Regional Intestinal Drug Permeability and Effects of Permeation Enhancers in Rat

Abstract

Sufficient colonic absorption is necessary for all systemically acting drugs in dosage forms that release the drug in the large intestine. Preclinically, colonic absorption is often investigated using the rat single-pass intestinal perfusion model. This model can determine intestinal permeability based on luminal drug disappearance, as well as the effect of permeation enhancers on drug permeability. However, it is uncertain how accurate the rat single-pass intestinal perfusion model predicts regional intestinal permeability and absorption in human. There is also a shortage of systematic in vivo investigations of the direct effect of permeation enhancers in the small and large intestine. In this rat single-pass intestinal perfusion study, the jejunal and colonic permeability of two low permeability drugs (atenolol and enalaprilat) and two high-permeability ones (ketoprofen and metoprolol) was determined based on plasma appearance. These values were compared to already available corresponding human data from a study conducted in our lab. The colonic effect of four permeation enhancers-sodium dodecyl sulfate, chitosan, ethylenediaminetetraacetic acid (EDTA), and caprate-on drug permeability and transport of chromium EDTA (an established clinical marker for intestinal barrier integrity) was determined. There was no difference in jejunal and colonic permeability determined from plasma appearance data of any of the four model drugs. This questions the validity of the rat single-pass intestinal perfusion model for predicting human regional intestinal permeability. It was also shown that the effect of permeation enhancers on drug permeability in the colon was similar to previously reported data from the rat jejunum, whereas the transport of chromium EDTA was significantly higher (p < 0.05) in the colon than in jejunum. Therefore, the use of permeation enhancers for increasing colonic drug permeability has greater risks than potential medical rewards, as indicated by the higher permeation of chromium EDTA compared to the drugs.

Keywords: absorption-modifying excipients; intestinal perfusion; oral peptide delivery; permeation enhancers; pharmaceutical development; regional intestinal permeability.

Figure 1

The mean ±SEM rat plasma concentration–time profiles (n = 30 for colon at pH 7.4, and n = 6 for the other three groups) of: (a) atenolol, (b) enalaprilat, (c) ketoprofen, and (d) metoprolol following single-pass jejunal and colonic perfusions of the pH 6.5 and 7.4 control solutions (0–60 min). These plasma data were used to determine regional intestinal basal P_eff values using Equation (2) (Table 2).

Figure 2

The mean ±SEM rat colonic plasma concentration–time profiles (n = 6) of: (a) atenolol, (b) enalaprilat, (c) ketoprofen, and (d) metoprolol following single-pass intestinal perfusions of a control solution for 60 min, followed by a 75-min perfusion of any of six test formulations containing a permeation enhancer (PE). The control solution and all test formulations contained 100 µM atenolol, enalaprilat, ketoprofen, and metoprolol. The control and test formulation perfusate pH was 6.5 for the PEs: sodium dodecyl sulfate (SDS) at 1 and 5 mg/mL, chitosan at 5 mg/mL, and ethylenediaminetetraacetic acid (EDTA) at 1 and 5 mg/mL. The control and test formulation perfusate pH was 7.4 for caprate at 10 mg/mL. All formulations were solutions, except caprate which was a suspension (its solubility at pH 7.4 is 5 mg/mL).

Figure 3

The P_eff ratio between the jejunum and colon at pH 6.5 in rat of atenolol, enalaprilat, ketoprofen, and metoprolol (Table 2). The historical human and dog P_eff ratios between the jejunum and colon at pH 6.5 of atenolol, enalaprilat (not human), ketoprofen, and metoprolol are also presented for species comparison [4,19].

Figure 4

The mean ±SEM rat jejunal (historical data) and colonic lumen-to-blood intestinal effective permeability (P_eff) ratio (n = 6) of: (a) atenolol, (b) enalaprilat, (c) ketoprofen, and (d) metoprolol, after intestinal perfusions of a control solution for 60 min, followed by a 75-min perfusion of any of six permeation enhancing (PE) test formulations [11,12]. The control and test formulation perfusate pH was 6.5 for the PEs: sodium dodecyl sulfate (SDS) at 1 and 5 mg/mL, chitosan at 5 mg/mL, and ethylenediaminetetraacetic acid (EDTA) at 1 and 5 mg/mL. The control and test formulation perfusate pH was 7.4 for caprate at 10 mg/mL. All formulations were solutions, except caprate which was a suspension (its solubility at pH 7.4 is 5 mg/mL). There is no jejunal historical data for metoprolol and only jejunal historical data for EDTA and caprate for enalaprilat. A * represents a significant difference in jejunal and colonic P_eff (two-way ANOVA, Holm–Sidak).

Figure 5

The mean ±SEM rat jejunal (historical data) and colonic blood-to-lumen ⁵¹Cr-EDTA clearance (CL_Cr-EDTA) ratio (n = 6), after intestinal perfusions of a control solution for 60 min, followed by a 75-min perfusion of any of six permeation enhancing (PE) test formulations. The control and test formulation perfusate pH was 6.5 for the PEs: sodium dodecyl sulfate (SDS) at 1 and 5 mg/mL, chitosan at 5 mg/mL, and ethylenediaminetetraacetic acid (EDTA) at 1 and 5 mg/mL. The control and test formulation perfusate pH was 7.4 for caprate at 10 mg/mL. All formulations were solutions, except caprate which was a suspension (its solubility at pH 7.4 is 5 mg/mL). A * represents a significant difference in jejunal and colonic CL_Cr-EDTA ratio (two-way ANOVA, Holm–Sidak).