Structural basis for dihydrofolate reductase inhibition

Abstract

Several molecules are well known to interact with nucleic acids by intercalation between paires of bases. Some of them, like ellipticine derivatives, have antitumoral activity, some other like, psoralen derivatives, have photosensitizing activity.X-Ray analysis of such molecules has been undertaken to increase the knowledge either, of the stacking possibilities between rings and bases or, of the precise geometry of photoproducts after irradiation. In this case, synthetic models were used (Ade-chain-Pso or Thy-chainpso) •The collected results allow to suggest a possible mecanism of photocyclisation when nucleic acids are irradiated in presence of psoralen derivatives. The specificity of acridines for tissues has long been known and various modifications have been effective as mutagens, carcinogens and antitumor agents. The introduction of a l-nitro group into the acridine nucleus gives a compound, ledakrin (nitracrine), that has been successfully used as an antitumor agent in humans in Poland. The structures of ledakrin [9-(3-dimethylaminopropylimino)-1-nitro-9,10-dihydroacridine, C-283J and several analogs have been determined by X-ray crystallographic techniques and show that the l-nitro group interferes sterically with the 9-substituted position, so causing buckling in the molecule. We present here data on such compounds as free bases or as acid salts; the type of distortions found in such l-nitro compounds are discussed. Our studies suggest that the l-nitro-9-amino substituents favor selection of the imino tautomeric form with a double bond from C(9) to the external 9-amino nitrogen atom. We propose that the imino tautomer for is important for antineoplastic activity. The results of docking these structures into a DNA (by computer graphics) will be discussed in light of the biological activities of these compounds. X-Research supported by the Polish Cancer Program (PR-6/2205), the Verband der Chemische Industrie, the American Chemical Society (BC-242), and the National Institutes of Health, U.S. Public Health Service (CA-10925). The effectiveness of the antifolates as chemotherapeutic agents against various bacterial and neoplastic diseases has spurred extensive research on dihydrofolate reductase (DHFR), the target enzyme, and on its interaction with these powerful inhibitors. As a result, abundant information has accumulated on DHFR including its three dimensional structure and a discription of its interaction with anti folate drugs at the molecular level Of particular interest to us has been the active-site interactions which account for the extremely tight binding of methotrexate, an inhibitor of DHFR which is in wi,despread clinical use. There is now strong evidence from x-ray diffraction ( and MTX (Bolin et.al., J. Biol. Chern. 257, 13650-13662 (1982) (Villafranca et.al., Science 222, 782-788 (1983)). The mutant DHFR shows no protonation of MTX in the binary complex but only a-'OO fold lower binding constant for MTX than the wild type enzyme. This indicates that the charge interaction is probably not ...