Structure–activity relationships of some taxoids as multidrug resistance modulator

1,7-Deoxy-4-deacetylbaccatin III (12) and its five analogues 6–9, 13, and their oxetane ring opened derivatives 14, 16, and 17, which were synthesized from taxinine, showed significant activity as MDR reversal agent by the assay of the calcein accumulation toward MDR human ovarian cancer 2780AD cells. The most effective compound 12 in this assay is actually efficient for the recovery of cytotoxic activity of paclitaxel (taxol®), adriamycin (ADM), and vincristine (VCR) toward MDR 2780AD cells at the same level toward parental 2780 cells. This activity of 12 is very interesting because baccatin III (4) has no such MDR reversal activity but has cytotoxic activity. The essential functional groups inducing such a difference in biological activity between 4 and 12 are 4α-acetoxyl for 4 and 4α-hydroxyl for 12. In seven compounds possessing MDR reversal activity, compound 12 is the most desirable compound for anti-MDR cancer reversal agent, because it has the highest accumulation ability of anticancer agent in MDR cancer cells and weak cytotoxic activity. Compounds 8 and 13 showed significant cytotoxic activity toward HepG2 and VA-13, respectively, as well as MDR reversal activity. They are expected to become lead compounds for new types of anticancer agent or anti-MDR cancer agent.