Development of anin vivorat screen model to predict pharmacokinetic interactions of CYP3A4 substrates

Influence of methylxanthines isolated from Bancha green tea on the pharmacokinetics of sildenafil in rats

BACKGROUND

Sildenafil is used to treat erectile dysfunction and pulmonary arterial hypertension and is metabolized in the liver mainly by CYP3A4, thus co-administration with drugs or herbal extracts that affect CYP3A4 activity may lead to drug-drug or drug-herb interactions, respectively. The aim of the present study was to evaluate the influence of single and multiple oral doses of methylxanthine fraction, isolated from Bancha green tea leaves on the pharmacokinetics of sildenafil in rats.

METHODS

Rats were given sildenafil alone as well as simultaneously with methylxanthines or ketoconazole. The plasma concentrations of sildenafil were measured with high-performance liquid chromatography method with ultraviolet detection. The pharmacokinetic parameters of sildenafil were calculated by non-compartmental analysis.

RESULTS

Concomitant use of sildenafil with a single oral dose of methylxanthines resulted in a decrease in C_max (p > 0.05), AUC_0-t (p < 0.05) and AUC_0-inf (p < 0.05), while the administration of sildenafil after methylxanthines pretreatment resulted in an increase in C_max (p < 0.0001), AUC_0-t (p < 0.0001) and AUC_0-inf (p < 0.001) compared to the sildenafil group. After co-administration of sildenafil and ketoconazole, a significant increase in C_max, AUC_0-t and AUC_0-inf was observed in both of the experiments.

CONCLUSION

Drug-herb interactions were observed when sildenafil was co-administered with Bancha methylxanthines in rats. Further in vivo studies about the potential drug interactions between sildenafil and methylxanthines, especially caffeine, are needed to clarify mechanisms underlying the observed changes in sildenafil pharmacokinetics.

Validation of a HPLC method for quantification of midazolam in rat plasma: Application during a Maytenus ilicifolia-drug interaction study

Midazolam (MDZ) is routinely employed as a marker compound of cytochrome P450 3A (CYP3A) activity. Despite the many HPLC-UV methods described to quantify MDZ in plasma, all of them use acetonitrile (ACN) or a mixture of methanol-isopropanol as organic solvent of the mobile phase. Since the ACN shortage in 2008, efforts have been made to replace this solvent during HPLC analysis. A simple, sensitive, accurate and repeatable HPLC-UV method (220 nm) was developed and validated to quantify MDZ in rat plasma using methanol instead. The method was applied during a herb-drug interaction study involving Maytenus ilicifolia, a Brazilian folk medicine used to treat gastric disorders. Plasma samples were alkalinized and MDZ plus alprazolam (internal standard) were extracted with diethyl ether. After solvent removal, the residue was reconstituted with methanol-water (1:1). The analyte was eluted throughout a C₁₈ column using sodium acetate buffer (10 mm, pH 7.4)-methanol (40:60, v/v). The precision at the lower limit of quantification never exceeded 19.40%, and 13.86% at the higher levels of quality control standards, whereas the accuracy ranged from -19.81 to 14.33%. The analytical curve was linear from 50 to 2,000 ng/ml. The activity of the hepatic CYP3A enzymes was not affected by the extract.

Humanized UGT2 and CYP3A transchromosomic rats for improved prediction of human drug metabolism

Genomically humanized animals overcoming species differences are invaluable for biomedical research. Although rats would be preferred over mice for several applications, generation of a humanized model is restricted to mice due to the difficulty of complex genetic manipulations in rats. In this study, we successfully generated humanized rats with megabase-sized gene clusters via combination of chromosome transfer using mouse artificial chromosome vector and genome editing technologies. In the humanized UGT2 and CYP3A transchromosomic rats described in this paper, the expression of the human genes, as well as the pharmacokinetics and metabolism of relevant probe substrates, accurately mimic the situation in humans. Thus, the advanced technologies can be used to generate fully humanized rats useful for biomedical research.

Although “genomically” humanized animals are invaluable tools for generating human disease models as well as for biomedical research, their development has been mainly restricted to mice via established transgenic-based and embryonic stem cell-based technologies. Since rats are widely used for studying human disease and for drug efficacy and toxicity testing, humanized rat models would be preferred over mice for several applications. However, the development of sophisticated humanized rat models has been hampered by the difficulty of complex genetic manipulations in rats. Additionally, several genes and gene clusters, which are megabase range in size, were difficult to introduce into rats with conventional technologies. As a proof of concept, we herein report the generation of genomically humanized rats expressing key human drug-metabolizing enzymes in the absence of their orthologous rat counterparts via the combination of chromosome transfer using mouse artificial chromosome (MAC) and genome editing technologies. About 1.5 Mb and 700 kb of the entire UDP glucuronosyltransferase family 2 and cytochrome P450 family 3 subfamily A genomic regions, respectively, were successfully introduced via the MACs into rats. The transchromosomic rats were combined with rats carrying deletions of the endogenous orthologous genes, achieved by genome editing. In the “transchromosomic humanized” rat strains, the gene expression, pharmacokinetics, and metabolism observed in humans were well reproduced. Thus, the combination of chromosome transfer and genome editing technologies can be used to generate fully humanized rats for improved prediction of the pharmacokinetics and drug–drug interactions in humans, and for basic research, drug discovery, and development.

Screening Strategies and Methods for Better Off-Target Liability Prediction and Identification of Small-Molecule Pharmaceuticals

Pharmaceutical discovery and development is a long and expensive process that, unfortunately, still results in a low success rate, with drug safety continuing to be a major impedance. Improved safety screening strategies and methods are needed to more effectively fill this critical gap. Recent advances in informatics are now making it possible to manage bigger data sets and integrate multiple sources of screening data in a manner that can potentially improve the selection of higher-quality drug candidates. Integrated screening paradigms have become the norm in Pharma, both in discovery screening and in the identification of off-target toxicity mechanisms during later-stage development. Furthermore, advances in computational methods are making in silico screens more relevant and suggest that they may represent a feasible option for augmenting the current screening paradigm. This paper outlines several fundamental methods of the current drug screening processes across Pharma and emerging techniques/technologies that promise to improve molecule selection. In addition, the authors discuss integrated screening strategies and provide examples of advanced screening paradigms.

Neuroprotective Effect of Coumarin Nasal Formulation: Kindling Model Assessment of Epilepsy

Epilepsy is a brain disorder characterized by sudden recurrent seizures. Considering the fact that epileptogenesis is a process that affects the quality of life, our goal is to delay the process of epileptogenesis and to increase the latency of epileptic attacks, offering better quality of life to patients. Traditional system of medicines has a promise in some of the medicines, which have been used for the treatment of epilepsy. One such medicinal plant is Eclipta alba (EA). According to Ayurvedic philosophy, the juice of leaves of EA is pounded with garlic and pepper for the treatment of epilepsy. Taking clue from the Ayurvedic system of medicines, we formulated coumarin fraction of EA, namely, coumarin nasal formulation (CNF) for its nasal delivery. CNF was analyzed by using high performance liquid chromatography (HPLC) and ultraviolet absorption spectroscopy for its drug content determination. In vitro drug release studies were performed in simulated nasal electrolyte solution (SNES) maintaining constant pH of 5.5 at 37°C. Irritation by CNF was evaluated using hen's egg test chorioallantoic membrane (HET-CAM) assay. Formulation was found to be non-irritant in HET-CAM assay. CNF was further assessed in vivo by measuring the progress and attainment of pentylenetetrazole (PTZ) kindling in mice. Neuronal changes were assessed by hematoxylin and eosin (H&E) and Nissl staining technique. Glial fibrillary acidic protein (GFAP) a neuroinflammatory marker and tumor necrosis factor alpha (TNF-α) an inflammatory marker were also measured. CNF (10 mg/kg, nasal route) when given as a pretreatment lowered seizure score and delayed the progression of seizure similar to diazepam. CNF decreased the PTZ induced oxidative damage, TNF-α as well as GFAP levels in the midbrain tissue particularly in hippocampus region. The results suggest that CNF may be a promising therapeutic approach to offer protection from sudden recurrent seizures alone or in combination with current drugs in management of epilepsy.

Novel pre-clinical methodologies for pharmacokinetic drug-drug interaction studies: spotlight on "humanized" animal models

Poly-therapy is common due to co-occurrence of several ailments in patients, leading to the elevated possibility of drug-drug interactions (DDI). Pharmacokinetic DDI often accounts for severe adverse drug reactions in patients resulting in withdrawal of drug from the market. Hence, the prediction of DDI is necessary at pre-clinical stage of drug development. Several human tissue and cell line-based in vitro systems are routinely used for screening metabolic and transporter pathways of investigational drugs and for predicting their clinical DDI potentials. However, ample constraints are associated with the in vitro systems and sometimes in vitro-in vivo extrapolation (IVIVE) fail to assess the risk of DDI in clinic. In vitro-in vivo correlation model in animals combined with human in vitro studies may be helpful in better prediction of clinical outcome. Native animal models vary remarkably from humans in drug metabolizing enzymes and transporters, hence, the interpretation of results from animal DDI studies is difficult. With the advent of modern molecular biology and engineering tools, novel pre-clinical animal models, namely, knockout rat/mouse, transgenic rat/mouse with humanized drug metabolizing enzymes and/or transporters and chimeric rat/mouse with humanized liver are developed. These models nearly simulate human-like drug metabolism and help to validate the in vivo relevance of the in vitro human DDI data. This review briefly discusses the application of such novel pre-clinical models for screening various type of DDI along with their advantages and limitations.

Assessment of CYP3A-mediated drug-drug interaction potential for victim drugs using an in vivo rat model

The present study aims to determine if an in vivo rat model of drug-drug interaction (DDI) could be useful to discriminate a sensitive (buspirone) from a 'non-sensitive' (verapamil) CYP3A substrate, using ketoconazole and ritonavir as perpetrator drugs. Prior to in vivo studies, ketoconazole and ritonavir were shown to inhibit midazolam hydroxylation with IC50 values of 350 ± 60 nm and 11 ± 3 nm, respectively, in rat liver microsomes (RLM). Buspirone and verapamil were also shown to be substrates of recombinant rat CYP3A1/3A2. In the rat model, the mean plasma AUC0-inf of buspirone (10 mg/kg, p.o.) was increased by 7.4-fold and 12.8-fold after co-administration with ketoconazole and ritonavir (20 mg/kg, p.o.), respectively. The mean plasma AUC0-inf of verapamil (10 mg/kg, p.o.) was increased by 3.0-fold and 4.8-fold after co-administration with ketoconazole and ritonavir (20 mg/kg, p.o.), respectively. Thus, the rat DDI model correctly identified buspirone as a sensitive CYP3A substrate (>5-fold AUC change) in contrast to verapamil. In addition, for both victim drugs, the extent of DDI when co-administered was greater with ritonavir compared with ketoconazole, in line with their in vitro CYP3A inhibition potency in RLM. In conclusion, our study extended the rat DDI model applicability to two additional victim/perpetrator pairs. In addition, we suggest that use of this model would increase our confidence in estimation of the DDI potential for victim drugs in early discovery.

Inhibitory Effects of Baicalin on the Expression and Activity of CYP3A Induce the Pharmacokinetic Changes of Midazolam in Rats

Baicalin, a flavonoid compound isolated from Scutellaria baicalensis, has been shown to possess antiinflammatory, antiviral, antitumour, and immune regulatory properties. The present study evaluated the potential herb-drug interaction between baicalin and midazolam in rats. Coadministration of a single dose of baicalin (0.225, 0.45, and 0.90 g/kg, i.v.) with midazolam (10 mg/kg, i.v.) in rats resulted in a dose-dependent decrease in clearance (CL) from 25%  (P < 0.05) to 34%  (P < 0.001) with an increase in AUC0-∞ from 47%  (P < 0.05) to 53%  (P < 0.01). Pretreatment of baicalin (0.90 g/kg, i.v., once daily for 7 days) also reduced midazolam CL by 43%  (P < 0.001), with an increase in AUC0-∞ by 87%  (P < 0.01). Multiple doses of baicalin decreased the expression of hepatic CYP3A2 by approximately 58%  (P < 0.01) and reduced midazolam 1'-hydroxylation by 23%  (P < 0.001) and 4'-hydroxylation by 21%  (P < 0.01) in the liver. In addition, baicalin competitively inhibited midazolam metabolism in rat liver microsomes in a concentration-dependent manner. Our data demonstrated that baicalin induced changes in the pharmacokinetics of midazolam in rats, which might be due to its inhibition of the hydroxylation activity and expression of CYP3A in the liver.

Probe ADME and test hypotheses: a PATH beyond clearance in vitro-in vivo correlations in early drug discovery

INTRODUCTION

In vitro cytochrome P450 (CYP450) metabolic profiling is pursued extensively to optimize drug properties. Still, the in vivo clearance of half of all new chemical entities (NCEs) remains poorly predicted by CYP450 metabolism, based on Novartis rat pharmacokinetic data. The conventional route to illuminating key drivers of in vivo clearance beyond hepatic metabolism is, frequently, the process of elimination, a time-consuming and sometimes resource-intensive practice. A more nimble and efficient diagnosis of drug clearance is imperative to support today's chemistry optimization.

AREAS COVERED

This article reviews in vitro-in vivo clearance correlation (IVIVC) analysis of drugs and NCEs including in silico advances, in vitro opportunities for clearance characterization and guidance for proper interpretation of clearance data. Potential mechanisms for under- and overestimation of in vivo clearance obtained from in vitro approaches are reviewed. The article offers insight into a practical PATH (Probe ADME and Test Hypotheses) for discovery data analysis that can enrich IVIVC development and guide more efficient use of the ADME-PK toolbox.

EXPERT OPINION

In vitro hepatic CYP450 stability measurements remain the most practical way to triage for high metabolic liabilities. Clearance is a complex process involving multiple mechanisms and many factors tend to be overlooked in routine correlation analyses. Equilibrium protein binding, intrinsic permeability and ionization may yield insight into distribution-limited clearance. In addition, hydrophobic character and transporter interaction can be valuable in diagnosing dominant clearance pathways. An integrated ADME approach to clearance interrogation is expected to help refine the in vitro-in silico strategies that guide medicinal chemistry.

Impact of nonlinear midazolam pharmacokinetics on the magnitude of the midazolam-ketoconazole interaction in rats

Numerous groups have described the rat as an in vivo model for the assessment and prediction of drug-drug interactions (DDIs) in humans involving the inhibition of cytochrome P450 3A forms. Even for a well-established substrate-inhibitor pair like midazolam-ketoconazole, however, the magnitude of the DDI in rats (e.g. 1.5- to 5-fold) does not relate to what is observed clinically (e.g. 5- to 16-fold). Because nonlinear substrate pharmacokinetics (PK) may result in a weaker interaction, it was hypothesized that the lower magnitude of interaction observed in rats was due to the saturation of metabolic pathway(s) of midazolam at the doses used (10-20 mg/kg). Therefore, the inhibitory effects of ketoconazole were reevaluated at lower oral (1 and 5 mg/kg) and intravenous (IV) (1 mg/kg) doses of midazolam. In support of the hypothesis, oral exposure at 5 mg/kg dose of midazolam was 18-fold higher compared to that at 1 mg/kg. Furthermore, when the interaction was investigated at the lower midazolam dose (1 mg/kg), ketoconazole increased the IV and oral exposure of midazolam by 7-fold and 11-fold, respectively. A weaker DDI (1.5- to 1.8-fold) was observed at the higher oral midazolam dose. Collectively, these results suggest that the lower reported interaction in rats is likely due to saturation of midazolam clearance at the doses used. Therefore, when the rat is used as a DDI model to screen and differentiate compounds, or predict CYP3A inhibition in humans, it is important to use low doses of midazolam and ensure linear PK.

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.

Oxidative demethylenation and subsequent glucuronidation are the major metabolic pathways of berberine in rats

Present study was designed to explore roles of metabolic clearance in the disposition of berberine (BBR) in rats, with a focus on oxidative metabolism and subsequent glucuronidation. Plasma from rats after intravenous administration of BBR was collected to identify and quantify BBR and its major metabolites. The major circulating metabolites of BBR were oxidative metabolites M1 (via demethylation) and M2 (via demethylenation) and their corresponding glucuronides, with M2-glucuronide approximately 24-fold higher than M1-glucuronide. Incubations with rat liver microsomes were conducted to examine formation kinetics of two oxidative metabolites-M1 and M2, and depletion kinetics of M1 and M2, leading to the formation of glucuronide conjugates. Efforts were also made to examine roles of key CYPs and UGTs isoforms responsible for BBR metabolism using known chemical inhibitors and/or substrates. In vitro, the formation of M1 and M2 were comparable and multiple CYP enzymes were involved. In contrast, the glucuronidation of M2 was much faster than that of M1. Inhibition studies using well-characterized UGT substrates suggested both M1 and M2 could be glucuronidated by UGT1A1 and UGT2B1 while M2 glucuronidation was favored by UGT1A1. In summary, oxidative demethylenation and the subsequent glucuronidation were the major metabolic pathways of BBR in rats.

Examination of CYP3A and P-glycoprotein-mediated drug-drug interactions using animal models

With the advent of polytherapy for cancer treatment it has become prudent to minimize, as much as possible, the potential for drug-drug interactions (DDI). Toward this end, the metabolic and transporter pathways involved in the disposition of a drug candidate (phenotyping) and potential for inhibition and induction of drug-metabolizing enzymes and transporters are evaluated in vitro. Such in vitro human data can be made available prior to human dosing and enable in vitro to in vivo-based predictions of clinical outcomes. Despite some success, however, in vitro systems are not dynamic and sometimes fail to predict drug-drug interactions for a variety of reasons. In comparison, relatively less effort has been made to evaluate predictions based on data derived from in vivo animal models. This chapter will attempt to summarize different examples from the literature where animal models have been used to predict cytochrome P450 3A (CYP3A)- and P-glycoprotein-based DDI. When employing data from animal models one needs to be aware of species differences in enzyme- and transporter-activity leading to differences in pharmacokinetics, clearance pathways as well as species differences in selectivity and affinity of probe substrates and inhibitors. Because of these differences, in vivo animal studies alone, cannot be predictive of human DDI. Despite these caveats, the information obtained from validated in vivo animal models may prove useful when used in conjunction with in vitro-in vivo extrapolation methods. Such an integrated data set can be used to select drug candidates with a reduced DDI potential.

In vitro evaluation of reversible and irreversible cytochrome P450 inhibition: current status on methodologies and their utility for predicting drug-drug interactions

It is widely accepted that today's practice of polypharmacy inevitably increases the incidence of drug-drug interactions (DDIs). Serious DDI is a major liability for any new chemical entity (NCE) entering the pharmaceutical market. As such, pharmaceutical companies employ various strategies to avoid problematic compounds for clinical development. A key cause for DDIs is the inhibition of cytochrome P450 enzymes (CYPs) that are responsible for metabolic clearance of many drugs. Screening for inhibition potency of CYPs by NCEs has therefore become a routine practice during the drug discovery stage. However, in order to make proper use of DDI data, an understanding of the strengths and weaknesses of the various experimental systems in current use is required. An illustrated review of experimental practices is presented with discussion of likely future developments. The combination of high quality in vitro data generation and the application of in vivo CYP inhibition modelling approaches should allow more informed decisions to be made in the search for drug molecules with acceptable DDI characteristics.