Absorption Models to Examine Bioavailability and Drug-Drug Interactions in Humans

Human drug absorption studies are warranted in many situations. The development of modified release formulations, the evaluation of different formulations designed to improve bioavailability by enhancing solubility or permeability, the investigation of mechanisms of drug-drug/food interactions in the gastrointestinal tract, and the determination of intestinal and biliary secretion in humans all require assessment of drug absorption/excretion in the human gastrointestinal tract. This chapter discusses in vivo approaches that can be used to assess absorption/bioavailability issues in the human gastrointestinal tract. The design and the application of remotely activated capsules and aspiration/perfusion catheters are reviewed, and the utility of human microdose studies is discussed. With the use of appropriate probes, these approaches also may be used to assess drug drug or drug food interactions, as well as interactions involving transporters and enzymes present in the gastrointestinal tract and/or the liver.