NONLINEAR PHARMACOKINETICS OF PROPAFENONE IN RATS AND HUMANS: APPLICATION OF A SUBSTRATE DEPLETION ASSAY USING HEPATOCYTES FOR ASSESSMENT OF NONLINEARITY

Cardiac-specific overexpression of CREM-IbΔC-X via CRISPR/Cas9 in mice presents a new model of atrial cardiomyopathy with spontaneous atrial fibrillation

Atrial cardiomyopathy (ACM) forms the substrate for atrial fibrillation (AF) and underlies the potential for atrial thrombus formation and subsequent stroke. However, generating stable animal models that accurately replicate the entire progression of atrial lesions, particularly the onset of AF, presents significant challenges. In the present study, we found that the isoform of CRE-binding protein modulator (CREM-IbΔC-X), which is involved in the regulation of cardiac development and atrial rhythm, was highly expressed in atrial biopsies from patients with AF. Building upon this finding, we employed CRISPR/Cas9 technology to create a mouse model with cardiac-specific overexpression of CREM-IbΔC-X (referred to as CS-CREM mice). This animal model effectively illustrated the development of ACM through electrophysiological and structural remodelings over time. Proteomics and Chip-qPCR analysis of atrial samples revealed significant upregulation of cell-matrix adhesion and extracellular matrix structural components, alongside significant downregulation of genes related to atrial functions in the CS-CREM mice. Furthermore, the corresponding responses to anti-arrhythmia drugs, i.e., amiodarone and propafenone, suggested that CS-CREM mice could serve as an ideal in vivo model for drug testing. Our study introduced a novel ACM model with spontaneous AF by cardiac-specifically overexpressing CREM-IbΔC-X in mice, providing valuable insights into the mechanisms and therapeutic targets of ACM.

The Effect of CYP2D6 Phenotypes on the Pharmacokinetics of Propafenone: A Systematic Review and Meta-Analysis

Propafenone (PPF) is a class 1C antiarrhythmic agent mainly metabolized by cytochrome (CYP) 2D6, CYP1A2, and CYP3A4. Previous studies have shown that CYP2D6 polymorphism influences the pharmacokinetics (PK) of PPF. However, the small sample sizes of PK studies can lead to less precise estimates of the PK parameters. Thus, this meta-analysis was performed to merge all current PK studies of PPF to determine the effects of the CYP2D6 phenotype more accurately on the PPF PK profile. We searched electronic databases for published studies to investigate the association between the PPF PK and CYP2D6 phenotype. Four PK-related outcomes were included: area under the time–concentration curve (AUC), maximum concentration (Cmax), apparent clearance (CL/F), and half-life (t1/2). A total of five studies were included in this meta-analysis (n = 56). Analyses were performed to compare PK parameters between poor metabolizers (PMs) versus extensive metabolizers (EMs). PPF has a non-linear pharmacokinetics; therefore, analyses were performed according to dose (300 mg and 400 mg). At 300 mg, the AUC mean (95% CI), Cmax, and t1/2 of PPF in PMs were 15.9 (12.5–19.2) µg·h/mL, 1.10 (0.796–1.40) µg/mL, and 12.8 (11.3–14.3) h, respectively; these values were 2.4-, 11.2-, and 4.7-fold higher than those in the EM group, respectively. At 400 mg, a comparison was performed between S- and R-enantiomers. The CL/F was approximately 1.4-fold higher for the R-form compared with the S-form, which was a significant difference. This study demonstrated that CYP2D6 metabolizer status could significantly affect the PPF PK profile. Adjusting the dose of PPF according to CYP2D6 phenotype would help to avoid adverse effects and ensure treatment efficacy.

Effect of Rythmol (propafenone HCl) administration during pregnancy in Wistar rats

Propafenone is a well-known Class 1C antiarrhythmic agent that has sodium channel blocking properties as well as the ability to block 13 other channels and a modest calcium antagonistic effect. Propafenone has a profound electrophysiologic effect on auxiliary atrioventricular circuits and in patients with atrioventricular nodal reentry tachycardia can obstruct conduction in the fast conducting pathway. Furthermore, propafenone is less likely than other Class 1C drugs to cause proarrhythmia. However, although this medicine can pass through the placenta, the effects during pregnancy remain unknown. Here, we investigated the potential teratogenic and genotoxic effects of Rythmol during rat development. Pregnant Wistar rats received 46.25 mg/kg body weight of propafenone daily by gavage from Gestation Day (GD) 5 to GD 19. At GD 20, the dams were dissected, and their fetuses were assessed via morphologic, skeletal, and histologic investigation. In addition, a comet assay was used to measure DNA impairment of fetal skull osteocytes and hepatic cells. The study showed that propafenone treatment of pregnant rats led to a marked decrease in gravid uterine weight, number of implants/litter, number of viable fetuses, and bodyweight of fetuses but a clear increase in placental weight and placental index in the treated group. Frequent morphologic abnormalities and severe ossification deficiency in the cranium bones were observed in the treatment group. Various histopathological changes were observed in the liver, kidney, and brain tissues of maternally treated fetuses. Similarly, propafenone induced DNA damage to examined samples. Thus, our study indicates that propafenone may be embryotoxic in humans.

A public-private partnership to enrich the development of in silico predictive models for pharmacokinetic and cardiotoxic properties

A novel framework for a public-private (PP) partnership was established by a national initiative of the Development of a Drug Discovery Informatics System, supported by the Japan Agency for Medical Research and Development (AMED). This informatics PP partnership consortium comprised private and public sectors. A database of pharmacokinetic (PK) and cardiotoxic properties was developed, with considerable expansion after integrating proprietary data from private-sector members. This database led to robust in silico prediction models with higher performance than those from the original database. This partnership is a unique example worldwide and could substantially strengthen drug discovery capabilities in both sectors.

Regioselective hydroxylation of an antiarrhythmic drug, propafenone, mediated by rat liver cytochrome P450 2D2 differs from that catalyzed by human P450 2D6

1. Propafenone, an antiarrhythmic drug, is a typical human cytochrome P450 (P450) 2D6 substrate used in preclinical studies. Here, propafenone oxidation by mammalian liver microsomes was investigated in vitro. 2. Liver microsomes from humans and marmosets preferentially mediated propafenone 5-hydroxylation, minipig, rat and mouse livers primarily mediated 4'-hydroxylation, but cynomolgus monkey and dog liver microsomes differently mediated N-despropylation. 3. Quinine, ketoconazole or anti-P450 2D antibodies suppressed propafenone 4'/5-hydroxylation in human and rat liver microsomes. Pretreatments with β-naphthoflavone or dexamethasone increased N-despropylation in rat livers. 4. Recombinant rat P450 2D2 efficiently catalysed propafenone 4'-hydroxylation in a substrate inhibition manner, comparable to rat liver microsomes, while human P450 2D6 displayed propafenone 5-hydroxylation. Human and rat P450 1A, 2C and 3A enzymes mediated propafenone N-despropylation with high capacities. 5. Carbon-4' of propafenone docked favourably into the active site of P450 2D2 based on an in silico model; in contrast, carbon-5 of propafenone docked into human P450 2D6. 6. These results suggest that the major roles of individual P450 2D enzymes in regioselective hydroxylations of propafenone differ between human and rat livers, while the minor roles of P450 1A, 2C and 3A enzymes for propafenone N-despropylation are similar in livers of both species.

A physiologically based pharmacokinetic model to predict the pharmacokinetics of highly protein-bound drugs and the impact of errors in plasma protein binding

Predicting the pharmacokinetics of highly protein-bound drugs is difficult. Also, since historical plasma protein binding data were often collected using unbuffered plasma, the resulting inaccurate binding data could contribute to incorrect predictions. This study uses a generic physiologically based pharmacokinetic (PBPK) model to predict human plasma concentration-time profiles for 22 highly protein-bound drugs. Tissue distribution was estimated from in vitro drug lipophilicity data, plasma protein binding and the blood: plasma ratio. Clearance was predicted with a well-stirred liver model. Underestimated hepatic clearance for acidic and neutral compounds was corrected by an empirical scaling factor. Predicted values (pharmacokinetic parameters, plasma concentration-time profile) were compared with observed data to evaluate the model accuracy. Of the 22 drugs, less than a 2-fold error was obtained for the terminal elimination half-life (t1/2 , 100% of drugs), peak plasma concentration (Cmax , 100%), area under the plasma concentration-time curve (AUC0-t , 95.4%), clearance (CLh , 95.4%), mean residence time (MRT, 95.4%) and steady state volume (Vss , 90.9%). The impact of fup errors on CLh and Vss prediction was evaluated. Errors in fup resulted in proportional errors in clearance prediction for low-clearance compounds, and in Vss prediction for high-volume neutral drugs. For high-volume basic drugs, errors in fup did not propagate to errors in Vss prediction. This is due to the cancellation of errors in the calculations for tissue partitioning of basic drugs. Overall, plasma profiles were well simulated with the present PBPK model. Copyright © 2016 John Wiley & Sons, Ltd.

Polymorphism of human cytochrome P450 enzymes and its clinical impact

Pharmacogenetics is the study of how interindividual variations in the DNA sequence of specific genes affect drug response. This article highlights current pharmacogenetic knowledge on important human drug-metabolizing cytochrome P450s (CYPs) to understand the large interindividual variability in drug clearance and responses in clinical practice. The human CYP superfamily contains 57 functional genes and 58 pseudogenes, with members of the 1, 2, and 3 families playing an important role in the metabolism of therapeutic drugs, other xenobiotics, and some endogenous compounds. Polymorphisms in the CYP family may have had the most impact on the fate of therapeutic drugs. CYP2D6, 2C19, and 2C9 polymorphisms account for the most frequent variations in phase I metabolism of drugs, since almost 80% of drugs in use today are metabolized by these enzymes. Approximately 5-14% of Caucasians, 0-5% Africans, and 0-1% of Asians lack CYP2D6 activity, and these individuals are known as poor metabolizers. CYP2C9 is another clinically significant enzyme that demonstrates multiple genetic variants with a potentially functional impact on the efficacy and adverse effects of drugs that are mainly eliminated by this enzyme. Studies into the CYP2C9 polymorphism have highlighted the importance of the CYP2C9*2 and *3 alleles. Extensive polymorphism also occurs in other CYP genes, such as CYP1A1, 2A6, 2A13, 2C8, 3A4, and 3A5. Since several of these CYPs (e.g., CYP1A1 and 1A2) play a role in the bioactivation of many procarcinogens, polymorphisms of these enzymes may contribute to the variable susceptibility to carcinogenesis. The distribution of the common variant alleles of CYP genes varies among different ethnic populations. Pharmacogenetics has the potential to achieve optimal quality use of medicines, and to improve the efficacy and safety of both prospective and currently available drugs. Further studies are warranted to explore the gene-dose, gene-concentration, and gene-response relationships for these important drug-metabolizing CYPs.

Usefulness of hepatocytes for evaluating the genetic polymorphism of CYP2D6 substrates

The usefulness of human hepatocytes for assessing CYP2D6-related genetic polymorphisms was investigated. Propranolol and propafenone, which undergo phase I and II biotransformations, were used as model substrates alongside metoprolol, which is only metabolized via oxidative pathways. The contributions of CYP2D6 to the primary metabolisms of the substrates were estimated from the quinidine-mediated inhibition of their depletion rate constants in human hepatocytes and liver microsomes. The contributions in hepatocytes were 19.2% for propranolol at 0.05 microM and 36.7--76.3% for propafenone at 0.05--1.0 microM, and smaller than the contribution in microsomes, unlike the case for metoprolol. The differences between microsomes and hepatocytes were attributable to conjugate formation. The CYP2D6 contributions in hepatocytes reflected the in vivo data. The relevance of the concentration-dependent involvement of CYP2D6 in propafenone metabolism in hepatocytes to the in vivo polymorphic profile and the applicability of hepatocytes for evaluating these polymorphisms are discussed.

Prediction of metabolic clearance of diclofenac in adjuvant-induced arthritis rats using a substrate depletion assay

1. The purpose of this study was to evaluate drug clearance measured by the metabolic intrinsic clearance (CL(int)) in a substrate depletion assay in comparison with the in vivo clearance (CL(tot)) observed in adjuvant-induced arthritis (AA) rats. 2. After intravenous administration of diclofenac as a model drug, CL(tot) was 2.8-fold higher in AA rats than in control rats. In two different substrate depletion assays with liver microsomes for glucuronidation and hydroxylation, the CL(int) values for glucuronidation was significantly decreased in AA rats to 60% of the value in control rats, whereas the CL(int) values for hydroxylation were similar. The unbound fraction of diclofenac in plasma (f(u, plasma)) was significantly higher (2.8-fold) in AA rats than in control rats. 3. Hepatic clearance predicted from the CL(int) values for both biotransformation pathways and f(u, plasma) was higher in AA rats than in control rats, with good consistency between predicted and observed values. The same results were obtained for experiments using hepatocytes. 4. The plasma protein-binding activities, rather than metabolic clearance, in both types of rats would be a determining factor in the pharmacokinetic behaviour differences between control and AA rats. 5. In summary, substrate depletion assays with liver microsomes and hepatocytes in combination with protein binding assessment can help to predict changes in pharmacokinetics under AA conditions.

Determination of xenobiotic intrinsic clearance in freshly isolated hepatocytes from rainbow trout (Oncorhynchus mykiss) and rat and its application in bioaccumulation assessment

Bioaccumulation in fish depends on the dynamics of various processes that involve fish uptake, storage, and elimination of xenobiotics. Elimination via fish biotransformation is a primary process that can be evaluated in an in vitro system to improve the performance of the prediction of xenobiotic bioaccumulation potentials. In this study, values of intrinsic clearance (CLint) of seven reference compounds (atrazine, molinate, 4,4-bis(dimethylamino)-benzophenone, 4-nonylphenol, 2,4-di-tert-butylphenol, trifluralin, benzo(a)pyrene) in hepatocytes freshly isolated from rainbow trout and rat were determined using a substrate depletion approach. Atrazine was metabolized in rat hepatocytes with a CLint value of 3.81 +/- 1.96 mL/h/ 10(6) cells, whereas in trout hepatocytes, the clearance was not significant until very high cell concentration was used and the rate was estimated to be approximately 0.002 mL/h/10(6) cells. Intrinsic clearance values for all other compounds were 5.5-78.5-fold lower in trout hepatocytes than those in rat hepatocytes. Trout hepatic clearance (CL(H)) values were extrapolated from the CLint values using a "well-stirred" liver model. Biotransformation rate constants (kMET) of the compounds in trout were subsequently estimated and used as inputs to a kinetic model for the prediction of bioconcentration factors (BCF) in fish. Compared to the BCF values predicted without consideration of fish biotransformation, the inclusion of estimated kMET values significantly improved fish BCF predictions for the reference compounds. This study demonstrates a framework for future bioaccumulation assessment of xenobiotics using combined information of the physical-chemical properties of the compounds and the biotransformation potentials of the compounds in fish.

Mechanistic Approaches to Volume of Distribution Predictions: Understanding the Processes

PURPOSE

To use recently developed mechanistic equations to predict tissue-to-plasma water partition coefficients (Kpus), apply these predictions to whole body unbound volume of distribution at steady state (Vu(ss)) determinations, and explain the differences in the extent of drug distribution both within and across the various compound classes.

MATERIALS AND METHODS

Vu(ss) values were predicted for 92 structurally diverse compounds in rats and 140 in humans by two approaches. The first approach incorporated Kpu values predicted for 13 tissues whereas the second was restricted to muscle.

RESULTS

The prediction accuracy was good for both approaches in rats and humans, with 64-78% and 82-92% of the predicted Vu(ss) values agreeing with in vivo data to within factors of +/-2 and 3, respectively.

CONCLUSIONS

Generic distribution processes were identified as lipid partitioning and dissolution where the former is higher for lipophilic unionised drugs. In addition, electrostatic interactions with acidic phospholipids can predominate for ionised bases when affinities (reflected by binding to constituents within blood) are high. For acidic drugs albumin binding dominates when plasma protein binding is high. This ability to explain drug distribution and link it to physicochemical properties can help guide the compound selection process.

A Theoretical Validation of the Substrate Depletion Approach to Determining Kinetic Parameters

The substrate depletion approach is an increasingly popular alternative to the traditional method (observing product formation) of determining the kinetic parameters (KM and Vmax) of an enzyme. Obach and Reed-Hagen (Drug Metab Dispos 30:831-837, 2002) used an empirical relationship between substrate depletion rate constants and initial substrate concentration to determine kinetic parameters for a number of cytochrome P450-catalyzed reactions. We present a proof that this relationship can be derived from the Michaelis-Menten equation and, therefore, that kinetic parameters obtained by the substrate depletion approach are equivalent and comparable to those obtained by the traditional product formation approach. Analysis of a simulated data set produced similar kinetic parameters regardless of which approach was used, confirming the theoretical result.