DIFFERENTIAL-EFFECTS OF PACLITAXEL (TAXOL) ANALOGS MODIFIED AT POSITIONS C-2, C-7, AND C-3' ON TUBULIN POLYMERIZATION AND POLYMER STABILIZATION - IDENTIFICATION OF A HYPERACTIVE PACLITAXEL DERIVATIVE

Our finding that an analog of paclitaxel (Taxol) modified at position C-2 (2-debenzoyl-2-(m-azidobenzoyl)paclitaxel) was substantially more active than paclitaxel in promoting tubulin assembly [Chaudhary et al. (1994) J. Am. Chem. Sec. 116, 4097-4098] led us to perform an analysi s of the modulating effects of microtubule-associated proteins, GTP, a nd temperature on assembly and polymer stability. The analog always sh owed superior activity to paclitaxel in inducing polymerization where it fails to occur without drug: probably indicating a greater ability than paclitaxel to ''hypernucleate'' assembly. In contrast, much small er differences in effects on polymer stability were observed. The anal ysis was extended to a large series of derivatives modified at positio ns C-2, C-7, C-10, and C-3', including docetaxel, a clinically importa nt analog of paclitaxel. While analog stabilization of polymer was fre quently observed, neither qualitative nor quantitative analysis of thi s property reliably predicted whether a compound would have enhanced h ypernucleation activity relative to that of paclitaxel. Stabilization was often observed at substoichiometric analog concentrations, while e ven superstoichiometric concentrations of most compounds failed to ind uce extensive tubulin polymerization at low temperatures or in the abs ence of microtubule-associated proteins or GTP. Docetaxel was intermed iate in activity between paclitaxel and 2-debenzoyl-2-(m-azidobenzoyl) paclitaxel in promoting assembly reactions, We conclude that the hyper nucleation of tubulin assembly and polymer stabilization observed with paclitaxel represent two distinct properties of the drug. Our finding s suggest that paclitaxel, docetaxel, and 2-debenzoyl-2-(m-azidobenzoy l)paclitaxel are able to interact with progressively smaller assemblag es of tubulin at low temperatures or in the absence of microtubule-ass ociated proteins or GTP.