Date of Award

2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Pharmaceutical Sciences

Department

Pharmaceutical Sciences

First Advisor

Grazia Stagni

Committee Chair and Members

Grazia Stagni, Chair

Dovenia Ponnoth

Tycho Heimbach

Sam Raney

Robert Stratford

Keywords

Bioavailability, Bioequivalence, In vitro in vivo correlations, Microdialysis, Pharmacokinetics

Abstract

The use of dermal microdialysis (dMD) to evaluate bioavailability (BA) and bioequivalence (BE) of topical dermatological drug products (TDDP) has shown promising results; however, studies conducted thus far have been inconclusive conceivably due to large inter- and intra-subject variability, in addition to the inability of short experiments to completely characterize the dermis pharmacokinetics (PK). By using multiple test sites on the same subject, and replicate probes at each test site, it is feasible to compare the dermal pharmacokinetics (dPK) of a drug from different products in parallel on the same subject, which in turn will help to control variability. Moreover, increasing the duration of the experiments to detect the terminal phase, would allow a better comparison between products. However, there are technical considerations related to the dMD experimental design that must be characterized and optimized to ensure that the in vivo dMD study is selective, sensitive, discriminating, reproducible and that the dMD probe performs consistently for the entire duration of the experiments. Studies were conducted in Yucatan mini-pigs using a generic cream (0.75%) and a generic gel (0.75%) formulation of the drug Metronidazole (MTZ) with known different In-Vitro Permeation Testing (IVPT) data profiles. MTZ is a small, water soluble molecule, with low protein binding. Thus, it is an ideal substrate to challenge the dMD technique. The goals of the first set of Yucatan mini-pig studies were to assess: (i) the minimum distance required between test sites to prevent cross-talk between probes (due to potential lateral diffusion of drug in the dermis from a different test site); (ii) the sensitivity of the dMD method to detect differences in the local concentration of the drug, which was modulated by applying different doses of the topical product (3, 10, and 30 mg/cm2); (iii) the stability of dMD probe sampling over the 48-hour study duration. The results of these study showed negligible lateral diffusion and undetectable systemic redistribution suggesting that the MTZ concentrations measured by the dMD probes can be specifically associated with the local bioavailability from the topical dose of the cream or gel, at that specific application site. The results also indicated that the dMD method was sensitive to differences in the in vivo topical bioavailability of MTZ from each product, and selective to differences in bioavailability between cream and gel products of the same strength. Although studies were 48 hours long, the terminal phase was still non-identifiable. Therefore, a second set of Yucatan mini pig studies were conducted with the goal to assess the effect of application-time (AT) on dermis exposure, which may be a possible strategy to shorten the length of an experiment and obtain an estimate of the terminal phase. The dPK from the dose of 10 mg/cm2 of the two, previously used, MTZ topical dermatological formulations applied for 6 hours and 12 hours was compared to non-removal (48-hr-AT). Total exposure from the gel formulation was independent of the application time, suggesting that most of the drug had permeated within 6 hours. Conversely, for the cream formulation, the exposure resulting from the 6-hr-AT was significantly lower than that with 12-hr-AT (p = 0.026) and 48-hr-AT (p = 0.006), suggesting that absorption into the skin from the cream is complete by 12-hr. However, dermis concentrations increased for an additional 6-8 hours suggesting that MTZ from the upper layers of the skin continue to reach the dermis after formulation removal. In this experiment, two additional dMD probes were inserted at a site distant from the formulations and were perfused with a known concentration of MTZ solution (retrodialysis phase). The perfusion solution was then switched to plain lactated Ringer’s solution (microdialysis phase) to estimate MTZ dermis elimination. Differences between terminal half-life at formulation sites and formulation-independent elimination t. suggests a flip/flop PK scenario. Data from these probes were used to estimate the dermis unit impulse response (dUIR). In vivo flux and in vivo cumulative amount permeated were estimated by deconvolution, for each formulation and AT, from the observed PK profiles with the dUIR. Comparison of these in vivo dMD data with in vitro permeation testing (IVPT) data led to the development of a Level A in vitro-in vivo relationship (IVIVR). The results of these studies further confirm the capability of the microdialysis technique to capture differences in the rate and extent of dermis absorption from topical dermatological formulations. In addition, availability of an IVIVR can help to streamline the development of generic TDDP and possibly reduce the cost of bringing a generic to market; however, more investigation must be done into the assumptions for this IVIVR approach.

Download

Share

COinS