J. Soc. Cosmet. Chem., 26, 581-592 (December 1975) An Animal Model for Estimating the Relative Sting Potential of Shampoos ROBERT W. SHANAHAN, M.S.* and CHARLES O. WARD, Ph.D.t Synopsis-Comparative studies of various experimental and commercially available SHAMPOOS, utilizing the MOUSE WRITHING TEST as an assessment of PAIN, DIS- COMFORT, or STINGING, showed a reasonable rank-order CORRELATION of MILD- NESS with that found through controlled EYE STING studies in MAN. In view of the poor predictive value of conventional animal primary irritancy studies in determining discomfort or eye stinging properties of shampoos and other cosmetic materials, the mouse writhing test can provide a valuable adjunct in predicting the potential for discom- fort and stinging in man. The pHs of 29 shampoos studied were between 5.5 and 7.7. This narrow range did not appear to influence the potential for discomfort in the mouse, eye irritancy in the rabbit, or eye stinging in humans. INTRODUCTION There is an important need for screening methods which can detect and distinguish the relative discomfort or sting potential of cosmetic formulations early in their developmental stages. Laden (1) has reported that despite extensive animal and human testing, it was sometimes observed that prototype formulations, which had low pri~ mary irritancy, still caused discomfort to the riser in terms of low levels of stinging. It appeared to him that the stinging potential of a formulation was often totally unrelated to its primary irritancy. Van Abbe (2) has also reported problems with standard rabbit and monkey eye tests in predicting potential eye irritancy and discomfort of a hair dressing product, which when test-marketed, caused an unusually high incidence of stinging, itching, and pain of the eyes following the complainants exposure to rain. *Clinical Evaluation Section, Department of Toxicology, Avon Products, Inc., Surfera, New York 10901. tGraduate Division, College of Pharmacy and Allied Health Professions, St. John's University, Jamaica, New York. 581

582 JOURNAL OF TIlE SOCIETY OF COSMETIC CHEMISTS Thus, conventional animal eye and skin irritancy tests have shown little correlation with that of the human perception of discomfort or stinging. Not only can there be a lack of evidence of irritation in rabbit eyes by substances that sting human eyes, there can also be little or no evidence of irritation in human eyes by substances that can still be perceived to be severely stinging. In an attempt to devise an animal model, which would better predict the stinging potential of materials intended for use on humans, Laden (1) devised a rat-tail test, which measured the time for the animal to "flick" its abraded tail from acidic solutions of varying concentrations. Although a promising ap- proach, the method suffered from large animal-to-animal and experiment- to-experiment variations. Flicking reactions by the animals for no particular reasons were often observed, and the effects of time following abrasion, along with difficulties in producing consistent degrees of abrasion, appeared to influence and limit the effectiveness of the procedure as a means for reliably estimating relative differences in stinging between materials. The purpose of this report is to present another method which can provide an early screening procedure for the detection of the relative discomfort po- tential of cosmetic products that may enter the eye. The preliminary work herein will focus on shampoo products for which special mildness properties are desired (e.g., "no sting," "no-tear," "baby" shampoos). METHOD The method is based on the mouse writhing test which has been previously used primarily as a pharmacologic screening procedure for the detection of analgesic drug activity (3-8). MOUSE WRITHING CRITERIA The writhing syndrome has been described as a stretching or squirming response and is considered an overt manifestation of the perception of pain by the animal. The test was originally developed by elicting pain responses in mice following the intraperitoneal injection of irritant materials such as iodinated contrast agents (3), bradykinin (4) and paraphenylquinone (5-8). In the present study, the response was characterized by contraction of the abdominal musculature and was often accompanied by torsion and flexion of the trunk and extension of one or both of the hind limbs. It usually occurred within seconds or at most 1 or 2 min following the injection of dilute shampoo solutions into the test mouse. The syndrome was observed to be either inter- mittent and repetitive in nature or exhibited as a relatively steady and pro- longed contraction of fl•e middorsal abdominal wall. In either case, the end point was easily recognizable and a characteristic response to discomfort or pain by the animal. Other overt symptomatology following the intraperitoneal injection of the shampoos included ataxia, high-stepping gait, reduced mo-