j. Soc. Cosmet. Chem., 46, 191-198 (July/August 1995) Stability of all-trans-retinol in cream T. TSUNODA and K. TAKABAYASHI, Laboratory of Development Research, Shiseido Research Center, 1050 Nippa-cho, Kohoku-ku, Yokohama-shi 223, Japan. Received February 1, 1995. Synopsis Among the retinoids, we chose retinol (vitamin A) and studied the stability behavior of all-trans-retinol in creams. We observed thermal isomerization from all-trans-retinol to 13-cis-retinol, and this isomerization was a function of increasing temperature and oil content in creams. In addition, when the possibility of contact between retinol and water in creams was enhanced, retinol could be easily dehydrated to anhydro- vitamin A. Therefore, a balance of oils and water is considered to be important for the stabilizing of retinol in cream. We found that the reduction of all-trans-retinol in cream was caused by compound factors: thermal isomerization by temperature, dehydration by water, decomposition by oxygen, and peroxide in the surfactants or oils used in creams. INTRODUCTION Among the retinoids, which are supposed to induce thickening of the epidermis and thin the stratum corneum and be effective for the treatment of skin diseases (1), retinol is regarded as desirable because of its lower stimulus compared to retinoic acid. How- ever, retinol is said to be unstable in light and oxygen. Some studies on the stability of oil-soluble retinoids in oil are reported (2-4), but in these studies the phenomenon of decrease of retinoids was observed under mixed conditions affected by light, oxygen, lipid peroxide, temperature, and water. Therefore, in order to clarify the influence of each condition and the stabilization countermeasures for application in skin-care cream, we studied the stability behavior of all-trans-retinol, which has a higher physiological activity than other isomers, by means of reverse-phase high-performance liquid chro- matography (HPLC). The possibilities of ultraviolet effects that can cause photo- isomerization (5) of retinol and oxygen effects that can cause oxidation and decompo- sition of retinol are negated by using pure yellow-colored fluorescent light, brown- colored bottles, and a glove box and argon gas blanket for preparing the samples, preservation, and measurement. MATERIALS AND METHODS STORAGE TEST Ten milliliters of 500 ppm retinol (3100 units/mg FLUKA) in ethanolic solution or 191

192 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS squatane (Nikko Chemicals) in a 20-ml brown-colored screw bottle sealed with a cap and 400 ppm of retinol in creams with 500 ppm butylated hydroxy toluene (BHT) (Wako Jyunyaku Kogyo Co.) in an aluminum tube sealed with a cap (about 15 g) were preserved under various temperature conditions. Table I shows the formula of sample water-in-oil creams. In order to investigate the influence of water and oil in cream on all-trans-retinol, we prepared water-oil creams, varying the relative quantities of the oil and water phases. Prepared retinol in ethanolic solution, and squalane and sample creams, were prepared in the absence of ultraviolet lights by using pure yellow-colored fluorescent light (National Co.) and in the absence of oxygen, using an argon gas blanket, blowing and bubbling with argon gas. CONTACT OF RETINOL WITH WATER Five hundred parts per million of retinol in 100%, 75%, and 50% ethanolic solutions were prepared and stored at 50øC for periods of ten days. In addition, 500 ppm of retinol in 50% ethanolic solution with 2% polyoxyethylene glycero! monoisostearate (60 mol) (Nihon Emulsion Co.), a surface-active agent, were prepared and compared with 500 ppm of retinol in 50% ethanolic solution without surface-active agent. This surface- active agent had to be fresh, and the sample for the storage test had to be prepared using blowing and bubbling argon. If not, the influence of peroxide in the surface-active agent (4) and that of oxygen could not be neglected. EFFECT OF ANTIOXIDANTS Butylated hydroxy toluene (BHT) (Wako Jyunyaku Kogyo Co.), butylated hydroxy anisole (BHA) (Wako Jyunyaku Kogyo Co.), vitamin E (Tokyo Kasei Kogyo Co.), and vitamin C (Wako Jyunyaku Kogyo Co.) were selected as antioxidants. Five hundred parts per million of retinol in ethanolic solution including 500 ppm of each of these antioxidants were stored at 50øC for periods of five days without using an argon gas blanket. When vitamin C was added to the retinol-ethanolic solution, the pH was adjusted at pH 7.0 with sodium hydroxide, since retinol is very unstable at low pH. Table I Formula of Sample Water-in-Oil Creams Cream C Ingredient Cream A Cream B (% by weight) Water 65.03 54.03 42.03 Glycerin 15 15 15 Oils 15 26 38 Stearyl alcohol (4) (4) (4) Petrolatum (4) (8) (12) Liquid petrolatum (7) (14) (22) Polyoxyethylene glyceryl monostearate 4.6 4.6 4.6 Ethyl parahydroxy benzoate 0.2 0.2 0.2 retinol 0.04 0.04 0.04 Butylated hydroxy toluene 0.05 0.05 0.05 perfume 0.08 0.08 0.08