Intestinal secretion of drugs. The role of P-glycoprotein and related drug efflux systems in limiting oral drug absorption

The developability classification system: application of biopharmaceutics concepts to formulation development

A revised classification system for oral drugs was developed using the biopharmaceutics classification system (BCS) as a starting point. The revised system is designed to have a greater focus on drug developability. Intestinal solubility, the compensatory nature of solubility and permeability in the small intestine and an estimate of the particle size needed to overcome dissolution rate limited absorption were all considered in the revised system. The system was then validated by comparison with literature on the in vivo performance of a number of test compounds. Observations on the test compounds were consistent with the revised classification, termed the developability classification system (DCS), showing it to be of greater value in predicting what factors are critical to in vivo performance than the widely used BCS.

In vitro and in vivo studies of lipid-based nanocarriers for oral N3-o-toluyl-fluorouracil delivery

Lipid-based drug delivery systems show great potential for enhancing oral bioavailability but have not been broadly applied, largely due to lack of general formulation guidance. In the previous studies, three different formulations including anionic SLNs, cationic SLNs, and liposomes were investigated and significantly enhanced the oral bioavailability of N3-o-toluyl-fluorouracil (TFu) compared with its aqueous suspension, which indicated their high potential as oral delivery carriers. In order to define which formulation is worthy of being further researched and developed, the studies on Caco-2 cell model and rat intestine were investigated. In both studies of crossing Caco-2 cell monolayers and the single-pass intestinal perfusion (SPIP) in rat, SLNs exhibited much more capability to enhance transport of TFu than liposomes. More specifically, in cell study, the P(app) values of cationic SLNs (p < 0.01) and anionic SLNs (p < 0.05) were significantly higher than liposomes. Especially the cationic SLNs present the most effective capacity. During SPIP study, both Ka and uptake percentage of these three different formulations followed a rank order: anionic SLN > cationic SLN > liposomes. In addition, the P(eff) of different nanocarriers in various intestinal segments indicated they all exhibited site-dependent absorption behavior. By comparing the transmucosal behavior of these nanocarriers in vitro and in vivo, the anionic SLNs were identified to be more effective in the transport of TFu and were worthy of being further researched and developed.

Population pharmacokinetic model of digoxin in older Chinese patients and its application in clinical practice

AIM

To establish a population pharmacokinetic (PPK) model of digoxin in older Chinese patients to provide a reference for individual medication in clinical practice.

METHODS

Serum concentrations of digoxin and clinically related data including gender, age, weight (WT), serum creatinine (Cr), alanine aminotransferase (ALT), aspartate aminotransferase (AST), blood urea nitrogen (BUN), albumin (ALB), and co-administration were retrospectively collected from 119 older patients taking digoxin orally for more than 7 d. NONMEM software was used to get PPK parameter values, to set up a final model, and to assess the models in clinical practice.

RESULTS

Spironolactone (SPI), WT, and Cr markedly affected the clearance rate of digoxin. The final model formula is Cl/F=5.9x[1-0.412 x SPI] x [1-0.0101x(WT-62.9)] x [1-0.0012x(Cr-126.8)] (L/h); Ka=1.63 (h(-1)); V(d)/F=550 (L). The population estimates for Cl/F and V(d)/F were 5.9 L/h and 550 L, respectively. The interindividual variabilities (CV) were 49.0% for Cl/F and 94.3% for V(d)/F. The residual variability (SD) between observed and predicted concentrations was 0.365 microg/L. The difference between the objective function value and the primitive function value was less than 3.84 (P>0.05) by intra-validation. Clinical applications indicated that the percent of difference between the predicted concentrations estimated by the PPK final model and the observed concentrations were -4.3%-+25%. Correlation analysis displayed that there was a linear correlation between observed and predicted values (y=1.35x+0.39, r=0.9639, P<0.0001).

CONCLUSION

The PPK final model of digoxin in older Chinese patients can be established using the NONMEM software, which can be applied in clinical practice.

Transport of hop bitter acids across intestinal Caco-2 cell monolayers

Several health-beneficial properties of hop bitter acids have been reported (inhibition of bone resorption and anticarcinogenic and anti-inflammatory activities); however, scientific data on the bioavailability of these compounds are lacking. As a first approach to study the bioavailability, the epithelial transport of hop alpha- and beta-acids across Caco-2 monolayers was investigated. Hop acids were added either to the apical or to the basolateral chamber and, at various time points, amounts transported to the receiving compartment were determined. The monolayer integrity control was performed by using marker compounds (atenolol and propranolol), transepithelial electrical resistance (TEER) measurement, and determination of the fluorescein efflux. The TEER and fluorescein efflux confirmed the preservation of the monolayer integrity. The membrane permeability of the alpha-acids (apparent permeability coefficients for apical to basolateral transport (P(appAB)) ranged from 14 x 10(-6) to 41 x 10(-6) cm/s) was determined to be substantially higher than that of the beta-acids (P(appAB) values ranging from 0.9 x 10(-6) to 2.1 x 10(-6) cm/s). Notably, the beta-acids exhibited significantly different bidirectional P(app) values with efflux ratios around 10. The involvement of carrier-mediated transport for beta-acids (active efflux pathway by P-gp, BCRP, and/or MRP-2 type efflux pumps) could be confirmed by transport experiments with specific inhibitors (verapamil and indomethacin). It appears that alpha-acids are efficiently absorbed, whereas the permeability of beta-acids is low. Limiting factors in the absorption of beta-acids could involve P-gp and MRP-2 type efflux transporters and phase II metabolism.

Modulating effect of polyethylene glycol on the intestinal transport and absorption of prednisolone, methylprednisolone and quinidine in rats by in-vitro and in-situ absorption studies

The effects of polyethylene glycol 20000 (PEG 20000) on the intestinal absorption of prednisolone, methylprednisolone and quinidine, three P-glycoprotein (P-gp) substrates, across the isolated rat intestinal membranes were examined by an in-vitro diffusion chamber system. The serosal-to-mucosal (secretory) transport of these P-gp substrates was greater than their mucosal-to-serosal (absorptive) transport, indicating that their net movement across the intestinal membranes was preferentially in the secretory direction. The polarized secretory transport of these drugs was remarkably diminished and their efflux ratios decreased in the presence of PEG 20000. In addition, PEG 20000 did not affect the transport of Lucifer yellow, a non-P-gp substrate. The intestinal membrane toxicity of PEG 20000 was evaluated by measuring the release of alkaline phosphatase (ALP) and protein from the intestinal membranes. The release of ALP and protein was enhanced in the presence of 20 mM sodium deoxycholate (NaDC), a positive control, while these biological parameters did not change in the presence of 0.1–5% (w/v) PEG 20000. These findings indicated that the intestinal membrane damage caused by PEG 20000 was not a main reason for the enhanced absorptive transport of these P-gp substrates in the presence of PEG 20000. Furthermore, the transepithelial electrical resistance (TEER) of rat jejunal membranes in the presence or absence of PEG 20000 was measured by a diffusion chamber method. PEG 20000 (0.1–5.0 % w/v) did not change the TEER values of the rat jejunal membranes, indicating that the increase in the absorptive transport of these P-gp substrates might not be due to the increased transport of these P-gp substrates via a paracellular pathway caused by PEG 20000. Finally, the effect of PEG 20000 on the intestinal absorption of quinidine was examined by an in-situ closed-loop method. The intestinal absorption of quinidine was significantly enhanced in the presence of 0.1-1.0% (w/v) PEG 20000. These findings suggest that PEG 20000 might be a useful excipient to improve the intestinal absorption of quinidine, which is mainly secreted by a P-gp-mediated efflux system in the intestine.

Cytochrome P450-mediated metabolism in the human gut wall

Objective

Although the human small intestine serves primarily as an absorptive organ for nutrients and water, it also has the ability to metabolise drugs. Interest in the small intestine as a drug-metabolising organ has been increasing since the realisation that it is probably the most important extrahepatic site of drug biotransformation.

Key findings

Among the metabolising enzymes present in the small intestinal mucosa, the cytochromes P450 (CYPs) are of particular importance, being responsible for the majority of phase I drug metabolism reactions. Many drug interactions involving induction or inhibition of CYP enzymes, in particular CYP3A, have been proposed to occur substantially at the level of the intestine rather than exclusively within the liver, as originally thought. CYP3A and CYP2C represent the major intestinal CYPs, accounting for approximately 80% and 18%, respectively, of total immunoquantified CYPs. CYP2J2 is also consistently expressed in the human gut wall. In the case of CYP1A1, large interindividual variation in the expression levels has been reported. Data for the intestinal expression of the polymorphic CYP2D6 are conflicting. Several other CYPs, including the common hepatic isoform CYP2E1, are expressed in the human small intestine to only a very low extent, if at all. The distribution of most CYP enzymes is not uniform along the human gastrointestinal tract, being generally higher in the proximal regions of the small intestine.

Summary

This article reviews the current state of knowledge of CYP enzyme expression in human small intestine, the role of the gut wall in CYP-mediated metabolism, and how this metabolism limits the bioavailability of orally administered drugs. Possible interactions between drugs and CYP activity in the small intestine are also discussed.

Impact of traditional medicinal plant extracts on antiretroviral drug absorption

ETHNOPHARMACOLOGICAL RELEVANCE

[corrected] Traditional herbal medicines are often used for the treatment of different diseases in developing countries, especially in the rural areas where a lack of an efficient primary health care system is usually experienced. Many patients infected with the human immunodeficiency virus are taking traditional herbal medicines in conjunction with their modern antiretroviral medication and drug-herb interactions can occur in these cases.

AIM OF THE STUDY

To investigate the effect of water extracts of two traditional medicinal plants, Hypoxis hemerocallidea and Sutherlandia frutescens as well as l-canavanine (a constituent of Sutherlandia frutescens) on the transport of nevirapine across human intestinal epithelial cells.

MATERIALS AND METHODS

Nevirapine transport in the apical to basolateral and basolateral to apical directions across Caco-2 cell monolayers was determined alone (normal control) and in the presence of verapamil (positive control), water extracts of Hypoxis hemerocallidea and Sutherlandia frutescens and an aqueous solution of l-canavanine. The cumulative transport and apparent permeability coefficient (P(app)) values were calculated and compared.

RESULTS

Nevirapine alone was substantially effluxed in the basolateral to apical direction across the intestinal epithelial cell monolayers, which was statistically significantly (p < or = 0.05) decreased by addition of verapamil, Hypoxis hemerocallidea extract and the l-canavinine solution. The effect of Sutherlandia frutescens on nevirapine transport was not statistically significantly different from the control.

CONCLUSIONS

Hypoxis hemerocallidea and l-canavanine interact with the efflux of nevirapine across intestinal epithelial cells and therefore can potentially increase the bioavailability of this antiretroviral drug when taken concomitantly.

Enhanced transport of P-glycoprotein substrate saquinavir in presence of thiolated chitosan

It was the aim of this study to investigate the effect of chitosan-4-thiobutylamidine (Ch-TBA) and reduced glutathione (GSH) on the absorption of P-glycoprotein (P-gp) and multidrug resistance protein (MRP) substrate saquinavir in vitro and in vivo. Bidirectional transport studies were performed with Caco-2 cell monolayers and additionally with freshly excised rat small intestinal mucosa mounted in Ussing type chambers. Furthermore, a delivery system based on Ch-TBA and GSH was evaluated in vivo in rats. The functional activity of the efflux pumps in Caco-2 cells and rat intestinal mucosa during the experiment was proven by the efflux ratio of saquinavir, which was 6.4 for Caco-2 cells and 2.1 for rat intestinal mucosa, respectively. Ch-TBA and particularly the combination of Ch-TBA with GSH enhanced apical (AP) absorption and decreased the secretory transport of saquinavir. In presence of 0.5% Ch-TBA and 0.5% GSH, the uptake of saquinavir was 1.6-fold improved in Caco-2 monolayer and 2.1-fold improved in rat intestinal mucosa. In vivo, the area under the plasma concentration time curve (AUC) of saquinavir was 1.4-fold and Cmax 1.6-fold increased, in comparison with control. Results of this study showed that Ch-TBA in combination with GSH can be an interesting tool for increasing the oral bioavailability of actively secreted compounds.

P-glycoprotein Potentiates CYP3A4-mediated Drug Disappearance during Caco-2 Intestinal Secretory Detoxification

Human intestinal Caco-2 cell monolayers grown in the presence of 1alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3) were used to test the hypothesis that drugs which interact with the apical efflux pump P-glycoprotein (Pgp) may enhance CYP3A4-mediated disappearance of substrates. 6beta-hydroxytestosterone production, a marker of CYP3A4 activity, was approximately 3- and 7-fold greater in 1,25(OH)2D3-treated cells compared to untreated cells when incubated with 50 and 500 microM testosterone, respectively, and was unaffected by the addition of digoxin to reduce Pgp activity. In the presence of digoxin, secretory transport of vinblastine and erythromycin, substrates for both Pgp and cytochrome P450 3A4 (CYP3A4), was significantly reduced, whereas absorptive transport was unaffected. In contrast, no directional transport of testosterone, a substrate for CYP3A4 only, was observed, either in the presence or absence of digoxin. Over 2 h, disappearance of erythromycin and vinblastine from the incubation medium was significantly greater from the basolateral than from the apical compartments. In the presence of digoxin, disappearance of both compounds from the basolateral, but not from the apical compartments, was significantly reduced. In contrast, disappearance of testosterone was unaffected by the addition of digoxin, demonstrating that the effect of digoxin on erythromycin and vinblastine disappearance was via inhibition of Pgp function, rather than on CYP3A4 activity. Thus, evidence is provided for Pgp/CYP3A4 co-substrates, Pgp potentiates CYP3A4-mediated drug disappearance during intestinal secretory detoxification.

Antibiotic exposure does not influence MRP2 functional expression in Caco-2 cells

Multidrug resistance-associated protein 2 (MRP2) is associated with active drug efflux and may influence oral bioavailability of common classes of drugs. MRP2 expression demonstrates plasticity. Caco-2 cells, a routine in vitro model for predicting oral bioavailability, are often cultured in media containing antibiotics. We have investigated whether exposure of Caco-2 cells to two common antibiotic regimes alters MRP2 functional expression. Caco-2 cells were grown in the presence or absence of either gentamicin or penicillin-streptomycin for up to 9 weeks. MRP2 functional activity was assessed by calcein efflux across the apical membrane. MRP2 protein expression was determined by immunoblots. Neither antibiotic regime resulted in consistent changes in calcein efflux across the apical membrane (reflecting MRP2 activity) or basolateral membrane (reflecting MRP3 and possibly MRP6 activity) of Caco-2 cells. MRP2 protein expression also showed no change in response to antibiotic exposure. Routine exposure of Caco-2 cells to penicillin-streptomycin or gentamicin does not affect apical MRP2 functional activity in intestinal enterocytic Caco-2 cells. Extrapolating these results to the situation in vivo suggests that the oral bioavailability of MRP substrates is not predicted to be influenced by recent courses of antibiotics.

Current Perspectives of Solubilization: Potential for Improved Bioavailability

This review focuses on the recent techniques of solubilization for the attainment of effective absorption and improved bioavailability. Solubilization may be affected due to cosolvent water interaction or altered crystal structure by cosolvent addition. Micellar solubilization could be affected by both ionic strength and pH. Addition of cosolvents to the surfactant solutions offers only a small advantage because of the decrease in the solubilization capacity of the micelles. Polymorphism is known to influence dissolution and bioavailability of the drugs. Molecular modeling study of cyclodextrin inclusion complexations can predict the inclusion modes, stoichiometry of the complex, and the relative complexing efficiency of cyclodextrins with various drug molecules.

Probiotic Pre-treatment Reduces Gliclazide Permeation (ex vivo) in Healthy Rats but Increases It in Diabetic Rats to the Level Seen in Untreated Healthy Rats

Aim

To investigate the influence of probiotic pre-treatment on the permeation of the antidiabetic drug gliclazide in healthy and diabetic rats.

Methods

Wistar rats (age 2–3 months, weight 350 ± 50 g) were randomly allocated into one of 4 groups (N = 16 each group): healthy control, healthy probiotic, diabetic control, and diabetic probiotic. Probiotics (75 mg/kg, equal quantities of Lactobacillus acidophilus, Bifidobacterium lactis, and Lactobacillus rhamnosus) were administered twice a day for three days to the appropriate groups after diabetes had been induced with alloxan i.v. 30 mg/kg. Rats were sacrificed, ileal tissues mounted in Ussing chambers and gliclazide (200 µg/mL) was administered for the measurement of the mucosal to serosal absorption Jss^(MtoS) and serosal to mucosal secretion Jss^(StoM) of gliclazide.

Results

Treatment of healthy rats with probiotics reduced Jss^(MtoS) of gliclazide from 1.2 ± 0.3 to 0.3 ± 0.1 µg/min/cm² (P < 0.01) and increased Jss^(StoM)from 0.6 ± 0.1 to 1.4 ± 0.3 (P < 0.01) resulting in net secretion while, in diabetic tissues, treatment with probiotics increased both Jss^(MtoS) and Jss^(StoM)fluxes of gliclazide to the comparable levels of healthy tissues resulting in net absorption.

Discussion

In healthy rats, the reduction in Jss^(MtoS) after probiotics administration could be explained by the production of bacterial metabolites that upregulate the mucosal efflux drug transporters Mrp2 that control gliclazide transport. In diabetic rats, the restored fluxes of gliclazide after probiotic treatment, suggests the normalization of the functionality of the drug transporters resulting in a net absorption.

Conclusion

Probiotics may alter gliclazide transport across rat ileal tissue studied ex vivo.

Scaling of in vitro membrane permeability to predict P-glycoprotein-mediated drug absorption in vivo

In a previous study, the concentration-dependent permeability of P-glycoprotein (P-gp) substrate drugs, quinidine, verapamil, and vinblastine, in several cell monolayers with different levels of P-gp expression was analyzed kinetically to obtain fundamental parameters for P-gp-mediated transport, V(max) and K(m(app)) values. Both V(max) and K(m(app)) values of each drug were found to show linear correlations with the expression level of P-gp. These findings imply the possibility of estimating the V(max) and K(m(app)) values of P-gp substrate drugs in the in vivo intestinal membrane on the basis of the P-gp expression level. In the present study, concentration-dependent drug permeability to the rat small intestines (upper jejunum and ileum) was simulated on the basis of V(max) and K(m(app)) values of each drug estimated from the P-gp expression level in the rat small intestines. To validate the predictability of these procedures, drug permeability in the rat small intestines was measured by the in situ single-pass perfusion method. It was confirmed that simulated permeability of each drug in the rat jejunum and ileum corresponded well with permeability measured by the in situ single-pass perfusion method. This study clearly demonstrated the potential to estimate the permeability of P-gp substrate drugs in the human intestine from its P-gp expression level and thus the possibility to predict the oral absorption of those drugs.

Lipid excipients and delivery systems for pharmaceutical development: a regulatory perspective

The use of lipid-based dosage forms for enhancement of drug absorption or delivery has drawn considerable interest from pharmaceutical scientists. The unique characteristics of these dosage forms, however, present significant challenges to pharmaceutical industry and regulatory agencies in many ways. For example, safety assessment is necessary when the use of a new lipid excipient is considered. An important question for lipid formulation is whether the drug remains in solubilised form along the gastrointestinal (GI) tract after it is administered. Certain lipid excipients and surfactants have been reported to change intestinal permeability or interfere with enzyme/transporter activity, thereby affecting drug bioavailability. The potential influence of biopharmaceutical and/or pathophysiological factors on the drug or lipid excipient(s) needs to be explored. For a complex lipid-based dosage form, the conventional in vitro dissolution methods may not be appropriate for predicting in vivo performance in view of the convoluted GI processing of the lipid vehicle and formulation Of paramount importance is to identify any gaps in the scientific understanding of lipid-based dosage forms so that regulatory issues can be addressed. More mechanistic studies should be encouraged to facilitate a better understanding of the pharmaceutical characteristics of lipid formulations and complex interactions between lipid excipient, drug and physiological environment. This review discusses some regulatory considerations in the use of lipid excipients and delivery systems for pharmaceutical development. Implications in the regulatory determination of pharmaceutical equivalence, bioequivalence and therapeutic equivalence are also illustrated.

Effect of P‐Glycoprotein Expression Levels on the Concentration‐Dependent Permeability of Drugs to the Cell Membrane

The purpose of this study is to develop a kinetic model that can predict the in vivo absorption of P-glycoprotein (P-gp) substrates from in vitro data. Apical (AP) to basal (BL) absorptive permeability of typical P-gp substrate drugs including quinidine, verapamil, vinblastine, and digoxin, were measured in several cell monolayers with different levels of P-gp expression, normal, P-gp induced, P-gp highly induced and MDR1-knockdown Caco-2 cells and MDR1-MDCKII cells. In all cell monolayers, AP to BL permeability of P-gp substrates increased when their AP concentration was increased, showing a sigmoid-type relationship to donor (AP) concentrations. At the higher concentration range, permeability reached a maximum value, suggesting saturation of P-gp-mediated efflux, and at the lower concentration range, permeability decreased depending on P-gp expression level. A simple kinetic model was applied to the permeability-concentration curve of each drug to obtain the fundamental parameters for P-gp-mediated transport, K(m(app)) and V(max). Both K(m(app)) and V(max) of each drug were found to show linear correlations with expression level of P-gp. This study clearly demonstrated the possibility to estimate the permeability of P-gp substrate drugs in human intestine from the expression level of P-gp, and thus the possibility to predict oral absorption of those drugs.

Effect of cell differentiation and passage number on the expression of efflux proteins in wild type and vinblastine-induced Caco-2 cell lines

The mRNA level expression of MDR1, MRP1-6, BCRP and CYP3A4 was determined by quantitative PCR in wild type (Caco-2WT) and vinblastine-treated (Caco-2VBL) Caco-2 cells at different passage levels (32-53). Differentiation increased the mRNA levels of MDR1, BCRP and all the MRPs except MRP4. Corresponding mRNA levels were observed in Caco-2WT and Caco-2VBL, except that the expression of MRD1 was higher in Caco-2VBL than in Caco-2WT cells. CYP3A4 was barely detected in either cell line. MDR1 functionality was studied using rhodamine123 and verapamil as a substrate-inhibitor pair. Corresponding to the observed differences in mRNA levels, MDR1 activity was higher in the Caco-2VBL cells. In Caco-2WT, MDR1 functionality was elevated at low passage numbers (32-35) compared to higher ones (49-53). Verapamil inhibited MDR1 efflux except at higher passage Caco-2WT cells, where no MDR1 activity could be observed. The results support the use of Caco-2VBL cells in MDR1 screening. The functional expression is higher than in Caco-2WT and remains consistent across the studied passages without major differences in mRNA levels of other efflux proteins. As both the passage number and the level of cell differentiation affect the expression profile of efflux proteins, short-term cell growth protocols should be evaluated accordingly.

Evaluation of human P-glycoprotein (MDR1/ABCB1) ATPase activity assay method by comparing with in vitro transport measurements: Michaelis-Menten kinetic analysis to estimate the affinity of P-glycoprotein to drugs

Human ABC transporter P-glycoprotein (P-gp/ABCB1) encoded by the multidrug resistance (MDR1) gene is recognized as one of the most important factors regulating pharmacokinetics of a number of clinically important drugs because of its function of extruding a wide range of structurally unrelated amphiphilic and hydrophobic drugs from the inside to the outside of cells in an ATP-driven mechanism. In the present study, we have evaluated the high-speed ATPase activity assay method by comparing with in vitro transport assay systems using MDR1-transfected MDR1-MDCK cells. Since substrate drugs were found to interfere with the photometric detection of inorganic phosphate (Pi) that was liberated according to the hydrolysis of ATP to ADP in ATPase activity assay, at first, a method in which the amount of Pi can be calculated correctly. Results demonstrate that the kinetic parameters obtained in ATPase activity assay are not necessarily correspond with those in in vitro transport assay, suggesting that these methods might detect the different processes of drug-P-gp interaction. The combining of the ATPase activity assay and in vitro transport technologies provides us the insight into mechanisms of the membrane transport of drugs by P-gp.

Lipid Formulation Strategies for Enhancing Intestinal Transport and Absorption of P-Glycoprotein (P-gp) Substrate Drugs: In vitro/In vivo Case Studies

The intestinal efflux pump, P-glycoprotein (P-gp), located in the apical membranes of intestinal absorptive cells, can reduce the bioavailability of a wide range of drugs which are substrates for this membrane transporter. In addition to anticancer and anti-HIV drugs, NCEs for other disease indications are P-gp substrates and there is considerable interest in inhibiting P-gp and thus increasing the bioavailability of these molecules. In this review article, an overview of P-gp and its role in drug transport and absorption will be presented first and then formulation strategies to effectively inhibit P-gp will be discussed and compared. These strategies independently and in combination, are: (a) coadministration of another P-gp substrate/specific inhibitor, and (b) incorporation of a nonspecific lipid and/or polymer excipient in the formulation. The first approach, although very effective in inhibiting P-gp, utilizes a second active compound in the formulation and thus imposes regulatory constraints and long development timelines on such combination products. Excipient inhibitors appear to have minimal nonspecific pharmacological activity and thus potential side effects of specific active compound inhibitors can be avoided. Case studies will be presented where specific active compounds, surfactants, polymers, and formulations incorporating these molecules are shown to significantly improve the intestinal absorption of poorly soluble and absorbed drugs as a result of P-gp inhibition and enhanced drug transport in vitro.

New 1,3-dioxolane and 1,3-dioxane derivatives as effective modulators to overcome multidrug resistance

Multidrug resistance (MDR) to antitumor agents represents a major obstacle to a successful chemotherapy of cancer. Overexpression of P-glycoprotein (p-gp) seems to be the major factor responsible for MDR. A large number of chemically unrelated compounds are known to interact with p-gp resulting in a decreasing resistance. In our efforts related to structure-activity studies of new potential MDR reversal agents we synthesized a series of compounds that differ in the aromatic core structure, the linker, and the basic moiety. For our search of new aromatic core structures we synthesized novel 2,2-diphenyl-1,3-dioxolane, 2,2- diphenyl-1,3-dioxane, and 4,5-diphenyl-1,3-dioxolane derivatives. A range of lipophilic linker structures and protonable basic moieties were synthesized and investigated to optimize the structure of the potential MDR-modulators. The compounds were tested in vitro using human Caco-2 cells. Both the cytotoxicity of the synthons and their ability to resensitize the cells were determined with a MTT assay. The results show that at low concentration various substances reverse tumor cell MDR. Some of the new structures show better effects than established modulators like trifluoperazine.

Effects of Labrasol and Other Pharmaceutical Excipients on the Intestinal Transport and Absorption of Rhodamine123, a P-Glycoprotein Substrate, in Rats

Effects of Labrasol and other pharmaceutical excipients on the intestinal transport and absorption of rhodamine123, a P-glycoprotein substrate (P-gp) were examined. Intestinal transport and absorption studies were examined by an in vitro diffusion chamber method and an in situ closed loop method. We evaluated the intestinal membrane damage produced by Labrasol by measuring the release of protein and alkaline phosphatase (ALP). Labrasol (0.075-0.1% (v/v)) increased the absorptive transport of rhodamine123 and decreased its secretory transport in the in vitro transport studies. However, Labrasol did not change the transport of Lucifer yellow, a non-P-gp substrate, suggesting that the effect of Labrasol on the transport of drugs was specific for rhodamine123. We observed almost no intestinal membrane damage in the presence of Labrasol. These findings suggest that the increase in the absorptive transport of rhodamine123 in the presence of Labrasol may not be due to its intestinal membrane damage. In the in situ absorption studies, we found that Labrasol (0.1% (v/v)) significantly enhanced the intestinal absorption of rhodamine123 in rats, although the absorption enhancing effect of Labrasol was much less than that of verapamil. These findings suggest that low concentrations of Labrasol might inhibit the function of P-gp in the intestine, thereby increasing intestinal absorption and bioavailability of P-gp substrates including rhodamine123. However, we may also consider the contribution to the enhanced intestinal absorption of rhodamine123 via a passive transport in addition to the inhibitory action of Labrasol for the function of P-gp in the intestine.

Expression of Organic Cation Transporter SLC22A16 in Human Endometria

SLC22A16 is one of newly isolated organic cation transporters, which is responsible for uptake and transport of adriamycin into cells. Adriamycin is one of the key drugs for treatment of endometrial cancer. Therefore, we examined expression of SLC22A16 in human endometrium and its disorders. Protein and mRNA expression levels of SLC22A16 were examined in 124 endometrial cancer specimens, 25 normal endometrial tissue samples (15 in proliferative phase, 10 in secretory phase), and 7 endometrial cancer cell lines using immunohistochemical analysis and reverse transcription-polymerase chain reaction. Changes in SLC22A16 mRNA expression level after progesterone exposure were also examined. Immunohistochemical analysis showed that SLC22A16 protein was highly expressed in endometrium during the normal secretory phase, but its level was significantly reduced in the proliferative phase. SLC22A16 protein was detected in 59 of 124 (48%) endometrial cancer specimens and 3 of 7 (43%) endometrial cancer cell lines. The mRNA levels measured by quantitative reverse transcription-polymerase chain reaction were comparable with levels of protein expression. Furthermore, SLC22A16 mRNA levels were increased in endometrial cancer cell lines in the presence of progesterone. In conclusion, SLC22A16 is expressed in various endometrial tissues. Its expression level is high during the secretory phase and may be regulated by progesterone. Our findings also suggest that it may be possible to use progestins to increase the response of endometrioid endometrial carcinoma with SLC22A16 expression to adriamycin-based chemotherapeutic regimens.

Effects of ischemia-reperfusion on the absorption and esterase metabolism of diltiazem in rat intestine

Intestinal ischemia-reperfusion (I/R) is a serious clinical condition that triggers a complex inflammatory response. Inflammatory processes affect some enzymatic systems related to intestinal drug metabolism and bioavailability. Diltiazem (DTZ) is a calcium channel blocker, which is extensively metabolised in the intestine by esterases and different CYP450 isoforms. The main biotransformation pathway of DTZ in rats is desacetylation by esterases. This study analysed the effect of I/R on intestinal absorption and metabolism of DTZ, focusing on esterase activity, through different methodologies, after 60 min of superior mesenteric artery occlusion and 30 min of reperfusion or sham surgical procedures. The rate of DTZ appearance in blood during in situ studies increased significantly in the I/R group (0.094+/-0.014 10(-5) cm/s vs 0.271+/-0.110 10(-5) cm/s) and the calculated metabolised fraction of DTZ decreased significantly, showing an important reduction in the desacetylase activity in the I/R group. These results were supported by microsomal incubations, where desacetylase activity was related to esterases by specific inhibition, using paraoxon and bis-nitrophenylphosphate, and also by studies in everted rings. DTZ metabolism was higher in the jejunum than in the ileum, the esterase activity being affected by I/R in both regions. The present findings suggest that I/R injury clearly affects the esterases' activity and modifies the amount of DTZ and its metabolites in blood during in situ perfusion. This modification of intestinal esterase activity could be important for the pharmacokinetic behaviour of other drugs and prodrugs after intestinal pathologies involving inflammation and oxidative stress.

In vivo evaluation of an oral delivery system for P-gp substrates based on thiolated chitosan

Recently, thiolated polymers, so called thiomers, have been reported to modulate drug absorption by inhibition of intestinal P-glycoprotein (P-gp). The aim of the present study was to provide a proof-of-principle for a delivery system based on thiolated chitosan in vivo in rats, using rhodamine-123 (Rho-123) as representative P-gp substrate. In vitro, the permeation enhancing effect of unmodified chitosan, chitosan-4 thiobutylamidine (Ch-TBA) and the combination of Ch-TBA with reduced glutathione (GSH) was evaluated by using freshly excised rat intestinal mucosa mounted in Ussing-type chambers. In comparison to buffer only, Rho-123 transport in presence of 0.5% (w/v) chitosan, 0.5% (w/v) Ch-TBA and the combination of 0.5% (w/v) Ch-TBA/0.5% (w/v) GSH, was 1.8-fold, 2.6-fold, 3.8-fold improved, respectively. Furthermore, enteric-coated tablets based on unmodified chitosan or Ch-TBA/GSH, were investigated in vivo. In rats, the Ch-TBA/GSH tablets increased the area under the plasma concentration time curve (AUC0-12) of Rho-123 by 217% in comparison to buffer control and by 58% in comparison to unmodified chitosan. This in vivo study showed that a delivery system based on thiolated chitosan significantly increased the oral bioavailability of P-gp substrate Rho-123.

The potential inhibitory effect of antiparasitic drugs and natural products on P-glycoprotein mediated efflux

The potential inhibitory effect on P-glycoprotein (Pgp) by antiparasitic drugs and natural compounds was investigated. Compounds were screened for Pgp interaction based on inhibition of Pgp mediated [3H]-taxol transport in Caco-2 cells. Bidirectional transport of selected inhibitors was further evaluated to identify potential Pgp substrates using the Caco-2 cells. Of 21 antiparasitics tested, 14 were found to inhibit Pgp mediated [3H]-taxol with K(iapp) values in the range 4-2000 microM. The antimalarial quinine was the most potent inhibitor with a K(iapp) of 4 microM. Of the 12 natural compounds tested, 3 inhibited [3H]-taxol transport with K(iapp) values in the range 50-400 microM. Quinine, amodiaquine, chloroquine, flavone, genistein, praziquantel, quercetin and thiabendazole were further investigated in bidirectional transport assays to determine whether they were substrates for Pgp. Transport of quinine in the secretory direction exceeded that in the absorptive direction and was saturable, suggesting quinine being a Pgp substrate. The rest of the compounds inhibiting Pgp showed no evidence of being Pgp substrates. In conclusion, we have demonstrated that a substantial number of antiparasitic and natural compounds, in a range of concentrations, are capable of inhibiting Pgp mediated [3H]-taxol efflux in Caco-2 cells, without being substrates and this may have implications for drug interactions with Pgp.

Is alpha-pinene a substrate for permeability-glycoprotein in wood rats?

alpha-Pinene is the dominant monoterpene in Juniperus monosperma. Wood rat species in the genus Neotoma that consume J. monosperma vary in their inclusion of it in their wild diet and in their tolerance of whole J. monosperma or alpha-pinene in laboratory feeding trials. A proposed mechanism for variable tolerance is a difference in absorption of alpha-pinene from the small intestine that is mediated by the intestinal transporter permeability glycoprotein (Pgp). To determine if alpha-pinene is a Pgp substrate, we tested whether it can competitively inhibit Pgp and thereby increase the accumulation of a known Pgp substrate (digoxin) in (1) everted sleeves of small intestine from Neotoma stephensi, a juniper specialist, N. albigula, a sympatric generalist that consumes juniper, N. cinerea, a more distantly related generalist, and Sprague-Dawley rats, and (2) in Caco-2 cells that over express Pgp. We also measured Pgp ATPase phosphate production in transfected insect membrane vesicles exposed to alpha-pinene. We found no significant increase in digoxin accumulation with competitive inhibition experiments, and no increase in phosphate production with transfected membranes, at any concentration of alpha-pinene up to 100 muM. To test whether other compounds in juniper affect Pgp activity, we acclimated five N. stephensi to a juniper diet for 5 d, but found no significant effect compared to animals on control diet. Our data suggest that alpha-pinene is not a Pgp substrate.

Application of Pharmacological Approaches to Plant–Mammal Interactions

The dominant theory in the field of mammalian herbivore-plant interactions is that intake, and therefore tolerance, of plant secondary metabolites (PSMs) is regulated by mechanisms that reduce absorption and increase detoxification of PSMs. Methods designed by pharmacologists to measure detoxification enzyme activity, metabolite excretion, and most recently, drug absorption, have been successfully applied by ecologists to study PSM intake in a variety of mammalian study systems. Here, we describe several pharmacological and molecular techniques used to investigate the fate of drugs in human that have potential to further advance knowledge of mammalian herbivore-plant interactions.

Efflux transporters as a novel herbivore countermechanism to plant chemical defenses

The recent discovery of efflux transporters in the gut has revolutionized our understanding of the absorption and bioavailability of pharmaceuticals and other xenobiotics in humans. Despite the celebrity of efflux transporters in the areas of pharmacology and medicine, their significance is only beginning to be realized in the area of plant-herbivore interactions. This review integrates reports on the importance of gut efflux transporters to diet selection by herbivores. The diets of herbivores are laden with toxic plant secondary metabolites (PSMs) that until recently were thought to be processed almost exclusively by detoxification enzymes in the liver. We describe how efflux transporters in the gut may play a critical role in regulating the absorption of PSMs in herbivores and dictating diet selection. Recent studies suggest that the role of efflux transporters in mediating diet selection in herbivores may be as critical as detoxification enzymes. In addition to diet selection, gut efflux transporters have implications for other aspects of plant-animal interactions. They may be significant components of the evolutionary arms race that influences chemical diversity in plants. Furthermore, in agricultural systems, gut efflux transporters may play an important role in the effectiveness of pesticides. This synthesis paper introduces a new direction in plant-herbivore interactions by providing a complementary mechanism, regulated absorption, to detoxification that may define tolerance to PSMs by herbivores.

Polyelectrolyte Nanoparticles with High Drug Loading Enhance the Oral Uptake of Hydrophobic Compounds

In the pharmaceutical industry, orally active compounds are required to have sufficient water solubility to enable dissolution within the gastrointestinal tract prior to absorption. Limited dissolution within the gastrointestinal tract often reduces the bioavailability of hydrophobic drugs. To improve gastrointestinal tract dissolution, nonaqueous solvents are often used in the form of emulsions and microemulsions. Here, we show that oil-free polyelectrolyte nanosystems (micellar dispersions and 100-300 nm particles) prepared from poly(ethylenimines) derivatized with cetyl chains and quaternary ammonium groups are able to encapsulate high levels of hydrophobic drug (0.20 g of drug per g of polymer) for over 9 months, as demonstrated using cyclosporine A (log P = 4.3). The polyelectrolytes facilitate the absorption of hydrophobic drugs within the gastrointestinal tract by promoting drug dissolution and by a hypothesized mechanism involving paracellular drug transport. Polyelectrolyte nanoparticle drug blood levels are similar to those obtained with commercial microemulsion formulations. The polyelectrolytes do not promote absorption by inhibition of the P-glycoprotein efflux pump.

Tetrazole compounds: The effect of structure and pH on Caco-2 cell permeability

A tetrazole ring is often used in drug discovery as a replacement for the carboxylic acid group. Previous work indicates that compounds containing a tetrazole moiety show asymmetric permeability in Caco-2 cells characteristic of an efflux transporter substrate. The aim of this study is to determine which transporters are responsible for polarization of transport of tetrazole-containing compounds in Caco-2 cells. Results indicate that only select compounds with tetrazole moieties display asymmetric transport. Three compounds (two commercial drug products and one druglike structure) were selected for further studies. Losartan appears to be primarily a P-glycoprotein (P-gp) substrate, as previously reported, but MRP inhibitors such as MK-571 and rifampicin also affect the difference between apical to basolateral and basolateral to apical transport. Pemirolast and phenyltetrazole derivative C are sensitive to P-gp inhibition, but transport seems to be mediated by one or more of the MRP family of transporters. Additionally, lowering the pH from 7.4 to 4.0 eliminates the polarization of permeability in Caco-2 cells. These studies indicate that some tetrazole compounds are susceptible to efflux, therefore caution should be used when choosing an appropriate functional group to replace carboxylic acids when synthesizing a drug candidate.

Glutathione and thiolated chitosan inhibit multidrug resistance P-glycoprotein activity in excised small intestine

The aim of the present study was to evaluate the influence of glutathione (GSH), the thiomer chitosan-4-thiobutylamidine (chitosan-TBA) and a combination of both compounds on P-glycoprotein (P-gp) activity. Permeation studies were performed with freshly excised guinea pig ileum mounted in Ussing chambers using the fluorescent dye rhodamine-123 (Rho-123) as P-gp substrate. Apparent permeability coefficients (Papp) as well as efflux ratios (secretory Papp/absorptive Papp) were calculated and compared with values gained from experiments with the well-established P-gp inhibitors terfenadine and verapamil. In the presence of terfenadine, verapamil as well as GSH, the absorptive transport of Rho-123 across intestinal tissue increased, while the secretory decreased with efflux ratios around 1.0. Chitosan-TBA and especially chitosan-TBA/GSH not only enhanced absorption of Rho-123, but also reduced the basolateral to apical secretion of Rho-123 resulting in efflux ratios of 1.1, 0.8 and 0.5. The study indicates that chitosan-TBA/GSH is a potentially valuable tool for inhibiting the ATPase activity of P-gp in the intestine.

Induction of Human P-Glycoprotein in Caco-2 cells: Development of a Highly Sensitive Assay System for P-Glycoprotein-Mediated Drug Transport

The aim of this work is to develop a highly sensitive assay system for P-gp-mediated transport by using two methods, induction of P-gp and short-term culture of Caco-2 cells. To induce P-gp in Caco-2 cells, cells were cultured in vinblastine-containing medium. The mRNA level of P-gp was approximately 7-fold higher in Caco-2 cells cultured with vinblastine (P-gp-induced Caco-2 cells) than in control cells. Western blot analysis showed a significant increase in P-gp expression. After cell differentiation, the mRNA level of P-gp was downregulated, however, P-gp-induced Caco-2 cells still possessed a 5.6-fold higher mRNA level of P-gp compared to control cells. Polarized transport of substrate drugs was greater in the monolayer of P-gp-induced cells than in that of control cells. Moreover, we found that P-gp expression in Caco-2 cells could be further enhanced by applying the higher concentration of vinblastine. Transport activity of P-gp in Caco-2 cells cultured with higher concentration of vinblastine was markedly higher than that in P-gp-induced Caco-2 cells and was comparable with that in MDR1-MDCKII cells. In conclusion, this study provided a stable and highly sensitive in vitro assay system that can identify compounds that are subject to P-gp-mediated efflux.

Herb-drug interactions: a literature review

Herbs are often administered in combination with therapeutic drugs, raising the potential of herb-drug interactions. An extensive review of the literature identified reported herb-drug interactions with clinical significance, many of which are from case reports and limited clinical observations. Cases have been published reporting enhanced anticoagulation and bleeding when patients on long-term warfarin therapy also took Salvia miltiorrhiza (danshen). Allium sativum (garlic) decreased the area under the plasma concentration-time curve (AUC) and maximum plasma concentration of saquinavir, but not ritonavir and paracetamol (acetaminophen), in volunteers. A. sativum increased the clotting time and international normalised ratio of warfarin and caused hypoglycaemia when taken with chlorpropamide. Ginkgo biloba (ginkgo) caused bleeding when combined with warfarin or aspirin (acetylsalicylic acid), raised blood pressure when combined with a thiazide diuretic and even caused coma when combined with trazodone in patients. Panax ginseng (ginseng) reduced the blood concentrations of alcohol (ethanol) and warfarin, and induced mania when used concomitantly with phenelzine, but ginseng increased the efficacy of influenza vaccination. Scutellaria baicalensis (huangqin) ameliorated irinotecan-induced gastrointestinal toxicity in cancer patients.Piper methysticum (kava) increased the 'off' periods in patients with parkinsonism taking levodopa and induced a semicomatose state when given concomitantly with alprazolam. Kava enhanced the hypnotic effect of alcohol in mice, but this was not observed in humans. Silybum marianum (milk thistle) decreased the trough concentrations of indinavir in humans. Piperine from black (Piper nigrum Linn) and long (P. longum Linn) peppers increased the AUC of phenytoin, propranolol and theophylline in healthy volunteers and plasma concentrations of rifamipicin (rifampin) in patients with pulmonary tuberculosis. Eleutheroccus senticosus (Siberian ginseng) increased the serum concentration of digoxin, but did not alter the pharmacokinetics of dextromethorphan and alprazolam in humans. Hypericum perforatum (hypericum; St John's wort) decreased the blood concentrations of ciclosporin (cyclosporin), midazolam, tacrolimus, amitriptyline, digoxin, indinavir, warfarin, phenprocoumon and theophylline, but did not alter the pharmacokinetics of carbamazepine, pravastatin, mycophenolate mofetil and dextromethorphan. Cases have been reported where decreased ciclosporin concentrations led to organ rejection. Hypericum also caused breakthrough bleeding and unplanned pregnancies when used concomitantly with oral contraceptives. It also caused serotonin syndrome when used in combination with selective serotonin reuptake inhibitors (e.g. sertraline and paroxetine). In conclusion, interactions between herbal medicines and prescribed drugs can occur and may lead to serious clinical consequences. There are other theoretical interactions indicated by preclinical data. Both pharmacokinetic and/or pharmacodynamic mechanisms have been considered to play a role in these interactions, although the underlying mechanisms for the altered drug effects and/or concentrations by concomitant herbal medicines are yet to be determined. The clinical importance of herb-drug interactions depends on many factors associated with the particular herb, drug and patient. Herbs should be appropriately labeled to alert consumers to potential interactions when concomitantly used with drugs, and to recommend a consultation with their general practitioners and other medical carers.

MDR1 genotype do not influence the absorption of a single oral dose of 100 mg talinolol in healthy Chinese males

OBJECTIVE

We investigated the linkage between SNPs in exon 12 (C1236T), exon 21 (G2677T/A) and exon 26 (C3435T) of MDR1, and explored the effect of linked polymorphism on the absorption of talinolol after a single oral dose of 100 mg.

METHODS

The genotype of 192 healthy Chinese volunteers was determined using PCR-RFLP with respect to the MDR1 alleles of interest, C1236T, G2677T/A and C3435T. Linkage disequilibrium was analyzed using PHASE software. Consecutive eligible subjects received a single oral dose of 100 mg talinolol. Venous blood samples were taken at intervals up to 60 h post dose for HPLC analysis of plasma concentration of talinolol to obtain a pharmacokinetic profile.

RESULTS

Linkage disequilibrium existed between exon 21 (G2677T/A) and exon 26 (C3435T), exon 12 (C1236T) and exon 21 (G2677T/A), but not between exon 12 (C1236T) and exon 21 (G2677T/A). AUC (0,3 h), AUC (0, infinity), Cmax and Cmax/AUC (0, infinity), used as indices of talinolol absorption, were not significantly different between the genotype groups of 2677GG/3435TT, 2677TT/3435TT, 2677GT/3435CT and 2677AT/3435CT. For these 4 groups, AUC(0,3 h) were 436.8 +/- 50.1, 510.1 +/- 86.3, 466.1 +/- 77.8 and 437.2 +/- 73.4 (microg x h/l) and the Cmax/AUC (0, infinity) were 0.097 +/- 0.018, 0.093 +/- 0.022, 0.105 +/- 0.014 and 0.102 +/- 0.027 (h(-1)), respectively. (P > 0.05).

CONCLUSIONS

The linked MDR1 polymorphisms in exon 21 G2677T/A and exon 26 C3435T apparently did not contribute to the absorptive pharmacokinetics of a single oral dose of 100 mg talinolol.

The role of plant secondary metabolites in mammalian herbivory: ecological perspectives

Plant secondary metabolites (PSM) have many ecological functions, but have long been considered as defences against pathogens or herbivores (vertebrate or invertebrate), reducing the likelihood and extent of attack. However, mammalian herbivores ingest many foods containing PSM and use both behavioural methods and physiological strategies to limit their negative effects. Most physiological counter-adaptations are inducible in response to ingested PSM, providing efficient protection against toxic effects. Possible positive effects of PSM include antioxidant and anthelminthic properties and complex formation between protein and condensed tannins that protects dietary protein from degradation by the symbiotic microflora of foregut fermenters, increasing its utilisation by the animal. This protein effect is probably only beneficial to animals under a narrow range of nutrient-rich conditions found mainly in agricultural systems. There are many examples of PSM causing food avoidance or reducing food intake, but there is as yet relatively little evidence for positive selection of them by herbivores. Although the feedback mechanisms relating the post-ingestive consequences of PSM to subsequent foraging behaviour are beginning to be understood, knowledge of the integration of behavioural and physiological strategies for regulating the effects of PSM is relatively poor. The opportunities for learned avoidance of PSM may be restricted in animals with complex diets that cannot associate a particular feedback signal with a given food type. A greater emphasis on the study of subclinical effects of PSM rather than acute effects, on pharmaco-kinetic studies in relation to behavioural studies and on the use of realistic experimental models is advocated.

Engineering polysaccharide-based polymeric micelles to enhance permeability of cyclosporin A across Caco-2 cells

PURPOSE

To assess and compare the effectiveness of two types of polysaccharide-based micelles as delivery vehicles for poorly water soluble drugs by monitoring their permeability across Caco-2 cell monolayers.

METHODS

Dextran (DEX) and hydroxypropylcellulose (HPC) were hydrophobically modified (HM) by grafting polyoxyethylene cetyl ether (POE-C16, 15 mol% and 5.4 mol%, respectively). The onset of micellization and mean diameter of polymeric micelles formed by HM-DEX and HM-HPC were determined by fluorescence spectroscopy and dynamic light scattering, respectively. Cyclosporin A (CsA)-loaded polymeric micelles were prepared by a dialysis procedure, and the amount of incorporated CsA was assayed by high performance liquid chromatography (HPLC). The stability of micelles in simulated gastric and intestinal fluids was studied as a function of contact time, and their cytotoxicity toward Caco-2 cells was evaluated using the MTT colorimetric assay. The bidirectional transport across Caco-2 cell monolayers of CsA entrapped in HM-DEX and HM-HPC micelles and of the polymers themselves was evaluated in the presence and absence of P-glycoprotein inhibitor.

RESULTS

The amount of CsA incorporated in HM-HPC and HM-DEX micelles reached 5.5 and 8.5% w/w, respectively (entrapment efficiency of 22% or more). The polymeric micelles exhibited high stability in gastric and intestinal fluids and no significant cytotoxicity toward Caco-2 cells. The apical to basal permeability of CsA across Caco-2 cells increased significantly when loaded in polymeric micelles compared to free CsA.

CONCLUSIONS

Polysaccharide-based polymeric micelles are promising carriers for the oral delivery of poorly water soluble drugs. In vitro tests indicate that, overall, HM-HPC micelles are more effective compared to HM-DEX micelles.

Intestinal capacity of P-glycoprotein is higher in the juniper specialist, Neotoma stephensi, than the sympatric generalist, Neotoma albigula

Permeability-glycoprotein (Pgp) is a membrane-bound, ATP-dependent, transport protein that excludes many cytotoxic compounds including plant metabolites and pollutants from the barrier epithelia of many tissues including the small intestine. We hypothesized that intestinal Pgp capacity would be higher in Neotoma stephensi, a specialist on Juniperus monosperma known to be high in plant toxins, than the sympatric generalist, Neotoma albigula, which consumes juniper in the field, but is unable to tolerate a high juniper diet. We measured Pgp activity as the difference in accumulation of a known Pgp substrate, digoxin, between everted sections of small intestine exposed to ethanol vehicle control and a maximal level of a known competitive inhibitor of Pgp, cyclosporin A. We estimated intestinal capacity by averaging Pgp activity along the intestine and multiplying by total small intestine mass. These first measures of Pgp in wild mammals show a significant difference among species with the juniper specialist, N. stephensi, exhibiting a 2.4 fold higher capacity than the generalist, N. albigula. This result suggests that Pgp may play a role in the ability of N. stephensi to tolerate juniper.

Selective Downregulation of the MDR1 Gene Product in Caco-2 Cells by Stable Transfection To Prove Its Relevance in Secretory Drug Transport

Considerable interest is focused on overcoming multidrug resistance (MDR) in cancer chemotherapy. The in vitro experiments to characterize P-glycoprotein's (P-gp) function and to decrease its effects have led to a variety of strategies such as addition of competitors or supplementation of the medium with oligonucleotides complementary to the 5'-end of the MDR1-mRNA. For the Caco-2 cell line, an in vitro model for absorption screening, expressing multiple transporters including P-gp, which pumps substances back into the apical solution, P-gp activity might mask other relevant transport proteins' activity. The objective of the present study was to construct a Caco-2 subline with reduced P-gp expression level. Caco-2 cells were transfected by electroporation with two different mammalian expression vectors, and the obtained subclones were investigated at RNA (Northern blotting, RT-PCR), protein (FACS analysis), and functional (transport studies) levels for reduction in P-gp expression. Northern blotting showed that the levels of transcription of the inserted gene were different among the several clones, but those results did not completely correlate with the FACS analysis for P-gp expression. The clones with the strongest reduction in P-gp expression detected by the FACS analysis also showed the lowest secretory fluxes of the P-gp substrate talinolol in transport studies. Repetition of FACS analysis after 7 and 24 months on 20 to 30 passage older subclones still showed reduction in P-gp expression and indicated that they are stably transfected. The new cell lines constructed in the present study provide the possibility to perform in vitro absorption studies in a cell system composed of differentiated enterocytes growing as a monolayer like the normal Caco-2 cell line but with a lower down to almost lacking expression of P-gp.

Oral Delivery of Peptide Drugs

A wide variety of peptide drugs are now produced on a commercial scale as a result of advances in the biotechnology field. Most of these therapeutic peptides are still administered by the parenteral route because of insufficient absorption from the gastrointestinal tract. Peptide drugs are usually indicated for chronic conditions, and the use of injections on a daily basis during long-term treatment has obvious drawbacks. In contrast to this inconvenient and potentially problematic method of drug administration, the oral route offers the advantages of self-administration with a high degree of patient acceptability and compliance. The main reasons for the low oral bioavailability of peptide drugs are pre-systemic enzymatic degradation and poor penetration of the intestinal mucosa. A considerable amount of research has focused on overcoming the challenges presented by these intestinal absorption barriers to provide effective oral delivery of peptide and protein drugs. Attempts to improve the oral bioavailability of peptide drugs have ranged from changing the physicochemical properties of peptide molecules to the inclusion of functional excipients in specially adapted drug delivery systems. However, the progress in developing an effective peptide delivery system has been hampered by factors such as the inherent toxicities of absorption-enhancing excipients, variation in absorption between individuals, and potentially high manufacturing costs. This review focuses on the intestinal barriers that compromise the systemic absorption of intact peptide and protein molecules and on the advanced technologies that have been developed to overcome the barriers to peptide drug absorption.