Absorption and excretion of drugs. XXVII. Effect of nonionic surface- active agents on rectal absorption of sulfonamides

Improvement of intestinal absorption of curcumin by cyclodextrins and the mechanisms underlying absorption enhancement

Curcumin is known to possess a wide range of pharmacological activities for the treatment of chronic or inflammatory diseases, Alzheimer's disease, and various cancers. However, the therapeutic efficacy of curcumin is restricted by its poor bioavailability after oral administration. In this study, the effects of various cyclodextrins on the intestinal absorption of curcumin were evaluated in rat intestine by an in situ closed-loop method. Among the tested cyclodextrins, 50 mM α-cyclodextrin significantly enhanced the absorption of curcumin without inducing any intestinal toxicity. The analysis of cellular transport across Caco-2 cell monolayers showed that 50 mM α-cyclodextrin reduced the transepithelial electrical resistance value of cell monolayers and improved the permeability of 5(6)-carboxyfluorescein, a poorly absorbable drug, which is mainly transported via a paracellular pathway. Furthermore, the western blotting analysis showed that α-cyclodextrin decreased the expression of claudin-4, a tight junction-associated protein, in brush border membrane vesicles. Additionally, α-cyclodextrin increased the membrane fluidity of lipid bilayers in brush border membrane vesicles and may also have promoted the permeation of drug molecules via a transcellular pathway. These results suggested that α-cyclodextrin might enhance the intestinal absorption of curcumin via both paracellular and transcellular pathways.

Modification of intestinal absorption of drugs by lipoidal adjuvants

The use of various lipids for the modification of intestinal absorption of lipophilic, hydrophilic, or macromolecular drugs is reviewed. The influence of lipids on drug absorption varies with the structure and physical state of incorporated lipids. The mechanisms of drug absorption across the intestine, that involve lipids in the state of emulsions, liposomes, and micelles, are discussed. The use of fusogenic lipids in the micellar state can be most effective in enabling the absorption of poorly absorbed drugs, such as antibiotics and macromolecules. Moreover, within the gastrointestinal tract their promoting ability is greatest in the colorectal region. Fusogenic lipids are also useful for selective lymphatic delivery of drugs with a macromolecular carrier.

Improvement of Large Intestinal Absorption of Insulin by Chemical Modification with Palmitic Acid in Rats

The intestinal absorption of 125I-labelled palmitoyl insulin was examined following administration into in-situ closed large intestinal loops of rats. When mono- and dipalmitoyl insulins (Palins-1 and Palins-2, respectively) were administered in polyoxyethylene hydrogenated castor oil (HCO 60) micellar system into intestinal loops, a marked increase in plasma radioactivity and a corresponding disappearance of residual radioactivity in the intestinal lumen were observed in the following rank order: Palins-2 > Palins-1 > native insulin. In addition, the derivatives were more stable than native insulin in the mucosal tissue homogenates of the large intestine. These results suggest that chemical modification of insulin with palmitic acid may not only increase the lipophilicity of insulin but also reduce its degradation, resulting in the increased transfer of insulin across the large intestinal mucous membrane. The linoleic acid-HCO 60 mixed micelles system did not have a significant effect on the large intestinal absorption of radioactivity associated with the lipophilic insulin analogues.

Role of formulation vehicles in taxane pharmacology

The non-ionic surfactants Cremophor EL (CrEL) and Tween 80, both used as formulation vehicles of many (anticancer) agents including paclitaxel and docetaxel, are not physiological inert compounds. We describe their biological properties, especially the toxic side effects, and their pharmacological properties, such as modulation of P-glycoprotein activity. In detail, we discuss their influence on the disposition of the solubilized drugs, with focus on CrEL and paclitaxel, and of concomitantly administered drugs. The ability of the surfactants to form micelles in aqueous solution as well as biological fluids (e.g. plasma) appears to be of great importance with respect to the pharmacokinetic behavior of the formulated drugs. Due to drug entrapment in the micelles, plasma concentrations and clearance of free drug change significant leading to alteration in pharmacodynamic characteristics. We conclude with some perspectives related to further investigation and development of alternative methods of administration.

Coulter Counter measurements of solubility and dissolution rate of sparingly soluble compounds using micellar solutions

A Coulter Counter, Model TAII, was used to determine both solubility and surface specific dissolution rate of two sparingly soluble materials suspended in micellar solutions. The equilibrium solubility increased linearly with surfactant concentration, thereby making it possible by extrapolation to characterize materials with an aqueous solubility down to 1 ppm or less. At high concentrations (greater than 0.1% w/v) the effect of surfactant concentration on the surface specific dissolution rate was less than that predicted from the increased bulk solubility.

Interaction of substituted benzoic acids with polysorbate 20 micelles

Equilibrium solubilities of a series of substituted benzoic acids in different concentrations of polysorbate 20 at controlled pH were measured. The maintenance of pH was achieved using a pH-stat assembly. A linear relationship was found between the amount of benzoic acid solubilized and surfactant concentration. As solubilizate polarity increased, the amount solubilized also increased. Solubility data were analyzed, and the interaction between solubilizate molecules and micelles was calculated in terms of partition coefficients of ionized and unionized molecules between aqueous and micellar phases. A linear relationship between pi values (log partition coefficients) of functional groups and aqueous-micellar partition coefficient was found.

The correlation between the in vitro and in vivo availability of salicylic acid for membrane transfer from polysorbate 20 solutions

The presence of 1·0% w/v polysorbate 20 in solutions of salicylic acid reduces the absorption of the drug from the rat stomach. This effect is the resultant of two opposing effects of polysorbate 20 on salicylic acid absorption. Firstly, the surfactant reduces salicylic acid absorption by solubilization and secondly the surfactant tends to enhance salicylic acid absorption by an alternate unidentified effect. Knowing the magnitude of the absorption-enhancing effect the overall effect on salicylic acid absorption can be calculated using equations that describe the availability of salicylic acid for transfer across a Cellophane membrane from polysorbate solutions.

A quantitative approach to the in vitro availability of drugs from some non-ionic surfactant solutions

Equations that can be used in calculating the theoretical dialysis rates of drugs from surfactant solutions have been derived. Theoretical dialysis rates for the dialysis of salicylic acid from polysorbate solutions at a range of pH levels are compared with those determined experimentally. At low pH levels there is good agreement between theoretical and experimental values. With increasing pH at each surfactant concentration the values diverge. A possible reason for this divergence is considered.

Effects of solubilization on drug diffusion

The diffusion of ephedrine, sulphathiazole, chloramphenicol, paracetamol, isoniazid and amphetamine in solutions of Tween 40, Tween 80 and cetomacrogol 1000 have been studied. With the exception of isoniazid, the observed diffusion coefficients, corrected for resistance to flow, depend upon the surfactant used and both the drug and surfactant concentration. The effect of drug solubilization upon the diffusion coefficient is also discussed.