Modulation of drug cytotoxicity in wild-type and multidrug-resistant tumor cells by stereoisomeric series of C-20'-vinblastine congeners that lack antimicrotubule activity.

Seven binary vinca alkaloid congeners were newly synthesized as the C14' or C16'(20') or C14'16'(20') stereoisomers of C20'-modified VBL. These congeners lacked detectable antimicrotubule activity in assays of polymerization of purified microtubule protein and of mitotic arrest induction. The compounds modulated the cytotoxicity of VBL, VCR, and DOX in sarcoma and colon-tumor cell lines. In wild-type cell lines, each congener elicited a concentration-dependent enhancement of cytotoxicity that was drug- and cell-type-selective. For example, C20'-deoxy C14'16'20'-epi VBL sensitized sarcoma S180 cells 19-fold to DOX and 11-fold to VCR but had no effect on VBL cytotoxicity. In the rat colon-cancer cell lines there was preferential enhancement of VCR cytotoxicity by most congeners. In two MDR cell strains of S180, the modulation potency of each congener was independent of specific drug or of resistance level. As a result, the amount of modulator (concentration) required for reversal was proportional to the drug-resistance level. Such properties were not displayed by the monomeric vinca alkaloid modulator vindoline. The potency of drug modulation in both wild-type and MDR cells strains was dependent on the stereoisomeric form of the congener and its C20'-substituents.