The long-term stability of reconstituted exametazime: a clinical and laboratory evaluation

99Tcm-exametazime has become recognized as an effective agent for in vitro leucocyte labelling. However, the material is relatively expensive and exhibits instability, being recommended for use within 30 min. Following subdivision and low-temperature storage of unlabelled exametazime we have evaluated long-term stability in a clinical setting and provided some laboratory support for the observations. Vials of exametazime were reconstituted with sodium chloride injection BP and subdivided into three aliquots prior to storage at -66 degrees C. At intervals up to 27 days they were used for routine leucocyte labelling with percentage labelling efficiency being recorded. Additional aliquots were tested for lipophilic primary complex and radiochemical impurities using thin layer chromatography over a similar period. Results on 92 patients gave a mean leucocyte labelling efficiency of 37% (range 10-78%). Chromatography supported the patient data. The percentage of primary exametazime remaining after 24 days' storage (55%) compared favourably with that in a freshly prepared aliquot, subdispensed but not stored (66%). Subdivision and refrigerated storage of unlabelled exametazime at -66 degrees C appears to preserve its radiolabelling capacity and the ability of the 99Tcm-exametazime to label leucocytes. This combination of long-term stability and improved cost-effectiveness may promote more widespread utilization of this pharmaceutical in clinical nuclear medicine.