ENERGETICS OF VINCA ALKALOID INTERACTIONS WITH TUBULIN ISOTYPES - IMPLICATIONS FOR DRUG EFFICACY AND TOXICITY

A number of vinca alkaloids, including vincristine, vinblastine, and v inorelbine, are currently used in cancer chemotherapy. These three vin ca alkaloids interact differently with a range of solid and hematologi c tumors. To test the possibility that the tubulin isotype composition is an important determinant in antineoplastic efficacy, we determined thermodynamic parameters for vinca alkaloid interactions with purifie d beta-tubulin isotypes, alpha beta II or alpha beta III, as well as m ixtures of alpha beta II and alpha beta III, alpha beta II and alpha b eta I&IV, or alpha beta III and alpha beta I&IV (referred to as isotyp e-depleted tubulin) by quantitative sedimentation velocity. Vincristin e-, vinblastine-, or vinorelbine-induced isotype self-association was studied at 25 degrees C in 10 mM Pipes, pH 6.9, 1 mM MgSO4, and 2 mM E GTA in the presence of 50 mu M GTP or GDP. For all three drugs, we obs erved no significant differences in overall affinities, K1K2, or in GD P enhancement of purified isotypes compared to unfractionated tubulin, suggesting that differential antitumor efficacy observed clinically f or these vinca alkaloids is not determined by tissue isotype compositi on. Small, but significant differences in the individual binding param eters, K-1 and K-2, are found in the vincristine data. In the presence of vincristine and GTP, K-1, the affinity of drug for tubulin heterod imers, tends to be larger for purified alpha beta II- or alpha beta II I-tubulin compared to unfractionated tubulin. Furthermore, the apparen t dimerization constant, K-2(app), physiologically significant drug co ncentrations is larger for these purified isotypes. When alpha beta II - and alpha beta III-tubulin are combined, the cooperativity between d rug binding and spiral formation approaches that of unfractionated PC- tubulin. These differences are not observed in the presence of vinblas tine or vinorelbine. The differences found with vincristine may be imp licated in the dose-limiting neurotoxicity found with this drug, but n ot found with vinblastine or vinorelbine. (C) 1998 Wiley-Liss, Inc.