INTERACTION OF A FLUORESCENT PACLITAXEL ANALOG WITH TUBULIN

T0 study the mechanism of binding of the antitumor agent paclitaxel to microtubules and tubulin, we have synthesized a fluorescent analogue of the drug. A dimethylamino group was introduced onto the 3'-N-benzoy l group of paclitaxel. This compound was synthesized from N-debenzoylp aclitaxel and 3-(dimethylamino)benzoyl chloride in 67% yield. -Debenzo yl-N-[3-(dimethylamino)benzoyl]-paclitaxel has activity similar to pac litaxel in inducing microtubule assembly and binds to tubulin at the p aclitaxel-binding site. Under assembly conditions, binding of this pac litaxel analogue to tubulin occurs in a time-dependent manner and is a ccompanied by a large increase in fluorescence intensity, as well as a large blue shift in the emission maximum. In addition, evidence is pr esented to show that this compound also binds to tubulin in the dimeri c state, but the binding affinity is much lower (K-d = 49 +/- 8 mu M a t 25 degrees C) than that reported for polymeric tubulin. The fluoresc ent paclitaxel analogue, with a high quantum yield, will be a useful t ool in studying the mechanism of paclitaxel binding to tubulin and the environment of the paclitaxel-binding site on tubulin.