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Comparison of CYP2D6 content and metoprolol oxidation between microsomes isolated from human livers and small
1Department of Pharmaceutics, University of Washington, Seattle 98195-7610, USA.
Purpose:
To assess the role of intestinal CYP2D6 in oral first-pass drug clearance by comparing the enzyme content and catalytic activity of a prototype CYP2D6 substrate, metoprolol, between microsomes prepared from human intestinal mucosa and from human livers.
Methods:
Microsomes were prepared from a panel of 31 human livers and 19 human intestinal jejunal mucosa. Microsomes were also obtained from the jejunum, duodenum and ileum of four other human intestines to assess regional distribution of intestinal CYP2D6. CYP2D6 content (pmole/mg microsomal protein) was determined by Western blot. CYP2D6 activity was measured by alpha-hydroxylation and O-demethylation of metoprolol.
Results:
Kinetic studies with microsomes from select livers (n = 6) and jejunal mucosa (n = 5) yielded K(M) estimates of 26 +/- 9 microM and 44 +/- 17 microM, respectively. The mean Vmax (per mg protein) for total formation of alpha-OH-M and ODM was 14-fold higher for the liver microsomes compared to the jejunal microsomes. Comparisons across intestinal regions showed that CYP2D6 protein content and catalytic activity were in the order ofjejunum > duodenum > ileum. Excluding the poor metabolizer genotype donors, CYP2D6 content varied 13- and 100-fold across the panels of human livers (n = 31) and jejunal mucosa (n = 19), respectively. Metoprolol alpha-hydroxylation activity and CYP2D6 content were highly correlated in the liver microsomes (r = 0.84, p < 0.001) and jejunal microsomes (r = 0.75, p < 0.05). Using the well-stirred model, the mean microsomal intrinsic clearance (i.e., Vmax/K(M)) for the livers and jejunum were scaled to predict their respective in vivo organ intrinsic clearance and first-pass extraction ratio. Hepatic and intestinal first-pass extractions of metoprolol were predicted to be 48% and 0.85%, respectively.
Conclusions:
A much lower abundance and activity of CYP2D6 are present in human intestinal mucosa than in human liver. Intestinal mucosal metabolism contributes minimally to the first-pass effect of orally administered CYP2D6 substrates, unless they have exceptionally high microsomal intrinsic clearances and/or long residence time in the intestinal epithelium.
Insights
Human intestinal CYP2D6 abundance and activity are significantly lower than in the liver. Intestinal metabolism plays a minimal role in the first-pass effect of oral CYP2D6 substrates.
Area of Science:
- Pharmacology
- Drug Metabolism
- Gastroenterology
Background:
- Cytochrome P450 2D6 (CYP2D6) is a key enzyme in drug metabolism.
- Understanding the role of intestinal CYP2D6 is crucial for oral drug bioavailability.
Purpose of the Study:
- To compare intestinal and hepatic CYP2D6 content and activity.
- To assess the contribution of intestinal CYP2D6 to oral first-pass drug clearance.
Main Methods:
- Microsomes from human livers and intestinal mucosa (jejunum, duodenum, ileum) were prepared.
- CYP2D6 content was measured by Western blot.
- CYP2D6 activity was assessed using metoprolol as a substrate.
Main Results:
- Intestinal CYP2D6 content and activity were substantially lower than hepatic.
- CYP2D6 activity and content were correlated in both tissues.
- Intestinal first-pass extraction of metoprolol was predicted to be minimal (0.85%).
Conclusions:
- Human intestinal CYP2D6 abundance and catalytic activity are much lower than in the liver.
- Intestinal metabolism contributes minimally to the first-pass effect of oral CYP2D6 substrates.