Micellar solubilisation of corticoids by nonionic surfactants.

Summary The availaibility of a drug in the presence of surfactants depends on the amount of free drug in solution. In previous work (1), we tried to measure the amount of free corticoïds (Dexamethasone, Fluprednylidene 21 - acetate and Betamethasone 17 - valerate) in topical preparations containing various amounts of non-ionic surfactants (ethers of polyoxyethyleneglycol. With this aim, experiments have been carried out to characterise the nature of the interaction between these drugs and the surfactants. We have studied the micellar solubilisation of the three steroïds by means of solubility measurements. From the results obtained, we have calculated the solubilising capacities of surfactants expressed on molar basis and we can assume that solubilisation probably takes place in the inner part of micelles. Calculations of solubility ratios (R) as a function of the percentage of surfactant enable determination of a binding capacity of the surfactants for the drugs. This value is a constant independent of the concentration of the surfactants. If the distribution law is obeyed, the interaction can also be expressed by a partition coefficient calculated from the results of the solubility measurements. To verify whether the distribution law really applies and if the interaction is independent of the amount of free drug in solution, we have employed, for Dexamethasone, the methods of equilibrium and differential dialysis.