Involvement of intestinal P-glycoprotein in the restricted absorption of methylprednisolone from rat small intestine

Integration of In Silico, In Vitro and In Situ Tools for the Preformulation and Characterization of a Novel Cardio-Neuroprotective Compound during the Early Stages of Drug Development

The main aim of this work is the biopharmaceutical characterization of a new hybrid benzodiazepine-dihydropyridine derivative, JM-20, derived with potent anti-ischemic and neuroprotective effects. In this study, the pKa and the pH-solubility profile were experimentally determined. Additionally, effective intestinal permeability was measured using three in vitro epithelial cell lines (MDCK, MDCK-MDR1 and Caco-2) and an in situ closed-loop intestinal perfusion technique. The results indicate that JM-20 is more soluble at acidic pH (9.18 ± 0.16); however, the Dose number (Do) was greater than 1, suggesting that it is a low-solubility compound. The permeability values obtained with in vitro cell lines as well as with the in situ perfusion method show that JM-20 is a highly permeable compound (Caco-2 value 3.8 × 10-5). The presence of an absorption carrier-mediated transport mechanism was also demonstrated, as well as the efflux effect of P-glycoprotein on the permeability values. Finally, JM-20 was provisionally classified as class 2 according to the biopharmaceutical classification system (BCS) due to its high intestinal permeability and low solubility. The potential good oral absorption of this compound could be limited by its solubility.

Downregulated expression of intestinal P-glycoprotein in rats with cisplatin-induced acute kidney injury causes amplification of its transport capacity to maintain "gatekeeper" function

The expression of transporters on the apical and basal membranes of renal proximal tubular cells are down- or upregulated to various extents under cisplatin (CDDP)-induced acute kidney injury (AKI). However, little is known about the changes in transporters in tissues other than the kidney under CDDP-induced AKI. This study aimed to investigate the modulation of the expression/function of intestinal efflux transporters, P-glycoprotein (P-gp) and breast cancer resistance protein (Bcrp), in CDDP-induced AKI rats. On day 3 after the intraperitoneal administration of CDDP (5 mg/kg) to rats, the expression levels of P-gp and Bcrp were compared with those of normal rats. Further, the absorption of three P-gp substrates (6α-methylprednisolone, rhodamine 123, and gatifloxacin) was evaluated in both groups using conventional loop techniques. In the CDDP-induced AKI rats, P-gp expression in the ileum was markedly decreased to approximately 38% of that in the normal rats. However, no significant changes in Bcrp expression were observed in the AKI rats. In contrast with the reduction in P-gp expression in the AKI rats, the absorption of the three P-gp substrates remained almost the same or decreased in the AKI group. The addition of verapamil (a potent P-gp inhibitor) increased the absorption of the three P-gp substrates to the values obtained from the normal rats. In conclusion, our results suggested that P-gp expression is downregulated in rats with CDDP-induced AKI but that P-gp maintains its potency as a "gatekeeper" against the absorption of xenobiotics by amplifying its individual transport capacity under these conditions.

Biopharmaceutics classification of puerarin and comparison of perfusion approaches in rats

The present study was conducted to characterize the biopharmaceutics classification system (BCS) category of puerarin in terms of intrinsic dissolution rate (IDR) and rat intestinal permeability and to investigate the poor intestinal absorption probably related to the drug metabolism in the gut wall of rats. Equilibrium solubility of puerarin was determined in various phosphate buffers and water, and IDR was estimated by measuring the dissolution of a non-disintegrating compact. Intestinal permeability (Peff and Pblood) of puerarin was determined using the technology of in situ single-pass intestinal perfusion (SPIP) and intestinal perfusion with venous sampling (IPVS) in fasted rats. Metabolism of puerarin in intestinal tissue was tested by S9 incubation in vitro. The aqueous solubility of puerarin in phosphate buffers and water was good with a maximum solubility of 7.56 mg/mL at pH 7.4. Obtained IDR values of puerarin were in the range of 0.360-1.088 mg/min/cm(2), with maximum and minimum IDR value of pH 7.4 and pH 4.0, respectively. The Peff was 1.252 × 10(-5)cm/s determined by SPIP and the Pblood was 0.068×10(-5)cm/s by IPVS in jejunum at puerarin 80 μg/mL. The metabolism rate of puerarin determined by the intestinal S9 fraction indicated that the gut wall metabolism of puerarin is one cause of poor absorption. According to the proposed classification of drugs and the results obtained from equilibrium solubility, IDR, Peff and Pblood, it is concluded that puerarin could be categorized IV drug of the BCS based on its low solubility and low intestinal permeability values.

ABC transporters in multidrug resistance and pharmacokinetics, and strategies for drug development

Multidrug resistance (MDR) is a serious problem that hampers the success of cancer pharmacotherapy. A common mechanism is the overexpression of ATP-binding cassette (ABC) efflux transporters in cancer cells such as P-glycoprotein (P-gp/ABCB1), multidrug resistance-associated protein 1 (MRP1/ABCC1) and breast cancer resistance protein (BCRP/ABCG2) that limit the exposure to anticancer drugs. One way to overcome MDR is to develop ABC efflux transporter inhibitors to sensitize cancer cells to chemotherapeutic drugs. The complete clinical trials thus far have showen that those tested chemosensitizers only add limited or no benefits to cancer patients. Some MDR modulators are merely toxic, and others induce unwanted drug-drug interactions. Actually, many ABC transporters are also expressed abundantly in the gastrointestinal tract, liver, kidney, brain and other normal tissues, and they largely determine drug absorption, distribution and excretion, and affect the overall pharmacokinetic properties of drugs in humans. In addition, ABC transporters such as P-gp, MRP1 and BCRP co-expressed in tumors show a broad and overlapped specificity for substrates and MDR modulators. Thus reliable preclinical assays and models are required for the assessment of transporter-mediated flux and potential effects on pharmacokinetics in drug development. In this review, we provide an overview of the role of ABC efflux transporters in MDR and pharmacokinetics. Preclinical assays for the assessment of drug transport and development of MDR modulators are also discussed.

The dual cyclooxygenase/5-lipoxygenase inhibitor licofelone attenuates p-glycoprotein-mediated drug resistance in the injured spinal cord

There are currently no proven effective treatments that can improve recovery of function in spinal cord injury (SCI) patients. Many therapeutic compounds have shown promise in pre-clinical studies, but clinical trials have been largely unsuccessful. P-glycoprotein (Pgp, Abcb1b) is a drug efflux transporter of the blood-spinal cord barrier that limits spinal cord penetration of blood-borne xenobiotics. Pathological Pgp upregulation in diseases such as cancer causes heightened resistance to a broad variety of therapeutic drugs. Importantly, several drugs that have been evaluated for the treatment of SCI, such as riluzole, are known substrates of Pgp. We therefore examined whether Pgp-mediated pharmacoresistance diminishes delivery of riluzole to the injured spinal cord. Following moderate contusion injury at T10 in male Sprague-Dawley rats, we observed a progressive, spatial spread of increased Pgp expression from 3 days to 10 months post-SCI. Spinal cord uptake of i.p.-delivered riluzole was significantly reduced following SCI in wild type but not Abcb1a-knockout rats, highlighting a critical role for Pgp in mediating drug resistance following SCI. Because inflammation can drive Pgp upregulation, we evaluated the ability of the new generation dual anti-inflammatory drug licofelone to promote spinal cord delivery of riluzole following SCI. We found that licofelone both reduced Pgp expression and enhanced riluzole bioavailability within the lesion site at 72 h post-SCI. This work highlights Pgp-mediated drug resistance as an important obstacle to therapeutic drug delivery for SCI, and suggests licofelone as a novel combinatorial treatment strategy to enhance therapeutic drug delivery to the injured spinal cord.

Oral and inhaled corticosteroids: differences in P-glycoprotein (ABCB1) mediated efflux

There is concern that P-glycoprotein mediated efflux contributes to steroid resistance. Therefore, this study examined bidirectional corticosteroid transport and induction capabilities for P-glycoprotein (P-gp) to understand which of the systemic and inhaled corticosteroids interacted with P-gp to the greatest extent. Hydrocortisone, prednisolone, prednisone, methylprednisolone, and dexamethasone represented systemically active drugs, while fluticasone propionate, beclomethasone dipropionate, ciclesonide and budesonide represented inhaled corticosteroids. Aldosterone and fludrocortisone represented mineralocorticoids. All drugs were detected using individually optimised HPLC protocols. Transport studies were conducted through Caco-2 monolayers. Hydrocortisone and aldosterone had efflux ratios below 1.5, while prednisone showed a P-gp mediated efflux ratio of only 1.8 compared to its active drug, prednisolone, with an efflux ratio of 4.5. Dexamethasone and beclomethasone had efflux ratios of 2.1 and 3.3 respectively, while this increased to 5.1 for methylprednisolone. Fluticasone showed an efflux ratio of 2.3. Protein expression studies suggested that all of the inhaled corticosteroids were able to induce P-gp expression, from 1.6 to 2 times control levels. Most of the systemic corticosteroids had higher passive permeability (>20×10(-6) cm/s) compared to the inhaled corticosteroids (>5×10(-6) cm/s), except for budesonide, with permeability similar to the systemic corticosteroids. Inhaled corticosteroids are not transported by P-gp to the same extent as systemic corticosteroids. However, they are able to induce P-gp production. Thus, inhaled corticosteroids may have greater interactions with other P-gp substrates, but P-gp itself is less likely to influence resistance to the drugs.

Cyclooxygenase-2 inhibitors prevent trinitrobenzene sulfonic acid-induced P-glycoprotein up-regulation in vitro and in vivo

Failed medical therapy is a common problem in inflammatory bowel disease. P-glycoprotein (P-gp), an efflux pump encoded by MDR1 (ABCB1) gene can actively pump drugs out of cells conferring the phenotype of multidrug resistance. Various studies evoked that cyclooxygenase (COX) system may be involved in the regulation of P-gp activity. Since COX-2 isoform is overexpressed in colic inflammatory states, we examined the inhibitory effect of COX-2-inhibitors on P-gp expression and function under COX-2 stimulated conditions mediated by trinitrobenzene sulfonic acid (TNBS) in vitro, in Caco-2 cells, and in TNBS-induced colitis in mice. COX-2 and P-gp expressions were evaluated by real-time PCR and western blot. The activity of P-gp was measured by intracellular accumulation of rhodamine123 (Rho123) in Caco-2 cells and by Rho123 efflux using the intestinal everted loop method in mice. We showed that COX-2 stimulation in Caco-2 cells by 0.1mM TNBS exposure for 24h induced P-gp protein expression and activity. This activation was reversed by simultaneous COX-2-inhibitor treatment. Moreover, this effect was reproduced in vivo, in mice, where an increased P-gp expression and activity were observed 24h post intra-rectal TNBS administration. Induced P-gp expression and activity could be blocked by the oral pre-treatment with indomethacin heptyl ester (IHE) (20mg/kg). Administration of indomethacin heptyl ester had also a protective effect in TNBS-induced colitis. Our observations suggest that the inhibition of P-gp by COX-2-inhibitors could contribute to the improvement of medical response and this finding may have relevance to medical treatment of inflammatory bowel disease patients.

Relationship between P-glycoprotein activity measured in peripheral blood mononuclear cells and indinavir bioavailability in healthy volunteers

Indinavir, a HIV-1 protease inhibitor, showed large inter-individual pharmacokinetic variability. It has been proposed as a substrate of P-glycoprotein (P-gp), an efflux transporter, that may contribute to limit indinavir bioavailability. A liquid formulation of indinavir was developed from indinavir capsules in order to study indinavir pharmacokinetics in healthy volunteers. Compartmental and noncompartmental analysis of indinavir plasma concentrations showed high inter-individual variability in terms of area under the curve (AUC) and maximal plasma concentration (C(max)). A significant negative association between AUC normalized to body weight (AUC x weight) and lymphocyte P-gp activity, using Rh123 efflux assay, was observed (p = 0.008; r = -0.75). AUC normalized to elimination rate constant (AUC x beta) also showed a significant negative relationship with lymphocyte P-gp activity (p = 0.03, r = -0.64). Apparent clearance (CL/[F x weight]) and volume of distribution (VD/[F x weight]) showed a positive correlation with P-gp activity. Conversely, elimination rate constant did not correlate with P-gp activity. Although there is not enough evidence of a correlation between lymphocitary and intestinal function of P-gp, our results suggest a relationship between a P-gp phenotype marker, Rh123 efflux assay in lymphocytes, and indinavir bioavailability.

The role of multi-drug resistance p-glycoprotein in glucocorticoid function: studies in animals and relevance in humans

Entry of glucocorticoid hormones into cells is tightly regulated by membrane transporters. One of these transporters, the multi-drug resistance p-glycoprotein, has been extensively described to confer treatment resistance to tumour cells as well as to regulate the intracellular levels of glucocorticoid hormones. Moreover, multi-drug resistance p-glycoprotein is also present on the endothelial cells of the blood-brain-barrier, and in neurones, where it limits the access of glucocorticoids to the brain. Finally, this transporter also has the ability to limit the entry of some antidepressants to the brain, with potential consequences for the clinical therapeutic effects of these drugs. This review will focus on the studies that have used multi-drug resistance p-glycoprotein knockout animals in such context, and will discuss the potential clinical relevance of these transporters for psychiatric disorders. In particular, we will discuss the reciprocal interactions between this transporter and antidepressants, both as its inhibitors and as its substrates. We believe that the interaction between antidepressants and multi-drug resistance p-glycoprotein is one of the most potentially exciting developments in psychopharmacological research.

Methoxypolyethylene Glycol-block-polycaprolactone Diblock Copolymers Reduce P-glycoprotein Efflux in the Absence of a Membrane Fluidization Effect while Stimulating P-glycoprotein ATPase Activity

We have previously shown that amphiphilic diblock copolymers composed of methoxypolyethylene glycol-b-polycaprolactone (MePEG-b-PCL) increased the cellular accumulation and reduced the basolateral to apical flux of the P-glycoprotein substrate, rhodamine 123 (R-123) in caco-2 cells. The purpose of this study was to investigate membrane perturbation effects of MePEG-b-PCL diblock copolymers with erythrocyte membranes and caco-2 cells and the effect on P-gp ATPase activity. The diblock copolymer MePEG(17)-b-PCL(5) induced increasing erythrocyte hemolysis at concentrations which correlated with increasing accumulation of R-123 into caco-2 cells. However, no increase in cellular accumulation of R-123 by non-P-gp expressing cells was observed, suggesting that diblock did not enhance the transmembrane passive diffusion of R-123, but that the accumulation enhancement effect of the diblock in caco-2 cells was likely mediated primarily via P-gp inhibition. Fluorescence anisotropy measurements of membrane fluidity and P-gp ATPase activity demonstrated that MePEG(17)-b-PCL(5) decreased caco-2 membrane fluidity while stimulating ATPase activity approximately threefold at concentrations that maximally enhanced R-123 caco-2 accumulation. These results suggest that inhibition of P-gp efflux by MePEG(17)-b-PCL(5) does not appear to be related to increases in membrane fluidity or through inhibition in P-gp ATPase activities, which are two commonly reported cellular effects for P-gp inhibition mediated by surfactants.

Efflux transporters in ulcerative colitis: decreased expression of BCRP (ABCG2) and Pgp (ABCB1)

BACKGROUND

Efflux transport proteins are important components of the intestinal barrier against bacterial toxins, carcinogens, and drugs. This investigation was conducted to determine the expression of Breast Cancer Resistance Protein (BCRP/ABCG2), P-glycoprotein (Pgp/MDR1/ABCB1), and Multidrug Resistance Protein 2 (MRP2/ABCC2) in the gut mucosa of patients with ulcerative colitis (UC).

METHODS

Patients were thoroughly diagnosed according to well-established clinical, endoscopic, and histologic criteria to be included in the group of patients with active UC (n = 16) or UC in remission (n = 17). Colonic and rectal mucosa from patients with UC were compared with tissues from control subjects (n = 15). The mRNA expression (TaqMan) of the efflux transporters and the proinflammatory cytokines interleukin (IL)-1beta and IL-6 was determined. Western blot was used in the analysis of protein expression and the tissue localization of BCRP was determined with confocal microscopy.

RESULTS

BCRP and Pgp expression was strongly reduced in individuals with active inflammation compared with controls and was negatively correlated with the levels of IL-6 mRNA. The BCRP staining of colonic epithelium seen in healthy mucosa was diminished in inflamed tissues, with concurrent disruption of epithelial F-actin structure.

CONCLUSIONS

Two of the efflux transporters of importance for the barrier function of the gut mucosa, Pgp and BCRP, are expressed at strongly reduced levels during active inflammation in patients with UC. Investigations are warranted to determine whether the low levels of efflux transporters during active UC contribute to altered transport and tissue exposure of carcinogens, bacterial toxins, and drugs.

Involvement of P-glycoprotein in the release of cytokines from peripheral blood mononuclear cells treated with methotrexate and dexamethasone

P-glycoprotein (P-gp), a product of the MDR1 gene, is an important factor in the turnover of many drugs and xenobiotics. Recent reports have suggested that P-gp can also be involved in the transport of cytokines. The aim of this study was to examine the role of P-gp in cytokine release from phytohaemagglutinin (PHA)-stimulated peripheral blood mononuclear cells (MNCs) as well as in the release of cytokines from MNCs treated with methotrexate (MTX) and dexamethasone (DEX). The study was carried out on PHA-stimulated MNC from 10 healthy subjects. Flow cytometry was applied to measure interleukin (IL)-2, IL-4, IL-6, IL-10, interferon (IFN)-gamma and tumour necrosis factor (TNF)-alpha levels in the culture supernatants. In the experiments verapamil (VER) and P-gp specific monoclonal antibodies (mAb) (clone 17F9) were used to inhibit P-gp function. P-gp inhibitors suppressed the release of IL-2, IL-4, IFN-gamma and TNF-alpha from PHA-stimulated MNC, whereas release of IL-6 and IL-10 remained unaffected. VER and mAb significantly decreased the release of IL-2, IL-4, TNF-alpha and INF-gamma in MNC cultures treated with MTX or DEX. The results of this study suggest that P-gp may be involved in the transmembrane transport of some cytokines. Moreover, it seems that blocking of P-gp function may influence the release of some cytokines from MNCs, displaying an additive inhibitory effect to DEX and MTX.

Reduced methylprednisolone clearance causing prolonged pharmacodynamics in a healthy subject was not associated with CYP3A5*3 allele or a change in diet composition

The influence of diet and genetics was investigated in a healthy white person who had distinctly low methylprednisolone clearance. Pharmacokinetic and pharmacodynamic parameter values were similar on 2 occasions during the consumption of a low-carbohydrate diet and a Weight Watchers diet, indicating that the decreased clearance was unlikely attributable to a change in diet composition. Although the subject was found to be homozygous for CYP3A5*3, genetic findings were not significant for a number of other CYP3A4 and CYP3A5 allelic variants. Because of the high prevalence of CYP3A5*3/*3 in whites and because 5 of 7 white control subjects are also homozygous for CYP3A5*3, this genotype cannot fully explain the reduced metabolism of the drug. Other genetic or contributing factors might have been involved. New polymerase chain reaction-based genotyping methods for functionally defective CYP3A5*6, *8, *9, and *10 alleles were developed in this study. These assays will be useful for CYP3A5 genotype analysis in future clinical studies.

The effect of 3435C>T MDR1 gene polymorphism on rheumatoid arthritis treatment with disease-modifying antirheumatic drugs

OBJECTIVE

Rheumatoid arthritis (RA) is a multifactorial disease, with immunological, genetical as well as environmental factors being implicated in its pathogenesis. Treatment of RA is based mainly on drugs modulating the course of the disease, e.g. methotrexate (MTX) or sulfasalazine (SL). The MDR1 gene product, P-glycoprotein (P-gp), is probably one of the most important and best defined transporters for drug delivery in humans. P-gp transports a wide range of substrates with diverse chemical structures, among them anticancer agents, cardiac drugs, and immunosuppressants. The aim of this study was to examine the effect of the 3435C>T MDR1 gene polymorphism on the efficacy of RA treatment with disease-modifying antirheumatic drugs, i.e. MTX plus methylprednisolone (MP), and SL.

METHODS

The study was carried out on 255 patients with RA treated according to two regimes: (1) MTX (7.5-15.0 mg weekly) plus low doses of MP (n=174), (2) SL (1.5-3 g daily, n=81).

RESULTS

The probability of remission of RA symptoms after MTX plus MP therapy was 4.65-fold higher in carriers of the TT genotype compared to patients with CC genotype (P=0.003, OR 4.65, 95%CI 1.66-13.05), whereas the probability of remission of RA symptoms in patients treated with SL was 2-fold higher in carriers of TT genotype compared to patients with CC genotype, but did not reach statistical significance (P=0.358, OR=2.00 95% CI=0.58-6.87).

CONCLUSION

The results from the present study suggest that the 3435C>T MDR1 gene polymorphism may influence the efficacy of RA therapy with disease-modifying antirheumatic drugs.

Limited interaction between tacrolimus and P-glycoprotein in the rat small intestine

The significance of intestinal P-glycoprotein (P-gp) in determining the oral bioavailability of tacrolimus has been still controversial. In this study, we reevaluated the interaction of tacrolimus with P-gp in the rat small intestine, by evaluating its absorption from the rat small intestine and its modulating effect on the absorption of known P-gp substrates (digoxin, methylprednisolone, and vinblastine). Intestinal absorption of tacrolimus itself was as extensive as other P-gp modulators such as cyclosporine and verapamil. While cyclosporine and verapamil significantly increased the absorption of methylprednisolone and vinblastine through potent inhibition of intestinal P-gp, tacrolimus failed to achieve this. When cyclosporine and tacrolimus were intravenously administered to rats, digoxin absorption was significantly increased by cyclosporine but not by tacrolimus. When tacrolimus was coadministered with clotrimazole, a specific CYP3A inhibitor, into the rat small intestine, the area under the curve of tacrolimus blood concentrations increased more than seven-fold compared with that of tacrolimus alone. Our present results strongly suggest that the interaction between tacrolimus and P-gp is limited in the rat small intestine and that extensive metabolism by CYP3A enzymes is more responsible for the low oral bioavailability of tacrolimus. It was considered that the extensive absorption of cyclosporine and verapamil was closely associated with their potent ability to inhibit intestinal P-gp.

Involvement of C3435T and G2677T multidrug resistance gene polymorphisms in release of cytokines from peripheral blood mononuclear cells treated with methotrexate and dexamethasone

P-Glycoprotein is a cell membrane-associated protein that transports a variety of exogenous (including drugs) and endogenous substances. P-Glycoprotein may also be involved in transmembrane transport of some endogenous proteins; thus, it may have physiological function in cytokine transport. Previous studies suggested that P-glycoprotein expression is genetically determined. The aim of this study was to examine involvement of multidrug resistance gene (MDR1) C3435T and G2677T polymorphisms in release of cytokines from phythemaglutynin (PHA)-stimulated peripheral blood mononuclear cells, as well as treated with methotrexate or dexamethasone. The release of cytokines: interleukin-2 (IL-2), IL-4, IL-6, IL-10, interferon-gamma (INF-gamma) and tumor necrosis factor-alpha (TNF-alpha) was determined in supernatants of mononuclear cell cultures from 72 healthy subjects, measured by flow cytometry. The release of INF-gamma, IL-2, IL-4 and TNF-alpha in cultures from subjects with 2677(T-T) 3435(T-T) haplotype pair was significantly decreased as compared to subjects with other haplotypes. There were no statistically significant differences in release of IL-6 and IL-10. The results of this study suggest an association between C3435T and G2677T MDR1 polymorphisms and transmembrane transport of some cytokines. Although the studied polymorphisms may be in linkage with polymorphisms of other transporters involved in cytokine release, it seems that the present results indirectly indicate involvement of P-glycoprotein in transport of some cytokines. Moreover, determination of C3435T and G2677T MDR1 polymorphisms might be useful in response prediction to therapy with methotrexate and dexamethasone.

Characterization of Intestinal Absorption and Enterohepatic Circulation of Mycophenolic Acid and Its 7-O-Glucuronide in Rats

To assess the mechanism of gastrointestinal disorders by mycophenolate mofetil (MMF), the intestinal absorption and enterohepatic circulation of mycophenolic acid (MPA), an active metabolite of MMF, and its 7-O-glucuronide (MPAG) were investigated using rat intestinal loops and a linked-rat model. The stability of MPAG in the intestinal fluids, the toxicity of MPA and MPAG to intestinal mucosa, and biliary excretion of MPAG in rats with acute renal failure (ARF) were also characterized. MPA was rapidly and extensively absorbed from the rat intestine whereas MPAG was much less absorbable. When MPA was administered intravenously to bile-donor rats, 1.2% of dose was excreted in bile of receiver rats exclusively as MPAG during 4 h. MPAG was minimally deconjugated in the intestinal fluids. MPAG, but not MPA, significantly enhanced the release of lactate dehydrogenase from intestinal mucosa. When MPA was intravenously administered to ARF rats, the biliary excretion of MPAG significantly increased, compared with that in normal rats. These results demonstrated that MPAG accumulated in the intestinal lumen following biliary excretion and exerted some toxic effect on the intestinal mucosa. It was also suggested that enterohepatic circulation of MPAG under renal dysfunction increased the risk of gastrointestinal disorders due to MPAG.

Regional Difference in P-glycoprotein Punction in Rat Intestine

It has been reported that inhibition of the P-glycoprotein (P-gp) results in the improved absorption of P-gp substrate in the intestinal tract. In fact, the increased permeability of P-gp substrate across the intestinal epithelium was observed following inhibition of P-gp in in vitro experiments. To develop the formulation containing P-gp inhibitor and P-gp substrate for practical use, it is necessary to know whether the results obtained in the in vitro experiments are reproducible at whole body level. It is also important to find out the regional difference of the P-gp activity in the intestinal tract. In this study, we examined whether verapamil, a specific inhibitor of P-gp, improves the absorption of rhodamine123 (Rho123), a substrate of P-gp, from the jejunum, ileum, and colon of rats using the in situ loop method. The water content in the loop decreased during the experiment, resulting in a significant change of the Rho123 concentration in the loop. Thus, to accurately determine the absorption rate of Rho123, it was necessary to measure the water movement. It was found that there was a regional difference in the water movement, i.e., greatest in colon, followed by ileum. Verapamil did not change the water movement in any intestinal regions. When the concentration of Rho123 in the loop was corrected by water movement, the Rho123 clearance was in the order of ileum (1.15 microL/min/cm), colon (0.83 microL/min/cm) and jejunum (0.47 microL/min/cm). In the presence of verapamil, the Rho123 clearance was significantly increased at jejunum and ileum but not in colon (ileum: 2.08 microL/min/cm, colon: 1.14 microL/min/cm, jejunum: 1.28 microL/min/cm). These results suggest that P-gp inhibits the drug absorption in jejunum and ileum. From these results, it is possible to evaluate the role of P-gp and its regional difference in the in situ experiments. In particular, the inhibition of P-gp results in an increase in absorption of the P-gp substrate limited to jejunum and ileum.

Impact of P-Glycoprotein-Mediated Intestinal Efflux Kinetics on Oral Bioavailability of P-Glycoprotein Substrates†

Studies of many P-glycoprotein (Pgp) substrates have demonstrated a significant effect of Pgp-mediated efflux on intestinal drug transport. However, most of these studies were designed to detect whether a particular drug is a Pgp substrate and thus were conducted at very low concentrations. We performed two simulations to evaluate the effect of Pgp-mediated efflux on oral drug absorption at various concentrations. In the first simulation, a steady-state model allowed us to predict whether the contribution of Pgp to oral drug absorption would be significant at clinically relevant concentrations. Our second simulation investigated the role of Pgp-mediated efflux in oral absorption with a dynamic compartmental absorption and transit model linked to a pharmacokinetic model. For high-solubility drugs, Pgp-mediated efflux altered the bioavailability only at drug concentrations corresponding to doses much lower than the usual clinical dose. The ratio of transporter-mediated transport to passive transport determined whether intestinal Pgp transporters would reduce the bioavailability of high-solubility drugs.

Identification of budesonide and prednisone as substrates of the intestinal drug efflux pump P-glycoprotein

Steroid resistance is a major problem in the management of patients with inflammatory bowel disease. In Crohn disease, poor response to corticosteroids has been related to increased expression of the drug efflux pump, P-glycoprotein. However, it has not been investigated thoroughly whether corticosteroids commonly used for drug therapy in inflammatory bowel disease are substrates of P-glycoprotein. We tested the hypothesis that budesonide and prednisone are substrates of P-glycoprotein thereby possibly contributing to variable therapeutic effects. Polarized, basal to apical transport of [3H]budesonide and [3H]prednisone was studied in monolayers of L-MDR1 cells (LLC-PK1 cells stably transfected with human MDR1 cDNA) and Caco-2 cells, both of which express P-glycoprotein in their apical membrane. Drug transport was measured during 4 hours at substrate concentrations of 5 microM. Net transport rates and permeability coefficients were calculated. Inhibition of P-glycoprotein-mediated transport across Caco-2 monolayers was determined after addition of the P-glycoprotein inhibitor PSC-833. The net transport rate from the basolateral to the apical side was significantly higher in L-MDR1 than in LLC-PK1 cells for both budesonide and prednisone. Apparent permeability coefficients of budesonide and prednisone reflected polarized transport from basal to apical. PSC-833 inhibited the polarized transport of both corticosteroids. In conclusion, budesonide and prednisone were identified as substrates of the intestinal drug efflux pump, P-glycoprotein. Therefore, drug secretion via P-glyco-protein into gut lumen might play a more important role in pharmacokinetics and pharmacodynamics of these corticosteroids than currently appreciated in gastroenterological practice.

Evidence of Efflux-Mediated and Saturable Absorption of Rifampicin in Rat Intestine Using the Ligated Loop and Everted Gut Sac Techniques

This study was carried out to explore whether efflux-mediated and saturable mechanisms play any role toward poor and variable intestinal absorption of rifampicin. In situ segmental permeability of rifampicin at various residence times was determined in rat gastrointestinal tract using the ligated loop technique. The involvement of efflux-mediated and saturable absorption of rifampicin was studied in rat intestine using the everted sac method. The samples were analyzed by a validated HPLC method. Rifampicin showed decreased permeability in jejunum and ileum with an increase in residence time. The permeation of rifampicin from the serosal to the mucosal side (secretion) was significantly higher than permeation from the mucosal to the serosal side (absorption) of jejunum and ileum. This indicated the involvement of efflux-mediated transport. Addition of verapamil, an inhibitor for P-glycoprotein (Pgp), multidrug resistance associated protein-2 (MRP-2), and other related transporters, increased absorption of rifampicin in jejunum and ileum by 2-3-fold and decreased secretion by almost 4-fold. The permeation rate (flux) of rifampicin through duodenum increased with concentration up to 300 microg/mL, becoming constant thereafter, indicating the existence of saturable absorption. There was no saturable permeation in jejunum and ileum. Thus the present study indicates the involvement of efflux-mediated and saturable absorption mechanisms of rifampicin in rat intestine, which act as barriers to the absorption of the drug. This explains the drug's poor absolute bioavailability. As Pgp varies from person to person to an extent of 2-8-fold, it can be one direct reason for the interindividual variable bioavailability shown by rifampicin.

β-Cyclodextrin as a suitable solubilizing agent for in situ absorption study of poorly water-soluble drugs

To evaluate the intestinal permeability of poorly water-soluble compounds, it is of importance to completely dissolve them in a medium and to avoid precipitation during experiments. This study was undertaken to find an agent possessing a high-solubilizing capacity and exhibiting minimal modulating impact on membrane integrity and absorption systems such as passive diffusion and carrier-mediated permeation. Phenytoin dissolution was compared in the presence of seven solubilizing agents at concentrations of 1, 2, or 5% using a centrifugation method. The capacity to dissolve phenytoin was great in beta-cyclodextrin (beta-CD) and hydroxypropyl beta-cyclodextrin, followed by Tween 80. Those of methanol, dimethyl sulfoxide, dimethyl acetoamide, and polyethylene glycol 400 were much lower than expected. One percent beta-CD did not alter the absorption of fluorescein isothiocyanate-dextran 4,000 or the release of protein and lactate dehydrogenase into in situ loop contents, suggesting that 1% beta-CD had no significant impact on the integrity of the intestinal membrane. One percent beta-CD also did not alter the absorption of caffeine, ceftibuten, or rhodamine 123 from in situ jejunal loops, indicating no interference with passive diffusion and active transports mediated by a peptide transporter and P-glycoprotein. In conclusion, 1% beta-CD is a suitable solubilizing agent for evaluating in situ intestinal absorption of poorly water-soluble compounds.

Structural determinants of P-glycoprotein-mediated transport of glucocorticoids

PURPOSE

The aim of this study was to determine requisite structural features for P-glycoprotein-mediated transport of a series of structurally related glucocorticoids (GCs).

METHODS

Transport experiments were conducted in wild-type and stably transfected MDRI LLC-PK cell line. Transport efficiency (Teff = Peff, B-->A / Peff, A-->B) in both cell lines was compared as a measure of passive diffusion and P-glycoprotein-mediated transepithelial transport for each steroid. Three-dimensional structure-activity relationships were built to determine how specific structural features within the steroids affect their P-gp-mediated efflux.

RESULTS

Mean (+/- SD) Teff in LLC-PK cells was 1.1 +/- 0.17, indicating that differences in structure and partition coefficient did not affect drug flux in the absence of P-glycoprotein. Teff in L-MDRI cells ranged from 3.6 to 26.6, demonstrating the importance of glucocorticoid structure to P-glycoprotein transport. The rank order of Teff in MDR1 cells was: methylprednisolone> prednisolone > betamethasone > dexamethasone/prednisone > cortisol. There was no correlation between individual Teff values and partition coefficient. 3D-QSAR models were built using CoMFA and CoMSIA with a q2 (r2) of 0.48 (0.99) and 0.41 (0.95), respectively.

CONCLUSIONS

Nonpolar bulky substituents around the C-6alpha position, as well as a hydrogen-bond donor at position C-11, enhance P-glycoprotein affinity and cellular efflux, whereas bulky substituents at C-16 diminish transporter affinity.

Classification analysis of P-glycoprotein substrate specificity

Prediction of P-glycoprotein substrate specificity (S(PGP)) can be viewed as a constituent part of a compound's "pharmaceutical profiling" in drug design. This task is difficult to achieve due to several factors that raised many contradictory opinions: (i) the disparity between the S(PGP) values obtained in different assays, (ii) the confusion between Pgp substrates and inhibitors, (iii) the confusion between lipophilicity and amphiphilicity of Pgp substrates, and (iv) the dilemma of describing class-specific relationships when Pgp has no binding sites of high ligand specificity. In this work, we compiled S(PGP) data for 1000 compounds. All data were represented in a binary format, assigning S(PGP) = 1 for substrates and S(PGP) = 0 for non-substrates. Each value was ranked according to the reliability of experimental assay. Two data sets were considered. Set 1 included 220 compounds with S(PGP) from polarized transport across MDR1 transfected cell monolayers. Set 2 included the entire list of 1000 compounds, with S(PGP) values of generally lower reliability. Both sets were analysed using a stepwise classification structure-activity relationship (C-SAR) method, leading to derivation of simple rules for crude estimation of S(PGP) values. The obtained rules are based on the following factors: (i) compound's size expressed through molar weight or volume, (ii) H-accepting given by the Abraham's beta (that can be crudely approximated by the sum of O and N atoms), and (iii) ionization given by the acid and base pKa values. Very roughly, S(PGP) can be estimated by the "rule of fours". Compounds with (N + O) > or = 8, MW > 400 and acid pKa > 4 are likely to be Pgp substrates, whereas compounds with (N + O) < or = 4, MW < 400 and base pKa < 8 are likely to be non-substrates. The obtained results support the view that Pgp functioning can be compared to a complex "mini-pharmacokinetic" system with fuzzy specificity. This system can be described by a probabilistic version of Abraham's solvation equation, suggesting a certain similarity between Pgp transport and chromatographic retention. The chromatographic model does not work in the case of "marginal" compounds with properties close to the "global" physicochemical cut-offs. In the latter case various class-specific rules must be considered. These can be associated with the "amphiphilicity" and "biological similarity" of compounds. The definition of class-specific effects entails construction of the knowledge base that can be very useful in ADME profiling of new drugs.

Profile of P‐glycoprotein Distribution in the Rat and Its Possible Influence on the Salbutamol Intestinal Absorption Irocess

The intrinsic absorption of salbutamol in different intestinal segments of the rat was measured and related with the corresponding intestinal P-glycoprotein (P-gp) expression levels. The apparent absorption rate constants (k(a), h(-1)) observed in each fraction by means of the "in situ" rat gut absorption method after perfusion of a 0.29-mM isotonic solution of salbutamol were used as absorption indexes. In a separate series of studies, a semiquantitative analysis of the mRNA expression of P-gp by means of polymerase chain reaction and Western blot with an antibody raised against the P-gp were also performed. The "in situ" k(a) values determined in the different segments (h(-1)) showed that the absorption is not homogeneous along the intestinal tract, that is, 0.499 +/- 0.054 for colon, 0.474 +/- 0.052 for the proximal segment, 0.345 +/- 0.014 for the mean, and 0.330 +/- 0.023 for the distal fraction. Addition of verapamil to the perfusion fluid did provide a better absorption of salbutamol in the distal segment. The analysis of the mRNA expression and levels of P-gp showed that the enzyme content in each section of the intestine was inversely related to salbutamol absorption.

Apocynum venetum Extract Does Not Induce CYP3A and P-Glycoprotein in Rats

We investigated the effect of Apocynum venetum L. extract (AV) on the activity of cytochrome P450 (CYP) 3A and P-glycoprotein (P-gp). The plasma concentration of nifedipine (NF), which is a substrate for CYP3A, did not change after oral administration with AV (3.3 mg/kg). Also, AV (3.3 and 33 mg/kg) did not affect the intestinal absorption of NF. In the rats treated with multiple administrations (15 mg/kg/d) of St. John's wort extract (SJW) for 2 weeks, the plasma concentration of NF after oral administration was significantly decreased. On the other hand, there was no significant differences in the pharmacokinetic parameters of NF between AV-treated (3.3 mg/kg/d) and none-treated rats. Furthermore, the intestinal absorption of methylprednisolone, which is a substrate for P-gp, was not affected by AV treatment for 2 weeks. These results suggest that, unlike SJW, the recommended dose of AV (3.3 mg/kg/d) would not influence hepatic CYP3A and intestinal P-gp in rats.

Studies on Intestinal Absorption of Sulpiride (3): Intestinal Absorption of Sulpiride in Rats

The aim of this study was to investigate whether the concomitant administration of the substrates or inhibitors of PEPT1, OCTN1, OCTN2, and P-glycoprotein affects the intestinal absorption of sulpiride in rats. The absorption of sulpiride from rat intestine was decreased by the substrates or inhibitors of PEPT1, OCTN1, and OCTN2. On the other hand, the absorption was increased by the substrates of P-glycoprotein. The effects of these concomitantly administered drugs on the pharmacokinetic behavior of sulpiride after oral administration in rats were investigated. Peak concentration (C(max)) and area under the plasma concentration-time curve (AUC(0-8 h)) of sulpiride were decreased by the concomitant administration of the substrates or inhibitors of PEPT1, OCTN1, and OCTN2. However, the same parameters were significantly increased by the concomitant administration of the substrates of P-glycoprotein. The present results suggest the possibility of drug-drug interaction during the absorption process in the small intestine due to the coadministration of sulpiride and these agents. These findings provide important information for preventing adverse effects and for ensuring the effectiveness of sulpiride and concomitantly administered drugs.

Efflux ratio cannot assess P-glycoprotein-mediated attenuation of absorptive transport: asymmetric effect of P-glycoprotein on absorptive and secretory transport across Caco-2 cell monolayers

PURPOSE

The purpose of this work was to determine whether P-glycoprotein (P-gp) modulates absorptive and secretory transport equally across polarized epithelium (i.e., Caco-2 cell monolayers) for structurally diverse P-gp substrates, a requirement for the use of the efflux ratio to quantify P-gp-mediated attenuation of absorption across intestinal epithelium.

METHODS

Studies were performed in Caco-2 cell monolayers. Apparent permeability (P(app)) in absorptive (P(app,AB)) and secretory (P(app,BA)) directions as well as efflux ratios (P(app,BA)/P(app,AB)) were determined for substrates as a function of concentration. Transport of these compounds (10 microM) was measured under normal conditions and in the presence of the P-gp inhibitor, GW918 (1 microM), to dissect the effect of P-gp on absorptive and secretory transport. Apparent biochemical constants of P-gp-mediated efflux activity were calculated for both transport directions.

RESULTS

Efflux ratios for rhodamine 123 and digoxin were comparable (approx. 10). However, transport studies in the presence of GW918 revealed that P-gp attenuated absorptive transport of digoxin by approx. 8-fold but had no effect on absorptive transport of rhodamine 123 (presumably because absorptive transport of rhodamine 123 occurs via paracellular route). The apparent Km for P-gp-mediated efflux of digoxin was > 6-fold larger in absorptive vs. secretory direction. For structurally diverse P-gp substrates (acebutolol, colchicine, digoxin, etoposide, methylprednisolone, prednisolone, quinidine, and talinolol) apparent Km was approximately 3 to 8-fold greater in absorptive vs. secretory transport direction, whereas apparent J(max) was somewhat similar in both transport directions.

CONCLUSIONS

P-gp-mediated efflux activity observed during absorptive and secretory transport was asymmetric for all substrates tested. For substrates that crossed polarized epithelium via transcellular pathway in both directions, this difference appears to be caused by greater apparent Km of P-gp-mediated efflux activity in absorptive vs. secretory direction. These results clearly suggest that use of efflux ratios could be misleading in predicting the extent to which P-gp attenuates the absorptive transport of substrates.

Novel experimental parameters to quantify the modulation of absorptive and secretory transport of compounds by P-glycoprotein in cell culture models of intestinal epithelium

PURPOSE

The purpose of this work was to elucidate the asymmetric effect of P-gp on modulation of absorptive and secretory transport of compounds across polarized epithelium, to develop experimental parameters to quantify P-gp-mediated modulation of absorptive and secretory transport, and to elucidate how P-gp-mediated modulation of transport is affected by passive diffusion properties, interaction of the substrate with P-gp, and P-gp expression.

METHODS

The permeability of a set of P-gp substrates was determined in absorptive and secretory directions in Madine-Darby Canine kidney (MDCK), Caco-2, and MDR-MDCK monolayers. The transport was also determined in the presence of GW918, a non-competitive P-gp inhibitor, to quantify the permeability without the influence of P-gp. From these two experimental permeability values in each direction, two new parameters, absorptive quotient (AQ) and the secretory quotient (SQ), were defined to express the functional activity of P-gp during absorptive and secretory transport, respectively. Western blot analysis was used to quantify P-gp expression in these monolayers and in normal human intestinal.

RESULTS

P-gp expression in Caco-2 and MDR-MDCK monolayers was comparable to that in normal intestine, and much less in MDCK cells. For all models, the substrates encompassed a wide range of apparent permeability due to passive diffusion (PPD). The parameters AQ and SQ, calculated for all compounds, assessed the attenuation in absorptive and enhancement of secretory transport, respectively, normalized to the permeability due to passive diffusion. Analysis of these parameters showed that 1) P-gp affected absorptive and secretory transport differentially and 2) compounds could be stratified into distinct groups with respect to the modulation of their absorptive and secretory transport by P-gp. Compounds could be identified whose absorptive transport was either strongly affected or poorly affected by changes in P-gp expression. For certain compounds, AQ values showed parabolic relationship with respect to passive diffusivity, and for others AQ was unaffected by changes in passive diffusivity.

CONCLUSIONS

The relationship between attenuation of absorptive transport and enhancement of secretory transport of compounds by P-gp is asymmetric, and different for different sets of compounds. The relationship between attenuation of absorption by P-gp and passive diffusivity of compounds, their interaction potential with P-gp, and levels of P-gp expression is complex; however, compounds can be classified into sets based on these relationships. A classification system that describes the functional activity of P-gp with respect to modulation of absorptive and secretory transport was developed from these results.

Intestinal absorption of cephalexin in diabetes mellitus model rats

We investigated the intestinal absorption and pharmacokinetics of cephalexin, as well as the intestinal H+/oligopeptide transporter PEPT1 mRNA and protein levels in type 1 and type 2 diabetic rats. Cephalexin disappearance from the duodenum loop was significantly lower in streptozotocin-induced type 1 diabetic rats and higher in hyperinsulinemic type 2 diabetic GK and Zucker-fa/fa (Zucker) rats, than in control rats. These results were speculated to be due to the enhancement of intestinal absorption of cephalexin in GK and Zucker rats. Intestinal PEPT1 mRNA levels were not significantly different between control and diabetic rats; however, the brush-border membrane vesicle PEPT1 protein levels were increased in GK and Zucker rats. After oral administration of cephalexin, plasma cephalexin concentrations and pharmacokinetic parameters, area under the concentration versus time curve from 0 to infinity, AUC(0-->infinity), and maximum plasma concentration, Cmax, in GK and Zucker rats were markedly higher than in control rats. From these findings, it is considered that intestinal absorption of drugs mediated by PEPT1 may be enhanced in hyperinsulinemic type 2 diabetes mellitus rats.

Kinetics of metabolism and degradation of mometasone furoate in rat biological fluids and tissues

Mometasone furoate (MF) is a potent glucocorticoid developed for the treatment of glucocorticoid-responsive inflammatory disorders. The in-vitro and ex-vivo kinetics of the degradation and metabolism of MF were studied in selected biological fluids of rat and subcellular fractions of different rat tissues. In-vitro, MF was found to degrade slowly into four products in serum and urine, and metabolized rapidly and extensively in rat liver, minimally in extrahepatic tissues, including intestine, stomach, lung and kidney. Further investigation found that the microsomal fraction was the major intracellular site of MF 6 beta-hydroxylation in rat liver. Using chemical inhibitors, CYP3A was found to be the major enzyme involved in the in-vitro MF 6 beta-hydroxylation in rat liver microsomes. Enzyme kinetic studies in rat liver microsomes showed that the overall metabolic process of MF followed biphasic Michaelis-Menten kinetics, while 6 beta-hydroxylation obeyed monophasic Michaelis-Menten kinetics. The kinetic parameters derived from the kinetic models along with the enzyme inhibition studies suggest that MF is mainly metabolized via 6 beta-hydroxylation mediated by CYP3A primarily, and also biotransformed via other pathway(s) catalysed by other enzymes in rat liver in-vitro.

Induction of CYP3As in HepG2 cells by several drugs. Association between induction of CYP3A4 and expression of glucocorticoid receptor

The cytochrome P-450 3A (CYP3A) enzyme family is responsible for most of the drug metabolism in the human liver. In this study, we demonstrated the inductive effects of phenobarbital, rifampicin, carbamazepine, phenytoin, prednisolone, ciclosporin and clotrimazole on CYP3A4, CYP3A5 and CYP3A7 mRNA expression, and established the relationship between the expression of human glucocorticoid receptor alpha (hGR) mRNA and the induction of CYP3A4 mRNA in cultured HepG2 cells by reverse transcription polymerase chain reaction (RT-PCR). Treatment with prednisolone, rifampicin and carbamazepine rapidly induced the level of CYP3A4 mRNA expression by 3- to 6-fold. However, phenytoin and phenobarbital gradually induced CYP3A4 mRNA level by 3 to 4-fold. The induction of CYP3A4 mRNA expression by clotrimazole and ciclosporin was negligible. Treatment with phenytoin, rifampicin, carbamazepine and ciclosporin induced approximately 2-fold increases in the expression of CYP3A5 mRNA, although prednisolone, phenytoin and clotrimazole had no effect. Treatment with rifampicin, phenytoin, clotrimazole and ciclosporin resulted in approximately a 2-fold induction of the CYP3A7 mRNA level. Treatment with rifampicin and ciclosporin induced the expression of hGRalpha mRNA significantly in comparison with controls, although the induction of hGRalpha mRNA following treatment with other drugs was negligible. In cluster analysis, the induced level of CYP3A4, CYP3A5, CYP3A7 and hGRalpha mRNA by these drugs could be classified into four major clusters. This suggested that each cluster might be associated with different mechanism(s) of induction by these drugs. Furthermore, we studied the associations between the expression of hGRalpha mRNA and the induced level of CYP3A4 mRNA by prednisolone and ciclosporin. Treatment with both prednisolone and ciclosporin showed synergistic effects on induction of CYP3A4 mRNA and, following treatment with both drugs, the expression level of CYP3A4 mRNA was 2-fold greater compared with prednisolone alone after the fifth day. Positive correlations were observed between the levels of hGRalpha mRNA expression and those of CYP3A4 mRNA. This observation shows that the regulation of CYP3A4 gene expression was hGRalpha-dependent and that ciclosporin may function as a regulator of expression via hGRalpha.

Physicochemical properties and transport of steroids across Caco-2 cells

PURPOSE

The purpose of this work was to study the relevant physicochemical properties for the absorption of steroids.

METHODS

Various physicochemical properties of steroids were calculated (molecular weight, ClogP, static polar surface area [PSA], etc.). Within this series of steroids, different pharmacological groups were defined. Based on the outcome of this survey, steroids were selected for the Caco-2 permeability study. The apparent permeability coefficients (Papp) were related to the calculated and measured physicochemical properties.

RESULTS

Between the defined groups of steroids, ClogP was the most discriminative descriptor. The steroids were well transported over the cell monolayers and the Papp was independent of the concentration and the transport direction. No relationship was found with the PSA; however, the Papp showed a weak inverse correlation with ClogP.

CONCLUSIONS

The molecular descriptors and Papp values showed that all steroids are well transported. The small differences in the Papp values showed a weak inverse correlation with ClogP: the hydrophilic steroids (ClogP approximately 0-2) tend to diffuse faster over the cell monolayers compared with the more hydrophobic steroids (ClogP approximately 5). The relationship with ClogP suggests that partitioning of steroids between the biologic membrane and the surrounding aqueous phase is one of the main mechanisms for absorption.

Expression of ATP-binding cassette transporter in human salivary ducts

P-glycoprotein expression has been observed in normal tissues as well as malignant tumours and thus does not appear to be induced by anticancer drugs. Knowledge of the distribution of ATP-binding cassette (ABC) transporters other than P-glycoprotein in normal salivary tissue is essential for understanding the physiological secretion or excretion of potentially toxic substances. Here the expression of ABC transporters was studied immunohistochemically in normal salivary gland tissue from nine patients. In striated duct cells, staining was strong for P-glycoprotein, multidrug resistance-associated protein (MRP) 1, MRP 2/canalicular multispecific organic anion transporter (cMOAT), and lung resistance-related protein (LRP). The staining intensity of acinar and intercalated duct cells for MRP 1 expression was distinct from that for MRP2/cMOAT, but was similar to that for P-glycoprotein. LRP was observed as particles between the nuclear and luminal membranes in the cytoplasm of intercalated duct cells. The expression of ABC transporters suggests that numerous transporters other than those studied might be isolated from normal salivary tissues. These observations indicate that these ABC transporters may not arise from any previous contact with anticancer drugs but are expressed physiologically. The achieved drug resistance as well as the physiological secretory function of ABC transporters could contribute to the responsiveness to chemotherapy of malignant salivary tumours.

P-glycoprotein-170 inhibition significantly reduces cortisol and ciclosporin efflux from human intestinal epithelial cells and T lymphocytes

AIM

To assess the role of P-glycoprotein-170 (P-gp) in transporting cortisol and ciclosporin from human intestinal epithelium and T lymphocytes.

METHODS

The effect of P-gp inhibitors (verapamil, 0-100 microM; PSC 833, 0-20 microM) on the intracellular accumulation of 3H-cortisol and 3H-ciclosporin was studied in confluent layers of human Caco-2 cells (n=6), a P-gp-dependent absorptive intestinal epithelial cell phenotype, and moderately resistant MDRhigh CEM/VBL 100 T cells (n=6). The transport of 3H-vinblastine, a strong multidrug resistance (MDR) substrate, and 3H-progesterone, a poor MDR substrate, was also studied.

RESULTS

Caco-2 cells had a 2.4-, 6.6-, 6.7- and 1.03-fold higher net basal to apical transport (efflux) of 3H-cortisol, 3H-ciclosporin, 3H-vinblastine and 3H-progesterone, respectively. PSC 833 (20 microM) reduced cortisol efflux by 69% (0.23 +/- 0.04 to 0.07 +/- 0.01 pmol/cm2/h, P < 0.05) and ciclosporin efflux by 76% (11.1 +/- 1.4 to 2.7 +/- 0.6 pmol/cm2/h, P < 0.001). MDRlow CEM T cells had a 1.4-, 1.9-, 3.2- and 1.02-fold higher intracellular accumulation of cortisol, ciclosporin, vinblastine and progesterone than MDRhigh CEM/VBL 100 T cells. Increasing concentrations of PSC 833 (> 0.1 microM) and verapamil (> 1 microM) restored the intracellular level of 3H-cortisol and 3H-ciclosporin in MDRhigh CEM/VBL 100 T cells to that of MDRlow CEM cells with little change in accumulation in the MDRlow parental cell line.

CONCLUSIONS

P-gp inhibitors significantly increase intracellular cortisol and ciclosporin levels in human intestinal epithelium and T lymphocytes in a dose-dependent manner, demonstrating a potential mechanism for overcoming poor response to immunosuppressant therapy in refractory inflammatory bowel disease.

Secretory transport of methylprednisolone possibly mediated by P-glycoprotein in Caco-2 cells

We recently reported that P-glycoprotein (MDR1) is capable of interfering with the absorption of methylprednisolone in the rat small intestine. This study was undertaken to examine the interaction between methylprednisolone and MDR1 using Caco-2 cells. The permeation of various steroid hormones (hydrocortisone, prednisolone, progesterone, beta-estradiol, and testosterone) was compared. The basolateral-to-apical (secretory) permeation of methylprednisolone was more than 3-fold greater than the apical-to-basolateral (absorptive) permeation. When verapamil (0.1 mm), a potent modulator of MDR1, was added to both apical and basolateral sides of Caco-2 cells, the absorptive permeation of methylprednisolone was increased and its secretory permeation was decreased. As a result, the secretory-oriented manner of methylprednisolone permeation almost completely disappeared. Prednisolone and hydrocortisone exhibited weaker secretory-oriented movement than did methylprednisolone. The secretory-oriented permeation of prednisolone and hydrocortisone was also diminished by the addition of verapamil. There was no significant directionality in progesterone permeation and the permeation of beta-estradiol and testosterone tended to be absorptive. These results appear to suggest that methylprednisolone, prednisolone, and hydrocortisone interact with MDR1 as the substrates. In contrast, there was no evidence that MDR1 was capable of potently interfering with the absorption of the sex hormones tested in this study, supporting our previous findings in the rat. It was further found that apically-added verapamil demonstrated a modulating effect on MDR1 function even at 5 microM.

Involvement of multidrug resistance-associated protein 2 in intestinal secretion of grepafloxacin in rats

We investigated the contribution of multidrug resistance-associated protein 2 (MRP2) to the secretory transport of grepafloxacin and compared its functional role with that of P-glycoprotein (P-gp) by using Sprague-Dawley rats (SDRs) and Eisai hyperbilirubinemic rats (EHBRs), in which MRP2 is hereditarily defective. In intestinal tissue from SDRs mounted in Ussing chambers, the level of transport in the direction from the serosal layer to the mucosal layer was twofold greater than that in the direction from the mucosal layer to the serosal layer. This secretory transport of grepafloxacin was diminished by both probenecid, an MRP2 inhibitor, and cyclosporine, a P-gp inhibitor. In intestinal tissue from EHBRs, the secretory transport of grepafloxacin was lower than that in intestinal tissue from SDRs and was inhibited by cyclosporine but not by probenecid. The absorption of grepafloxacin from intestinal loops in SDRs was in the order of duodenum > jejunum > ileum and was increased by cyclosporine but not by probenecid. The absorption in EHBRs was not higher than that in SDRs. The intestinal secretory clearance in SDRs after intravenous administration of grepafloxacin was shown to be greater for the ileum than for the duodenum, which is in good agreement with the previously reported regional expression profile of MRP2 mRNA. The intestinal secretory clearance was lower in EHBRs than in SDRs. Accordingly, in addition to P-gp, MRP2 might play a role in the secretory transport of grepafloxacin. The function of MRP2 in facilitating grepafloxacin transport in the secretory direction is more pronounced both in vitro and in vivo, while the restriction of entry from the lumen into the cell by MRP2 seems to be negligible, compared with that by P-gp, in the case of grepafloxacin.

In vitro intestinal permeability of factor Xa inhibitors: influence of chemical structure on passive transport and susceptibility to efflux

PURPOSE

To study the in vitro intestinal permeability of a number of newly synthesised factor Xa inhibitors to better understand the poor oral absorption of these compounds.

METHODS

The bidirectional transport of the fXa inhibitors was studied in the Caco-2 cell model and isolated rat ileal tissue. An attempt was made to characterize efflux mechanisms with the help of commonly used substrates and inhibitors of various transport proteins. In addition, the transport of the fXa inhibitors was studied in MDCK cells transfected with the human MDR1 gene and expressing large amounts of P-glycoprotein (Pgp).

RESULTS

The in vitro absorptive permeability was low for all but one of the fXa inhibitors. For compounds with non-substituted amidine, a charge (due to ionisation at neutral pH) may have resulted in poor membrane partitioning. Neutral compounds with substituted amidines were effluxed from the epithelial cells. The significance of the secretion process was illustrated by the results obtained for a neutral analogue showing high absorptive Caco-2 cell permeability that was not obviated by efflux. Transport inhibition studies in Caco-2 and permeability studies in the MDR1-transfected MDCK cells consistently showed that Pgp is not involved in the secretion of fXa inhibitors. Besides efflux, metabolic liability limited the permeation of the neutral lipophilic analogues with a carbamate ester.

CONCLUSIONS

Poor intestinal permeability may be an important factor in the incomplete oral absorption of the bisbenzimidazole-type fXa inhibitors. Poor permeability may be related to poor membrane partitioning for hydrophilic analogues, whereas susceptibility to efflux transports and gastro-intestinal enzymatic degradation may limit the permeability of some of the neutral less hydrophilic derivatives.

Ontogeny of P-glycoprotein in mouse intestine, liver, and kidney

BACKGROUND

P-glycoprotein (Pgp) is an ATP-dependent, integral plasma-membrane efflux pump that is constitutively expressed on (i) adult apical brush-border epithelial cells of the intestine, (ii) the bile canalicular face of hepatocytes, and (iii) the brush border epithelium of renal proximal tubules. This Pgp tissue distribution and localization affects the absorption, distribution, metabolism, and excretion of Pgp substrates. Little is known regarding the ontogeny of Pgp expression in these tissues.

METHODS

Postnatal expression of Pgp on brush border membranes of small intestine, liver, and kidney as a function of maturity from birth through adulthood was determined using Western immunoblotting and immunohistochemical techniques. Tissue was isolated from FVB mice at four different ages: day of life 0 (D0), day of life 7 (D7), day of life 21 (D21), and adult (Ad). The relative expression of Pgp protein on Western immunoblots was assessed by scanning densitometry and indexed as a percentage (mean+/-SEM) of the adult levels.

RESULTS

On Western immunoblots, Pgp expression was limited at birth (19+/-6% of Ad) and increased significantly with maturation in intestine (ANOVA, P<0.005). In contrast, hepatic (113+/-12% of Ad) and renal (96+/-15% of Ad) Pgp expression were at adult levels at birth. The tissue-specific developmental pattern of Pgp expression was confirmed by immunohistochemistry.

CONCLUSIONS

We conclude that Pgp is expressed in a tissue-specific and developmentally regulated fashion and speculate that developmental modulation of intestine-Pgp expression may affect the oral bioavailability of Pgp substrates.

Effect of itraconazole on the pharmacokinetics of prednisolone and methylprednisolone and cortisol secretion in healthy subjects

AIMS

Itraconazole is a potent inhibitor of CYP3A4 activity and is often used in combination with corticosteroids. Since the latter are partly metabolized by CYP3A4, we studied the interaction between itraconazole, prednisone and methylprednisolone in healthy male subjects.

METHODS

The effects of 4 days administration of oral itraconazole (400 mg on the first day then 200 mg day-1 for 3 days) on the pharmacokinetics of prednisolone after a single oral dose of prednisone (60 mg) and the pharmacokinetics of methylprednisolone after single oral dose of methylprednisolone (48 mg) were studied in 14 healthy male subjects in a two-period cross-over trial. Plasma cortisol concentrations were determined as a pharmacodynamic index.

RESULTS

Itraconazole increased the mean area under the methylprednisolone concentration-time curve from 2773 ng ml-1 h to 7011 ng ml-1 h (P < 0.001) and the elimination half-life from 3.2 h to 5.5 h (P < 0.001). The pharmacokinetics of prednisolone were unchanged. Cortisol concentrations at 24 h were lower after administration of methylprednisolone with itraconazole than after methylprednisolone alone (24 ng ml-1 vs 109 ng ml-1, P < 0.001).

CONCLUSIONS

Itraconazole increased methylprednisolone concentrations markedly with enhanced suppression of endogenous cortisol secretion, but had no effect on prednisolone pharmacokinetics. The pharmacokinetic interaction between methylprednisolone and itraconazole is probably related to inhibition of hepatic CYP3A4 activity by itraconazole.

Improved intestinal absorption of sulpiride in rats with synchronized oral delivery systems

The goals of this study were to examine whether formulations, capable of releasing sulpiride (SP) in synchrony with the p-Glycoprotein (P-gp) inhibitors, verapamil (Ver) or quinidine (Qn) can increase SP relative bioavailability and to suggest a rationale approach for oral administration of SP. Jejunum of anesthetized rats was perfused with 200 or 400 microg/ml of SP either alone or together with 98 microg/ml of Ver. It was observed that while an increasing SP concentration did not cause an increase in SP blood levels, the addition of Ver or Qn to the perfusion solution caused a profound increase in SP absorption. Erodible matrix tablets, exhibiting a range of erosion rates, were prepared by manipulating the ratios of hydroxypropylmethylcellulose (HPMC) in the matrices. The tablets were designed to release the low water soluble SP and the highly water soluble Qn concomitantly over 1, 2 or 4 h. In all cases, the synchronous release increased SP bioavailability after intra-intestinal administration. The increase varied from 2.6- to 3.9-fold for the fast and the slow release formulations, respectively (compared with a control administration of a powdered mixture of SP and Qn). It is speculated that the poor oral bioavailability of SP was caused by brush border P-gp efflux. Synchronous release delivery systems of SP containing also the P-gp inhibitor Qn were able to increase SP bioavailability after intestinal administration in the rat. It is concluded that oral bioavailability of poorly absorbed drugs, in which absorption is restricted by gut wall secretory transport, may be improved by formulating them with functional adjuvants in synchronous release drug carriers.

Region-dependent disappearance of vinblastine in rat small intestine and characterization of its P-glycoprotein-mediated efflux system

This study was aimed to characterize the absorption behavior of vinblastine (VLB), a well-known substrate of P-glycoprotein (P-gp), from rat small intestine, especially focusing on the regional-dependence of its efflux mediated by P-gp. VLB disappeared from duodenal and ileal loops of male Wistar rats fairly rapidly (30-60% in 30 min). In contrast, its disappearance from the jejunal loop was almost negligible and in some rats >100% of the jejunal dose was recovered. The radioactivity derived from [3H]VLB, which was absorbed from duodenum and ileum, was detected in the jejunal region. The jejunal appearance of radioactivity was increased when unlabeled VLB was present in the region in advance. The basolateral-to-apical transport of [3H]VLB across Caco-2 cell monolayers was greater when unlabeled VLB was added to the apical medium than when VLB-free buffer was applied to the apical side. When verapamil or cyclosporin A, potent modulators of P-gp, was added to the apical medium together with unlabeled VLB, enhanced basolateral-to-apical transport of [3H]VLB was disappeared. It is suggested that VLB absorption is strongly restricted by P-gp, especially in the jejunal region of the rat small intestine, and that the secretory transport via intestinal P-gp may be subject to trans-stimulation. Moreover, intravenously administered methylprednisolone and intramuscularly administered progesterone significantly enhanced the absorption of VLB, suggesting that parenterally administered P-gp modulators could influence the intestinal absorption of P-gp substrates.

P-glycoprotein, secretory transport, and other barriers to the oral delivery of anti-HIV drugs

Orally administered anti-HIV drugs must be adequately and consistently absorbed for therapy to be successful. This review discusses the barriers to achieving oral bioavailability for the currently available anti-HIV drugs. Most reverse transcriptase inhibitors have good oral bioavailabilities. Didanosine bioavailability could be reduced by acid instability, first-pass hepatic metabolism, and possibly poor intestinal permeation. Bioavailability of zidovudine is also reduced by first-pass metabolism. The non-nucleoside reverse transcriptase inhibitors have oral bioavailabilities most probably limited by poor aqueous solubility. For each of the currently marketed HIV protease inhibitors, solubility, intestinal permeability, and first-pass metabolism could contribute to reducing oral bioavailability. The intestinal permeabilities of these agents is influenced by secretory transport. In vitro, secretory transport, which appears to be P-glycoprotein-mediated, is much greater than permeation in the absorptive direction for indinavir, nelfinavir, ritonavir, and saquinavir. The mechanisms of secretory intestinal transport are reviewed, and the factors that may influence the impact of secretory transport in vivo are considered.

P-glycoprotein and drug therapy in organ transplantation

The role of multidrug resistance and P-glycoprotein (P-gp) in the development of drug-resistant tumor cells has been extensively studied. As more knowledge on the physiological functions of P-gp has accumulated, the effects of P-gp modulation on the pharmacokinetics and the pharmacodynamics of many drugs have become apparent. Solid organ transplant recipients receive numerous medications that are substrates for P-gp. The objective of this review is to discuss the effects of P-gp modulation on the pharmacokinetics and the pharmacodynamics of immunosuppressive agents such as cyclosporine, tacrolimus, sirolimus, and corticosteroids. Pharmacokinetic alterations may occur in drug absorption since P-gp is in the small bowel, in drug distribution since P-gp functions in the blood-brain barrier, in drug metabolism since P-gp and cytochrome P450 3A have linked functions, and in drug elimination since P-gp is in the bile canaliculi and renal tubules. A link between P-gp and organ rejection has been speculated since upregulation of the P-gp pump may restrict immunosuppressant drug entry into immunocompetent cells. A further understanding of P-gp regulation upon chronic exposure to P-gp substrates and inhibitors and the potential administration of selective P-gp inhibitors will enhance our ability to use potent immunosuppressive drugs in organ transplant patients.

Pharmacokinetics and metabolism in early drug discovery

The need for high-throughput approaches in absorption, distribution, metabolism and excretion studies is driven by the impact of high-speed chemistry and pharmacological screening. Perhaps an even greater impact is that these studies will, in the future, provide large data sets that can be used to predict biological events related to absorption, bioavailability and metabolism of drugs. Through linking of in silico and in vitro methods, considerable progress has recently been made towards this future perspective. Despite this progress, these approaches do not yet replace in vivo methods.