Specific activity and molar activity

Specific activity is the activity of a radionuclide divided by the total mass of the element (or the labelled compound) present, expressed in units such as GBq/mg; molar activity is the same idea per mole (GBq/µmol) and is the preferred term when the labelled molecule, not the element, is what matters clinically. Because no radiolabelling reaction is perfectly efficient, every batch of a radiopharmaceutical contains some non-radioactive ('cold') copies of the same targeting molecule alongside the radioactive ones, and specific activity quantifies that ratio.

The practical stakes are receptor saturation: for a receptor-targeted tracer or therapy (an SSTR or PSMA ligand, for example), the cold carrier competes with the radiolabelled molecule for a limited number of binding sites, so low specific activity can reduce tumour uptake per unit of injected mass. The qualitative terms carrier-free, no-carrier-added (n.c.a.) and carrier-added describe roughly how much stable-isotope carrier is present without stating a number; a 2018 international nomenclature consensus recommends always reporting the measured specific or molar activity rather than relying on these qualitative labels alone.

Specific activity is determined experimentally, commonly by HPLC (comparing the radioactive peak to a UV mass calibration curve) or, theoretically, from the decay constant and the assumption of no carrier at all. It is a batch-release and formulation parameter tracked in GMP manufacture of PET and therapeutic radiometal agents, and it is specified in clinical trial protocols for tracers such as 18F-fluoromisonidazole or 89Zr-antibody conjugates.