GUINEA PIG ASSAY OF PHOTOSENSITIZING POTENTIAL 125 more than 1 in 10,000 is not acceptable from a safety or business point of view. An incidence of 1 in 100,000 or less is the desirable goal for such products. This report will be limited to a discussion of topical germicides for use in soaps and cosmetic products and to a description of a new lab- oratory procedure for assessing photosensitizing potential of topical agents on guinea pigs. TOPICAL CJERMICIDES Topical germicides to be fully acceptable for use in soaps and cosmetics must have the following properties: 2. 3. 4. 5. 6. The following attep. tion: Effectiveness against skin bacteria at low concentrations. Substantivity to skin. Compatibility with vehicles. Stability to light and heat. Activity in presence of soil. Safety--not a primary irritant, sensitizer, or photosensitizer. topical germicides have received the greatest 1. 2,2'-methylenehis (3,4,6-trichlorophenol) (Hexachlorophene) 2. 3,4',$-Tribromosalieylanilide (TBS) 3. 3,4,4'-trichlorocarbanilide (TCC) 4. 2,2'-thiohis (4,6-diehlorophenol) (Bithionol) 5. 3,3',4',$-tetrachlorosalicylanilide (TCSA) Tetrachlorosalicylanilide (TCSA), a germicide used in soaps, has been reported both in England and the United States to be a photo- sensitizer and has been promptly withdrawn from the market. This is an example of a biological agent that has passed conventional toxicolog- ical screening tests but was found to cause adverse skin reactions in some consumers after market introduction of soap containing TCSA. Bi- thionol has experienced similar difficulties but to a lesser degree and affecting a much smaller segment of the population. Jillson and Baughman (9, 10) reported on the photosensitizing action of bithionol on subjects characterized as persistent light reactors (PLR). Both TCSA- and bithionol-sensitized subjects were reported to be cross photosensi- tized, in some instances, to germicides like hexachlorophene and certain polybrominated salicylanilides.

126 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS In view of these unexpected incidents of photosensitization by some germicides, it has become very evident that an animal assay procedure to screen out weak photosensitizers would be a valuable aid in the con- tinuing search for safe and effective topical agents for use in soaps and cosmetics. Such a procedure has been developed. The guinea pig is the animal of choice for studying sensitization phenomena, and it has proved to be useful for checking out potential photosensitizers. The key to this procedure is to apply the test agent under "abuse" conditions. Solvents are used that have the effect of altering the skin defenses, thus making the guinea pig more vulnerable to the action of irritants. Three major skin defenses that provide pro- tection against environmental stress, both physical and chemical, are (a) the skin barrier in the stratum corneum (b) the water binding capacity of the stratum corneum and (c) the buffering capacity of the skin. It is believed that the normal subject is capable of effectively combating weak irritants that come in contact with the skin, but the hypersensitive person is prone to react to low-order irritants because of poor skin defenses. The test procedure to be described is, therefore, based on the concept that the test animal should be exposed to the topical agent under conditions of stress that undermine the skin defenses. This is accomplished by administering the test material in organic sol- vents known to reduce barrier efficiency, water binding capacity, and buffering capacity. PROCEDURE EMPLOYED FOR DETERMINING PHOTOSENSITIZATION POTENTIAL OF TOPICAL AGENTS ON CxUINEA PIG SKIN This test is comprised of two stages with an optional third stage. 1st Stage Young adult guinea pigs, usually six to a group, are shaven with electric clippers on the upper dorsal area (cervical region). To this site is applied 0.05 ml. of a 2% solution of the test agent in absolute alcohol. This is repeated daily for a total of five applications. After each applicaq tion the guinea pigs are ultraviolet-irradiated for 15 minutes, employing a Sun Lamp (GE or Westinghouse) from a distance of 18 in. The animals for this exposure are contained in a box with dimensions 15•/• X 13•/• and 8 in. high. Skin readings for evidence of erythema are made 24 hours after each application as follows: