Complementary roles of antibody affinity and specificity for in vivo diagnostic cardiovascular targeting: How specific is antimyosin for irreversible myocardial damage?

Background. Identification of irreversible myocyte injury with antimyosin antibody imaging depends on both antibody specificity and affinity. To characterize the role of antibody affinity, we performed studies in dogs with acute coronary occlusion followed by reperfusion using 3 monoclonal antimyosin antibodies with different affinities. Methods and Results. Dogs with experimental reperfused acute myocardial infarction were injected with 2 high-affinity radiolabeled monoclonal antimyosin Fab fragments (R11D10 and 2G42D7), 1 low-affinity antimyosin Fab (3H31E6), and a nonspecific Fab. The left lateral gamma images at 5 H were used to assess the infarct (I) to blood (B) region of interest (ROI) count density ratios by computer planimetry. All infarcts were confirmed by in vivo imaging with ²⁰¹Tl for perfusion defects as well as by postmortem histochemical staining. The mean I/B ROI (± standard deviation [SD]) for R11D10 (1.701 ± 0.376) was not significantly different from that of 2G42D7 (1.501 ± 0.267, P = NS), but both were significantly greater than that of 3H31E6 Fab (0.85 ± 0.12, P = .0001 and .0012, respectively). The I/B ROI of 3H31E6 Fab was similar to that of nonspecific Fab (0.75 to 0.77 range). Radiolabeled R11D10 and 2G42D7 were unequivocally positive by gamma imaging in all infarcts by 5 H. No infarcts were visualized with 3H31E6 or nonspecific Fab. Conclusions. The low-affinity antibody, despite its specificity for cardiac myosin, cannot be used to image the infarct zone. Therefore immunoscintigraphic diagnosis of irreversible myocardial injury with radiolabeled antimyosin Fab is doubly specific because in vivo visualization required both specificity and high enough affinity of the antibody.