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

Tufts University, Бостон, Georgia, United States
Drug Metabolism and Disposition (Impact Factor: 3.25). 04/2003; 31(3):289-93. DOI: 10.1124/dmd.31.3.289
Source: PubMed


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.

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    • "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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    ABSTRACT: Background 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. Methods 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. Results 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%. Conclusions 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.
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    • "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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    Full-text · Article · Sep 2011 · Drug metabolism and disposition: the biological fate of chemicals
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