Prediction of metabolic clearance using fresh human hepatocytes: Comparison with cryopreserved hepatocytes and hepatic microsomes for five benzodiazepines

'New/Designer Benzodiazepines': An Analysis of the Literature and Psychonauts' Trip Reports

BACKGROUND

NPS belonging to the benzodiazepine (BZD) class, e.g., 'legal/designer BZDs'/'research chemicals', have recently emerged in the drug (mainly online/virtual) market.

OBJECTIVE

While certain NPS belonging to the BZD class possess pharmacological profiles similar to controlled pharmaceutical BZDs, clinical and pharmacological profiles of current emerging BZDs are still not well-described. Therefore, there is a need to increase clinicians'/public health knowledge/awareness, to incentive harm reduction strategies.

METHOD

A comprehensive overview was carried out by using the EMCDDA/EDND database regularly monitored by our research team, by specifically looking at the 'new BZDs' so far notified. Furthermore, given the limitation of peer-reviewed data published so far, a nonparticipant multilingual qualitative netnographic study was conducted to obtain further clinical/pharmacological/ toxicological data, including psychonauts' online trip reports.

RESULTS

First designer BZDs appeared as NPS around 2007. So far, 29 designer BZDs have been notified to the EMCDDA, being some of them extremely powerful, also at lower dosages. They are sold as tablets/powder/pellets/capsules/blotters/liquids, at very affordable prices, and variably administered. Some are also sold on the illicit drugmarket as counterfeit forms of traditional BZDs or as either adulterants or diluents in heroin or other synthetic opioids/cannabinoids. Nowadays, there is no guarantee of the quality of designer BZDs composition/purification and, hence, most NPS consumers may be inadvertently exposed to unsafe and harmful compounds.

CONCLUSION

Given the limited information on their pharmacology/toxicity, variations in dosage, onset of effects, combination of substances, potency, and general patient or individual variability, the concomitant use of these substances with other drugs entails several and unpredictable risks.

Comprehensive Evaluation of Organotypic and Microphysiological Liver Models for Prediction of Drug-Induced Liver Injury

Drug-induced liver injury (DILI) is a major concern for the pharmaceutical industry and constitutes one of the most important reasons for the termination of promising drug development projects. Reliable prediction of DILI liability in preclinical stages is difficult, as current experimental model systems do not accurately reflect the molecular phenotype and functionality of the human liver. As a result, multiple drugs that passed preclinical safety evaluations failed due to liver toxicity in clinical trials or postmarketing stages in recent years. To improve the selection of molecules that are taken forward into the clinics, the development of more predictive in vitro systems that enable high-throughput screening of hepatotoxic liabilities and allow for investigative studies into DILI mechanisms has gained growing interest. Specifically, it became increasingly clear that the choice of cell types and culture method both constitute important parameters that affect the predictive power of test systems. In this review, we present current 3D culture paradigms for hepatotoxicity tests and critically evaluate their utility and performance for DILI prediction. In addition, we highlight possibilities of these emerging platforms for mechanistic evaluations of selected drug candidates and present current research directions towards the further improvement of preclinical liver safety tests. We conclude that organotypic and microphysiological liver systems have provided an important step towards more reliable DILI prediction. Furthermore, we expect that the increasing availability of comprehensive benchmarking studies will facilitate model dissemination that might eventually result in their regulatory acceptance.

Interlaboratory Variability in Human Hepatocyte Intrinsic Clearance Values and Trends with Physicochemical Properties

PURPOSE

To examine the interlaboratory variability in CL_int values generated with human hepatocytes and determine trends in variability and clearance prediction accuracy using physicochemical and pharmacokinetic parameters.

METHODS

Data for 50 compounds from 14 papers were compiled with physicochemical and pharmacokinetic parameter values taken from various sources.

RESULTS

Coefficients of variation were as high as 99.8% for individual compounds and variation was not dependent on the number of prediction values included in the analysis. When examining median values, it appeared that compounds with a lower number of rotatable bonds had more variability. When examining prediction uniformity, those compounds with uniform in vivo underpredictions had higher CL_{int, in vivo} values, while those with non-uniform predictions typically had lower CL_{int, in vivo} values. Of the compounds with uniform predictions, only a small number were uniformly predicted accurately. Based on this limited dataset, less lipophilic, lower intrinsic clearance, and lower protein binding compounds yield more accurate clearance predictions.

CONCLUSIONS

Caution should be taken when compiling in vitro CL_int values from different laboratories as variations in experimental procedures (such as extent of shaking during incubation) may yield different predictions for the same compound. The majority of compounds with uniform in vitro values had predictions that were inaccurate, emphasizing the need for a better mechanistic understanding of IVIVE. The non-uniform predictions, often with low turnover compounds, reaffirmed the experimental challenges for drugs in this clearance range. Separating new chemical entities by lipophilicity, intrinsic clearance, and protein binding may help instill more confidence in IVIVE predictions.

3D engineered In vitro hepatospheroids for studying drug toxicity and metabolism

Drug toxicity is one of the reasons for late stage drug attrition, because of hepatotoxicity. Various in vitro liver models like primary human hepatocytes, immortalized human hepatic cell lines, liver slices and microsomes have been used; but limited by viability, hepatic gene expression and function. The 3D-engineered construct of hepatocyte-like-cells (HLCs) differentiated from stem cells, may provide a limitless source of hepatocytes with improved reproducibility. Towards this end, we used hepatospheroids (diameter=50-80μm) differentiated from human-umbilical-cord-mesenchymal stem cells (hUC-MSCs) on 3D scaffold GEVAC (Gelatin-vinyl-acetate-copolymer) as in vitro model for studying drug metabolism/toxicity. Our data demonstrated that hUC-MSCs-derived-hepatospheroids cultured on GEVAC expressed significantly higher drug-metabolizing enzymes (CYPs) both at mRNA and activity level compared to 2D culture, using HR-LC/MS. We further showed that hepatospheroids convert phenacetin (by CYP1A2) and testosterone (by CYP3A4) to their human-specific metabolites acetaminophen and 6β-hydroxytestosterone with a predictive clearance rate of 0.011ml/h/10⁶ cells and 0.021ml/h/10⁶ cells respectively, according to first-order kinetics. Hepatotoxicity was confirmed by exposing hepatospheroids to ethanol and acetaminophen; ROS generation, cell viability, cytoskeleton structure, elevation of liver function enzymes, i.e. AST and ALT, was analyzed. To the best of our knowledge, this is the first report to use hUC-MSCs-derived-hepatospheroids on GEVAC as in vitro model for drug metabolism/toxicity study; which can replace the conventional 2D-models used in drug development.

Metabolites replace the parent drug in the drug arena. The cases of fonazepam and nifoxipam

Fonazepam (desmethylflunitrazepam) and nifoxipam (3-hydroxy-desmethylflunitrazepam) are benzodiazepine derivatives and active metabolites of flunitrazepam. They recently invaded the drug arena as substances of abuse and alerted the forensic community after being seized in powder and tablet forms in Europe between 2014 and 2016. A review of all the existing knowledge of fonazepam and nifoxipam is reported, concerning their chemistry, synthesis, pharmacology and toxicology, prevalence/use, biotransformation and their analysis in biological samples. To our knowledge, fonazepam and nifoxipam-related intoxications, lethal or not, have not been reported in the scientific literature. All the available information was gathered through a detailed search of PubMed and the World Wide Web.

Comparison of methods for the prediction of human clearance from hepatocyte intrinsic clearance for a set of reference compounds and an external evaluation set

1. We compared direct scaling, regression model equation and the so-called "Poulin et al." methods to scale clearance (CL) from in vitro intrinsic clearance (CL_int) measured in human hepatocytes using two sets of compounds. One reference set comprised of 20 compounds with known elimination pathways and one external evaluation set based on 17 compounds development in Merck (MS). 2. A 90% prospective confidence interval was calculated using the reference set. This interval was found relevant for the regression equation method. The three outliers identified were justified on the basis of their elimination mechanism. 3. The direct scaling method showed a systematic underestimation of clearance in both the reference and evaluation sets. The "Poulin et al." and the regression equation methods showed no obvious bias in either the reference or evaluation sets. 4. The regression model equation was slightly superior to the "Poulin et al." method in the reference set and showed a better absolute average fold error (AAFE) of value 1.3 compared to 1.6. A larger difference was observed in the evaluation set were the regression method and "Poulin et al." resulted in an AAFE of 1.7 and 2.6, respectively (removing the three compounds with known issues mentioned above). A similar pattern was observed for the correlation coefficient. Based on these data we suggest the regression equation method combined with a prospective confidence interval as the first choice for the extrapolation of human in vivo hepatic metabolic clearance from in vitro systems.

Rationalizing underprediction of drug clearance from enzyme and transporter kinetic data: from in vitro tools to mechanistic modeling

Over the years, there has been an increase in the number and quality of available in vitro tools for the assessment of clearance. Complexity of data analysis and modelling of corresponding in vitro data has increased in an analogous manner, in particular for the simultaneous characterization of transporter and metabolism kinetics, together with intracellular binding and passive diffusion. In the current chapter, the impact of different factors on the in vitro-in vivo extrapolation of clearance will be addressed in a stepwise manner, from the selection of the most adequate in vitro system and experimental design/condition to the corresponding modelling of data generated. The application of static or physiologically based pharmacokinetic models in the prediction of clearance will be discussed, highlighting limitations and current challenges of some of the approaches. Particular focus will be on the ability of in vitro and in silico predictive tools to overcome the trend of clearance underprediction. Improvements made as a result of inclusion of extrahepatic metabolism and consideration of transporter-metabolism interplay across different organs will be discussed.

Assessment of cytochrome P450 (1A2, 2B6, 2C9 and 3A4) induction in cryopreserved human hepatocytes cultured in 48-well plates using the cocktail strategy

1. A fast, straightforward and cost-effective assay was validated for the assessment of CYP induction in cryopreserved human hepatocytes cultured in 48-well plates. The cocktail strategy (in situ incubation) was used to assess the induction of CYP1A2, CYP2B6, CYP2C9 and CYP3A4 by using the recommended probe substrate, i.e. phenacetin, bupropion, diclofenac and midazolam, respectively. 2. Cryopreserved human hepatocytes were treated for 72 h with prototypical reference inducers, β-naphthoflavone (25 µM), phenobarbital (500 µM) and rifampicin (10 µM) as positive controls for CYP induction. The use of a cocktail strategy has been validated and compared to the classical approach (single incubation). The need of using phase II inhibitor (salicylamide) in CYP induction assay was also investigated. 3. By using three different batches of cryopreserved human hepatocytes and our conditions of incubations, we showed that there was no relevant drug-drug interaction using the cocktail strategy. The same conclusions were observed when a broad range of enzyme activity has to be assessed (wide range of reference inducers, i.e. EC50-Emax experiment). In addition, the interassay reproducibility assessment showed that the day-to-day variability was minimal. 4. In summary, the study showed that the conditions used (probe substrates, concentration of probe substrate and time of incubation) for the cocktail approach were appropriate for investigations of CYP induction potential of new chemical entities. In addition, it was also clear that the use of salicylamide in the incubation media was not mandatory and could generate drug-drug interactions. For this reason, we recommend to not use salicylamide in CYP induction assay.

A comprehensive evaluation of metabolic activity and intrinsic clearance in suspensions and monolayer cultures of cryopreserved primary human hepatocytes

Primary human hepatocytes are widely used for metabolic stability evaluations. However, there are limited data directly comparing phase I and phase II drug-metabolizing enzymes in fresh and cryopreserved hepatocytes prepared from the same human donor liver. We evaluated the metabolic competency of human hepatocytes prepared from seven donor tissues before and after cryopreservation. Temporal-dependent enzyme activity in suspension and matched adherent cultures of primary human hepatocytes was also assessed. Cryopreservation of hepatocytes resulted in statistically significant increases in activities of CYP1A2, CYP2B6, CYP2C9, CYP2D6, and CYP3A but not CYP2C8, CYP2C19, FMO, UGT, and SULT, relative to fresh hepatocytes. In suspension cultures of hepatocytes, enzyme stabilities were as follows: UGT<CYP3A<CYP1A2<CYP2D6<CYP2C9<SULT. CYP1A2 and CYP3A enzyme stability was significantly greater in plated cells relative to suspension with mean enzyme inactivation time values of 2.69 ± 0.39 and 1.62 ± 0.09 h in suspension and 21.3 ± 2.1 and 28.8 ± 20.4 h in culture, respectively. These data demonstrate that cryopreservation is not detrimental to primary human hepatocytes enzyme activities, indicate time-dependent changes in metabolic activity in both suspension and adherent cultures, and support the utility of adherent cultures of cryopreserved hepatocytes for prediction of metabolic clearance for low-clearance drugs.

Metabolite formation kinetics and intrinsic clearance of phenacetin, tolbutamide, alprazolam, and midazolam in adenoviral cytochrome P450-transfected HepG2 cells and comparison with hepatocytes and in vivo

Cryopreserved human hepatocytes and other in vitro systems often underpredict in vivo intrinsic clearance (CL(int)). The aim of this study was to explore the potential utility of HepG2 cells transduced with adenovirus vectors expressing a single cytochrome P450 enzyme (Ad-CYP1A2, Ad-CYP2C9, or Ad-CYP3A4) for metabolic clearance predictions. The kinetics of metabolite formation from phenacetin, tolbutamide, and alprazolam and midazolam, selected as substrates probes for CYP1A2, CYP2C9, and CYP3A4, respectively, were characterized in this in vitro system. The magnitude of the K(m) or S(50) values observed in Ad-P450 cells was similar to those found in the literature for other human liver-derived systems. For each substrate, CL(int) (or CL(max)), values from Ad-P450 systems were scaled to human hepatocytes in primary culture using the relative activity factor (RAF) approach. Scaled Ad-P450 CL(int) values were approximately 3- to 6-fold higher (for phenacetin O-deethylation, tolbutamide 4-hydroxylation, and alprazolam 4-hydroxyaltion) or lower (midazolam 1'-hydroxylation) than those reported for human cryopreserved hepatocytes in suspension. Comparison with the in vivo data reveals that Ad-P450 cells provide a favorable prediction of CL(int) for the substrates studied (in a range of 20-200% in vivo observed CL(int)). This is an improvement compared with the consistent underpredictions (<10-50% in in vivo observed CL(int)) found in cryopreserved hepatocyte studies with the same substrates. These results suggest that the Ad-P450 cell is a promising in vitro system for clearance predictions of P450-metabolized drugs.

Role of membrane transporters in the safety profile of drugs

It has increasingly been recognized that few molecules move across the cell membrane without the assistance of transporter proteins. Large superfamilies of transporter proteins have been identified in every living cell, including microorganisms and mitochondria. This report reviews the role of transporters in physiology and pharmacology, and identifies where this may have an impact on drug efficacy and toxicity. This new understanding will require a fresh appreciation of pharmacokinetics and drug effects, as the current paradigms are based largely on the assumption that drug molecules have a reasonable unrestricted permeability across membranes. Rather than just focusing on clearance changes and central compartment pharmacokinetics, it will become increasingly necessary to examine the peripheral tissue distribution of drugs to more accurately predict drug efficacy and toxicity.

Prediction of hepatic clearance in human from in vitro data for successful drug development

The in vivo metabolic clearance in human has been successfully predicted by using in vitro data of metabolic stability in cryopreserved preparations of human hepatocytes. In the predictions by human hepatocytes, the systematic underpredictions of in vivo clearance have been commonly observed among different datasets. The regression-based scaling factor for the in vitro-to-in vivo extrapolation has mitigated discrepancy between in vitro prediction and in vivo observation. In addition to the elimination by metabolic degradation, the important roles of transporter-mediated hepatic uptake and canalicular excretion have been increasingly recognized as a rate-determining step in the hepatic clearance. It has been, therefore, proposed that the in vitro assessment should allow the evaluation of clearances for both transporter(s)-mediated uptake/excretion and metabolic degradation. This review first outlines the limited ability of subcellular fractions such as liver microsomes to predict hepatic clearance in vivo. It highlights the advantages of cryopreserved human hepatocytes as one of the versatile in vitro systems for the prediction of in vivo metabolic clearance in human at the early development stage. The following section discusses the mechanisms underlying the systematic underprediction of in vivo intrinsic clearance by hepatocytes. It leads to the proposal for the assessment of hepatic uptake clearance as one of the kinetically important determinants for accurate predictions of hepatic clearance in human. The judicious combination of advanced technologies and understandings for the drug disposition allows us to rationally optimize new chemical entities to the drug candidate with higher probability of success during the clinical development.