In vitro studies were conducted using Franz diffusion cells (0.79 cm2)
and split-thickness human cadaver skin as previously described [16].
Briefly, receptor compartments contained 4.5 mL of Dulbecco's
phosphate-buffered saline (PBS), pH 7.4, with 0.02% sodium azide preservative,
maintained at 37 °C in aluminum blocks and magnetically
stirred. A tritiated water screening test was used to confirm skin integrity,
ensuring an intact, normal, barrier function for in vitro skin studies.
Skin samples with 3H2O permeation values greater than 1.56 μL/cm2 as
determined by liquid scintillation counting (Beckman LS6500) were
rejected as excessively permeable [17]. Samples were rank ordered
based on 3H2O permeation results and randomly assigned to a treatment
group [18]. After an overnight equilibration the formulations described
above were applied to each cell at a dose of 3 μL/cell, using a positive
displacement pipette and a pre-wetted glass rod for uniform spreading.