Loperamide is a peripherally acting antidiarrheal opioid with some affinity for P-glycoprotein (P-gp). One of the main reasons for its lack of central nervous system (CNS) activity is a combination first-pass metabolism and P-gp-mediated efflux preventing brain penetration. It was assumed that P-gp would also have a similar effect at the intestinal tract, limiting loperamide systemic absorption. However, previous in vitro studies had not determined loperamide flux using pH gradients present in the intestinal tract. Hence, our aim was to determine the influence of pH gradient conditions on the gastrointestinal uptake of loperamide, including any changes to its P-gp-mediated efflux.
Methods: Cellular uptake and transcellular transport were determined after exposure to various concentrations of loperamide (2-50 microM) with and without the presence of active efflux protein inhibitors. Loperamide was detected at 214 nm using high-performance liquid chromatography (HPLC) protocols.
Results: Bidirectional transport studies of 10 microM loperamide with a pH 6.0/7.4 apical (Ap)-to-basolateral (Bas) gradient showed efflux to be 17-fold higher than influx (10 ng/cm2/min Bas-->Ap compared to 0.6 for Ap-->Bas). This differential was much greater than when examined at pH 7.4/7.4 (only two-fold higher). The potent P-gp inhibitor, PSC-833, had only a moderate effect at blocking loperamide efflux under pH gradient conditions, yet could equilibrate bidirectional transport at pH 7.4. This suggested the presence of significant P-gp independent mechanisms, preventing loperamide access to the basolateral chamber. Amiloride and 5-(N-ethyl-N-isopropyl) amiloride had some effect on reducing efflux, hence the Na(+)--H(+) antiporter may have some involvement. Accumulation of loperamide into Caco-2 cells reduced almost 70% at pH 6.0 compared to pH 7.4, yet P-gp was always able to approximately double the equilibrium concentration in the cells within a defined pH study. This showed that P-gp was not affected by pH conditions.
Conclusions: P-gp-mediated efflux of loperamide is supplemented under pH gradient conditions. Hence, drugs used to decrease acid secretion in the stomach could result in higher plasma loperamide levels based on our in vitro system reflecting the in vivo environment. The addition of a P-gp inhibitor could potentially further increase the gastrointestinal absorption of loperamide.