Category: Preclinical Development
Purpose: Dermal microdialysis (dMD) quantitatively measures the rate and extent at which a topically administered drug becomes available in dermis, at or near the site of action in the skin. The observed exposure in dermis is the result of the rate at which the drug reaches the dermis as well as disposition (distribution and elimination) within dermis. However, in order to build efficient predictive in-vitro/in-vivo (IVIVC) models, it would be desirable to measure the in-vivo flux (the equivalent of the input-rate for oral administrations) independent of local disposition. Typically, a systemic disposition function (Unit Impulse Response, UIR) is estimated from IV administrations, a type of drug delivery that by-passes the absorption step. By analogy, we sought to estimate the disposition of a drug in skin, by delivering the drug directly into dermis using a dMD probe. The objectives of this study were: (i) to estimate the dermis disposition function (UIR) of metronidazole (MTZ) with a combination of microdialysis/retrodialysis techniques, and (ii) to calculate the in-vivo flux and cumulative amount input by deconvoluting the dermis concentration profiles that were obtained following application of MTZ topical formulations with the estimated UIR.
Methods: An experiment was conducted to assess the effect of formulation removal on dermis exposure from the application of two MTZ topical dermatological formulations. In 3 Yucatan mini-pigs, twenty probes were placed under formulation application sites and two additional dMD probes were inserted into the dorsum of each pig at a site distant from the formulations. The formulation application sites were wiped off at pre-determined times of 6hr, 12hr, and no wipe off. The two additional dMD probes were perfused with a 40 ng/mL solution of MTZ at a flow rate of 0.5µL/min for 10 hours (retrodialysis phase). The perfusion solution was then switched to plain lactated ringer solution and MTZ – D3 solution as internal standard (microdialysis phase). Samples were collected every hour until the end of the experiment (48 hr) and analyzed for MTZ content. Dermis clearance was calculated from the retrodialysis data at steady state by Equation 1, where X2.5-9.5 is the amount delivered by the probe between 2.5 to 9.5 hours, and AUC2.5-9.5 is the area under the curve from 2.5 to 9.5 hours.
ClDermis = (X2.5-9.5)/(AUC2.5-9.5) Equation 1
The amount delivered was calculated as:
X2.5-9.5 = (Cperfusate - Css) x Vperfused(2.5-9.5) Equation 2
Where Cperfusate is the MTZ concentration in the perfusate or Cin, Css is the concentration of the MTZ in dialysate exiting the probe, Vperfusate is the volume of solution perfused through the probes in that seven hours. The dermis apparent volume of distribution was calculated as V=Cl/k where k is the slope of the elimination phase. The dermis disposition function (UIR) was built as follows:
UIR = (1/V)e-kt Equation 3
The Phoenix® (Certara, Princeton, NY) deconvolution module was used to calculate the cumulative amount delivered to the skin and the input-rate.
Results: Dermis concentrations declined mono-exponentially. Dermis elimination half-life of MTZ was 1.35 hr (0.5) (geometric mean (CV%)) whereas half-life of MTZ at the formulation sites for the 6hr, 12hr and no-wipe off were 9.01 (30.29), 10.51 (34.06), 10.32 (32.84) hours for the gel and 7.32 (25.95), 10.50 (57.75), 23.86 (86.74) hours for the cream, respectively. The significantly longer half-life of the formulations’ sites compared to that of the formulation independent half-life suggests that the rate of absorption is the limiting process for the apparent elimination of MTZ from the dermis, apparently demonstrating flip/flop kinetics, typically defined as an increased terminal elimination half-life following oral dosing of a drug, as compared to its intravenous half-life. These results indicate that MTZ permeation across the upper layers of the skin is a long, sustained process. The average dermis apparent volume of distribution and clearance were 0.12 0.06 mL, and 0.057 0.03 mL/hr, respectively. Figure 1 shows the in-vivo flux and cumulative amount input profiles sorted per formulation and wipe-off scheme.
Conclusion: The estimation of dermis disposition is a promising tool by which to estimate in-vivo flux and cumulative amount inputs that can be directly compared with in-vitro permeation (IVPT) data. The characterization of the absorption process independent of local disposition may be helpful for the development of quantitative IVIVC using IVPT and dMD data to improve the comprehension of in-vivo percutaneous permeation that is important for the successful development of new formulations and for the assessment of bioequivalence of topical dermatological products.
Sharareh Senemar– Ph.D student, Long Island University, Brooklyn, New York
Tannaz Ramezanli– Pharmacologist, Division of Therapeutic Performance, Office of Research and Standards, Office of Generic Drugs, Center for Drug Evaluation and Research, U.S. Food and Drug Administration, Silver Spring, Maryland, USA, Silver Spring, Maryland
Priyanka Ghosh– Pharmacologist, US Food and Drug Administration, Silver Spring, Maryland
Sam Raney– Lead for Topical & Transdermal Drug Products, U.S. FDA, Silver Spring, Maryland
Grazia Stagni– Associate professor, Long Island University, Brooklyn, New York