Stereo-specific cytotoxic effects of gossypol enantiomers and gossypolone in tumour cell lines.

The naturally occurring compound, gossypol, has been previously used as a male oral contraceptive, for the treatment of benign gynaecological conditions and cancer patients. Long-term daily dosing with gossypol is associated with minimal side effects and no myelosuppression. Since gossypol exhibits atropisomerism due to the restricted rotation about the 2,2' carbon bond, we have isolated the l- and d-isomers by Schiff's base formation using a chiral amine and regenerated the enantiomers by acid hydrolysis. The enantiomers and the proposed oxidative metabolite, gossypolone, were characterized by HPLC, 1H-NMR and optical rotation. The cytotoxicity was assessed in cell cultures derived from melanoma, lung, breast, cervix, and leukaemia using the MTT viability assay. The cytotoxicity of gossypolone was similar to racemic gossypol in five out of the six cell lines studied. The l-enantiomer of gossypol induced a dose-dependent cell kill in all cell lines with a mean IC50 of 20 microM and was significantly more potent than racemic gossypol, the d-enantiomer of gossypol and gossypolone. In addition, when the leukaemia line was exposed to l-gossypol (0.5-10 microM) over a 4-day period, a schedule-dependent decrease in cell viability was observed. l-Gossypol was also compared with respective drugs used to treat patients with melanoma, lung cancer and leukaemia. The data indicate that l-gossypol was significantly more active than cisplatin, melphalan and dacarbazine in the two melanoma lines, cisplatin and daunorubicin in the lung line and hydroxyurea and busulphan in the leukaemia line. Preliminary studies using one melanoma line showed that the l-isomer induced cell shrinkage, membrane blebbing and DNA fragmentation, characteristics suggestive of apoptotic cell death.