How monoclonal antibodies behave differently from small molecules
Monoclonal antibodies are large proteins, so they do not behave like conventional small-molecule drugs. They are too big to cross the gut wall intact, which is why they are given by injection or infusion rather than as tablets. They are also not metabolised by the liver's cytochrome P450 enzymes; instead they are broken down into amino acids by general protein-degradation pathways, so the classic drug-drug interactions seen with small molecules are largely absent.
Their distribution is mostly confined to the blood and extracellular fluid because their size limits movement into tissues and cells.
FcRn recycling and long half-lives
Many therapeutic antibodies persist in the body for weeks. A key reason is the neonatal Fc receptor, FcRn, which binds antibodies taken up into cells and returns them to the circulation instead of letting them be degraded. This salvage pathway is why dosing intervals are often measured in weeks rather than hours, and engineering the FcRn interaction is one way developers extend or shorten an antibody's persistence.
Clearance can speed up when the antibody binds an abundant target, a phenomenon called target-mediated drug disposition. This is general educational information about antibody pharmacology, not medical advice.