In silico modeling for the nonlinear absorption kinetics of UK-343,664: a P-gp and CYP3A4 substrate

The aim of this work was to extrapolate in vitro and preclinical animal data to simulate the pharmacokinetic parameters of UK-343,664, a P-glycoprotein (P-gp) and CYP3A4 substrate, in human. In addition, we aimed to develop a simulation model to demonstrate the involvement and the controversial complex interaction of intestinal P-gp and CYP3A4 in its nonlinear absorption, first-pass extraction, and pharmacokinetics using the advanced compartmental absorption and transit (ACAT) model. Finally, we aimed to compare the results predicted from the model to the reported findings in human clinical studies. In situ perfusion, allometric scaling, PBPK Rodger mechanistic approach, in vitro metabolism, and fitting to in vivo data were used to mechanistically explain the absorption, distribution and metabolism, respectively. GastroPlus was used to build the integrated simulation model in human for UK-343,664 to mechanistically explain the observed clinical data at 30, 100, 200, 400, and 800 mg oral doses. The measured in vitro value for CYP3A4 K(m) (465 μM) in rCYPs was converted to units of μg/mL, corrected for assumed microsomal binding (17.8%) and applied to all metabolic processes. The measured in vitro values of V(max) for CYP3A4 (38.9 pmol/min/pmol), 2C8, 2C9, 2C19, and 2D6 were used along with the in vitro CYP3A4 K(m) to simulate liver first pass extraction and systemic clearance. The measured in vitro values of V(max) for CYP3A4 and 2D6 were used along with the in vitro CYP3A4 K(m) to simulate gut first pass extraction. V(max) and K(m) values for P-gp were obtained by fitting to in vivo data and used to simulate gut efflux transport activity. Investigation of the interaction mechanism of P-gp and CYP3A4 in the intestine was achieved by comparing the influence of a virtual knockout of P-gp or gut metabolism on the fraction absorbed, fraction reaching the portal vein, and fraction metabolized in the gut. Comparison between simulation and in vivo results showed that the in silico simulation provided a mechanistic explanation of the observed nonlinear absorption kinetics of UK-343,664 in human following its administration in the range of 30-800 mg as oral solutions. The simulation results of the pharmacokinetic parameters, AUC and C(max), by GastroPlus were comparable with those observed in vivo. This simulation model is one possible mechanistic explanation of the observed in vivo data and suggests that the nonlinear dose dependence could be attributed to saturation of both the efflux transport by P-gp and the intestinal metabolism. However, the concentration ranges for either protein saturation did not overlap and resulted in much greater than dose proportional increases in AUC. At low doses, producing intraenterocyte concentrations below the fitted value of K(m) for P-gp, the influence of P-gp appears to be protective and results in a lower fraction of gut 3A4 metabolism. At higher doses, as P-gp becomes saturated the fraction of gut 3A4 extraction increases, and eventually at the highest doses, where 3A4 becomes saturated, the fraction of gut 3A4 extraction again decreases. Such a complex interpretation of this in vitro-in vivo extrapolation (IVIVE) is another example of the value and insight obtained by physiologically based absorption simulation.     

Potential role for P-glycoprotein in the non-proportional pharmacokinetics of UK-343,664 in man

1. UK-343,664 is a potent and specific PDE5 inhibitor. Following single oral doses to human volunteers, it exhibited non-proportional pharmacokinetics over the dose range 30-800 mg. Over this 27-fold dose range, Cmax and AUCt increased 247- and 287-fold respectively. The half-life (4-6 h) was similar at all doses. No systemic exposure was quantifiable at doses <10 mg. 2. UK-343,664 is a lipophilic molecule (log D7.4 = 3.1) and as such is expected to be cleared mainly by metabolism. Based on studies with expressed human P450 enzymes it was concluded that the metabolism of UK-343,664 was predominantly mediated by CYP3A4. With a moderate Km = 76 microM for this enzyme, saturation of first-pass metabolism alone was considered unlikely to account for the non-proportional pharmacokinetics. 3. UK-343,664 showed high affinity for P-glycoprotein in vitro, with a Km = 7.3 microM. In transport studies in LLC-PK1 cell monolayers transfected with P-glycoprotein, UK343,664 showed marked polarized transport which was concentration dependent. 4. The high affinity of UK-343,664 for P-glycoprotein is considered to be the primary source of the non-proportional pharmacokinetic profile observed in man.     

Regional transport and metabolism of ropivacaine and its CYP3A4 metabolite PPX in human intestine

The major aim of this study was to investigate the CYP3A4 metabolism and polarized transport of ropivacaine and its metabolite 2',6'-pipecoloxylidide (PPX) in tissue specimens from the human small and large intestine. Ropivacaine has been shown to be effective in the treatment of ulcerative colitis in human colon. This study was conducted using a modified Ussing-chamber technique with specimens from jejunum, ileum and colon collected from 11 patients. The local kinetics of ropivacaine and PPX were assessed from their concentration-time profiles in mucosal and serosal compartments. The permeability (P(app)) in the absorptive direction for both ropivacaine and PPX increased regionally in the order jejunum < ileum < colon. Ropivacaine was not found to be subjected to any carrier-mediated intestinal efflux. However, the CYP3A4 metabolite left the human enterocyte in a polarized manner and both the extent of CYP3A4 metabolism of ropivacaine and the extrusion of its metabolite to the mucosal chamber were more efficient in jejunum than in ileum. P-glycoprotein was probably not involved in the metabolite extrusion. No other metabolite than PPX was found. This in-vitro study with human intestinal tissues provides new mechanistic insights into regional transport and metabolism of drugs.     

Effect of ketoconazole on the pharmacokinetics of ornidazole--a possible role of p-glycoprotein and CYP3A

The influence of ketoconazole, a modulator of P-glycoprotein (P-gp), on the exsorption of ornidazole from everted sacs of rat intestine (duodenum, jejunum and ileum) was investigated. The effect of ketoconazole pretreatment on the pharmacokinetics of ornidazole was also studied in eight healthy human volunteers. After overnight fasting ornidazole 500 mg was administered before and after pretreatment with ketoconazole 200 mg once daily for 7 days. Serum samples were analyzed by reversed phase HPLC. Significant differences were observed in pharmacokinetic parameters C(max), AUC(0-t), AUC(0-infinity), T(max) and clearance. Ornidazole is believed to be metabolized through CYP3A and it has considerable intestinal efflux, which was observed from the in vitro study. The altered pharmacokinetic parameters can be attributed to ornidazole efflux from the blood to the intestine and its metabolism by CYP3A in the intestine.     

Decreased intestinal CYP3A and P-glycoprotein activities in rats with adjuvant arthritis

Adjuvant-induced arthritis (AA) rats have been used as an animal model for rheumatoid arthritis. Several studies have shown that the pharmacokinetics of a number of drugs are altered in AA rats. We investigated the effects of AA on the barrier functions of the intestine using a rat model. Intestinal CYP3A activities (midazolam 1'-hydroxylation and 7-benzyloxy-4-(trifluoromethyl)-coumarin 7-hydroxylation) in AA rats were significantly decreased compared with those in normal rats, with marked decrease observed in the upper segment of intestine. Intestinal P-glycoprotein (P-gp) activity at upper segment was also significantly decreased in AA rats to 60% of that in normal rats, and the other segments (middle and lower) of intestine also exhibited tendencies toward decrease in P-gp activity. This decrease was supported by the finding that levels of mdr1a mRNA and P-gp protein were decreased in AA rats. No significant differences were observed in intestinal paracellular and transcellular permeability between AA and normal rats. These results suggest that intestinal CYP3A and P-gp activities are decreased in AA rats, and that the pharmacokinetics and bioavailabilities of drugs whose membrane permeation is limited by intestinal CYP3A and/or P-gp may be altered in rheumatic diseases.     

Effect of experimental acute renal and hepatic failure on absorption of tacrolimus in rat small intestine

The objective of this study is to evaluate the effect of acute renal or hepatic failure on the intestinal absorption of tacrolimus. Simultaneous perfusion study in rat small intestine revealed that the extent of absorption into blood vessels was decreased in the jejunum and the ileum of rat of acute renal failure due to the decrease in the uptake of tacrolimus into enterocytes. In contrast, there observed no significant changes in tacrolimus absorption in rat of acute hepatic failure. Since it has been reported that tacrolimus absorption is regulated mainly by Cytochrome P-450 (CYP) mediated metabolism in the jejunum, but by P-glycoprotein (P-gp) mediated efflux in the ileum, these factors might contribute to the changes in intestinal absorption of tacrolimus in rat of acute renal failure. Enzyme inhibitor, ketoconazole, was co-perfused with tacrolimus to specify the effect of CYP and P-gp. However, since ketoconazole failed to recover the permeability in the jejunum and ileum of rat of acute renal failure, it is considered that the changes in CYP or P-gp functions might not be involved in the decreased uptake of tacrolimus. This type of kinetic study in rats should be valuable to identify the precise mechanisms of drug absorption and the effects of various diseases on it, such as acute renal or hepatic failure.     

Potential role for P-glycoprotein in the nonproportional pharmacokinetics of UK-343,664 in man

1. UK-343,664 is a potent and specific PDE5 inhibitor. Following single oral doses to human volunteers, it exhibited non-proportional pharmacokinetics over the dose range 30-800mg. Over this 27-fold dose range, C_max and AUC_t increased 247- and 287-fold respectively. The half-life (4-6?h) was similar at all doses. No systemic exposure was quantifiable at doses &;lt;10?mg. 2. UK-343,664 is a lipophilic molecule (log D_7.4 = 3.1) and as such is expected to be cleared mainly by metabolism. Based on studies with expressed human P450 enzymes it was concluded that the metabolism of UK-343,664 was predominantly mediated by CYP3A4. With a moderate K_m     

Expression of clinically relevant drug‐metabolizing enzymes along the human intestine and their correlation to drug transporters and nuclear receptors: An intra‐subject analysis

The oral bioavailability of many drugs is highly influenced not only by hepatic but also by intestinal biotransformation. To estimate the impact of intestinal phase I and II metabolism on oral drug absorption, knowledge on the expression levels of the respective enzymes is an essential prerequisite. In addition, the potential interplay of metabolism and transport contributes to drug disposition. Both mechanisms may be subjected to coordinative regulation by nuclear receptors, leading to unwanted drug‐drug interactions due to induction of intestinal metabolism and transport. Thus, it was the aim of this study to comprehensively analyse the regional expression of clinically relevant phase I and II enzymes along the entire human intestine and to correlate these data to expression data of drug transporters and nuclear receptors of pharmacokinetic relevance. Gene expression of 11 drug‐metabolizing enzymes (CYP2B6, 2C8, 2C9, 2C19, 2D6, 3A4, 3A5, SULT1A, UGT1A, UGT2B7, UGT2B15) was studied in duodenum, jejunum, ileum and colon from six organ donors by real‐time RT‐PCR. Enzyme expression was correlated with expression data of the nuclear receptors PXR, CAR and FXR as well as drug transporters observed in the same cohort. Intestinal expression of all studied metabolizing enzymes was significantly higher in the small intestine compared to colonic tissue. CYP2B6, CYP2C9, CYP2C19, CYP2D6, CYP3A4/5, SULT1A, UGT1A and UGT2B7 expression increased from the duodenum to jejunum but was markedly lower in the ileum. In the small intestine, that is, the predominant site of drug absorption, the highest expression has been observed for CYP3A4, CYP2C9, SULT1A and UGT1A. In addition, significant correlations were found between several enzymes and PXR as well as ABC transporters in the small intestine. In conclusion, the observed substantial site‐dependent intestinal expression of several enzymes may explain regional differences in intestinal drug absorption. The detected correlations between intestinal enzymes, transporters and nuclear receptors provide indirect evidence for their coordinative expression, regulation and function in the human small intestine.     

Active secretion and enterocytic drug metabolism barriers to drug absorption

Intestinal phase I metabolism and active extrusion of absorbed drug have only recently been recognized as major determinants of oral drug bioavailability. Both CYP3A4, the major phase I drug metabolizing enzyme in humans, and the multidrug efflux pump, P-glycoprotein (P-gp), are present at high levels in the villus enterocytes of the small intestine, the primary site of absorption for orally administered drugs. Moreover, these proteins are induced by many of the same compounds and demonstrate a broad overlap in substrate and inhibitor specificities, suggesting that they act as a concerted barrier to drug absorption. Clinical studies have demonstrated that inhibition of CYP3A4-mediated intestinal metabolism can significantly improve the oral bioavailability of a wide range of drugs. Intestinal P-gp is a major route of elimination for both orally and intravenously administered anticancer drugs in animal models, and experiments with the Caco-2 cell line have provided strong evidence that inhibition of intestinal P-gp is another means by which oral drug bioavailability could be enhanced.     

Contributions of hepatic and intestinal metabolism and P-glycoprotein to cyclosporine and tacrolimus oral drug delivery

The objective of this section is to evaluate the contributions of hepatic metabolism, intestinal metabolism and intestinal p-glycoprotein to the pharmacokinetics of orally administered cyclosporine and tacrolimus. Cyclosporine and tacrolimus are metabolized primarily by cytochrome P450 3A4 (CYP3A4) in the liver and small intestine. There is also evidence that cyclosporine is metabolized to a lesser extent by cytochrome P450 3A5 (CYP3A5). Cyclosporine and tacrolimus are also substrates for p-glycoprotein, which acts as a counter-transport pump, actively transporting cyclosporine and tacrolimus back into the intestinal lumen. Traditional teaching of clinical drug metabolism has been that hepatic metabolism is of primary importance, and other sites of metabolism play a relatively minor role. It appears as though intestinal metabolism plays a much greater role in the pharmacokinetics of orally administered drugs than previously thought. Intestinal metabolism may account for as much as 50% of oral cyclosporine metabolism. There are at least two components of intestinal metabolism for cyclosporine and tacrolimus, intestinal CYP3A4/CYP3A5 and intestinal p-glycoprotein activities. The quantity of intestinal enzymes, although highly variable, do not appear to be the key to explaining the variability of oral cyclosporine pharmacokinetics in kidney transplant patients. However, the quantity of intestinal p-glycoprotein accounts for approximately 17% of the variability in oral cyclosporine pharmacokinetics. It may be that p-glycoprotein maximizes drug exposure to intestinal enzymes, thus decreasing the importance of enzyme quantity. Since cyclosporine's FDA approval in 1983, there have been many reports of clinically significant drug interactions of other agents when given concomitantly with cyclosporine. With the FDA approval of tacrolimus in 1994, a similar pattern of clinically significant drug interactions appears to be emerging. It seems that compounds that alter (either induce or inhibit) CYP3A4 and/or p-glycoprotein will alter the oral pharmacokinetics of cyclosporine and tacrolimus. It should be expected that, until further data are available, the drugs which interact with cyclosporine will also interact with tacrolimus.     

The impact of P-glycoprotein efflux on enterocyte residence time and enterocyte-based metabolism of verapamil.

P-glycoprotein (P-gp) can limit the intestinal permeability of a number of compounds and may therefore influence their exposure to metabolizing enzymes within the enterocyte (e.g. cytochrome P450 3A, CYP 3A). In this study, the intestinal metabolic profile of verapamil, the influence of P-gp anti-transport on the cellular residence time of verapamil, and the impact of this change in residence time on the extent of enterocyte-based metabolism have been investigated in-vitro, utilizing segments of rat jejunum and side-by-side diffusion chambers. Verapamil exhibited concentration-dependent P-gp efflux and CYP 3A metabolism. The P-gp efflux of verapamil (1 microM) increased the cellular residence time across the intestinal membrane (approximately 3-fold) in the mucosal to serosal (m to s) direction relative to serosal to mucosal (s to m), yielding significantly greater metabolism (approximately 2-fold), presumably as a result of the prolonged exposure to CYP 3A. Intestinal metabolism of verapamil generated not only norverapamil, but resulted also in the formation of an N-dealkylated product (D-617). Norverapamil and D-617 accumulated significantly in mucosal chambers, relative to serosal chambers, over the time course of the experiment. Based on these in-vitro data, it was apparent that P-gp efflux prolonged the cellular residence time of verapamil (m to s) and therefore increased the extent of intestinal metabolism, and also played a role in metabolite secretion from within the enterocyte.     

CYP3A in horse intestines

The intestinal enterocytes provide the initial site for cytochrome P450 (CYP)-mediated metabolism of orally absorbed xenobiotics. In man and some animal species, the CYP3A subfamily is highly expressed in the intestines and considered to be important in the first-pass metabolism of drugs and other xenobiotics. The aim of the present study was to investigate the mRNA expression, immunohistochemical localization and catalytic activity of CYP3A in the intestines of horse. Real-time RT-PCR analyses showed that the highest CYP3A mRNA expression was present in the duodenum with a decreasing level towards jejunum, ileum, cecum, and colon. The CYP3A mRNA expression in the liver was similar as in the anterior part of the jejunum, but about 4.5 times lower than in the anterior part of the duodenum. Immunohistochemistry showed CYP3A immunoreactivity in the cytoplasm of the enterocytes, which decreased distally along the intestinal tract. CYP3A-dependent metabolic activity rose slightly from the anterior to the distal part of the duodenum and the anterior part of the jejunum and then declined to the middle and distal parts of the jejunum and the ileum, cecum, and colon. Our results suggest that CYP3A in the small intestine plays a major role in first-pass metabolism and may affect bioavailability and therapeutic efficiency of some orally administrated drugs in horse.     

Inhibitory effect of docosahexaenoic acid (DHA) on the intestinal metabolism of midazolam: in vitro and in vivo studies in rats

The aim of this study was to evaluate the effects of docosahexaenoic acid (DHA) on the intestinal cytochrome P450 isoenzyme (CYP3A) and P-glycoprotein (P-gp) functions using midazolam and rhodamine-123 as specific substrates of CYP3A and P-gp, respectively. Perfused everted intestinal segments from rats were employed to determine the effects of DHA on midazolam metabolism and rhodamine-123 transport. In addition, the effects of DHA on in vitro midazolam metabolism in rat intestinal microsomes and on midazolam bioavailability in rats were examined. The intestinal extraction ratio (ER G) of midazolam was determined to be 0.43 and decreased significantly to 0.12, 0.07, and 0.06 in the presence of 50, 100, and 200 microM DHA, respectively, in a concentration-dependent manner. The results from an in vitro study using rat intestinal microsomes demonstrated that DHA competitively inhibited the intestinal CYP3A activity with Ki of 15.7 and 27.1 microM for the formations of 1'-OH midazolam and 4-OH midazolam, respectively. Moreover, the oral administration of DHA (100mg/kg) increased the AUC infinity, Cmax, and oral bioavailability (F) of midazolam by about 50% in rats, without affecting the T 1/2, V dss/F, or CL tot/F. In contrast, DHA did not change the serosal-to-mucosal transport of rhodamine-123 in the perfused everted intestine and oral administration of DHA (100mg/kg) had no influence on the pharmacokinetics of intravenously administered midazolam in rats, thus suggesting that DHA has little effect on the intestinal P-gp activity and hepatic clearance of midazolam. This study provided the first direct evidence to show that DHA has an inhibitory effect on the intestinal pre-systemic metabolism of a CYP3A substrate and that DHA has little, if any, effect on the P-gp activity in the gut.     

Poor oral bioavailability of a promising anticancer agent andrographolide is due to extensive metabolism and efflux by P-glycoprotein

Andrographolide (AP), isolated from Andrographis paniculata (Burm. F.) Nees, is an anticancer agent with significant clinical potential. This study determined its oral bioavailability and how intestinal disposition affects its bioavailability. Pharmacokinetics was evaluated in rats. Intestinal disposition was determined using a single-pass rat intestinal perfusion model and the cultured Caco-2 cells and Madin-Darby canine kidney II cells over expressing human P-gp (MDR1-MDCKII). Absolute bioavailability of AP was 2.67%. In the duodenum and jejunum, AP was rapidly metabolized to a sulfonate, identified as 14-deoxy-12-sulfo- andrographolide. AP was also rapidly metabolized by liver S9 fraction and in blank perfusates collected from duodenum and jejunum. The apparent permeability (P(app) ) of AP from basolateral (B) to apical (A) (4.94 × 10 cm/s) in the Caco-2 model was four times higher than the P(app) from A to B (1.14 × 10(-5) cm/s). Moreover, AP was significantly more permeable in the B to A direction than the opposite direction in MDR1-MDCKII cells. In the perfusion model, the effective permeability (P*(eff) ) for AP was highest in the duodenum, followed by jejunum, and then ileum and colon. In the ileum and colon, the P*(eff) for AP was significantly increased by verapamil, a P-glycoprotein (P-gp) inhibitor. AP has poor oral bioavailability because of its rapid biotransformation and efflux by P-gp.     

Limited interaction between tacrolimus and P-glycoprotein in the rat small intestine.

The significance of intestinal P-glycoprotein (P-gp) in determining the oral bioavailability of tacrolimus has been still controversial. In this study, we reevaluated the interaction of tacrolimus with P-gp in the rat small intestine, by evaluating its absorption from the rat small intestine and its modulating effect on the absorption of known P-gp substrates (digoxin, methylprednisolone, and vinblastine). Intestinal absorption of tacrolimus itself was as extensive as other P-gp modulators such as cyclosporine and verapamil. While cyclosporine and verapamil significantly increased the absorption of methylprednisolone and vinblastine through potent inhibition of intestinal P-gp, tacrolimus failed to achieve this. When cyclosporine and tacrolimus were intravenously administered to rats, digoxin absorption was significantly increased by cyclosporine but not by tacrolimus. When tacrolimus was coadministered with clotrimazole, a specific CYP3A inhibitor, into the rat small intestine, the area under the curve of tacrolimus blood concentrations increased more than seven-fold compared with that of tacrolimus alone. Our present results strongly suggest that the interaction between tacrolimus and P-gp is limited in the rat small intestine and that extensive metabolism by CYP3A enzymes is more responsible for the low oral bioavailability of tacrolimus. It was considered that the extensive absorption of cyclosporine and verapamil was closely associated with their potent ability to inhibit intestinal P-gp.     

Permeability dominates in vivo intestinal absorption of P-gp substrate with high solubility and high permeability

Three purposes are presented in this study: (1) to study the in vivo regional dependent intestinal absorption of a P-gp substrate with high solubility and high permeability, (2) to study the gene expression difference in the various regions of the intestine, and (3) to study the contributions of P-gp or any other transporters for the absorption of a P-gp substrate. The in vivo permeability of verapamil and propranolol were determined by single-pass in situ intestinal perfusion in rat. The gene expression profiles were measured using Affymetrix GeneChip. Correlation analysis between drug in vivo permeability and expression of 3500 genes was performed with nonparametric bootstrap and ANOVA analysis. The permeability of verapamil and propranolol did not demonstrate regional dependency even though significant differences in gene expression were observed in various regions of the intestine. Verapamil permeability significantly correlates with propranolol permeability in both jejunum and ileum, but did not correlate with the permeability of other hydrophilic compounds (valacyclovir, acyclovir, and phenylalanine). Four different regions (duodenum, jejunum, ileum, and colon) showed distinct gene expression patterns with more than 70-499 genes showing at least 5-fold expression differences. Interestingly, P-gp expression is gradually increased by 6-fold from the duodenum to colon. Despite the distinct gene expression patterns in the various regions of the intestine, verapamil permeability did not correlate with any gene expression from 3500 expressed genes in the intestine. A 2-6-fold P-gp expression difference did not seem to associate verapamil permeability in the various intestinal regions in vivo. These data suggest that P-gp plays a minimal role in the in vivo intestinal absorption process of verapamil with high water solubility and high membrane permeability. The intestinal absorption of verapamil in vivo is primarily dominated by its high permeability. However, it is important to note that the findings in this paper do not undermine the importance of P-gp in oral drug bioavailability, drug disposition from the liver, drug efflux from the blood-brain barrier, and drug-drug interaction.     

CYP3A4,CYP3A5和MDR1基因多态性对环孢素处置的影响

环孢素是一个广泛用于器官移植患者的免疫抑制剂,具有治疗指数窄,不同个体间药代动力学差异较大的特点。它主要通过肝脏和小肠的CYP3A4和CYP3A5代谢;同时它又是药物转运体的底物。不同个体间药物代谢酶和转运体活性的差异可能是造成不同器官移植患者环孢素药代动力学差异的主要原因。而遗传因素即编码药物代谢酶和转运体基因序列的差异可能是其产生活性差异的分子机制。因此,从编码药物代谢酶和转运体的基因入手,可能会为器官移植患者提供最优的治疗方案。     

Intestinal contribution to the presystemic elimination of beta-phenethylamine in the rat

The metabolism of beta-phenethylamine (PEA) has been investigated in intestinal, liver and lung homogenates from the rat. Metabolic activity was ranked liver much greater than lung much greater than duodenum greater than jejunum greater than ileum greater than colon. Absorption and metabolism of PEA was also studied using in-situ intestinal loops from rats. Absorption of PEA appeared to be a passively mediated process with metabolism occurring in jejunum, ileum and colon. When isolated loops of jejunum were used, phenylacetic acid was the only metabolite formed. Presystemic elimination of PEA in anaesthetized rats, was largely attributable to the intestine and lung with a relatively small hepatic contribution.