Simplified and efficient labeling of human platelets in plasma using indium-111-2-mercaptopyridine-N-oxide: preparation and evaluation.

The need for a gamma-emitting radioactive agent that will label platelets in plasma efficiently by a procedure that can be used uniformly in each laboratory is well recognized. A water soluble sodium salt of 2-Mercaptopyridine-N-oxide (Merc) was evaluated that quantitatively chelated 111ln at a pH range of 0.7 to 7.4, and allowed greater than 80% incorporation of 111ln in platelets in plasma. This was dependent on pH, Merc concentration, and platelet concentration. Platelets were labeled using either preformed [111ln]Merc or incubating platelets with 2.5 micrograms dry Merc first and then with 111ln. The latter method provided a simple kit procedure that labeled platelets with negligible alteration of in vitro aggregability. In dogs, labeled platelets had normal survival (7.5 days), 66 +/- 6.6% recovery, detected vascular thrombi (thrombi/blood = 59.4), and pulmonary emboli (PE/blood = 46.2) by scintigraphy. In the kit procedure, the use of Merc compared favorably to that of oxine and tropolone.