Definition
Pharmacokinetics describes what an organism does to a compound, summarised as ADME: absorption, distribution, metabolism and elimination. PK studies measure concentrations in plasma or tissue at a series of time points and derive parameters such as maximum concentration (Cmax), time to maximum (Tmax), area under the curve (AUC), clearance, volume of distribution and elimination half-life. Pharmacodynamics, by contrast, describes what the compound does to the organism. Peptides typically have short half-lives because of proteolysis and renal filtration, which is why analogues use D-amino acids, lipidation or PEGylation. Peptide concentrations are usually measured by LC-MS/MS or immunoassay, and PK parameters often differ substantially between species.