Role of Human Liver Microsomes in In Vitro Metabolism of Drugs—A Review

Novel 4-aryl-pyrido[1,2-c]pyrimidines with dual SSRI and 5-HT1A activity. Part 4

This project describes the synthesis, pharmacological and pharmacodynamic tests on two series of novel derivatives of 2H-pyrido[1,2-c]pyrimidine with potential binary binding to 5-HT1A receptors and SSRI + serotonin transporters. The influence of piperidinyl-indole (8.1-8.7) and tetrahydropyridinyl-indole (8.8-8.32) residues and indole 5-position substituents (R3 = Br, Cl, F) present in the pharmacophore element of ligands on their binding to both molecular targets was tested. A considerable impact of piperidinyl-indole residue on binding to both targets was confirmed and compounds with a high binding affinity were identified: Ki 5-HT1A = 12.4 nM; Ki SERT = 15.6 nM 8.1; Ki 5-HT1A = 5.6 nM; Ki SERT = 20.7 nM 8.7, while the presence of a tetrahydropyridinyl-indole residue was found to reduce the affinity of ligands to 5-HT1AR. The presence of chlorine (R3) in this series resulted in a notable reduction in binding to both targets (5-HT1A and SERT). Selected compounds had their metabolic stability in a first-pass test (human liver microsomes, NADPH) determined in vitro, and R1 and R2 substituents present on the terminal residue of pyrido[1,2-c]pyrimidine were recognized as having an impact on stability.

Biotransformation in vitro: An essential consideration in the quantitative in vitro-to-in vivo extrapolation (QIVIVE) of toxicity data

Early consideration of the multiplicity of factors that govern the biological fate of foreign compounds in living systems is a necessary prerequisite for the quantitative in vitro-in vivo extrapolation (QIVIVE) of toxicity data. Substantial technological advances in in vitro methodologies have facilitated the study of in vitro metabolism and the further use of such data for in vivo prediction. However, extrapolation to in vivo with a comfortable degree of confidence, requires continuous progress in the field to address challenges such as e.g., in vitro evaluation of chemical-chemical interactions, accounting for individual variability but also analytical challenges for ensuring sensitive measurement technologies. This paper discusses the current status of in vitro metabolism studies for QIVIVE extrapolation, serving today's hazard and risk assessment needs. A short overview of the methodologies for in vitro metabolism studies is given. Furthermore, recommendations for priority research and other activities are provided to ensure further widespread uptake of in vitro metabolism methods in 21st century toxicology. The need for more streamlined and explicitly described integrated approaches to reflect the physiology and the related dynamic and kinetic processes of the human body is highlighted i.e., using in vitro data in combination with in silico approaches.

An in-vitro cocktail assay for assessing compound-mediated inhibition of six major cytochrome P450 enzymes

An efficient screening assay was developed and validated for simultaneous assessment of compound-mediated inhibition of six major human cytochrome P450 (CYP) enzymes. This method employed a cocktail of six probe substrates (i.e., phenacetin, amodiaquine, diclofenac, S-mephenytoin, dextromethorphan and midazolam for CYP1A2, 2C8, 2C9, 2C19, 2D6 and 3A4, respectively) as well as individual prototypical inhibitors of the six CYP enzymes in human liver microsomes under optimized incubation conditions. The corresponding marker metabolites (i.e., acetaminophen, N-desethylamodiaquine, 4-OH-diclofenac, 4-OH-S-mephenytoin, dextrorphan and 1-OH-midazolam) in the incubates were quantified using LC–MS/MS methods either by an internal standard (IS) calibration curve or a simplified analyte-to-IS peak area ratio approach. The results showed that the IC₅₀ values determined by the cocktail approach were in good agreement with those obtained by the individual substrate approach as well as those reported in the literature. Besides, no remarkable difference was observed between the two quantification approaches. In conclusion, this new cocktail assay can be used for reliable screening of compound-mediated CYP inhibition.

A non-michaelian behavior of the in vitro metabolism of the pentacyclic triterpene alfa and beta amyrins by employing rat liver microsomes

Pharmacological studies employing alpha and beta amyrin have demonstrated potential application in several biological activities suggesting their application as promising drugs. In the early drug development, metabolism studies may give important parameters regarding the efficacy and safety of the drug candidate. Therefore, the aim of this work was to determine the enzymatic kinetic parameters of these pentacyclic triterpenes. Chromatographic analyzes were performed using a Shimadzu GC-MS system. The resolution of amyrins was achieved with a DB5-MS column of 0.25 μM film thickness, 30.0 cm length and 0.25 mm diameter. At this condition, the retention times of beta- and alpha-amyrin were 21.3 and 20.2 min, respectively. The proposed method showed to be linear over the concentration range of 0.16-42.18 μM for beta amyrin and 0.11-28.12 μM for alpha amyrin. The lowest concentration quantified by the validated method was 0.16 μM for beta and 0.11 μM for alpha amyrin. The stability study showed that amyrins were stable at room temperature for 12h and at 37°C for 1h. The absolute recovery of the amyrin isomers from the rat microsome was 54.3-59.2%. The enzymatic kinetics presented sigmoidal plots. It was observed a Vmax=0.698 ± 0.022 μmol/mg protein/min, S50=4.4 μM and Hill coefficient of 2.7 ± 0.17 for alpha amyrin and a Vmax=0.775 ± 0.034 μmol/mg protein/min, S50=7.0 μM and Hill coefficient of 2.5 ± 0.21 for beta amyrin. The obtained results give the first clues regarding amyrin metabolism and suggests a more detailed study conducted employing isolated CYP isoforms.

Cunninghamella as a microbiological model for metabolism of histamine H(3) receptor antagonist 1-[3-(4-tert-butylphenoxy)propyl]piperidine

The aim of the study was to analyze the ability of the microorganism Cunninghamella to carry out the biotransformation of 1-[3-(4-tert-butylphenoxy)propyl]piperidine (DL76) and to compare the obtained results with in silico models. Biotransformation was carried out by three strains of filamentous fungus: Cunninghamella echinulata, Cunninghamella blakesleeana, and Cunninghamella elegans. Most probable direction of DL76 metabolic transition was the oxidation of the methyl group in the tert-butyl moiety leading to the formation of the metabolite with I° alcohol properties. This kind of reaction was conducted by all three strains tested. However, only in the case of C. blakesleeana that biotransformation product had a structure of carboxylic acid. CYP2C19 was identified by Metasite software to be the isoform of major importance in the oxidation process in the tert-butyl moiety of DL76. In silico data coincide with the results of experiments conducted in vitro. It was confirmed that Cunninghamella fungi are a very good model to study the metabolism of xenobiotics. The computational methods and microbial models of metabolism can be used as useful tools in early ADME-Tox assays in the process of developing new drug candidates.

Generation of metabolites by an automated online metabolism method using human liver microsomes with subsequent identification by LC-MS(n), and metabolism of 11 cathinones

Human liver microsomes (HLMs) are used to simulate human xenobiotic metabolism in vitro. In forensic and clinical toxicology, HLMs are popularly used to study the metabolism of new designer drugs for example. In this work, we present an automated online extraction system we developed for HLM experiments, which was compared to a classical offline approach. Furthermore, we present studies on the metabolism of 11 cathinones; for eight of these, the metabolism has not previously been reported. Metabolites were identified based on MS(2) and MS(3) scans. Fifty-three substances encompassing various classes of drugs were employed to compare the established offline and the new online methods. The metabolism of each of the following 11 cathinones was studied using the new method: 3,4-methylenedioxy-N-benzylcathinone, benzedrone, butylone, dimethylcathinone, ethylone, flephedrone, methedrone, methylone, methylethylcathinone, naphyrone, and pentylone. The agreement between the offline and the online methods was good; a total of 158 metabolites were identified. Using only the offline method, 156 (98.7%) metabolites were identified, while 151 (95.6%) were identified using only the online method. The metabolic pathways identified for the 11 cathinones included the reduction of the keto group, desalkylation, hydroxylation, and desmethylenation in cathinones containing a methylenedioxy moiety. Our method provides a straightforward approach to identifying metabolites which can then be added to the library utilized by our clinical toxicological screening method. The performance of our method compares well with that of an established offline HLM procedure, but is as automated as possible.

Comparison of cytochrome P450 concentrations and metabolic activities in porcine hepatic microsomes prepared with two different methods

In the present study, porcine liver microsomes prepared by a conventional ultracentrifugation method were compared with microsomes prepared by a calcium aggregation method. Protein concentrations and activities of several cytochrome P450 enzymes were measured. It was concluded that using a calcium aggregation method for microsome preparation resulted in lower activities of porcine 7-ethoxyresorufin O-deethylase (EROD), 7-methoxyresorufin O-demethylase (MROD), 7-pentoxyresorufin O-depentylase (PROD) and p-nitrophenol hydroxylase (PNPH), compared to ultracentrifugation. Protein concentrations of CYP1A2 and CYP2E1, measured by Western blot, were similar in the microsomes prepared by the two methods, whereas CYP2A protein concentrations were significantly lower in the microsomes prepared by the calcium aggregation method. The choice of homogenization buffer (TRIS with addition of either 250 mM sucrose or 2mM EDTA) did not affect either individual CYP450 protein concentration or the rates of CYP450-mediated reactions. Freeze/thawing of microsomes did not affect the activities of EROD, MROD, COH and PNPH in the microsomes, indicating the stability of the measured isoforms following three cycles of freezing/thawing. A reduction in the activity of PROD was observed after the third freeze/thawing cycles of the microsomes prepared by both methods.