Effects of novel taxanes SB-T-1213 and IDN5109 on tubulin polymerization and mitosis

SB-T-1213 and IDN5109 are semisynthetic, orally available taxanes that are up to 400-fold more active than paclitaxel against drug-resistant cells. IDN5109 is in clinical trials. We investigated the primary target for SB-T-1213 and IDN5109 and whether the compounds interact with microtubules differently than paclitaxel. Unlike paclitaxel, at 1-10 μM both novel taxanes initiate microtubule polymerization in vitro with no lag. They enhance polymerization equally or more potently than paclitaxel. SB-T-1213 induces unusual microtubules with attached extra protofilaments or open sheets, and IDN5109 induces large protofilamentous sheets. Both inhibit HeLa cell proliferation, block mitosis at the metaphase/anaphase transition, bundle microtubules at high drug concentrations, and induce abnormal metaphase spindles and apoptosis. They target microtubules but alter their polymerization and structure differently than paclitaxel. These differences may play a role in their enhanced cytotoxicity and efficacy.