Optimum conditions for radiolabelling human granulocytes and mixed leucocytes with 111In-tropolonate

To determine the optimum conditions for the in vitro radiolabelling of human granulocytes with ¹¹¹In-tropolonate for clinical studies, the factors which affected the amount of ¹¹¹In bound to the cells, the labelling efficiency (LE), were measured. These included the tropolone concentration, labelling medium and cell concentration. The tropolone concentration was dependant on the amount of plasma in the labelling medium; with 90% ACD plasma it was 4×10^-4 M and with Hepes saline buffer it was 4×10^-5 M. Using these tropolone concentrations and a low granulocyte concentration of 1×10⁷ ml^-1, the LE in 90% ACD plasma was 29% and in buffer was 74%. However, increasing the cell concentration to 1×10⁸ ml^-1 gave a LE of 90% in buffer and plasma. The optimum conditions for clinical studies involved incubating granulocytes, or mixed leucocytes as a source of granulocytes, at a cell concentration of at least 5×10⁷ cell/ml in 1 ml ACD plasma, pH 7–7.6 with 0.1 ml tropolone at 4.4×10^-3 M mixed with no more than 100 l ¹¹¹InCl₃ for 15 min at room temperature. Under these conditions more than 96% of the ¹¹¹In was taken up by the granulocytes and only 3% of the ¹¹¹In was released from the labelled cells during a 30 min incubation in plasma. ¹¹¹In-tropolonate is therefore an efficient agent for stably radiolabelling human granulocytes in plasma for clinical studies.