Uptake/efflux transport of tramadol enantiomers and O-desmethyl-tramadol: focus on P-glycoprotein

Abstract

The analgesic effect of tramadol (TMD) results from the monoaminergic effect of its two enantiomers, (+)-TMD and (-)-TMD as well as its opioid metabolite (+)-O-desmethyl-tramadol (M1). P-glycoprotein (P-gp) might be of importance in the analgesic and tolerability profile variability of TMD. Our study investigated the involvement of P-gp in the transepithelial transport of (+)-TMD, (-)-TMD and M1, using a Caco-2 cell monolayer model. The bidirectional transport of racemic TMD and M1 (1-100 microM) across the monolayers was investigated at two pH conditions (pH 6.8/7.4 and 7.4/7.4) in the presence and absence of P-gp inhibitor cyclosporine A (10 microM) and assessed with the more potent and specific P-gp inhibitor GF120918 (4 microM). Analytical quantification was performed by liquid chromatography coupled to the fluorescence detector. A net secretion of (+)-TMD, (-)-TMD and M1 was observed when a pH gradient was applied (TR: P(app)(B - A)/P(app)(A - B): 1.8-2.7; P < 0.05). However, the bidirectional transport of all compounds was equal in the non-gradient system. In the presence of P-gp inhibitors, a slight but significant increase of secretory flux was observed (up to 26%; P < 0.05) at both pH conditions. In conclusion, (+)-TMD, (-)-TMD and M1 are not P-gp substrates. However, proton-based efflux pumps may be involved in limiting the gastrointestinal absorption of TMD enantiomers as well as enhancing TMD enantiomers and M1 renal excretion. A possible involvement of uptake carriers in the transepithelial transport of TMD enantiomers and M1 is suggested.

Fig. 1

Major in vivo metabolic pathways for tramadol.

Fig. 2

Transepithelial transport of rhodamine 123 across Caco-2 cell monolayers and the effect of cyclosporine A. Rhodamine 123 (5 µM) was added to the apical side (open columns) or the basolateral side (solid columns) of the monolayers in the presence (+cyclosporine) or absence (control) of 10 µM cyclosporine A at pH 6.8/7.4. Data are the mean ± SD of 3 experiments. *P < 0.05.

Fig. 3

Transepithelial transport of (+)-tramadol and (−)-tramadol across Caco-2 cell monolayers, the effect of cyclosporine A and the influence of pH conditions. (+)-tramadol (I) and (−)-tramadol (II) (10 µM) were added to the apical side (open columns) or the basolateral side (solid columns) of the monolayers in the presence (+cyclosporine) or absence (control) of 10 µM cyclosporine A at pH 6.8/7.4 (A) and pH 7.4/7.4 (B). Data are the mean ± SD of three experiments. *P < 0.05.

Fig. 4

Transepithelial transport of O-desmethyl-tramadol across Caco-2 cell monolayers, the effect of cyclosporine A and the influence of pH conditions. O-desmethyl-tramadol (10 µM) was added to the apical side (open columns) or the basolateral side (solid columns) of the monolayers in the presence (+cyclosporine) or absence (control) of 10 µM cyclosporine A at pH 6.8/7.4 (A) and pH 7.4/7.4 (B). Data are the mean ± SD of three experiments. *P < 0.05.