Novel actions of the antitumor drugs vinflunine and vinorelbine on microtubules.

Vinflunine is a novel Vinca alkaloid presently in Phase I clinical trials. In preclinical studies, it exhibited superior antitumor activity to that of other Vinca alkaloids, including vinorelbine from which it was synthetically derived. Vinca alkaloids appear to inhibit cell proliferation by affecting the dynamics of spindle microtubules. Here we have analyzed the effects of vinflunine and vinorelbine on microtubule dynamic instability and treadmilling and found that these newer drugs exert effects on microtubule dynamics that differ significantly from those of the classic Vinca alkaloid, vinblastine. The major effects of vinflunine and vinorelbine on dynamic instability were a slowing of the microtubule growth rate, an increase in growth duration, and a reduction in shortening duration. In marked contrast to the action of vinblastine, they neither reduced the rate of shortening nor increased the percentage of time the microtubules spent in an attenuated state, neither growing nor shortening detectably. In addition, vinflunine and vinorelbine suppressed treadmilling, but less strongly than vinblastine. The diverse actions of these drugs on microtubules are likely to produce different effects on mitotic spindle function, leading to different effects on cell cycle progression and cell killing. Nontumor cells with normal checkpoint proteins may tolerate the relatively less powerful inhibitory effects of vinflunine and vinorelbine on microtubule dynamics better than the more powerful effects of vinblastine. Thus the unique constellation of effects of vinflunine and vinorelbine on dynamic instability and treadmilling may contribute to their superior antitumor efficacies.