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•l. Soc. Cosmet. Chem., 36, 303-311 (July/August 1985) Comparative study of propylene glycol and caprylic/capric triglyceride vehicles for topical application DINESH PATEL,* DON WELSH, and MICHAEL BAKER, Alcon Laboratories, Inc., Fort Worth, TX 76134. Received October 17, 1984. Presented at the American Pharmaceutical Association Academy of Pharmaceutical Sciences 36th National Meeting, Montreal, May 5-10, 1984. Synopsis The influence of propylene glycol and caprylic/capric triglyceride on the in vitro percutaneous absorption and on the in vivo vasoconstriction activity of various steroids was studied. Propylene glycol is widely used as a solvent for topical applications. Although the use of propylene glycol improves drug solubility and bioavailability in topical formulations, propylene glycol has been reported to cause irritation and/or sen- sitization when the concentration exceeds ten percent. Caprylic/capric triglyceride has been shown to be a non-irritating and non-sensitizing solvent. In the in vitro study, caprylic/capric triglyceride vehicles gave slightly higher steady-state penetration flux--with a maximum being a four-fold difference with diflo- rasone diacetate. There were no statistically significant differences between the vehicles in the in vivo vasoconstriction assay. From this study, it is apparent that caprylic/capric triglyceride is an acceptable vehicle for topical applications and is the vehicle of choice in optimized formulations requiring high solvent concentrations. INTRODUCTION Propylene glycol is widely used as a solvent for topical application (1-14). Incorpo- ration of propylene glycol into a vehicle base may modify the barrier properties in the stratum corneum, facilitating the passage of the active drug into the skin (15). Polano et al. (9) found that propylene glycol carries at least a portion of the steroid through the epidermis membrane. As propylene glycol is a humectant, it increases stratum corneum hydration, resulting in an increase in the penetration rate across the skin. Although the use of propylene glycol improves drug solubility and bioavailability in topical formulations, propylene glycol has been reported to cause irritation and/or sensitization when the concentration exceeds ten percent (16-28). Using the Draize skin test (16) and the chamber scarification test (28), propylene glycol has been classified as a moderate irritant. The Draize test (16) also indicates that propylene glycol might be a sensitizer. Present address: Therapeutics Technologies, Inc., Salt Lake City, Utah. 3O3