Cemadotin (
LU103793) (NSC D-669356) is a water-soluble synthetic analogue of
dolastatin 15 that inhibits cell proliferation in vitro and the growth of human
tumor xenografts.
Cemadotin is in phase II clinical trials as a promising
cancer chemotherapeutic agent. The
drug blocks cells at mitosis. Its primary mode of action has been unclear but is believed to involve an action on microtubules. We have found that
cemadotin binds to
tubulin and strongly suppresses microtubule dynamics. Scatchard analysis of
cemadotin binding to
tubulin indicated that there are two affinity classes of
cemadotin-binding sites with Kd values of 19.4 microM and 136 microM.
Cemadotin did not inhibit the binding of
vinblastine to
tubulin, and, conversely,
vinblastine did not inhibit the binding of
cemadotin to
tubulin. By quantitative video microscopy of individual microtubules, we found that
cemadotin strongly suppressed dynamic instability of microtubules assembled to steady state using bovine brain
tubulin devoid of
microtubule-associated proteins. It reduced the rate and extent of growing and shortening, increased the rescue frequency, and increased the percentage of time the microtubules spent in an attenuated or paused state, neither growing nor shortening detectably. At the lowest effective
cemadotin concentrations, dynamics were suppressed in the absence of significant microtubule depolymerization. The results suggest that
cemadotin exerts its antitumor activity by suppressing spindle microtubule dynamics through a distinct molecular mechanism by binding at a novel site in
tubulin.