Abstract
Biotransformation of phenacetin via O-deethylation to acetaminophen, an index reaction reflecting activity of Cytochrome P450-1A2, was studied in microsomal preparations from a series of human livers. Acetaminophen formation was consistent with a double Michaelis-Menten system, with low-Km (mean Km1 = 68 μM) and high-Km (mean Km2 = 7691 μM) components. The low-Km enzyme accounted for an average of 96% of estimated intrinsic clearance, and was predicted to contribute more than 50% of net reaction velocity at phenacetin concentrations less than 2000 μM. Among index inhibitor probes, α-naphthoflavone was a highly potent inhibitor of the low-Km enzyme (Ki1 = 0.013 μM); furafylline also was a moderately active inhibitor (Ki1 = 4.4 μM), but its inhibiting potency was increased by preincubation with microsomes. Ketoconazole was a relatively weak inhibitor (Ki1 = 32 μM); quinidine and cimetidine showed minimal inhibiting activity. Among six selective serotonin reuptake inhibitor (SSRI) antidepressants, fluvoxamine was a potent inhibitor of 1A2 (mean Ki1 = 0.24 μM). The other SSRIs were more than tenfold less potent. Mean Ki1 values were: fluoxetine, 4.4 μM; norfluoxetine, 15.9 μM; sertraline, 8.8 μM; desmethylsertraline, 9.5μM; paroxetine, 5.5 μM. The antidepressant nefazodone and four of its metabolites (meta-chloro-phenylpiperazine, two hydroxylated derivatives, and a triazoledione) were very weak inhibitors of P450-1A2. Venlafaxine and its O- and N-desmethyl metabolites showed minimal inhibitory activity.
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Received: 18 March 1996/Final version: 10 July 1996
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von Moltke, L., Greenblatt, D., Duan, S. et al. Phenacetin O-deethylation by human liver microsomes in vitro: inhibition by chemical probes, SSRI antidepressants, nefazodone and venlafaxine. Psychopharmacology 128, 398–407 (1996). https://doi.org/10.1007/s002130050149
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DOI: https://doi.org/10.1007/s002130050149