Synthesis and Structure-Activity Relationships of New Antitumor Taxoids. Effects of Cyclohexyl Substitution at the C-3' and/or C-2 of Taxotere (Docetaxel)

Synthesis and cytotoxicity of the new analogs (11-13) of docetaxel possessing cyclohexyl groups instead of phenyl groups at the C-3' and/or C-2 benzoate positions are described. The C-2 cyclohexanecarboxylate analog of paclitaxel (15) is also synthesized for comparison. The potency of these new taxoids were examined for their inhibitory activity for microtubule disassembly and also for their cytotoxicity against murine P388 leukemia cell line as well as doxorubicin-resistant P388 leukemia cell line (P388/Dox). It is found that 3'-dephenyl-3'-cyclohexyldocetaxel (11) (0.72T) and 2-(hexahydro)docetaxel (12) (0.85T) possess strong inhibitory activity for microtubule disassembly equivalent to docetaxel (0.7T), which is more potent than paclitaxel (1.0T). The results clearly indicate that phenyl or an aromatic group at C-3' or C-2 is not a requisite for strong binding to the microtubules. This finding has opened an avenue for development of new nonaromatic analogs of docetaxel and paclitaxel. 3'-Dephenyl-3'-cyclohexyl-2-(hexahydro)docetaxel (13) (2T) turns out to be a substantially weaker inhibitor. The cytotoxicities of 11-13 against P388 are, however, in the same range that is 8-12 times weaker than docetaxel and 4-6 times weaker than paclitaxel, i.e., 13 shows equivalent cytotoxicity to that of 11 or 12 in spite of much lower microtubule disassembly inhibitory activity. The cytotoxicities of these new taxoids against the P388/Dox cell line are only 2-2.5 times lower than that of docetaxel. The potency of 2-(hexahydro)paclitaxel (15) for these assays is much lower than the docetaxel counterpart 12. The significant loss of activity in vivo against B16 melanoma is observed for 11-13, i.e., 11 is only marginal (T/C = 38% at 20 mg/kg/day), and 12 and 13 are inactive (T/C = 76% and 79%, respectively). This could be ascribed to faster metabolism, faster excretion or other bioavailability problems.