Physico-chemical determinants of dermal drug delivery: effects of the number and substitution pattern of polar groups.

The aim of this study was to investigate the effect of number and substitution pattern of -OH groups of a set of phenols on the in vitro permeation of heat-separated human epidermis. The diffusion was calculated from Log(D/x)=logk(p)-0.59logK(oct)+0.024 (D, diffusion coefficient; x, pathlength; k(p), permeability coefficient (cm/h); and K(oct), octanol-water partition coefficient). The main factors reducing D were the dipolar and hydrogen bonding capabilities of the permeants quantified as their Hansen partial solubility parameters delta(p) and delta(h). These parameters are significantly reduced by the degree of symmetry of the molecule, so that phloroglucinol (1,3,5-benzenetriol), with three -OH groups, diffuses more rapidly that phenol. When symmetry is absent, as in 1,2,4-benzenetriol, the number of -OH groups results in very slow diffusion. D/x (cm/h) was related to the combined solubility parameter delta(a) defined as radical(delta(p)(2)+delta(h)(2)) by: (D/x)=0.0024-0.000065delta(a) (n=7, R(2)=0.70, P=0.012).