Investigations to determine the optimum conditions for radiolabelling human platelets with 99Tcm-hexamethyl propylene amine oxime (99Tcm-HM-PAO)

99Tcm-hexamethyl propylene amine oxime (99Tcm-HM-PAO) has been evaluated as an agent to radiolabel human platelets in vitro. The rate of uptake of this lipophilic complex, the effect of HM-PAO, plasma and platelet concentration were measured to determine the optimum conditions for radiolabelling platelets for short-term clinical investigations of thromboses. The complex was made according to the manufacturer's instructions and immediately added to isolated platelets in vitro. The rate of labelling was slower than for leucocytes, reaching a plateau after approximately 40 min at room temperature (RT). Increasing the temperature to 37 degrees C did not increase the labelling efficiency (LE). Addition of plasma to platelets at a cell concentration of 1 x 10(9) [corrected] ml-1 reduced the LE from 66% in saline to 52% in 20% ACD-plasma. However, increasing the platelet concentration from 5 x 10(8) to 2 x 10(9) ml-1, increased the LE from 9 to 76% for platelets labelled in 20% plasma for 30 min at RT. The in vitro stability of the 99Tcm in the labelled cells showed that 7% of the radioactivity were immediately released from the platelets and a further 13% were eluted during a 60 min incubation in plasma at 37 degrees C. It has been concluded that incubation of platelets at RT with 99Tcm-HM-PAO containing 80 micrograms ml-1 HM-PAO at a cell concentration of 1 x 10(9) ml-1 or greater, results in a high LE, with more than 85% of the 99Tcm being retained by the platelets during a 60 min incubation in plasma.