Pharmacokinetics

Pharmacokinetics

Pharmacokinetics (PK) is the part of pharmacology that studies what the body does to a drug. Pharmacodynamics, by contrast, studies what the drug does to the body. PK is usually described with the ADME framework: absorption (how the drug gets into the blood), distribution (how it spreads through the body), metabolism (how it is chemically changed) and excretion (how it is removed). Together these set how much active drug reaches its targets over time.

For injected peptide drugs like retatrutide, absorption happens from the injection site under the skin into the bloodstream. How fast and how completely this happens affects the time to peak blood level (Tmax) and the bioavailability. Distribution depends on protein binding, especially to albumin, which retatrutide boosts with a fatty acid chain attached to the peptide. Peptide drugs are broken down mainly by general protein-digesting enzymes rather than the liver’s cytochrome P450 enzymes, which lowers the risk of drug-drug interactions. Removal happens through the kidneys and protein breakdown.

PK data are essential for choosing dosing regimens. For retatrutide, PK studies established the roughly six-day half-life that supports weekly dosing and shaped the dose-escalation schedules used in Phase 2 and Phase 3 trials. PK modeling also helps predict drug levels in special groups, such as people with kidney or liver problems.