Because the significance of P-glycoprotein in the in-vivo secretion of beta-blockers in intestinal epithelial cells is unclear, the secretory mechanism for beta-blockers and other drugs has been evaluated. Uptake of the beta-blockers acebutolol, celiprolol, nadolol and timolol, and the antiarrhythmic agent, quinidine by the multidrug-resistant leukaemic cell line variant K562/ADM was significantly lower than that by drug-sensitive K562 cells, suggesting that these beta-blockers are transported by P-glycoprotein out of cells. The reduced uptake of acebutolol by the drug-resistant K562/ADM cells was reversed by treating the cells with anti-P-glycoprotein monoclonal antibody, MRK16, whereas no such alteration in uptake was observed for drug-sensitive K562 cells. Acebutolol uptake by K562/ADM cells was, moreover, markedly enhanced, in a concentration-dependent manner, in the presence of the specific P-glycoprotein inhibitors, MS-209 and cyclosporin. Caco-2 cells were used for evaluation of the role of P-glycoprotein in intestinal permeability to drugs in-vitro. Basolateral-to-apical transport of acebutolol was twice that in the reverse direction. A similar polarized flux was also observed in the transport of vinblastine, but not in that of acetamide or mannitol. When in-vivo intestinal absorption was evaluated by the rat jejunal loop method, with simultaneous intravenous administration of a P-glycoprotein inhibitor, cyclosporin, intestinal absorption of both acebutolol and vinblastine increased 2.6- and 2.2-fold, respectively, but no such enhancement was observed in the absorption of acetamide. The effect of cyclosporin on the intestinal absorption of several drugs was further examined, and the extent of the contribution of P-glycoprotein as an absorption barrier to those drugs was evaluated. ATP depletion by occlusion of the superior mesenteric artery resulted in a clear increase in epithelial permeability to vinblastine, but not to 3-O-methylglucose or acetamide, indicating that vinblastine is secreted by ATP-dependent P-glycoprotein into the lumen. These findings demonstrate that P-glycoprotein plays a role as an absorption barrier by transporting several drugs from intestinal cells into the lumen.

Although recent evidence suggests that certain beta-lactam antibiotics are absorbed via a specific transport mechanism, its nature is unclear. To confirm whether peptide transport in the rat can be largely ascribed to the intestinal oligopeptide transporter PepT1, the transporter has been functionally characterized and its significance in the intestinal absorption of beta-lactam antibiotics was evaluated. For evaluation of transport activity complementary RNA (cRNA) of rat PepT1 was synthesized in-vitro and expressed in Xenopus laevis oocytes. cRNA induced uptake of several beta-lactam antibiotics and the dipeptide [14C]glycylsarcosine; this was specifically inhibited by various dipeptides and tripeptides but not by their constituent amino acids or by tetra- or pentapeptides. The transport activity of PepT1 for beta-lactam antibiotics correlated well with their in-vivo intestinal transport and absorption. Furthermore, mutual inhibitory effects on uptake were observed between glyclsarcosine and beta-lactam antibiotics. Hybrid depletion of the functional expression of rat PepT1 in oocytes injected with rat intestinal epithelial total mRNA was studied using an antisense oligonucleotide corresponding to the 5'-coding region of PepT1. In oocytes injected with rat mRNA pre-hybridized with the antisense oligonucleotide against rat PepT1, the uptake of [14C]glycylsarcosine was almost completely abolished, whereas its uptake was not influenced by a sense oligonucleotide for the same region of PepT1. Similarly, the uptake of beta-lactam antibiotics was also reduced by the antisense oligonucleotide against rat PepT1. These results demonstrate that the intestinal proton-coupled oligopeptide transporter PepT1 plays a predominant role in the carrier-mediated intestinal absorption of beta-lactam antibiotics and native oligopeptides in the rat.

Furosemide (frusemide) is a weakly acidic diuretic drug. Its absorption is poor and variable, in part due to its restricted sites of absorption, mainly the stomach. The narrow absorption window of this drug can be explained by pH partition theory. The purpose of this study was to investigate the feasibility of widening the absorption window of furosemide by controlling the pH in distal portions of the gastrointestinal tract with officially used additives. Methacrylate copolymer (Eudragit L100-55), hydroxypropylmethylcellulose phthalate (HP-55) and hydroxypropylmethylcellulose acetate succinate (AS-MF) were selected as additives. The pH of suspensions of these additives was about 4, and the pH was adjusted to about 6-7 by the addition of NaOH. The Eudragit L100-55 suspension was found to be the most resistant to NaOH titration. When Eudragit L100-55 was used in an in-situ ileal loop experiment in rats, the pH of the intestinal contents was significantly reduced, from 7.9+/-0.1 to 5.7+/-0.1, and the plasma concentration of furosemide 15 min after administration was about 3 times higher than that in controls, 1.81+/- 0.42microg mL(-1) vs 0.63+/-0.08 microg mL(-1). However, the plasma concentration of [14C] mannitol was not changed by the co-administration of Eudragit L100-55. Furthermore, the AUC of furosemide was significantly increased by a factor of about 1.6 relative to that in controls by the co-administration of Eudragit L100-55, to 21.4+/-4.0 microg h mL(-1) from 13.3+/- 3.9 microg h mL(-1), and the gastrointestinal pH in the midgut and ileum was significantly reduced, with most of the furosemide remaining in these segments at 2 h following the oral administration of furosemide with Eudragit L100-55 to rats. These findings clearly demonstrate that the addition of Eudragit L100-55 can increase the absorption of furosemide in distal portions of the gastrointestinal tract. In conclusion, it is feasible to widen the absorption window of furosemide by controlling the pH in distal portions of the gastrointestinal tract by the co-administration of Eudragit L100-55.

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