NONCLASSICAL 5-SUBSTITUTED TETRAHYDROQUINAZOLINES AS POTENTIAL INHIBITORS OF THYMIDYLATE SYNTHASE

Classical inhibitors of thymidylate synthase such as N-10-propargyl-5, 8-dideazafolic acid (1), n-6-ylmethyl)-N-methylamino]-2-thenoyl)-L-glu tamic acid (ZD1694, 2) and rolo[2,3-d]pyrimidin-5-yl)ethylbenzoyl]-L-g lutamic acid (LY231514, 3) while potent, suffer from a number of poten tial disadvantages, such as impaired uptake due to an alteration of th e active transport system required for their cellular uptake, as well as formation of long acting, non-effluxing polyglutamates via the acti on of folylpolyglutamate synthetase, which are responsible for toxicit y. To overcome some of the disadvantages of classical inhibitors, ther e has been considerable interest in the synthesis and evaluation of no nclassical thymidylate synthase inhibitors, which could enter cells vi a passive diffusion. In an attempt to elucidate the role of saturation of the B-ring of nonclassical, quinazoline antifolate inhibitors of t hymidylate synthase, analogues 7-17 were designed. Analogues 13-17 whi ch contain a methyl group at the 7-position, were synthesized in an at tempt to align the methyl group in an orientation which allows interac tion with tryptophan-80 in the active site of thymidylate synthase. Th e synthesis of these analogues was achieved via the reaction of guanid ine with the appropriately substituted cyclohexanone-ketoester. These ketoesters were in turn synthesized via a Michael addition of the appr opriate thiophenol with 2-carbethoxycyclohexen-1-one or 5-methyl-2-car bethoxycyclohexen-1-one to afford a mixture of diastereomers. The most inhibitory compound was the 3,4-dichloro, 7-methyl derivative 17 whic h inhibited the Escherichia coil and Pneumocystis carinii thymidylate syntheses 50% at 5 x 10(5) M. Our results confirm the importance of th e 7-CH3 group and electron withdrawing groups on the aromatic side cha in for thymidylate synthase inhibition.