Abstract
The development of a reliable in vitro permeation system necessitates the use of a precise and accurate method of quantifying tb~ amount of permeant partitioning from the membrane into the cell receptor phase. Aqueous donor and receptor chamber fluids have ~en used in the majority of reported investigations, which makes quantitative permeant analysis relatively facile. Alternatively, radiolabelled diffusants have been used and flux rates monitored by scintillation counting, obviating the need for chromatographic separation of the receptor-phase components. However, this technique is not applicable when nonlabelled compounds or commercial dosage forms are to be evaluated by a cell system. Furthermore, several studies indicate that aqueous receptor phases may not present an optimal partitioning environment for certain lipophilic permeants (1~), thereby impairing accurate flux monitoring due to limited diffusant solubility. Several attempts have thetefore been made to improve the partitioning environment within these systems, by the addition of surfactants for example (4). A lipophilic receptor environment appears beneficial for corticosteroid partitioning, and thus, the use of isopropyl myristate has been investigated because of its bipolar properties that tend to mimic the biochemical composition of the skin (5,6). Betamethasone 17-valerate and its 21-valerate degradation product are highly soluble in isopropyl myristate and this nonaqueous solvent willnot augmentC-17-to-C-21ester degradation reactions. The UV absorbance of the receptor-phase solutions has been measured directly by spectrophotometry without prior chromat9graphic'separation (7-10). However, compounds other than the diffusant of interest may permeate from the donor compartment, or may leach fromthe membrane, and may contribute to this measured absorbance. Therefore,
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