FORMULATION FACTORS AFFECTING RELEASE OF DRUG FROM TOPICAL FORMULATIONS .1. EFFECT OF EMULSION-TYPE UPON IN-VITRO DELIVERY OF ETHYL P-AMINOBENZOATE

There are numerous kinetic and thermodynamic processes which occur during the transport of a topically applied drug from within its vehicle to the site of action in the skin. The present study compares the release and permeation of a model compound, ethyl p-aminobenzoate, from water, mineral oil, a mineral oil in water (O/W) emulsion, and a water in mineral oil (W/O) emulsion. These formulations are compared for differences in transport characteristics using in vitro diffusion studies and a synthetic polydimethylsiloxane membrane. Two factors appear prominent in influencing the release kinetics of the drug from its formulation. The first is the observation that the drug is released 2 times stower from mineral oil than from water at approximately equivalent thermodynamic activities, and the second is the appreciable lowering of the thermodynamic activity as a result of substantial micellar solubilization of the drug in the aqueous phase of the O/W emulsion. The thermodynamic activity effect predominants, resulting in a lower release rate from the O/W emulsion as compared to the W/O emulsion.