Influence of side-chain charge on hepatic transport of bile acids and bile acid analogues

New Kids on the Block: Bile Salt Conjugates of Microbial Origin

Biotransformation of host bile salts by gut microbes results in generation of secondary bile salt species that have biological and physicochemical properties that are distinct from the parent compounds. There is increased awareness that a bile salt–gut microbiome axis modulates various processes in the host, including innate and adaptive immunity, by interaction of microbial bile salt metabolites with host receptors. Omics and targeted approaches have vastly expanded the number and repertoire of secondary bile salt species. A new class of microbial bile salt metabolites was reported in 2020 and comprises bile salts that are conjugated by microbial enzymes. Amino acids other than those employed by host enzymes (glycine and taurine) are used as substrates in the formation of these microbial bile salt conjugates (MBSCs). Leucocholic acid, phenylalanocholic acid and tyrosocholic acid were the first MBSCs identified in mice and humans. The number of distinct MBSCs is now approaching 50, with variation both at the level of bile salt and amino acid employed for conjugation. Evidence is emerging that MBSC generation is a common feature of human gut bacteria, and initial links with disease states have been reported. In this review, we discuss this intriguing new class of secondary bile salts, with yet enigmatic function.

Exploitation of bile acid transport systems in prodrug design

The enterohepatic circulation of bile acids is one of the most efficient recycling routes in the human body. It is a complex process involving numerous transport proteins, which serve to transport bile acids from the small intestine into portal circulation, from the portal circulation into the hepatocyte, from the hepatocyte into the bile, and from the gall bladder to the small intestine. The tremendous transport capacity and organ specificity of enterohepatic circulation combined with versatile derivatization possibilities, rigid steroidal backbone, enantiomeric purity, availability, and low cost have made bile acids attractive tools in designing pharmacological hybrid molecules and prodrugs with the view of improving intestinal absorption, increasing the metabolic stability of pharmaceuticals, specifically targeting drugs to organs involved in enterohepatic circulation, as well as sustaining therapeutically reasonable systemic concentrations of active agents. This article briefly describes bile acid transport proteins involved in enterohepatic circulation, summarizes the key factors affecting on the transport by these proteins, and reviews the use of bile acids and their derivatives in designing prodrugs capable of exploiting the bile acid transport system.

Enterohepatic circulation: physiological, pharmacokinetic and clinical implications

Enterohepatic recycling occurs by biliary excretion and intestinal reabsorption of a solute, sometimes with hepatic conjugation and intestinal deconjugation. Cycling is often associated with multiple peaks and a longer apparent half-life in a plasma concentration-time profile. Factors affecting biliary excretion include drug characteristics (chemical structure, polarity and molecular size), transport across sinusoidal plasma membrane and canniculae membranes, biotransformation and possible reabsorption from intrahepatic bile ductules. Intestinal reabsorption to complete the enterohepatic cycle may depend on hydrolysis of a drug conjugate by gut bacteria. Bioavailability is also affected by the extent of intestinal absorption, gut-wall P-glycoprotein efflux and gut-wall metabolism. Recently, there has been a considerable increase in our understanding of the role of transporters, of gene expression of intestinal and hepatic enzymes, and of hepatic zonation. Drugs, disease and genetics may result in induced or inhibited activity of transporters and metabolising enzymes. Reduced expression of one transporter, for example hepatic canalicular multidrug resistance-associated protein (MRP) 2, is often associated with enhanced expression of others, for example the usually quiescent basolateral efflux MRP3, to limit hepatic toxicity. In addition, physiologically relevant pharmacokinetic models, which describe enterohepatic recirculation in terms of its determinants (such as sporadic gall bladder emptying), have been developed. In general, enterohepatic recirculation may prolong the pharmacological effect of certain drugs and drug metabolites. Of particular importance is the potential amplifying effect of enterohepatic variability in defining differences in the bioavailability, apparent volume of distribution and clearance of a given compound. Genetic abnormalities, disease states, orally administered adsorbents and certain coadministered drugs all affect enterohepatic recycling.

Squalamine: an aminosterol antibiotic from the shark

In recent years, a variety of low molecular weight antibiotics have been isolated from diverse animal species. These agents, which include peptides, lipids, and alkaloids, exhibit antibiotic activity against environmental microbes and are thought to play a role in innate immunity. We report here the discovery of a broad-spectrum steroidal antibiotic isolated from tissues of the dogfish shark Squalus acanthias. This water-soluble antibiotic, which we have named squalamine, exhibits potent bactericidal activity against both Gram-negative and Gram-positive bacteria. In addition, squalamine is fungicidal and induces osmotic lysis of protozoa. The chemical structure of the antibiotic 3 beta-N-1-(N-[3-(4-aminobutyl)]- 1,3-diaminopropane)-7 alpha,24 zeta-dihydroxy-5 alpha-cholestane 24-sulfate has been determined by fast atom bombardment mass spectroscopy and NMR. Squalamine is a cationic steroid characterized by a condensation of an anionic bile salt intermediate with spermidine. The discovery of squalamine in the shark implicates a steroid as a potential host-defense agent in vertebrates and provides insights into the chemical design of a family of broad-spectrum antibiotics.

Properties of the canalicular bile acid transport system in rat liver

4,4-Di-isothiocyanostilbene-2,2'-disulphonic acid inhibition of taurocholate efflux from canalicular vesicles was used to demonstrate that potential driven and 'carrier'-mediated canalicular excretion of taurocholate occur via a common, rather than two separate, pathways. This electrogenic canalicular bile acid 'carrier' preferentially transports trihydroxylated and conjugated dihydroxylated bile acids, but not the unphysiological oxo bile acids, and possibly extends its substrate specificity to other amphipathic molecules such as sulphobromophthalein.