BCS class IV drugs BCS drugs Biopharmaceutics Classification System DDI Franz cell Franz–PAMPA GastroPlus ICH IVIVE Medicine PBBM PBPK Pharmaceutical industries Phoenix WinNonlin absorption-modifying excipients bioequivalence biomimetic membrane biopharmaceutics biopharmaceutics classification system clinical studies colon absorption combined zero- and first-order absorption controlled release drug product disintegration dissolution dissolution test drug absorption drug permeability drug solubility drug solubility/dissolution