INFLUENCE OF PH ON LIDOCAINE PENETRATION THROUGH HUMAN AND HAIRLESS MOUSE SKIN INVITRO

The permeation of radiolabeled lidocaine through full-thickness hairless mouse skin and dermatomed human epidermis was studied under infinite-dose conditions using a flow-through diffusion apparatus. The donors consisted of 5% suspensions in 40% propylene glycol gelled with 1% hydroxypropyl cellulose and preserved with 0.25% chlorobutanol. The pH was adjusted to several values and the solubility in each donor was measured. Steady-state flux values were subjected to mathematical treatment to estimate the permeability coefficients (P) of the ionized and unionized forms of lidocaine. The permeability coefficient for unionized lidocaine through hairless mouse skin was no more than 15 times greater than for the ionized form; a 50-fold difference was obtained with human epidermis. The permeability differences indicate that hairless mouse skin is not an adequate in vitro substitute for human skin when considering lidocaine penetration.