In vitro identification of metabolic pathways and cytochrome P450 enzymes involved in the metabolism of etoperidone

The Therapeutic Potential of 2-{[4-(2-methoxyphenyl)piperazin-1-yl]alkyl}-1H-benzo[d]imidazoles as Ligands for Alpha1-Adrenergic Receptor - Comparative In Silico and In Vitro Study

Adrenergic receptors are among the most studied G protein-coupled receptors. Activation or blockade of these receptors is a major therapeutic approach for the treatment of numerous disorders such as cardiac hypertrophy, congestive heart failure, hypertension, angina pectoris, cardiac arrhythmias, depression, benign prostate hyperplasia, anaphylaxis, asthma, and hyperthyroidism. Among all nine cloned adrenoceptor subtypes and the subsequent development of animal models, a significant target for various neurological conditions treatment is alpha1-adrenergic receptors. 2-{[4-(2-Methoxyphenyl)piperazin-1-yl]alkyl}-1H-benzo[d]imidazoles, their 5 substituted derivatives, and structurally similar, arylpiperazine based alpha1-adrenergic receptors antagonists (trazodone, naftopidil, and urapidil) have been subjects of comparative analysis. Most of the novel compounds showed alpha1-adrenergic affinity in the range from 22 nM to 250 nM. The in silico docking and molecular dynamics simulations, binding data together with absorption, distribution, metabolism, and excretion (ADME) calculations identified the promising lead compounds. The results brought out the conclusions which allowed us to propose a rationale for the activity of these molecules and to highlight six compounds (2-5, 8, and 12) that exhibited an acceptable pharmacokinetic profile to the advanced investigation as the potential alpha1-adrenergic receptor antagonists.

In vitro identification of cytochrome P450 enzymes responsible for drug metabolism

Metabolism catalyzed by the cytochrome P450 enzymes (CYPs) represents the most important pathway for drug metabolism and elimination in humans. Identification of the CYPs responsible for metabolism of existing and novel drugs is critical for the prediction of adverse reactions caused by drug-drug interactions or individual genetic polymorphism. An integrated approach is described for CYP-mediated metabolic reaction phenotyping using both recombinant enzymes and human liver microsomes in combination of selective inhibitors or inhibitory antibodies. The in vitro method described includes screening of recombinant CYPs for metabolic activity, chemical inhibition or antibody neutralization, and correlation analysis with isoform-selective marker activities. The primary focus is on identification of the most common enzymes including CYP1A2, 2C9, 2C19, 2D6, and 3A4, although the same strategy could potentially be used for identification of other isoforms.

Current awareness of piperazines: pharmacology and toxicology

Although many piperazine derivatives exist, only a limited number have been studied, whereby they have been found to be generally stimulant in nature resulting from dopaminergic, noradrenergic, and predominantly serotoninergic effects in the brain. Reported toxic effects include agitation, anxiety, cardiac symptoms (e.g. tachycardia) and sometimes seizures. As for many drugs, they are primarily metabolized by cytochrome P450 with subsequent possible glucuronidation and/or sulfation. Their abuse has been relatively recently observed in the last decade with only a few identified in biological fluid (primarily 1-benzylpiperazine (BZP) and 1-(3-trifluoromethylphenyl)piperazine (3-TFMPP)) despite publications of a number of analytical methods. Even when detected, however, the toxicological significance of their presence is often difficult to ascertain as many cases involve other drugs as well as a wide and overlapping range of concentrations found in blood (both in life and after death). This paper reviews the current pharmacological and toxicological information for piperazine derivatives and also includes new ante-mortem and post-mortem blood data.

Organophosphorus pesticide degradation product in vitro metabolic stability and time-course uptake and elimination in rats following oral and intravenous dosing

Levels of urinary dialkylphosphates (DAPs) are currently used as a biomarker of human exposure to organophosphorus insecticides (OPs). It is known that OPs degrade on food commodities to DAPs at levels that approach or exceed those of the parent OP. However, little has been reported on the extent of DAP absorption, distribution, metabolism and excretion. The metabolic stability of O,O-dimethylphosphate (DMP) was assessed using pooled human and rat hepatic microsomes. Time-course samples were collected over 2 h and analyzed by LC-MS/MS. It was found that DMP was not metabolized by rat or pooled human hepatic microsomes. Male Sprague-Dawley rats were administered DMP at 20 mg kg(-1) via oral gavage and i.v. injection. Time-course plasma and urine samples were collected and analyzed by LC-MS/MS. DMP oral bioavailability was found to be 107 ± 39% and the amount of orally administered dose recovered in the urine was 30 ± 9.9% by 48 h. The in vitro metabolic stability, high bioavailability and extent of DMP urinary excretion following oral exposure in a rat model suggests that measurement of DMP as a biomarker of OP exposure may lead to overestimation of human exposure.

Detection of a novel reactive metabolite of diclofenac: evidence for CYP2C9-mediated bioactivation via arene oxides

A new glutathione adduct (M4) was tentatively identified, likely as 2'-hydroxy-3'-(glutathione-S-yl)-monoclofenac, using liquid chromatography-tandem mass spectrometry analysis of incubations of diclofenac with human liver microsomes. The same conjugate was not detected in incubations with either rat or monkey liver microsomes. Formation of M4 was mediated specifically by CYP2C9 in human liver microsomes, as evidenced by the following observations: 1) cDNA-expressed CYP2C9-catalyzing formation of M4; 2) inhibition of M4 formation by sulfaphenazole, a CYP2C9-selective inhibitor; and 3) strong correlation between the production of M4 and CYP2C9-mediated tolbutamide 4-hydroxylase activities in a panel of human liver microsome samples. Formation of M4 suggests the existence of a new reactive intermediate as diclofenac-2',3'-oxide. A tentative pathway states that diclofenac is oxidized to diclofenac-2',3'-oxide that reacts with glutathione (GSH) to form a thioether conjugate at the C-3' position, followed by a concomitant loss of chlorine to give rise to M4. Furthermore, a likely mechanism leading to the formation of diclofenac oxides is rationalized: CYP2C9-catalyzed oxidation at the C-3' position of the dichlorophenyl ring to form a cationic sigma-complex that subsequently results in diclofenac-3',4'-oxide and diclofenac-2',3'-oxide; the former oxide is converted to 4'-hydroxy-diclofenac as a major metabolite and can be trapped by GSH to produce 4'-hydroxy-3'-glutathione-S-yl diclofenac (M2), whereas the latter oxide forms 3'-hydroxy-diclofenac and can be trapped by GSH to produce M4. This mechanism is consistent with the structural modeling of the CYP2C9-diclofenac complex, which reveals that both the C-3' and C-4' of the dichlorophenyl ring are proximate to the heme group.

Metabolic assessment in liver microsomes by co-activating cytochrome P450s and UDP-glycosyltransferases

A "dual-activity" microsomal system in which both CYPs and UGTs were active was evaluated for studies of metabolic stability and in-vitro metabolite profiling. In this "dual-activity" system, alamethicin, a pore-forming peptide, was used to activate UGTs in human liver microsomes without affecting CYP activity. Interference studies indicated that CYP cofactors had little effect on UGT surrogate activity as measured by glucuronidation of acetaminophen and trifluoperazine. Further, UGT cofactor, UDPGA (< 2 mM), did not inhibit the marker activity of five major CYPs including 1A2, 2C9, 2C19, 2D6 and 3A4, suggesting that both oxidation and glucuronidation can be co-activated in microsomes. In a comparison study, compounds with significant glucuronidation showed distinct stability profiles in the "dual-activity" system, compared to the conventional microsomal incubation in which only CYPs were active. For compounds with minor or no glucuronidation, the metabolic stability remained similar between the "dual-activity" system and the conventional microsomal incubation. The feasibility of this "dual-activity" system utilized for metabolite profiling was also investigated using tramadol as a model drug. It was found that oxidative metabolites of tramadol generated in the "dual-activity" system matched those detected in the conventional microsomal incubation. However, tramadol glucuronide was observed in the "dual-activity" system but not in the conventional micromosal incubation. Results clearly suggest that the "dual-activity" system is a valuable in vitro model for metabolism studies in drug discovery.