MECHANISM OF ACTION OF THE UNUSUALLY POTENT MICROTUBULE INHIBITOR CRYPTOPHYCIN-1

Cryptophycin 1 is a remarkably potent antiproliferative compound that shows excellent antitumor activity against mammary, colon, and pancrea tic adenocarcinomas in mouse xenographs. At picomolar concentrations, cryptophycin 1 blocks cells in the G(2)/M phase of the cell cycle by a n apparent action on microtubules. The compound binds to tubulin, inhi bits microtubule polymerization, and depolymerizes preformed microtubu les in vitro. Its exceptionally powerful antitumor activity (many-fold greater than paclitaxel or the vinca alkaloids) raises important ques tions about its mechanism of action. By quantitative video microscopy, we examined the effects of cryptophycin 1 on the dynamics of individu al microtubules assembled to steady state from bovine brain tubulin. A t low nanomolar concentrations, in the absence of net microtubule depo lymerization, cryptophycin 1 potently stabilized microtubule dynamics. It reduced the rate and extent of microtubule shortening and growing and increased the frequency of rescue. The results suggest that crypto phycin 1 exerts its antiproliferative and antimitotic activity by bind ing reversibly and with high affinity to the ends of microtubules, per haps in the form of a tubulin-cryptophycin 1 complex, resulting in the most potent suppression of microtubule dynamics yet described.