Differential Effects of Ethyl 5-Amino-2-methyl-1,2-dihydro-3-phenylpyrido[3,4-b]pyrazin-7-yl Carbamate Analogs Modified at Position C2 on Tubulin Polymerization, Binding, and Conformational Changes

NSC 613863 (R)-(+) and NSC 613862 (S)-(-) (CI980) are two chiral isomers of ethyl 5-amino 2-methyl-1,2-dihydro-3-phenylpyrido[3,4-b]pyrazin-7-yl carbamate which have potent antitubulin activity. The S-isomer is a more potent antimitotic compound than the R-isomer, and the two isomers differ markedly in binding to tubulin [Leynadier, D., Peyrot, V., Sarrazin, M., Briand, C., Andreu, J. M., Rener, G. A., & Temple, C., Jr. (1993) Biochemistry 32, 10675-10682]. To understand the origin of such differences, we studied the interactions of three R- and S-isomer structural analogs which differ in C2 (the chiral carbon), i.e., C179, NSC 337238, and NSC 330770. C179 is a methylated dehydrogenated achiral compound. It bound to tubulin with an apparent affinity Ka of (2.29 +/- 0.17) x 10(4) M-1, inhibited tubulin polymerization in vitro at a half-inhibitory concentration (IC50) of 100 microM, and presented no GTPase activity. The substitution of -CH3 by -H leads to the NSC 337238 compound. It bound to tubulin with a higher affinity [Ka = (2.62 +/- 0.35) x 10(5) M-1] and inhibited tubulin polymerization at a lower concentration (IC50 = 14 microM). It presented no GTPase activity and induced the formation of abnormal polymers at a protein critical concentration (Cr) of 2 mg mL-1. NSC 330770, a demethylated hydrogenated molecule, interacted strongly with tubulin [Ka = (3.30 +/- 0.56) x 10(6) M-1].