PERCUTANEOUS ABSORPTION 509 Sulphonamides do not appear to follow these general rules. Thus, the determination of the rate of penetration of sulphanilamide, sulphathiazole, sulphadiazine and sodium sulphacetamide applied to the shaved backs of guinea-pigs revealed that there was no difference in the rate of absorption when o/w or w/o emulsions were used (164, 165). Clearly, absorption of the compound from ointment bases is impossible to predict with any reasonable degree of accuracy. Only experimentation using the compound and the base intended for its incorporation can be of assistance in this respect. 'State of hydration' of stratum corneum Observations on human beings have shown that conditions of high relative humidity enhance the damage produced to the skin by toxic chemi- cals. Cullumbine (18) and Kenshaw (166)observed that the vesicant pro- perties of certain war gases was greater when the persons exposed were sweating freely. The influence of the water content of the stratum corneum on percutaneous absorption was studied in detail using both in vitro and in vivo techniques. Cronin and Stoughton (167) investigated the passage of •4C-labelled ethylnicotinate through excised human skin supported over a chamber containing saline. They found that immersion of the skin in water of varying temperatures prior to mounting on the chamber resulted in a five-fold increase in the passage of ethyl nicotinate. In vivo experiments are in agreement with these observations. The concentration of topically applied ethylnicotinate necessary to produce a predetermined degree of erythema on the forearm of human volunteers was reduced by 5-10 times if the forearm was previously soaked in warm or cool water (168). Soaking of the forearm of human volunteers has also been shown to increase the percutaneous absorption of steroids. In a study (132, 169) •4C-labelled trimethalone in 95• alcohol was applied to a small area of the forearm, allowed to remain for 30 min and then washed off with a standard washing procedure. Previous hydration of the arm resulted in a five-fold increase in the absorption of the glucocorticosteroid. A more effective absorption of other steroids also takes place if they are applied under occlusive dressings. A suspicion that this might be so was expressed by clinicians who observed a better clinical response to topical glucocorticosteroids when the site of application was covered by a water- impermeable covering (170, 171). This suspicion was confirmed quantita- tively (79, 80). Using graded concentrations of a glucocorticosteroid on

510 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS each arm, and covering one arm with an occlusive dressing while the other arm was protected by an elevated, perforated (non-occlusive) guard, it was found that the occluded arm showed vasoconstriction with glucocortico- steroid concentrations 100 times less than those required to give equivalent vasoconstrictor responses on the non-occluded arm. A similar conclusion was reached by Scoggins and Kligman (172) in seven patients with dermatitis, using plasma cortisol levels as an index of' the suppression of' pituitary adrenal axis by the percutaneously absorbed synthetic steroids. The authors applied a weighed amount of a commercial cream containing asteroid mixture of known composition (prednisone, triamcinolone, fluocinolone acetonide, methylprednisone acetate and hydrocortisone) on areas of dermatitis and covered a number of these areas with an occlusive dressing, the rest were left uncovered. Under these con- ditions absorption of corticosteroids through diseased skin covered by an occlusive dressing is considerably higher than that from similarly affected but uncovered skin. The authors observed that: 'without an occlusive dressing, systemically significant amounts of these substances are absorbed only if the dose applied is very large'. Occlusion of the site of application in humans was found by Feldmann and Maibach (173) to increase 10-fold the amount of hydrocortisone absorbed. In these experiments, occlusion of the site increased the percutaneous absorption irrespective of the type of vehicle employed. No comments were made as to the possible mechanism responsible for the increased absorption from the occluded site. Covering the skin with an impermeable dressing prevents evaporation of sweat and of insensible perspiration resulting in an increased hydration of the stratum corneum. One would presume that this increase in the water content of the stratum corneum is responsible for the increased absorption in accordance with the results of the other authors already mentioned. The site at which water is retained in the stratum corneum is not clearly defined. Recent work (174) suggests that it is retained in the intercellular space in association with macromolecules possibly of a mucopolysaccharide nature. SPECIES VARIATION The rabbit, guinea-pig and rat are the most commonly used species for studying percutaneous absorption but the mouse and pig are used on occasions.