Absorption of ACE inhibitors from small intestine and colon

Pharmacophore-based discovery of ligands for drug transporters

The ability to identify ligands for drug transporters is an important step in drug discovery and development. It can both improve accurate profiling of lead pharmacokinetic properties and assist in the discovery of new chemical entities targeting transporters. In silico approaches, especially pharmacophore-based database screening methods have great potential in improving the throughput of current transporter ligand identification assays, leading to a higher hit rate by focusing in vitro testing to the most promising hits. In this review, the potential of different in silico methods in transporter ligand identification studies are compared and summarized with an emphasis on pharmacophore modeling. Various implementations of pharmacophore model generation, database compilation and flexible screening algorithms are also introduced. Recent successful utilization of database searching with pharmacophores to identify novel ligands for the pharmaceutically significant transporters hPepT1, P-gp, BCRP, MRP1 and DAT are reviewed and the challenges encountered with current approaches are discussed.

Dog Colonoscopy Model for Predicting Human Colon Absorption

PURPOSE

This study was conducted to develop and validate a dog colon model that predicts colon permeability in humans.

METHODS

The following compounds were studied: Class 1 highly soluble (HS)/highly permeable (HP): aminophylline, propranolol, CP-409092; Class 2 LS/HP: nifedipine; trovafloxacin, sertraline; Class 3 HS/LP: azithromycin, atenolol, CP-331684, CP-424391; Class 4 LS/LP: CJ-13610. Administration to dogs was made 30 cm cranial to the anal sphincter with a lubricated Schott Model VFS-5 flexible endoscope. The bioavailability of the compound following the colon administration in dogs, relative to the same formulation administered orally (relative bioavailability), was determined.

RESULTS

Except for atenolol, a small hydrophillic molecule, the relative bioavailability from administration to the colon of the dog correlated well with the following compound properties: high solubility and high, passive permeability > high solubility, low permeability > low solubility, high, passive permeability approximately low solubility, low permeability.

CONCLUSION

The dog colon model is proposed as a surrogate for human intubation studies when the controlled release candidate falls in BCS Classes 2 (LS/HP), 3 (HS/LP), and 4 (LS/LP). However, no human intubation or dog colon studies are required for Class 1 (HS/HP), as these compounds are likely to be well absorbed from the colon.

Pharmacokinetics and pharmacokinetic/pharmacodynamic relationships for angiotensin-converting enzyme inhibitors

The pharmacokinetic (PK) properties and the pharmacokinetic/pharmacodynamic (PK/PD) relationships for the angiotensin-converting enzyme (ACE) inhibitors (ACEIs), such as enalaprilat, benazeprilat, imidaprilat and ramiprilat, differ from those of conventional drugs. This is because of their immediate and saturable binding to an ACE pool which is partly circulating (and contributing to the measured plasma concentration), and partly noncirculating (tissular), being anchored to the endothelium of vessels and not measurable by the analytical technique. A physiologically based model is required to allow appropriate interpretation of the different phases of the disposition curve of ACEI. The protracted terminal phase observed for all ACEIs is not a conventional elimination phase but a phase dependent on ACEI dissociation from ACE. In contrast, the phase which reflects ACEI elimination (and which is interpreted as a distribution phase for a conventional drug) has a short half-life, thus explaining the absence of drug accumulation during repeated dosing and mild kidney failure. ACE inhibition is the surrogate endpoint generally selected for establishing a PK/PD relationship and for simulating dosage regimen scenarios in order to decide on the appropriate dosage regimen for ACEIs.

Current perspectives on established and putative mammalian oligopeptide transporters

Peptides and peptide-based drugs are increasingly being utilized as therapeutic agents for the treatment of numerous disorders. The increasing development of peptide-based therapeutic agents is largely due to technological advances including the advent of combinatorial peptide libraries, peptide synthesis strategies, and peptidomimetic design. Peptides and peptide-based agents have a broad range of potential clinical applications in the treatment of many disorders including AIDS, hypertension, and cancer. Peptides are generally hydrophilic and often exhibit poor passive transcellular diffusion across biological barriers. Insights into strategies for increasing their intestinal absorption have been derived from the numerous studies demonstrating that the absorption of protein digestion products occurs primarily in the form of small di- and tripeptides. The characterization of the pathways of intestinal, transepithelial transport of peptides and peptide-based drugs have demonstrated that a significant degree of absorption occurs through the role of proteins within the proton-coupled, oligopeptide transporter (POT) family. Considerable focus has been traditionally placed on Peptide Transporter 1 (PepT1) as the main mammalian POT member regulating intestinal peptide absorption. Recently, several new POT members, including Peptide/Histidine Transporter 1 (PHT1) and Peptide/Histidine Transporter 2 (PHT2) and their splice variants have been identified. This has led to an increased need for new experimental methods enabling better characterization of the biophysical and biochemical barriers and the role of these POT isoforms in mediating peptide-based drug transport.

Effect of buffer media composition on the solubility and effective permeability coefficient of ibuprofen

The effect of perfusion medium composition on the two important biopharmaceutical parameters drug solubility and permeability was determined for ibuprofen. Eight commonly used buffers were examined. Equilibrium solubility, buffer capacity profiles and permeability coefficients, using the in situ rat gut perfusion model, were determined for each medium at 37 degrees C. The solubility of ibuprofen differed sixfold over the range of buffer systems studied. The differences in solubility were associated with different pHs of the buffers when saturated with drug and also the presence of micelles and divalent ions. The solubility of ibuprofen in FeSSIF was significantly higher than predicted from the pH due to micellisation, while that in Krebs was significantly lower due to ibuprofen-calcium salt formation. Buffer capacities varied over a 40-fold range. The pK(a) values of the buffer components were determined from the buffer capacity versus pH profiles and were in good agreement with the thermodynamic values when corrected for temperature and ionic strength. Smaller, but statistically significant differences in P(app) values for ibuprofen were also observed between some of the buffers. During perfusion, pHs of the perfusate samples gradually changed over time towards a median value of approximately 6.5. HBSS gave a P(app) approximately 50% greater than that observed in PBS 7.4. Physicochemical factors such as medium pH, buffer capacity and osmolarity should be considered when determining the P(app) values of ionisable compounds. Care needs to be exercised when comparing P(app) values from different laboratories as buffer composition can have a significant effect on both solubility and permeability of a drug, whose ionisation is substantially changed over the pH range of the buffers. Despite the high amount ionised, ibuprofen appears to be well absorbed and it can be classified as a highly permeable drug.

Structural biology and function of solute transporters: implications for identifying and designing substrates

Solute carrier (SLC) proteins have critical physiological roles in nutrient transport and may be utilized as a mechanism to increase drug absorption. However, we have little understanding of these proteins at the molecular level due to the absence of high-resolution crystal structures. Numerous efforts have been made in characterizing the peptide transporter (PepT1) and the apical sodium dependent bile acid transporter (ASBT) that are important for both their native transporter function as well as targets to increase absorption and act as therapeutic targets. In vitro and computational approaches have been applied to gain some insight into these transporters with some success. This represents an opportunity for optimizing molecules as substrates for the solute transporters and providing a further screening system for drug discovery. Clearly the future growth in knowledge of SLC function will be led by integrated in vitro and in silico approaches.

Modeling of active transport systems

Transport proteins have critical physiological roles in nutrient transport and may be utilized as a mechanism to increase drug absorption. However, we have little understanding of these proteins at the molecular level due to the absence of high-resolution crystal structures. Numerous efforts have been made to characterize the P-glycoprotein efflux pump, the peptide transporter (PepT1) and the apical sodium-dependent transporter (ASBT) which are important not only for their native transporter function but also as drug targets to increase absorption and bioactivity. In vitro and computational approaches have been applied to gain some insight into these transporters with some success. This represents an opportunity for optimizing molecules as substrates for the solute transporters and providing a further screening system for drug discovery. Clearly the future growth in knowledge of transporter function will be led by integrated in vitro and in silico approaches.

Epidermal growth factor inhibits glycylsarcosine transport and hPepT1 expression in a human intestinal cell line

The human intestinal cell line Caco-2 was used as a model system to study the effects of epidermal growth factor (EGF) on peptide transport. EGF decreased apical-to-basolateral fluxes of [(14)C]glycylsarcosine ([(14)C]Gly-Sar) up to 50.2 +/- 3.6% (n = 6) of control values. Kinetic analysis of the fluxes showed that maximal flux (V(max)) of transepithelial transport decreased from 3.00 +/- 0.17 nmol x cm(-2) x min(-1) in control cells to 0.50 +/- 0.07 nmol x cm(-2) x min(-1) in cells treated with 5 ng/ml EGF (n = 6, P < 0.01). The apparent Michaelis-Menten constant (K(m)) was 2.71 +/- 0.31 mM (n = 6) in control cells and 1.89 +/- 0.28 mM (n = 6, not significantly different from control) in EGF-treated cells. Similarly, apical uptake of [(14)C]Gly-Sar decreased in cells treated with EGF, with an ED(50) value of 0.36 +/- 0.06 ng/ml (n = 6) EGF and a maximal inhibition of 80 +/- 0.02% (n = 6). V(max) decreased from 2.61 +/- 0.4 to 1.06 +/- 0.1 nmol x cm(-2) x min(-1) (n = 3, P < 0.05), whereas K(m) remained constant. Basolateral Gly-Sar uptake showed no changes in V(max) or K(m) after EGF treatment (n = 3). RT-PCR showed a decrease in hPepT1 mRNA (using glucose-6-phosphate dehydrogenase mRNA as control) in cells treated with EGF. Western blotting indicated a decrease in hPepT1 protein in cell lysates. We conclude that EGF treatment decreases Gly-Sar transport in Caco-2 cells by decreasing the number of peptide transporter molecules in the apical membrane.

Model prodrugs for the intestinal oligopeptide transporter: model drug release in aqueous solution and in various biological media

The human intestinal di/tri-peptide carrier, hPepT1, has been suggested as a target for increasing intestinal transport of low permeability compounds by creating prodrugs designed for the transporter. Model ester prodrugs using the stabilized dipeptides D-Glu-Ala and D-Asp-Ala as pro-moieties for benzyl alcohol have been shown to have affinity for hPepT1. Furthermore, in aqueous solution at pH 5.5 to 10, the release of the model drug seems to be controlled by a specific base-catalyzed hydrolysis, indicating that the compounds may remain relatively stable in the upper small intestinal lumen with a pH of approximately 6.0, but still release the model drug at the intercellular and blood pH of approximately 7.4. Even though benzyl alcohol is not a low molecular weight drug molecule, these results indicate that the dipeptide prodrug principle is a promising drug delivery concept. However, the physico-chemical properties such as electronegativity, solubility, and log P of the drug molecule may also have an influence on the potential of these kinds of prodrugs. The purpose of the present study is to investigate whether the model drug electronegativity, estimated as Taft substitution parameter (sigma*) may influence the acid, water or base catalyzed model drug release rates, when released from series of D-Glu-Ala and D-Asp-Ala pro-moieties. Release rates were investigated in both aqueous solutions with varying pH, ionic strength, and buffer concentrations as well as in in vitro biological media. The release rates of all the investigated model drug molecules followed first-order kinetics and were dependent on buffer concentration, pH, ionic strength, and model drug electronegativity. The electronegativity of the model drug influenced acid, water and base catalyzed release from D-Asp-Ala and D-Glu-Ala pro-moieties. The model drug was generally released faster from D-Asp-Ala- than from the D-Glu-Ala pro-moieties. In biological media the release rate was also dependent on the electronegativity of the model drug. These results demonstrate that the model drug electronegativity, estimated as Taft (sigma*) values, has a significant influence on the release rate of the model drug.

A rapid screening system to determine drug affinities for the intestinal dipeptide transporter 2: affinities of ACE inhibitors

PURPOSE

To assess the affinities of a series of ACE inhibitors for the di/tri/oligopeptide transport system (DTS) using a rapid in vitro system.

METHODS

Monolayers of Caco-2 cells were cultured in plastic wells for 7-9 days and the uptake of Gly-[3H]L-Pro was used as an affinity probe. Gly-[3H]L-Pro (50 nM), together with excess L-Pro (10 mM), to suppress uptake of any [3H]L-Pro produced by degradation of the probe, was incubated with the test compound (usually 1 mM) at pH 6 for 3-mins. The uptake of radiolabel was determined by liquid scintillation counting.

RESULTS

A 2-dimensional six-domain model of the transporter based on the structure of a phosphinate ACE inhibitor (SQ-29852) was constructed to facilitate interpretation of the competitor affinities. The SQ-29852 molecule was divided into six binding domains (A-F) based on functional groups within these regions and the effects of structural variation in four of these domains (A, C-E) were explored. A series of dipeptide-like compounds varying within specific domains were selected from a large number of commercially available ACE inhibitors and SQ-29852 analogues. Domain A had a preference for an uncharged group, with bulky hydrophobic groups reducing affinity. Domain C exhibited a preference for a positive charge over a neutral function, with the space this functional group occupies contributing to affinity. Domain D favoured lipophilic residues and domain E retained activity when the carboxylic acid was esterified.

CONCLUSION

The test system is able to reveal structure-activity relationships of peptidomimetic agents and may well serve as a design tool to optimise affinity for the DTS.

Intestinal peptide transport systems and oral drug availability

The intestinal peptide transport system has broad substrate specificities. In addition to its physiological function of absorbing di- and tripeptides resulting from the digestion of dietary proteins, this transport system also absorbs some orally administered peptidomimetic drugs, including beta-lactam antibiotics, angiotensin converting enzyme inhibitors, renin inhibitors, bestatin, thrombin inhibitors, and thyrotropin-releasing hormone and its analogues. There have been several studies on the mechanism and substrate structure-affinity relationship for this transport system. Rapid progress has been made recently in studies on the molecular basis of the intestinal peptide transport system. A protein apparently involved in peptide transport has been isolated from rabbit small intestines, and genes for human intestinal peptide transporters have been cloned, sequenced and functionally expressed. This review summarizes these studies and addresses the pharmaceutical potential of the intestinal peptide transport system.

Drug, meal and formulation interactions influencing drug absorption after oral administration. Clinical implications

Drug-drug, drug-formulation and drug-meal interactions are of clinical concern for orally administered drugs that possess a narrow therapeutic index. This review presents the current status of information regarding interactions which may influence the gastrointestinal (GI) absorption of orally administered drugs. Absorption interactions have been classified on the basis of rate-limiting processes. These processes are put in the context of drug and formulation physicochemical properties and oral input influences on variable GI physiology. Interaction categorisation makes use of a biopharmaceutical classification system based on drug aqueous solubility and membrane permeability and their contributions towards absorption variability. Overlaying this classification it is important to be aware of the effect that the magnitudes of drug dosage and volume of fluid administration can have on interactions involving a solubility rate limits. GI regional differences in membrane permeability are fundamental to the rational development of extended release dosage forms as well as to predicting interaction effects on absorption from immediate release dosage forms. The effect of meals on the regional-dependent intestinal elimination of drugs and their involvement in drug absorption interactions is also discussed. Although the clinical significance of such interactions is certainly dependent on the narrowness of the drug therapeutic index, clinical aspects of absorption delays and therapeutic failures resulting from various interactions are also important.

Colonic drug targeting

Specific targeting of drugs to the colon is recognized to have several therapeutic advantages. Drugs which are destroyed by the stomach acid and/or metabolized by pancreatic enzymes are slightly affected in the colon, and sustained colonic release of drugs can be useful in the treatment of nocturnal asthma, angina and arthritis. Treatment of colonic diseases such as ulcerative colitis, colorectal cancer and Crohn's disease is more effective with direct delivery of drugs to the affected area. Likewise, colonic delivery of vermicides and colonic diagnostic agents require smaller doses. This article is aimed at providing insight into the design considerations and evaluation of colonic drug delivery systems. For this purpose, the anatomy and physiology of the lower gastrointestinal tract are surveyed. Furthermore, the biopharmaceutical aspects are considered in relation to drug absorption in the colon and hence various approaches to colon-specific drug delivery are discussed.

Molecular determinants of recognition for the intestinal peptide carrier

Computer-aided conformational analysis was used to characterize the pharmacophore for the intestinal peptide carrier. The active analog approach to pharmacophore building was applied as implemented in the SYBYL software package. Conformational analysis and MOPAC calculations were used to determine the lowest energy conformation of carrier substrates, as well as the conformations of compounds that displayed a common pharmacophoric geometry (i.e., inhibitors and inactive structural analogs). A pharmacophore map was calculated, and based on structural mutualities and functional topology, three substrate groups were suggested: compounds that bind to the transporter and are transferred across the membrane; compounds that show affinity for the peptide carrier (i.e., known to inhibit transport of substrates) but are not transferred across the membrane; and compounds that contain the pharmacophoric geometry but show no affinity for the carrier. Affinity for the peptide transporter can be diminished or abolished in either of three ways: esterification of the free carboxylic acid moiety; introduction of a second negative group; and intramolecular steric hindrance of the free carboxylic acid by either side chains with a positively charged nitrogen function or groups capable of hydrogen bond formation.

Colonic absorption of antiepileptic agents

PURPOSE

To evaluate a canine intestinal accessport model to study colonic absorption of drugs. The antiepileptic drugs phenytoin and gabapentin were chosen to study absorption of a lipophilic and hydrophilic compound, respectively.

METHODS

Drug plasma level-time plots were generated subsequent to small intestinal and colonic drug administration of both drugs. The poorly water-soluble phenytoin was administered in two doses to evaluate the impact of dissolution rate limits on colonic absorption. Maximal plasma concentration (Cmax) and area under the plasma level-time curve (AUC) were used to assess the relative contribution of colonic absorption to plasma levels.

RESULTS

Whereas colonic gabapentin AUC and Cmax were only 0.25 and 0.15 of those seen after small intestinal administration, colonic phenytoin AUC and Cmax were one half and equivalent to, respectively, those observed for small intestinal administration. Furthermore, colonic administration of a higher phenytoin dose showed secondary maxima and continued increases in drug plasma levels with time.

CONCLUSIONS

Colonic gabapentin absorption is poor compared with upper intestinal absorption, consistent with membrane transport rate limits to the absorption of this hydrophilic AED. Peak phenytoin plasma levels from colonic and small intestinal administration are comparable, indicating membrane transport does not limit absorption of this lipophilic agent. Continued plasma-level increases from higher phenytoin doses are consistent with dissolution-rate control of drug absorption in the colon. We suggest that colonic absorption provides a greater potential for toxicity from phenytoin overdose as a function of continued drug dissolution than for gabapentin overdose.

Distribution of the dipeptide transporter system along the gastrointestinal tract of rats based on absorption of a stable and specific probe, SQ-29852

Peptidic drugs such as beta-lactam aminocephalosporin antibiotics (e.g., cephalexin) and the ACE inhibitors lisinopril, quinapril, and benzazepril are apparently absorbed, at least in part, by the intestinal dipeptide transporter system (DTS). Although many properties of the DTS have been elucidated, including isolation of the carrier protein, little is known about the distribution of this transporter along the gastrointestinal (GI) tract. The objectives of the present study were to (1) validate that SQ-29852 (a lysylproline ACE inhibitor) is a stable and specific probe for evaluation of the DTS in rats and (2) provide fundamental in vivo information on the distribution of the DTS along the GI tract of rats. Most of the previous studies that explored the location of the DTS typically involved either in vitro uptake or in situ disappearance of unstable or nonspecific probes. SQ-29852, on the other hand, is an ideal probe for evaluation of the DTS because it is chemically and metabolically stable and it is absorbed almost exclusively by the DTS. SQ-29852 appears to be a specific probe for the DTS because the dose-dependent reduction in absorption from about 60% to less than 8% (3 and 3000 mg/kg, respectively) suggests that at least 85% of an orally administered low dose of SQ-29852 is absorbed by a saturable process, which was shown previously to be the DTS. [14C]SQ-29852 was administered by gavage to intact rats and via an indwelling cannula in one of the following sections of the intestine: duodenum, jejunum, ileum and proximal colon (n = 4 for each site). On the basis of the recovery of [14C]SQ-29852 in urine, the DTS is apparently distributed throughout the entire GI tract of rats, including the proximal colon. The present results are consistent with previously reported results on the absorption of natural dipeptides in humans and rats and immunohistochemical evaluation in rats; however, they disagree with a recent report in humans with amoxicillin. This difference is discussed in terms of the specificity and stability of various drugs that have been used as probes of the DTS.