Function Of Bile Salt Concentration Research Articles

Impact of Drug-Rich Colloids of Itraconazole and HPMCAS on Membrane Flux in Vitro and Oral Bioavailability in Rats.

Improving the oral absorption of compounds with low aqueous solubility is a common challenge that often requires an enabling technology. Frequently, oral absorption can be improved by formulating the compound as an amorphous solid dispersion (ASD). Upon dissolution, an ASD can reach a higher concentration of unbound drug than the crystalline form, and often generates a large number of sub-micrometer, rapidly dissolving drug-rich colloids. These drug-rich colloids have the potential to decrease the diffusional resistance across the unstirred water layer of the intestinal tract (UWL) by acting as rapidly diffusing shuttles for unbound drug. In a prior study utilizing a membrane flux assay, we demonstrated that, for itraconazole, increasing the concentration of drug-rich colloids increased membrane flux in vitro. In this study, we evaluate spray-dried amorphous solid dispersions (SDDs) of itraconazole with hydroxypropyl methylcellulose acetate succinate (HPMCAS) to study the impact of varying concentrations of drug-rich colloids on the oral absorption of itraconazole in rats, and to quantify their impact on in vitro flux as a function of bile salt concentration. When Sporanox and itraconazole/AFFINISOL High Productivity HPMCAS SDDs were dosed in rats, the maximum absorption rate for each formulation rank-ordered with membrane flux in vitro. The relative maximum absorption rate in vivo correlated well with the in vitro flux measured in 2% SIF (26.8 mM bile acid concentration), a representative bile acid concentration for rats. In vitro it was found that as the bile salt concentration increases, the importance of colloids for improving UWL permeability is diminished. We demonstrate that drug-containing micelles and colloids both contribute to aqueous boundary layer diffusion in proportion to their diffusion coefficient and drug loading. These data suggest that, for compounds with very low aqueous solubility and high epithelial permeability, designing amorphous formulations that produce colloids on dissolution may be a viable approach to improve oral bioavailability.

Bile acids affect liver mitochondrial bioenergetics: possible relevance for cholestasis therapy.

It has been pointed out that intracellular accumulation of bile acids cause hepatocyte injury in cholestatic disease process. This study was aimed to test if cytotoxicity of these compounds is mediated through mitochondria dysfunction. Bile acids effects on isolated rat liver mitochondrial were analyzed by monitoring changes in membrane potential and mitochondrial respiration, as well as alterations in H(+) membrane permeability and mitochondrial permeability transition pore induction. Increasing concentrations of the bile acids litocholic (LCA), deoxycholic (DCA), ursodeoxycholic (UDCA), chenodeoxycholic (CDCA), glycochenodeoxycholic (GCDC), or taurochenodeoxycholic (TCDC) decrease transmembrane potential (delta psi) developed upon succinate energization. These compounds also decreased state 3 respiration and enhanced state 4. We have also demonstrated that the observed concentration-dependent stimulation of state 4 by LCA, DCA, CDCA, TCDC, and GCDC, is associated with an enhanced permeability of mitochondria to H(+). Addition of LCA, DCA, CDCA, TCDC, GCDC, and UDCA to mitochondria energized with succinate resulted in a dose-dependent membrane depolarization and stimulation of mitochondrial permeability transition. Tauroursodeoxycholate (TUDC) elicited no significant effect on succinate-supported mitochondrial bioenergetics. In contrast, in the presence of glycoursodeoxycholic (GUDC), delta psi increases as a function of bile salt concentration. The results of this investigation demonstrate that at toxicologically relevant concentrations, most but not all bile acids alter mitochondrial bioenergetics, so impairment of mitochondrial function can be clinically relevant for patients with cholestasis.

A physicochemical Basis for the Effect of Food on the Absolute Oral Bioavailability of Halofantrine

AbstractHalofantrine hydrochloride (Hf·HCl) is a highly lipophilic phenanthrenemethanol antimalarial. The poor and erratic absorption of Hf after oral administration has been implicated in some treatment failures. Food increases the oral bioavailability of Hf in humans approximately 3–5–fold, although neither the absolute bioavailability nor the basis for the food effect has been fully defined. In this study, the mean (± SD, n=3) absolute oral bioavailability of 250mg Hf·HCl tablets in beagles was 8.6±5.3% in the fasted state, which increased approximately 12-fold when administered postprandially. These data indicate that Hf·HCl is efficiently absorbed from the postprandial intestinal environment. The solubility and intrinsic dissolution rate of Hf·HCl were investigated as a function of bile salt concentration (sodium taurocholate, NaTC, 0–30mM) and micellar composition (4:1 NaTC:lecithin). At premicellar (fasted) concentrations of NaTC (<5mM), the solubility and intrinsic dissolution rate were very low (<15 μg/mL; <0.01 μg s−1 cm−2). At NaTC concentrations typical of the postprandial state, the solubility and dissolution rate improved dramatically. For example, solubility in 30mM NaTC increased approximately 1000-fold relative to buffer control, with even greater enhancement (3000-fold) associated with mixed micellar systems. These data suggest that the improved absorption of Hf·HCl in the fed state is most likely due to the increased solubilization and dissolution of the drug in the presence of bile salt mixed micelles.

Estimation of the increase in solubility of drugs as a function of bile salt concentration.

The objective of this study was to develop a model to predict the extent to which bile salts can enhance the solubility of a drug, based on the physicochemical properties of the compound. The ability to predict bile salt solubilization of poorly soluble drugs would be a key component in determining which drugs will exhibit fed vs. fasted differences in drug absorption. A correlation between the logarithm of the octanol/water partition coefficient [log P] of six steroidal compounds and their solubilities in the presence of various concentrations of sodium taurocholate at 37 degrees C, log [SR] = 2.234 + 0.606 log [P] (r2 = 0.987) where SR is the ratio of the stabilization capacity of the bile salt to the solubilization capacity of water for the drug, was used to predict the solubility of the compounds in presence of sodium taurocholate were then measured. The predicted solubilities were within 10% of the experimentally observed solubilities for griseofulvin, cyclosporin A and pentazocine. The model overpredicted the solubility of phenytoin and diazepam in 15 mM sodium taurocholate solution by a factor of 1.33 and 1.62 respectively. The expected increase in solubility as a function of bile salt concentration can be estimated on the basis of the partition coefficient and aqueous solubility of the compound.

Bile salt-induced disintegration of egg phosphatidylcholine liposomes: A kinetic study based on turbidity changes.

The disintegration kinetics of egg phosphatidylcholine small unilamellar liposomes in various bile salts (nine species) were investigated by monitoring turbidity changes with a stopped-flow apparatus. The pseudo-first-order rate constants obtained as a function of bile salt concentration (up to 25 mM) were analyzed based on a two-step model in which a penetration-saturation step of bile salt into the bilayer and a lamellar-micellar transition step were assumed for the disintegration mechanism of the bilayer. The order of the rate of the penetration-saturation step, which is assumed to be a measure of the disintegration ability, was as follows: SCDOC greater than SDOC greater than STCDOC greater than STDOC greater than STC greater than SC greater than SGCDOC greater than SGDOC greater than SGC. The results indicated that (1) the dihydroxy bile salts have a greater disintegration ability than the corresponding trihydroxy bile salts, (2) the chenodeoxy bile salts have a greater ability than the corresponding deoxy-bile salts regardless of non-conjugated or conjugated form, (3) the taurine conjugates always have a greater ability than the glycine conjugates. The penetration-saturation rate of the bile salts against the lipid bilayer depends considerably on the chemical nature of each bile salt, varying by a factor of about 10(5). In the conjugated bile salts alone, they were in a narrower range of a factor of 10(3). The physical integrity of liposomes can hardly be maintained in the bile salt-rich intestinal tract but the resulting mixed micelles may contribute substantially to solubilization and enhanced delivery of drugs.

A two-step model of disintegration kinetics of liposomes in bile salts.

The disintegration kinetics of egg phosphatidylcholine small unilamellar liposomes (SUV) in sodium deoxycholate (SDOC) and sodium cholate (SC) was investigated as a mimic of the disintegration behavior of orally administered liposomes in the intestine. The disintegrating action of the bile salts was followed by measuring turbidity changes with a stopped-flow apparatus, from which the pseudo-first-order disintegration rate constant (kobs) was calculated as a function of bile salt concentration (up to 25mM). The disintegration rate of the SUV in SDOC was considerably higher than that in SC, and the kobs increased with a tendency to reach a plateau in SDOC but not in SC. At 20 mM bile salts, the disintegration half-life of the SUV (1.5 mM phosphorus) was 0.003s in SDOC and was 0.69s in SC. A model in which the penetration-saturation step of bile salt molecules and the subsequent lamellar-micellar transition step are involved was applied to analyses of the disintegration kinetics. The results showed that SDOC molecules penetrate into the bilayer structure faster than SC molecules by a factor of 5×102. This is probably due to the difference of molecular surface avaliable for hydrophobic interaction.

Distribution of Bile Salts Between 1-Octanol and Aqueous Buffer

The distribution of four bile salts: sodium cholate (I), sodium deoxycholate (II), sodium chenodeoxycholate (III), and sodium urso- deoxycholate (IV), between aqueous buffer and 1-octanol has been measured as a function of temperature between 25 and 55° and as a function of bile salt concentration at concentrations <0.1 mole/liter in the aqueous phase. The distribution isotherms obtained have been explained on the basis of reversible association in the aqueous phase. The treatment assumes that the bile acid exists as a monomer in the organic phase, which is verified by vapor pressure osmometry. A graphical method has been employed to estimate the association constants in the aqueous phase for the various equilibria encountered. An aggregation number of four for IV and 12 for I, II, and III has been estimated. From the results, thermodynamic functions associated with the transfer of each of the bile salts from water to octanol and those associated with association processes in the aqueous phase were calculated. These results are consistent with previous findings that the premicellar association of bile salts occurs by hydrophobic interaction. The thermodynamics of transfer of bile salts revealed an unfavorable enthalpic and favorable entropic contribution for all four bile salts. However, for IV, which is an epimer of III, both enthalpic and entropic contributions are reduced, compared to III, suggesting a pronounced effect of stereochemical orientation on hydrophobic interaction.

Fluorescent probe and NMR studies of the aggregation of bile salts in aqueous solution

The binding of the fluorescent probes 1-anilino-8-naphthalene sulfonate and dansyl cadaverine to the sodium salts of cholic, deoxycholic and dehydrocholic acids has been investigated. Enhanced probe solubilisation accompanies aggregation. Monitoring of fluorescence intensities as a function of bile salt concentration permits the detection of primary micelle formation, as well as secondary association. The transition concentrations obtained by fluorescence are in good agreement with values determined for the critical micelle concentrations, by other methods. Differences in the behaviour of cholate and deoxycholate have been noted. Fluorescence polarisation studies of 1,6-diphenyl-1,3,5-hexatriene solubilised in bile salt micelles suggest a higher microviscosity for the interior of the deoxycholate micelle as compared to cholate. 1H NMR studies of deoxycholate over the range 1–100 mg/ml suggest that micelle formation leads to a greater immobilisation of the C 18 and C 19 methyl groups as compared to the C 21 methyl group. Well resolved 13C resonances are observed for all three steroids even at high concentration. Both fluorescence and NMR studies confirm that dehydrocholate does not aggregate.

Physiologic Surface-Active Agents and Drug Absorption III: Effect of Bile Salt on Drug Absorption in Goldfish

Sodium taurodeoxycholate significantly increases the uptake of nonionized 4-aminoantipyrine in goldfish. Studies of drug uptake as a function of bile salt concentration suggest that sodium taurodeoxycholate exerts an essentially all-or-none effect which is consistent with a mechanism involving rapid adsorption of the bile salt on the external membranes of the fish and consequent alteration of membrane permeability when a minimum “surface” concentration is reached. Evidence is also presented to demonstrate that the adsorption of 4-aminoantipyrine, in the presence and absence of sodium taurodeoxycholate, involves a passive process.