Bertelsen KM, Venkatakrishnan K, von Moltke LL, Obach RS, Greenblatt DJ. Apparent mechanism-based inhibition of human CYP2D6 in vitro by paroxetine: comparison with fluoxetine and quinidine. Drug Metab Disp 31: 289-293
Paroxetine, a selective serotonin reuptake inhibitor, is a potent inhibitor of cytochrome P450 2D6 (CYP2D6) activity, but the mechanism of inhibition is not established. To determine whether preincubation affects the inhibition of human liver microsomal dextromethorphan demethylation activity by paroxetine, we used a two-step incubation scheme in which all of the enzyme assay components, minus substrate, are preincubated with paroxetine. The kinetic parameters of inhibition were also estimated by varying the time of preincubation as well as the concentration of inhibitor. From these data, a Kitz-Wilson plot was constructed, allowing the estimation of both an apparent inactivator concentration required for half-maximal inactivation (K(I)) and the maximal rate constant of inactivation (k(INACT)) value for this interaction. Preincubation of paroxetine with human liver microsomes caused an approximately 8-fold reduction in the IC(50) value (0.34 versus 2.54 microM). Time-dependent inhibition was demonstrated with an apparent K(I) of 4.85 microM and an apparent k(INACT) value of 0.17 min(-1). Spectral scanning of CYP2D6 with paroxetine yielded an increase in absorbance at 456 nm suggesting paroxetine inactivation of CYP2D6 via the formation of a metabolite intermediate complex. This pattern is consistent with the metabolism of the methylenedioxy substituent in paroxetine; such substituents may produce mechanism-based inactivation of cytochrome P450 enzymes. In contrast, quinidine and fluoxetine, both of which are inhibitors of CYP2D6 activity, did not exhibit a preincubation-dependent increase in inhibitory potency. These data are consistent with mechanism-based inhibition of CYP2D6 by paroxetine but not by quinidine or fluoxetine.
Available from: Bardia Jamali
- "It has been reported that HHMA can be oxidized to related ortho quinone compounds that react with nucleophilic groups of macromolecules or form neurotoxins
[8,18]. Moreover, HHMA can change to carbene intermediate and covalently bind to heme iron of CYP2D6 in the biological systems
[18,19]. Beside HHMA, dopamine, which is structurally the most similar compound to HHMA, is oxidized in the presence of MnO2 and NaIO4 to the reactive ortho quinone compounds which covalently bind to cysteinyl group of proteins and impair their functions. "
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Ecstasy is one of the popular illicit drugs in the world and its usage has been recently increased in Iran. This compound can destroy the serotonergic neurons and produces cognitive and psychopathology diseases. 3,4-dihydroxymethamphetamine (HHMA) which is the main metabolite of this compound, seems to be responsible for this effect. However, no consensus has been reached among the researchers about its role. This disagreement between the researches may be due to failure in determination of HHMA as free form in physiological fluids. In this study, the stability of this crucial metabolite of ecstasy was examined in different mediums.
The stability of HHMA was studied in the perfusion medium and water at 100 and 10 ng/mL concentrations. Moreover, the effect of temperature (0–25°C), pH (3–10), calcium chloride (0–150 g/L) and ethylenediaminetetraacetic acid (EDTA) on the stability of HHMA was also examined.
Our result suggested that the free form of HHMA could be degraded in the perfusion medium. The rate of this degradation has direct proportion to temperature (at 25°C = 0.037 min-1 and at 0°C = 0.002 min-1). Calcium chloride and sodium bicarbonate are two responsible components in this instability. Moreover, the alkaline pHs and increasing the shaking time can accelerate this effect. Although, while degradation was prevented at pH=3, EDTA could only reduce this rate about 30%.
Calcium cation can act as an accelerator of HHMA degradation. Therefore, the perfusion medium should not contain Ca2+ and the pH of medium is better to be adjusted at acidic range. Since, the internal cellular source of calcium is endoplasmic reticulum system, it can be assumed that, this cation may change HHMA and dopamine to reactive compounds that can bind covalently to the cysteinyl group of biological compounds and damage cellular components.
DARU-JOURNAL OF FACULTY OF PHARMACY 01/2013; 21(1):9. DOI:10.1186/2008-2231-21-9 · 1.64 Impact Factor
Available from: Hannu Raunio
- "The phosphodiesterase-5 inhibitor tadalafil and the selective serotonin reuptake inhibitor paroxetine are examples of drugs that contain MDP. These two drugs are known to form MIC with human CYP3A4 and CYP2D6, respectively, resulting in mechanism-based inactivation of these enzymes (Bertelsen et al., 2003; Ring et al., 2005; Kalgutkar et al., 2007). We reported recently a novel fluorometric progress curve analysis approach for rapid identification of TDI of CYP2C19 (Salminen et al., 2011a). "
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ABSTRACT: Several in vitro criteria were used to assess whether three methylenedioxyphenyl (MDP) compounds, the isoquinoline alkaloids bulbocapnine, canadine, and protopine, are mechanism-based inactivators of CYP2C19. The recently reported fluorometric CYP2C19 progress curve analysis approach was applied first to determine whether these alkaloids demonstrate time-dependent inhibition. In this experiment, bulbocapnine, canadine, and protopine displayed time dependence and saturation in their inactivation kinetics with K(I) and k(inact) values of 72.4 ± 14.7 μM and 0.38 ± 0.036 min(-1), 2.1 ± 0.63 μM and 0.18 ± 0.015 min(-1), and 7.1 ± 2.3 μM and 0.24 ± 0.021 min(-1), respectively. Additional studies were performed to determine whether other specific criteria for mechanism-based inactivation were fulfilled: NADPH dependence, irreversibility, and involvement of a catalytic step in the enzyme inactivation. CYP2C19 activity was not significantly restored by dialysis when it had been inactivated by the alkaloids in the presence of a NADPH-regenerating system, and a metabolic-intermediate complex-associated increase in absorbance at approximately 455 nm was observed. In conclusion, the CYP2C19 progress curve analysis method revealed time-dependent inhibition by these alkaloids, and additional experiments confirmed its quasi-irreversible nature. This study revealed that the CYP2C19 progress curve analysis method is useful for identifying novel mechanism-based inactivators and yields a wealth of information in one run. The alkaloids bulbocapnine, canadine, and protopine, present in herbal medicines, are new mechanism-based inactivators and the first MDP compounds exhibiting quasi-irreversible inactivation of CYP2C19.
Drug metabolism and disposition: the biological fate of chemicals 09/2011; 39(12):2283-9. DOI:10.1124/dmd.111.041319 · 3.25 Impact Factor
Available from: Rafael de la Torre
- "NEUROTOXIC THIOETHER ADDUCTS OF MDMA IN HUMAN URINE CYP2D6 is inactivated within 1 h after a recreational dose of MDMA (Yang et al., 2006; O'Mathúna et al., 2008) and because when CYP2D6 is inhibited before MDMA intake with paroxetine, a potent mechanism-based inactivator of this enzyme (Bertelsen et al., 2003), only a 30% increase of the area under the curve of MDMA in plasma is observed (Segura et al., 2005). Prolonging the urine collection after MDMA intake would assist in assessing the correlation (or lack thereof) between CYP2D6 polymorphism and N-Ac-5-Cys adduct formation. "
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ABSTRACT: 3,4-Methylenedioxymethamphetamine (MDMA, Ecstasy) is a widely misused synthetic amphetamine derivative and a serotonergic neurotoxicant in animal models and possibly humans. The underlying mechanism of neurotoxicity involves the formation of reactive oxygen species although their source remains unclear. It has been postulated that MDMA-induced neurotoxicity is mediated via the formation of bioreactive metabolites. In particular, the primary catechol metabolites, 3,4-dihydroxymethamphetamine (HHMA) and 3,4-dihydroxyamphetamine (HHA), subsequently cause the formation of glutathione and N-acetylcysteine conjugates, which retain the ability to redox cycle and are serotonergic neurotoxicants in rats. Although the presence of such metabolites has been recently demonstrated in rat brain microdialysate, their formation in humans has not been reported. The present study describes the detection of 5-(N-acetylcystein-S-yl)-3,4-dihydroxymethamphetamine (N-Ac-5-Cys-HHMA) and 5-(N-acetylcystein-S-yl)-3,4-dihydroxyamphetamine (N-Ac-5-Cys-HHA) in human urine of 15 recreational users of MDMA (1.5 mg/kg) in a controlled setting. The results reveal that in the first 4 h after MDMA ingestion approximately 0.002% of the administered dose was recovered as thioether adducts. Genetic polymorphisms in CYP2D6 and catechol-O-methyltransferase expression, the combination of which are major determinants of steady-state levels of HHMA and 4-hydroxy-3-methoxyamphetamine, probably explain the interindividual variability seen in the recovery of N-Ac-5-Cys-HHMA and N-Ac-5-Cys-HHA. In summary, the formation of neurotoxic thioether adducts of MDMA has been demonstrated for the first time in humans. The findings lend weight to the hypothesis that the bioactivation of MDMA to neurotoxic metabolites is a relevant pathway to neurotoxicity in humans.
Drug metabolism and disposition: the biological fate of chemicals 05/2009; 37(7):1448-55. DOI:10.1124/dmd.108.026393 · 3.25 Impact Factor
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