Radiolabelling quality control
Before any radioactive drug is given to a patient, a quick chemistry test checks that the radioactive atom is properly and completely attached to the drug molecule, rather than floating free, because free radioactivity goes to the wrong place in the body.
Overview
Radiochemical purity (RCP) is the proportion of a radiopharmaceutical's total radioactivity that is present in the intended, correctly labelled chemical form, and it is distinct from radionuclidic purity (correct radioisotope, free of parent or other radioactive contaminants such as 99Mo breakthrough) and chemical purity (non-radioactive contaminants such as aluminium ion breakthrough from a generator column). RCP is most commonly measured by instant thin-layer chromatography (iTLC or ITLC-SG): a small spot of the radiopharmaceutical is applied near the base of a silica-impregnated glass-fibre or paper strip, developed in a specified mobile phase, then either scanned with a radiochromatogram scanner or cut into segments and counted in a gamma counter or dose calibrator, with RCP calculated as the percentage of counts in the desired product peak versus total counts on the strip. Typical release specifications require RCP of at least 90-95%, with the exact limit, solvent system and strip type defined by the product's package insert, USP monograph or Ph. Eur. monograph, and a failing preparation must be rejected and not administered.
High-performance liquid chromatography (radio-HPLC) offers finer chromatographic resolution and can detect degradation products from radiolysis that iTLC misses; one comparative study of 177Lu-PSMA found that reducing RCP by 50% (through induced radiolysis) reduced the labelled peptide's cell-binding capacity by 27%, and that only HPLC, not iTLC, identified the radiolysis peaks responsible, underscoring that the two methods are complementary rather than interchangeable (iTLC remains preferred for fast, sensitive detection of free/unbound radiometal at release, while HPLC is used for detailed impurity profiling and method validation). For alpha-emitter products such as 225Ac-labelled peptides, additional considerations apply, including waiting for radioactive equilibrium between 225Ac and its gamma-emitting daughters (213Bi, 221Fr) before iTLC readout, and confirming RCP by both radio-iTLC and radio-HPLC (typical release criteria of at least 95% by radio-iTLC).
How it works
A small spot of the finished radiopharmaceutical is run on a chromatography strip or through an HPLC column, which physically separates the correctly labelled drug from any unbound or degraded radioactivity; counting how much radioactivity ends up in the product's peak versus everywhere else gives the percentage that is in the intended chemical form, which must clear a preset limit before the dose can be released for injection.
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