0022-3565/97/2833-1552$03.00/0
THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS
Copyright © 1997 by The American Society for Pharmacology and Experimental Therapeutics
JPET 283:1552–1562, 1997
Vol. 283, No. 3
Printed in U.S.A.
Characterization of Interintestinal and Intraintestinal Variations
in Human CYP3A-Dependent Metabolism1
MARY F. PAINE, MEHRANEH KHALIGHI, JEANNINE M. FISHER, DANNY D. SHEN, KENT L. KUNZE,
CHRISTOPHER L. MARSH, JAMES D. PERKINS and KENNETH E. THUMMEL
Departments of Pharmaceutics (M.F.P., M.K., J.M.F., D.D.S., K.E.T.), Medicinal Chemistry (K.L.K.) and Surgery (C.L.M., J.D.P.),
University of Washington, Seattle, Washington
Accepted for publication August 27, 1997
Human cytochrome P450 3A isoforms (CYP3A) metabolize
a number of widely prescribed, structurally diverse drugs
that belong to a variety of therapeutic classes (Guengerich,
1995; Wrighton and Stevens, 1992). CYP3A4 and CYP3A5
are the major isoforms expressed in adults. Although found
in many tissues throughout the body, their relative levels of
expression are greatest in the liver and villus epithelium of
the small intestine. On average, CYP3A composes 25% to
30% of total hepatic cytochromes P450 (Shimada et al., 1994;
Wrighton and Stevens, 1992) and an even larger percentage
Received for publication April 28, 1997.
1
This work was supported in part by National Institutes of Health Grants
GM48349, GM32165 and ES07033 and a fellowship from the American Foundation for Pharmaceutical Education.
similar trend for the intestinal regions; median duodenal intrinsic clearance was comparable to hepatic intrinsic clearance
(157 and 200 ml/min/mg, respectively). Vmax correlated with
CYP3A content for all tissues except the ileum. Duodenal and
jejunal Vmax and CYP3A content varied by .30-fold among
donors. Microsomes prepared from every other 1-foot section
of six intestines were also analyzed for CYP3A as well as for
two coenzymes. In general, CYP3A activity, CYP3A content
and CYP reductase activity rose slightly from duodenum to
middle jejunum and then declined to distal jejunum and ileum.
Cytochrome b5 content and cytochrome b5 reductase activity
varied little throughout the intestinal tract. Regional intrinsic
midazolam 19-hydroxylation clearance was greatest for the jejunum, followed by the duodenum and ileum (144, 50 and 19
ml/min, respectively). Collectively, these results demonstrate
that the upper small intestine serves as the major site for
intestinal CYP3A-mediated first-pass metabolism and provides
a rationale for interindividual differences in oral bioavailability
for some CYP3A substrates.
of total small intestinal cytochromes P450 (De Waziers et al.,
1990; Watkins et al., 1987). In addition, CYP3A content in
both organs is highly variable among individuals. Lown et al.
(1994) found CYP3A4 protein to vary .11-fold in S9 fractions
prepared from 20 human duodenal biopsies, and Shimada et
al. (1994) found a similar variability in liver microsomes
prepared from 60 human samples.
Due to the anatomic arrangement of the small intestine
and liver, drugs may encounter sequential, CYP3A-mediated
first-pass metabolism when taken orally (Thummel et al.,
1997). Historically, the liver was considered the major site of
CYP3A-dependent first-pass metabolic extraction. Recent in
vitro and in vivo studies, however, suggest that mucosal villi
of the small intestine can be of equal or greater importance
ABBREVIATIONS: CYP, cytochrome P450; MDZ, midazolam; 19-OH MDZ, 19-hydroxymidazolam; 4-OH MDZ, 4-hydroxymidazolam; S9, supernatant fraction at 9000 3 g; SDS, sodium dodecyl sulfate; PAGE, polyacrylamide gel electrophoresis; TEMED, N,N,N9,N9-tetra-methyl-ethylenediamine; BCIP-NBT, 5-bromo-4-chloro-3-indoyl phosphate and nitroblue tetrazolium; IOD, integrated optical density; PMSF, phenylmethylsulfonyl
fluoride; EDTA, ethylenediamine tetraacetic acid; ANOVA, analysis of variance.
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ABSTRACT
Cytochrome P450 3A (CYP3A) metabolizes a diverse array of
clinically important drugs. For some of these (e.g., cyclosporine, verapamil, midazolam), CYP3A in the intestinal mucosa
contributes to their extensive and variable first-pass extraction.
To further characterize this phenomenon, we measured CYP3A
content and catalytic activity toward the probe substrate midazolam in mucosa isolated from duodenal, jejunal and ileal sections of 20 human donor intestines. For comparison, the same
measurements were performed for 20 human donor livers, eight
of which were obtained from the same donors as eight of the
intestines. Excellent correlations existed between homogenate
and microsomal CYP3A content for the three intestinal regions.
Median microsomal CYP3A content was greatest in the duodenum and lowest in the ileum (31 vs. 17 pmol/mg of protein).
With respect to midazolam 19-hydroxylation kinetics, the median Km for each intestinal region was similar to the median
hepatic Km, ;4 mM. In contrast, the median Vmax decreased
from liver to duodenum to jejunum to ileum (850 vs. 644 vs. 426
vs. 68 pmol/min/mg). Intrinsic clearance (Vmax/Km) followed a
1997
1553
Variability in Human Intestinal CYP3A
Methods
Chemicals. MDZ, 19-OH MDZ and 4-OH MDZ were kindly provided by Drs. William Garland and Bruce Mico (Roche Laboratories,
Nutley, NJ). Cytochrome c (horse heart type VI), NADPH (reduced
form, tetrasodium salt), NADH, EDTA, PMSF and anti-rabbit IgG
alkaline phosphatase conjugate were purchased from Sigma Chemical (St. Louis, MO). N-Methyl-N-(t-butyl-dimethylsilyl)trifluoroacetamide was purchased from Pierce Chemical (Rockford, IL). Acetonitrile and ethyl acetate were purchased from Fisher Scientific (Santa
Clara, CA). SDS-PAGE reagents (SDS, acrylamide, ammonium persulfate, TEMED) were purchased from BioRad (Hercules, CA). Nitrocellulose was purchased from Schleicher & Schuell (Keene, NH).
BCIP-NBT was purchased from Kirkegaard and Perry (Gaithersburg, MD). All other chemicals were of reagent grade or better.
Organ procurement. The collection and use of human donor
tissue for research were approved by the University of Washington
Human Subjects Review Board. During procurement, the liver and
small intestine were dissected in preparation for removal and then
perfused in situ with cold (4°C) University of Wisconsin solution
(Viaspan). The organs were kept cold with topical iced saline until
removal from the abdominal cavity. After retrieval and transport of
each intestine (in “iced saline”) to our laboratory, the organ was cut
into 1-foot sections, flushed with ;60 ml cold saline and weighed.
Sections of mucosa were exposed by a longitudinal cut and carefully
scraped free from the remainder of the gut tissue using a glass slide.
Mucosal scrapings from each section were then placed separately
into 45 ml conical polypropylene tubes, weighed, immediately frozen
in liquid nitrogen and stored at 270°C. All processing of tissue was
performed on a bed of ice. Full-length small intestines from a total of
20 donors were used for study. The time elapsed between vascular
cross-clamp (start of cold ischemia) and the freezing of all mucosal
scrapings was usually ,3 hr. Two intestines were obtained from
out-of-state donors (Alaska and Oregon). Transit and processing
time of these organs were ,5 hr. Procured livers were kept on ice for
a more variable period of time, 12 to 24 hr, before processing. After
transfer to our laboratory, the organs were cut into ;10 g pieces and
snap-frozen in liquid nitrogen. Tissue from a total of 20 livers was
used for study. Six exhibited fatty infiltration in .25% of parenchymal cells. Three livers were obtained from out-of-state donors (Idaho,
Montana and Alaska), and eight livers were from the same donors as
8 of the 20 intestines. There were an equal number of male and
female intestine donors, but 12 female and 8 male liver donors. The
age range for both liver and intestine donors was 10 to 70 years, and
nearly all were Caucasian.
Organ donors typically receive numerous medications before procurement, including cardiovascular agents (dobutamine, dopamine,
ephedrine, lidocaine, nitroprusside, procainamide, phenylephrine,
vasopressin), antibiotics (cephalosporins, clindamycin, ampicillin/
sulbactam, gentamicin, vancomycin), insulin and medications to
treat brain injury, such as mannitol, dexamethasone and phenytoin.
These last two drugs, which had been administered to two intestine
and three liver donors, can induce CYP3A expression.
TABLE 1
Concordance between CYP3A4 and CYP3A5 expression among
matched livers and intestines
Liver
Intestine
Liver-intestine Pair
HL-146/HI-24
HL-147/HI-26
HL-148/HI-27
HL-149/HI-29
HL-150/HI-30
HL-151/HI-36
HL-152/HI-37
HL-153/HI-38
CYP3A4
CYP3A5
CYP3A4
CYP3A5
1
1
1
1
1
1
1
1
2
1
2
2
1
2
2
2
1
1
1
1
1
1
1
1
2
2
2
2
1
2
2
2
Paired liver (HL-) and small intestine (HI-) were procured from the same donor.
Positive symbols denote the detection of a band that comigrated with authentic
standard, following Western blot procedures described in the text. Negative
symbols denote the absence of a readily detectable band.
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for some drugs such as cyclosporine (Gomez et al., 1995;
Hebert et al., 1992; Kolars et al., 1991; Webber et al., 1992)
and verapamil (Fromm et al., 1996). We also conducted a
series of human studies that indicate extensive intestinal
first-pass metabolism of the sedative/hypnotic agent MDZ.
MDZ is eliminated entirely (.97%) by oxidative biotransformation reactions catalyzed by the CYP3A subfamily (Fabre et
al., 1988; Gorski et al., 1994; Kronbach et al., 1989). A pharmacokinetic analysis of intravenous and oral MDZ disposition in healthy volunteers (Thummel et al., 1996) suggested
that the low oral bioavailability observed for this drug (mean,
30 6 10%) was the result of comparable extraction ratios for
the liver (mean, 44 6 14%) and intestine (mean, 43 6 24%).
Direct measurements of first-pass MDZ extraction during the
anhepatic phase of liver transplant operations confirmed an
identical mean gut wall extraction fraction (43 6 18%) (Paine
et al., 1996). Both in vivo MDZ studies revealed large interindividual differences in hepatic and intestinal extraction
ratios. Hepatic extraction ranged from 22% to 76%, whereas
intestinal extraction ranged from ;0% to 77% (healthy volunteer study) and from 14% to 59% (anhepatic study). These
interindividual variations are consistent with variably expressed CYP3A content and associated in vitro MDZ 19-hydroxylation activity measured in human liver microsomes
(Kronbach et al., 1989) and in S9 fractions prepared from
human duodenal biopsies (Lown et al., 1994).
The intravenous formulation of MDZ (Versed), administered orally or directly into the duodenum, is absorbed rapidly from the most proximal region of the small intestine.
Other CYP3A substrates that have slow dissolution or administered in sustained-release formulations are absorbed
throughout the small intestinal tract. Because mucosal
CYP3A4 content is reportedly lower in jejunum and ileum
compared with duodenum (De Waziers et al., 1990), first-pass
intestinal metabolic extraction may depend on the absorption
characteristics of the drug formulation; that is, first-pass
intestinal metabolism may be reduced when drug is absorbed
at more distal sites of the small intestine. However, very
little is known about the extent of interindividual variability
in CYP3A expression in epithelium distal to the duodenum
as well as the relative catalytic capacities of duodenal, jejunal and ileal CYP3A.
We characterized CYP3A protein content and catalytic activity, as well as two CYP coenzymes (NADPH-dependent
cytochrome P450 reductase and cytochrome b5), along the
entire length of six different human donor small intestines.
In addition, we determined the relative metabolic capacity
(MDZ intrinsic clearance) of the three regions (duodenum,
jejunum and ileum) in 15 donor small intestines. For comparison, parallel analyses were performed with 20 human
livers. Based on these in vitro results, we estimated wholeorgan intrinsic clearances using a conventional flow model
(Pond and Tozer, 1984; Wilkinson, 1987) and considered
their correlation to gut and liver extraction of MDZ in vivo.
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Paine et al.
BCIP-NBT according to the manufacturer’s instructions. A protein
band that comigrated with the CYP3A4 standard was detected in all
samples. A second, slightly higher-molecular-weight protein band
was also detected in 20% of livers and intestines; this was presumed
to be CYP3A5. All blots were scanned (Howtek Scanmaster 31) and
quantified for CYP3A4 and CYP3A5 by densitometry using the software programs Visage and Whole Band Analysis (v4.6M and v2.4,
respectively; Millipore BioImage Products, Ann Arbor, MI). Purified
CYP3A4 was used as the reference standard.
Hepatic and intestinal microsomal incubations. An internal
standard mixture containing 15N3-labeled MDZ metabolites (i.e.,
19-OH MDZ and 4-OH MDZ) was prepared by incubating 6 nmol of
cytochrome P450 (using HL-122 microsomes) with 100 mg of 15N3MDZ and 12 mg of NADPH (final concentration, ;1.5 mM) in potassium phosphate buffer (0.1 M, pH 7.4, in a final volume of 8 ml) at
37°C. After 10 min, the reaction was stopped by the addition of 8 ml
of Na2CO3 (0.1 M, pH 12). The compounds were extracted twice with
20 ml of ethyl acetate, and the solvent was evaporated to dryness
under a stream of nitrogen. The remaining solid was then reconstituted in 20 ml of methanol, split into two 10-ml aliquots and stored
at 220°C.
To determine longitudinal distribution of intestinal CYP3A activity, microsomes prepared from every other 1-foot sections of six
whole small intestines were analyzed for 19-OH MDZ formation rate.
Duplicate incubation mixtures containing 100 to 200 mg of microsomal protein (metabolite formation was found to be linear with microsomal protein up to 500 mg of protein) and 100 ml of 80 mM MDZ
in potassium phosphate buffer (total volume of 0.9 ml) were preincubated at 37°C for 5 min. The reaction was initiated by the addition
of 100 ml of 10 mM NADPH; final concentrations of MDZ and
NADPH were 8 mM and 1 mM, respectively. The reaction was terminated after 4 min by the addition of 1 ml of Na2CO3 (final pH ;11).
Alkalinized samples were spiked with 100 ml of a 1:5 dilution of the
internal standard mixture (in distilled, deionized water), which represented ;50 and ;10 ng of 15N3-labeled 19-OH MDZ and 4-OH
MDZ, respectively. The metabolites were extracted with 5 ml of ethyl
acetate, the solvent was removed under nitrogen and the concentrated extracts were dissolved in 100 ml of derivatizing reagent [10%
N-methyl-N-(t-butyl-dimethylsilyl)trifluoroacetamide in acetonitrile]. The samples were then transferred to autoinjector vials and
sealed before being heated at 80°C for 2 hr before analysis.
Michaelis-Menten parameters Vmax and Km for 19-OH MDZ formation were determined in microsomes prepared from all livers and
from the three regions of 15 intestines. Incubation conditions were
the same as described earlier except that eight MDZ concentrations
were used (0, 0.25, 0.5, 1, 2, 4 and 8 mM) and microsomal protein
ranged from 20 to 50 mg and from 50 to 300 mg for liver and intestine,
respectively. Initial estimates of Vmax and Km were determined by
applying the Eadie-Hofstee transformation for a unienzyme system
to the product formation rate data. Final parameter estimates were
obtained by nonlinear least-squares regression of unweighted data
using PCNONLIN (v4.2, SCI Software, Lexington, KY). The unbound intrinsic clearance, Clin,mic, was calculated by dividing Vmax by
K m.
Because CYP3A5-containing livers were found to have higher
product ratios (19-OH MDZ/4-OH MDZ) than those without CYP3A5
(Gorski et al., 1994), the catalytic activities of three CYP3A5-containing jejunal samples were further examined by measuring the
same product ratios and comparing them with three intestines that
did not contain CYP3A5. Similar incubations were also carried out
with six livers. Incubation conditions were again the same as those
described earlier except that four MDZ concentrations were used:
0.25, 1, 4 and 8 mM.
Incubates were analyzed for 4-OH MDZ and/or 19-OH MDZ by
selective ion gas chromatography-negative chemical ionization mass
spectrometry (GC/NCI-MS) as previously described (Paine et al.,
1996) except that molecular ions with m/z 455 and 460 (corresponding to the unlabeled and 15N3-labeled 37Cl isotope of 19-OH MDZ,
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Preparation of hepatic and intestinal microsomes. Hepatic
microsomes were prepared from 20 livers as described previously
(Thummel et al., 1993), except that 0.25 M sucrose containing 1 mM
EDTA was used as the storage solution. Intestinal microsomes were
prepared from every other 1-foot section of six whole small intestines
and from a 1-foot section of each of the three regions (duodenum,
jejunum and ileum) of an additional 14 whole small intestines as
described previously (Thummel et al., 1996) with some modifications.
Briefly, homogenizing buffer (10 mM potassium phosphate, pH 7.4,
containing 0.25 M sucrose, 1 mM EDTA and 0.1 mM PMSF) was
added directly to the frozen conical tubes containing the mucosal
scrapings (5-fold v/v dilution). After the tissue was thawed, the
mixture was transferred to and homogenized in a 55-ml glass
Wheaton tube using a motor-driven Teflon-tipped pestle (8–10
strokes). The total volume of homogenate was recorded, and two 1-ml
aliquots were saved. Homogenate was transferred to 30-ml centrifuge tubes, which were spun at 600 3 g for 5 min and then at
11,000 3 g for 15 min. The resulting supernatant was filtered
through sterile gauze into 60-ml centrifuge tubes, which were spun
at 110,000 3 g for 70 min. After saving the supernatant for unrelated
studies, the remaining pellet was resuspended in wash buffer (10
mM potassium phosphate, pH 7.4, containing 1 mM EDTA and 0.1
mM PMSF) and centrifuged again at 110,000 3 g for 70 min. The
washed pellet was resuspended in storage solution (0.25 M sucrose,
pH 7.4, containing 1 mM EDTA and 0.1 mM PMSF) to a final protein
concentration of 10 to 30 mg/ml. Aliquots of the final microsomal
suspension (;0.25 ml) were stored at 270°C. Protein concentrations
were determined according to the method of Lowry et al. (1951) using
bovine serum albumin as the reference standard.
Spectrophotometric assays. Total cytochrome P450 and cytochrome b5 concentrations in hepatic and intestinal microsomes were
measured according to the method of Omura and Sato (1964) using
extinction coefficients of 91 and 185 mM21 cm21, respectively. Microsomal cytochrome P450 reductase and cytochrome b5 reductase
activities were measured by determining the rates of NADPH- and
NADH-dependent cytochrome c reduction, respectively, as described
previously (Kurzban and Strobel, 1986). Briefly, 100 mg of microsomal protein, cytochrome c and either NADPH or NADH were mixed
with potassium phosphate buffer (0.3 M, pH 7.7) to a final 1-ml
volume in a cuvette. The final concentrations of cytochrome c,
NADPH and NADH were 40 mM, 1 mM and 0.64 mM, respectively.
Immediately after the addition of NADPH or NADH, the change in
absorbance (550–538 nm) was measured every 3 sec over 60 sec at
25°C. The rates of cytochrome c reduction were calculated using an
extinction coefficient of 21 mM21 cm21 (Van Gelder and Slater,
1962).
Western blot analysis of CYP3A4 and CYP3A5. Both homogenate and microsomes were analyzed for these two proteins. Intestinal microsomes, intestinal homogenate and hepatic microsomes
were diluted in sample buffer (60 mM Tris z HCl, 25% glycerol, 0.2%
Emulgen 911, pH 7.4) to final concentrations of 20 to 50, 50 and 5
mg/20 ml, respectively. After boiling each sample for 2 min, 20 ml was
loaded onto 0.1% SDS-9% acrylamide gels, and the proteins were
separated as described previously (Favreau et al., 1987). Purified
CYP3A4 standards (Kharasch and Thummel, 1993) were also prepared and run in parallel with the microsomal or homogenate samples. At the end of the run (;3.5 hr), the proteins were electrophoretically transferred to nitrocellulose; the sheet was rinsed twice
with PBS and stored in blocking buffer (2% nonfat dry milk and 2%
Triton X-100 in PBS) overnight. The following morning, the nitrocellulose sheets were incubated with a selective rabbit polyclonal
anti-CYP3A4 IgG (Kharasch and Thummel, 1993) at a final concentration of 1 mg/ml in blocking buffer for 2 hr at room temperature.
The sheets were washed twice with blocking buffer and then incubated with the secondary antibody, anti-rabbit IgG alkaline phosphatase conjugate (1:1000), for 2 hr. The sheets were washed twice
with blocking buffer, twice with PBS and twice with 0.1 M Tris buffer
(pH 7.4). The proteins of interest were visualized with the addition of
Vol. 283
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Variability in Human Intestinal CYP3A
Regional CYP3A 5 Regional wet weight (g)
3 mucosal recovery (g mucosa/g wet weight) (1)
3 mucosal CYP3A (pmol/g mucosa)
where mucosal CYP3A is homogenate protein (mg/g mucosa) 3 homogenate CYP3A (pmol/mg of protein). Total regional intrinsic clearance (Clin,region) was then calculated from Clin,mic, and regional
CYP3A was calculated according to the following equation:
Clin,region 5 Clin,mic (ml/min/mg of protein)
4 microsomal CYP3A (pmol/mg of protein)
(2)
3 regional CYP3A (pmol)
Because mucosal masses were not recorded for every region, from
which microsomal CYP3A and intrinsic clearance were determined,
median values from equation 1 and for microsomal intrinsic clearance were used for equation 2. From these calculations, we obtained
a single estimate of 19-OH MDZ formation clearance for each of the
three regions of human small intestine. For comparison, total intrinsic clearance for the liver was calculated using a liver weight of
1500 g (Snyder et al., 1975), the median hepatic Clin,mic, and assuming an average microsomal protein recovery of 52.5 mg/g liver (Iwatsubo et al., 1997). This recovery value was obtained from hepatocyte
cultures, which we believe is more accurate than that obtained from
recovered microsomal protein.
To determine an in vitro-in vivo “scaling factor” for each intestinal
region (i.e., a number that could be used to scale microsomal intrinsic
clearance for any substrate to total regional clearance), we estimated
the total amount of microsomal protein in each region for the seven
intestines from which we had complete data sets. Assuming that
microsomal CYP3A content from a 1-foot section was representative
of the entire intestinal region, the scaling factor for each region was
calculated as follows:
Regional microsomal protein (mg) 5 Regional weight (g)
3 mucosal recovery (g mucosa/g wet weight)
3 mucosal CYP3A (pmol/g mucosa)
(3)
4 microsomal CYP3A (pmol/mg protein)
Microsomal and homogenate CYP3A content in the duodenum,
jejunum and ileum from 10 of the 20 donor organs were used to
determine the percent yield of microsomal CYP3A from homogenate
according to the following equation:
% Yield 5
Microsomal CYP3A (pmol/mg protein)
3 total microsomal protein (mg)
homogenate CYP3A (pmol/mg protein)
3 total homogenate protein (mg)
(4)
Statistical analysis. All statistical analyses were performed using Sigmastat for Windows (v1.0, Jandel Corp., San Rafael, CA).
Because several data sets failed the homogeneity-of-variance and
normality tests (Levene Median and Kolmogorov-Smirnov tests, respectively), nonparametric methods were used for the majority of
analyses. Average values were reported for comparisons of 19-OH
MDZ with 4-OH MDZ ratios because the sample sizes were ,5.
Spearman correlation coefficients (rs) were considered significant if
the P-value was ,0.05. One-way ANOVA on ranks (Kruskal-Wallis)
was used to determine whether a difference existed in the various
kinetic parameter estimates and CYP3A contents among the liver,
duodenum, jejunum and ileum (and ignoring that each set of three
intestinal regions came from the same donor and that eight livers
and intestines were matched; this resulted in a less powerful but
more conservative test). For the three intestines from which we could
not obtain reliable parameter estimates for one or two regions (due to
low CYP3A activity), median regional Km values were assumed along
with Vmax and Clin,mic values that were ranked lower than the corresponding regional minimum Vmax and Clin,mic. Likewise, for intestinal regions for which the protein band was too faint to be detected
by the densitometer, a ranking lower than the regional minimum
was assumed. Further, if ANOVA revealed a significant difference
(P , 0.05), then either the Student-Newman-Keuls test (for equal
sample sizes) or Dunn’s test (for unequal samples sizes) was used to
determine which group medians differed from the others. To compare the three intestinal regions, taking into account that each set of
three came from the same donor, three pairwise Wilcoxon signedrank tests were used along with a Bonferroni-corrected level of
significance, 0.017 (i.e., 0.05/3).
Results
Total CYP, cytochrome b5 and CYP3A contents. Western blot analysis of microsomes from 20 livers and small
intestines showed the presence of CYP3A4 protein in every
organ and CYP3A5 protein in four livers and four intestines.
A representative blot illustrating the continuous expression
of CYP3A5 along the entire length of small intestine is shown
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respectively) and base peak fragment ions ([M-tBu(CH3)2SiOH]2)
with m/z 323 and 328 (corresponding to the unlabeled and 15N3labeled 37Cl isotope of 4-OH MDZ, respectively) were monitored. The
GC column retention time for 4-OH MDZ was slightly shorter than
19-OH MDZ (12.7 vs. 14.0 min). The two metabolites were quantified
by comparing peak area ratios with standard curves prepared by the
addition of known amounts of 4-OH MDZ (0.3–30 pmol) and/or 19-OH
MDZ (1.5–300 pmol) and 100 ml of internal standard to phosphate
buffer. The interassay coefficient of variation in the slopes from the
standard curves for 4-OH MDZ and 19-OH MDZ were 8.1% and 5.2%,
respectively.
Determination of a correction factor for homogenate protein. With Western blot analysis, we compared the slope obtained
from a standard curve using purified CYP3A4 only (i.e., 0, 0.5, 1 and
2 pmol) with that using purified CYP3A4 spiked into 50 mg of jejunal
homogenate protein (i.e., 50 mg of protein plus 0.5, 1 or 2 pmol). The
slope (change in IOD/change in pmol CYP3A4 added) from the
CYP3A4-only curve was higher than that obtained from the standard
addition curve. Therefore, to correct for this “matrix effect,” five
additional jejunal homogenates were randomly chosen and subjected
to Western blot analysis as described previously. Again, the slopes
from the purified CYP3A4-only standard curves were consistently
higher than those from the standard addition curves. The correction
factor (slope without homogenate/slope with homogenate) for the six
homogenates averaged 1.26 6 0.29. The median was 1.23 and was
used to correct for the median amount of homogenate CYP3A determined from earlier blots. Only corrected medians are reported. We
did not find a noticeable matrix effect from microsomal protein and
thus did not apply a correction factor to median amounts of microsomal CYP3A.
Calculation of total CYP3A recovery and organ intrinsic
clearance. Based on a previous study of the physical characteristics
(i.e., weight and length) of human small intestine (Snyder et al.,
1975), we assigned the first 1-foot section as the duodenum, sections
2 to 9 as the jejunum and the remaining sections as the ileum. The
total mucosal scrapings mass per gram of intestinal tissue was
recorded for each section of seven organs, and regional (duodenal,
jejunum, ileal) wet weights were determined accordingly. Total
CYP3A in each of the three regions was then calculated using the
following equation:
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Paine et al.
Fig. 1. Western blot of microsomes prepared from HI-30 showing the
presence of CYP3A4 and CYP3A5 protein along the entire length of
small intestine. The nitrocellulose sheet was developed with a polyclonal anti-CYP3A4 IgG. Lanes 1 to 3, purified human CYP3A4 (0.5, 1
and 2 pmol, respectively); lane 4, duodenum; lanes 5 to 8, proximal to
distal jejunum; lanes 9 to 12, proximal to distal ileum. Lanes 4 to 12
were loaded with 50 mg of protein.
Furthermore, Dunn’s test revealed a difference between the
liver and each intestinal region (P , .05), but there was no
statistical difference in microsomal CYP3A content for all
pairwise comparisons among the three intestinal regions.
For intestinal regions from which we were able to measure
total CYP content, CYP3A represented 63%, 49% and 88%
(median percentages) of total small intestinal cytochromes
P450 in the duodenum, jejunum and ileum, respectively. In
comparison, CYP3A represented 17% of total hepatic cytochromes P450 (median value for 20 liver microsomal samples).
MDZ 1*-hydroxylation kinetics. Fifteen intestines were
examined for regional (duodenal, jejunal and ileal) MDZ 19hydroxylation kinetics. However, in some regions of three
intestines, the amount of product formed was below the limit
of quantification at low substrate concentrations and, in
some cases, at all substrate concentrations. Thus, we could
not obtain reliable kinetic parameter estimates in these
cases. Representative Eadie-Hofstee plots of 19-OH MDZ formation kinetics for each of the three intestinal regions of one
small intestine are shown in figure 2. All 20 liver microsomal
samples produced amounts of product that were readily
quantifiable (at all substrate concentrations), so we were able
to obtain reliable kinetic parameter estimates. Box plots of
Km, Vmax and Clin,mic for liver and small intestine are shown
in figure 3. Median Km values were 3.7, 3.8, 3.7 and 4.5 for
liver, duodenum, jejunum and ileum, respectively. ANOVA
revealed no differences in Km values among the four tissues
(P 5 .15). Moreover, no statistical difference was found if the
liver data were excluded from the analysis (P . .05 for all
pairwise comparisons). Median Vmax values were 850, 644,
426 and 68 pmol/min/mg for liver, duodenum, jejunum and
ileum, respectively. Not surprisingly, ANOVA revealed a statistically significant difference in Vmax among the four tissues (P 5 .0001). With Dunn’s test, only the ileum-liver
difference was significant. However, Wilcoxon signed-rank
tests revealed a significant difference between duodenum
and ileum (P 5 .002) and jejunum and ileum (P 5 .005), but
not between duodenum and jejunum (P 5 .14). Median intrinsic clearances (Clin,mic) for liver, duodenum, jejunum and
ileum were 200, 157, 85 and 14 ml/min/mg. ANOVA with
subsequent Dunn’s test and Wilcoxon signed-rank tests revealed similar contrasts as those described for Vmax.
Similar to our findings with CYP3A content, there was a
large degree of interdonor variability in Clin,mic for liver and
duodenal, jejunal and ileal regions (28-, 29-, 22- and 18-fold,
respectively). There were significant correlations between
hepatic microsomal CYP3A content and both Vmax and Clin,mic (rs 5 .80, P , .001 and rs 5 .86, P , .001, respectively).
The same correlations were also significant for duodenum
Fig. 2. Microsomal MDZ 19-hydroxylation kinetics for HI-32. EadieHofstee plots are shown for each region of small intestine. Symbols
represent observed values (average of duplicate incubations), and solid
lines represent regression lines.
Downloaded from jpet.aspetjournals.org at ASPET Journals on June 17, 2017
in figure 1. Concordant hepatic-intestinal expression of
CYP3A4 and CYP3A5 was observed in seven of the eight
matched organs (table 1). Six of the seven concordant organs
expressed CYP3A4 only, whereas one pair expressed both
CYP3A4 and CYP3A5. For the single discordant pair (HL147/HI-26), both proteins were detected in the liver, but only
CYP3A4 was detected in the intestine. Also, although a modest CYP3A4 protein band was detected in the jejunum and
ileum of HI-26, no band was detected (by the densitometer)
for the duodenum. The CYP3A5-to-CYP3A4 IOD ratio in
CYP3A5-positive organs was less than unity in all except one
liver and two ileal sections of its intestinal mate. This interpretation may be biased because the detection antibody was
raised against purified CYP3A4 protein.
The median total CYP, cytochrome b5 and total CYP3A
contents for the 20 liver microsomal samples were 0.37 nmol/
mg, 0.48 nmol/mg and 69.7 pmol/mg, respectively, with
ranges of 0.21 to 0.58 nmol/mg, 0.34 to 0.65 nmol/mg and 4.1
to 262.0 pmol/mg. Due to limited and/or very dilute quantities of several intestinal microsomes, we were not able to
obtain CYP difference spectra for all samples. Nevertheless,
the median total CYP contents for seven duodenal, six jejunal
and two ileal samples were 0.06 (range, 0.03–0.21), 0.07
(range, 0.04–0.11) and 0.05 (range, 0.03–0.07) nmol/mg, with
an overall median of 0.06 nmol/mg. Median cytochrome b5
contents for a larger number of intestinal samples (n 5 13)
were 0.19, 0.19 and 0.26 nmol/mg for duodenum, jejunum
and ileum, respectively, with ranges of 0.12 to 0.39, 0.13 to
0.40 and 0.15 to 0.49 nmol/mg. Median yields of microsomal
CYP3A protein from mucosal homogenates were 10.6%,
18.2% and 27.0% for duodenum, jejunum and ileum, respectively. The correlations between microsomal and homogenate
CYP3A content were excellent (rs 5 .92, rs 5 .93 and rs 5 .83,
P , .001, for duodenum, jejunum and ileum, respectively),
suggesting that recovered microsomes from each intestinal
region contained a fair representation of total mucosal
CYP3A protein.
Median microsomal CYP3A protein contents for each intestinal region were 30.6, 22.6, and 16.6 pmol/mg with ranges
of ,3.0 to 90.8, 2.1 to 98.0 and ,1.9 to 59.5 pmol/mg, respectively (n 5 20). Median duodenal, jejunal and ileal values
were 44%, 32% and 24% of the median total hepatic CYP3A
content, respectively. ANOVA revealed a significant difference in microsomal CYP3A content among the four tissues.
Vol. 283
1997
1557
and jejunum but not for ileum. With respect to Vmax, the
correlation coefficients were rs 5 .84 (P , .001) for duodenum, rs 5 .69 (P 5 .004) for jejunum and rs 5 .59 (P 5 .05) for
ileum. For Clin,mic, the correlation coefficients were rs 5 .85
(P , .001) for duodenum, rs 5 .61 (P 5 .015) for jejunum and
rs 5 .29 (P 5 .35) for ileum.
Distribution of intestinal CYP3A, cytochrome b5 and
CYP reductase activities. To further define the CYP3A
gradient along the entire small intestine and determine the
role of coenzymes in the regional variation of CYP3A activity,
the following were measured along the entire length of six
small intestines (HI-29, -30, -31, -32, -33 and -35): CYP3A
catalytic activity (19-OH MDZ formation rate), CYP3A protein content, NADPH- and NADH-dependent cytochrome c
reduction rates (as measures of CYP reductase and cytochrome b5 reductase activity, respectively) and cytochrome b5
protein content. For four of six intestines (HI-30, -32, -33 and
-35), CYP3A catalytic activity increased from duodenum to
middle jejunum and then decreased to distal ileum (fig. 4).
For HI-31, activity was highest in the first 1-foot section
(duodenum) and declined thereafter. For HI-29, activity was
low for the first 5 to 6 feet of bowel, increased dramatically in
midjejunum and declined thereafter. Fold-differences between the section with the highest activity vs. that with the
lowest activity (distal ileum in all cases) were 3-fold (HI-31
and HI-35) to 14-fold (HI-29, excluding duodenum; HI-32) to
25-fold (HI-30 and HI-33). Intraintestinal variability in
CYP3A protein content was much less than that for CYP3A
activity and ranged from 1.5-fold (HI-31) to 4.6-fold (HI-30).
Correlations between CYP3A activity and protein content
within an intestine were significant for only four of the six
intestines (HI-30, -32, -33 and -35; rs 5 .83, .98, .71 and .90,
respectively).
To compare distribution patterns across individuals, all
measurements within each donor intestine were normalized
to the peak value. Median values, by foot-section, were then
determined among the six intestines (fig. 5). The variations
in CYP3A activity and protein content paralleled each other
throughout the duodenum and jejunum and began to diverge
Fig. 4. Longitudinal MDZ 19-hydroxylation rates (at
8 mM MDZ) for six different donor intestines. Foot
sections 1 to 15 represent duodenal to ileal ends.
For simplicity, only the first eight odd sections are
shown. For those intestines that had more than
eight odd sections (30, 32 and 35), the ensuing
distal value was less than or very near the preceding value.
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Fig. 3. Box plots of Km (top), Vmax (middle) and Clin,mic (bottom) in
microsomes prepared from liver (L), duodenum (D), jejunum (J) and
ileum (I). Each box represents the 25th to 75th percentile, and the
central line represents the median. Upper and lower “whiskers” represent the largest and smallest nonoutlying values, respectively. F, Outliers (values between 1.5 and 3 box-lengths from the upper or lower
edge). E, Extremes (values .3 box-lengths from the upper or lower
edge). * Median significantly different from liver median (P , .05,
Kruskal-Wallis). ** Median significantly different from duodenal and jejunal medians (P , .01, Wilcoxon signed-rank test). See text for explanation of statistical tests.
Variability in Human Intestinal CYP3A
1558
Paine et al.
Vol. 283
TABLE 2
Mean MDZ product ratios
Intestine
Liver
1-OH/4-OH
MDZ
CYP3A52
CYP3A51
mM
0.25
1
4
8
1-OH/4-OH
MDZ
CYP3A52
CYP3A51
10.2
(1.3)
9.8
(1.2)
7.8
(0.8)
6.2
(0.6)
24.0
(11.9)
23.5
(9.8)
16.8
(5.7)
12.3
(3.5)
mM
8.3
(0.8)
8.5
(0.4)
7.0
(0.2)
5.7
(0.1)
12.6
(2.5)
11.0
(1.0)
9.7
(2.1)
7.8
(1.7)
0.25
1
4
8
Standard deviations are shown in parentheses.
at the ileum. CYP3A activity continued to decline, whereas
CYP3A content remained relatively constant.
In general, CYP reductase activity paralleled CYP3A activity; moreover, it exhibited similar intraindividual variabilities as for CYP3A protein, from 2-fold (HI-31 and HI-33) to
4-fold (HI-30). Cytochrome b5 content and cytochrome
b5reductase activity followed more even patterns of distribution across regions (fig. 5). Consequently, there was less
intraindividual variability in these measurements (up to 1.9fold for cytochrome b5 content and up to 3.5-fold for cytochrome b5 reductase activity). There was a trend for cytochrome b5 content to increase towards the ileum (fig. 5).
1*-OH MDZ to 4-OH MDZ ratios. MDZ metabolite ratios
(19-OH MDZ/4-OH MDZ) were measured in microsomes prepared from six jejunums and six livers. Three livers and three
intestines with the highest CYP3A5/CYP3A4 IOD ratios (HI30, -31 and -35 and HL-125, -127 and -150) were selected and
compared with randomly chosen CYP3A5-negative organs
(HI-19, -20 and -28 and HL-107, -122 and -148). Results are
summarized in table 2. Ratios for CYP3A5-positive jejunums
were higher than CYP3A5-negative jejunums at all four substrate concentrations examined. Interestingly, the jejunum
with the highest CYP3A5/CYP3A4 IOD ratio (0.64 for HI-30)
had the highest product ratio at all substrate concentrations.
Product ratios for CYP3A5-positive livers were also higher
than CYP3A5-negative livers at all four substrate concentrations examined (except for HL-125 at 0.25 mM MDZ). Mean
product ratios for CYP3A5-negative livers were similar to the
corresponding ratios for CYP3A5-negative jejuna. In contrast, the mean product ratios for CYP3A5-positive livers
were greater than the corresponding ratios for CYP3A5-positive jejuna.
Fig. 6. Estimated amounts of total CYP3A and total intrinsic clearance
for duodenum, jejunum and ileum. For comparison, estimated total
hepatic CYP3A and intrinsic clearance were 5490 nmol and 15.8 l/min,
respectively.
Downloaded from jpet.aspetjournals.org at ASPET Journals on June 17, 2017
Fig. 5. Median values, by section, for microsomal CYP3A activity,
CYP3A protein content, CYP reductase activity (top) and cytochrome
b5 and cytochrome b5 reductase activity (bottom) for six different small
intestines. Foot section numbers represent the same as described for
figure 4. All measurements within each donor intestine were normalized
to the peak value for that donor.
Total regional intestinal intrinsic clearances and
scaling factors. Total intrinsic clearances for duodenum,
jejunum and ileum (Clin,region) were estimated using microsomal intrinsic clearance (Clin,mic), microsomal CYP3A content and total regional CYP3A data. Total regional CYP3A
was calculated using total wet weights of each region, mucosal recovery (as percent of tissue wet weight) and mucosal
CYP3A content (based on corrected homogenate CYP3A content and homogenate mass/g mucosa). Because we had incomplete data collection, median values for microsomal intrinsic clearance (n 5 15), microsomal CYP3A content (n 5
20), regional wet weight (n 5 7), mucosal recovery (n 5 7) and
mucosal CYP3A content (n 5 10) were used for our calculations. Median regional wet weights were 79, 411 and 319 g
for duodenum, jejunum and ileum, respectively. Mucosal
masses represented 23%, 16% and 12% of total wet weights.
Total median CYP3A contents were 445, 463 and 391 pmol/g
mucosa, or 102, 74 and 47 pmol/g wet weight. Total regional
CYP3A was thus 9.7, 38.4 and 22.4 nmol for duodenum,
jejunum and ileum, respectively (fig. 6). Median Clin,mic values were 157, 85 and 14 ml/min/mg. Therefore, from equation
2, Clin,region values were calculated to be 50.1, 144.3 and 19.3
ml/min, respectively. The sum of these three values, total gut
1997
Variability in Human Intestinal CYP3A
intrinsic clearance, was 213.7 ml/min. For comparison, we
calculated total hepatic CYP3A to be 5490 nmol (69.7 pmol
CYP3A/mg 3 52.5 mg/g 3 1500 g 3 1 nmol/1000 pmol) and
an intrinsic clearance of 15.8 l/min (0.2 ml/min/mg 3 52.5
mg/g 3 1500 g 3 1 liter/1000 ml).
From the seven intestines for which we had complete data
sets, the total microsomal protein mass (i.e., a scaling factor)
was calculated for each region. Median values were 375, 1584
and 1019 mg for duodenum, jejunum and ileum, respectively.
Therefore, the total microsomal mass for the whole intestine
was 2977 mg. For comparison, the total microsomal mass for
the liver was estimated to be 78,750 mg (52.5 mg/g 3 1500 g),
or 78.8 g.
Discussion
lower interindividual variability were observed. In contrast,
the ranges of CYP3A content we observed for different donors
were considerable (3–90, 2–98 and 2–38 pmol/mg for duodenum, jejunum, and ileum, respectively) and are likely to be
even larger because we were not able to accurately quantify
some very faint protein bands. The source of this interindividual variability is not completely known. It could not be
explained by length of hospitalization, prior drug therapy,
age or sex of the donor population. Therefore, based on the
similar degree of variability in the present study, in which
the intestinal mucosae had been largely unexposed to potential dietary modulators for at least 48 hr before procurement,
and that reported for duodenal biopsies from healthy volunteers (Lown et al., 1994), we speculate that these interindividual differences are derived largely from homeostatic
mechanisms (e.g., enterocyte differentiation, hormonal control of gene expression and enterocyte turnover).
Rates of intestinal NADPH-dependent cytochrome c reduction (range, 12–150 nmol/min/mg across all sections of the six
intestines studied) agreed with those reported for 54 liver
microsomal preparations (Schmucker et al., 1990; range, 38 –
135 nmol/min/mg) and six ileal microsomal preparations
(Pacifici et al., 1989; range, 32–106 nmol/min/mg). Interestingly, HI-33 had the lowest CYP reductase activity, ,30
nmol/min/mg for all sections, which may explain why this
intestine had low CYP3A activity despite moderate CYP3A
content. Furthermore, the three most proximal and one most
distal section of HI-29 and the four ileal sections of HI-30 also
had low CYP reductase activity (,30 nmol/min/mg). Again,
this may explain why we were not able to obtain kinetic
parameter estimates from some regions of these intestines.
With respect to intraintestinal comparisons, a significant
correlation between CYP3A content and catalytic activity
was observed for four of the six full-length organs examined,
indicating that for the majority of individuals, enzyme function parallels protein content. Low intraintestinal variability
in CYP3A content and activity (HI-31) or low CYP reductase
activity (HI-29) may explain the poor correlations observed
for the remaining two intestines. Furthermore, correlations
between duodenal and jejunal Vmax (rs 5 .81) or Clin,mic (rs 5
.76) for the larger set of 15 donors were highly significant
(P , 0.0001), suggesting that for most subjects, an analysis of
duodenal pinch biopsies will provide representative metabolic information for the entire proximal intestine.
19-OH MDZ was the dominant metabolite in all intestine
and liver microsomal preparations and at all MDZ concentrations examined (0.25– 8.0 mM). This is in agreement with
previous reports of high 19-/4-OH MDZ ratios for hepatic
microsomes when substrate concentrations were ,10 mM
(Gorski et al., 1994; Kronbach et al., 1989). Collectively, these
in vitro findings are consistent with in vivo observations
(Heizmann and Ziegler, 1981) that the 4-hydroxylation pathway contributes little to the systemic metabolism of MDZ in
vivo because plasma MDZ concentrations are typically well
below 1 mM.
The similarity in Km values for MDZ 19-hydroxylation (;4
mM) for the three intestinal regions and liver, together with
strong correlations between Vmax and CYP3A content in both
organs (except for ileum), suggest that hepatic and proximal
gut CYP3A are functionally equivalent. Although not statistically different, ileal Km values tended to be higher than
corresponding duodenal and jejunal values. This, along with
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Results from this study provide the first comprehensive
characterization of intraintestinal and interintestinal variability in CYP3A expression and metabolic function. Although previous studies involved either a limited sample size
(De Waziers et al., 1990; McKinnon et al., 1995; Peters and
Kremers, 1989; Prueksaritanont et al., 1996; Thummel et al.,
1996) or collection of duodenal tissue only (Kivisto et al.,
1996; Lown et al., 1994), we examined 20 full-length intestines. Nearly all were obtained from donors who had been
hospitalized for no more than 4 days before procurement (one
donor, HI-33, was hospitalized for 12 days). The organs were
also procured under carefully controlled conditions, with a
limited period of cold ischemia time and no warm ischemia
time. Thus, although not identical to biopsy tissue from
healthy volunteers, we believe that potential differences between donor and biopsy tissue with respect to mucosal
CYP3A stability (protein content and catalytic activity) were
kept to a minimum.
Our findings that CYP3A is the major cytochrome P450
expressed in all three regions of the human small intestine
fully agree with results reported by Watkins et al. (1987) and
De Waziers et al. (1990). In contrast, CYP3A represented a
smaller percentage of total hepatic CYP compared with the
mean percentage reported by Shimada et al. (1994) (17% vs.
29%). For a fairer comparison, we calculated a mean percentage of 20%, which is still lower than the estimate of previous
investigators. The discrepancy is due to a slightly higher
mean total CYP content and a lower mean CYP3A content
observed in the present study compared with those from the
earlier study (0.38 vs. 0.34 nmol/mg and 83 vs. 96 pmol/mg,
respectively). Nevertheless, small intestinal CYP3A represents a larger percentage of total small intestinal CYP compared with hepatic CYP3A as a percentage of total hepatic
CYP.
We found large interindividual variability in CYP3A content for all three regions of the small intestine, supporting
the findings reported by Lown et al. (1994), who measured
CYP3A content in 20 duodenal pinch biopsies. Moreover, we
corroborated the results reported by De Waziers et al. (1990),
who observed a progressive decline in microsomal CYP3A
content from duodenum to jejunum to ileum. The regional
differences we observed, however, were not as great. The
previous investigators relied on small and uneven sample
sizes (n 5 2, 6 and 5 for duodenum, jejunum and ileum,
respectively); thus, larger interregional differences and a
1559
1560
Paine et al.
gradually decreases from proximal to distal ends, is ;8 times
longer than the duodenum and thus has the greatest total
amount of CYP3A. Total intrinsic clearances followed a similar trend except that the duodenal value was higher than the
ileal value (50 vs. 19 ml/min). Again, this is reflective of a
metabolically active duodenum and a relatively deficient ileum.
Because the total hepatic intrinsic clearance was .70
times that for the small intestine (15.8 and 0.21 l/min for
liver and intestine, respectively), it appears that the gut
should contribute very little to the metabolism of MDZ in
vivo. However, recent studies provide convincing evidence
that both organs contribute equally, on average, to the firstpass metabolism of this drug (Paine et al., 1996; Thummel et
al., 1996). Although unbound intrinsic clearance is an important determinant of first-pass extraction by both the liver
and intestine, there is no a priori reason to believe that the
relationships between the extraction ratio and the unbound
intrinsic clearance, organ blood flow and plasma protein
binding should be the same for all organs of elimination
(Wilkinson, 1987). The effect of plasma protein binding and
hepatic blood flow on hepatic extraction is fairly well defined,
but their impact on gut extraction is unclear. Although exposure of absorbed drug molecules to enterocytic CYP3A
might be obligatory for drugs absorbed transcellularly, drug
access to hepatic CYP3A depends on the translocation of
unbound drug molecules across the sinusoidal membrane
into the parenchymal cell. In addition, although blood flow
should govern the residence time of drug molecules in the
sinusoid and in the intestinal villus, limiting their exposure
to CYP3A, the ratio of blood flow to intrinsic clearance may
be greater for liver than for the intestinal mucosa.
If we assume a liver plasma flow of 0.78 l/min (MDZ does
not partition appreciably into erythrocytes), an unbound
fraction of 0.02 (Thummel et al., 1996) and a total hepatic
unbound intrinsic clearance of 15.8 l/min and apply these to
the well-stirred model for hepatic clearance (Rowland et al.,
1973), an in vivo 19-OH MDZ formation clearance of 0.23
l/min was predicted. By performing the same calculations for
small intestine and assuming a total wet weight of 809 g, a
mucosal plasma flow of 0.16 l/min (Hultén et al., 1977), the
same unbound fraction and a total unbound intrinsic clearance of 0.21 l/min, an in vivo systemic clearance of 4.1 3 1023
l/min was predicted. This, together with the predicted hepatic 19-OH MDZ formation clearance (0.23 l/min) being in
excellent agreement with the average and median systemic
formation clearance we observed in healthy volunteers (0.26
and 0.24 l/min, respectively; Thummel et al., 1996), confirms
our finding that the gut contributes little to the systemic
clearance of MDZ (Paine et al., 1996).
In vivo organ extraction ratios based on systemic delivery
of drug were predicted to be 0.29 and 0.03 for liver and small
intestine, respectively. Both are within the range of values
observed after intravenous administration (Paine et al.,
1996; Thummel et al., 1996). Pond and Tozer (1984) and
Mistry and Houston (1987) have suggested that the same
relationship for intestinal systemic extraction should apply
to intestinal first-pass extraction. However, the gut prediction is .10 times lower than the average first-pass extraction
ratio observed after intraduodenal administration (0.43;
Paine et al., 1996). If the unbound fraction is excluded from
the gut calculation, the extraction ratio increases to 0.57,
Downloaded from jpet.aspetjournals.org at ASPET Journals on June 17, 2017
a poor correlation between ileal Vmax and CYP3A content,
implies that ileal CYP3A behaves differently from more proximal CYP3A. Data from our longitudinal studies further support this belief, where median ileal CYP3A activity decreased
to a greater extent than did CYP3A content (fig. 5). Median,
normalized CYP3A activity decreased 42% from sections 11
to 13 and 47% from sections 13 to 15, whereas CYP3A content decreased 10% and 19%, respectively. Overall, the data
indicate a much lower metabolic capacity for the distal compared with the proximal small intestine.
Although we observed a 1-in-5 frequency of CYP3A5 expression in both small intestine and liver, Lown et al. (1994)
reported a frequency of 70% for small intestine, and Jounaı̈di
et al. (1996) reported a frequency of 74% for liver. This
discrepancy is likely due to different protein detection methods (the other investigators used the more sensitive enhanced chemiluminescence technique plus prolonged exposure). Nevertheless, in agreement with these earlier studies,
CYP3A5 was generally a minor component of total CYP3A,
which intimates that for most individuals, CYP3A5 plays a
minor role in the intestinal and hepatic first-pass extraction
of CYP3A substrates in vivo.
Before our analysis of matched liver and small intestinal
tissue, we expected the factors regulating the maintenance of
CYP3A5 protein in a given individual to be operative in both
organs. This did not always appear to be the case, however.
The one discordant pair exhibited a clear immunoreactive
CYP3A5 band for liver but not for intestine. Although the
reason for this is unclear, it is possible that this intestine
received an insult either before or during procurement, leading to enzyme degradation and a decrease in CYP3A5 below
the limit of detection. Interestingly, for the two intestines in
which the CYP3A5 band was quantifiable (HI-30 and HI-31),
the CYP3A5-to-CYP3A4 ratio decreased from duodenum to
jejunum and then increased in ileum to values comparable to
or greater than those observed for the duodenum. This is
consistent with previous reports of predominantly CYP3A5
expression in the stomach and colon (Gervot et al., 1996;
Kolars et al., 1994; Peters and Kremers, 1989).
There was no clear trend between the eight matched livers
and intestines with respect to MDZ 19-hydroxylation; that is,
if the liver had a high Clin,mic, the intestine did not necessarily have a high Clin,mic. An extreme case is the pair HL-147
and HI-26, where the liver exhibited one of the highest hepatic Vmax and Clin,mic values and the intestine had virtually
no metabolic activity. Another case is the pair HL-148 and
HI-27, where the liver had moderately high Vmax and Clin,mic
values and the intestine had low Vmax and Clin,mic values.
Some pairs did exhibit parallel CYP3A activities, such as
HL-146/HI-24 and HL-150/HI-30. Collectively, these findings
support in vivo observations reported by others (Lown et al.,
1994), who found that hepatic and intestinal CYP3A (measured as the erythromycin breath test and 19-OH MDZ formation in duodenal biopsies, respectively) are not coordinately regulated.
We estimated the duodenum to contain almost half the
amount of CYP3A compared with the ileum (9.7 vs. 22 nmol),
which is striking given that the ileum is ;10 times longer
than the duodenum (;10 feet vs. 1 foot; Snyder et al., 1975).
The discrepancy lies in the duodenal mucosa being extremely
rich in villi, the tips of which are lined with mature CYP3Acontaining enterocytes. The jejunum, whose villus density
Vol. 283
1997
Acknowledgments
The authors wish to thank the Northwest Organ Procurement
Agency for their assistance in the collection of donor tissues.
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which is closer to the first-pass gut extraction ratio observed
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it plays a negligible role in determining the first-pass intestinal extraction of the drug.
In summary, our extensive characterization of intestinal
CYP3A metabolic capacity revealed that interorgan variability is much greater than intraorgan variability with
respect to CYP3A expression and catalytic activity. This
may account for the large interindividual differences observed in the oral bioavailabilities of some CYP3A substrates. Furthermore, our findings indicate that the upper
small intestine (duodenum and proximal jejunum) serves
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unpredictable bioavailabilities. Alternatively, the low metabolic capacity of the ileum implies that slow-release preparations may largely be “spared” an intestinal first-pass
effect. Finally, a comparison of predicted extraction ratios
for the small intestine and liver suggests that protein
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apply to first-pass metabolic extraction by the intestine.
Variability in Human Intestinal CYP3A
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Send reprint requests to: Kenneth E. Thummel, Ph.D., Department of
Pharmaceutics, Box 357610, University of Washington, Seattle, WA 98195.
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