Population pharmacokinetics of terfenadine

Prediction of hepatic drug clearance with a human microfluidic four-cell liver acinus microphysiology system

Predicting human hepatic clearance remains a fundamental challenge in both pharmaceutical drug development and toxicological assessments of environmental chemicals, with concerns about both accuracy and precision of in vitro-derived estimates. Suggested sources of these issues have included differences in experimental protocols, differences in cell sourcing, and use of a single cell type, liver parenchymal cells (hepatocytes). Here we investigate the ability of human microfluidic four-cell liver acinus microphysiology system (LAMPS) to make predictions as to hepatic clearance for seven representative compounds: Caffeine, Pioglitazone, Rosiglitazone, Terfenadine, Tolcapone, Troglitazone, and Trovafloxacin. The model, whose reproducibility was recently confirmed in an inter-lab comparison, was constructed using primary human hepatocytes or human induced pluripotent stem cell (iPSC)-derived hepatocytes and 3 human cell lines for the endothelial, Kupffer and stellate cells. We calculated hepatic clearance estimates derived from experiments using LAMPS or traditional 2D cultures and compared the outcomes with both in vivo human clinical study-derived and in vitro human hepatocyte suspension culture-derived values reported in the literature. We found that, compared to in vivo clinically-derived values, the LAMPS model with iPSC-derived hepatocytes had higher precision as compared to primary cells in suspension or 2D culture, but, consistent with previous studies in other microphysiological systems, tended to underestimate in vivo clearance. Overall, these results suggest that use of LAMPS and iPSC-derived hepatocytes together with an empirical scaling factor warrants additional study with a larger set of compounds, as it has the potential to provide more accurate and precise estimates of hepatic clearance.

MitoToxy assay: A novel cell-based method for the assessment of metabolic toxicity in a multiwell plate format using a lactate FRET nanosensor, Laconic

Mitochondrial toxicity is a primary source of pre-clinical drug attrition, black box warning and post-market drug withdrawal. Methods that detect mitochondrial toxicity as early as possible during the drug development process are required. Here we introduce a new method for detecting mitochondrial toxicity based on MDA-MB-231 cells stably expressing the genetically encoded FRET lactate indicator, Laconic. The method takes advantage of the high cytosolic lactate accumulation observed during mitochondrial stress, regardless of the specific toxicity mechanism, explained by compensatory glycolytic activation. Using a standard multi-well plate reader, dose-response curve experiments allowed the sensitivity of the methodology to detect metabolic toxicity induced by classical mitochondrial toxicants. Suitability for high-throughput screening applications was evaluated resulting in a Z’-factor > 0.5 and CV% < 20 inter-assay variability. A pilot screening allowed sensitive detection of commercial drugs that were previously withdrawn from the market due to liver/cardiac toxicity issues, such as camptothecin, ciglitazone, troglitazone, rosiglitazone, and terfenadine, in ten minutes. We envisage that the availability of this technology, based on a fluorescent genetically encoded indicator, will allow direct assessment of mitochondrial metabolism, and will make the early detection of mitochondrial toxicity in the drug development process possible, saving time and resources.

Second-Generation Histamine H1 Receptor Antagonists Suppress Delayed Rectifier K+-Channel Currents in Murine Thymocytes

BACKGROUND/AIMS

Voltage-dependent potassium channels (Kv1.3) are predominantly expressed in lymphocyte plasma membranes. These channels are critical for the activation and proliferation of lymphocytes. Since second-generation antihistamines are lipophilic and exert immunomodulatory effects, they are thought to affect the lymphocyte Kv1.3-channel currents.

METHODS

Using the patch-clamp whole-cell recording technique in murine thymocytes, we tested the effects of second-generation antihistamines, such as cetirizine, fexofenadine, azelastine, and terfenadine, on the channel currents and the membrane capacitance.

RESULTS

These drugs suppressed the peak and the pulse-end currents of the channels, although the effects of azelastine and terfenadine on the peak currents were more marked than those of cetirizine and fexofenadine. Both azelastine and terfenadine significantly lowered the membrane capacitance. Since these drugs did not affect the process of endocytosis in lymphocytes, they were thought to have interacted directly with the plasma membranes.

CONCLUSIONS

Our study revealed for the first time that second-generation antihistamines, including cetirizine, fexofenadine, azelastine, and terfenadine, exert suppressive effects on lymphocyte Kv1.3-channels. The efficacy of these drugs may be related to their immunomodulatory mechanisms that reduce the synthesis of inflammatory cytokine.

Virtual Clinical Trial Toward Polytherapy Safety Assessment: Combination of Physiologically Based Pharmacokinetic/Pharmacodynamic-Based Modeling and Simulation Approach With Drug-Drug Interactions Involving Terfenadine as an Example

A Quantitative Systems Pharmacology approach was utilized to predict the cardiac consequences of drug-drug interaction (DDI) at the population level. The Simcyp in vitro-in vivo correlation and physiologically based pharmacokinetic platform was used to predict the pharmacokinetic profile of terfenadine following co-administration of the drug. Electrophysiological effects were simulated using the Cardiac Safety Simulator. The modulation of ion channel activity was dependent on the inhibitory potential of drugs on the main cardiac ion channels and a simulated free heart tissue concentration. ten Tusscher's human ventricular cardiomyocyte model was used to simulate the pseudo-ECG traces and further predict the pharmacodynamic consequences of DDI. Consistent with clinical observations, predicted plasma concentration profiles of terfenadine show considerable intra-subject variability with recorded C_max values below 5 ng/mL for most virtual subjects. The pharmacokinetic and pharmacodynamic effects of inhibitors were predicted with reasonable accuracy. In all cases, a combination of the physiologically based pharmacokinetic and physiology-based pharmacodynamic models was able to differentiate between the terfenadine alone and terfenadine + inhibitor scenario. The range of QT prolongation was comparable in the clinical and virtual studies. The results indicate that mechanistic in vitro-in vivo correlation can be applied to predict the clinical effects of DDI even without comprehensive knowledge on all mechanisms contributing to the interaction.

Cytochrome P450 Enzyme Metabolites in Lead Discovery and Development

The cytochrome P450 (CYP) enzymes are a versatile superfamily of heme-containing monooxygenases, perhaps best known for their role in the oxidation of xenobiotic compounds. However, due to their unique oxidative chemistry, CYPs are also important in natural product drug discovery and in the generation of active metabolites with unique therapeutic properties. New tools for the analysis and production of CYP metabolites, including microscale analytical technologies and combinatorial biosynthesis, are providing medicinal chemists with the opportunity to use CYPs as a novel platform for lead discovery and development. In this review, we will highlight some of the recent examples of drug leads identified from CYP metabolites and the exciting possibilities of using CYPs as catalysts for future drug discovery.

Pharmacologically active drug metabolites: impact on drug discovery and pharmacotherapy

Metabolism represents the most prevalent mechanism for drug clearance. Many drugs are converted to metabolites that can retain the intrinsic affinity of the parent drug for the pharmacological target. Drug metabolism redox reactions such as heteroatom dealkylations, hydroxylations, heteroatom oxygenations, reductions, and dehydrogenations can yield active metabolites, and in rare cases even conjugation reactions can yield an active metabolite. To understand the contribution of an active metabolite to efficacy relative to the contribution of the parent drug, the target affinity, functional activity, plasma protein binding, membrane permeability, and pharmacokinetics of the active metabolite and parent drug must be known. Underlying pharmacokinetic principles and clearance concepts are used to describe the dispositional behavior of metabolites in vivo. A method to rapidly identify active metabolites in drug research is described. Finally, over 100 examples of drugs with active metabolites are discussed with regard to the importance of the metabolite(s) in efficacy and safety.

Long-term bradycardia caused by atrioventricular block can remodel the canine heart to detect the histamine H1 blocker terfenadine-induced torsades de pointes arrhythmias

Although a second-generation histamine H(1) blocker terfenadine induced torsades de pointes (TdP) arrhythmias in patients via the blockade of a rapid component of delayed rectifier K(+) current (I(Kr)), such action of terfenadine has not been detected in previous animal models. We analysed the potential of the canine persistent atrioventricular block heart, a new in vivo proarrhythmia model, to detect a torsadogenic effect of terfenadine of an oral dose of 3 or 30 mg kg(-1). The doses can provide therapeutic to supra-therapeutic plasma concentrations as an anti-histamine. In 2 weeks of bradycardiac heart model, there were no significant changes in any of the electrocardiogram parameters after the administration of both doses of terfenadine. In 4-6 weeks of bradycardiac heart model, the low dose of terfenadine hardly affected any of the electrocardiogram parameters except that it induced TdP in one out of six animals. The high dose significantly decreased the atrial rate and ventricular rate, prolonged the QT interval, and induced TdP in five out of six animals. Moreover, temporal variability of repolarization increased after the high-dose administration. These results suggest that long-term bradycardia caused by atrioventricular block can remodel the canine heart to detect terfenadine-induced TdP.

Factors affecting the clinical development of cytochrome p450 3A substrates

The objective of this review is to evaluate the risks associated with the discovery and development of cytochrome p450 (CYP) 3A substrates. CYP3A is the most abundant p450 enzyme in human liver and is highly expressed in the intestinal tract. The enzyme contributes substantially to metabolism of approximately 50% of currently marketed drugs that undergo oxidative metabolism. As a result, drug-drug interactions involving inhibitors of CYP3A-mediated metabolism can be of great clinical consequence. It is the position of the authors that, because of the factors responsible for the broad substrate specificity of CYP3A, discovery and development of compounds across a large and broad portfolio that are completely devoid of CYP3A metabolism is not feasible. Thus, it is important that scientifically valid approaches to the discovery and development of compounds metabolised by CYP3A be realised. The clinical relevance of CYP3A metabolism is dependent on a multitude of factors that include the degree of intestinal and hepatic CYP3A-mediated first-pass extraction, the therapeutic index of the compound and the adverse event associated with inhibition of CYP3A metabolism. Thus, a better understanding of the disposition of a CYP3A-metabolised compound relative to the projected or observed therapeutic index (or safety margin) can provide ample evidence to support the continued development of a CYP3A substrate. This document will highlight current practices as well as the benefits and risks associated with those practices.

Genetic contribution to variable human CYP3A-mediated metabolism

The human CYP3A subfamily plays a dominant role in the metabolic elimination of more drugs than any other biotransformation enzyme. CYP3A enzyme is localized in the liver and small intestine and thus contributes to first-pass and systemic metabolism. CYP3A expression varies as much as 40-fold in liver and small intestine donor tissues. CYP3A-dependent in vivo drug clearance appears to be unimodally distributed which suggests multi-genic or complex gene-environment causes of variability. Interindividual differences in enzyme expression may be due to several factors including: variable homeostatic control mechanisms, disease states that alter homeostasis, up- or down-regulation by environmental stimuli (such as smoking, drug intake, or diet), and genetic mutations. This review summarizes the current understanding and implications of genetic variation in the CYP3A enzymes. Unlike other human P450s (CYP2D6, CYP2C19) there is no evidence of a 'null' allele for CYP3A4. More than 30 SNPs (single nucleotide polymorphisms) have been identified in the CYP3A4 gene. Generally, variants in the coding regions of CYP3A4 occur at allele frequencies <5% and appear as heterozygous with the wild-type allele. These coding variants may contribute to but are not likely to be the major cause of inter-individual differences in CYP3A-dependent clearance, because of the low allele frequencies and limited alterations in enzyme expression or catalytic function. The most common variant, CYP3A4*1B, is an A-392G transition in the 5'-flanking region with an allele frequency ranging from 0% (Chinese and Japanese) to 45% (African-Americans). Studies have not linked CYP3A4*1B with alterations in CYP3A substrate metabolism. In contrast, there are several reports about its association with various disease states including prostate cancer, secondary leukemias, and early puberty. Linkage disequilibrium between CYP3A4*1B and another CYP3A allele (CYP3A5*1) may be the true cause of the clinical phenotype. CYP3A5 is polymorphically expressed in adults with readily detectable expression in about 10-20% in Caucasians, 33% in Japanese and 55% in African-Americans. The primary causal mutation for its polymorphic expression (CYP3A5*3) confers low CYP3A5 protein expression as a result of improper mRNA splicing and reduced translation of a functional protein. The CYP3A5*3 allele frequency varies from approximately 50% in African-Americans to 90% in Caucasians. Functionally, microsomes from a CYP3A5*3/*3 liver contain very low CYP3A5 protein and display on average reduced catalytic activity towards midazolam. Additional intronic or exonic mutations (CYP3A5*5, *6, and *7) may alter splicing and result in premature stop codons or exon deletion. Several CYP3A5 coding variants have been described, but occur at relatively low allelic frequencies and their functional significance has not been established. As CYP3A5 is the primary extrahepatic CYP3A isoform, its polymorphic expression may be implicated in disease risk and the metabolism of endogenous steroids or xenobiotics in these tissues (e.g., lung, kidney, prostate, breast, leukocytes). CYP3A7 is considered to be the major fetal liver CYP3A enzyme. Although hepatic CYP3A7 expression appears to be significantly down-regulated after birth, protein and mRNA have been detected in adults. Recently, increased CYP3A7 mRNA expression has been associated with the replacement of a 60-bp segment of the CYP3A7 promoter with a homologous segment in the CYP3A4 promoter (CYP3A7*1C allele). This mutational swap confers increased gene transcription due to an enhanced interaction between activated PXR:RXRalpha complex and its cognate response element (ER-6). The genetic basis for polymorphic expression of CYP3A5 and CYP3A7 has now been established. Moreover, the substrate specificity and product regioselectivity of these isoforms can differ from that of CYP3A4, such that the impact of CYP3A5 and CYP3A7 polymorphic expression on drug disposition will be drug dependent. In addition to genetic variation, other factors that may also affect CYher factors that may also affect CYP3A expression include: tissue-specific splicing (as reported for prostate CYP3A5), variable control of gene transcription by endogenous molecules (circulating hormones) and exogenous molecules (diet or environment), and genetic variations in proteins that may regulate constitutive and inducible CYP3A expression (nuclear hormone receptors). Thus, the complex regulatory pathways, environmentally susceptible milieu of the CYP3A enzymes, and as yet undetermined genetic haplotypes, may confound evaluation of the effect of individual CYP3A genetic variations on drug disposition, efficacy and safety.

Cooperativity of alpha-naphthoflavone in cytochrome P450 3A-dependent drug oxidation activities in hepatic and intestinal microsomes from mouse and human

1. The effects of several CYP3A substrates (alpha-naphthoflavone (alphaNF), terfenadine, midazolam, erythromycin) on nifedipine oxidation and testosterone 6beta-hydroxylation activities were investigated in hepatic and intestinal microsomes from mouse and human. 2. alphaNF (10 microM) and terfenadine (100 microM) inhibited nifedipine oxidation activities (at substrate concentration of 100 microM) in mouse hepatic microsomes to approximately 50%, but not in mouse intestinal microsomes. alphaNF (30 microM) stimulated nifedipine oxidation activities in mouse and human intestinal microsomes and in human hepatic microsomes to approximately 1.3-1.8-fold. Inhibitory potencies (50% inhibition concentration, IC50) of midazolam and erythromycin for nifedipine oxidations were calculated to be approximately 90 microM in human intestinal microsomes. In contrast, testosterone (100 microM) stimulated the nifedipine oxidation activities approximately 1.5-fold in hepatic and intestinal microsomes from mouse and human. 3. alphaNF showed different effects on the kinetic parameters including the Hill coefficients of nifedipine oxidation and testosterone 6beta-hydroxylation catalysed by hepatic and intestinal microsomes from mouse and human. Cooperativity in nifedipine oxidation was increased by the addition of alphaNF to pooled human hepatic microsomes, but little effects of alphaNF could be observed in individual human intestinal microsomes. 4. These results suggest that CYP3A enzymes in liver and intestine might have different characteristics and that observations from hepatic microsomes should not be directly applicable to intestine metabolism in some cases. Studies of drug-drug interactions of CYP3A substrates are recommended to be performed using intestinal samples.

Analysis of hydroxylated and N-dealkylated metabolites of terfenadine in microsomal incubates by liquid chromatography--mass spectrometry

This report describes an assay for the H(1)-receptor antagonist, terfenadine, and its two primary metabolites, terfenadine alcohol (TOH) and azacyclonol (AZ), using positive-ion, electrospray ionization-liquid chromatography-mass spectrometry. The assay was developed in support of kinetic studies of terfenadine oxidative metabolism in human liver and intestinal microsomes, which required quantification of incubate metabolites at low nanomolar concentrations. Terfenadine metabolites were extracted from basified microsomal incubates into methylene chloride. Reconstituted extracts were subject to liquid chromatographic separation on a cyano-reverse phase column. The [M+H]+ ions of terfenadine, terfenadine metabolites, and internal standard were monitored in the effluent by quadrupole mass spectrometry. The assay demonstrated linearity over an incubate concentration range of 5-250 and 12.5-1250 ng/ml for the metabolites and the parent drug, respectively. The respective limits of detection and quantitation for all three analytes were 1.5 and 5 ng/ml of microsomal incubate. Replicate analysis of quality control samples exhibited intra-day coefficients of variation ranging from 3.3% to 7.8% for the three analytes. The corresponding inter-day coefficients of variation ranged from 4.2% to 8.6%. The reproducibility and sensitivity of the assay, combined with the selectivity of mass spectrometric detection, should allow an accurate kinetic characterization of terfenadine oxidation mediated by the high affinity CYP3A enzymes in human liver and intestinal microsomes.

Pharmacokinetic-pharmacodynamic analysis of the arrhythmogenic potency of a novel antiallergic agent, ebastine, in rats

Ebastine (EBS), a novel nonsedative antiallergic agent, is similar to terfenadine in its chemical structure. However, clinical arrhythmogenicity of EBS remains controversial. In this study, we evaluated the possible arrhythmogenic potency of EBS as assessed by QT prolongation from a pharmacokinetic-pharmacodynamic viewpoint in comparison with that of terfenadine. EBS was intravenously infused into anesthetized rats at a rate of 3.0 or 10 mg/kg/h for 60 min, and electrocardiographic effects were continuously monitored from lead II. The plasma concentrations of EBS and its major metabolite, carebastine, were also measured under the same conditions. When intravenously administered, EBS exhibited QT prolongation in an infusion rate-dependent manner, with a lag time. Pharmacokinetic-pharmacodynamic analysis of EBS based on the effect-compartment model revealed values of EC50, Emax and EC(10 ms), (where 10 ms of QT prolongation was evoked) of 0.73 microg/mL, 12.5 ms and 2.90 microg/mL, respectively. The EC(10 ms) value of EBS was five times higher than that of terfenadine reported previously (Ohtani et al., J. Pharm. Pharmacol., 49, 458-462 (1997)). In conclusion, EBS was suggested to be less arrhythmogenic than terfenadine.

Atorvastatin does not produce a clinically significant effect on the pharmacokinetics of terfenadine

The effect of atorvastatin, a CYP3A4 substrate, on the pharmacokinetics of terfenadine and its carboxylic acid metabolite, fexofenadine, were evaluated. Single 120-mg doses of terfenadine were given 2 weeks apart to healthy volunteers with 80-mg daily doses of atorvastatin administered from 7 days before through 2 days after the second terfenadine dose. Concentrations of terfenadine and fexofenadine were measured for 72 hours after each terfenadine dose. Administration of terfenadine alone or in combination with atorvastatin produced no alterations in the QTc interval. For terfenadine, atorvastatin coadministration produced an 8% decrease in maximum concentration (Cmax), a 35% increase in area under the concentration-time curve extrapolated to infinity (AUC0-infinity), and a 2% decrease in elimination half-life (t1/2). For fexofenadine, atorvastatin coadministration produced a 16% decrease in Cmax, a 2% decrease in AUC0-infinity and a 51 % increase in t1/2. None of these changes achieved statistical significance. Coadministration of atorvastatin with terfenadine does not result in a clinically significant drug interaction. Because 80 mg is the highest atorvastatin dose used clinically, drug interactions mediated by CYP3A4 inhibition are unlikely in clinical practice.