Specific alterations in the biological activities of C-20'-modified vinblastine congeners

Both the anti-tumor and toxic activities of the vinca alkaloid dimers, vinblastine (VBL) and vincristine (VCR), may reside at the level of their known cellular target, the microtubule system. The contributions made by each of the various actions of these alkaloids on this system are unknown. We have used new, complete synthetic methodologies to create a series of eight C-20' alkyl congeners of VBL and have examined these compounds for their abilities to (1) inhibit microtubule assembly, (2) disassemble preformed microtubules, and (3) induce spiral aggregate formation, using purified brain microtubule protein. By combining turbidimetric and electron microscopic techniques, we discovered that each of the various effects of VBL on the microtubule system in vitro was amenable to alteration by specific modification at this single molecular site. In addition, we report two new aberrations of VBL action—the induction of spirals by a concentration of congener below 1 μM and the formation of “opened” microtubules polymerized in the presence of congener. The relationship between anti-microtubule action in vitro and the cellular activities of growth inhibition and mitotic arrest by the congeners was examined in leukemic and colon cancer cell lines. In general, we found that both cellular perturbations were correlated to the ability of the congeners to inhibit microtubule polymerization rather than to the actions of spiral formation or microtubule disassembly. These results are a breakthrough in the structure/function relationship of the vinca alkaloid dimers and should provide the means to determine the role of specific anti-microtubule activities to the complex biological actions of these natural product drugs.