Monoclonal Antibody to Cardiac Myosin: Imaging of Experimental Myocardial Infarction

Monoclonal antibody R11D10 to human cardiac myosin, which also cross-reacted with canine cardiac myosin, was used to demonstrate in vivo localization and visualization by gamma scintigraphy of experimental myocardial infarction. R11D10 Fab with a Ka of 5 X 10(8) M-1 was labeled with technetium-99m (99mTc) by the dithionite reduction method of technetium pertechnetate, via a bifunctional chelating agent, diethylene triamine pentaacetic acid (DTPA). Uptake of 99mTc R11D10 Fab in the infarct can be visualized as early as 2 h after intravenous administration. Comparison of R11D10 uptake to thallium-201, an analogue of potassium which is sequestered by normal myocardium, showed an inverse relation (r = -0.75, -0.87, -0.89), similar to that obtained with 125I labeled polyclonal antimyosin Fab. Ratios of R11D10 Fab in the infarct to normal myocardium were as high as 30:1 where access of antibody to antigen was not blood flow limited. However, with severe blood-flow restriction, the ratios were lower at about 10:1. Despite the theoretical limitation of a single epitope per myosin molecule available for binding by R11D10 Fab, the immense excess of myosin in the infarcted myocardium allowed adequate concentration of radiolabeled R11D10 for visualization of the infarct by external gamma scintiscanning.