Biological half-life

Biological half-life is the time required for the amount of a substance present in an organ or the whole body to fall to half its original value through biological elimination processes alone, that is, metabolic breakdown, renal or hepatobiliary excretion, or other clearance mechanisms, independent of any radioactive decay of an attached isotope. It is a pharmacokinetic property of the molecule (and of the specific tissue considered), not of the radionuclide, and it is influenced by physiological factors such as renal function, hydration and hepatic clearance.

Biological half-life is measured directly from a decay-corrected time-activity curve, or inferred from the biodistribution of a stable-isotope-labelled or otherwise trackable analogue of the compound. In radiopharmaceutical dosimetry it is combined with the physical (radioactive) half-life to give the effective half-life, which is the quantity that actually determines how quickly the radioactivity, and therefore the absorbed dose rate, declines in a given organ or tumour after administration.

Because biological half-life is independent of the isotope used, the same targeting molecule with different attached radionuclides (a diagnostic and a therapeutic pair) is expected to share very similar tissue biological half-lives, an assumption that underlies using a diagnostic imaging agent's kinetics (for example gallium-68 or copper-64 labelled) to predict dosimetry for its therapeutic counterpart (lutetium-177 or actinium-225 labelled) in theranostic pairs.