Date of Award


Document Type


Degree Name

Doctor of Pharmaceutical Sciences - Pharmaceutics


Pharmaceutical Sciences

First Advisor

Grazia Stagni

Committee Chair and Members

Grazia Stagni, Chair

Kenza Benzeroual

Tannaz Ramezanli

Tycho Heimbach


Dermal pharmacokinetics, IVIVR, Microdialysis, Pharmaceutics, Pharmacokinetic, Topical dermatological drug products


There is no standard technique to measure skin pharmacokinetics (PK) and therefore to evaluate bioequivalence (BE) for topical dermatological drug products (TDDPs). Generic companies are required to conduct costly clinical endpoint studies to evaluate BE. For this reason, regulatory agencies are looking for more efficient methods to measure the drug bioavailability of TDDPs. It is about 40 years since microdialysis was developed as a tool to study tissue biochemistry in animals, specifically, neurotransmitter release in the rodent brain, and 30 years since it was applied to study skin PK. However, improvements in the dermal microdialysis (dMD) technique are required to become accepted by the regulatory agencies for the assessment of dermal BE. The purpose of this dissertation is to improve the experimental design and to address methodological deficiencies in order to lower data variability to obtain an acceptable BE estimate for TDDPs Recommendations include extended sampling duration timepoints to adequately characterize the dermal PK-profile, inclusion of a probe-performance marker to the perfusate to account for variation between samples, the precise control of the dMD probe membrane length along with increasing the number of replicate probes/application sites. Studies were performed in rabbits with Metronidazole (MTZ) TDDPs as a tool compound. MTZ is a hydrophilic, low protein-bound drug that is an ideal substrate for dMD recovery. The experiments were designed to evaluate four MTZ TDDP simultaneously in the same rabbit: gel-reference, gel-test, cream-reference, and cream-test, where the test products were US-FDA approved generics. Results of these studies show that the dMD was capable to clearly differentiate among MTZ formulations and evaluate non-BE for them. Moreover, the point estimate of bioavailability parameters, Ln AUC (extent) and Ln Cmax (rate) of reference/test ratios for each formulation were within BE limits. However, studies were not statistically powered to bring all 90% confidence interval (CI) of bioavailability parameters inside the BE boundaries (N=7). Another set of experiments were conducted using TDDP containing equal amounts of lidocaine (LDC) and prilocaine (PLC). LDC and PLC are molecules more lipophilic than MTZ and with higher protein binding, therefore they are more challenging for the dMD technique. For LDC/PLC products we investigated the capability of dMD to discriminate amongst escalating doses of a brand cream (EMLA). Moreover, the combination of dMD with sensitive analytical equipment, LC-MS/MS allowed the detection of skin metabolites. The experiment was designed to adequately characterize concentration-time profiles of LDC and PLC in a range of brand cream doses. The brand gel in one dose was administrated as a negative control formulation. DMD was able to sensitively assess the dose-response of LDC and PLC and their corresponding metabolites in the range of doses. The results demonstrated there is disproportionality in the exposure of each analyte and even its metabolite in the range of defined doses, although, both LDC and PLC were administrated in the same dose and same formulation. The exposure of PLC was lower than the ones for LDC while the main metabolite of PLC (Orto-Toluidine) was higher than the exposure of LDC’s metabolite, monoethylglyclxylidide, (MEGX). Moreover, dMD was able to differentiate among two formulations from only three experiments. These results suggest that dMD, with properly controlled variables, can accurately characterize the bioavailability of different groups of TDDPs and it should be further considered in the evaluation of TDDPs’ BE. Another goal was to apply a novel type of study, retrodialysis/ microdialysis (dermal infusion) to investigate disposition parameters like clearance, the volume of distribution, and half-life of MTZ independent of the dermal absorption procedure. The result of this study was deconvolved to investigate absorption functions like input rate, cumulative amount, and fractional input. The comparison of these results with the ones acquired from identical experiments in Yucatan-mini pig helped to compare and understand the role of the skin character in the absorption process. Moreover, the results of the cumulative amount and calculated dermal bioavailability (F) parameters confirmed the BE results. In the next step, the IVPT data of humans were utilized to explore the correlation of human in vitro data with in vivo rabbit data. The graphed Levy plots of different formulations in the test and reference products demonstrated a correlation for each product. Therefore, the IVPT data for each formulation was convolved with the MTZ dermal unit impulse response (dUIR) of the rabbit, and the prediction error for each formulation was calculated. The resulting plasma concentration profiles of brand cream and generic gel can predict the extent of absorption of the rabbit model with a percentage error less than60% in each formulation. The developed IVIVR adequately predicted the extent of absorption for different formulations but not the rate.