Evaluation of the stablilisation of Technetium-99m exametazime utilising cobalt chloride: preliminary results

A limited post-reconstitution shelf-life of 30 min for Technetium-99m exametazime (Ceretec(R)) restricts its clinical availability. Several attempts have been made to extend the 30 min shelf-life with varying success and a review of published methods concludes that although stabilisation is possible, careful analysis of the radiochemical purity is necessary before use. A method for stabilising 99mTc-exametazime using chloride was investigated. Preparation of the 99mTc-exametazime was as per manufacturer's instructions and cobalt chloride hexahydrate (Aldrich) was used to prepare a 100 μg/mL (0.42 mM) solution. Vials were reconstituted with eluate containing approximately 2 GBq of 99mTc diluted to 5 mL with normal saline. A stabilised 99mTc-exametazime solution was prepared by adding 2mL (200 μg) of the CoCI2.6H2O solution to the Ceretec vial, 2 min post-reconstitution of the 99mTc-exametazime. Analyses of the 99mTc-exametazime solutions were performed at various time points up to six hours post-reconstitution, and the methods used were instant thin layer chromatography (ITLC) and high-performance liquid chromatography (HPLC). Stability of the stabilised 99mTc-exametazime solution was studied in the original manufacturer's glass vial and in disposable syringes. Using the radiochemical purity specification of not less than 80 per cent lipophilic complex, the results indicate that the stabilised radiopharmaceutical which was stable for at least six hours in the original glass vial (n=6), was stable for only two to four hours in a lubricated syringe (Terumo) (n=3) but stable for six hours in an non-lubricated syringe (Discardit II, BD) (n=3). This preliminary investigation indicates that 99mTc-exametazime stabilised with cobalt chloride hexahydrate appears to be stable for at least six hours in both the original vial and non-lubricated syringes