Comparison of CYP2D6 content and metoprolol oxidation between microsomes isolated from human livers and small intestines

Purpose. To assess the role of intestinal CYP2D6 in oral first-pass drug cl earance by comparing the enzyme content and catalytic activity of a prototy pe CYP2D6 substrate, metoprolol, between microsomes prepared from human int estinal mucosa and from human livers. Methods. Microsomes were prepared from a panel of 31 human livers and 19 hu man intestinal jejunal mucosa. Microsomes were also obtained from the jejun um, duodenum and ileum of four other human intestines to assess regional di stribution of intestinal CYP2D6. CYP2D6 content (pmole/mg microsomal protei n) was determined by Western blot. CYP2D6 activity was measured by alpha-hy droxylation and O-demethylation of metoprolol. Results. Kinetic studies with microsomes from select livers (n = 6) and jej unal mucosa (n = 5) yielded K-M estimates of 26 +/- 9 mu M and 44 +/- 17 mu M, respectively. The mean V-max (per mg protein) for total formation of al pha-OH-M and ODM was 14-fold higher for the liver microsomes compared to th e jejunal microsomes. Comparisons across intestinal regions showed that CYP 2D6 protein content and catalytic activity were in the order of jejunum > d uodenum > ileum. Excluding the poor metabolizer genotype donors, CYP2D6 con tent varied 13- and 100-fold across the panels of human livers (n = 31) and jejunal mucosa (n = 19), respectively. Metoprolol ol-hydroxylation activit y 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., V-max/K-M) fo r the livers and jejunum were scaled to predict their respective in vivo or gan intrinsic clearance and first-pass extraction ratio. Hepatic and intest inal 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 h uman intestinal mucosa than in human liver. Intestinal mucosal metabolism c ontributes minimally to the first-pass effect of orally administered CYP2D6 substrates, unless they have exceptionally high microsomal intrinsic clear ances and/or long residence time in the intestinal epithelium.