Topical Bioequivalence Determination Using Near-Infrared Spectroscopy

Econazole nitrate topical creams are a specific example of a drug product class that requires bioequivalence testing method development. Preliminary data demonstrate that econazole nitrate has an absorbance peak near 1670 nm that coincides with absorbance minima of skin. This peak suggests that imaging of the drug in skin is possible. The preliminary principal component analysis of these spectra shows that untreated skin spectra, solvent treated skin sample spectra, and solvent+drug treated skin spectra cluster in different regions of hyperspace in at least two of the largest principal components. This difference will be used to map the location and concentration of econazole nitrate in skin. Three concentrations (75%, 100%, 125%) of drug in vehicle will be applied to skin in Franz-type diffusion cells. The treatments will remain on the skin for different time periods from one to twenty-four hours. The surface formulation will be carefully removed from the skin immediately before the NIR measurement is made. Scans of the untreated skin, vehicle (placebo) treated skin, and pure drug will be taken for comparison. Treatment exposure times and drug concentrations will be optimized until a significant dose-response relationship can be established, and inter-treatment variability is determined. Minimum analytical detection of the drug will be determined in these initial studies. Standard treatment procedures will be established and followed as previously described (Pirot et a/., Stinchcomb et a/.). The skin tissues will be exposed to the same concentration of drug solution for three different time periods. Then the formulation will be removed, the skin NIR scanned, and then analyzed for drug content by tissue extraction and HPLC quantification. This study will be an attempt to find the approximate minimum time required (Tmax-ss)for drug to saturate the stratum corneum, as described in the FDA draft guidance for topical drug products (FDA 1998). When the proper exposure time is identified, the elimination of the compound from the stratum corneum can be studied. Once this proof-of-concept in vitro study is complete, further human subject studies with this NIR method will allow correlation of results from in vitro and in vivo treatments. NIR imaging BAIBE DPK assessment will eventually be attempted even in diseased/damaged skin, which has generally been avoided in DPK assessment that required removal of the stratum corneum by tape stripping or other means.