Radiochemical purity (RCP)

Radiochemical purity is the proportion, usually expressed as a percentage, of the total radioactivity in a radiopharmaceutical preparation that is present in the intended chemical form, that is, correctly bound to the targeting molecule in the right molecular position. It is measured by radio-HPLC or radio-thin-layer chromatography, comparing the area or counts of the desired peak to the total radioactive signal across all peaks, and it is a mandatory batch-release and stability test under pharmacopoeial standards such as the European Pharmacopoeia.

Radiochemical impurities, most often free (unbound) radiometal or radiolysis breakdown products, have a different biodistribution from the intended drug and can degrade image quality in diagnostics or misdirect dose in therapeutics. EANM technical guidance generally asks for RCP above 95% for most diagnostic tracers, though some agents such as 99mTc-HMPAO tolerate lower purity (above 80%) because of their inherent chemistry. For 177Lu-labelled therapeutics, radiochemical purity typically starts near 98% at the end of synthesis and is tracked over the product's shelf-life because it falls with time owing to radiolysis, driven by beta particles from the decaying 177Lu generating reactive free radicals in the aqueous formulation; antioxidant 'radioprotectants' such as ascorbate and gentisic acid are added specifically to slow this decline (see Radiolysis).

Radiochemical purity is distinct from radionuclidic purity, which asks whether the radioactivity comes from the correct radionuclide at all, and from chemical purity, which concerns non-radioactive contaminants.