Background: Establishing the bioequivalence of topical drug products is a costly and time-consuming process since, with few exceptions, clinical efficacy trials are required. Objective: To develop a surrogate for clinical bioequivalence testing through evaluation of the kinetics of drug absorption in vitro through excised human skin. Methods: The percutaneous absorption of seven approved generic topical drug products was compared with their corresponding reference products during preclinical development using the Franz diffusion cell. Thereafter, following the conduct of bioequivalence trials and regulatory approval of these products in the United States, clinical data became available to which the in vitro data were compared. Results: In six of the seven cases the in vitro test:reference ratio for total absorption was close to one and indicated that the products were equivalent, in agreement with the clinical data. Results from the seventh case, in which the test:reference ratio was only 0.63, indicated that the in vitro model actually had greater sensitivity than the clinical method to detect small differences between products. Conclusion: These data demonstrate the relevance and predictive power of the in vitro human skin model and strongly support its use as a surrogate for in vivo bioequivalence studies.
Aims: To examine the existing literature to determine the degree to which percutaneous absorption data obtained using the excised human skin model match those obtained from living man. Methods: The scientific literature was reviewed to collect data on compounds whose percutaneous absorption through human skin had been measured under both in vitro and in vivo conditions. The in vitro-in vivo (IVIV) correlation was evaluated by computing the in vitro/in vivo ratio using total absorption (percent of applied dose) as the metric for comparison. Results: A total of 92 data sets were collected from 30 published studies. The average IVIV ratio across all values was 1.6, though for any single data set there could be a nearly 20-fold difference between the in vitro and in vivo values. In 85% of the cases, however, the difference was less than 3-fold. The correlation was significantly improved when data were excluded from studies in which the protocols for both studies were not fully harmonized. For harmonized data sets the average IVIV ratio was 0.96 and there was a less than 2-fold difference between the in vitro and in vivo results for any one compound, with IVIV ratios ranging from 0.58 to 1.28. The dominant factors leading to exclusion of data were the use of skin from different anatomical sites and vehicles of differing composition. Conclusions: Percutaneous absorption data obtained from the excised human skin model closely approximate those obtained from living man when the two study protocols are appropriately matched.
The pharmacokinetic approach has accelerated the development of high-quality generic medicines with extraordinary cost savings, transforming the pharmaceutical industry and healthcare system in the USA. While this is true for systemically absorbed drug products, the availability of generic versions of topical dermatological products remains constrained due to the limited methods accepted for bioequivalence evaluation of these products. The current review explores the possibility of developing appropriate bioequivalence approaches based on pharmacokinetic principles for topical dermatological products. This review focuses on the strengths and limitations of the three most promising pharmacokinetics-based methods to evaluate the performance and bioequivalence of topical dermatological products, which include in vivo skin stripping, in vivo microdialysis, and in vitro permeation testing (IVPT) with excised human skin. It is hoped that recent advances in pharmaceutical and regulatory science will facilitate the development of robust bioequivalence approaches for these dosage forms, enable more efficient methodologies to compare the performance of new drug products in certain pre-approval or post-approval change situations, and promote the availability of high-quality generic versions of topical dermatological products.
Although the activation of calcium-independent phospholipase A2 (PLA2) enzymes has been described in the heart, the pathogenetic role of this enzyme(s) in hypoxic cell injury has not been previously examined in any tissue. Therefore, we characterized the time course of activation of calcium-independent PLA2 using both plasmalogen and diacylglycerophospholipid substrates during hypoxia in rabbit proximal tubules and examined whether inhibition of calcium-independent PLA2 activity is associated with a cytoprotective effect. Subjecting rabbit proximal tubules to hypoxia for 5 min resulted in at least a threefold increase in cytosolic calcium-independent PLA2, which was selective for plasmalogen substrates (control 444±69 vs hypoxia 1,675±194 pmol* mg protein-' -min-', n = 5). In contrast, no changes in PLA2 activity were observed in the presence of 4 mM EGTA in the membrane fraction using plasmenylcholine substrates. 20 min of hypoxia resulted in an increase in arachidonate from 3±1 to 28±4 ng/mg protein and lactate dehydrogenase release from 7.5±2% to 38±5%, n = 4.Pretreatment of proximal tubules with 10 ,gM Compound I, a specific inhibitor of calcium-independent PLA2, resulted in reduction in the magnitude of both hypoxia-induced arachidonic acid release (11±3 ng/mg protein) and lactate dehydrogenase release (18±4%). Our data indicate that a significant fraction of PLA2 activity in the proximal tubule is calcium-independent and selective for plasmalogen substrates. Furthermore, the activation ofthis enzyme plays an important role in the pathogenesis of membrane injury during hypoxia in the proximal tubule. (J. Clin. Invest. 1994. 93:1609-1615.) Key words: phospholipid hydrolysis -ischemic cell injury -hypoxia * calciumindependent phospholipase A2
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