Role and Relevance of PEPT1 in Intestinal Absorption and Pharmacokinetics of 5-Aminolevulinic Acid.

Abstract

Peptide transporter 1 (PepT1), a member of the proton-coupled oligopeptide transporter family, is known to transport di-/tri-peptides and peptidomimetics across biological membranes. Due to its abundant expression in small intestine, broad substrate specificity and high transport capacity, PepT1 is considered an ideal oral drug delivery target and plays a pivotal role in transporting numerous pharmacological compounds. The therapeutic agent 5-aminolevulinic acid (5-ALA) is widely applied in photodynamic therapy and fluorescence diagnosis for the treatment of various cancers and non-malignant diseases. In this project, experimental results from in situ perfusion and in vivo pharmacokinetic studies in wildtype and PepT1 knockout mice offer a deeper understanding of the PepT1-mediated intestinal absorption of 5-ALA. The effective permeability of 5-ALA was evaluated as a function of drug concentration, potential inhibitors and regional segments of the intestines using in situ perfusions. The results from in situ perfusions indicated that PepT1 accounted for approximately 90% of 5-ALA permeability in mouse small intestine. The differential segmental permeability of 5-ALA was consistent with PepT1 expression along the intestinal segments, in which high permeabilities were observed in the duodenum, jejunum and ileum of wildtype mice with little permeability in colon. In contrast, the residual permeability of 5-ALA in the duodenum, jejunum, and ileum of PepT1 knockout mice was only about 10% of that in wildtype mice and similar to that of colon permeability. The contribution of other transporters, including the amino acid transporter PAT1, in mediating the intestinal permeability of 5-ALA was minor at best. After oral administration (0.2 and 2 micro mol/g) of 5-ALA, the maximum plasma concentration (Cmax) and area under the plasma concentration-time curve (AUC) of 5-ALA were decreased approximately 2-fold in PepT1 knockout mice as compared to wildtype mice. The tissue distribution results after 0.2 micro mol/g 5-ALA oral dose and intravenous pharmacokinetic study (0.01 micro mol/g) revealed that PepT1 had marginal, if any, effect on the in vivo disposition of 5-ALA. In conclusion, research in this dissertation project offered solid evidence in defining the significant role of PepT1 on the intestinal absorption and systemic exposure of 5-ALA after oral dosing.