Pharmacokinetics.

How the body absorbs, distributes, metabolizes, and clears a compound.

Pharmacokinetics, usually shortened to PK, describes what the body does to a compound over time, conventionally split into absorption, distribution, metabolism, and excretion. It answers questions such as how quickly a compound enters the blood, how widely it spreads into tissues, how it is broken down, and how long it takes to leave. For peptides the PK story is unusual compared with conventional small-molecule drugs. Absorption is dominated by the delivery route because oral routes largely fail. Distribution is limited because large, water-soluble molecules do not cross membranes or the blood-brain barrier easily. Metabolism happens through peptidase enzymes that cut amino-acid chains rather than through the liver enzyme systems that dominate small-molecule metabolism, which changes which drug interactions are plausible. And clearance often depends on kidney filtration and on receptor-mediated uptake, meaning the target itself can help remove the drug. Understanding PK is how you make sense of design choices such as albumin binding, PEGylation, and depot formulations, all of which exist to slow one step of this pipeline. The pairing to keep straight is that pharmacokinetics is what the body does to the compound, while pharmacodynamics is what the compound does to the body.