Abstract
The effect of dexmedetomidine (DEX) on cytochrome P4502D6 (CYP2D6)-dependent dextromethorphan O-demethylase (DEXTROase) activity was studied using native human liver microsomes. DEX (0.01–4.0 μM) inhibited DEXTROase activity (IC50 = 1.8 ± 0.25 μM; mean ± SD; N = 5 livers) and was less potent than quinidine (QND), a prototypical and clinically relevant CYP2D6 inhibitor (IC50 = 0.22 ± 0.02 μM; mean Ki = 0.07 μM). Similar results were obtained with human B-lymphoblast microsomes containing cDNA-expressed CYP2D6 (DEX, IC50 = 2.2 μM; QND, IC50 = 0.15 μM). Formal kinetic analyses indicated that DEX was a reversible mixed (competitive/noncompetitive) inhibitor of DEXTROase activity in human liver microsomes, where Kies >Ki and α > 1 (Ki = 0.4 ± 0.2 μM; Kies = 2.3 ± 0.9 μM; α = 8.1 ± 6.8; N = 3 livers). In addition, DEX elicited a Type llb difference spectrum (λmax ∼436 nm; λmin ∼414 nm) when added to cDNA-expressed CYP2D6 under aerobic (oxidized) conditions. These data indicated that DEX was able to bind reversibly to the heme (ferric) iron of CYP2D6. It is postulated that binding occursvia the 4(5)-substituted imidazole moiety. In this instance, binding was characterized by a spectral dissociation constant (Ks ) of 0.4 μM that was identical to theKi obtained with native human liver microsomes.
Footnotes
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Send reprint requests to: Dr. A. David Rodrigues, Drug Metabolism I, Merck Research Laboratories, Sumneytown Pike, P.O. Box 4, WP26-A 2044, West Point, PA 19486-0004.
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↵2 DEX (0.9–85 ng/ml) binding to human plasma protein in vitro is 93.7%; J. M. Machinist, Ph.D. (Abbott Laboratories), unpublished observations.
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↵3 Intersubject variability is low (coefficient of variation ≤ 22%) and the peak concentration of DEX in plasma, after a loading dose, is not more than 2-fold greater than that at steady state.
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↵4 M. Karol, Ph.D. (Abbott Laboratories), personal communication.
- Abbreviations used are::
- DEX
- dexmedetomidine, (±)-4-[1-(2, 3-dimethylphenyl)ethyl]-1H-imidazole
- CYP
- cytochrome P450
- CYP2D6
- cytochrome P4502D6
- QND
- quinidine
- DEXTRO
- dextromethorphan
- DEXTROase
- [O-methyl-14C]DEXTRO O-demethylase
- Vmax
- apparent maximal initial reaction velocity
- KM
- apparent Michaelis constant
- [drug]plasma
- concentration of drug (free and protein bound) in plasma
- Ks
- apparent spectral dissociation constant
- ΔAmax
- apparent maximal absorbance difference elicited by DEX ([DEX]free) upon addition to CYP2D6
- ΔAmax′
- apparent maximal absorbance difference elicited by DEX ([DEX]total) upon addition to CYP2D6
- [CYP]
- cytochrome P450 concentration
- [DEX]free
- concentration of free DEX (drug nominally in equilibrium with enzyme)
- [DEX]total
- concentration of total DEX added to the cuvette
- IC50
- concentration of drug required to inhibit activity by 50%
- Ki
- apparent inhibition constant (dissociation constant of the enzyme inhibitor, or EI, complex)
- Kies
- apparent dissociation constant of the enyzyme-inhibitor-substrate, or EIS, complex
- SERT
- sertraline
- FLUV
- fluvoxamine
- The American Society for Pharmacology and Experimental Therapeutics
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