Rao, Shyam A.

Platelets have been labeled with a new, neutral, lipidsoluble metal-complex of Indium-111-tropolone (In-TPL). Unlike oxine, tropolone is soluble in isotonic saline. Oil/saline partition of In-111-oxine and In-TPL are 0.5 and 23.7 respectively. Platelet labeling with In-TPL was performed in both ACD-plasma and ACD-saline media within two hours' time. Increasing concentration of tropolone, citrate ion, and plasma proteins decreases plateletlabeling efficiency. Effects of pH, temperature, platelet concentration, and calcium ion concentration on platelet labeling were studied. In optimum platelet-labeling conditions, canine, rabbit, porcine, and human platelets have been labeled with a consistent labeling efficiency of 80-90%. The optimum labeling conditions are In-TPL in ACD- saline and ACD-plasma at tropolone concentration of 5 and 10 μg/ml respectively and 30 minutes’ incubation of platelets with the tracer at room temperature. A kit formulation for convenient routine preparation of In-111-labeled platelets has been developed. The function of the In-TPL- labeled platelets has been studied by their aggregability with adenosine diphosphate. The capacity of In-111-labeled platelets to aggregate in vitro does not correlate well with their ability to circulate in vivo. No adverse effect of In-TPL on platelets has been observed by studies of biodistribution, recovery, and platelet survival in dogs. Comparable mean platelet survival times of four repeated studies in five dogs were obtained in both ACD-saline and ACD-plasma. These results indicate that Indium-111 platelets labeled by tropolone carrier may be preferable to oxine carrier for studies of platelet survival, imaging sites of endothelial damage, and thrombus formation.

Subcellular distribution of In-111 in In-111-labeled platelets (In-P1) determines the biodistribution and dosimetry of In-111 after the in vivo lysis of In-P1. Human platelets were labeled with In-111-oxine (In-Ox) and In- 111-troPolone (In-TPL) in ACD/saline (A/S) and plasma (P). In-P1 was lysed after 6 repeated freezing and thawing procedures. A fraction of In-P1-lysate (In-P1-LS) was filtered (0.22 ym) and passed through a calibrated Sephadex G-100 column for estimation of molecular weight (Mol. wt.) of In-bound Pl-protein. The In-P1-LS was injected in 15 rabbits and biodistribution (% I.D.) performed at 24 hours after I.V. administration. Results are tabulated below: 80 ± 11% and 70 ± 10% of In was bound to soluble platelet protein when platelets were labeled in plasma and A/S respectively. Mol. wt. of In-bound platelet protein was estimated as 52,000 ± 5,000 daltons. Biodistribution shows that In-PI-LS labeled in A/S localizes more in RES than plasma; whole body retention is also longer. These studies shed further light on the binding sites of In in platelets labeled in A/S or plasma media and their ultimate fate after administration