Substantivity of sunscreens-preparation and evaluation of some quaternary ammonium benzophenone derivatives.

ABSTRACT

Synopsis In continuation of previous studies on keratin-substantive sunscreens, two series of quaternary ammonium compounds, derived from 2,4-dihydroxy and from 4-hydroxybenzophenone and containing O-alkyl and N-alkyl chains of different length were prepared and submitted to substantivity tests on wool, to microbiological tests on two bacterial and two fungal species, and to tests for allergenicity/irritation on human skin. All compounds were rapidly adsorbed by wool in aqueous solution as expected, the maximum amounts adsorbed at equilibrium (S(max)), indicative of the skin substantivity, were greatest for the C12 N-alkyl derivatives, and showed a progressive decrease with increasing length of the N-alkyl substituent. No substantial substantivity differences were detected in four C12 quaternary derivatives, differing by the presence (or absence) of a 2-OH group in the benzophenone moiety and by the length of the O-alkyl (n= 2 or 3) side chain. When tested for antimicrobial activity, however, only the 4-hydroxybenzophenone C12 quaternary derivatives showed an order of activity comparable with that of two reference compounds. The conditions leading to a maximal antibacterial activity in the present compounds appeared to be the absence of the 2-OH group, and a shorter O-alkyl side chain. None of the new quaternary derivatives showed skin-irritant properties. The results of this investigation, while confirming the peak substantivity of C12 quaternary derivatives, indicate the possibility of dissociating, in a quaternary ammonium sunscreen, the skin substantivity from other unwanted side-effects, and offer some guidelines for the development of safer agents of this type.