P-glycoprotein increases from proximal to distal regions of human small intestine

Mass spectrometry-based targeted proteomics as a tool to elucidate the expression and function of intestinal drug transporters

Intestinal transporter proteins affect the oral bioavailability of many drugs in a significant manner. In order to estimate or predict their impact on oral drug absorption, data on their intestinal expression levels are needed. So far, predominantly mRNA expression data are available which are not necessarily correlated with the respective protein content. All available protein data were assessed by immunoblotting techniques such as Western blotting which both possess a number of limitations for reliable protein quantification. In contrast to this, mass spectrometry-based targeted proteomics may represent a promising alternative method to provide comprehensive protein expression data. In this review, we will summarize so far available intestinal mRNA and protein expression data for relevant human multidrug transporters. Moreover, recently observed mass spectrometry-based targeted proteomic data will be presented and discussed with respect to potential functional consequences. Associated to this, we will provide a short tutorial how to set up these methods and emphasize critical aspects in method development. Finally, potential limitations and pitfalls of this emerging technique will be discussed. From our perspective, LC-MS/MS-based targeted proteomics represents a valuable new method to comprehensively analyse the intestinal expression of transporter proteins. The resulting expression data are expected to improve our understanding about the intestinal processing of drugs.

A mechanistic physiologically based pharmacokinetic-enzyme turnover model involving both intestine and liver to predict CYP3A induction-mediated drug-drug interactions

Cytochrome P450 (CYP) 3A induction-mediated drug-drug interaction (DDI) is one of the major concerns in drug development and clinical practice. The aim of the present study was to develop a novel mechanistic physiologically based pharmacokinetic (PBPK)-enzyme turnover model involving both intestinal and hepatic CYP3A induction to quantitatively predict magnitude of CYP3A induction-mediated DDIs from in vitro data. The contribution of intestinal P-glycoprotein (P-gp) was also incorporated into the PBPK model. First, the pharmacokinetic profiles of three inducers and 14 CYP3A substrates were predicted successfully using the developed model, with the predicted area under the plasma concentration-time curve (AUC) [area under the plasma concentration-time curve] and the peak concentration (Cmax ) [the peak concentration] in accordance with reported values. The model was further applied to predict DDIs between the three inducers and 14 CYP3A substrates. Results showed that predicted AUC and Cmax ratios in the presence and absence of inducer were within twofold of observed values for 17 (74%) of the 23 DDI studies, and for 14 (82%) of the 17 DDI studies, respectively. All the results gave us a conclusion that the developed mechanistic PBPK-enzyme turnover model showed great advantages on quantitative prediction of CYP3A induction-mediated DDIs.

Investigation of the effect of the uneven distribution of CYP3A4 and P-glycoprotein in the intestine on the barrier function against xenobiotics: a simulation study

CYP3A4 and P-glycoprotein (P-gp) have similar substrate specificities and work together to form an intestinal absorption barrier against xenobiotics. Previous reports have indicated that CYP3A4 expression decreases gradually, whereas P-gp expression increases, from the upper to lower small intestine. The physiological rationale for this uneven distribution of CYP3A4 and P-gp as a barrier against xenobiotics has not been determined. To clarify the effect of these distribution patterns on barrier function, we constructed a mathematical model that included passive membrane permeation, P-gp-mediated apical efflux, and CYP3A4-mediated metabolism, and we simulated the effects of these distribution patterns on the fraction absorbed of co-substrates without changing their overall activities. The simulation showed that the physiological distribution patterns of both CYP3A4 and P-gp result in the lowest fraction absorbed, but not for drugs with low CYP3A4 and high P-gp-mediated clearances. These results suggest that the distribution pattern of CYP3A4 is especially important for the barrier function. On the other hand, physiological distribution pattern of P-gp exerts the maximum barrier function for dual good substrates for P-gp and CYP3A4, but even distribution of P-gp mostly suppresses the intestinal absorption of good P-gp, but poor CYP3A4 substrates.

From gut to kidney: transporting and metabolizing calcineurin-inhibitors in solid organ transplantation

Since their introduction circa 35 years ago, calcineurin-inhibitors (CNI) have become the cornerstone of immunosuppressive therapy in solid organ transplantation. However, CNI's possess a narrow therapeutic index with potential severe consequences of drug under- or overexposure. This demands a meticulous policy of Therapeutic Drug Monitoring (TDM) to optimize outcome. In clinical practice optimal dosing is difficult to achieve due to important inter- and intraindividual variation in CNI pharmacokinetics. A complex and often interdependent set of factors appears relevant in determining drug exposure. These include recipient characteristics such as age, race, body composition, organ function, and food intake, but also graft-related characteristics such as: size, donor-age, and time after transplantation can be important. Fundamental (in vitro) and clinical studies have pointed out the intrinsic relation between the aforementioned variables and the functional capacity of enzymes and transporters involved in CNI metabolism, primarily located in intestine, liver and kidney. Commonly occurring polymorphisms in genes responsible for CNI metabolism (CYP3A4, CYP3A5, CYP3A7, PXR, POR, ABCB1 (P-gp) and possibly UGT) are able to explain an important part of interindividual variability. In particular, a highly prevalent SNP in CYP3A5 has proven to be an important determinant of CNI dose requirements and drug-dose-interactions. In addition, a discrepancy in genotype between graft and receptor has to be taken into account. Furthermore, common phenomena in solid organ transplantation such as inflammation, ischemia- reperfusion injury, graft function, co-medication, altered food intake and intestinal motility can have a differential effect on the expression enzymes and transporters involved in CNI metabolism. Notwithstanding the built-up knowledge, predicting individual CNI pharmacokinetics and dose requirements on the basis of current clinical and experimental data remains a challenge.

Vitamin, mineral, and drug absorption following bariatric surgery

The prevalence of obesity continues to rise throughout the world. Increasingly, bariatric surgery is used for those with morbid obesity as a pivotal approach to achieve weight loss. Along with substantial weight loss, malabsorption of essential vitamins, minerals, and drugs also occurs. Therefore, more than ever, a better understanding of the physiology and mechanisms by which these deficiencies occur is essential. We review the normal physiology of vitamin, mineral, and drug absorption. This is followed by a description of currently performed bariatric surgeries in the United States. A detailed review of specific nutrient and mineral deficiency states is presented, based on the most significant studies published in the last two decades. Of note, screening and supplementation recommendations have been included. Drug absorption data after these procedures is presented and discussed. Studies were identified by searching the Cochrane Registry and MEDLINE using relevant search terms, as well as through review of the reference section of included manuscripts.

CONCLUSIONS

Bariatric surgery can be effectively used to achieve sustainable weight-loss in morbidly obese patients. It simultaneously brings forth important functional consequences on nutrient deficiencies and drug absorption that clinician's must be aware of. Further prospective, randomized research on specific procedures and deficiencies is required.

Investigating the enteroenteric recirculation of apixaban, a factor Xa inhibitor: administration of activated charcoal to bile duct-cannulated rats and dogs receiving an intravenous dose and use of drug transporter knockout rats

The study described here investigated the impact of intestinal excretion (IE; excretion of drug directly from circulation to intestinal lumen), enteroenteric recirculation (EER), and renal tubule recirculation (RTR) on apixaban pharmacokinetics and disposition. The experimental approaches involve integrating apixaban elimination pathways with pharmacokinetic profiles obtained from bile duct-cannulated (BDC) rats and dogs receiving i.v. doses together with oral administration of activated charcoal (AC). Additionally, the role of P-gp (P-glycoprotein; abcb1) and BCRP (breast cancer resistance protein; abcg2) in apixaban disposition was evaluated in experiments using transporter inhibitors and transporter knockout (KO) rats. Approximately 20-50% of an apixaban i.v. dose was found in feces of BDC rats and dogs, suggesting IE leading to fecal elimination and intestinal clearance (IC). The fecal elimination, IC, and systemic clearance of apixaban were increased upon AC administration in both BDC rats and dogs and were decreased in BDC rats dosed with GF-120918, a dual BCRP and P-gp inhibitor). BCRP appeared to play a more important role for absorption and intestinal and renal elimination of apixaban than P-gp in transporter-KO rats after oral and i.v. dosing, which led to a higher level of active renal excretion in rat than other species. These data demonstrate that apixaban undergoes IE, EER, and RTR that are facilitated by efflux transporters. Intestinal reabsorption of apixaban could be interrupted by AC even at 3 hours post-drug dose in dogs (late charcoal effect). This study demonstrates that the intestine is an organ for direct clearance and redistribution of apixaban. The IE, EER, and RTR contribute to overall pharmacokinetic profiles of apixaban. IE as a clearance pathway, balanced with metabolism and renal excretion, helps decrease the impacts of intrinsic (renal or hepatic impairment) and extrinsic (drug-drug interactions) factors on apixaban disposition.

Systemic Availability of the Active Metabolite Hydroxy-Fasudil After Administration of Fasudil to Different Sites of the Human Gastrointestinal Tract

This study evaluated the gastrointestinal absorption of fasudil, a novel Rho kinase inhibitor for the treatment of stable angina, at different sites using remote-controlled capsules and assessed the feasibility of developing an extended-release formulation. Ten healthy male volunteers were enrolled, and 8 subjects completed this single-dose, open-label, randomized, 5-way crossover study. Forty milligrams of fasudil HCl was administered as solution to the distal ileum and ascending colon, as powder to the ascending colon, and orally as an immediate-release tablet and solution. All treatments were well-tolerated and no serious adverse events were observed. The mean systemic availabilities of M3 relative to the oral solution were 1.04 (distal ileum, solution), 1.14 (ascending colon, solution), 1.27 (ascending colon, powder) and 1.04 (oral tablet), indicating similar systemic availability of M3 after administration of fasudil HCl to different gastrointestinal sites. The results suggest that development of a once-a-day extended-release formulation for fasudil HCl should be readily achievable.

Trends in oral drug bioavailability following bariatric surgery: examining the variable extent of impact on exposure of different drug classes

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

Changes to oral drug bioavailability have been observed post bariatric surgery. However, the magnitude and the direction of changes have not been assessed systematically to provide insights into the parameters governing the observed trends. Understanding these can help with dose adjustments.

WHAT THIS STUDY ADDS

Analysis of drug characteristics based on a biopharmaceutical classification system is not adequate to explain observed trends in altered oral drug bioavailability following bariatric surgery, although the findings suggest solubility to play an important role.

AIMS

To identify the most commonly prescribed drugs in a bariatric surgery population and to assess existing evidence regarding trends in oral drug bioavailability post bariatric surgery.

METHODS

A retrospective audit was undertaken to document commonly prescribed drugs amongst patients undergoing bariatric surgery in an NHS hospital in the UK and to assess practice for drug administration following bariatric surgery. The available literature was examined for trends relating to drug permeability and solubility with regards to the Biopharmaceutics Classification System (BCS) and main route of elimination.

RESULTS

No significant difference in the 'post/pre surgery oral drug exposure ratio' (ppR) was apparent between BCS class I to IV drugs, with regards to dose number (Do) or main route of elimination. Drugs classified as 'solubility limited' displayed an overall reduction as compared with 'freely soluble' compounds, as well as an unaltered and increased ppR.

CONCLUSION

Clinical studies establishing guidelines for commonly prescribed drugs, and the monitoring of drugs exhibiting a narrow therapeutic window or without a readily assessed clinical endpoint, are warranted. Using mechanistically based pharmacokinetic modelling for simulating the multivariate nature of changes in drug exposure may serve as a useful tool in the further understanding of postoperative trends in oral drug exposure and in developing practical clinical guidance.

Thiomer-coated liposomes harbor permeation enhancing and efflux pump inhibitory properties

An ideal oral drug carrier should facilitate drug delivery to the gastrointestinal tract and its absorption into the systemic circulation. To meet these requirements, we developed a thiomer-coated liposomal delivery system composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and a maleimide-functionalized lipid, to which chitosan-thioglycolic acid (CS-TGA) was covalently coupled. In addition to conventional 77 kDa CS-TGA (CS-TGA77), we tested the 150 kDa homologue (CS-TGA150) as well as an S-protected version of this polymer (CS-TGA150-MNA), in which some of the free SH-groups are conjugated with 6-mercaptonicotinamide to protect them from oxidation. Coupling of CS-TGA to the liposomal surface led to an increase in the particle size of at least 150 nm and an increase in the zeta potential from approximately − 33 mV to a maximum of about + 36 mV, depending on the polymer. As revealed by fluorescence dequenching the formulations have a storage stability of at least two weeks without releasing any encapsulated compounds. In simulated gastric fluid, the system was shown to be stable over 24 h, while in simulated intestinal fluid, a slow, sustained release of encapsulated compounds was observed. According to our experiments, thiomer-coated liposomes did not induce immunogenic reactions after an oral administration to mice. To evaluate the permeation enhancing and efflux pump inhibiting properties of CS-TGA coated liposomes we monitored the transport of fluoresceinisothiocyanate-dextran (FD₄) and rhodamine-123 (Rho-123), respectively, through rat small intestine. Permeation studies showed a 2.8-fold higher permeation of FD₄ in the presence of CS-TGA77 coated liposomes and an even 4-fold higher permeation in the presence of CSA-TGA150-MNA coated liposomes. The latter also performed best when we evaluated P-glycoprotein inhibiting properties by monitoring the transport of Rho-123, revealing a 4.2-fold enhancement respective to the buffer control. Taken together, thiomer-coated liposomes were shown to protect encapsulated drugs in the stomach, slowly release them in the small intestine and enhance their absorption through the intestinal tissue by opening tight junctions and inhibiting efflux pumps.

MDR1-P-glycoprotein behaves as an oncofetal protein that promotes cell survival in gastric cancer cells

P-glycoprotein (P-gp), traditionally linked to cancer poor prognosis and multidrug resistance, is undetectable in normal gastric mucosa and overexpressed in gastric cancer (GC). We propose that P-gp may be involved in Helicobacter pylori (Hp)-related gastric carcinogenesis by inhibiting apoptosis. Aim of the study was to evaluate the expression of P-gp in fetal stomach and in Hp-related gastric carcinogenesis, the epigenetic control of the multi-drug resistance-1 (MDR1) gene, the localization and interaction between P-gp and Bcl-x(L) and the effect of the selective silencing of P-gp on cell survival. P-gp and Bcl-xl expression was evaluated by immunohistochemistry on 28 spontaneously abortive human fetuses, 66 Hp-negative subjects, 138 Hp-positive chronic gastritis (CG) of whom 28 with intestinal metaplasia (IM) and 45 intestinal type GCs. P-gp/Bcl-x(L) colocalization was investigated by confocal immunofluorescence microscopy and protein-protein interaction by co-immunoprecipitation, in basal conditions and after stress-induced apoptosis, in GC cell lines AGS and MKN-28 and hepatocellular carcinoma cell line Hep-G2. The role of P-gp in controlling apoptosis was evaluated by knocking down its expression with a specific small interfering RNAs in stressed AGS and MKN-28 cell lines. P-gp is expressed in the gastric mucosa of all human fetuses while, it is undetectable in adult normal mucosa and re-expressed in 30/110 Hp-positive non-IM-CG, 28/28 IM-CG and 40/45 GCs. P-gp expression directly correlates with that of Bcl-x(L) and with the promoter hypomethylation of the MDR1 gene. In GC cell lines, P-gp is localized on the plasma membrane and mitochondria where it colocalizes with Bcl-x(L). Co-immunoprecipitation confirms the physical interaction between P-gp and Bcl-x(L) in AGS, MKN-28 and Hep-G2, at both basal level and after stress-induced apoptosis. The selective silencing of P-gp sensitizes GC cells to stress-induced apoptosis. P-gp behaves as an oncofetal protein that, by cross-talking with Bcl-x(L), acts as an anti-apoptotic agent in Hp-related gastric carcinogenesis.

Murine P-glycoprotein deficiency alters intestinal injury repair and blunts lipopolysaccharide-induced radioprotection

P-glycoprotein (P-gp) has been reported to increase stem cell proliferation and regulate apoptosis. Absence of P-gp results in decreased repair of intestinal epithelial cells after chemical injury. To further explore the mechanisms involved in the effects of P-gp on intestinal injury and repair, we used the well-characterized radiation injury model. In this model, injury repair is mediated by production of prostaglandins (PGE(2)) and lipopolysaccharide (LPS) has been shown to confer radioprotection. B6.mdr1a(-/-) mice and wild-type controls were subjected to 12 Gy total body X-ray irradiation and surviving crypts in the proximal jejunum and distal colon were evaluated 3.5 days after irradiation. B6.mdr1a(-/-) mice exhibited normal baseline stem cell proliferation and COX dependent crypt regeneration after irradiation. However, radiation induced apoptosis was increased and LPS-induced radioprotection was blunted in the C57BL6.mdr1a(-/-) distal colon, compared to B6 wild-type controls. The LPS treatment induced gene expression of the radioprotective cytokine IL-1α, in B6 wild-type controls but not in B6.mdr1a(-/-) animals. Lipopolysaccharid-induced radioprotection was absent in IL-1R1(-/-) animals, indicating a role for IL-1α in radioprotection, and demonstrating that P-gp deficiency interferes with IL-1α gene expression in response to systemic exposure to LPS.

A mechanistic pharmacokinetic model to assess modified oral drug bioavailability post bariatric surgery in morbidly obese patients: interplay between CYP3A gut wall metabolism, permeability and dissolution

Objectives

Due to the multi-factorial physiological implications of bariatric surgery, attempts to explain trends in oral bioavailability following bariatric surgery using singular attributes of drugs or simplified categorisations such as the biopharmaceutics classification system have been unsuccessful. So we have attempted to use mechanistic models to assess changes to bioavailability of model drugs.

Methods

Pharmacokinetic post bariatric surgery models were created for Roux-en-Y gastric bypass, biliopancreatic diversion with duodenal switch, sleeve gastrectomy and jejunoileal bypass, through altering the ‘Advanced Dissolution Absorption and Metabolism’ (ADAM) model incorporated into the Simcyp® Simulator. Post to pre surgical simulations were carried out for five drugs with varying characteristics regarding their gut wall metabolism, dissolution and permeability (simvastatin, omeprazole, diclofenac, fluconazole and ciprofloxacin).

Key findings

The trends in oral bioavailability pre to post surgery were found to be dependent on a combination of drug parameters, including solubility, permeability and gastrointestinal metabolism as well as the surgical procedure carried out.

Conclusions

In the absence of clinical studies, the ability to project the direction and the magnitude of changes in bioavailability of drug therapy, using evidence-based mechanistic pharmacokinetic in silico models would be of significant value in guiding prescribers to make the necessary adjustments to dosage regimens for an increasing population of patients who are undergoing bariatric surgery.

The fraction dose absorbed, in humans, and high jejunal human permeability relationship

The drug intestinal permeability (P(eff)) measure has been widely used as one of the main factors governing both the rate and/or extent of drug absorption (F(abs)) in humans following oral administration. In this communication we emphasize the complexity behind and the care that must be taken with this in vivo P(eff) measurement. Intestinal permeability, considering the whole of the human intestine, is more complex than generally recognized, and this can lead to misjudgment regarding F(abs) and P(eff) in various settings, e.g. drug discovery, formulation design, drug development and regulation. Setting the adequate standard for the low/high permeability class boundary, the different experimental methods for the permeability measurement, and segmental-dependent permeability throughout the human intestine due to different mechanisms are some of the main points that are discussed. Overall, the use of jejunal P(eff) as a surrogate for extent of absorption is sound and scientifically justified; a compound with high jejunal P(eff) will have high F(abs), eliminating the risk for misclassification as a BCS class I drug. Much more care should be taken, however, when jejunal P(eff) does not support a high-permeability classification; a thorough examination may reveal high-permeability after all, attributable to e.g. segmental-dependent permeability due to degree of ionization or transporter expression. In this situation, the use of multiple permeability experimental methods, including the use of metabolism, which except for luminal degradation requires absorption, is prudent and encouraged.

Relative bioavailability and pharmacodynamic effects of methantheline compared with atropine in healthy subjects

OBJECTIVES

Methantheline is a strong muscarinic receptor blocking drug used in the treatment of overactive bladder syndrome, hypersalivation and hyperhidrosis. To provide basic information on the pharmacokinetics, magnitude of pharmacodynamic (PD) effects and their correlations with plasma concentrations, we performed a clinical study in 12 healthy subjects receiving methantheline as immediate-release coated tablets (IR) or in watery solution (SOL) in comparison with atropine and placebo tablets.

METHODS

The pharmacokinetics and influence of methantheline, atropine and placebo on salivation and accommodation and pupil function (pupillometry: diameter, response to light flash) were studied in a randomized, controlled study after the administration of 100 mg methantheline bromide as IR and in SOL (phase 1) and 1.0 mg atropine sulphate and placebo (phase 2).

RESULTS

Methantheline reached maximum plasma concentrations of approximately 25 ng/ml after 2.5-3 h and was eliminated at an apparent half-life of approximately 2 h. There was no pharmacokinetic (PK) bioequivalence of methantheline IR and SOL. The ratio IR/SOL (90 % confidence interval) were 0.892 (0.532-1.493) for AUC(0-∞) and 0.905 (0.516-1.584) for maximum plasma concentration. The PD effects of both forms were nearly equivalent with a IR/SOL ratio of 1.015 (0.815-1.262) for salivation, which is the most susceptible characteristic. Methantheline reduced salivation at a potency (methantheline concentration at half maximum effects, EC₅₀) of 5.5 ng/ml in accordance with it plasma concentration. The antimuscarinic effects observed after methantheline administration were stronger and persisted longer than those following the administration of atropine.

CONCLUSIONS

Methantheline is slowly absorbed but rapidly eliminated in humans, and it exerts a strong effect on salivation which is closely associated with its plasma concentrations following a standard sigmoid PD model. Immediate-release tablets and a watery solution of methantheline are equivalent in terms of major PD effects (salivation, pupil function, heart rate) despite its high PK variability.

Absorption enhancement studies of clopidogrel hydrogen sulphate in rat everted gut sacs

Objectives

Clopidogrel, a thienopyridine antiplatelet agent, is a poor aqueous soluble compound and a P-glycoprotein (P-gp) efflux pump substrate. These two factors are responsible for its incomplete intestinal absorption. In this study, we have attempted to enhance the absorption of clopidogrel by improving its solubility and by inhibiting intestinal P-gp activity.

Methods

Solubility enhancement was achieved by preparing solid dispersions. Quinidine and naringin were selected as P-gp inhibitors, whilst tartaric acid was selected as the intestinal absorption enhancer. Absorption studies were performed using the everted gut sac model prepared from rat jejunum. The determination of clopidogrel was performed by high performance liquid chromatography.

Key findings

We noticed an enhancement of clopidogrel absorption by improving its solubility or by inhibiting the P-gp activity. The greatest results were obtained for solid dispersions in the presence of P-gp inhibitors at their highest concentrations, with an absorption improvement of 3.41- and 3.91-fold for naringin (15 mg/kg) and quinidine (200 µm), respectively. However, no clopidogrel absorption enhancement occurred in the presence of tartaric acid.

Conclusions

Naringin, a natural compound which has no undesirable side effects as compared with quinidine, could be used as a pharmaceutical excipient in the presence of clopidogrel solid dispersions to increase clopidogrel intestinal absorption and therefore its oral bioavailability.

Transporter-mediated drug-drug interactions

Drug-drug interactions are a serious clinical issue. An important mechanism underlying drug-drug interactions is induction or inhibition of drug transporters that mediate the cellular uptake and efflux of xenobiotics. Especially drug transporters of the small intestine, liver and kidney are major determinants of the pharmacokinetic profile of drugs. Transporter-mediated drug-drug interactions in these three organs can considerably influence the pharmacokinetics and clinical effects of drugs. In this article, we focus on probe drugs lacking significant metabolism to highlight mechanisms of interactions of selected intestinal, hepatic and renal drug transporters (e.g., organic anion transporting polypeptide [OATP] 1A2, OATP2B1, OATP1B1, OATP1B3, P-gp, organic anion transporter [OAT] 1, OAT3, breast cancer resistance protein [BCRP], organic cation transporter [OCT] 2 and multidrug and toxin extrusion protein [MATE] 1). Genotype-dependent drug-drug interactions are also discussed.

Physiologically based pharmacokinetic modeling of intestinal first-pass metabolism of CYP3A substrates with high intestinal extraction

Prediction of intestinal availability (F(G)), in conjunction with hepatic metabolism, is of considerable importance in drug disposition to assess oral clearance and liability to drug-drug interactions. In the current study, F(G) predictions were performed within a physiologically based pharmacokinetic (PBPK) model using in vitro permeability and clearance data. The prediction success was assessed in comparison with the Q(Gut) model. In addition, apparent oral clearance values, predicted using the PBPK model, were compared with in vivo observations from meta-analyses. Finally, unbound intrinsic clearance values (CLu(int)) were determined for 12 CYP3A substrates in eight individual human jejunal microsome (HJM) samples to assess interindividual variability in intestinal intrinsic clearance and subsequent F(G) predictions. Overall, the PBPK model improved F(G) predictions in comparison with the Q(Gut) model; this was apparent by a reduced bias and increased precision. In particular, F(G) predictions of indinavir, saquinavir, and terfenadine were model-dependent. The predicted oral clearance values of the drugs investigated ranged from 8.79 to 6320 l/h for tacrolimus and simvastatin, respectively, and were overall within 3-fold of the observed data with the exception of indinavir, atorvastatin, and buspirone. The individual HJM CLu(int) values ranged from 17 to 14,000 μl · min(-1) · mg(-1) for atorvastatin and saquinavir, respectively, and corresponding interindividual variability in CLu(int) estimates ranged from 41 to 67%. These in vitro data resulted in predicted F(G) values ranging from 0.03 to 0.94 for simvastatin and indinavir, respectively. The largest interindividual variability of F(G) was predicted for terfenadine (65%) in contrast with the low variability in the case of indinavir (3%).

Assessment of levetiracetam bioavailability from targeted sites in the human intestine using remotely activated capsules and gamma scintigraphy: Open-label, single-dose, randomized, four-way crossover study in healthy male volunteers

BACKGROUND

Levetiracetam is a broad-spectrum antiepileptic drug that binds to synaptic vesicle protein SV2A. Levetiracetam is indicated in the adjunctive treatment of partial-onset seizures, myoclonic seizures, and generalized tonic-clonic seizures. It is also approved in Europe as monotherapy for newly diagnosed partial-onset seizures. A Phase I clinical pharmacology trial was conducted during preregistration clinical development to better understand the regional gastrointestinal (GI) absorption of levetiracetam.

OBJECTIVE

This study evaluated the relative bioavailability of levetiracetam in various regions of the GI tract using a noninvasive, remote-controlled capsule device providing targeted drug delivery, relative to that after oral administration, and explored the drug's absorption characteristics in healthy volunteers.

METHODS

Pharmacokinetic data were obtained from healthy men aged 18 to 65 years in an open-label, single-dose, randomized, 4-way crossover study. Treatments included levetiracetam 250 mg administered as an immediate-release tablet and capsule delivery of 250 mg drug substance (levetiracetam powder without excipients) to the proximal small bowel, distal small bowel, and ascending colon. The location of the capsule in the GI tract was monitored using γ-scintigraphic imaging. Blood samples for plasma levetiracetam concentration were collected before dosing; at 10, 20, 30, and 45 minutes; and at 1, 1.5, 2, 3, 6, 9, 12, 16, 20, and 24 hours after tablet intake or after capsule activation. Pharmacokinetic parameters C(max), T(max), AUC₀₋(last), AUC₀₋(∞) and t(½) were calculated using noncompartmental methods. Tolerability was determined using clinical assessment, monitoring of vital signs, laboratory analysis, and interviews with the volunteers regarding adverse events.

RESULTS

Nine healthy men, 7 whites and 2 Asians, were enrolled (mean [SD] age, 31 [14] years; weight, 77 [5] kg; height, 176 [6] cm). Six volunteers completed all 4 treatments. Seven adverse events (headache [3], lethargy [2], tachycardia [1], and contusion [1]) were reported in 5 volunteers, but only 2 (headache and lethargy) were judged by the investigator to be possibly drug related. The geometric mean (%CV) AUC(0-last) values of levetiracetam delivered in the proximal small bowel, distal small bowel, ascending colon, and stomach (oral tablet) were 58.2 (9.3%), 59.6 (8.9%), 51.5 (12.0%), and 59.0 (7.4%) μg · h/mL, respectively. Values for bioavailability in the proximal small bowel, distal small bowel, and ascending colon relative to the tablet were 98.5% (95% CI, 89.7%-108.2%), 100.8% (95% CI, 91.4%-111.1%), and 87.1% (95% CI, 77.9%-97.5%).

CONCLUSION

After delivery in the proximal small bowel, distal small bowel, or ascending colon, the systemic bioavailability of levetiracetam (AUC), but not C(max) and T(max), appeared comparable to that after oral administration and thus appeared site independent in this small group of healthy fasting men.

In vivo probes of drug transport: commonly used probe drugs to assess function of intestinal P-glycoprotein (ABCB1) in humans

Intestinal P-glycoprotein (P-gp, ABCB1) may significantly influence drug absorption and elimination. Its expression and function is highly variable, regio-selective and influenced by genetic polymorphisms, drug interactions and intestinal diseases. An in vivo probe drug for intestinal P-gp should a registered, safe and well tolerated nonmetabolized selective substrate with low protein binding for which P-gp is rate-limiting during absorption. Other P-gp dependent processes should be of minor influence. The mechanism(s) and kinetics of intestinal uptake must be identified and quantified. Moreover, the release properties of the dosage form should be known. So far, the cardiac glycoside digoxin and the ß₁-selective blocker talinolol have been used in mechanistic clinical studies, because they meet most of these criteria. Digoxin and talinolol are suitable in vivo probe drugs for intestinal P-gp under the precondition, that they are used as tools in carefully designed pharmacokinetic studies with adequate biometrically planning of the sample size and that several limitations are considered in interpreting and discussion of the study results.

Role of p-glycoprotein in region-specific gastrointestinal absorption of talinolol in rats

P-Glycoprotein (PGP) is nonuniformly distributed along the gastrointestinal (GI) tract; however, the data regarding regional differences in PGP function in the intestine are controversial. The aim of this work was to investigate the role of PGP efflux in region-specific absorption of talinolol from the GI tract in rats. Plasma talinolol concentrations were measured after several modes of administration, including high (40 mg/kg) and low (4 mg/kg) dose levels, to different segments of the GI tract (stomach versus colon), and codosing with PGP inhibitors (verapamil or cyclosporine). The bioavailability (F) of talinolol after high-dose administration to the stomach was significantly greater than that achieved by the low dose (approximately 18 versus 2%). Coadministration of low-dose talinolol with cyclosporine increased F by approximately 5-fold (p < 0.01). For the high dose, codosing with PGP inhibitors did not increase the extent of absorption. Talinolol demonstrated poor colonic absorption that was significantly increased by coadministration with cyclosporine (F = 0.76 versus 8.1%). Oral verapamil significantly increased systemic clearance and the steady state volume of distribution of intravenous talinolol. A semiphysiological model was developed that successfully captured the pharmacokinetic profiles of talinolol after various modes of administration. PGP-mediated efflux appears to be a major factor responsible for GI region-specific absorption of talinolol in rats, and gastroretentive dosage forms may provide an advantage in the delivery of talinolol and PGP substrate drugs.

Pre-clinical evidence of enhanced oral bioavailability of the P-glycoprotein substrate talinolol in combination with morin

Most known interactions between herbal extracts and drugs involve the inhibition of drug-metabolizing enzymes, but little is yet known about the possible role of transporters in these interactions. In order to evaluate the effect of one of such prominent flavonoids, morin, on P-glycoprotein related efflux carriers, measurements of transport characteristics through Ussing chambers, in situ perfusion and in vivo drug absorption studies were performed with the transported, yet not metabolized model compound talinolol.This study investigated the effects of orally administered morin (1.0, 2.5 and 5.0 mg kg(-1)), on the pharmacokinetics of orally (10 mg kg(-1)) and intravenously (1.0 mg kg(-1)) administered talinolol in rats. In the presence of morin, the pharmacokinetic parameters of talinolol were significantly altered in the oral group but not in the intravenous group. The presence of 2.5 and 5.0 mg kg(-1) of morin significantly increased (1.8-2.0 fold, p<0.01) the area under the plasma concentration-time curve and the peak plasma concentration (2.3-3.0 fold, p<0.01) of orally administered talinolol. The absolute bioavailability (F %) of talinolol in the rats pretreated with morin was significantly higher (89.09-98.29%, p<0.01) than the control (52.14%). Talinolol demonstrated asymmetric transport across rat ileum with significantly greater basolateral-to-apical (B-A) permeability than that in the apical-to-basolateral (A-B) direction. The addition of morin resulted in a concentration dependent effect, especially on the secretory transport of talinolol.The present study demonstrates that morin bears the ability to interfere with secretory intestinal transport processes. This might be due to an interaction with P-glycoprotein.

Predictions of metabolic drug-drug interactions using physiologically based modelling: Two cytochrome P450 3A4 substrates coadministered with ketoconazole or verapamil

Nowadays, evaluation of potential risk of metabolic drug-drug interactions (mDDIs) is of high importance within the pharmaceutical industry, in order to improve safety and reduce the attrition rate of new drugs. Accurate and early prediction of mDDIs has become essential for drug research and development, and in vitro experiments designed to evaluate potential mDDIs are systematically included in the drug development plan prior to clinical assessment. The aim of this study was to illustrate the value and limitations of the classical and new approaches available to predict risks of DDIs in the research and development processes. The interaction of cytochrome P450 (CYP) 3A4 inhibitors (ketoconazole and verapamil) with midazolam was predicted using the inhibitor concentration/inhibition constant ([I]/K(i)) approach, the static approach with added variability (Simcyp(R)), and whole-body physiologically based pharmacokinetic (WB-PBPK) modelling (acslXtreme(R)). Then an in-house reference drug was used to challenge the different approaches based on the midazolam experience. Predicted values (pharmacokinetic parameters, the area under the plasma concentration-time curve [AUC] ratio and plasma concentrations) were compared with observed values obtained after intravenous and oral administration in order to assess the accuracy of the prediction methods. With the [I]/K(i) approach, the interaction risk was always overpredicted for the midazolam substrate, regardless of its route of administration and the coadministered inhibitor. However, the predictions were always satisfactory (within 2-fold) for the reference drug. For the Simcyp(R) calculations, two of the three interaction results for midazolam were overpredicted, both when midazolam was given orally, whereas the prediction obtained when midazolam was administered intravenously was satisfactory. For the reference drug, all predictions could be considered satisfactory. For the WB-PBPK approach, all predictions were satisfactory, regardless of the substrate, route of administration, dose and coadministered inhibitor. DDI risk predictions are performed throughout the research and development processes and are now fully integrated into decision-making processes. The regulatory approach is useful to provide alerts, even at a very early stage of drug development. The 'steady state' approach in Simcyp(R) improves the prediction by using physiological knowledge and mechanistic assumptions. The DDI predictions are very useful, as they provide a range of AUC ratios that include individuals at the extremes of the population, in addition to the 'average tendency'. Finally, the WB-PBPK approach improves the predictions by simulating the concentration-time profiles and calculating the related pharmacokinetic parameters, taking into account the time of administration of each drug - but it requires a good understanding of the absorption, distribution, metabolism and excretion properties of the compound.

The transit of dosage forms through the small intestine

The human small intestine, with its enormous absorptive surface area, is invariably the principal site of drug absorption. Hence, the residence time of a dosage form in this part of the gut can have a great influence on the absorption of the contained drug. Various methods have been employed to monitor the gastrointestinal transit of pharmaceutical dosage forms, but the use of gamma-scintigraphy has superceded all the other methods. However, careful consideration of the time interval for image acquisition and proper analysis of the scintigraphic data are important for obtaining reliable results. Most studies reported the mean small intestinal transit time of various dosage forms to be about 3-4h, being closely similar to that of food and water. The value does not appear to be influenced by their physical state nor the presence of food, but the timing of food intake following administration of the dosage forms can influence the small intestinal transit time. While the mean small intestinal transit time is quite consistent among dosage forms and studies, individual values can vary widely. There are differing opinions regarding the effect of density and size of dosage forms on their small intestinal transit properties. Some common excipients employed in pharmaceutical formulations can affect the small intestinal transit and drug absorption. There is currently a lack of studies regarding the effects of excipients, as well as the timing of food intake on the small intestinal transit of dosage forms and drug absorption.

Modulation of the activity of ABC transporters (P-glycoprotein, MRP2, BCRP) by flavonoids and drug response

The present article aims to review the up-to-date information on the most recent studies of the interaction of flavonoids with ABC transporters, in particular the drug pharmacokinetic consequences of such a relationship. In addition, the modulation of the expression of the ABC transporters by flavonoids is also illustrated. Flavonoids are a large group of plant polyphenols present extensively in our daily diets and herbal products. High intake of isoflavones has been associated with a variety of beneficial effects on several common diseases. These polyphenols interact with ABC drug transporters involved in drug resistance and drug absorption, distribution and excretion. A number of studies have demonstrated inhibition of drug transporters by flavonoids. This flavonoid-ABC-transporter interaction could be beneficial for poorly absorbed drugs but could also result in severe drug intoxication, especially drugs with a narrow therapeutic window. On the other hand, flavonoids are themselves substrates of ABC transporters. These proteins can affect the oral availability and tissue distribution of these compounds, modifying their beneficial effects. The challenge is to find a suitable way to predict harmful drug-flavonoid interactions mediated by these transporters.

The Effects of Pregnenolone 16α-Carbonitrile Dosing on Digoxin Pharmacokinetics and Intestinal Absorption in the Rat

The effect of Pgp induction in rats by pregnenolone 16α-carbonitrile (PCN) (3 days, 35 mg/kg/d, p.o.) on digoxin pharmacokinetics and intestinal transport has been assessed. After intravenous or oral digoxin dosing the arterial and hepatic portal vein (oral) AUC_(0-24h) were significantly reduced by PCN pre-treatment. Biliary digoxin clearance increased 2-fold following PCN treatment. PCN significantly increased net digoxin secretion (2.05- and 4.5-fold respectively) in ileum and colon but not in duodenum or jejunum. This increased secretion correlated with increased Pgp protein expression in ileum and colon. Both intestinal and biliary excretion therefore contribute to altered digoxin disposition following PCN.

Interplay of transporters and enzymes in drug and metabolite processing

This review highlights the "interplay" between enzymes and transporters, essential components of eliminating organs for drug removal. The understanding of the interplay is important in terms of deciphering the change of one eliminatory pathway on compensatory mechanisms in drug disposal, and, ultimately, their importance in drug-drug interactions. Controversy existed on the explanation underlying the interplay between transporters and enzymes in the Caco-2 cell monolayer or cell culture systems, but less so on eliminating organs such as the intestine and liver. For the Caco-2 system, the increase in the mean residence time (MRT) accompanying increased secretion had been construed as the basis for increased metabolism. We hold the opposite view and assert that increased secretion should evoke a decrease in metabolism due to the competition between the enzyme and apical efflux transporter for the drug within the cell. To illustrate this point, simulations on the MRT, fraction of dose metabolized (f(met)) and the extraction ratio (ER) as defined by various investigators under linear and nonlinear metabolic conditions were compared to observed data and the trends upon induction/inhibition of secretion. The conclusion is that the f(met) is the more appropriate index to reflect the extent of metabolism in transporter-enzyme interplay, since the parameter captures drug metabolism in the cell when its contents in the apical, cell, and basolateral compartments or the entire dose is considered to be available for metabolism. This parameter for metabolism (f(met)) bears a reciprocal relationship to the secretory intrinsic clearance and is in concordance with the notion that both the enzyme and apical transporter compete for the cellular substrate within. For the liver and intestine, several physiologically based pharmacokinetic (PBPK) models that contain transporters and enzymes were utilized, together with the solved equations for the area under the curve (AUC), metabolic, excretory, and total clearance (CL) to shed meaningful insight of how the inhibition of one pathway can result in a higher AUC and therefore a reduced total clearance for drug, but a higher apparent clearance of the alternate pathway; induction of the same pathway would lead to an increased total clearance but decreased drug AUC, and reduced clearance of the alternate pathway. The use of an increased MRT to explain increased extents of metabolism upon increased apical excretion is not tenable in these organs or "open systems" since the MRT of drug in the cell is reduced with irreversible loss from biliary excretion or hastened gastrointestinal transit of the secreted drug in the lumen. Data in the literature for the Caco-2 system, knockout animals and organ perfusion systems were discussed in relation to these concepts on clearance based on fundamental, pharmacokinetic theory. The shortcomings in data interpretation were discussed. The general conclusion is that a reciprocal relationship exists between the clearances related to enzymes and apical transporters due to their competition for the substrate within the cell, and is a relationship independent of the MRT of drug in the system.

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.

A systematic review of drug absorption following bariatric surgery and its theoretical implications

Demand for bariatric surgery has risen exponentially and bariatric patients often have multiple indications for post-operative pharmacotherapy. The purpose of this study was to systematically review the published literature examining the effect of bariatric surgery on drug absorption. Studies were sought through searches of MEDLINE, EMBASE, the Cochrane Controlled Trials Registry and hand searches of reference lists. Two reviewers independently assessed studies for inclusion. Twenty-six studies (15 case reports/case series evaluating 12 different agents and 11 non-randomized controlled studies examining 15 different agents) were found. Evidence for diminished drug absorption was found in 15/22 studies involving jejunoileal bypass, 1/3 studies of gastric bypass/gastroplasty and 0/1 studies examining biliopancreatic diversion. The effect of bariatric surgery on drug absorption appears drug-specific. Drugs that are intrinsically poorly absorbed, highly lipophilic and/or undergo enterohepatic recirculation exhibited the greatest potential for malabsorption. The most consistent evidence for diminished absorption was found for cyclosporine, thyroxine, phenytoin and rifampin. Reduced drug absorption may occur post-bariatric surgery and this effect appears drug-specific. Individual dose-adjustment and therapeutic monitoring may be required. Rigorously conducted controlled studies are needed to evaluate the effect of modern bariatric procedures on drug absorption.

Pharmacology of morphine in obese patients: clinical implications

Morphine is an analgesic drug used to treat acute and chronic pain. Obesity is frequently associated with pain of various origins (e.g. arthritis, fibromyalgia, cancer), which increases the need for analgesic drugs. Obesity changes drug pharmacokinetics, and for certain drugs, specific modalities of prescription have been proposed for obese patients. However, scant data are available regarding the pharmacokinetics and pharmacodynamics of morphine in obesity. Prescription of morphine depends on pain relief but the occurrence of respiratory adverse effects correlates with obesity, and is not currently taken into account. Variations in the volume of distribution, elimination half-life and oral clearance of morphine, as well as recent advances in the respective roles of drug-metabolizing enzymes, catechol-O-methyltransferase and the mu opioid receptor in morphine pharmacokinetics and pharmacodynamics, may contribute to differences between obese and non-obese patients. In addition, drug-drug interactions may alter the disposition of morphine and its glucuronide metabolites, which may either increase the risk of adverse effects or reduce drug efficacy.