Mechanistic analysis and experimental verification of bicarbonate-controlled enteric coat dissolution: Potential in vivo implications

Enteric coatings have shown in vivo dissolution rates that are poorly predicted by traditional in vitro tests, with the in vivo dissolution being considerably slower than in vitro. To provide a more mechanistic understanding of this, the dependence of the release properties of various enteric-coated (EC) products on bulk pH and bicarbonate molarity was investigated. It was found that, at presumably in vivo-relevant values, the bicarbonate molarity is a more significant determinant of the dissolution profile than the bulk pH. The findings also indicate that this steep relationship between the dissolution of enteric coatings and bicarbonate molarity limits those coatings' performance in vivo. This is attributed to the relatively low bicarbonate molarities in human intestinal fluids. Further, the hydration and dehydrations kinetics of carbonic acid and carbon dioxide are not sufficiently rapid to reach equilibrium in the diffusion layer surrounding a dissolving ionizable solid. This results in the effective pKa of bicarbonate in the diffusion layer being lower than that determined potentiometrically at equilibrium in the bulk surrounding fluid. These results demonstrate the importance of thoroughly investigating the intestinal bicarbonate concentrations and using bicarbonate buffers or properly designed surrogates (if possible) when evaluating enteric drug products during product development and quality control.

Mechanistic Oral Absorption Modeling and Simulation for Formulation Development and Bioequivalence Evaluation: Report of an FDA Public Workshop

On May 19, 2016, the US Food and Drug Administration (FDA) hosted a public workshop, entitled “Mechanistic Oral Absorption Modeling and Simulation for Formulation Development and Bioequivalence Evaluation.”1 The topic of mechanistic oral absorption modeling, which is one of the major applications of physiologically based pharmacokinetic (PBPK) modeling and simulation, focuses on predicting oral absorption by mechanistically integrating gastrointestinal transit, dissolution, and permeation processes, incorporating systems, active pharmaceutical ingredient (API), and the drug product information, into a systemic mathematical whole‐body framework.2

Toward Biopredictive Dissolution for Enteric Coated Dosage Forms

The aim of this work was to develop a phosphate buffer based dissolution method for enteric-coated formulations with improved biopredictivity for fasted conditions. Two commercially available enteric-coated aspirin products were used as model formulations (Aspirin Protect 300 mg, and Walgreens Aspirin 325 mg). The disintegration performance of these products in a physiological 8 mM pH 6.5 bicarbonate buffer (representing the conditions in the proximal small intestine) was used as a standard to optimize the employed phosphate buffer molarity. To account for the fact that a pH and buffer molarity gradient exists along the small intestine, the introduction of such a gradient was proposed for products with prolonged lag times (when it leads to a release lower than 75% in the first hour post acid stage) in the proposed buffer. This would allow the method also to predict the performance of later-disintegrating products. Dissolution performance using the accordingly developed method was compared to that observed when using two well-established dissolution methods: the United States Pharmacopeia (USP) method and blank fasted state simulated intestinal fluid (FaSSIF). The resulting dissolution profiles were convoluted using GastroPlus software to obtain predicted pharmacokinetic profiles. A pharmacokinetic study on healthy human volunteers was performed to evaluate the predictions made by the different dissolution setups. The novel method provided the best prediction, by a relatively wide margin, for the difference between the lag times of the two tested formulations, indicating its being able to predict the post gastric emptying onset of drug release with reasonable accuracy. Both the new and the blank FaSSIF methods showed potential for establishing in vitro-in vivo correlation (IVIVC) concerning the prediction of Cmax and AUC0-24 (prediction errors not more than 20%). However, these predictions are strongly affected by the highly variable first pass metabolism necessitating the evaluation of an absorption rate metric that is more independent of the first-pass effect. The Cmax/AUC0-24 ratio was selected for this purpose. Regarding this metric's predictions, the new method provided very good prediction of the two products' performances relative to each other (only 1.05% prediction error in this regard), while its predictions for the individual products' values in absolute terms were borderline, narrowly missing the regulatory 20% prediction error limits (21.51% for Aspirin Protect and 22.58% for Walgreens Aspirin). The blank FaSSIF-based method provided good Cmax/AUC0-24 ratio prediction, in absolute terms, for Aspirin Protect (9.05% prediction error), but its prediction for Walgreens Aspirin (33.97% prediction error) was overwhelmingly poor. Thus it gave practically the same average but much higher maximum prediction errors compared to the new method, and it was strongly overdiscriminating as for predicting their performances relative to one another. The USP method, despite not being overdiscriminating, provided poor predictions of the individual products' Cmax/AUC0-24 ratios. This indicates that, overall, the new method is of improved biopredictivity compared to established methods.

Biowaiver monographs for immediate release solid oral dosage forms: diclofenac sodium and diclofenac potassium

Literature data are reviewed regarding the scientific advisability of allowing a waiver of in vivo bioequivalence (BE) testing for the approval of immediate release (IR) solid oral dosage forms containing either diclofenac potassium and diclofenac sodium. Within the biopharmaceutics classification system (BCS), diclofenac potassium and diclofenac sodium are each BCS class II active pharmaceutical ingredients (APIs). However, a biowaiver can be recommended for IR drug products of each salt form, due to their therapeutic use, therapeutic index, pharmacokinetic properties, potential for excipient interactions, and performance in reported BE/bioavailability (BA) studies, provided: (a) test and comparator contain the same diclofenac salt; (b) the dosage form of the test and comparator is identical; (c) the test product contains only excipients present in diclofenac drug products approved in ICH or associated countries in the same dosage form, for instance as presented in this paper; (d) test drug product and comparator dissolve 85% in 30 min or less in 900 mL buffer pH 6.8, using the paddle apparatus at 75 rpm or the basket apparatus at 100 rpm; and (e) test product and comparator show dissolution profile similarity in pH 1.2, 4.5, and 6.8.

Biowaiver Monographs for Immediate Release Solid Oral Dosage Forms: Ethambutol Dihydrochloride**This paper reflects the scientific opinion of the authors and not the policies of regulating agencies.**A project of the International Pharmaceutical Federation FIP, Groupe BCS, http://www.fip.org/bcs

Literature data relevant to the decision to allow a waiver of in vivo bioequivalence (BE) testing for the approval of immediate release (IR) solid oral dosage forms containing ethambutol dihydrochloride as the only active pharmaceutical ingredient (API) are reviewed. Ethambutol dihydrochloride is a Biopharmaceutics Classification System (BCS) Class III drug with permeability properties approaching the border between BCS Class I and III. BE problems of ethambutol formulations containing different excipients and different dosages forms have not been reported and hence the risk of bioinequivalence caused by excipients is low. Ethambutol has a narrow therapeutic index related to ocular toxicity. However, as long as the prescribers' information of the test product stipulates the need for regular monitoring of ocular toxicity, the additional patient risk is deemed acceptable. It is concluded that a biowaiver can be recommended for IR solid oral dosage forms provided that the test product (a) contains only excipients present in ethambutol IR solid oral drug products approved in ICH or associated countries, for instance as presented in this paper, (b) complies with the criteria for "very rapidly dissolving" and (c) has a prescribers' information indicating the need for testing the patient's vision prior to initiating ethambutol therapy and regularly during therapy.

Biowaiver Monographs for Immediate Release Solid Oral Dosage Forms: Isoniazid**A project of the International Pharmaceutical Federation FIP, Groupe BCS, www.fip.org/bcs

Literature data relevant to the decision to allow a waiver of in vivo bioequivalence (BE) testing for the approval of immediate release (IR) solid oral dosage forms containing isoniazid as the only active pharmaceutical ingredient (API) are reviewed. Isoniazid's solubility and permeability characteristics according to the Biopharmaceutics Classification System (BCS), as well as its therapeutic use and therapeutic index, its pharmacokinetic properties, data related to the possibility of excipient interactions and reported BE/bioavailability (BA) problems were taken into consideration. Isoniazid is "highly soluble" but data on its oral absorption and permeability are inconclusive, suggesting this API to be on the borderline of BCS Class I and III. For a number of excipients, an interaction with the permeability is extreme unlikely, but lactose and other deoxidizing saccharides can form condensation products with isoniazid, which may be less permeable than the free API. A biowaiver is recommended for IR solid oral drug products containing isoniazid as the sole API, provided that the test product meets the WHO requirements for "very rapidly dissolving" and contains only the excipients commonly used in isoniazid products, as listed in this article. Lactose and/or other deoxidizing saccharides containing formulations should be subjected to an in vivo BE study.

Rapid in vivo dissolution of ketoprofen: implications on the biopharmaceutics classification system

The aim of this paper was to investigate the in vivo dissolution behavior of ketoprofen, a Class II drug according to the Biopharmaceutics Classification System (BCS), in the upper small intestine of dogs. An intubations method was used, where no blocking balloons were used to prevent luminal drug transport along the GI tract. Our design allowed the drug to be transported freely to more distal parts of the GI tract and also, it was supported by a pharmacokinetic study. Pharmacokinetic parameters of ketoprofen were determined in dogs after administering approximately 0.27 mg kg(-1) (solution) or approximately 1.47 mg kg(-1) (suspension) in oral bolus doses. There were not statistical significant differences in plasma concentrations for both formulations, either in the maximum concentrations C(max) or AUC following oral dose administration. The rapid disappearance of ketoprofen from the intestinal lumen, reflected by low mass recovery in the supernatant and sediment of the collected intestinal fluid samples, in comparison to that recovery of the non-absorbable marker phenol red, suggests that ketoprofen is emptying into the small intestine and is rapidly dissolved and absorbed. In this study, the in vivo results clearly show that the absorption rate of ketoprofen is not dissolution limited; therefore ketoprofen would be essentially equivalent to Class I drugs and could be considered for waiver of bioavailability and bioequivalence testing.

Biowaiver Monographs for Immediate Release Solid Oral Dosage Forms: Amitriptyline Hydrochloride**This paper reflects the scientific opinion of the authors and not the policies of regulating agencies

Literature data relevant to the decision to allow a waiver of in vivo bioequivalence (BE) testing for the approval of immediate release (IR) solid oral dosage forms containing amitriptyline hydrochloride are reviewed. Its therapeutic uses, its pharmacokinetic properties, the possibility of excipient interactions and reported BE/bioavailability (BA) problems are also taken into consideration. Literature data indicates that amitriptyline hydrochloride is a highly permeable active pharmaceutical ingredient (API). Data on the solubility according to the current Biopharmaceutics Classification System (BCS) were not fully available and consequently amitriptyline hydrochloride could not be definitively assigned to either BCS Class I or BCS Class II. But all evidence taken together, a biowaiver can currently be recommended provided that IR tablets are formulated with excipients used in existing approved products and that the dissolution meets the criteria defined in the Guidances.

Biowaiver Monographs for Immediate Release Solid Oral Dosage Forms: Cimetidine**This paper reflects the scientific opinion of the authors and not the policies of regulating agencies

Literature data relevant to the decision to allow a waiver of in vivo bioequivalence (BE) testing for the approval of immediate release (IR) solid oral dosage forms containing cimetidine are reviewed. According to the current Biopharmaceutics Classification System (BCS), cimetidine would be assigned to Class III. Cimetidine's therapeutic use and therapeutic index, its pharmacokinetic properties, data related to the possibility of excipient interactions, and reported BE/bioavailability (BA) problems were also taken into consideration. On the basis of the overall evidence, a biowaiver can be recommended for cimetidine IR products, provided that the test product contains only those excipients reported in this paper in their usual amounts, and that the test and the comparator drug products both are "rapidly dissolving" as per BCS.

Biowaiver monographs for immediate release solid oral dosage forms: Acetaminophen (paracetamol)

Literature data are reviewed on the properties of acetaminophen (paracetamol) related to the biopharmaceutics classification system (BCS). According to the current BCS criteria, acetaminophen is BCS Class III compound. Differences in composition seldom, if ever, have an effect on the extent of absorption. However, some studies show differences in rate of absorption between brands and formulations. In particular, sodium bicarbonate, present in some drug products, was reported to give an increase in the rate of absorption, probably caused by an effect on gastric emptying. In view of Marketing Authorizations (MAs) given in a number of countries to acetaminophen drug products with rapid onset of action, it is concluded that differences in rate of absorption were considered therapeutically not relevant by the Health Authorities. Moreover, in view of its therapeutic use, its wide therapeutic index and its uncomplicated pharmacokinetic properties, in vitro dissolution data collected according to the relevant Guidances can be safely used for declaring bioequivalence (BE) of two acetaminophen formulations. Therefore, accepting a biowaiver for immediate release (IR) acetaminophen solid oral drug products is considered scientifically justified, if the test product contains only those excipients reported in this paper in their usual amounts and the test product is rapidly dissolving, as well as the test product fulfils the criterion of similarity of dissolution profiles to the reference product.

Biowaiver monographs for immediate release solid oral dosage forms based on biopharmaceutics classification system (BCS) literature data: Verapamil hydrochloride, propranolol hydrochloride, and atenolol

Literature data related to the Biopharmaceutics Classification System (BCS) are presented on verapamil hydrochloride, propranolol hydrochloride, and atenolol in the form of BCS-monographs. Data on the qualitative composition of immediate release (IR) tablets containing these active substances with a Marketing Authorization (MA) in the Netherlands (NL) are also provided; in view of these MA's the assumption was made that these tablets were bioequivalent to the innovator product. The development of a database with BCS-related data is announced by the International Pharmaceutical Federation (FIP).

Enhanced cytosolic delivery of plasmid DNA by a sulfhydryl-activatable listeriolysin O/protamine conjugate utilizing cellular reducing potential

Listeriolysin O (LLO), a sulfhydryl-activated pore-forming protein from Listeria monocytogenes, was tested and utilized for promoting plasmid DNA (pDNA) delivery into the cytosol of cells in culture. To render pDNA-complexing capability to LLO, the unique cysteine 484 of LLO was conjugated to polycationic peptide protamine (PN) at a 1:1 molar ratio through a reversible, endosome-labile disulfide bond. The sulfhydryl-oxidized LLO construct, LLO-s-s-PN, completely lacked its pore-forming activity, yet regained its original activity upon reduction. The enhanced cytosolic delivery using this construct therefore relies on the requisite reduction of the disulfide bond in LLO-s-s-PN by endogenous cellular reducing capacity. Condensed PN/pDNA complexes incorporating LLO-s-s-PN were tested for their enhanced gene delivery capability monitoring reporter gene expression in HEK293, RAW264.7, P388D1 cell lines and bone-marrow-derived macrophages in the presence of serum. Dramatic enhancement was observed for all tested complexes with varying weight ratios. The effect was most prominent at 0.64-0.80 (w/w) of PN/pDNA upon replacing 1-4% of PN with LLO-s-s-PN, resulting in approximately three orders of magnitude higher luciferase expression compared to PN/pDNA without apparent toxicity. These results demonstrate that incorporation of endosomolytic LLO into pDNA delivery systems in a controlled fashion is a promising approach of enhancing delivery into the cytosol of target cells in gene delivery strategies.

The effect of amiloride on the in vivo effective permeability of amoxicillin in human jejunum: experience from a regional perfusion technique

The purpose of this human intestinal perfusion study (in vivo) was twofold. Firstly, we aimed to determine the effective in vivo jejunal permeability (P(eff)) of amoxicillin and to classify it according to the Biopharmaceutics Classification System (BCS). Secondly, we investigated the acute effect of amiloride on the jejunal P(eff) of amoxicillin. Amoxicillin, a beta-lactam antibiotic, has been reported to be absorbed across the intestinal mucosa by both passive diffusion and active transport. A regional single-pass perfusion of the jejunum was performed using a Loc-I-Gut perfusion tube in 14 healthy volunteers. Each perfusion lasted for 200 min and was divided into two periods of 100 min each. The concentration of amoxicillin entering the jejunal segment was 300 mg/l in both periods, and amiloride, an inhibitor of the Na+/H+ exchanger, was added at 25 mg/l in period 2. The concentrations of amoxicillin and amiloride in the outlet jejunal perfusate were measured with two different HPLC-methods. Antipyrine and [14C]PEG 4000 were added as internal standards to the perfusion solution. Amiloride had no significant effect on the jejunal P(eff) of amoxicillin. The human in vivo jejunal P(eff) for amoxicillin was 0.34+/-0.11 x 10(-4) and 0.46+/-0.12 x 10(-4) cm/s in periods 1 and 2, respectively. The high jejunal P(eff) for amiloride was 1.63+/-0.51 x 10(-4) cm/s which predicts an intestinal absorption of more than 90%. Following the BCS amoxicillin was classified as a low P(eff) drug, and amiloride had no acute effect on the in vivo jejunal P(eff) of amoxicillin.

Novel method to assess gastric emptying in humans: the Pellet Gastric Emptying Test

To further validate the Pellet Gastric Emptying Test (PGET) as a marker of gastric emptying, a randomized, four-way crossover study was conducted with 12 healthy subjects. The study consisted of oral co-administration of enteric coated caffeine (CAFF) and acetaminophen (APAP) pellets in four treatment phases: Same Size (100 kcal), Fasted, Small Liquid Meal (100 kcal), and Standard Meal (847 kcal). The time of first appearance of measurable drug marker in plasma, t(initial), was taken as the emptying time for the markers. Co-administration of same size enteric coated pellets of CAFF and APAP (0.7 mm in diameter) revealed no statistically significant differences in t(initial) values indicating that emptying was dependent only on size and not on chemical make-up of the pellets. Co-administration of different size pellets indicated that the smaller 0.7-mm diameter (CAFF) pellets were emptied and absorbed significantly earlier than the larger 3.6-mm diameter (APAP) pellets with both the Small Liquid Meal (by 35 min) and the Standard Meal (by 33 min) (P<0.05). The differences in emptying of the pellets were not significant in the Fasted Phase. The results suggest that the pellet gastric emptying test could prove useful in monitoring changes in transit times in the fasted and fed states and their impact on drug absorption.

Correlation between epithelial cell permeability of cephalexin and expression of intestinal oligopeptide transporter

The proton-coupled oligopeptide transporter (PEPT1) has been shown to mediate mucosal cell transport of di- and tripeptide, and some peptidomimetic drugs. In this study, we determined the correlation between PEPT1 protein expression and the permeability of cephalexin, a substrate of PEPT1, in human PEPT1 (hPEPT1)-overexpressed Caco-2 cells (Caco-2/hPEPT1 cells) and rat jejunum. Caco-2/hPEPT1 cells with various levels of hPEPT1 expression were established by an adenoviral transfection system. The effective intestinal permeability (P(eff)) in rat jejunum was evaluated using a single pass in situ perfusion method. The level of PEPT1 in Caco-2/hPEPT1 cells and rat intestinal mucosal samples was quantitated by densitometry after immunoblotting and enhanced chemiluminescence detection. In Caco-2/hPEPT1 cells, an excellent correlation was observed between cephalexin uptake and hPEPT1 expression (R2 = 0.96, P < 0.005). This demonstrates that cephalexin uptake is directly proportional to hPEPT1 expression. In the rat perfusion study, the mean P(eff) +/- S.D. (n = 15) of cephalexin was 3.89 +/- 1.63 x 10(-5) cm/s. A very significant correlation between PEPT1 expression and cephalexin permeability with an R2 = 0.63 (P < 0.001) was observed. This indicates that the variation in PEPT1 expression is one of the major factors accounting for variable intestinal cephalexin absorption. To our knowledge, this is the most direct evidence that variation of PEPT1 expression is correlated with absorption permeability variation of peptide-like compounds in vitro and in vivo.

Human jejunal permeability of two polar drugs: cimetidine and ranitidine

PURPOSE

To determine the human jejunal permeability of cimetidine and ranitidine using a regional jejunal perfusion approach, and to integrate such determinations with previous efforts to establish a baseline correlation between permeability and fraction dose absorbed in humans for soluble drugs.

METHODS

A sterile multi-channel perfusion tube, Loc-I-Gut, was inserted orally and positioned in the proximal region of the jejunum. A solution containing cimetidine or ranitidine and phenylalanine, propranolol, PEG 400, and PEG 4000 was perfused through a 10 cm jejunal segment in 6 and 8 subjects, respectively.

RESULTS

The mean Peff (+/- se) of cimetidine and ranitidine averaged over both phases were 0.30 (0.045) and 0.27 (0.062) x 10(-4) cm/s, respectively, and the differences between the two were found to be statistically insignificant. The mean permeabilities for propranolol, phenylalanine, and PEG 400 averaged over both phases and studies were 3.88 (0.72), 3.36 (0.50), and 0.56 (0.08) x 10(-4) cm/s, respectively. The differences in permeability for a given marker were not significant between phases or between the two studies.

CONCLUSIONS

The 10-fold lower permeabilities found for cimetidine and ranitidine in this study, compared to propranolol and phenylalanine, appear to be consistent with their less than complete absorption in humans.

Time-dependent oral absorption models

The plasma concentration-time profiles following oral administration of drugs are often irregular and cannot be interpreted easily with conventional models based on first- or zero-order absorption kinetics and lag time. Six new models were developed using a time-dependent absorption rate coefficient, ka(t), wherein the time dependency was varied to account for the dynamic processes such as changes in fluid absorption or secretion, in absorption surface area, and in motility with time, in the gastrointestinal tract. In the present study, the plasma concentration profiles of propranolol obtained in human subjects following oral dosing were analyzed using the newly derived models based on mass balance and compared with the conventional models. Nonlinear regression analysis indicated that the conventional compartment model including lag time (CLAG model) could not predict the rapid initial increase in plasma concentration after dosing and the predicted Cmax values were much lower than that observed. On the other hand, all models with the time-dependent absorption rate coefficient, ka(t), were superior to the CLAG model in predicting plasma concentration profiles. Based on Akaike's Information Criterion (AIC), the fluid absorption model without lag time (FA model) exhibited the best overall fit to the data. The two-phase model including lag time, TPLAG model was also found to be a good model judging from the values of sum of squares. This model also described the irregular profiles of plasma concentration with time and frequently predicted Cmax values satisfactorily. A comparison of the absorption rate profiles also suggested that the TPLAG model is better at prediction of irregular absorption kinetics than the FA model. In conclusion, the incorporation of a time-dependent absorption rate coefficient ka(t) allows the prediction of nonlinear absorption characteristics in a more reliable manner.

Protein Denaturation during Freezing and Thawing in Phosphate Buffer Systems: Monomeric and Tetrameric β-Galactosidase

During freezing in sodium and potassium phosphate (NaP and KP) buffer solutions, changes in pH may impact the stability of proteins. Since the degradation pathways for the model proteins, monomeric and tetrameric beta-galactosidase (beta-gal), chosen for this study are governed by conformational changes (i.e., physical instability) as opposed to chemical transformations, we explored how the stresses of freezing and thawing alter the protein's native structure and if preservation of the native conformation during freeze-thawing is a requisite for optimal recovery of activity. During freezing in NaP buffer, a significant pH decrease from 7.0 to as low as 3.8 was observed due to the selective precipitation of the disodium phosphate; however, the pH during freezing in KP buffer only increased by at most 0.3 pH units. pH-induced inactivation was evident as seen by the lower recovery of activity when freeze-thawing in NaP buffer as compared to KP buffer for both sources of beta-gal. In addition, we investigated the effects of cooling rate and warming rate on the recovery of activity for monomeric and tetrameric beta-gal. Optimal recovery of activity for the NaP samples was obtained when the processing protocol involved a fast cool/fast warm combination, which minimizes exposure to acidic conditions and concentrated solutes. Alterations in the native secondary structure of monomeric beta-gal as measured by infrared spectroscopy were more significant when freezing and thawing in NaP buffer as opposed to KP buffer. Conformational and activity analyses indicate that pH changes during freezing in NaP buffer contribute to denaturation of beta-gal. These results suggest that proteins formulated in NaP buffer should be frozen and thawed rapidly to minimize exposure to low pH and high buffer salts.

A peptide prodrug approach for improving bisphosphonate oral absorption

This work was aimed at improving the absorption of bisphosphonates by targeting carrier systems in the intestine and the intestinal peptide carrier system (hPEPT1), in particular. (14)C-Labeled pamidronate and alendronate as well as radiolabeled and "cold" peptidyl-bisphosphonates, Pro-[(3)H]Phe-[(14)C]pamidronate, and Pro-[(3)H]Phe-[(14)C]alendronate were synthesized. In situ single-pass perfusion studies revealed competitive inhibition of transport by Pro-Phe, suggesting peptide carrier-mediated transport. Prodrug transport in the Caco-2 cell line was significantly better than that of the parent drugs, and the prodrugs exhibited high affinity to the intestinal tissue. Oral administration of the dipeptidyl prodrugs resulted in a 3-fold increase in drug absorption following oral administration in rats, and the bioavailability of Pro-Phe-alendronate was 3.3 (F(TIBIA)) and 1.9 (F(URINE)) times higher than that of the parent drug. The results indicate that the oral absorption of bisphosphonates can be improved by peptidyl prodrugs via the hPEPT1; however, other transporters may also be involved.

Modern bioavailability, bioequivalence and biopharmaceutics classification system. New scientific approaches to international regulatory standards

In the last decade, the regulatory bioequivalence (BE) requirements of drug products have undergone major changes. The introduction of the biopharmaceutics drug classification system (BCS) into the guidelines of the Food and Drug Administration (FDA) is a major step forward to classify the biopharmaceutical properties of drugs and drug products. Based on mechanistic approaches to the drug absorption and dissolution processes, the BCS enables the regulatory bodies to simplify and improve the drug approval process. The knowledge of the BCS characteristics of a drug in a formulation can also be utilized by the formulation scientist to develop a more optimized dosage form based on fundamental mechanistic, rather than empirical, information. This report gives a brief overview of the BCS and its implications.

Drug transport and targeting. Intestinal transport

A wide variety of transporters are found in the intestine, and are involved in the membrane transport of daily nutrients as well as drugs. These intestinal transporters are located in the brush border membrane as well as basolateral membrane. Each transporter exhibits its own substrate specificity, and some have broader specificities than others. In addition, the distribution and characteristics of the intestinal transporters exhibit regional differences along the intestine, implying diverse physiologic functions and in some cases pathologic responses. Indeed several genetic disorders have been shown to result from deficient intestinal transporters. The development of prodrugs that target to intestinal transporters has been successful in improving oral absorption. For example, the intestinal peptide transporter is utilized in order to increase the bioavailability of several classes of peptidomimetic drugs, especially ACE inhibitors and beta-lactam antibiotics. The bioavailability of poorly absorbed drugs can be improved by utilization of the transporters responsible for the intestinal absorption of various solutes and/or by inhibiting the transporter involved in the efflux system. Recent advances in gene cloning and molecular biology techniques make it possible to study the characteristics and distribution of transporters at the molecular level. Based on molecular characterizations of membrane transporters and accumulated biochemical data on their specificities and kinetics, structural modification and targeting of a specific transporter is a promising strategy for the design of drugs that improve bioavailability and tissue distribution.

Overview of membrane transport

Pharmaceutical scientists increasingly utilize transporters for drug delivery and targeting. The biological barriers to drug delivery can basically be divided into epithelial, endothelial, elimination, and target cell barriers. Membrane transporters play an important role in drug entrance and exit from the body. In addition, it is possible to utilize transporters for drug delivery, e.g., improving oral absorption via the peptide transporter. Identification, a better understanding of their transport characteristics, and the regulation of the membrane transporters will allow the development of better drug delivery strategies.

Dissolution testing as a prognostic tool for oral drug absorption: dissolution behavior of glibenclamide

PURPOSE

The dissolution behavior of two commercially available glibenclamide formulations was tested in various media. The aim of the study was to investigate whether the use of biorelevant dissolution media (BDM) would be advantageous over the use of standard media for predicting the in vivo performance of the two formulations.

METHODS

The dissolution tests were performed using USP 23 apparatus 2. Conventional buffers and USP media were compared with two BDM containing different amounts of lecithin and sodium taurocholate.

RESULTS

The dissolution of two drug powders was highly dependent on wetting, particle size, pH, and the composition of the medium used. In addition, the dissolution behavior of the two glibenclamide formulations showed differences in all media tested. The dissolution results of the two formulations were compared with those from an in vivo bioequivalence study undertaken by the central quality control laboratory of the German pharmacists (ZL). The bioequivalence criterion set by the ZL requires more than 80% drug release within 10 minutes. Results in FaSSIF, one of the BDMs, met the ZL criterion and this medium was also able to discriminate between the two formulations. This was not the case for the other media tested.

CONCLUSIONS

The study indicates that BDM are better able to discriminate between glibenclamide formulations than standard dissolution media.

Immune response with biodegradable nanospheres and alum: studies in rabbits using staphylococcal enterotoxin B-toxoid

In this study, the adjuvant effect of the sustained release biodegradable nanospheres (100-150 nm in diameter) has been compared with alum. Nanospheres were formulated using a biodegradable polylactic polyglycolic acid copolymer (PLGA, 50:50) containing Staphylococcal Enterotoxin B (SEB) toxoid as a model vaccine antigen. Systemic immune response of the nanospheres containing toxoid was studied in rabbits by subcutaneous immunization. The data demonstrated that approximately 30% of the toxoid activity was lost following its encapsulation into nanospheres. Under in vitro conditions, nanospheres demonstrated sustained release of the toxoid. However, only 20% of the antigenic toxoid was released over the first 2 weeks of the release study. Immunization of animals with equal doses of toxoid, either using nanospheres or alum induced a comparable systemic immune response (IgG, IgM and IgA). The immune response reached a maximum level at 7 weeks post-immunization, which then gradually declined with time. The booster dose of toxoid at 19 weeks, either using alum or nanospheres induced similar immune response in both the groups, but was greater than the primary immune response. The studies, thus, suggest that biodegradable nanospheres could be used as a vaccine adjuvant.

Dissolution of ionizable water-insoluble drugs: the combined effect of pH and surfactant

This study reports the results of the combined effect of pH and surfactant on the dissolution of piroxicam (PX), an ionizable water-insoluble drug in physiological pH. The intrinsic dissolution rate (J(total)) of PX was measured in the pH range from 4.0 to 7.8 with 0%, 0.5%, and 2.0% sodium lauryl sulfate (SLS) using the rotating disk apparatus. Solubility (c(total)) was also measured in the same pH and SLS concentration ranges. A simple additive model including an ionization (PX <--> H(+) + PX(-)) and two micellar solubilization equilibria (PX + micelle <--> [PX](micelle), PX(-) + micelle <--> [PX(-)](micelle)) were considered in the convective diffusion reaction model. J(total) and c(total) of PX increased with increasing pH and SLS concentration in an approximately additive manner. Nonlinear regression analysis showed that observed experimental data were well described with the proposed model (r(2) = 0.86, P < 0.001 for J(total) and r(2) = 0.98, P < 0.001 for c(total)). The pK(a) value of 5.63 +/- 0.02 estimated from c(total) agreed well with the reported value. The micellar solubilization equilibrium coefficient for the unionized drug was estimated to be 348 +/- 77 L/mol, while the value for the ionized drug was nearly equal to zero. The diffusion coefficients of the species PX, PX(-), and [PX](micelle) were estimated from the experimental results as (0. 93 +/- 0.35) x 10(-5), (1.4 +/- 0.30) x 10(-5), and (0.59 +/- 0.21) x 10(-5) cm(2)/s, respectively. The total flux enhancement is less than the total solubility enhancement due to the smaller diffusion coefficients of the micellar species. This model may be useful in predicting the dissolution of an ionizable water insoluble drug as a function of pH and surfactant and for establishing in vitro-in vivo correlations, IVIVC, for maintaining bioequivalence of drug products.

Caco-2 versus Caco-2/HT29-MTX co-cultured cell lines: permeabilities via diffusion, inside- and outside-directed carrier-mediated transport

PURPOSE

The objective of this study was a systematic characterization and evaluation of cell culture models based on mixtures of Caco-2/HT29-MTX co-cultures for their use in screening for drug absorption and intestinal permeability in comparison to the properties of the respective mono-cultures.

METHODS

Co-cultures of Caco-2 cells (absorptive-type) and HT29-MTX cells (goblet-type) were set up. Three different co-cultures (initial seeding ratios Caco-2/HT29-MTX: 90/10, 70/30, and 50/50) were grown on permeable filter supports, and monolayers were used for permeability studies with model compounds for paracellular absorption (atenolol, furosemide, H334/75, mannitol, terbutaline), transcellular absorption (antipyrine, ketoprofen, metoprolol, piroxicam), carrier-mediated absorption (D-glucose, Gly-Pro, and L-phenylalanine) as well as substrates for carrier-mediated secretion via P-glycoprotein (cimetidine and talinolol). Electrophysiological and microscopic controls were performed to characterize the cell cultures.

RESULTS

For compounds undergoing passive intestinal absorption permeabilities were generally higher in co-cultures than in Caco-2 monolayers, yielding highest values in pure HT29-MTX monolayers. This difference was most obvious for compounds transported via the paracellular pathway, where HT29-MTX cells may be up to 30 times more permeable than Caco-2 cells, whereas for lipophilic and highly permeable compounds, the difference in permeability values was less obvious. For drugs undergoing intestinal secretion mediated by P-glycoprotein, co-cultivation of Caco-2 cells with HT29-MTX cells led to increased apical to basolateral permeability which was decreased in the opposite direction, consistent with the fact that HT29-MTX cells do not express P-glycoprotein. When a carrier-mediated absorption mechanism is involved, the permeabilities observed were lower than the values reported for human small intestine and co-cultivation of HT29-MTX cells with Caco-2 cells resulted in even lower values as compared to the plain Caco-2 cultures.

CONCLUSIONS

Co-cultures of HT29-MTX and Caco-2 cells offer the opportunity of modifying the permeability barrier of the cell monolayers both with respect to paracellular resistance and secretory transport via P-gp. Thus, in special cases, they allow more flexibility in adapting the in vitro system to the in vivo situation as compared to the monocultures. Another advantage is the obvious robustness of the method with respect to the reproducibility of the results. A problem remaining, however, is the quantitative expression of carriers involved in intestinal uptake of many nutrients and drugs.

A pH- and ionic strength-responsive polypeptide hydrogel: synthesis, characterization, and preliminary protein release studies

A novel polypeptide hydrogel has been synthesized by crosslinking poly(L-glutamic acid) (PLG) with poly(ethylene glycol) (PEG). The PLG-PEG hydrogel was shown to be highly hydrophilic, and the extent of swelling varied with pH, increasing at higher ionization of the PLG. Aside from electrostatic effects, such as ion-ion repulsion and internal ion osmotic pressure, circular dichroism studies showed that swelling response to pH also is affected by secondary structural attributes associated with the polypeptide backbone. Modification of the polypeptide by changing its hydrophobicity and degree of ionization was an effective method for altering the overall extent of pH-responsive swelling. Rapid de-swelling (contraction) was observed when the PLG-PEG hydrogel was transferred from high to low pH buffer solution, and this swelling/de-swelling behavior was reversible over repeated cycles. Drug release from swollen hydrogels was examined using the model protein lysozyme. Rapid de-swelling of the hydrogel was found to be an effective means of facilitating lysozyme release. The crosslinking of synthetic polypeptides with PEG appears to be a highly versatile approach to the preparation of pH-responsive biodegradable hydrogels.

Setting bioequivalence requirements for drug development based on preclinical data: optimizing oral drug delivery systems

The recently proposed Biopharmaceutics Classification System can be used to classify drugs and set standards for scale-up and post-approval changes as well as standards for in vitro/in vivo correlation for immediate and controlled release products. This classification scheme is based on determining the underlying process that is controlling the drug absorption rate and extent, namely, drug solubility and intestinal membrane permeability. Theoretical analysis and experimental results suggest that a permeability/solubility classification scheme can be used to set more rationale drug standards. In particular, high solubility/high permeability, rapidly dissolving drugs may be regulated on the basis of a single point rapid dissolution test while low solubility dissolution rate limited drugs can be regulated based on an in vitro dissolution test that reflects the in vivo dissolution process. This dissolution test may include multiple time points, media change, as well as surfactants in order to reflect the in vivo dissolution process and would be used by the manufacturer for requesting a waiver from a bioequivalence (BE) trial. For controlled release products, the regulation of bioequivalence standards is more complex due to the potential differences in position-dependent permeability/solubility and metabolism of drugs along the gastrointestinal tract. These differences may result in drug absorption rates that are highly transit time dependent. This paper will present the current status of the biopharmaceutic drug classification scheme, the underlying developed data base and its application to optimizing IR and CR products.

Function and immunolocalization of overexpressed human intestinal H+/peptide cotransporter in adenovirus-transduced Caco-2 cells

PURPOSE

To determine the localization of the human intestinal H+/peptide cotransporter (hPepT1) and its function in intestinal epithelial cells after adenoviral transduction.

METHODS

Caco-2 cells grown on Transwell membrane filters were transduced with a recombinant replication-deficient adenovirus carrying the hPepT1 gene. The transport of Gly-Sar across both apical and basolateral membranes was measured after adenoviral transduction as a function of pH, temperature, inhibitors, and substrate concentration. The localization of hPepT1 was examined by immunocytochemistry using confocal laser scanning microscopy.

RESULTS

The apical-to-basolateral and basolateral-to-apical transport of Gly-Sar in Caco-2 cells after viral transduction was increased 3.3 and 3.5-fold, respectively. The similar magnitude of Gly-Sar permeability from either direction indicates involvement of identical transport pathways in both membranes. This was further confirmed by immunocytochemistry showing that hPepT1 was localized in the apical and basolateral membrane of Caco-2 cells after adenoviral transduction. In both directions, Gly-Sar transport was enhanced in the presence of a pH gradient. In addition, the basolateral-to-apical Gly-Sar transport was dependent on temperature, multiplicity of infection (MOI), and Gly-Sar concentration. It was inhibited in the presence of excess Gly-Pro and cephalexin.

CONCLUSIONS

Caco-2 cell monolayers represent an appropriate model to study gene expression in intestinal epithelial cells. Transport characteristics of Gly-Sar from the basolateral to the apical side in adenovirus-transduced Caco-2 cells are in agreement with those from the apical to the basolateral side, indicating that hPepT1 is also expressed in the basolateral membrane and displays a similar level of transport enhancement after adenovirus mediated hPepT1 gene expression.

A compartmental absorption and transit model for estimating oral drug absorption

This report describes a compartmental absorption and transit model to estimate the fraction of dose absorbed and the rate of drug absorption for passively transported drugs in immediate release products. The model considers simultaneous small intestinal transit flow and drug absorption. Both analytical and numerical methods were utilized to solve the model equations. It was found that the fraction of dose absorbed can be estimated by F(a) = 1-(1+0.54 P(eff))(-7), where P(eff) is the human effective permeability in cm/h. A good correlation was found between the fraction of dose absorbed and the effective permeability for ten drugs covering a wide range of absorption characteristics. The model was able to explain the oral plasma concentration profiles of atenolol.

Evidence for overlapping substrate specificity between large neutral amino acid (LNAA) and dipeptide (hPEPT1) transporters for PD 158473, an NMDA antagonist

PURPOSE

The objective of this research was to investigate the substrate specificity of large neutral amino acid carrier (LNAA) and di/tripeptide (hPEPT1) transporters with respect to PD 158473, an NMDA antagonist.

METHODS

Cellular uptake studies were carried out using two types of Chinese Hamster Ovary (CHO). CHO-K1 cells represent the wild type with inherent large neutral amino acid (LNAA) activity. CHO-PEPT1 cells were generated by stable transfection of hPEPT1 gene into CHO cells. Therefore, these cells possess both LNAA activity and di/tripeptide transporter activities as a result of the transfection. Cellular uptake of PD 158473 was quantified using a HPLC method previously developed in our laboratory.

RESULTS

The utility of the CHO-PEPT1 cell model was demonstrated by determining the uptake kinetics of Gly-Sar, a prototypical dipeptide transporter substrate. Uptake kinetics of PD 158473 displayed two carrier-mediated transport components in CHO-PEPT1 cells, while in CHO-K1 cells the relationship was consistent with classic one component Michaelis-Menten kinetics. These results confirmed the affinity of PD 158473 for both LNAA and di/tripeptide transporters. Further, results from inhibition experiments using these two cell types indicate that the high affinity-low capacity system was the LNAA carrier and the low affinity-high capacity carrier was the di/tripeptide transporter.

CONCLUSIONS

This study demonstrates overlapping substrate specificity between LNAA carrier and di/tripeptide transporter (hPEPT1) for PD 158473, an amino acid analog. Establishing Structure Transport Relationship (STR) for this overlap will aid in a design strategy for increasing oral absorption or targeting specific drugs to selected tissues.

CHO/hPEPT1 cells overexpressing the human peptide transporter (hPEPT1) as an alternative in vitro model for peptidomimetic drugs

The present study characterized Chinese hamster ovary cells overexpressing a human intestinal peptide transporter, CHO/hPEPT1 cells, as an in vitro model for peptidomimetic drugs. The kinetic parameters of Gly-Sar uptake were determined in three different cell culture systems such as untransfected CHO cells (CHO-K1), transfected CHO cells (CHO/hPEPT1) and Caco-2 cells. Vmax in CHO/hPEPT1 cells was approximately 3-fold higher than those in Caco-2 cells and CHO-K1 cells, while Km values were similar in all cases. The uptake of beta-lactam antibiotics in CHO/hPEPT1 cells was three to twelve fold higher than that in CHO-K1 cells, indicating that CHO/hPEPT1 cells significantly enhanced the peptide transport activity. However, amino acid drugs also exhibited high cellular uptake in both CHO-K1 and CHO/hPEPT1 cells due to the high background level of amino acid transporters. Thus, cellular uptake study in CHO/hPEPT1 cells is not sensitive enough to distinguish the peptidyl drugs from amino acid drugs. The potential of CHO/hPEPT1 cells as an in vitro model for peptidomimetic drugs was also examined through the inhibition study on Gly-Sar uptake. Peptidomimetic drugs such as beta-lactam antibiotics and enalapril significantly inhibited Gly-Sar uptake whereas the nonpeptidyl compounds, L-dopa and alpha-methyldopa, did not compete with Gly-Sar for cellular uptake within the therapeutic concentrations. In conclusion, the present study demonstrates the further characterization of CHO/hPEPT1 cells as an uptake model as well as inhibition study and suggests their utility as an alternative in vitro model for drug candidates targeting the hPEPT1 transporter.

Modified polypeptides containing gamma-benzyl glutamic acid as drug delivery platforms

We previously reported the development of diffusion-controlled biodegradable polypeptides for drug delivery purposes. In this paper, we describe the synthesis of three modified polypeptides that contain gamma-benzyl glutamic acid as the common structural backbone. The properties of these polymers were characterized with regard to their potential application as drug delivery platforms. Procainamide hydrochloride, a hydrophilic drug, and protamine sulfate, a low molecular weight protein, were used as model drugs for examining release rate profiles from these polymers. The homopolymer of poly(gamma-benzyl-L-glutamic acid), PBLG, showed a highly helical configuration and a moderate release rate of procainamide. Modification of structural attributes by random copolymerization of the D- and L- isomers of gamma-benzyl glutamic acid produced poly(gamma-benzyl-D,L-glutamic acid), PBDLG, which displayed a significantly slower release of procainamide when compared to PBLG. The modification of polymer bulk hydrophobicity by copolymerization of PBLG (A) with poly(ethylene glycol) (B) yielded an ABA triblock copolymer exhibiting much faster release rates for both procainamide and protamine than those demonstrated by the other two polymers. Using this triblock copolymer, protamine release rates ranging from 2 weeks to approximately 2 months were obtained by simply varying the polymer processing conditions and protein particle size. A nearly complete release of protein was obtained from the triblock copolymer blends and this occurred without reliance upon degradation of the polymer backbone. Fickian diffusion-controlled release mechanisms were implied for release of procainamide and protamine from these polypeptide formulations based on the linear relationship displayed between cumulative drug release and the square root of time.

The effect of in vivo dissolution, gastric emptying rate, and intestinal transit time on the peak concentration and area-under-the-curve of drugs with different gastrointestinal permeabilities

PURPOSE

To theoretically investigate the impact of gastric emptying half-time, intestinal transit time and the time for 85% in vivo dissolution on the peak concentration and area-under-the curve of model drugs.

METHODS

Simulations were performed using mathematical models of gastrointestinal physiology and pharmacokinetics of model drugs with different gastrointestinal permeability. They were used to investigate the effect of different permutations of gastric emptying times, intestinal transit times, dissolution rates and effective permeabilities on the maximum plasma drug concentration and the area-under-the-curve of immediate release tablets relative to an oral solution (i.e., Cmax(tablet)/Cmax(solution) and AUC(tablet)/AUC(solution)).

RESULTS

The higher the permeability of the drug, the more sensitive the Cmax ratio is to dissolution rate and gastric emptying rate. As the intestinal transit time becomes more rapid, the sensitivity to T85% dissolution time and gastric emptying half-time increases. There is less dependence for the AUC ratio on the gastric emptying time and dissolution rate.

CONCLUSIONS

Under the assumptions of the models, the criterion of 85% dissolution in 15 minutes (T85%) for classifying a rapidly dissolving drug product is relatively conservative since the Cmax ratio exceeded 0.8 for a T85% dissolution time of one hour and a gastric emptying half-time faster than 0.2 hour over a wide range of permeabilities.

Intestinal transport of gentamicin with a novel, glycosteroid drug transport agent

PURPOSE

The objective was to investigate the ability of a glycosteroid (TC002) to increase the oral bioavailability of gentamicin.

METHODS

Admixtures of gentamicin and TC002 were administered to the rat ileum by injection and to dogs by ileal or jejunal externalized ports, or PO. Bioavailability of gentamicin was determined by HPLC. 3H-TC002 was injected via externalized cannulas into rat ileum or jejunum, or PO and its distribution and elimination was determined. The metabolism of TC002 in rats was evaluated by solid phase extraction and HPLC analysis of plasma, urine and feces following oral or intestinal administration.

RESULTS

The bioavailability of gentamicin was substantially increased in the presence of TC002 in both rats and dogs. The level of absorption was dependent on the concentration of TC002 and site of administration. Greatest absorption occurred following ileal orjejunal administration. TC002 was significantly more efficacious than sodium taurocholate, but similar in cytotoxicity. TC002 remained primarily in the GI tract following oral or intestinal administration and cleared rapidly from the body. It was only partly metabolized in the GI tract, but was rapidly and completely converted to its metabolite in plasma and urine.

CONCLUSIONS

TC002 shows promise as a new drug transport agent for promoting intestinal absorption of polar molecules such as gentamicin.

In vitro stability and intestinal absorption characteristics of hexapeptide endothelin receptor antagonists

Endothelins are potent vasoconstrictor peptides which have a wide range of tissue distribution and three receptor subtypes (ET(A), ET(B) and ET(C)). Among the linear hexapeptide ET(A)/ET(B) receptor antagonists, PD 145065 (Ac-D-Bhg-L-Leu-L-Asp-L-Ile-L-Ile-L-Trp, Bhg = (10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5-yl)-Gly) and PD 156252 (Ac-D-Bhg-L-Leu-L-Asp-L-Ile-(N-methyl)-L-Ile-L-Trp) were selected to evaluate the metabolic stability and intestinal absorption in the absence and/or in the presence of protease inhibitors. In vitro stability of both compounds was investigated in fresh plasma, lumenal perfusate, intestinal and liver homogenates. PD 156252 was more stable than PD 145065 in intestinal tissue homogenate (63.4% vs. 20.5% remaining) and liver homogenate (74.4% vs. 35.5% remaining), while both compounds showed relatively good stability in the fresh plasma (94.5% vs. 86.7% remaining) and lumenal perfusate (85.8% vs. 72.3% remaining). The effect of protease inhibitors on the degradation of PD 145065 and PD 156252 was also investigated. Amastatin, thiorphan, chymostatin and the mixture of these three inhibitors were effective in reducing the degradation of both compounds. The pharmacokinetic parameters of PD 156252, calculated by using a non-compartmental model, were 6.95 min (terminal half-life), 191 mL (Vss), and 25.5 mL/min (Cl(tot)) after intravenous administration in rats. The intestinal absorption of PD 156252 in rats was evaluated in the absence and/or in the presence of protease inhibitors. The results indicate that the major elimination pathway of PD 156252 appears to be the biliary excretion and protease inhibitors increase the intestinal absorption of PD 156252 through increasing metabolic stability.

Membrane topology of the human dipeptide transporter, hPEPT1, determined by epitope insertions

We have used epitope insertion to analyze the transmembrane topology of the human H+-dipeptide symporter hPEPT1. An epitope tag, EYMPME (EE), was inserted into different locations at amino acids 39, 78, 106, 412, and 708 of hPEPT1 by site-directed mutagenesis. The functional integrity of the tagged protein was tested by measuring its dipeptide transport activity in transfected Cos7 cells. Further, cells expressing hPEPT1 or EE-tagged hPEPT1 derivatives were labeled with an anti-EE-monoclonal antibody (anti-EE-mAb) or an antiserum raised against the carboxyl terminus of hPEPT1 (anti-hPEPT1) and examined by immunofluorescence confocal microscopy. EE106-, 412-, and 708-hPEPT1 transported the dipeptide tracer as well as wild-type hPEPT1. Tags at position 106 and 412 were shown to be extracellular because they were accessible to anti-epitope antibody in nonpermeabilized cells. In contrast, the carboxyl-terminal domain and EE708 were shown to be intracellular since they were only accessible to the antibodies in permeabilized cells. These results are consistent with a 12-transmembrane domain (TMD) topological model of PEPT1. Epitope insertions at regions linking the putative TMD1 and TMD2, and TMD2 and TMD3 (EE39- and EE78-hPEPT1), abolished the dipeptide transport into the cells. In transfected Cos7 cells, these tagged proteins remained largely intracellular rather than at the plasma membrane. These results suggest that the integrity of these regions is essential for transporter trafficking and/or function. Thus, the topology of the amino-terminal portion, including putative TMD1 and -2, remains to be clarified.

Beta cyclodextrins enhance adenoviral-mediated gene delivery to the intestine

PURPOSE

In general, the intestinal epithelium is quite refractory to viral and non-viral methods of gene transfer. In this report, various cyclodextrin formulations were tested for their ability to enhance adenoviral transduction efficiency in two models of the intestinal epithelium: differentiated Caco-2 cells and rat jejunum.

METHODS

Transduction efficiency of replication-deficient adenovirus type 5 vectors encoded with either the E. coli beta-galactosidase or the jellyfish green fluorescent protein gene was assessed by X-gal staining or visualization of fluorescence 48 hours after infection. In vivo experiments were performed using an intestinal loop ligation technique.

RESULTS

Several formulations of neutral and positively charged beta cyclodextrins significantly enhanced adenoviral-mediated gene transfer in the selected models. The cyclodextrin formulations studied increased adenoviral transduction in the intestine by enhancing both viral binding and internalization. Viral binding was significantly increased on cell membranes treated with positively charged cyclodextrins, as seen with confocal microscopy and rhodamine-labeled virus. Permeability studies and TEER readings revealed that the most successful formulations gently disrupt cell membranes. This enhances internalization of viral particles and results in increased levels of gene expression.

CONCLUSIONS

These formulations can be of value in gene transfer to cells and tissues in which adenoviral infection is limited due to a lack of fiber and alpha(v) integrin receptors. They are simple to prepare and do not affect the ability of the virus to transduce target cells.

Cellular uptake mechanism of amino acid ester prodrugs in Caco-2/hPEPT1 cells overexpressing a human peptide transporter

PURPOSE

This study characterized the cellular uptake mechanism and hydrolysis of the amino acid ester prodrugs of nucleoside antiviral drugs in the transiently transfected Caco-2 cells overexpressing a human intestinal peptide transporter, hPEPT1 (Caco-2/hPEPT1 cells).

METHODS

Amino acid ester prodrugs of acyclovir and AZT were synthesized and their apical membrane permeability and hydrolysis were evaluated in Caco-2/hPEPT1 cells. The cellular uptake mechanism of prodrugs was investigated through the competitive inhibition study in Caco-2/hPEPT1 cells.

RESULTS

L-Valyl ester of acyclovir (L-Val-ACV) was approximately ten fold more permeable across the apical membrane than acyclovir and four times more permeable than D-valyl ester of acyclovir (D-Val-ACV). Correspondingly, L-valyl ester of AZT (L- Val-AZT) exhibited three fold higher cellular uptake than AZT. Therefore, amino acid ester prodrugs significantly increased the cellular uptake of the parent drugs and exhibited the D,L-stereoselectivity. Furthermore, prodrugs were rapidly hydrolyzed to the parent drugs by the intracellular hydrolysis, following the apical membrane transport. In the inhibition studies, cephalexin and small dipeptides strongly inhibited the cellular uptake of L-Val-ACV while L-valine had no effect, indicating that the peptide transporter is primarily responsible for the apical membrane transport of L-Val-ACV. In addition, the cellular uptake of L-Val-ACV was five times higher in Caco-2/hPEPT1 cells than the uptake in the untransfected Caco-2 cells, implying the cellular uptake of L-Val-ACV was related to the enhancement of the peptide transport activity in Caco-2/hPEPT1 cells.

CONCLUSIONS

Caco-2/hPEPT1 system is an efficient in vitro model for the uptake study of peptidyl derivatives. Amino acid ester prodrugs significantly improved the cellular uptake of the parent drugs via peptide transport mechanism and were rapidly converted to the active parent drugs by the intracellular hydrolysis.

Overexpression of human intestinal oligopeptide transporter in mammalian cells via adenoviral transduction

PURPOSE

Our goals are to establish an in vitro screening system and to evaluate a new approach in improving oral absorption of peptides and peptide-like drugs by overexpression of the human intestinal oligopeptide transporter (hPepT1). This study characterizes the expression of hPepT1 in human intestinal Caco-2 cells, rat intestinal epithelial cells (IEC-18), and human cervix epithelial cells (Hela) after adenoviral transduction.

METHODS

A recombinant replication-deficient adenovirus carrying the hPepT1 gene was made and used as a vector for the expression of hPepT1. The increase in the uptake permeability of cephalexin and Gly-Sar was determined. The effects of time, dose, apical pH, and substrate specificity were evaluated.

RESULTS

A significant increase in the uptake permeability of Gly-Sar and cephalexin was found in all three cell lines after viral transduction. The increase of Gly-Sar permeability in Hela. IEC-18, and Caco-2 cells was 85-, 46-, and 15-fold respectively. Immunoblotting using an antibody against hPepT1 detected high levels of a 85-98-kDa protein in all three infected cell lines. Substrate permeability was dependent on time of infection, inward pH gradients, and multiplicity of infection (MOI). Decreased infectivity and lower hPepT1 expression were observed in differentiated Caco-2 cells. The uptake was inhibited by dipeptides and beta-lactam antibiotics but not amino acids.

CONCLUSIONS

Adenoviral infected Hela cells displayed a pronounced level of hPepT1 expression with a low background and high specificity to dipeptides. These features make this system a useful tool for screening of potential substrates. The success of overexpression of hPepT1 in Caco-2 and IEC-18 cells may lead to a novel approach in improving oral absorption of peptides and peptidornimetic drugs.

First-pass metabolism of peptide drugs in rat perfused liver

To elucidate the extent and mechanisms of the first-pass metabolism of peptide drugs in the liver after oral administration, a liver perfusion study was performed in rats using metkephamid, a stable analogue of methionine enkephalin, and thyrotropin-releasing hormone (TRH), as model peptides. The fraction of intact metkephamid recovered after single-pass constant perfusion through rat liver reached steady-state very quickly, and it was concluded that metkephamid was hydrolysed enzymatically at the surface of hepatocytes or endothelial cells of microvessels, or both, rather than being taken up by hepatocytes. The fraction of metkephamid recovered intact was approximately 40% under protein-free conditions but increased to 70-75% on addition of bovine serum albumin (BSA) to the perfusate. The fraction of metkephamid bound to BSA was approximately 50% under these conditions, implying that only the free fraction of metkephamid in the plasma was metabolized in the liver. Calculations based on the tube model showed that approximately 30-35% of metkephamid absorbed from the intestine undergoes first-pass metabolism before entering the systemic circulation in-vivo. In contrast, the fraction of TRH metabolized in the liver was less than 10%, indicating a remarkably low contribution of first-pass metabolism to the bioavailability of TRH. These results show that hepatic first-pass metabolism of metkephamid contributes to its low systemic bioavailability. After intestinal absorption free metkephamid is rapidly hydrolysed on the surface of hepatocytes or endothelial cells, rather than being taken up by hepatocytes. This information has important implications in the oral delivery of many kinds of peptide.