Structure-activity studies on gossypol in tumor cell lines

Gossypol [(2,2′-binaphthalene)-8,8′-dicarboxaldehyde-1,1′,6,6′,7,7′-hexahydroxy-5,5′-diisopropyl-3,3′-dimethyl] 1a is a naturally occurring compound extracted from the cotton plant and has been extensively studied as an oral male contraceptive. Its favorable toxicity profile, and the more recent demonstration of anti-tumor activity in animals and humans, prompted us to investigate the role of the aldehyde groups in a structure-activity study in cultured tumor cells. Four racemic compounds were evaluatedgossypol 1a, gossypolone 2, the bis Schiffʼs base of L-phenylalanine methyl ester with gossypol (bis Schiffʼs base) 1c and apogossypol 1b. The former two compounds both retain the aldehyde functional groups at positions 8 and 8′ of the molecule whilst in the latter two compounds the aldehydes are blocked or absent, respectively. In addition, the l- and d-isomers of gossypol 1a, the bis Schiffʼs base 1c and the half Schiffʼs base 1d (one aldehyde blocked) were tested. The cell lines studied included melanoma (SK-mel-19), cervix (Sihas), small cell lung (H69) and myelogenous leukemia (K562). Cytotoxicity was measured using the MTT and flow cytometric viability assays. Racemic gossypol 1a and gossypolone 2 induced similar dose-dependent decreases in cell viability in all the cell lines with IC50 values of 23-46 and 28-50 μM, respectively. In contrast, the racemic bis Schiffʼs base derivative of gossypol 1c and apogossypol 1b showed minimal activity in any cell line up to 50 μM. The l-enantiomer of gossypol 1a was significantly more active than the d-enantiomer (IC50 of 20 versus >50 μM, respectively). When one aldehyde of either enantiomer was blocked 1d cytoxicity was comparable to the l-enantiomer of gossypol. The data suggest that only one aldehyde group is required for the cytotoxicity of gossypol 1a, irrespective of the stereo-configuration