Background:
Multidrug resistant (MDR) is one of the important obstacles in cancer chemotherapy. The over- expression of ATP-binding cassette (ABC) transporters is purposed to be one of the major cause of MDR cancers. Among the efflux transporters, ABC transporter-subfamily B member 1 (ABCB1 / P-gp) has been demonstrated to play a key role. Therefore, identifying ABCB1 inhibitors may contribute to the successful chemotherapy.

Aim:
The aim of the present study is to evaluate the influence of caffeine on ABCB1 efflux function, and the potential of caffeine used as an ABCB1 inhibitor to overcome MDR cancers.

Methods:
HEK-293 cells were stable transfected human ABCB1 gene by lipofectamine 2000 and selected by hygromycin B. Calcein-AM uptake assay and rhodamine123 accumulation assay were performed to examine the influence of caffeine on ABCB1 function. Intracellular fluorescence accumulation was also recorded by fluorescence microscopy.

Results:
The intracellular fluorescent calcein intensity was increased in the presence of 10, 20, 50mM caffeine with a dose-dependent manner. These observations were also supported by images of fluorescence microscopy. As for the rhodamine123 efflux assay, caffeine could significantly inhibit the efflux of rhodamine123 by P-gp.

Conclusions:
These results demonstrated that caffeine may be a potential P-gp inhibitor. The molecular mechanisms and kinetic assays of how caffeine interacts with P-gp still need further studies.