AbstractIn an effort to inhibit P‐glycoprotein (P‐gp) efflux function with greater activity and less side effects, the combined effect of CJX2 and verapamil (Ver) was evaluated isobolographically in fixed ratio combinations of 1:1, 1:2, 1:4, 1:8, and 1:10 in doxorubicin‐resistant human myelogenous leukemia (K562/DOX ) cells and rat brain microvessel endothelial cells (RBMEC). The results displayed that mixtures of both drugs at the fixed ratios of 1:1, 1:2, 1:4, 1:8, and 1:10 exerted synergistic interactions, indicating that when the two blockers that bind P‐gp on separate membrane sites were combined, each contributes to the overall interaction with P‐gp, leading to the greater effect than that seen by either agent alone. Drug Dev Res, 2009. © 2009 Wiley‐Liss, Inc.