390 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS depilatory ? Secondly, we have the question of whether or not the skin should be covered during part of the test or throughout the period of observa- tion. Thirdly, how should we score the effects so that we can make relative comparisons between different substances ? Lastly, we may need to discuss the need or value of scarifying the skin before applying the test material. Many of us use the rabbit test and in the pharmaceutical industry we arranged a collaborative trial between a number of companies. The results were remarkably in agreement--that is to say we all successfully placed three substances in order of irritant properties, but what we are not so clear about is the ultimate meanings of these tests when applied to man. We certainly know that if nitric acid is put on the skin of a rabbit and burns a hole in it, we may expect the same effect in man! But when a substance in a rabbit produces a certain amount of erythema, will this apply in man ? There are numerous examples where one gets opposed answers between man and rabbit. Another technique that we have used successfully for testing skin toxicity is to inject the material intradermally into the skin of the guineapig. One observes the reactions which occur after a period of a few hours, and subse- quently over a period of 7 days. By a system of standard reference pictures, which were supplied by Dr. Paget of I.C.I., we are able to grade the severity of the reaction on a scoring system. Such a test is intermediate in sensitivity between the rabbit skin test and the rabbit eye test, and I personally like it because it does have some humane value in preventing us putting in the eye of a rabbit something which obviously is going to cause a great deal of damage. In the rabbit eye test, the test substance is applied, usually in solution, into the eye and the subsequent reaction which occurs is observed. Again there are a variety of disciplines. Some people follow the original method of Draize, instil the solution into the eye and simply wash it out again within a few seconds. Others leave the solution in the eye and observe the effects which occur. The question arises how often one should look at the eye--at hourly intervals ? Overnight ? And so forth. Lastly, a few words about sensitization studies. I do not think any of us in this country would regard an animal test for sensitivity to be of any real value. Methods have been described where the material is injected into the skin of the guineapig over a period of time, and then subsequently re-tested about a fortnight after. These tests may detect the more potent sensitizers but we are more concerned with those peculiar in man. These require human patch tests. In addition to the local toxicity tests of a compound when applied to the skin, we have to consider the systemic effects which follow after absorption of toxic substances through the skin. These occur as distant toxic effects.

TESTING DRUGS FOR DERMAL TOXICITY 391 The intact dermis forms a natural barrier to absorption of compounds through the skin but skin absorption can be altered due to changes in formulation and also the state of the skin itself. If the skin is damaged then the dermal barrier is removed and considerable absorption can occur. The effects may be studied by placing the substance on the skin and observing the subsequent toxic effects which occur. This problem may be studied in a rather simpler way. We determine the amount of material we are likely to put on the skin. This is related to the chronic toxicity of the compound when given by the oral route in a normal feeding trial. If only a fraction of 1% of that substance in the formulation is to be applied to the skin, it does not seem necessary to carry out long-term skin toxicity tests. Another approach is to determine the amount of the substance absorbed and to relate this to the known toxicity of the compound. Thus we can assess the safety margin. If we wish to carry out toxicity tests following dermal application, problems do arise. We must be quite certain that oral contamination does not occur. There are substances we put on the skin which when taken orally would be highly toxic. Lastly I would draw attention to danger of carcinogenicity with sub-. stances applied to the skin. This is a very real problem we have to face. Here we need to apply the substance to the skin, usually the mouse or the rabbit, over a period of 12 months and then continue our observations of them for a future year. Again we look for local effects, but we must remember that a carcinogen may be absorbed through the skin and produce distant carcinogenic effects. We need not cover the skin to prevent oral contamina- tion. DISCUSSION MR. J. SOUTHWOOD: I would like to have your opinion on the value of' tests for carcinogenicity by the subcutaneous route. How would you view a substance which shows activity by this route and not by skin application ? TH• L•CTUR•R: We all know that when we inject substances into the rat subcutaneously we may get false positive results or at least we think so. Even injecting a solution of glucose subcutaneously for a period of time can cause rumour-formation in the rat. We therefore have to be very careful in controlling these things and interpreting them. I feel that if there is a suspicion of carcinogenicity we should avoid the use of the substance unless there is a very real need for it. In the use of a medicinal substance, which is essential for a medical treatment, we may have to take a certain amount of risk--no one would stop using X-rays because there is a danger of carcinogenicity. We just have to accept a degree of risk.