The effect of tyrosine conjugation on the critical micellar concentration of free and glycine-conjugated bile salts

Dual Functionality of Bile Acid: Physical Stabilization of Drugs in the Amorphous Form and Solubility Enhancement in Solution

Drugs containing an amino aromatic nitrogen moiety were stabilized in the amorphous form by the surfactant cholic acid (CA). Coamorphous systems of lamotrigine (LAM), pyrimethamine (PYR), and trimethoprim (TRI) were each prepared with CA. Drug-CA interactions, investigated by IR and solid-sate NMR spectroscopy, revealed deprotonation of the carboxylic acid group in CA and the protonation of the most basic nitrogen of the drug. The coamorphous systems exhibited exceptional physical stability and resisted crystallization at (i) elevated temperatures (>100 °C) and (ii) accelerated storage conditions, 40 °C/75% relative humidity for 15 months. The dissolution performance of each coamorphous system was compared with the respective crystalline drug based on the area under the curve (AUC) of the concentration-time profiles. A 25-fold increase in AUC was observed in the PYR-CA coamorphous system. The solubility enhancement is attributed not only due to drug amorphization but also due to solubilization by CA. The supramolecular synthon approach, through a drug-CA interaction, yielded physically stable coamorphous systems with enhanced aqueous drug solubility.

Absorption-Enhancing Effects of Bile Salts

Bile salts are ionic amphiphilic compounds with a steroid skeleton. Among the most important physiological properties of bile salts are lipid transport by solubilization and transport of some drugs through hydrophobic barriers. Bile salts have been extensively studied to enhance transepithelial permeability for different marker molecules and drugs. They readily agglomerate at concentrations above their critical micelle concentration (CMC). The mechanism of absorption enhancement by bile salts appears to be complex. The aim of the present article was to review bile salt structure and their application as absorption enhancers and the probable mechanism for increasing permeation based on previous studies.

Hydrogelation of bile acid–peptide conjugates and in situ synthesis of silver and gold nanoparticles in the hydrogel matrix

A number of bile acid–peptide conjugates were synthesized and their hydrogelation properties were studied. These gels were used as scaffolds to in situ make Ag and Au nanoparticle–gel hybrids.

Thermodynamic and structural characterization of zwitterionic micelles of the membrane protein solubilizing amidosulfobetaine surfactants ASB-14 and ASB-16

Surface tension and isothermal titration calorimetry (ITC) were used to determine the critical micelle concentration (cmc) of the zwitterionic amidosulfobetaine surfactants ASB-14 and ASB-16 (linear-alkylamidopropyldimethylammoniopropanosulfonates) at 25 °C. The cmc and the heat of micellization were determined from 15 to 75 °C by ITC for both surfactants. The increase in temperature caused significant changes in the enthalpy and in the entropy of micellization, with small changes in the standard Gibbs energy (ΔG(mic)), which is consistent to an enthalpy−entropy compensation with a compensatory temperature of 311 K (ASB-14) and 314 K (ASB-16). In the studied temperature range, the heat capacity of micellization (ΔC(p)(mic)) was essentially constant. The experimental ΔC(p)(mic) was lower than that expected if only hydrophobic interactions were considered, suggesting that polar interactions at the head groups are of significant importance in the thermodynamics of micelle formation by these surfactants. Indeed, a NMR NOESY spectrum showed NOEs that are improbable to occur within the same monomer, resulting from interactions at the polar head groups involving more than one monomer. The ITC and NMR results indicate a tilt in the polar headgroup favoring the polar interactions. We have also observed COSY correlations typical of dipolar interactions that could be recovered with the partial alignment of the molecule in solution, which results in an anisotropic tumbling. The anisotropy suggested an ellipsoidal shape of the micelles, which results in a positive magnetic susceptibility, and ultimately in orientation induced by the magnetic field. Such an ellipsoidal shape was confirmed from results obtained by SAXS experiments that revealed aggregation numbers of 108 and 168 for ASB-14 and ASB-16 micelles, respectively. This study characterizes an interesting micelle system that can be used in the study of membrane proteins by solution NMR spectroscopy.

Structural biology and function of solute transporters: implications for identifying and designing substrates

Solute carrier (SLC) proteins have critical physiological roles in nutrient transport and may be utilized as a mechanism to increase drug absorption. However, we have little understanding of these proteins at the molecular level due to the absence of high-resolution crystal structures. Numerous efforts have been made in characterizing the peptide transporter (PepT1) and the apical sodium dependent bile acid transporter (ASBT) that are important for both their native transporter function as well as targets to increase absorption and act as therapeutic targets. In vitro and computational approaches have been applied to gain some insight into these transporters with some success. This represents an opportunity for optimizing molecules as substrates for the solute transporters and providing a further screening system for drug discovery. Clearly the future growth in knowledge of SLC function will be led by integrated in vitro and in silico approaches.

Modeling of active transport systems

Transport proteins have critical physiological roles in nutrient transport and may be utilized as a mechanism to increase drug absorption. However, we have little understanding of these proteins at the molecular level due to the absence of high-resolution crystal structures. Numerous efforts have been made to characterize the P-glycoprotein efflux pump, the peptide transporter (PepT1) and the apical sodium-dependent transporter (ASBT) which are important not only for their native transporter function but also as drug targets to increase absorption and bioactivity. In vitro and computational approaches have been applied to gain some insight into these transporters with some success. This represents an opportunity for optimizing molecules as substrates for the solute transporters and providing a further screening system for drug discovery. Clearly the future growth in knowledge of transporter function will be led by integrated in vitro and in silico approaches.

Synthesis, physical and biological properties of lithocholyl-lysyl-fluorescein: a fluorescent monohydroxy bile salt analogue with cholestatic properties

We have synthesised and characterised a fluorescent monohydroxy bile salt analogue, lithocholyl-lysyl-fluorescein and compared its physical and biological properties with those of lithocholate, glycolithocholate, sulpholithocholate, lithocholic acid glucuronide and taurocholate. The synthetic method used excess N-epsilon-CBZ-L-lysine methyl ester hydrochloride and lithocholic acid via N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinolone (EEDQ) to give lithocholyl-lysine. Lithocholyl-lysyl-fluorescein (LLF) was then prepared using equimolar amounts of lithocholyl-lysine and fluorescein isothiocyanate (FITC) in bicarbonate buffer. LLF retained an apple green fluorescence, similar to that of fluorescein. Unlike lithocholate, the critical micellar concentrations (CMCs) of LLF, glycolithocholate (GLC), lithocholic acid glucuronide (LG) and sulpholithocholic acid (SLC) were similar. HPLC retention times (tRs) of LLF and GLC were similar with a ratio of LLF/GLC of 1.05. In contrast, the tR of SLC (6.52 min) but not of LG (21.2 min) was more comparable to that of taurocholate (5.73 min). In rats under pentobarbital anaesthesia, the plasma half-life (t(1/2alpha)) (min) was 4.5 +/- 1.3 (n = 6) for LLF, 2.9 +/- 0.4 (n = 5) for [14C]sulpholithocholate (14C-SLC) and 4.3 +/- 0.3 (min) for [14C]lithocholic acid glucuronide (14C-LG). Plasma clearances of 14C-SLC, LLF and 14C-LG were 15.5 +/- 2.2 (n = 6), 18.1 +/- 4.2 (n = 6) and 17.8 +/- 0.5 ml/min/kg (n = 6) (P = 0.15), respectively. Biliary excretion in bile-fistula rats gave cumulative 20 min biliary output as a percentage of injected dose as follows: LLF, 71.6 +/- 0.8% (n = 10); 14C-SLC, 75.5 +/- 2.8% (n = 6) and 14C-LG, 61.7 +/- 0.5% (n = 6) (P = NS). Peak biliary excretion rates, given as % dose/2 min, were 10.2 +/- 0.3 for LLF, 13.5 +/- 0.6 for 14C-SLC and 12.8 +/- 0.4 for 14C-LG. In another group of bile-fistula rats, a 3.0 micromol/500 microl saline i.v. bolus of LLF caused a 15.4 +/- 1.9% decrease in bile flow and, similarly, sodium lithocholate in a solution of albumin caused a 17.9 +/- 1.8% (P = NS) diminution in bile flow. Despite the similar cholestatic properties of LLF and lithocholate, LLF was more soluble than lithocholate, with a relative retention time on HPLC similar to that of GLC. LLF is a divalent 'unipolar' anionic fluorescent monohydroxy bile salt analogue with physical, biological and cholestatic properties that are similar to those of lithocholate, glycolithocholate and their derivatives and thus offers a potentially useful probe for studying mechanisms of monohydroxy bile salt-induced cholestasis at the hepatocellular level.