure-activity relationships and their possible significance are discussed.