Interaction of the antitumor compound cryptophycin-52 with tubulin

Cryptophycin-52 (LY355703) is currently undergoing clinical evaluation for cancer chemotherapy. It is a potent suppressor of microtubule dynamics in vitro, and low picomolar concentrations appear to inhibit cancer cell proliferation at mitosis by stabilizing spindle microtubules. In the present study, using [(3)H]cryptophycin-52, we found that the compound bound to tubulin at a single high-affinity site [apparent K(a) (3.6 +/- 1) x 10(6) L/mol, 34 degrees C]. The binding of cryptophycin-52 to tubulin was rapid, not appreciably temperature-dependent, and very poorly reversible. However, we could remove [(3)H]cryptophycin-52 from [(3)H]cryptophycin-52-tubulin complex by denaturing the complex with either urea treatment or boiling. These data suggest that the binding of cryptophycin-52 to tubulin is not covalent. A van't Hoff plot of the binding data indicated that the binding of cryptophycin-52 to tubulin is primarily entropy-driven with a minimum enthalpy contribution. In addition, cryptophycin-52 perturbed the far-ultraviolet circular dichroic spectrum of tubulin and it inhibited the colchicine-induced guanosine triphosphatase activity of tubulin, indicating that its binding to tubulin induces a conformational change in the tubulin. Competition experiments with vinblastine suggest that the binding site for crytophycin-52 may overlap with the vinblastine binding site.