Subcellular distribution of epothilones in human tumor cells

Epothilones are a new class of natural and potent antineoplastic agents tha t stabilize microtubules. Although 12,13-epoxide derivatives are potent ant iproliferative agents, the activities of the corresponding 12,13-olefin ana logs are significantly decreased. These data were confirmed for two new ana logs, 6-propyl-EpoB (pEB) and 6-propyl-EpoD (pED), in comparison with the n atural compounds EpoB/EpoD, by using human A431, MCF7, and MDR1-overexpress ing NCl/Adr cells. By using tritiated pEB/pED, compound uptake, release, an d nuclear accumulation were investigated in A431 and NCl/Adr cells. In thes e cells, epothilones can principally be recognized and exported by Verapami l-sensitive efflux pumps, which are not identical to MDR1. The degree of ex port depends on the structure, olefin vs. epoxide-analog, and also on the i ntracellular drug concentration. The accumulation of pED used at 3.5 or 70 nM, respectively, was increased in the presence of 10 muM Verapamil in both cell lines 2- to 8-fold. In contrast, the intracellular levels of pEB were affected by Verapamil only at 3.5 nM pEB in NCl/Adr (2-fold) and not in A4 31 cells. In addition, strong nuclear accumulation was observed for pEB (40 -50%) but not paclitaxel or pED (5-15%) in both cell lines. Our study sugge sts that differences in growth inhibitory efficacy between epoxide and olef in analogs may be based on different mechanisms of drug accumulation and su bcellular distribution.