The synthesis and in vivo antigen targeting of a novel iron oxide compound were studied. A monocrystalline iron oxide nanoparticle (MION) was synthesized that contains a small (mean diameter, 2.9 nm +/- 0.9) single crystal core, passes through capillary membranes, and exhibits superparamagnetism. The MION was attached to antimyosin Fab (R11D10) and used for immunospecific magnetic resonance (MR) imaging of cardiac infarcts One hour after intravenous administration of MION-R11D10 in rats (100 mumol/kg), a marked decrease in the signal intensity of infarcted myocardium was observed. Immunohistochemical correlation confirmed the specific binding of the immunoconjugate to infarcted, but not to normal, myocardium. No decrease in cardiac signal intensity was observed when unconjugated MION was administered intravenously. The results indicate the feasibility of immunospecific MR imaging in living organisms.

The less than optimal accumulation of immunoliposome-associated reagents at target sites has often been attributed to the rapid in vivo clearance of immunoliposomes from the blood. In an attempt to overcome the drawback of rapid clearance and use the targeting potential of immunoliposomes, we have prepared long-circulating, 111In-labeled immunoliposomes. Targeting properties and enhanced circulation times were demonstrated in a rabbit model of acute experimental myocardial infarct. The specificity of liposomes for newly exposed intracellular cardiac myosin at the necrotic sites was achieved by incorporating monoclonal antimyosin antibody. Extended circulation times were achieved by cocoating the antimyosin-liposomes with polyethylene glycol (PEG). The half-life of the immunoliposomes was 40 min, which increased to 200 min with 4% mol PEG and to approximately 1000 min with 10% mol PEG. The degree of binding of modified immunoliposomes at the target sites was also dependent on the concentration of PEG incorporated at the liposome surface. This study demonstrates the accumulation of long-circulating targeted liposomes at the area of acute rabbit experimental myocardial infarction.

A B S T R A C T Specific localization of purified antibody against cardiac myosin has been demonstrated in areas of altered myocardial membrane permeability after experimental myocardial infarction. Intravenously administered radioiodine-labeled antimyosin was selectively localized in infarcted myocardium of seven dogs 24 h after coronary occlusion. The mean ratio (±tSE) of antimyosin antibody in infarcted to normal myocardium in the center of the infarct was 4.2±0.4 for endocardial and 2.9±0.3 for epicardial layers. By utilizing (Fab')2 fragments of antimyosin obtained by pepsin digestion of purified antibody, the ratio of uptake was increased in eight dogs to 6.' ±0.6 in the endocardial and 3.3 +0.4 in the epicardial layers at the infarct center 24 h after occlusion. These ratios were further increased in the infarct center to 13.8+1.2 in the endocardial and 7.3±0.8 in the epicardial layers when eight dogs were sacrificed 72 h after coronary occlusion.The specificity of antimyosin (Fab')2 localization in infarcted myocardium was demonstrated in four dogs by simultaneous intravenous administration of MI-labeled antimyosin (Fab')2 and "'I-labeled normal rabbit gamma globulin (Fab')2. Nonspecific trapping of normal rabbit IgG (Fab')2 was observed to be about 38% of total antimyosin (Fab') 2 uptake in the central zone of infarction.Regional blood flow was related to antimyosin (Fab')2 uptake in infarcted myocardium by utilizing simultaneous administration of 'Sr-labeled microspheres. An inverse exponential relationship between antimyosin (Fab')2 uptake and regional blood flow was observed (r = 0.85).

The Fab fragments of antimyosin antibodies, labeled with 9mTc, were used in the scintigraphic examination of 30 patients with myocardial infarction. The ability to detect necrosis and determine its extent from the antimyosin scan were compared with the results of quantitative regional wall motion analysis by contrast ventriculography at 10 to 14 days and 99mTc-pyrophosphate imaging. Antimyosin images recorded by planar and single photon-emission computed tomography (SPECT) delineated areas of myocardial necrosis in 27 of 30 patients (90%) compared with a 91% sensitivity of pyrophosphate in 21 of 23 patients. Infarct size was determined by both antimyosin and pyrophosphate SPECT images. Results by both techniques showed a significant correlation with computer-derived hypokinetic segment length (r = .79 for both, p = .002) and peak creatine kinase (r = .9 for both, p < .01). Although sensitivity for and correlations with markers of necrosis were similar with both techniques, infarct size by pyrophosphate SPECT was 1.7 times larger than infarct size by antimyosin SPECT (p < .01). Certain zones in the infarct area were differentially labeled; the nature and irreversibility of injury within these zones remains to be clarified. Circulation 74, No. 3, 501-508, 1986. WE PREVIOUSLY described a method for localizing and quantifying regions of myocardial necrosis based on the binding of radiolabeled myosin-specific antibodies to cells that have lost the integrity of their plasma membranes.",2 Unlike other methods that are dependent on blood flow, concentration of antibody in the center of an infarct is greater than that at the periphery, with antibody concentration inversely proportional to blood flow.

Right ventricular endomyocardial biopsy currently remains the procedure of choice for identifying patients with symptomatic heart failure due to myocarditis from the larger population with idiopathic dilated cardiomyopathy. Despite its specificity, the sensitivity of right ventricular biopsy remains uncertain because of the focal or multifocal nature of the disease. Because myocyte necrosis is an obligate component of myocarditis, the use of indium-111 antimyosin imaging was evaluated in 82 patients with suspected myocarditis. Seventy-four patients had dilated cardiomyopathy of less than 1 year's duration (mean left ventricular ejection fraction 0.30 +/- 0.02); eight patients had normal left ventricular function (mean ejection fraction 0.59 +/- 0.03). Symptoms at presentation included congestive heart failure (92%), chest pain mimicking myocardial infarction (6%) and life-threatening ventricular tachyarrhythmias (2%). All patients underwent planar and single photon emission computed tomographic (SPECT) cardiac imaging after injection of indium-111-labeled antimyosin antibody fragments and right ventricular biopsy within 48 h of imaging. Antimyosin images were interpreted as either abnormal or normal and correlated with biopsy results. On the basis of the right ventricular histologic examination, the sensitivity of antimyosin imaging was 83%, specificity 53% and predictive value of a normal scan 92%. Improvement in left ventricular function occurred within 6 months of treatment in 54% of patients with an abnormal antimyosin scan compared with 18% of those with a normal scan (p less than 0.01). Antimyosin cardiac imaging may be useful for the initial evaluation of patients with dilated and nondilated cardiomyopathy and clinically suspected myocarditis.(ABSTRACT TRUNCATED AT 250 WORDS)