Experimental models of permeability in animals, excised tissues, cell monolayers, and artificial membranes are important during drug discovery and development as permeability is one of several factors affecting the intestinal absorption of oral drug products. The utility of these models is demonstrated by their ability to predict a drug’s in vivo intestinal absorption. Within the various permeability models, there are differences in the performance of the assays, along with variability in animal species, tissue sources, and cell types, resulting in a variety of experimental permeability values for the same drug among laboratories. This has led to a need for assay standardization within laboratories to ensure applicability in the drug development process. Method suitability provides a generalized approach to standardize and validate a permeability model within a laboratory. First, assay methodology is optimized and validated for its various experimental parameters along with acceptance criteria for the assay. Second, the suitability of the model is demonstrated by a rank order relationship between experimental permeability values and human extent of absorption of known model compounds. Lastly, standard compounds are employed to classify a test drug’s intestinal permeability and ensure assay reproducibility and quality. This review will provide examples of the different aspects method suitability for in situ (intestinal perfusions), ex vivo (everted intestinal sacs, diffusion chambers), and in vitro (cell monolayers, artificial membranes) experimental permeability models. Through assay standardization, reference standards, and acceptance criteria, method suitability assures the dependability of experimental data to predict a drug’s intestinal permeability during discovery, development, and regulatory application.

This study was undertaken to categorize representative fluoroquinolone drug substance permeability based on the methods outlined in the Food and Drug Administration's biopharmaceutic classification system (BCS) Guidance for Industry. The permeability of ciprofloxacin, levofloxacin, lomefloxacin, and ofloxacin was measured in an in vitro Caco-2 assay with previously demonstrated method suitability. The permeability class and efflux potential were ascertained by comparing test drug results with standard compounds (metoprolol, atenolol, labetalol, and rhodamine-123). All 4 quinolones drugs demonstrated concentration-dependent permeability, indicating active drug transport. In comparing absorptive versus secretive in vitro transport, the tested fluoroquinolones were found to be subject to efflux in varying degrees (ciprofloxacin > lomefloxacin > rhodamine 123 > levofloxacin > ofloxacin). Based on comparison to labetalol, the high permeability internal standard, ciprofloxacin was classified as a low permeability drug, whereas lomefloxacin, levofloxacin, and ofloxacin were classified as high permeability drugs. The in vitro permeability results matched human in vivo data based on absolute bioavailabilities. This laboratory exercise demonstrated the applicability of an in vitro permeability method for classifying drugs as outlined in the BCS Guidance.