Category: Formulation and Quality
Metronidazole is used to treat moderate to severe rosacea, a chronic, inflammatory skin condition and is commercially available in different formulations at the same strength. The purpose of the present study was to characterize the rate and extent of metronidazole absorption through excised human abdominal skin from topical drug products using an in vitro permeation test (IVPT). The IVPT studies were conducted to collect cutaneous pharmacokinetics (PK) information to assess bioavailability and support an evaluation of bioequivalence (BE) for tested topical formulations. The ultimate goal of this work is to develop a consistent testing protocol for the evaluation of BE for topical products using characterization-based approaches.
Methods: Metronidazole topical gel, 0.75% (reference product) was used as the reference and a generic metronidazole topical gel, 0.75% and generic metronidazole topical cream, 0.75% cream, were used as positive and negative control, respectively, for evaluating BE. The drug products were rubbed on 0.95 cm2areas of skin pieces with a standard inverted high-performance liquid chromatography (HPLC) vial after dispensing a pre-determined amount of 13.5 µL gel and 13 µL cream to achieve target dose of 10 mg/cm2. The IVPT was conducted with a PermeGear®In-Line flow-through diffusion cell system (PermeGear; Hellertown, PA). Skin surface temperature was maintained at 32.0 ± 1.0°C with a circulating water bath, verified prior to dosing using a traceable® infrared thermometer. Isotonic phosphate buffer (pH 7.4 ± 0.1) was used as the receiver solution; pumped continuously through diffusion cells at a rate of 1 rpm (approximately 1.0 mL/h). The fraction collector was programmed to automatically collect samples every 2 h for 24 h. Metronidazole concentrations were analyzed using a validated HPLC method. In this study, each of the three metronidazole topical products was tested on ex vivo human skin from the same five different donors, with four replicate skin sections/donor/product. Data are presented as mean ± SEM, n=5 donors. Bioequivalence was assessed using a mixed scaled approach similar to the one adopted from FDA/CDER in the case of highly variable drugs.
Results: Flux profiles are presented in Figure 1. Cutaneous PK of metronidazole was similar between the gels, while cutaneous PK of generic metronidazole cream was different compared to both gels. Following the early peak in metronidazole flux from the gels, flux from gels declined after the 4 h time point. In contrast, the flux profile of generic metronidazole cream showed an increasing flux during the initial 12 h followed by a relatively steady flux during the remaining 24 h study. Maximum flux (Jmax) (SEM) was observed at 4 h for both metronidazole gels, 0.69 (0.38) μg/cm2h for the reference gel and 0.85 (0.44) μg/cm2h for generic gel. The Jmaxfor the generic cream was observed at ≥12 h.
Highest total cumulative amount of metronidazole delivered through the skin per unit area was observed from the generic cream (15.0 ± 7.31 μg) compared to 7.30 ± 1.88 μg and 7.76 ± 2.07 μg for the reference and generic gels, respectively. Cumulative drug absorption across the skin from three metronidazole products, expressed as percentage of applied dose is shown in Figure 2. Total drug absorption at 24 h was similar between the reference gel (9.3%) and generic gel (9.7%) while total absorption from the generic cream (18.7%) was nearly doubled compared to each gel product. Relative bioavailability (BA) was 1.03 ± 0.04 and 1.62 ± 0.40 for generic gel and generic cream, respectively assuming BA of 1 for the reference product. Mass balance results indicated a high mean recovery of 88% for reference gel, 90% for generic gel and 90% for generic cream.
Three topical drug products with the same metronidazole concentration were compared using IVPT studies. The in vitro cutaneous PK results for the gels with similar formulation compositions (comparable to each other) and a cream (distinct from both gels) were consistent with the expectation that differences in physical and structural attributes between topical semisolid drug products (e.g., between a gel and cream) can alter the BA of metronidazole. These expectations were also consistent with the statistical assessment of BE using the mixed scaled criterion mentioned earlier. Results also suggested that IVPT studies may help support an evaluation of BE for topical drug products, since the IVPT results appropriately showed the two gels (positive controls for BE relative to each other) had a similar rate and extent of metronidazole delivery and discriminated the cutaneous PK of the cream (negative control for BE relative to reference gel) as being different from both gels. The IVPT method employed in the present study provides an alternative way to assess BA. A harmonized in vivo study with healthy human subjects is in progress for the development of in vitro-in vivo correlation.
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
Dana Hammell– Research Fellow, University of Maryland, Baltimore, Baltimore, Maryland
Audra Stinchcomb– Professor, University of Maryland Baltimore, Baltimore, Maryland
Elena Rantou– Division of Biostatistics VIII, Office of Biostatistics, Office of Translational Sciences, Center for Drug Evaluation and Research, U.S. Food and Drug Administration, Silver Spring, Maryland