The natural product (−)-dictyostatin is a microtubule stabilizing agent that potently inhibits the growth of human cancer cells including paclitaxel-resistant clones. Extensive structure-activity relationship studies have revealed several regions of the molecule that could be altered without loss of activity. The most potent synthetic dictyostatin analog described to date, 6-epi-dictyostatin, has in vivo antitumor activity against human breast cancer xenografts superior to paclitaxel. Despite their encouraging preclinical activities, the complex chemical structure of the dictyostatins presents a major obstacle in their development into novel antineoplastic therapies. We recently reported a streamlined synthesis of 16-desmethyl-25,26 dihydrodictyostatins and found several agents that compared with 6-epi-dictyostatin retained nanomolar activity in cellular microtubule bundling assays but showed cross-resistance to paclitaxel in cells with mutations in beta-tubulin. Extending these studies, we applied the new, highly convergent synthesis to generate 25,26-dihydrodictyostatin and 6-epi-25,26-dihydrodictyostatin. Both compounds were potent microtubule perturbing agents that induced mitotic arrest and microtubule assembly in vitro and in intact cells. In vitro radioligand binding studies showed that 25,26-dihydrodictyostatin and its C-6 epimer were able to displace [3H]paclitaxel and [14C]epothilone B from microtubules with potencies comparable to (−)-dictyostatin and discodermolide. Both compounds inhibited the growth of paclitaxel- and epothilone B-resistant cell lines at low nanomolar concentrations, synergized with paclitaxel in MDA-MB-231 human breast cancer cells, and had antiangiogenic activity in transgenic zebrafish larvae. The data identify 25,26-dihydrodictyostatin and 6-epi-25,26-dihydrodictyostatin as candidates for scale-up synthesis and further preclinical development.
Keywords: dictyostatin, high-content screening, multidrug resistance, zebrafish, angiogenesis