NPB304 affected P-gp function and increased paclitaxel accumulation in P-gp-overexpressing resistant cells and tumor tissue.

<p>(A) P-gp was overexpressed in resistant breast cancer cells compared with parent cells. NPB304 treatment had no effect on P-gp expression. (B) NPB304 treatment increased Rh123 accumulation in MX-1/paclitaxel cells and MCF-7/paclitaxel cells. Resistant cells were treated with the indicated drugs for 3 h, and 10 µM Rh123 was subsequently added and incubated for an additional 30 min. The mean fluorescence intensity of intracellular Rh123 was determined by flow cytometry. Cells that were not exposed to NPB304 served as negative controls. The relative value was the ratio of the mean fluorescence intensity of intracellular Rh123 of the groups and that of the negative controls. The data represent the mean ± SD. *p<0.05, **p<0.01. (C) NPB304 treatment promoted bidirectional permeability but decreased the efflux ratio of paclitaxel in a Caco-2 monolayer model. Caco-2 monolayers were treated with paclitaxel (5 µM) alone and paclitaxel combined with NPB304 or Vrp on both sides. The concentration of paclitaxel on the opposite side was determined at 3 h. The data represent the mean ± SD. The efflux ratio was the ratio of BL to AP and AP to BL. (D) NPB304 increased the intracellular concentration of paclitaxel on both sides. The Caco-2 cells were harvested to determine the intracellular concentration of paclitaxel using LC-MS-MS after 3 h. (E) NPB304 increased the intratumoral concentration of paclitaxel without reducing the P-gp expression. Four nude mice tumors with equal weights were harvested from the treatment groups to detect the expression of P-gp by Western blotting and the intratumoral concentration of paclitaxel using an LC-MS/MS assay. *p<0.05, **p<0.01.