By Kenneth R. Brouwer
In vitro systems are used extensively in the drug discovery and drug development process. Utilizing minimal amounts of drug, they facilitate rapid screening for specific compound properties (efficacy, toxicity, distribution, elimination) and also help determine the mechanism of drug action. These simplified systems focus on the drug concentration in the incubation medium (e.g., microsomes, membrane vesicles), which may or may not be relevant to the concentration that occurs in vivo at the site of efficacy, toxicity, or elimination.
The liver is the primary organ of metabolism and disposition for most drugs. Blood from the gastrointestinal tract flows via the portal vein through the liver and into the systemic circulation. Drugs are taken up into the hepatocytes through passive diffusion and/or active uptake by transport proteins present on the basolateral membrane of the hepatocytes. Once inside the hepatocyte, drugs may undergo metabolism, and the drug and/or generated metabolites may be effluxed across the canalicular membrane into the bile or across the basolateral membrane into the systemic circulation. The intracellular concentration of drugs and their metabolites are the primary factors that influence processes that occur inside the cell. In the liver, these processes can include efficacy (if the liver is the target organ), drug interactions (metabolic or efflux transporter based), toxicity, and regulatory processes (e.g., induction and/or inhibition of transporters or metabolic enzymes and other processes regulated by nuclear receptors).
The intracellular concentration is the driving force for any process that occurs inside the cell. If the in vitro system that is being used to predict the effect does not recapitulate the relevant in vivo concentration, then it will be impossible to predict the in vivo effect. This is true whether the effect is pharmacological or toxicological, and even if one is trying to predict in vivo metabolism or interactions with drugs or endogenous compounds. All of these rely heavily on in vivo-relevant intracellular concentrations, not only for drugs, but also for endogenous compounds.
Development of in vitro systems that better reproduce the in vivo-relevant intracellular concentrations may decrease the need for animal studies, which may lack human predictability due to species differences in transport, metabolism, or other factors. The cover article in the April issue of the AAPS Newsmagazine examines how an in vitro system might have the best potential to predict in vivo effects and interactions. Read Intracellular Drug Concentrations: A Critical Consideration for In Vitro Assays, from the PPDM section, and then participate in the discussion point below.
Based on your experience in drug development, have you seen differences in metabolism, transport, toxicity, or pharmacology between acute and chronic administration that are not predicted from your in vitro studies? How could your in vitro experiments be redesigned to address this?