Indium-III: a New Radionuclide Label for Studying Human Platelet Kinetics

Indium-III. when complexed with 8-hydroxyquinoline (oxine), has been employed as a radioactive platelet label for thrombus imaging in animals and man. The short half-life (2.8 d) and high yield of gamma photons of 111In make it ideal for in vitro counting and external imaging. To evaluate its suitability for studies of platelet turnover in man, platelet kinetic studies were carried out on 10 healthy volunteers using 111In-and 51Cr-platelets concurrently. For 111In labelling, platelets were harvested by differential centrifugation from 43 ml of whole blood drawn into acid-citrate dextrose (ACD) solution. The platelets were washed and suspended in a mixture of ACD and isotonic saline and then incubated with 111In-oxine, rewashed, and suspended in plasma for reinfusion. 51Cr labelling was performed using standard methods. Mean labelling efficiency was 73% with 111In and 6.5% with 51Cr. In vitro studies demonstrated minimal release, elution, and reutilization of the 111In label. There was no significant difference in the aggregation response of 111In- and 51Cr-platelets to ADP and collagen. The in vivo recovery of 111In-platelets was approximately 50% greater than that of 51Cr-platelets whereas the platelet life spans were similar. These results indicate that 111In labelled platelets may be useful for thromobokinetic studies in man. The new method offers the advantages of reduced blood requirements, higher labelling efficiency, and the ability to perform external imaging of platelet distribution in vivo.