Inhibition of murine cytochrome P4501A by tacrine: In vitro studies

Joanna Z. Peng, Rory P. Remmel, Ronald J. Sawchuk

Research output: Contribution to journalArticle

17 Citations (Scopus)

Abstract

Tacrine, a cholinesterase inhibitor, was approved for the treatment of Alzheimer's disease. Oxidative metabolism of tacrine occurs by CYP1A-catalyzed hydroxylation. In rats, it was observed that the area under the curve (AUC) of the second oral dose was consistently higher than the AUC after the first oral dose, which was not due to the accumulation of the drug in the plasma from the first dose. This finding suggested inhibition of the enzyme during metabolism or inhibition by a metabolite. The inhibitory mechanism was studied in liver and intestinal microsomes prepared from 3-methylcholanthrene-treated rats and with recombinant CYP1A1 and CYP1A2. Preincubation of CYP1A2 with tacrine and NADPH revealed a time-dependent inhibition of 7-ethoxyresorufin O-de-ethylation with a Ki of 1.94 μM and a kinact of 0.091 min-1. No time-dependent inhibition was observed with CYP1A1 or with 1-hydroxytacrine or 2-hydroxytacrine. Tacrine metabolism catalyzed by CYP1A was also carried out, and the partition ratio was estimated to be 22. A modified Michaelis-Menten equation involving mechanism-based inhibition was derived and used to analyze the data. Reasonable parameter fits were obtained indicating that this equation is suitable to describe metabolism data when the substrate is a mechanism-based inhibitor of the enzyme. The probable inactivation mechanism involves either hydrogen atom abstraction to produce a carbon-centered radical intermediate at the benzylic position or insertion of OH+ into a C-H bond with subsequent loss of water to produce a carbocation. Rapid rearrangement of the carbocation or radical and subsequent covalent binding of the tacrine moiety would result in enzyme inactivation.

Original languageEnglish (US)
Pages (from-to)805-812
Number of pages8
JournalDrug Metabolism and Disposition
Volume32
Issue number8
DOIs
StatePublished - Aug 1 2004

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Tacrine
Cytochromes
Cytochrome P-450 CYP1A2
Cytochrome P-450 CYP1A1
Area Under Curve
Methylcholanthrene
Cholinesterase Inhibitors
Liver Microsomes
Enzyme Inhibitors
Enzymes
Hydroxylation
NADP
Hydrogen
Alzheimer Disease
Carbon
In Vitro Techniques
Inhibition (Psychology)
Water
Pharmaceutical Preparations

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Inhibition of murine cytochrome P4501A by tacrine : In vitro studies. / Peng, Joanna Z.; Remmel, Rory P.; Sawchuk, Ronald J.

In: Drug Metabolism and Disposition, Vol. 32, No. 8, 01.08.2004, p. 805-812.

Research output: Contribution to journalArticle

Peng, Joanna Z. ; Remmel, Rory P. ; Sawchuk, Ronald J. / Inhibition of murine cytochrome P4501A by tacrine : In vitro studies. In: Drug Metabolism and Disposition. 2004 ; Vol. 32, No. 8. pp. 805-812.
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