Structural requirements of bile acid transporters: C-3 and C-7 modifications of steroidal hydroxyl groups

Research progress in the application of bile acid-drug conjugates: A "trojan horse" strategy

Bile acid transporters are highly expressed in intestinal cells and hepatocytes, and they determine the uptake of drugs in cells by modulating cellular entry and exit. In order to improve the oral bioavailability of drugs and investigate the potential application prospects of drugs used to target cancer, numerous studies have adopted these transporters to identify prodrug strategies. Through the connection of covalent bonds between drugs and bile acids, the resulting bile acid-drug conjugates continue to be recognized as similar to natural unmodified bile acid and is translocated by the transporter. The present mini-review provides a brief summary of recent progress of the application of bile acid-drug conjugates based primarily on ASBT, NTCP, and OATP, with the hope of contributing to subsequent research.

Insulin- and cholic acid-loaded zein/casein-dextran nanoparticles enhance the oral absorption and hypoglycemic effect of insulin

Diabetes mellitus is the most common metabolic disease in the world. Herein, insulin- and cholic acid-loaded zein nanoparticles with dextran surfaces were fabricated to enhance the oral absorptions of insulin in the intestine and in the liver which is the primary action organ of endogenous insulin. In the nanoparticles, zein acted as cement to embed insulin, cholic acid and casein by hydrophobic interactions. The hydrophilic dextran conjugated to casein by the Maillard reaction was located on the nanoparticle surface. The nanoparticles had an insulin loading efficiency of 74.6%, a cholic acid loading efficiency of 55.1% and a hydrodynamic diameter of 267 nm. The dextran significantly increased the disperse stability of the nanoparticles, protected the loaded insulin from hydrolysis in digestive juices, and increased the trans-mucus permeability of the insulin. The embedded cholic acid molecules were consecutively exposed to the surface when the nanoparticles were gradually eroded by proteases. The exposed cholic acid promoted the absorptions of the nanoparticles in the ileum and liver via bile acid transporters. The effect of pretreated lymphatic transport inhibitor cycloheximide revealed that about half of the nanoparticles were transported via the intestinal lymphatic transport pathway and the other half of the nanoparticles were transported via portal blood absorption. The oral pharmacological bioavailability of the nanoparticles in type I diabetic mice was 12.5-20.5%. This study demonstrates that nanoparticles are a promising oral delivery system for insulin.

Structural modifications that increase gut restriction of bile acid derivatives

Bile acid derivatives have been investigated as possible therapeutics for a wide array of conditions, including several for which gut-restricted analogs would likely be preferred. These include the prevention of Clostridioides difficile infection (CDI) and the treatment of inflammatory bowel disease (IBD). The design of gut-restricted bile acid analogs, however, is complicated by the highly efficient enterohepatic circulation system that typically reabsorbs these compounds from the digestive tract for subsequent return to the liver. Herein, we report that incorporation of a sulfate group at the 7-position of the bile acid scaffold reduces oral bioavailability and increases fecal recovery in two pairs of compounds designed to inhibit the germination of C. difficile spores. A different approach was necessary for designing gut-restricted bile acid-based TGR5 agonists for the treatment of IBD, as the incorporation of a 7-sulfate group reduces activity at this receptor. Instead, building on our previous discovery that incorporation of a 7-methoxy group into chenodeoxycholic acid derivatives greatly increases their TGR5 receptor potency, we determined that an N-methyl-d-glucamine group could be conjugated to the scaffold to obtain a compound with an excellent mix of potency at the TGR5 receptor, low oral exposure, and good fecal recovery.

Intestinal Absorption of Bile Acids in Health and Disease

The intestinal reclamation of bile acids is crucial for the maintenance of their enterohepatic circulation. The majority of bile acids are actively absorbed via specific transport proteins that are highly expressed in the distal ileum. The uptake of bile acids by intestinal epithelial cells modulates the activation of cytosolic and membrane receptors such as the farnesoid X receptor (FXR) and G protein-coupled bile acid receptor 1 (GPBAR1), which has a profound effect on hepatic synthesis of bile acids as well as glucose and lipid metabolism. Extensive research has focused on delineating the processes of bile acid absorption and determining the contribution of dysregulated ileal signaling in the development of intestinal and hepatic disorders. For example, a decrease in the levels of the bile acid-induced ileal hormone FGF15/19 is implicated in bile acid-induced diarrhea (BAD). Conversely, the increase in bile acid absorption with subsequent overload of bile acids could be involved in the pathophysiology of liver and metabolic disorders such as fatty liver diseases and type 2 diabetes mellitus. This review article will attempt to provide a comprehensive overview of the mechanisms involved in the intestinal handling of bile acids, the pathological implications of disrupted intestinal bile acid homeostasis, and the potential therapeutic targets for the treatment of bile acid-related disorders. Published 2020. Compr Physiol 10:21-56, 2020.

Mechanism of a Novel Camptothecin-Deoxycholic Acid Derivate Induced Apoptosis against Human Liver Cancer HepG2 Cells and Human Colon Cancer HCT116 Cells

BACKGROUND

Camptothecin (CPT) is known as an anticancer drug in traditional Chinese medicine. However, due to the lack of targeting, low solubility, and instability of CPT, its therapeutic applications are hampered. Therefore, we synthesized a series of CPT-bile acid analogues that obtained a national patent to improve their tumour-targeting chemotherapeutic effects on liver or colon cancers. Among these analogues, the compound G2 shows high antitumor activity with enhanced liver targeting and improved oral absorption. It is significant to further investigate the possible anticancer mechanism of G2 for its further clinical research and application.

OBJECTIVE

We aimed to unearth the anticancer mechanism of G2 in HepG2 and HCT116 cells.

METHODS

Cell viability was measured using MTT assay; cell cycle, Mitochondrial Membrane Potential (MMP), and cell apoptosis were detected by flow cytometer; ROS was measured by Fluorescent Microplate Reader; the mRNA and protein levels of cell cycle-related and apoptosis-associated proteins were examined by RT-PCR and western blot, respectively.

RESULTS

We found that G2 inhibited cells proliferation of HepG2 and HCT116 remarkably in a dosedependent manner. Moreover, G2-treatment led to S and G2/M phase arrest in both cells, which could be elucidated by the change of mRNA levels of p21, p27 and Cyclin E and the increased protein level of p21. G2 also induced dramatically ROS accumulated and MMP decreased, which contributed to the apoptosis through activation of both the extrinsic and intrinsic pathways via changing the genes and proteins expression involved in apoptosis pathway in both of HepG2 and HCT116 cells.

CONCLUSION

These findings suggested that the apoptosis in both cell lines induced by G2 was related to the extrinsic and intrinsic pathways.

Co-delivery doxorubicin and silybin for anti-hepatoma via enhanced oral hepatic-targeted efficiency

Background

To establish the combination of doxorubicin (DOX) and silybin (SLB) in oral hepatic-targeting liposomes with the goal of reducing cardiotoxic side effects and improve oral hepatoma treatment.

Methods

Distearoylphosphatidylethanolamine–polyethylene glycol–cholic acid-modified liposomes (CA-LP) were used to encapsulate DOX and SLB (CA-LP–DOX/SLB), and the hepatic targeting, efficacy against hepatoma and cardioprotective effects were evaluated by cell toxicity, scratch and apoptosis in vitro studies, and pharmacokinetics and pharmacodynamics in vivo studies.

Results

In vitro cell studies showed that CA-LP–DOX/SLB inhibited HepG2 cell proliferation and HCC97H cell migration, and protected H9c2 cells. In vivo pharmacokinetics demonstrated that the CA-LP–DOX/SLB-treated group showed higher liver accumulation and lower heart accumulation of DOX relative to those in the CA-LP–DOX and LP–DOX-treated groups. In vivo pharmacodynamic studies showed that the CA-LP–DOX/SLB-treated group not only efficiently inhibited growth but also induced significantly less tissue damage than that observed in the CA-LP–DOX-treated group.

Conclusion

Concurrent administration of DOX and SLB via CA-LP provided a viable strategy to mitigate acute DOX-induced cardiotoxicity.

Liver-targeted delivery of insulin-loaded nanoparticles via enterohepatic circulation of bile acids

Liver is the primary acting site of insulin. In this study, we developed innovative nanoparticles for oral and liver-targeted delivery of insulin by using enterohepatic circulation of bile acids. The nanoparticles were produced from cholic acid and quaternary ammonium modified chitosan derivative and hydroxypropyl methylcellulose phthalate (HPMCP). The nanoparticles had a diameter of 239 nm, an insulin loading efficiency of 90.9%, and a loading capacity of 18.2%. Cell culture studies revealed that the cholic acid groups effectively enhanced the transport of the nanoparticles through Caco-2 cell monolayer and greatly increased the absorption of the nanoparticles in HepG-2 cells via bile acid transporter mechanism. Ex vivo fluorescence images of ileum section, gastrointestinal tract, and liver demonstrated that the HPMCP increased the mucoadhesion of the nanoparticles in ileum, and the cholic acid groups facilitated the absorptions of the nanoparticles in both ileum and liver by use of bile acid transporters via enterohepatic circulation of bile acids. The therapy for diabetic mice displayed that the oral nanoparticle group could maintain hypoglycemic effect for more than 24 h and its pharmacological availability was about 30% compared with the insulin injection group. For the first time, this study demonstrates that using enterohepatic circulation of bile acids is an effective strategy for oral delivery of insulin.

New therapies target the toxic consequences of cholestatic liver disease

In most cholestatic liver diseases the primary cholestasis-causing lesions are located in the biliary tree and may be of (auto)immune origin. Bile salts are responsible for the secondary toxic consequences. Bile salt and nuclear hormone directed therapies primarily aim at improving this secondary toxic injury. In primary biliary cholangitis, trials show statistically significant responses on biochemical endpoints. Preclinical studies suggest that FXR- and PPAR-agonists, inhibitors of the apical sodium-dependent bile salt transporter (ASBT-inhibitors) and the C₂₃ UDCA derivative nor-UDCA are promising agents for the treatment of primary sclerosing cholangitis (PSC). Area covered: Pharmaceuticals that interfere with bile salt signaling in humans for the treatment of chronic cholestatic liver disease are reviewed. Expert commentary: Nuclear hormone receptors, bile salt transport proteins and receptors provide targets for novel therapies of cholestatic liver disease. These drugs show positive results on biochemical endpoints. For histological endpoints, survival and transplant-free survival, long-term trials are needed. For relief of symptoms, such as fatigue and pruritus, these drugs have yet to prove their value.

Advanced Prodrug Strategies in Nucleoside and Non-Nucleoside Antiviral Agents: A Review of the Recent Five Years

Background: Poor pharmacokinetic profiles and resistance are the main two drawbacks from which currently used antiviral agents suffer, thus make them excellent targets for research, especially in the presence of viral pandemics such as HIV and hepatitis C. Methods: The strategies employed in the studies covered in this review were sorted by the type of drug synthesized into ester prodrugs, targeted delivery prodrugs, macromolecular prodrugs, other nucleoside conjugates, and non-nucleoside drugs. Results: Utilizing the ester prodrug approach a novel isopropyl ester prodrug was found to be potent HIV integrase inhibitor. Further, employing the targeted delivery prodrug zanamivir and valine ester prodrug was made and shown a sole delivery of zanamivir. Additionally, VivaGel, a dendrimer macromolecular prodrug, was found to be very efficient and is now undergoing clinical trials. Conclusions: Of all the strategies employed (ester, targeted delivery, macromolecular, protides and nucleoside analogues, and non-nucleoside analogues prodrugs), the most promising are nucleoside analogues and macromolecular prodrugs. The macromolecular prodrug VivaGel works by two mechanisms: envelope mediated and receptor mediated disruption. Nucleotide analogues have witnessed productive era in the recent past few years. The era of non-interferon based treatment of hepatitis (through direct inhibitors of NS5A) has dawned.

Synthesis and Biological Evaluation of Bile Acid Analogues Inhibitory to Clostridium difficile Spore Germination

Standard antibiotic-based strategies for the treatment of Clostridium difficile infections disrupt indigenous microbiota and commonly fail to eradicate bacterial spores, two key factors that allow recurrence of infection. As an alternative approach to controlling C. difficile infection, a series of bile acid derivatives have been prepared that inhibit taurocholate-induced spore germination. These analogues have been evaluated in a highly virulent NAP1 strain using optical density and phase-contrast microscopy assays. Heterocycle substitutions at C24 were well-tolerated and several tetrazole-containing derivatives were highly potent inhibitors in both assays, with complete inhibition of spore germination observed at 10-25 μM. To limit intestinal absorption, C7-sulfated analogues designed to avoid active and passive transport pathways were prepared. One of these derivatives, compound 21b, was found to be a potent inhibitor of C. difficile spore germination and poorly permeable in a Caco-2 model of intestinal epithelial absorption, suggesting that it is likely to be gut-restricted.

Transporter-targeted cholic acid-cytarabine conjugates for improved oral absorption

Cytarabine has a poor oral absorption due to its rapid deamination and poor membrane permeability. Bile acid transporters are highly expressed both in enterocytes and hepatocytes and to increase the oral bioavailability and investigate the potential application of cytarabine for liver cancers, a transporter- recognizing prodrug strategy was applied to design and synthesize four conjugates of cytarabine with cholic acid (CA), chenodeoxycholic acid (CDCA), hyodeoxycholic acid (HDCA) and ursodeoxycholic acid (UDCA). The anticancer activities against HepG2 cells were evaluated by MTT assay and the role of bile acid transporters during cellular transport was investigated in a competitive inhibition experiment. The in vitro and in vivo metabolic stabilities of these conjugates were studied in rat plasma and liver homogenates. Finally, an oral bioavailability study was conducted in rats. All the cholic acid-cytarabine conjugates (40μM) showed potent antiproliferative activities (up to 70%) against HepG2 cells after incubation for 48h. The addition of bile acids could markedly reduce the antitumor activities of these conjugates. The N(4)-ursodeoxycholic acid conjugate of cytarabine (compound 5) exhibited optimal stability (t1/2=90min) in vitro and a 3.9-fold prolonged half-life of cytarabine in vivo. More importantly, compound 5 increased the oral bioavailability 2-fold compared with cytarabine. The results of the present study suggest that the prodrug strategy based on the bile acid transporters is suitable for improving the oral absorption and the clinical application of cytarabine.

Enhanced hepatic-targeted delivery via oral administration using nanoliposomes functionalized with a novel DSPE–PEG–cholic acid conjugate

DSPE–PEG–cholic acid modified nanoliposomes interacted with cholic acid receptor on intestine epithelial cells and hepatocytes specifically for oral hepatic targeting.

Genetic variations of bile salt transporters

Bile salt transporters directly or indirectly influence biological processes through physicochemical or signalling properties of bile salts. The coordinated action of uptake and efflux transporters in polarized epithelial cells of the liver, biliary tree, small intestine and kidney determine bile salt concentrations in different compartments of the body. Genetic variations of bile salt transporters lead to clinical relevant phenotypes of varying severity ranging from a predisposition for drug-induced liver injury to rapidly progressing end-stage liver disease. This review focuses on the impact of genetic variations of bile salt transporters including BSEP, NTCP, ASBT and OSTα/β and discusses approaches for transporter analysis.

Synthesis and Evaluation of Bile Acid-Ribavirin Conjugates as Prodrugs to Target the Liver

Ribavirin is used to treat hepatitis C but causes serious hemolytic anemia. The objective of the study was to develop a ribavirin prodrug to achieve liver-specific drug delivery and to reduce its off-target effect in red blood cells (RBCs). The approach aimed to target the human sodium taurocholate cotransporting polypeptide (NTCP), which is a bile acid transporter predominately expressed in the liver. Six prodrugs with ribavirin conjugation at C-3 or C-24 of the bile acids were synthesized. In vitro uptake studies indicated that all six prodrugs were NTCP substrates. Metabolic studies in vitro indicated that ribavirin-l-Val-glycochenodeoxycholic acid (GCDCA) was able to release ribavirin in the mouse liver S9 fraction. Additionally, in vitro studies showed that ribavirin in RBC was reduced by 16.7-fold from prodrug compared with parent drug incubation. Moreover, almost no prodrug was present in RBC. In vivo study in mice also showed that ribavirin-l-Val-GCDCA could provide almost the same ribavirin exposure in the liver as ribavirin administration, but with about 1.8-fold less exposure of ribavirin in RBC, plasma, and kidney. Overall, the study suggested that ribavirin-l-Val-GCDCA has the potential to achieve ribavirin-specific liver delivery.

Modern prodrug design for targeted oral drug delivery

The molecular information that became available over the past two decades significantly influenced the field of drug design and delivery at large, and the prodrug approach in particular. While the traditional prodrug approach was aimed at altering various physiochemical parameters, e.g., lipophilicity and charge state, the modern approach to prodrug design considers molecular/cellular factors, e.g., membrane influx/efflux transporters and cellular protein expression and distribution. This novel targeted-prodrug approach is aimed to exploit carrier-mediated transport for enhanced intestinal permeability, as well as specific enzymes to promote activation of the prodrug and liberation of the free parent drug. The purpose of this article is to provide a concise overview of this modern prodrug approach, with useful successful examples for its utilization. In the past the prodrug approach used to be viewed as a last option strategy, after all other possible solutions were exhausted; nowadays this is no longer the case, and in fact, the prodrug approach should be considered already in the very earliest development stages. Indeed, the prodrug approach becomes more and more popular and successful. A mechanistic prodrug design that aims to allow intestinal permeability by specific transporters, as well as activation by specific enzymes, may greatly improve the prodrug efficiency, and allow for novel oral treatment options.

Quantitative NTCP pharmacophore and lack of association between DILI and NTCP Inhibition

The human sodium taurocholate cotransporting polypeptide (NTCP) is a hepatic bile acid transporter. Inhibition of NTCP uptake may potentially also prevent hepatitis B virus (HBV) infection. The first objective was to develop a quantitative pharmacophore for NTCP inhibition. Recent studies showed that hepatotoxic drugs could inhibit bile acid uptake into hepatocytes, without inhibiting canalicular efflux, and cause bile acid elevation in plasma. Hence, a second objective was to examine whether NTCP inhibition is associated with drug induced liver injury (DILI). Twenty-seven drugs from our previous study were used as the training set to develop a quantitative pharmacophore. From secondary screening from a drug database, six retrieved drugs and three drugs not retrieved by the model were tested for NTCP inhibition. Tertiary screening involved drugs known to cause DILI and not cause DILI. Overall, ninety-four drugs were assessed for hepatotoxicity and were assessed relative to NTCP inhibition. The quantitative pharmacophore possessed one hydrogen bond acceptor, one hydrogen bond donor, a hydrophobic feature, and excluded volumes. From 94 drugs, NTCP inhibitors and non-inhibitors were approximately equally distributed across the drugs of most DILI concern, less DILI concern, and no DILI concern, indicating no relationship between NTCP inhibition and DILI risk. Hence, an approach to treat HBV via NTCP inhibition is not expected to be associated with DILI.

Synthesis and in vitro evaluation of bile acid prodrugs of floxuridine to target the liver

Floxuridine is often used to treat metastatic liver disease and is given as an infusion directly into the hepatic artery to increase the amount of intact drug that reaches the liver. The objective of this work was to design and synthesize prodrugs of floxuridine through conjugation to chenodeoxycholic acid (CDCA) to target the liver via the bile acid liver uptake transporter Na(+)/taurocholate cotransporting polypeptide (NTCP, SLC10A1). Two isomeric prodrugs of floxuridine were synthesized: floxuridine 3'glutamic acid-CDCA and floxuridine 5'-glutamic acid-CDCA. Both were potent inhibitors and substrates of NTCP. Floxuridine 3'glutamic acid-CDCA showed Ki=6.86±1.37 μM, Km=10.7±2.1 μM, and passive permeability=0.663(±0.121)×10(-7) cm/s while floxuridine 5'-glutamic acid-CDCA showed Ki=0.397±0.038 μM, Km=40.4±15.2 μM, and passive permeability=1.72(±0.18)×10(-7) cm/s. Floxuridine itself had a higher passively permeability of 7.54(±0.45)×10(-7) cm/s in the same cell line, indicating that both prodrugs have the potential for lower non-specific effects than the drug alone. Prodrugs were stable in rat plasma (t=3 h), but quickly released in rat liver s9 fraction, suggesting future in vivo evaluation.

Design and evaluation of a novel trifluorinated imaging agent for assessment of bile acid transport using fluorine magnetic resonance imaging

Previously, we developed a trifluorinated bile acid, CA-lys-TFA, with the objective of noninvasively assessing bile acid transport in vivo using (19) F magnetic resonance imaging (MRI). CA-lys-TFA was successfully imaged in the mouse gallbladder, but was susceptible to deconjugation in vitro by choloylglycine hydrolase (CGH), a bacterial bile acid deconjugating enzyme found in the terminal ileum and colon. The objective of the present study was to develop a novel trifluorinated bile acid resistant to deconjugation by CGH. CA-sar-TFMA was designed, synthesized, and tested for in vitro transport properties, stability, imaging properties, and its ability to differentially accumulate in the gallbladders of normal mice, compared with mice with known impaired bile acid transport (deficient in the apical sodium-dependent bile acid transporter, ASBT). CA-sar-TFMA was a potent inhibitor and substrate of ASBT and the Na(+) /taurocholate cotransporting polypeptide. Stability was favorable in all conditions tested, including the presence of CGH. CA-sar-TFMA was successfully imaged and accumulated at 16.1-fold higher concentrations in gallbladders from wild-type mice compared with those from Asbt-deficient mice. Our results support the potential of using MRI with CA-sar-TFMA as a noninvasive method to assess bile acid transport in vivo.

Key discoveries in bile acid chemistry and biology and their clinical applications: history of the last eight decades

During the last 80 years there have been extraordinary advances in our knowledge of the chemistry and biology of bile acids. We present here a brief history of the major achievements as we perceive them. Bernal, a physicist, determined the X-ray structure of cholesterol crystals, and his data together with the vast chemical studies of Wieland and Windaus enabled the correct structure of the steroid nucleus to be deduced. Today, C24 and C27 bile acids together with C27 bile alcohols constitute most of the bile acid "family". Patterns of bile acid hydroxylation and conjugation are summarized. Bile acid measurement encompasses the techniques of GC, HPLC, and MS, as well as enzymatic, bioluminescent, and competitive binding methods. The enterohepatic circulation of bile acids results from vectorial transport of bile acids by the ileal enterocyte and hepatocyte; the key transporters have been cloned. Bile acids are amphipathic, self-associate in solution, and form mixed micelles with polar lipids, phosphatidylcholine in bile, and fatty acids in intestinal content during triglyceride digestion. The rise and decline of dissolution of cholesterol gallstones by the ingestion of 3,7-dihydroxy bile acids is chronicled. Scientists from throughout the world have contributed to these achievements.

Sodium-dependent bile salt transporters of the SLC10A transporter family: more than solute transporters

The SLC10A transporter gene family consists of seven members and substrates transported by three members (SLC10A1, SLC10A2 and SLC10A6) are Na(+)-dependent. SLC10A1 (sodium taurocholate cotransporting polypeptide [NTCP]) and SLC10A2 (apical sodium-dependent bile salt transporter [ASBT]) transport bile salts and play an important role in maintaining enterohepatic circulation of bile salts. Solutes other than bile salts are also transported by NTCP. However, ASBT has not been shown to be a transporter for non-bile salt substrates. While the transport function of NTCP can potentially be used as liver function test, interpretation of such a test may be complicated by altered expression of NTCP in diseases and presence of drugs that may inhibit NTCP function. Transport of bile salts by NTCP and ASBT is inhibited by a number of drugs and it appears that ASBT is more permissive to drug inhibition than NTCP. The clinical significance of this inhibition in drug disposition and drug-drug interaction remains to be determined. Both NCTP and ASBT undergo post-translational regulations that involve phosphorylation/dephosphorylation, translocation to and retrieval from the plasma membrane and degradation by the ubiquitin-proteasome system. These posttranslational regulations are mediated via signaling pathways involving cAMP, calcium, nitric oxide, phosphoinositide-3-kinase (PI3K), protein kinase C (PKC) and protein phosphatases. There appears to be species difference in the substrate specificity and the regulation of plasma membrane localization of human and rodent NTCP. These differences should be taken into account when extrapolating rodent data for human clinical relevance and developing novel therapies. NTCP has recently been shown to play an important role in HBV and HDV infection by serving as a receptor for entry of these viruses into hepatocytes.

Sodium-dependent bile salt transporters of the SLC10A transporter family: more than solute transporters

The SLC10A transporter gene family consists of seven members and substrates transported by three members (SLC10A1, SLC10A2 and SLC10A6) are Na(+)-dependent. SLC10A1 (sodium taurocholate cotransporting polypeptide [NTCP]) and SLC10A2 (apical sodium-dependent bile salt transporter [ASBT]) transport bile salts and play an important role in maintaining enterohepatic circulation of bile salts. Solutes other than bile salts are also transported by NTCP. However, ASBT has not been shown to be a transporter for non-bile salt substrates. While the transport function of NTCP can potentially be used as liver function test, interpretation of such a test may be complicated by altered expression of NTCP in diseases and presence of drugs that may inhibit NTCP function. Transport of bile salts by NTCP and ASBT is inhibited by a number of drugs and it appears that ASBT is more permissive to drug inhibition than NTCP. The clinical significance of this inhibition in drug disposition and drug-drug interaction remains to be determined. Both NCTP and ASBT undergo post-translational regulations that involve phosphorylation/dephosphorylation, translocation to and retrieval from the plasma membrane and degradation by the ubiquitin-proteasome system. These posttranslational regulations are mediated via signaling pathways involving cAMP, calcium, nitric oxide, phosphoinositide-3-kinase (PI3K), protein kinase C (PKC) and protein phosphatases. There appears to be species difference in the substrate specificity and the regulation of plasma membrane localization of human and rodent NTCP. These differences should be taken into account when extrapolating rodent data for human clinical relevance and developing novel therapies. NTCP has recently been shown to play an important role in HBV and HDV infection by serving as a receptor for entry of these viruses into hepatocytes.

Thermodynamic and solution state NMR characterization of the binding of secondary and conjugated bile acids to STARD5

STARD5 is a member of the STARD4 sub-family of START domain containing proteins specialized in the non-vesicular transport of lipids and sterols. We recently reported that STARD5 binds primary bile acids. Herein, we report on the biophysical and structural characterization of the binding of secondary and conjugated bile acids by STARD5 at physiological concentrations. We found that the absence of the 7α-OH group and its epimerization increase the affinity of secondary bile acids for STARD5. According to NMR titration and molecular modeling, the affinity depends mainly on the number and positions of the steroid ring hydroxyl groups and to a lesser extent on the presence or type of bile acid side-chain conjugation. Primary and secondary bile acids have different binding modes and display different positioning within the STARD5 binding pocket. The relative STARD5 affinity for the different bile acids studied is: DCA>LCA>CDCA>GDCA>TDCA>CA>UDCA. TCA and GCA do not bind significantly to STARD5. The impact of the ligand chemical structure on the thermodynamics of binding is discussed. The discovery of these new ligands suggests that STARD5 is involved in the cellular response elicited by bile acids and offers many entry points to decipher its physiological role.

The SLC10 carrier family: transport functions and molecular structure

The SLC10 family represents seven genes containing 1-12 exons that encode proteins in humans with sequence lengths of 348-477 amino acids. Although termed solute carriers (SLCs), only three out of seven (i.e. SLC10A1, SLC10A2, and SLC10A6) show sodium-dependent uptake of organic substrates across the cell membrane. These include the uptake of bile salts, sulfated steroids, sulfated thyroidal hormones, and certain statin drugs by SLC10A1 (Na(+)-taurocholate cotransporting polypeptide (NTCP)), the uptake of bile salts by SLC10A2 (apical sodium-dependent bile acid transporter (ASBT)), and uptake of sulfated steroids and sulfated taurolithocholate by SLC10A6 (sodium-dependent organic anion transporter (SOAT)). The other members of the family are orphan carriers not all localized in the cell membrane. The name "bile acid transporter family" arose because the first two SLC10 members (NTCP and ASBT) are carriers for bile salts that establish their enterohepatic circulation. In recent years, information has been obtained on their 2D and 3D membrane topology, structure-transport relationships, and on the ligand and sodium-binding sites. For SLC10A2, the putative 3D morphology was deduced from the crystal structure of a bacterial SLC10A2 analog, ASBT(NM). This information was used in this chapter to calculate the putative 3D structure of NTCP. This review provides first an introduction to recent knowledge about bile acid synthesis and newly found bile acid hormonal functions, and then describes step-by-step each individual member of the family in terms of expression, localization, substrate pattern, as well as protein topology with emphasis on the three functional SLC10 carrier members.

Structure-activity relationship for FDA approved drugs as inhibitors of the human sodium taurocholate cotransporting polypeptide (NTCP)

The hepatic bile acid uptake transporter sodium taurocholate cotransporting polypeptide (NTCP) is less well characterized than its ileal paralog, the apical sodium dependent bile acid transporter (ASBT), in terms of drug inhibition requirements. The objectives of this study were (a) to identify FDA approved drugs that inhibit human NTCP, (b) to develop pharmacophore and Bayesian computational models for NTCP inhibition, and (c) to compare NTCP and ASBT transport inhibition requirements. A series of NTCP inhibition studies were performed using FDA approved drugs, in concert with iterative computational model development. Screening studies identified 27 drugs as novel NTCP inhibitors, including irbesartan (Ki = 11.9 μM) and ezetimibe (Ki = 25.0 μM). The common feature pharmacophore indicated that two hydrophobes and one hydrogen bond acceptor were important for inhibition of NTCP. From 72 drugs screened in vitro, a total of 31 drugs inhibited NTCP, while 51 drugs (i.e., more than half) inhibited ASBT. Hence, while there was inhibitor overlap, ASBT unexpectedly was more permissive to drug inhibition than was NTCP, and this may be related to NTCP possessing fewer pharmacophore features. Findings reflected that a combination of computational and in vitro approaches enriched the understanding of these poorly characterized transporters and yielded additional chemical probes for possible drug-transporter interaction determinations.

Design and characterization of a novel fluorinated magnetic resonance imaging agent for functional analysis of bile Acid transporter activity

PURPOSE

To synthesize a trifluorinated bile acid that can be used for (19)F magnetic resonance imaging (MRI) of bile acid enterohepatic circulation, characterize its in vitro transporter affinity, stability, and (19)F-MRI signal, and assess its ability to concentrate in the gallbladder of C57BL/6 mice.

METHODS

Target compound CA-lys-TFA was synthesized and tested for affinity toward the apical sodium dependent bile acid transporter (hASBT) and the Na+/taurocholate cotransporting polypeptide (hNTCP). In a pilot study, fasted mice were gavaged with vehicle control, 150 mg/kg or 300 mg/kg CA-lys-TFA. CA-lys-TFA in gallbladder, liver and plasma at t = 5 h was quantified. Additionally, a 24-h time course (24 mice across eight time points) was studied using 50 mg/kg CA-lys-TFA.

RESULTS

CA-lys-TFA was a potent substrate of hASBT (Kt = 39.4 μM, normalized Vmax = 0.853) and hNTCP (Kt = 8.99 μM, normalized Vmax = 0.281). (19)F MRI phantom imaging showed linear signal-concentration dependence. In vivo studies showed that rapid accumulation of CA-lys-TFA in the gallbladder was maximal within 4-7 h.

CONCLUSIONS

These findings suggest that CA-lys-TFA, a fluorinated non-radioactive bile acid analogue, has potential for use in MRI to measure in vivo bile acid transport and diagnose bile acid malabsorption and other conditions associated with impaired bile acid transport.