Can celecoxib affect P-glycoprotein-mediated drug efflux? A microPET study.

P-glycoprotein (Pgp) is an efflux pump that protects vital organs like the brain from toxic substances, but which is also associated with therapy resistance. The anti-inflammatory drug celecoxib potentiates the efficacy of several cytostatic and neurotropic drugs that are known Pgp substrates. To clarify whether Pgp is involved in the sensitizing effect of celecoxib, we investigated in vivo whether celecoxib is a substrate of Pgp and whether it can affect the efflux activity of the pump. In control rats and in rats treated with the Pgp modulator cyclosporin A (CsA), cerebral accumulation of radiolabeled [(11)C]celecoxib was investigated by ex vivo biodistribution and micro-positron emission tomography imaging. In addition, the effect of unlabeled celecoxib and CsA (positive control) on the cerebral uptake of the Pgp substrate [(11)C]verapamil was studied. [(11)C]Celecoxib uptake in rat brain was relatively high and homogeneously distributed. Treatment of rats with CsA only marginally increased cerebral tracer uptake, which is most likely due to reduced tracer clearance from plasma. [(11)C]Verapamil brain uptake was more than 10-fold higher after treatment with CsA. In contrast, a high dose of celecoxib increased cerebral [(11)C]verapamil uptake only twofold, which was accompanied by a similar increase in tracer concentration in plasma. This study shows that celecoxib is not a substrate of Pgp and does not substantially affect the Pgp-mediated efflux of [(11)C]verapamil. Therefore, celecoxib-induced augmentation of the efficacy of chemotherapeutic and neurotropic drugs must be due to another mechanism than modulation of Pgp-mediated drug efflux.