Indium-111 myosin-specific antibodies and technetium-99m pyrophosphate in the detection of acute cardiac rejection of transplanted hearts: studies in a heterotopic rat heart model.

111In-labelled myosin-specific antibodies were evaluated as an indicator of early changes in acute rejection in a rat heart heterotopic transplant model. Uptake of antibodies was measured in allograft and isograft hearts of animals undergoing different regimens of cyclosporine treatment and compared with the uptake of technetium-99m pyrophosphate. The data were correlated with histological estimation of the severity of myocyte necrosis and signs of early rejection (venous cuffing and endocardial inflammation, indicators of perivascular infiltrate and intermyocyte extension, respectively). Myocyte necrosis in transplanted hearts was reflected by increases in technetium-99m pyrophosphate accumulation (r = 0.88) but was poorly correlated with labelled antibody uptake (r = 0.58). There was no positive correlation between the degree of early cardiac rejection and uptake of either of the radiopharmaceuticals: accumulation of the labelled antibodies paradoxically declined with increased histological severity scores, whereas that of technetium-99m pyrophosphate remained unchanged. Cyclosporine treatment augmented the uptake of labelled antibodies in transplanted hearts. This may be due to alterations in plasma membrane permeability brought about by the drug, resulting in a rise in antibody binding to intracellular myosin.