STUDIES ON THE PERMEABILITY OF THE SKIN TO MERCURY* By EDWIN P. LAUO From the Division of Pharmacology, Food and Drug Aidministration, Federal Security Atgency, 14,•ashington 25, D.C. THe. CZU•.ST•O• of permeabil- ity of the skin should hold par- ticular interest for the. cosmetic chemist. This is so because the usual older criteria of acceptability of skin preparations, namely good physical properties, may no longer be adequate in characterizing an ointment or lotion. We should like to know, for exampie,'whether am- moniated mercury applied as a surface blektch penetrates the skin, to what extent propylene glycol, or glycerol functions as a percutaneous vehicle, etc. I realize that the use of mercurials in cosmetic chemistry probably has a very limited interest. Therefore, in presenting the problem of mer- cury penetration in considerable de- tail, I wish to emphasize that the individual findings are important only to the extent that they help to illustrate the principles and tech- niques involved. My chief interest, therefore, is to demonstrate a new method of approach in evaluating the properties of ointments, and I hope that these studies may stimu- * Presented at the May 13,194•7, Meeting, New York City. late further work in this direction. The work on the skin perme- ability .of mercury which is to be reported here is the outgrowth of studies made during the war under OfFice of Scientific Research and Development contracts. The ob- ject of the study was to reinvesti- gate the use of mercurial ointments for venereal prophylaxis of skin and mucous membranes, and to de- velop more efficient preparations. This involved the screening of a considerable number of ointments containing mercury in the form. of calomel. For a calomel ointment to be acceptable, it had to possess (1) good mechanical properties, such as stability, spreadability, resist- ance to drying, etc. (2) a minimum of irritating properties (3) ability to penetrate the skin in amounts thei'apeutically effective both locally and systemically. The last of these three criteria was the one upon which Xve concentrated our atten- tion. That the skin is permeable to mercury has been well established, and its therapeutic usefulness ade- quately investigated by Cole and his

82 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS associates (1, 2, 3). Proof of the cutaneous penetration of mercury has rested chiefly upon the finding of mercury in the urine, although systemic symptoms of mercurialism have also been frequently noted. While clinical studies on humans can be carried on with reasonable care to prevent contamination .of excreta with mercury, this is not so easy with animals. Furthermore, in the screening of a large number of ointments, a rapid means for quantitative differentiation was very necessary. In this respect, the urinary excretion method left much to be desired and, in fact, could be regarded as hardly suf•ciently quan- titative. Because of the fact that the new methods which we have adapted for the assay of mercurial ointments have applicability for other heavy metals, as well as the routine testing of numerous cosmetic preparations, it seems advisable to describe the procedures somewhat in :detail. A. METHODS FOR CUTANEOUS APPLICATION Obviously stringent precautions are necessary in determining cu- taneous permeability in order to avoid serious errors. There must be assurance that the substance under study enters the body • ia the skin and only • ia this route. All oral contact thr. ough lickir•g, etc., must, therefore, be avoided. In addition, if examination ofexcreta is to be made, ways must be found to prevent contamination by a flaking or running off of the test substance from the skin into the collection trays. Rabbits used in our study were immobilized in a special head-hip stock (4). Hair from the back and flanks was removed by clipper. Shaving or chemical methods of depiliation were avoided because of the possibility of injury to the skin. An area of 4 X 6 in. [155 (cm.) 2 which is 8 per cent of the total body surface of a 2.5-kg. rabbit] was laid off saddle fashion from the back around to each flank. To avoid creeping of ointment the area was limited by strips of tape. An excess of ointment (4 gm.) was inucted with a glass rod for 3 min, utes. Without removing the excess ointment the inunction site was then covered with rubberized cloth and the animal kept in the stock for 24 hours. Because of greater 'availability and ease of handling, the rat proved to be more desirable as a test animal than the rabbit. But since it was not feasible to confine rats in indi- vidual stocks like rabbits, a tech- nique for wrapping the animals was developed which proved rapid and effective. The animals were lightly etherized and the hair removed around the trunk between shoulders and hips. An area of 1i/2 X 3 in. [29 (cm.) 2 which is 8 per cent of the total body surface of a 250-gin. rat] was laid off on the back in a manner similar to that for the rab- bit. An excess of ointment (0.4 'gm.) was inuncted with glass rod for 3 minutes. After inunction, and without removal of the excess oint-