j. Cosmet. Sci,, 52, 225-236 (July/August 2001) Effect of formulation on the delivery and metabolism of -tocopheryl acetate MEERA RANGARAJAN and JOEL L. ZATZ, Schering Plough Research institute, Kenilworth, NJ 07033-0530 (M.R.), and Rutgers-The State University of New Jersey, Department of Pharmaceutics, College of Pharmacy, 160 Fmlinghuysen Road, Piscataway, NJ 08854-8020 •. R. Z.). Accepted for publication April 15, 2001. Data contained in this paper were presented at the Annual Scientific Seminar of the Society of Cosmetic Chemists, Toronto, Ontario, May 11-12, 2000. Synopsis The effect of delivery system on the permeation and metabolism of o•-tocopheryl acetate (o•-TAc) was studied in micro-Yucatan pig skin, which closely resembles human skin. Various o•-tocopheryl acetate formulations, including a simple isopropyl myristate (IPM) solution, an o/w emulsion, microemulsions, which diffbred in their oily phase content, and alcoholic and hydroalcoholic gels were made. A suitable HPLC method was developed and validated to separate and quantify o•-TAc and o•-tocopherol (o•-T). Dulbecco's modified phosphate-buffered saline with 3% bovine serum albumin (DMPBS-BSA 3%) served as the receptor media to ensure tissue viability and to maintain skin conditions. Finite doses (5 pl) of the formulations were applied to viable pig skin using a statistically approved randomized complete block design. Data were analyzed using Tukey's studentized range test, and interday variability was estimated using an F-test. About 70% of the active was recovered from the wash, representing the amount adhering to the surface of the skin. o•-TAc underwent metabolism in pig skin to the active antioxidant, o•-T. The identity of the HPLC peaks were confirmed by spiking studies using known standards. The extent of metabolism was found to be formulation-dependent. No o•-T was, however, detected in the stratum corneum. A higher extent of metabolism was obtained for the IPM solution, a microemulsion containing IPM as the oily phase, and the hydroalcoholic gel, when calculated based on the percent of total o•-TAc permeated in the viable skin. Metabolism occurred in pig skin to the extent of 15-20% in terms of the total amount of o•-TAc permeated in the viable skin and stratum corneum. Thus the topical delivery and metabolism ofo•-TAc were found to be dependent on formulation. INTRODUCTION The involvement of free radicals in skin damage including sunburn, photoaging, and photocarcinogenesis has led to the use of antioxidants such as o•-tocopherol (o•-T) in various skin preparations. o•-T acts primarily as a lipophilic radical-scavenging antioxi- dant and suppresses chain initiation and/or chain propagation steps by donating its 225

226 JOURNAL OF COSMETIC SCIENCE 6-phenolic hydrogen to the oxygen radicals. The free OH group functions as a scavenger of free radicals or singlet oxygen, usually being oxidized itself in the process. The hydroxyl group can be protected from oxidation by esterification with the carboxyl group of an organic acid, usually acetic acid, forming esters such as acetate or succinate derivatives, which are devoid of antioxidant activity. tx-Tocopheryl acetate, a liquid oil, when applied to the skin, acts as a prodrug and has to release the active tx-T by an enzyme-catalyzed hydrolytic reaction. The skin is a viable membrane that can metabolize topically applied substances before they reach the systemic circulation. The metabolizing potential of skin for actives has been estimated to be about 2% of that of liver (1). Cutaneous metabolism can signifi- cantly influence delivery through skin. Trevithick and Mitton (2) showed that about 4.5-5% of tx-TAc was hydrolyzed to free tx-T in mouse skin within 24 hours. Norkus et al. (3) reported that following repeated application of ot-TAc to mouse skin, the concentration of free tx-T in the skin was elevated. We found tx-TAc to be metabolized into free tx-T in viable pig skin, although no metabolism was detected in the stratum corneum (4). However, Alberts et al. (5) have documented substantial skin (epidermal and dermal) absorption of tx-TAc without evidence of bioconversion within the skin to its unesterified form. Micro-Yucatan pig skin was chosen as the animal model for these in vitro studies. Fundamental knowledge of skin metabolism has been obtained by work on the kinetics of epidermal proliferation, and Meyer and Neurand (6) have pointed out similarities between the domestic pig and human epidermal tissues. The authors also found non-specific esterases to be located in the epidermis, subcutis, hair follicles, dermis, and the sebaceous and apocrine glands of pig skin. The formulation vehicle may modify properties of the stratum corneum (e.g., increased hydration), which could influence the penetration of active ingredients (7). The effect of the delivery system on the permeation and metabolism of tx-TAc and tx-T has not been systematically characterized thus far. The extent and speed with which a pharmacologi- cally active substance penetrates the skin depends on the effect that the three compo- nents (vehicle, skin, and active) exert on the diffusion process. The active incorporated in the vehicle should reach the surface of the skin at a suitable rate and concentration. If the site of action lies in the deeper layers of the epidermis or below, the substance must cross the stratum corneum. In addition, it is imperative that a dermal prodrug be metabolized in the skin to release the active molecule. This study was done to (i) formulate tx-TAc in various topical delivery systems and (ii) find the effect of formu- lation factors on the permeation and metabolism of tx-TAc. MATERIALS AND METHODS CHEMICALS AND INSTRUMENTS D-tx-tocopheryl acetate, Covitol © 1360, was obtained as a gift from Henkel Nutrition and Health Group (Illinois). D-tx-tocopherol was obtained as a gift from Archer Daniels Midland Company (Illinois). The following chemicals were obtained directly from the manufacturer and used without purification: SD alcohol (Eastman, Tennessee) isopropyl myristate and mineral oil (Sigma Chemical Company, New Jersey) diisopropyl adipate and isocetyl alcohol, Ceraphyl © 230 and Ceraphyl ©, respectively (ISP Vandyk, New