Why is it Challenging to Predict Intestinal Drug Absorption and Oral Bioavailability in Human Using Rat Model

PhRMA CPCDC initiative on predictive models of human pharmacokinetics, part 4: prediction of plasma concentration-time profiles in human from in vivo preclinical data by using the Wajima approach

The objective of this study was to evaluate the performance of the Wajima allometry (Css -MRT) approach published in the literature, which is used to predict the human plasma concentration-time profiles from a scaling of preclinical species data. A diverse and blinded dataset of 108 compounds from PhRMA member companies was used in this evaluation. The human intravenous (i.v.) and oral (p.o.) pharmacokinetics (PK) data were available for 18 and 107 drugs, respectively. Three different scenarios were adopted for prediction of human PK profiles. In the first scenario, human clearance (CL) and steady-state volume of distribution (Vss ) were predicted by unbound fraction corrected intercept method (FCIM) and Øie-Tozer (OT) approaches, respectively. Quantitative structure activity relationship (QSAR)-based approaches (TSrat-dog ) based on compound descriptors together with rat and dog data were utilized in the second scenario. Finally, in the third scenario, CL and Vss were predicted using the FCIM and Jansson approaches, respectively. For the prediction of oral pharmacokinetics, the human bioavailability and absorption rate constant were assumed as the average of preclinical species. Various statistical techniques were used for assessing the accuracy of the simulation scenarios. The human CL and Vss were predicted within a threefold error range for about 75% of the i.v. drugs. However, the accuracy in predicting key p.o. PK parameters appeared to be lower with only 58% of simulations falling within threefold of observed parameters. The overall ability of the Css -MRT approach to predict the curve shape of the profile was in general poor and ranged between low to medium level of confidence for most of the predictions based on the selected criteria.

Effects of lovastatin on the pharmacokinetics of diltiazem and its main metabolite, desacetyldiltiazem, in rats: possible role of cytochrome P450 3A4 and P-glycoprotein inhibition by lovastatin

OBJECTIVES

The purpose of this study was to examine the effects of lovastatin on cytochrome P450 (CYP) 3A4 and P-glycoprotein (P-gp) in vitro and then to determine the effects of lovastatin on the pharmacokinetics of diltiazem and its main metabolite, desacetyldiltiazem, in rats.

METHODS

The pharmacokinetic parameters of diltiazem and desacetyldiltiazem were determined after orally administering diltiazem (12 mg/kg) to rats in the presence and absence of lovastatin (0.3 and 1.0 mg/kg). The effect of lovastatin on P-gp as well as CYP3A4 activity was also evaluated.

KEY FINDINGS

Lovastatin inhibited CYP3A4 enzyme activity with a 50% inhibition concentration of 6.06 µM. In addition, lovastatin significantly enhanced the cellular accumulation of rhodamine-123 in MCF-7/ADR cells overexpressing P-gp. Compared with the control (given diltiazem alone), the presence of lovastatin significantly altered the pharmacokinetic parameters of diltiazem. The areas under the plasma concentration-time curve (AUC) and the peak concentration of diltiazem were significantly increased (P < 0.05, 1.0 mg/kg) in the presence of lovastatin. Consequently, the absolute bioavailability values of diltiazem in the presence of lovastatin (11.1% at 1.0 mg/kg) were significantly higher (P < 0.05) than that of the control group (7.6%). The metabolite-parent AUC ratio in the presence of lovastatin (1.0 mg/kg) was significantly (P < 0.05) decreased compared with the control group.

CONCLUSIONS

It might be considered that lovastatin resulted in reducing the first-pass metabolism in the intestine and/or in the liver via inhibition of CYP3A4 and increasing the absorption of diltiazem in the intestine via inhibition of P-gp by lovastatin.

Effects of myricetin, an anticancer compound, on the bioavailability and pharmacokinetics of tamoxifen and its main metabolite, 4-hydroxytamoxifen, in rats

This study examined the effect of myricetin, an anticancer compound, on the bioavailability and pharmacokinetics of tamoxifen and its metabolite, 4-hydroxytamoxifen, in rats. The effect of myricetin on P-glycoprotein (P-gp), cytochrome P450 (CYP)3A4 and 2C9 activity was evaluated. Myricetin inhibited CYP3A4 and 2C9 activity with IC(50) values of 7.81 and 13.5 μM, respectively, and significantly inhibited P-gp activity in a concentration-dependent manner. Pharmacokinetic parameters of tamoxifen and 4-hydroxytamoxifen were determined in rats after oral (10 mg/kg) and intravenous (2 mg/kg) administration of tamoxifen in the presence and absence of myricetin (0.4, 2, and 8 mg/kg). Compared with the oral control group (given tamoxifen alone), the area under the plasma concentration-time curve (AUC(0-∞)) and the peak plasma concentration (C (max)) of tamoxifen were significantly (P < 0.05, 2 mg/kg; P < 0.01, 8 mg/kg) increased by 41.8-74.4 and 48.4-81.7%, respectively. Consequently, the absolute bioavailability (AB) of tamoxifen with myricetin (2 and 8 mg/kg) was 29.0-35.7%, which was significantly enhanced (P < 0.05 for 2 mg/kg, P < 0.01 for 8 mg/kg) compared with the oral control group (20.4%). Moreover, the relative bioavailability (RB) of tamoxifen was 1.14- to 1.74-fold greater than that of the control group. The metabolite-parent AUC ratio (MR) was significantly reduced (P < 0.05, 8 mg/kg), implying that the formation of 4-hydroxytamoxifen was considerably affected by myricetin. The enhanced bioavailability of tamoxifen might be mainly due to inhibition of the CYP3A4- and CYP2C9-mediated metabolism of tamoxifen in the small intestine and/or in the liver, and inhibition of P-gp efflux pump in the small intestine by myricetin.

[Bioavailability of oral drug formulations and methods for its improvement]

The recent studies in nanotechnology resulted in the development of novel formulations with improved bioavailability. This is especially important for oral administered drugs as the most convenient formulations. The current review deals with the processes occurring at the gastro-intestinal (GI) tract and their influence on the drug form. The increase of bioavailability of the drug may be achieved through designing novel formulations according to the specific drug properties. They include capsules that release pharmaceutical agents at various parts of the GI tract, floating systems that prolong the presence of the drug in the GI tract, dispersed forms with surface-active soluble polymers, micelles that carry poor-soluble drugs inside their non-polar core, agents that facilitate tight junction opening, such as caprate and chitosan, and lipid-based formulations. The own data show the stimulating influence of phospholipid nanoparticles on peroral absorption of drug indomethacin in rats and on passage of transport marker and drugs through Caco-2 cell monolayer in vitro. The review summarizes current understanding of factors that influence the bioavailability of the oral drug forms, currently used models for pharmacokinetic studies, and various approaches to developing novel pharmaceutical forms that increase the bioavailability of the drugs.

Synergistic influence of Abcb1 and Abcc2 on disposition and sterol lowering effects of ezetimibe in rats

Pharmacokinetics of the sterol-lowering drug ezetimibe (EZ) is influenced by intestinal ABCB1 and ABCC2. This study in Lew.1W rats with "chemical" and genetic Abcb1 and Abcc2 deficiency was initiated to evaluate the individual contribution of both efflux carriers to the overall disposition and sterol-lowering effects of EZ. Disposition and sterol-lowering effects of EZ (5 mg/kg, 14 days) were measured in wild-type (WT) and Abcc2-deficient (Abcc2-) rats (N = 8 per group) and in animals treated with PSC833 (20 mg/kg) to generate "chemical" Abcb1-deficiency (Abcb1-, Abcb1-/Abcc2-). EZ serum levels decreased in the order WT (3.11 +/- 1.09 ng/mL), Abcb1- (1.94 +/- 1.10 ng/mL), Abcc2- (1.42 +/- 0.42 ng/mL, p = 0.003 vs. WT), Abcb1-/Abcc2- (1.17 +/- 0.53 ng/mL, p = 0.002 vs. WT) whereas the serum EZ glucuronide levels increased as follows: WT (23.2 +/- 24.6 ng/mL), Abcb1- (119 +/- 74.5 ng/mL, p = 0.002 vs. WT), Abcc2- (195+/-76.5 ng/mL, p < 0.001 vs. WT), Abcb1-/Abcc2- (676 +/- 207 ng/mL, p < 0.001 vs. WT, Abcb1- and Abcc2-). Abcb1 and Abcc2 protein deficiency resulted synergistically in lower fecal but increased renal excretion of total EZ although to a much lower extent. The sterol-lowering effects of EZ were significantly correlated to serum levels of EZ. In conclusion, Abcb1 and Abcc2 deficiency leads to lower levels of the active EZ and in turn to decreased sterol-lowering effects.

Coexistence of passive and carrier-mediated processes in drug transport

The permeability of biological membranes is one of the most important determinants of the pharmacokinetic processes of a drug. Although it is often accepted that many drug substances are transported across biological membranes by passive transcellular diffusion, a recent hypothesis speculated that carrier-mediated mechanisms might account for the majority of membrane drug transport processes in biological systems. Based on evidence of the physicochemical characteristics and of in vitro and in vivo findings for marketed drugs, as well as results from real-life discovery and development projects, we present the view that both passive transcellular processes and carrier-mediated processes coexist and contribute to drug transport activities across biological membranes.

Prevention strategies for antimicrobial resistance: a systematic review of the literature

Antibiotics offer great benefits by reducing the duration and severity of illnesses and aiding in infection transmission control. With this being said, the inexorable process of antimicrobial drug resistance is to some degree unavoidable. Although drug resistance will likely persist and is to be expected, the overall level can be dramatically decreased with increased attention to antibiotic overuse and the pharmacokinetic and pharmacodynamic properties of different drug formulations, and the use of proper hygiene and protective barriers. Implementation of such practices as microbial surveillance and prophylaxis has been shown to result in decreased hospital length of stay, health care costs and mortality due to drug-resistant infections. This review will summarize current progress in preventative techniques aimed at reducing the incidence of infection by antimicrobial-resistant bacteria and the emergence and spread of antimicrobial-resistant strains. By employing a variety of prevention strategies, including proper personal hygiene, prescreening for carrier status before hospital admission, disinfection of hospital rooms, and careful monitoring of antimicrobial prescribing, marked progress can be achieved in the control of drug-resistant pathogens, which can translate into more effective antimicrobial therapy.

Cross-species comparisons of the pharmacokinetics of ibudilast

To enable clinical development of ibudilast for new indications, its pharmacokinetics were characterized in mice, rats, rabbits, dogs, cynomolgus monkeys, and minipigs. Animal pharmacokinetics were compared with a separate study in healthy volunteers. Following oral dosing, the dose-normalized area under the curve (AUC) (DN-AUC(24h)) in humans is 896 ((ng*h ml(-1))/(mg kg(-1))), and in animals ranges from 0.3 to 87. The variability among species cannot be explained by intrinsic clearance, which in intravenous dosing experiments shows only moderate interspecies variation (13-41 l h(-1) m(-2)). A portal vein rat pharmacokinetics model suggested that differences in first-pass gut clearance may explain some of the interspecies variation in oral bioavailability. Ibudilast shows auto-induction of metabolism in some animals, but not in humans. Plasma protein binding in humans and some animals is greater than or equal to 95%. The primary metabolite 6,7-dihyrdodiol-ibudilast is measurable and has been quantitated in plasma from animals and humans. Finally, biodistribution studies show that ibudilast distributes rapidly, extensively, and reversibly to the central nervous system.

Use of in vivo animal models to assess pharmacokinetic drug-drug interactions

Animal models are used commonly in various stages of drug discovery and development to aid in the prospective assessment of drug-drug interaction (DDI) potential and the understanding of the underlying mechanism for DDI of a drug candidate. In vivo assessments in an appropriate animal model can be very valuable, when used in combination with in vitro systems, to help verify in vivo relevance of the in vitro animal-based results, and thus substantiate the extrapolation of in vitro human data to clinical outcomes. From a pharmacokinetic standpoint, a key consideration for rational selection of an animal model is based on broad similarities to humans in important physiological and biochemical parameters governing drug absorption, distribution, metabolism or excretion (ADME) processes in question for both the perpetrator and victim drugs. Equally critical are specific in vitro and/or in vivo experiments to demonstrate those similarities, usually both qualitative and quantitative, in the ADME properties/processes under investigation. In this review, theoretical basis and specific examples are presented to illustrate the utility of the animal models in assessing the potential and understanding the mechanisms of DDIs.

Functional cell models of the gut and their applications in food microbiology--a review

Animal experimentation has a long tradition for risk assessment of new drugs before they reach the clinic. To reduce expensive animal experimentation, attempts have been made to build inexpensive and convenient intestinal functional cell models to study toxicity and bioavailability of new substances along with providing relevant models to study interactions between the host, pathogens and intestinal microflora. We review the available cell lines and models of the intestine and their potential uses. Tumor derived cell lines such as Caco-2, T84 and HT-29 are widely used despite many drawbacks, which are discussed with respect to complexity of the gut, where various cell types interact with commensal microbiota and gut-associated lymphoid tissue. To address this complexity, 3D models of human and animal gut represent a promising in vitro system to mimic in vivo situation without the use of transformed cell lines.

Bioavailability of lycopene in the rat: the role of intestinal lymphatic transport

Objectives

As a natural antioxidant derived from dietary sources, lycopene has attracted considerable attention as a potent chemopreventative agent. Lycopene is an extremely lipophilic compound and absorption from dietary sources is estimated to be low and highly variable. As a result, plasma lycopene concentrations are poorly correlated with dietary intake of lycopene rich food stuffs. The development of an oral formulation remains a challenge that requires a better understanding of the mechanisms involved in the intestinal absorption of this compound.

Methods

The solubility of lycopene in simulated physiological fluids and bile salt mixed micelle formulations was determined. The extent of intestinal lymphatic transport and the absolute bioavailability of lycopene from a range of biorelevant media was evaluated in a mesenteric lymph duct cannulated anaesthetised rat model.

Results

The absolute bioavailability of lycopene after 8 h was 1.85 ± 0.39%. The overall extent of the intestinal lymphatic transport was in the range of 0.6–3.4% of the administered dose. A strong positive correlation (r2 &gt; 0.9) between intestinal lycopene levels and intestinal triglyceride levels was demonstrated.

Conclusions

The intestinal lymphatic route is the major uptake mechanism of lycopene from the gastrointestinal tract. Lycopene transport in intestinal lymph was closely associated with triglyceride transport in the lymph. Formulation strategies designed to promote intestinal lymphatic uptake, such as lipid-based formulations containing long-chain fatty acids (LCFA) or lecithin, may serve to enhance oral bioavailability of lycopene.

Prediction of human pharmacokinetics—gut-wall metabolism

Intestinal mucosal cells operate with different metabolic and transport activity, and not all of them are involved in drug absorption and metabolism. The fraction of these cells involved is dependent on the absorption characteristics of compounds and is difficult to predict (it is probably small). The cells also appear comparably impermeable. This shows a limited applicability of microsome intrinsic clearance (CLint)-data for prediction of gut-wall metabolism, and the difficulty to predict the gut-wall CL (CLGW) and extraction ratio (EGW). The objectives of this review were to evaluate determinants and methods for prediction of first-pass and systemic EGW and CLGW in man, and if required and possible, develop new simple prediction methodology. Animal gut-wall metabolism data do not appear reliable for scaling to man. In general, the systemic CLGW is low compared with the hepatic CL. For a moderately extracted CYP3A4-substrate with high permeability, midazolam, the gut-wall/hepatic CL-ratio is only 1/35. This suggests (as a general rule) that systemic CLGW can be neglected when predicting the total CL. First-pass EGW could be of importance, especially for substrates of CYP3A4 and conjugating enzymes. For several reasons, including those presented above and that blood flow based models are not applicable in the absorptive direction, it seems poorly predicted with available methodology. Prediction errors are large (several-fold on average; maximum-15-fold). A new simple first-pass EGW-prediction method that compensates for regional and local differences in absorption and metabolic activity has been developed. It has been based on human cell in-vitro CLint and fractional absorption from the small intestine for reference (including verapamil) and test substances, and in-vivo first-pass EGW-data for reference substances. First-pass EGW-values for CYP3A4-substrates with various degrees of gastrointestinal uptake and CLint and a CYP2D6-substrate were well-predicted (negligible errors). More high quality in-vitro CLint- and in-vivo EGW-data are required for further validation of the method.

Prediction of human pharmacokinetics—evaluation of methods for prediction of hepatic metabolic clearance

Methods for prediction of hepatic clearance (CLH) in man have been evaluated. A physiologically-based in-vitro to in-vivo (PB-IVIV) method with human unbound fraction in blood (fu,bl) and hepatocyte intrinsic clearance (CLint)-data has a good rationale and appears to give the best predictions (maximum ∼2-fold errors; &lt; 25% errors for half of CL-predictions; appropriate ranking). Inclusion of an empirical scaling factor is, however, needed, and reasons include the use of cryopreserved hepatocytes with low activity, and inappropriate CLint- and fu,bl-estimation methods. Thus, an improvement of this methodology is possible and required. Neglect of fu,bl or incorporation of incubation binding does not seem appropriate. When microsome CLint-data are used with this approach, the CLH is underpredicted by 5- to 9-fold on average, and a 106-fold underprediction (attrition potential) has been observed. The poor performance could probably be related to permeation, binding and low metabolic activity. Inclusion of scaling factors and neglect of fu,bl for basic and neutral compounds improve microsome predictions. The performance is, however, still not satisfactory. Allometry incorrectly assumes that the determinants for CLH relate to body weight and overpredicts human liver blood flow rate. Consequently, allometric methods have poor predictability. Simple allometry has an average overprediction potential, &gt; 2-fold errors for ∼1/3 of predictions, and 140-fold underprediction to 5800-fold overprediction (potential safety risk) range. In-silico methodologies are available, but these need further development. Acceptable prediction errors for compounds with low and high CLH should be ∼50 and ∼10%, respectively. In conclusion, it is recommended that PB-IVIV with human hepatocyte CLint and fu,bl is applied and improved, limits for acceptable errors are decreased, and that animal CLH-studies and allometry are avoided.

Fasting increases tobramycin oral absorption in mice

The pharmacokinetics of the aminoglycoside tobramycin was evaluated after oral administration to fed or fasting (15 h) mice. As expected, under normal feeding conditions, oral absorption was negligible; however, fasting induced a dramatic increase in tobramycin bioavailability. The dual-sugar test with lactulose and l-rhamnose confirmed increased small bowel permeability via the paracellular route in fasting animals. When experiments aimed at increasing the oral bioavailability of hydrophilic compounds are performed, timing of fasting should be extremely accurate.

Comparative gene expression of intestinal metabolizing enzymes

The purpose of this study was to compare the expression profiles of drug-metabolizing enzymes in the intestine of mouse, rat and human. Total RNA was isolated from the duodenum and the mRNA expression was measured using Affymetrix GeneChip oligonucleotide arrays. Detected genes from the intestine of mouse, rat and human were ca. 60% of 22690 sequences, 40% of 8739 and 47% of 12559, respectively. Total genes of metabolizing enzymes subjected in this study were 95, 33 and 68 genes in mouse, rat and human, respectively. Of phase I enzymes, the mouse exhibited abundant gene expressions for Cyp3a25, Cyp4v3, Cyp2d26, followed by Cyp2b20, Cyp2c65 and Cyp4f14, whereas, the rat showed higher expression profiles of Cyp3a9, Cyp2b19, Cyp4f1, Cyp17a1, Cyp2d18, Cyp27a1 and Cyp4f6. However, the highly expressed P450 enzymes were CYP3A4, CYP3A5, CYP4F3, CYP2C18, CYP2C9, CYP2D6, CYP3A7, CYP11B1 and CYP2B6 in the human. For phase II enzymes, glucuronosyltransferase Ugt1a6, glutathione S-transferases Gstp1, Gstm3 and Gsta2, sulfotransferase Sult1b1 and acyltransferase Dgat1 were highly expressed in the mouse. The rat revealed predominant expression of glucuronosyltransferases Ugt1a1 and Ugt1a7, sulfotransferase Sult1b1, acetyltransferase Dlat and acyltransferase Dgat1. On the other hand, in human, glucuronosyltransferases UGT2B15 and UGT2B17, glutathione S-transferases MGST3, GSTP1, GSTA2 and GSTM4, sulfotransferases ST1A3 and SULT1A2, acetyltransferases SAT1 and CRAT, and acyltransferase AGPAT2 were dominantly detected. Therefore, current data indicated substantial interspecies differences in the pattern of intestinal gene expression both for P450 enzymes and phase II drug-metabolizing enzymes. This genomic database is expected to improve our understanding of interspecies variations in estimating intestinal prehepatic clearance of oral drugs.

Impact of transporters in oral absorption: a case study of in vitro and in vivo organic anion absorption

A key determinant for oral bioavailability of a drug candidate is the intestinal epithelial permeation of the drug candidate. This intestinal permeation may be affected by interactions on membrane transporters expressed in the intestinal epithelial cells. The purpose of the present study was to investigate whether transporters were involved in the intestinal absorption of an organic anion A275 and to compare the impact of interactions related to transporters in the Caco-2 cell model versus the in vivo rat model of intestinal absorption. In both models, it was investigated whether intestinal permeation of A275 was concentration dependent and affected by inhibitors or competitive organic anions. Interactions related to transporters in intestinal permeation was clearly demonstrated in the Caco-2 cell model but was not directly evident for in vivo rat absorption. However, an observed biphasic in vivo absorption and a large intervariability between rats might mask a dose-dependent absorption of A275. To avoid these suggested interactions, a dose of at least 10 mg/kg, which saturates the intestinal transporters involved in A275 absorption, should be administered, but at doses below that the risk of such drug interactions should be taken into account.

IVIVC in oral absorption for fenofibrate immediate release tablets using a dissolution/permeation system

The usefulness of a dissolution/permeation (D/P) system to predict the in vivo performance of solid dosage forms containing the poorly soluble drug, fenofibrate, was studied. Biorelevant dissolution media simulating the fasted and fed state conditions of the human gastrointestinal tract were used in order to simulate the effect of food on the absorption of fenofibrate. Moreover, the results obtained from the D/P system were correlated with pharmacokinetic parameters obtained following in vivo studies in rats. The in vitro parameter (amount permeated in the D/P system) reflected well the in vivo performance in rats in terms of AUC and C(max) of fenofibric acid. This study thus demonstrates the potential of the D/P system as valuable tool for absorption screening of dosage forms for poorly soluble drugs.

The role of transporters in the pharmacokinetics of orally administered drugs

Drug transporters are recognized as key players in the processes of drug absorption, distribution, metabolism, and elimination. The localization of uptake and efflux transporters in organs responsible for drug biotransformation and excretion gives transporter proteins a unique gatekeeper function in controlling drug access to metabolizing enzymes and excretory pathways. This review seeks to discuss the influence intestinal and hepatic drug transporters have on pharmacokinetic parameters, including bioavailability, exposure, clearance, volume of distribution, and half-life, for orally dosed drugs. This review also describes in detail the Biopharmaceutics Drug Disposition Classification System (BDDCS) and explains how many of the effects drug transporters exert on oral drug pharmacokinetic parameters can be predicted by this classification scheme.

Mechanistic approaches to predicting oral drug absorption

Modeling and simulation of oral drug absorption have been widely used in drug discovery, development, and regulation. Predictive absorption models are used to determine the rate and extent of oral drug absorption, facilitate lead drug candidate selection, establish formulation development strategy, and support the development of regulatory policies. This review highlights the development of recent drug absorption models including dispersion and compartmental models. The compartmental models include the compartmental absorption and transit model; Grass model; gastrointestinal transit absorption model; advanced compartmental absorption and transit model; and advanced dissolution, absorption, and metabolism model. Compared to the early absorption models, the above models developed or extended since the mid-1990s have demonstrated greatly improved predictive performance by accounting for multiple factors such as drug degradation, gastric emptying, intestinal transit, first-pass metabolism, and intestinal transport. For future model development, more heterogeneous features of the gastrointestinal tract (villous blood flow, metabolizing enzymes, and transporters), food effects, and drug-drug interactions should be fully characterized and taken into consideration. Moreover, predicting population inter- and intravariability in oral drug absorption can be useful and important for the evaluation of clinical safety and efficacy of drugs. Establishing databases and libraries that contain accurate pharmaceutical and pharmacokinetic information for commercialized and uncommercialized drugs may also be helpful for model development and validation.

Prediction of human pharmacokinetics from preclinical information: comparative accuracy of quantitative prediction approaches

Quantitative prediction of human pharmacokinetics is critical in assessing the viability of drug candidates and in determining first-in-human dosing. Numerous prediction methodologies, incorporating both in vitro and preclinical in vivo data, have been developed in recent years, each with advantages and disadvantages. However, the lack of a comprehensive data set, both preclinical and clinical, has limited efforts to evaluate the optimal strategy (or strategies) that results in quantitative predictions of human pharmacokinetics. To address this issue, the authors conducted a retrospective analysis using 50 proprietary compounds for which in vitro, preclinical pharmacokinetic data and oral single-dose human pharmacokinetic data were available. Five predictive strategies, involving either allometry or use of unbound intrinsic clearance from microsomes or hepatocytes, were then compared for their ability to predict human oral clearance, half-life through predictions of systemic clearance, volume of distribution, and bioavailability. Use of a single-species scaling approach with rat, dog, or monkey was as accurate as or more accurate than using multiple-species allometry. For those compounds cleared almost exclusively by P450-mediated pathways, scaling from human liver microsomes was as predictive as single-species scaling of clearance based on data from rat, dog, or monkey. These data suggest that use of predictive methods involving either single-species in vivo data or in vitro human liver microsomes can quantitatively predict human in vivo pharmacokinetics and suggest the possibility of streamlining the predictive methodology through use of a single species or use only of human in vitro microsomal preparations.

Combination therapy of advanced invasive pulmonary aspergillosis in transiently neutropenic rats using human pharmacokinetic equivalent doses of voriconazole and anidulafungin

At present, voriconazole (VOR) is the drug of first choice for treating invasive pulmonary aspergillosis (IPA). However, particularly in advanced stages of disease and in the severely immunocompromised host, the mortality remains substantial. The combination of VOR with an echinocandin may improve the therapeutic outcome. We investigate here whether combining VOR and anidulafungin (ANI) in advanced IPA in transiently neutropenic rats results in a higher therapeutic efficacy. Since VOR is metabolized more rapidly in rodents than in humans, dosage adjustment for VOR is necessary to obtain an area under the plasma concentration-time curve (AUC) in rodents that is equivalent to that of humans. In this study, the pharmacokinetics of VOR and ANI in rats were elucidated, and dosage schedules were applied that produced AUCs similar to those of humans. The developed dose schedules were well tolerated by the rats, without effects on renal and hepatic functions. VOR showed excellent efficacy in early IPA (100% rat survival). In advanced IPA, VOR was less efficacious (50% rat survival), whereas a significant decrease in galactomannan concentrations in lungs and sera was found in surviving rats. ANI administered in advanced IPA resulted in 22% rat survival, and the serum concentrations of fungal galactomannan were slightly but not significantly decreased. The addition of ANI to VOR did not result in significantly increased therapeutic efficacy in advanced IPA, resulting in 67% rat survival and a significant decrease in galactomannan concentration in serum. In conclusion, VOR monotherapy is therapeutically effective in the treatment of advanced-stage IPA and superior to the use of ANI. Combining both agents does not significantly improve the therapeutic outcome.

Vascular perfused segments of human intestine as a tool for drug absorption

Blood-based vascular perfusion of isolated segments of human jejunum was developed as a tool for drug absorption studies before clinical trials. Acceptance criteria for viable human gut preparations included stable blood flow, arterial pressure, glucose utilization, active peristalsis, oxygen uptake, less than 3% absorption of a 70,000 mol. wt. dextran, and a ratio of first-order absorption rate constants (k(a)) of antipyrine to terbutaline of > or =1.4. Mannitol absorption was less than that of antipyrine but larger than that of terbutaline and could not be used as a negative control in absorption studies with human intestine. In separate perfusions (n = 3) a cassette of nine drugs was administered into the gut lumen, and the net absorption of each drug into the circulation was measured over 75 min. Using the mean values of k(a), the test compounds could be ranked into four groups: group 1: sulfasalazine and furosemide, k(a) = 3.9 to 4.0 x 10(-3) min(-1); group 2: cimetidine, timolol, nadolol, and ranitidine, k(a) = 6.4 to 8.3 x 10(-3) min(-1); group 3: atenolol and metoprolol, k(a) = 9.6 x 10(-3) min(-1); and group 4: theophylline, k(a) = 17.5 x 10(-3) min(-1). The rationale for evaluating yet another oral absorption system was as follows: first, a human gut segment with an intact vascular system is the closest system available to a clinical trial without performing one; and second, the data generated would be a direct measure of net drug transport from the gut lumen into the vascular circulation under near physiological conditions, which is not possible in models lacking a blood supply.

Potential role of intestinal first-pass metabolism in the prediction of drug-drug interactions

BACKGROUND

The contribution of intestine to the magnitude of drug-drug interactions (DDI) may be significant, considering high levels of inhibitors in the gut lumen achieved during absorption and the abundance of metabolic enzymes in the mature enterocytes. Intestinal inhibition is incorporated in the DDI prediction models as the ratio of the intestinal wall availability in the presence and absence of the inhibitor (F(G)(') and F(G), respectively).

OBJECTIVE

This review will focus on the ability of the current approaches to estimate the extent of intestinal DDI accurately, addressing predominantly the most abundant intestinal P450 enzyme, CYP3A4.

METHODS

Considering the sensitivity of the DDI prediction models to the accuracy of the F(G) estimates, the current study focuses on 3 different in vitro and in vivo approaches to assess this parameter.

RESULTS/CONCLUSION

The advantages and limitations of each of F(G) methods are outlined. Accurate assessment of this parameter is essential for the prediction of human drug clearance and drug-drug interaction potential.

Applications of physiologically based absorption models in drug discovery and development

This article describes the use of physiologically based models of intestinal drug absorption to guide the research and development of new drugs. Applications range from lead optimization in the drug discovery phase through clinical candidate selection and extrapolation to human to phase 2 formulation development. Early simulations in preclinical species integrate multiple screening data and add value by transforming these individual properties into a prediction of in vivo absorption. Comparison of simulations to plasma levels measured after oral dosing in animals highlights unexpected behavior, and parameter sensitivity analysis can explore the impact of uncertainties in key properties, point toward factors which are limiting absorption and contribute to assessment of compound developability. Physiological models provide reliable prediction of human absorption and with refinement based on phase 1 data are useful guides to further market formulation development. Improvements in the accuracy of simulations are expected as better in vitro methods generate more in vivo relevant solubility and permeability data, and modeling will play a central role in the development of more predictive methods for transporter-related effects on drug absorption.

Closing the gaps: a full scan of the intestinal expression of p-glycoprotein, breast cancer resistance protein, and multidrug resistance-associated protein 2 in male and female rats

Intestinal ATP binding cassette (ABC) transporters may affect the bioavailability and effectiveness of orally administered drugs. Available studies on regional expression of intestinal efflux transporters were done with selected intestinal segments only and inconsistent with regard to the variability of transporter expression and the course of expression along the intestine. For an evaluation of the consistency between mRNA and protein expression, relative expression levels of P-glycoprotein (Pgp; ABCB1), breast cancer resistance protein (Bcrp; ABCG2), and multidrug resistance-associated protein (Mrp) 2 (ABCC2) were determined using quantitative real-time-polymerase chain reaction and Western blot in rat intestinal segments from duodenum, jejunum, ileum, and colon. In addition, the protein expression of Pgp, Bcrp, and Mrp2 from the entire rat intestine was studied by a complete 3-cm segmentation to evaluate the predictive power of expression analyses from selected intestinal segments. Pgp showed an increase from proximal to distal regions, Bcrp showed an arcuate pattern with highest expression toward the end of small intestine, and Mrp2 decreased along the intestinal axis from proximal to distal parts. No gender specific differences could be observed. Regarding the concordance of mRNA and protein expression, Pgp and Bcrp mRNA samples allow good estimations about the corresponding protein expression (for Pgp limited to the mdr1a isoform), but for Mrp2, pronounced deviation could be observed. All transporters showed considerable intra- and interindividual variability, especially at the protein level, making it problematic to take transporter expressions of small sections exemplary for general assumptions on intestinal abundances.