Computer simulation of portal venous shunting and other isolated hepatobiliary defects of the enterohepatic circulation of bile acids using a physiological pharmacokinetic model

Model-based data analysis of individual human postprandial plasma bile acid responses indicates a major role for the gallbladder and intestine

BACKGROUND

Bile acids are multifaceted metabolic compounds that signal to cholesterol, glucose, and lipid homeostasis via receptors like the Farnesoid X Receptor (FXR) and transmembrane Takeda G protein-coupled receptor 5 (TGR5). The postprandial increase in plasma bile acid concentrations is therefore a potential metabolic signal. However, this postprandial response has a high interindividual variability. Such variability may affect bile acid receptor activation.

METHODS

In this study, we analyzed the inter- and intraindividual variability of fasting and postprandial bile acid concentrations during three identical meals on separate days in eight healthy lean male subjects using a statistical and mathematical approach.

MAIN FINDINGS

The postprandial bile acid responses exhibited large interindividual and intraindividual variability. The individual mathematical models, which represent the enterohepatic circulation of bile acids in each subject, suggest that interindividual variability results from quantitative and qualitative differences of distal active uptake, colon transit, and microbial bile acid transformation. Conversely, intraindividual variations in gallbladder kinetics can explain intraindividual differences in the postprandial responses.

CONCLUSIONS

We conclude that there is considerable inter- and intraindividual variation in postprandial plasma bile acid levels. The presented personalized approach is a promising tool to identify unique characteristics of underlying physiological processes and can be applied to investigate bile acid metabolism in pathophysiological conditions.

A Physiology-Based Model of Human Bile Acid Metabolism for Predicting Bile Acid Tissue Levels After Drug Administration in Healthy Subjects and BRIC Type 2 Patients

Drug-induced liver injury (DILI) is a matter of concern in the course of drug development and patient safety, often leading to discontinuation of drug-development programs or early withdrawal of drugs from market. Hepatocellular toxicity or impairment of bile acid (BA) metabolism, known as cholestasis, are the two clinical forms of DILI. Whole-body physiology-based modelling allows a mechanistic investigation of the physiological processes leading to cholestasis in man. Objectives of the present study were: (1) the development of a physiology-based model of the human BA metabolism, (2) population-based model validation and characterisation, and (3) the prediction and quantification of altered BA levels in special genotype subgroups and after drug administration. The developed physiology-based bile acid (PBBA) model describes the systemic BA circulation in humans and includes mechanistically relevant active and passive processes such as the hepatic synthesis, gallbladder emptying, transition through the gastrointestinal tract, reabsorption into the liver, distribution within the whole body, and excretion via urine and faeces. The kinetics of active processes were determined for the exemplary BA glycochenodeoxycholic acid (GCDCA) based on blood plasma concentration-time profiles. The robustness of our PBBA model was verified with population simulations of healthy individuals. In addition to plasma levels, the possibility to estimate BA concentrations in relevant tissues like the intracellular space of the liver enhance the mechanistic understanding of cholestasis. We analysed BA levels in various tissues of Benign Recurrent Intrahepatic Cholestasis type 2 (BRIC2) patients and our simulations suggest a higher susceptibility of BRIC2 patients toward cholestatic DILI due to BA accumulation in the liver. The effect of drugs on systemic BA levels were simulated for cyclosporine A (CsA). Our results confirmed the higher risk of DILI after CsA administration in healthy and BRIC2 patients. The presented PBBA model enhances our mechanistic understanding underlying cholestasis and drug-induced alterations of BA levels in blood and organs. The developed PBBA model might be applied in the future to anticipate potential risk of cholestasis in patients.

In Silico Analysis Identifies Intestinal Transit as a Key Determinant of Systemic Bile Acid Metabolism

Bile acids fulfill a variety of metabolic functions including regulation of glucose and lipid metabolism. Since changes of bile acid metabolism accompany obesity, Type 2 Diabetes Mellitus and bariatric surgery, there is great interest in their role in metabolic health. Here, we developed a mathematical model of systemic bile acid metabolism, and subsequently performed in silico analyses to gain quantitative insight into the factors determining plasma bile acid measurements. Intestinal transit was found to have a surprisingly central role in plasma bile acid appearance, as was evidenced by both the necessity of detailed intestinal transit functions for a physiological description of bile acid metabolism as well as the importance of the intestinal transit parameters in determining plasma measurements. The central role of intestinal transit is further highlighted by the dependency of the early phase of the dynamic response of plasma bile acids after a meal to intestinal propulsion.

Animal models to study bile acid metabolism

The use of animal models, particularly genetically modified mice, continues to play a critical role in studying the relationship between bile acid metabolism and human liver disease. Over the past 20 years, these studies have been instrumental in elucidating the major pathways responsible for bile acid biosynthesis and enterohepatic cycling, and the molecular mechanisms regulating those pathways. This work also revealed bile acid differences between species, particularly in the composition, physicochemical properties, and signaling potential of the bile acid pool. These species differences may limit the ability to translate findings regarding bile acid-related disease processes from mice to humans. In this review, we focus primarily on mouse models and also briefly discuss dietary or surgical models commonly used to study the basic mechanisms underlying bile acid metabolism. Important phenotypic species differences in bile acid metabolism between mice and humans are highlighted.

Hepatic uptake of conjugated bile acids is mediated by both sodium taurocholate cotransporting polypeptide and organic anion transporting polypeptides and modulated by intestinal sensing of plasma bile acid levels in mice

The Na⁺ -taurocholate cotransporting polypeptide (NTCP/SLC10A1) is believed to be pivotal for hepatic uptake of conjugated bile acids. However, plasma bile acid levels are normal in a subset of NTCP knockout mice and in mice treated with myrcludex B, a specific NTCP inhibitor. Here, we elucidated which transport proteins mediate the hepatic uptake of conjugated bile acids and demonstrated intestinal sensing of elevated bile acid levels in plasma in mice. Mice or healthy volunteers were treated with myrcludex B. Hepatic bile acid uptake kinetics were determined in wild-type (WT), organic anion transporting polypeptide (OATP) knockout mice (lacking Slco1a/1b isoforms), and human OATP1B1-transgenic mice. Effects of fibroblast growth factor 19 (FGF19) on hepatic transporter mRNA levels were assessed in rat hepatoma cells and in mice by peptide injection or adeno-associated virus-mediated overexpression. NTCP inhibition using myrcludex B had only moderate effects on bile acid kinetics in WT mice, but completely inhibited active transport of conjugated bile acid species in OATP knockout mice. Cholesterol 7α-hydroxylase Cyp7a1 expression was strongly down-regulated upon prolonged inhibition of hepatic uptake of conjugated bile acids. Fgf15 (mouse counterpart of FGF19) expression was induced in hypercholanemic OATP and NTCP knockout mice, as well as in myrcludex B-treated cholestatic mice, whereas plasma FGF19 was not induced in humans treated with myrcludex B. Fgf15/FGF19 expression was induced in polarized human enterocyte-models and mouse organoids by basolateral incubation with a high concentration (1 mM) of conjugated bile acids.

CONCLUSION

NTCP and OATPs contribute to hepatic uptake of conjugated bile acids in mice, whereas the predominant uptake in humans is NTCP mediated. Enterocytes sense highly elevated levels of (conjugated) bile acids in the systemic circulation to induce FGF15/19, which modulates hepatic bile acid synthesis and uptake. (Hepatology 2017;66:1631-1643).

Bile acids: trying to understand their chemistry and biology with the hope of helping patients

An informal review of the author's five decades of research on the chemistry and biology of bile acids in health and disease is presented. The review begins with a discussion of bile acid structure and its remarkable diversity in vertebrates. Methods for tagging bile acids with tritium for metabolic or transport studies are summarized. Bile acids solubilize polar lipids in mixed micelles; progress in elucidating the structure of the mixed micelle is discussed. Extensive studies on bile acid metabolism in humans have permitted the development of physiological pharmacokinetic models that can be used to simulate bile acid metabolism. Consequences of defective bile acid biosynthesis and transport have been clarified, and therapy has been developed. Methods for measuring bile acids have been improved. The rise and fall of medical and contact dissolution of cholesterol gallstones is chronicled. Finally, principles of therapy with bile acid agonists and antagonists are given. Advances in understanding bile acid biology and chemistry have helped to improve the lives of patients with hepatobiliary or digestive disease.

A recirculatory model with enterohepatic circulation by measuring portal and systemic blood concentration difference

The present study describes a recirculatory model for the evaluation of pharmacokinetic characteristics of drugs possessing enterohepatic circulation (EHC). The advantage of the model is to separately define the extent and rate of absorption for the dosage and EHC after oral administration. Cephradine was used as a model drug and was intravenously or orally administered to rats. Portal and systemic bloods were simultaneously collected in order to estimate various local moments after defining the global moments obtained by non-compartment analysis. For the zero-order moments, bioavailability (BA), the hepatic recovery ratio (Fh), the sum of the local absorption ratio for the dosage and recirculatory local absorption ratio for EHC (F(a)po), and the recirculatory local absorption ratio for EHC (F(a)ehc) after oral administration were estimated to be 95.6, 77.9, 172, and 71.5%, respectively. These data indicate that a complete absorption and substantial EHC contribute high oral exposure of cephradine. For the first-order moments, the sum of the mean local absorption times for the dosage and EHC (t(a)po) and the mean transit time for a single pass of EHC (tc) were 2.50 and 0.117 hr, suggesting a rapid EHC of cephradine compared with the absorption from the dosage. With this model, the absorption rate-time profiles for the dosage and EHC were separately simulated by using a program of nonlinear least squares (MULTI) with fast inverse Laplace transform (FILT). The cumulative biliary excretion ratio (Fbile) calculated by the model was in good agreement with the experimental value obtained in the bile ductcannulated rats. These results suggest that the model proposed in this study would be useful for evaluating the extent and rate of ECH along with absorption from the dosage after oral administration of drugs.

Serum bile acid concentrations in mild liver cirrhosis

Fasting serum conjugated bile salt concentrations were measured in a group of 20 patients with moderate post-hepatitis cirrhosis. Twenty healthy volunteers were used as controls. The individual conjugated bile acids were analyzed by a specific and sensitive method which couples reversed-phase high-performance liquid chromatography with mass spectrometry. Significantly elevated levels of the total and individual conjugated bile acids were found in cirrhotic patients. The predominant serum bile acids were conjugates of chenodeoxycholic acid. The conjugates of lithocholic acid were also increased; in subjects with normal liver function, on the contrary, they were found only in traces.

Serum concentrations and excretion of bile acids in cirrhosis

Bile acid concentrations in serum, and urinary and faecal excretion of bile acids have been studied in ten patients with liver cirrhosis as a consequence of alcohol abuse. Eight of the patients were categorized as Child group A, whereas the remaining two patients comprised Child group C. Individual bile acids were isolated and identified by gas chromatography coupled to mass spectrometry. Total fasting serum bile acid concentrations were elevated in all patients, but not correlated to conventional tests of liver function. Eight of the patients had increased urinary excretion of bile acids. Faecal bile acid-excretion was highly variable between patients, and also between Child's group A and C patients. Total fasting serum bile acid concentrations were not correlated to either urinary, faecal, or total bile acid excretion (= synthesis of bile acids) or to the ratio between urinary and faecal excretion of bile acids. The daily synthesis of bile acids showed a large overlap between Child's group A and C patients. The percentage of chenodeoxycholic acid and its metabolites relative to total daily excretion of bile acids did not correlate, indicating that the synthesis pathways for the primary bile acids does not systematically change in relation to the rate of synthesis. We conclude that even in mild cirrhosis, serum bile acid concentrations are elevated. However, no consistent changes in synthesis of bile acids or synthesis pathways was observed in such patients.

Evaluation of portal-systemic shunting in rats from mesenteric and splenic beds

In rats with partial portal vein ligation, 95 +/- 0.9% of the splenic blood flow is shunted from the portal to the systemic circulation when an intrasplenic injection of microspheres is used to determine the degree of shunting. Despite this magnitude of portal-systemic shunting, several biochemical and endocrine consequences of portal-systemic shunting occur at levels below what is expected for the degree of shunting found. In an effort to resolve these discordant findings, shunting from both the splenic and the mesenteric bed was studied in anesthetized portal hypertensive rats with various degrees and/or duration of portal vein stenosis. The shunting from the mesenteric bed averaged 66.7 +/- 29.9% (range 5.1-99.1%) and was influenced both by the degree and duration of portal vein stenosis. In contrast, shunting from the splenic bed averaged 97.3 +/- 4.0% (range 79-99.9%) and demonstrated no variation between groups determined by the degree of portal vein stenosis. The shunting from the splenic bed was consistently greater than that found from the mesenteric bed. Mesenteric but not splenic shunting correlated with serum bile acid levels. Mesenteric shunting was related inversely to the weight-adjusted liver mass and to serum testosterone levels. Based upon these data obtained in portal hypertensive rats, it is concluded that splenic injections of microspheres overestimate portal-systemic shunting. In contrast, mesenteric injections of microspheres yield values for shunting that correlate well with independently determined biochemical and endocrine consequences of shunting. These observations support the validity of the mesenteric shunting measurements obtained.

The effect of portal-systemic shunting on hepatic sex hormone receptors in male rats

Signs of feminization are seen in men with cirrhosis of alcoholic but also of nonalcoholic origin even in the absence of markedly increased plasma estrogen levels. Recently identified alterations of hepatic sex hormone receptor levels have provided a hypothetical mechanism for the pathogenesis of the feminization seen in cirrhotic men. The aim of the present study was to determine the effect of experimental portal-systemic shunting in adult male rats on hepatic sex hormone receptor levels, plasma sex hormones, and two markers for sex hormone action in the liver. The following alterations were found in male rats with surgically created portacaval shunts compared with sham-operated controls: the hepatic content of cytosolic estrogen receptors was reduced by 35% and the cytosolic androgen receptors content by 59%; plasma levels of estradiol increased 6.7-fold while those of testosterone were reduced by 71%; the estrogen-responsive ceruloplasmin levels were decreased by 31% and the androgen-responsive male-specific estrogen binder by 72%. Based on these data, it can be concluded that portal-systemic shunting reduces the hepatic cytoplasmic content of several sex hormone related proteins. These changes are paralleled by a decreased estrogen responsiveness of the liver, as evidenced by the plasma ceruloplasmin level.