FIRST-PASS EFFECTS OF VERAPAMIL ON THE INTESTINAL ABSORPTION AND LIVER DISPOSITION OF FEXOFENADINE IN THE PORCINE MODEL

Induced mammary cancer in rat models: pathogenesis, genetics, and relevance to female breast cancer

Mammary cancer, or breast cancer in women, is a polygenic disease with a complex etiopathogenesis. While much remains elusive regarding its origin, it is well established that chemical carcinogens and endogenous estrogens contribute significantly to the initiation and progression of this disease. Rats have been useful models to study induced mammary cancer. They develop mammary tumors with comparable histopathology to humans and exhibit differences in resistance or susceptibility to mammary cancer depending on strain. While some rat strains (e.g., Sprague-Dawley) readily form mammary tumors following treatment with the chemical carcinogen, 7,12-dimethylbenz[a]-anthracene (DMBA), other strains (e.g., Copenhagen) are resistant to DMBA-induced mammary carcinogenesis. Genetic linkage in inbred strains has identified strain-specific quantitative trait loci (QTLs) affecting mammary tumors, via mechanisms that act together to promote or attenuate, and include 24 QTLs controlling the outcome of chemical induction, 10 QTLs controlling the outcome of estrogen induction, and 4 QTLs controlling the outcome of irradiation induction. Moreover, and based on shared factors affecting mammary cancer etiopathogenesis between rats and humans, including orthologous risk regions between both species, rats have served as useful models for identifying methods for breast cancer prediction and treatment. These studies in rats, combined with alternative animal models that more closely mimic advanced stages of breast cancer and/or human lifestyles, will further improve our understanding of this complex disease.

Large Animal Models of Breast Cancer

In this mini review the status, advantages, and disadvantages of large animal modeling of breast cancer (BC) will be discussed. While most older studies of large animal BC models utilized canine and feline subjects, more recently there has been interest in development of porcine BC models, with some early promising results for modeling human disease. Widely used rodent models of BC were briefly reviewed to give context to the work on the large animal BC models. Availability of large animal BC models could provide additional tools for BC research, including availability of human-sized subjects and BC models with greater biologic relevance.

Porcine Prediction of Pharmacokinetic Parameters in People: A Pig in a Poke?

The minipig has become an animal of considerable interest in preclinical drug development. It has been used in toxicology research and in examining/establishing regulatory guidelines as a nonrodent animal model. We have reviewed some basic issues that one would want to consider in the development and testing of any animal model for humans. The pig is a reasonable alternative to the dog, but there are some clear limitations and unexplained disparities in the literature, which require further study; primary among these is the need for standardization in choice of breed and sex and routine protocols. The minipig offers numerous advantages over other established animal models, and it has similarities to the human with regard to anatomy, physiology, and biochemistry. The gastrointestinal tract is structurally and functionally similar to humans. This appears to be true for enzymes and transporters in the gut as well, but more study is needed. One major concern is assessment of oral drug absorption, especially with regard to potential food effects due to gastric emptying differences, yet this does not appear to be a consistent observation. Hepatic metabolism seems to reflect enzymatic patterns in humans, with some differences. Kidney function seems similar to humans but requires further study. We have analyzed literature data that suggest the pig would offer a reasonable model for human oral bioavailability and for allometric predictions of clearance. The minipig appears to be the model for dermal absorption in humans, and we discuss this in terms of literature data and our own in-house experience.

Tacrine sinusoidal uptake and biliary excretion in sandwich-cultured primary rat hepatocytes

PURPOSE. The knowledge of hepatic disposition kinetics of tacrine, a first cholinesterase inhibitor was approved by FDA for the treatment of Alzheimer's disease (AD), would help to understand its hepatotoxicity, its therapeutic effect, and improve the management of patients with AD. The current study aims to characterize tacrine hepatic transport kinetics and study the role of organic cation transporters (OCTs), P-glycoprotein (P-gp) and multidrug resistance-associated protein (MRP2) in tacrine sinusoidal uptake and biliary excretion. METHODS. Modulation of tacrine hepatic uptake and efflux, biliary excretion index (BEI%), were performed in sandwich-cultured primary rat hepatocytes (SCHs) using transporters inhibitors. Conformation of the integrity of SCHs model was established by capturing images with light-contrast and fluorescence microscopy. RESULTS. Tacrine uptake in SCHs was carrier-mediated process and saturable with apparent Km of 31.5±9.6 µM and Vmax of 908±72 pmol/min/mg protein. Tetraethyl ammonium (TEA), cimetidine and verapamil significantly reduced tacrine uptake with more pronounced effect observed with verapamil which caused 3-fold reduction in tacrine uptake, indicating role for OCTs. Tacrine has a biliary excretion in SCHs with maximum BEI% value of 22.9±1.9% at 10 min of incubation. Addition of MK571 and valspodar decreased the BEI% of tacrine by 40 and 60% suggesting roles for canalicular MRP2 and P-gp, respectively. CONCLUSIONS. Our results show that in addition to metabolism, tacrine hepatic disposition is carrier-mediated process mediated by sinusoidal OCTs, and canalicular MRP2 and P-gp.

Direct In Vivo Human Intestinal Permeability (Peff ) Determined with Different Clinical Perfusion and Intubation Methods

Regional in vivo human intestinal effective permeability (Peff ) is calculated by measuring the disappearance rate of substances during intestinal perfusion. Peff is the most relevant parameter in the prediction of rate and extent of drug absorption from all parts of the intestine. Today, human intestinal perfusions are not performed on a routine basis in drug development. Therefore, it would be beneficial to increase the accuracy of the in vitro and in silico tools used to evaluate the intestinal Peff of novel drugs. This review compiles historical Peff data from 273 individual measurements of 80 substances from 61 studies performed in all parts of the human intestinal tract. These substances include: drugs, monosaccharaides, amino acids, dipeptides, vitamins, steroids, bile acids, ions, fatty acids, and water. The review also discusses the determination and prediction of Peff using in vitro and in silico methods such as quantitative structure-activity relationship, Caco-2, Ussing chamber, animal intestinal perfusion, and physiologically based pharmacokinetic (PBPK) modeling. Finally, we briefly outline how to acquire accurate human intestinal Peff data by deconvolution of plasma concentration-time profiles following regional intestinal bolus dosing.

Effects of verapamil on the pharmacokinetics and hepatobiliary disposition of fexofenadine in pigs

The pharmacokinetics (PK) of fexofenadine (FEX) in pigs were investigated with the focus on exploring the interplay between hepatic transport and metabolism when administered intravenously (iv) alone or with verapamil. The in vivo pig model enabled simultaneous sampling from plasma (pre-liver, post-liver and peripheral), bile and urine. Each animal was administered FEX 35mg iv alone or with verapamil 35mg. Plasma, bile and urine were analyzed with liquid chromatography-tandem mass spectrometry. Non-compartmental analysis (NCA) was used to estimate traditional PK parameters. In addition, a physiologically based pharmacokinetic (PBPK) model consisting of 11 compartments (6 tissues +5 sample sites) was applied for mechanistic elucidation and estimation of individual PK parameters. FEX had a terminal half-life of 1.7h and a liver extraction of 3%. The fraction of the administered dose of unchanged FEX excreted into the bile was 25% and the bile exposure was more than 100 times higher than the portal vein total plasma exposure, indicating carrier-mediated (CM) disposition processes in the liver. 23% of the administered dose of FEX was excreted unchanged in the urine. An increase in FEX plasma exposure (+50%) and a decrease in renal clearance (-61%) were detected by NCA as a direct effect of concomitant administration of verapamil. However, analysis of the PBPK model also revealed that biliary clearance was significantly inhibited (-53%) by verapamil. In addition, PBPK analysis established that metabolism and CM uptake were important factors in the disposition of FEX in the liver. In conclusion, this study demonstrated that CM transport of FEX in both liver and kidneys was inhibited by a single dose of verapamil.

The effects of lipiodol and cyclosporin A on the hepatobiliary disposition of doxorubicin in pigs

Doxorubicin (DOX) emulsified in Lipiodol (LIP) is used as local palliative treatment for unresectable intermediate stage hepatocellular carcinoma. The objective of this study was to examine the poorly understood effects of the main excipient in the drug delivery system, LIP, alone or together with cyclosporin A (CsA), on the in vivo liver disposition of DOX and its active metabolite doxorubicinol (DOXol). The advanced, multi-sampling-site, acute pig model was used; samples were collected from three blood vessels (v. portae, v. hepatica and v. femoralis), bile and urine. The four treatment groups (TI-TIV) all received two intravenous 5 min infusions of DOX into an ear vein: at 0 and 200 min. Before the second dose, the pigs received a portal vein infusion of saline (TI), LIP (TII), CsA (TIII) or LIP and CsA (TIV). Concentrations of DOX and DOXol were analyzed using UPLC-MS/MS. The developed multicompartment model described the distribution of DOX and DOXol in plasma, bile and urine. LIP did not affect the pharmacokinetics of DOX or DOXol. CsA (TIII and TIV) had no effect on the plasma pharmacokinetics of DOX, but a 2-fold increase in exposure to DOXol and a significant decrease in hepatobiliary clearance of DOX and DOXol were observed. Model simulations supported that CsA inhibits 99% of canalicular biliary secretion of both DOX and DOXol, but does not affect the metabolism of DOX to DOXol. In conclusion, LIP did not directly interact with transporters, enzymes and/or biological membranes important for the hepatobiliary disposition of DOX.

Human in vivo regional intestinal permeability: importance for pharmaceutical drug development

Both the development and regulation of pharmaceutical dosage forms have undergone significant improvements and development over the past 25 years, due primarily to the extensive application of the biopharmaceutical classification system (BCS). The Biopharmaceutics Drug Disposition Classification System, which was published in 2005, has also been a useful resource for predicting the influence of transporters in several pharmacokinetic processes. However, there remains a need for the pharmaceutical industry to develop reliable in vitro/in vivo correlations and in silico methods for predicting the rate and extent of complex gastrointestinal (GI) absorption, the bioavailability, and the plasma concentration-time curves for orally administered drug products. Accordingly, a more rational approach is required, one in which high quality in vitro or in silico characterizations of active pharmaceutical ingredients and formulations are integrated into physiologically based in silico biopharmaceutics models to capture the full complexity of GI drug absorption. The need for better understanding of the in vivo GI process has recently become evident after an unsuccessful attempt to predict the GI absorption of BCS class II and IV drugs. Reliable data on the in vivo permeability of the human intestine (Peff) from various intestinal regions is recognized as one of the key biopharmaceutical requirements when developing in silico GI biopharmaceutics models with improved predictive accuracy. The Peff values for human jejunum and ileum, based on historical open, single-pass, perfusion studies are presented in this review. The main objective of this review is to summarize and discuss the relevance and current status of these human in vivo regional intestinal permeability values.

Investigation of hepatobiliary disposition of doxorubicin following intrahepatic delivery of different dosage forms

Unresectable, intermediate stage hepatocellular carcinoma (HCC) is often treated palliatively in humans by doxorubicin (DOX). The drug is administered either as a drug-emulsified-in-Lipiodol (DLIP) or as drug loaded into drug eluting beads (DEB), and both formulations are administered intrahepatically. However, several aspects of their in vivo performance in the liver are still not well-understood. In this study, DLIP and DEB were investigated regarding the local and systemic pharmacokinetics (PK) of DOX and its primary metabolite doxorubicinol (DOXol). An advanced PK-multisampling site acute in vivo pig model was used for simultaneous sampling in the portal, hepatic, and femoral veins and the bile duct. The study had a randomized, parallel design with four treatment groups (TI-TIV). TI (n = 4) was used as control and received an intravenous (i.v.) infusion of DOX as a solution. TII and TIII were given a local injection in the hepatic artery with DLIP (n = 4) or DEB (n = 4), respectively. TIV (n = 2) received local injections of DLIP in the hepatic artery and bile duct simultaneously. All samples were analyzed for concentrations of DOX and DOXol with UPLC-MS/MS. Compared to DLIP, the systemic exposure for DOX with DEB was reduced (p < 0.05), in agreement with a slower in vivo release. The approximated intracellular bioavailability of DOX during 6 h appeared to be lower for DEB than DLIP. Following i.v. infusion (55 min), DOX had a liver extraction of 41 (28-53)%, and the fraction of the dose eliminated in bile of DOX and DOXol was 20 (15-22)% and 4.2 (3.2-5.2)%, respectively. The AUCbile/AUCVP for DOX and DOXol was 640 (580-660) and 5000 (3900-5400), respectively. In conclusion, DLIP might initially deliver a higher hepatocellular concentration of DOX than DEB as a consequence of its higher in vivo release rate. Thus, DLIP delivery results in higher intracellular peak concentrations that might correlate with better anticancer effects, but also higher systemic drug exposure and safety issues.

Prediction of human pharmacokinetics-biliary and intestinal clearance and enterohepatic circulation

The main objective was to evaluate and propose methods for predicting biliary clearance (CL(bile)) and enterohepatic circulation (EHC) of intact drugs in man. Another aim was to evaluate to role of intestinal drug secretion and propose a method for prediction of intestinal secretion CL (CL(i)). Animal data poorly predict the CL and CL(bile) of biliary excreted drugs, and the suggested molecular weight threshold for bile excretion as the dominant elimination route does not seem to hold. Active transport, low metabolic intrinsic CL (CL(int)) and, as an approximation, permeability (P(e)) less than that of metoprolol is required for substantial CL(bile) to occur. The typical EHC plasma concentration vs time profile (multiple peaks) is demonstrated for many low metabolic CL(int)-compounds with efflux and moderate to high intestinal P(e) and fraction absorbed. Physiologically-based in-vitro to in-vivo (PB-IVIV) methodology with in-vitro intrinsic CL(bile)-data obtained with sandwich-cultured human hepatocytes has generated 2- and 5-fold underpredictions for two compounds with intermediate to high CL(bile). This is despite not considering the unbound fraction. Possible explanations include low transporter activity and diffusion limitations in the in-vitro experiments. Intestinal reabsorption and EHC were also neglected in these predictions and in-vivo CL(bile) estimations. The sandwich model and these reference data are still very useful. Consideration of an empirical scaling factor and a newly developed approach that accounts for intestinal reabsorption and EHC could potentially lead to improved PB-IVIV predictions of CL(bile). Apparently, no attempts have been made to predict CL(i). Elimination via the intestinal route does not appear to be of great importance for the few compounds with available data, but could be equally as important as bile excretion. Net secretion in-vitro P(e) and newly estimated in-vivo intrinsic CL(i) data for digoxin and rosuvastatin could be useful for approximation of CL(i) of other compounds.

Biopharmaceutical challenges associated with drugs with low aqueous solubility--the potential impact of lipid-based formulations

The percentage of new chemical entities synthesised with low aqueous solubility and high therapeutic efficacy is growing, this presents major challenges for the drug delivery scientists. The role of physicochemical properties in identification of suitable drug candidates for oral lipid-based delivery systems is discussed. A knowledge of the interplay of physicochemical and biopharmaceutical drug properties with the physiological environment of the gastro-intestinal tract (GIT), as a prerequisite to successful formulation design, is reviewed. The importance of excipient selection with an emphasis on bioactive excipients is stressed. The need for more examples of in vitro-in vivo correlations as a means of maximizing the development potential and commercial future for lipid-based formulations, and, promoting confidence within the industry for these delivery systems is highlighted.

Intestinal permeability and its relevance for absorption and elimination

Human jejunal permeability (P(eff)) is determined in the intestinal region with the highest expression of carrier proteins and largest surface area. Intestinal P(eff) are often based on multiple parallel transport processes. Site-specific jejunal P(eff) cannot reflect the permeability along the intestinal tract, but they are useful for approximating the fraction oral dose absorbed. It seems like drugs with a jejunal P(eff) > 1.5 x 10(-4) cm s(-1) will be completely absorbed no matter which transport mechanism(s) are utilized. Many drugs that are significantly effluxed in vitro have a rapid and complete intestinal absorption (i.e. >85%) mediated by passive transcellular diffusion. The determined jejunal P(eff) for drugs transported mainly by absorptive carriers (such as peptide and amino acid transporters) will accurately predict the fraction of the dose absorbed as a consequence of the regional expression. The data also show that: (1) the human intestinal epithelium has a large resistance towards large and hydrophilic compounds; and (2) the paracellular route has a low contribution for compounds larger than approximately molecular weight 200. There is a need for more exploratory in vivo studies to clarify drug absorption and first-pass extraction along the intestine. One is encouraged to develop in vivo perfusion techniques for more distal parts of the gastrointestinal tract in humans. This would stimulate the development of more relevant and complex in vitro absorption models and form the basis for an accurate physiologically based pharmacokinetic modelling of oral drug absorption.

The effect of pancreatic and biliary depletion on the in vivo pharmacokinetics of digoxin in pigs

Several transporter systems in the liver and intestine are known to change their expression and function during cholestatic disease states. The objective of the present in vivo study was to investigate the effect of biliary depletion, as a method to mimic cholestasis, on the bioavailability and disposition of digoxin in biliary and pancreatic duct cannulated pigs. The study was divided in two parts. In the first part, a solution of 10 microg/kg digoxin was administered intravenously to the cannulated pigs with intact enterohepatic circulation (Control) and during depletion of the bile and pancreatic juice. In the second part, the same dose of digoxin was administered intraduodenally with intact enterohepatic circulation (Control) and during depletion of either bile or pancreatic juice or both. Biliary depletion decreased the flow of bile and pancreas juice as well as the amount of digoxin appearing in the bile. Deprivation of both bile and pancreas juice significantly increased the bioavailability of digoxin, the plasma AUC after enteral administration increased from 17.6+/-4.2 nmol/lh (Control) to 29.6+/-8.3 nmol/lh (P<0.05). The biliary clearance decreased significantly, from 0.22+/-0.11 l/h/kg (Control) to 0.04+/-0.03 l/h/kg during pancreatic and biliary depletion (P<0.05). There was a significant decrease in elimination half-life (P<0.05) and volume of distribution (P<0.01) during the depletion experiments while the systemic clearance remained unchanged. The results clearly suggest that biliary depletion trigger a short-term downregulation, most likely posttranscriptionally mediated, of a sinusoidal uptake transporter in the liver, possibly a pig ortholog of OATP.