Antiproliferative mechanism of action of cryptophycin-52: Kinetic stabilization of microtubule dynamics by high-affinity binding to microtubule ends

Cryptophycin-52 (LY355703) is a new synthetic member of the cryptophycin family of antimitotic antitumor agents that is currently undergoing clinical evaluation. At high concentrations (≥10 times the IC₅₀), cryptophycin-52 blocked HeLa cell proliferation at mitosis by depolymerizing spindle microtubules and disrupting chromosome organization. However, low concentrations of cryptophycin-52 inhibited cell proliferation at mitosis (IC₅₀ = 11 pM) without significantly altering spindle microtubule mass or organization. Cryptophycin-52 appears to be the most potent suppressor of microtubule dynamics found thus far. It suppressed the dynamic instability behavior of individual microtubules in vitro (IC₅₀ = 20 nM), reducing the rate and extent of shortening and growing without significantly reducing polymer mass or mean microtubule length. Using [³H]cryptophycin-52, we found that the compound bound to microtubule ends in vitro with high affinity (K_d, 47 nM, maximum of ≈19.5 cryptophycin-52 molecules per microtubule). By analyzing the effects of cryptophycin-52 on dynamics in relation to its binding to microtubules, we determined that ≈5–6 molecules of cryptophycin-52 bound to a microtubule were sufficient to decrease dynamicity by 50%. Cryptophycin-52 became concentrated in cells 730-fold, and the resulting intracellular cryptophycin-52 concentration was similar to that required to stabilize microtubule dynamics in vitro. The data suggest that cryptophycin-52 potently perturbs kinetic events at microtubule ends that are required for microtubule function during mitosis and that it acts by forming a reversible cryptophycin-52-tubulin stabilizing cap at microtubule ends.