⁹⁹ᵐTc Radiochemistry & Cold Kits

• Free ⁹⁹ᵐTcO₄⁻ behaves like pertechnetate — thyroid, salivary and gastric uptake add background — which is why kits contain excess Sn²⁺ and are sealed under nitrogen.
• Air oxidises Sn²⁺ to Sn⁴⁺, lowering the reducing capacity and raising free pertechnetate; hence antioxidants, nitrogen headspace and prompt use.
• Residual Sn²⁺ from a recent ⁹⁹ᵐTc kit (for example a bone scan) can label red cells in vivo, giving blood-pool activity on a later pertechnetate study.
• ⁹⁹ᵐTc-MAA: at least 90% of particles must be 10–90 µm (USP) or 10–100 µm (Ph. Eur.), with none larger than 150 µm, so that particles lodge in pre-capillary arterioles without blocking larger vessels.

• Stannous ion does not label the tracer — it only reduces Tc⁷⁺; complexes such as DTPA, MDP and gluconate contain no tin.
• 'More tin is better' is wrong: excess Sn²⁺ hydrolyses and co-precipitates reduced technetium as a colloid, lowering the yield.
• Pertechnetate (Tc⁷⁺) cannot label most ligands directly — reduction comes first (sulfur colloid is the exception, with technetium precipitated as the heptasulfide).
• One agent can hold technetium in different oxidation states at different pH (e.g. HEDP: +3 in acid, +5 in alkaline solution).

• Ph. Eur. ⁹⁹ᵐTc-macrosalb (MAA): at least 90% of particles 10–100 µm and none above 150 µm (USP: 10–90 µm, none above 150 µm); radiochemical purity is tested by filtration — at least 90% of the radioactivity must be retained on a 3-µm polycarbonate filter.
• Exametazime forms a lipophilic primary complex that slowly converts to a hydrophilic secondary complex which does not cross the blood–brain barrier. Hence the labelled limits: RCP >80%, eluate no more than 2 h old from a generator eluted within 24 h, use within 30 min unstabilised or 4 h when stabilised.
• Hydrolysed-reduced technetium forms when reduced technetium meets too little free ligand (or is hydrolysed in water); it behaves as a colloid, so it is trapped by the reticulo-endothelial system and shows as liver and spleen activity.
• The kit's stannous excess is deliberate: at 10⁻⁹ M technetium, oxygen introduced with the eluate or through the septum would otherwise consume the reductant. Adding air, or eluate with oxidants, is a classic cause of high free pertechnetate.
• Sulfur colloid is prepared by heating acidified thiosulfate so that colloidal sulfur precipitates with technetium heptasulfide; aluminium ions in the eluate flocculate the colloid into larger particles that are trapped in the lungs.
• The EANM cGRPP guideline treats kit reconstitution as an aseptic manipulation in a grade A environment and asks that the quality-control procedure in the SmPC be performed at least for each new batch of kits; routine sterility and endotoxin testing is not required for licensed kit preparations, but aseptic technique is monitored with media fills.
• Particle number matters as much as size for MAA: the number of particles in a dose scales with the fraction of the vial's activity drawn up and rises as the preparation decays, so the particle count per dose must be calculated at the time of injection.

Sources: Jensen et al. 2022 (Molecules 27:3997) · Ceretec SmPC / US prescribing information · Saha, Fundamentals of Nuclear Pharmacy, 7th ed. (2018), Ch. 6–7 · Sulfur colloid kit US label (2019) · Gillings et al. 2021 (PMID 33580358)