Excerpt

Antimyosin antibody was created in response to a challenge Edgar Haber, MD, Chief of Cardiology at the Massachusetts General Hospital, gave his post-doctoral student, Ban An Khaw, PhD to develop a technique to specifically identify acute myocardial necrosis. Since Dr. Khaw had completed his training in immunology, his thoughts led him to consider creating an antibody that would recognize one of the least soluble proteins in the body, the heavy chain of cardiac myosin. Drs. Khaw and Haber reasoned that areas of severely damaged myocardium lose cell membrane integrity, allowing macromolecules such as enzymes and troponin to leak out, and allow macromolecules, such as antibodies recognizing insoluble cardiac myosin, to leak in and bind to the residual myosin protein at the site of damage. After numerous challenges were overcome, Khaw, Haber, and their colleagues Beller and Smith published a seminal article in The Journal of Clinical Investigation titled: ‘Localization of cardiac myosin-specific antibody in myocardial infarction’1 describing localization of intact radiolabeled polyclonal antimyosin antibody and localization of a radiolabeled (Fab′)₂ antimyosin antibody fragment in experimental canine infarction. Although the concept worked, the intact antibody (molecular weight ~ 150 kDa) had a long residence time in the circulation, requiring a delay between antibody injection and imaging. To increase the rate of blood clearance, the antimyosin antibody was partially digested to produce the (Fab′)₂ fragment (molecular weight ~ 110 kDa). Even the lower molecular weight (Fab′)₂ required waiting up to 48 hours for sufficient blood clearance to record diagnostic images in experimental animals. To further decrease the interval between injection and imaging, Dr. Khaw purified the Fab (molecular weight ~ 50 kDa).2 Although the Fab had the advantage of faster blood clearance, it had the disadvantage of decreased affinity for the heavy chain of cardiac myosin, since the Fab has only a single antigen recognition arm, while the larger (Fab′)₂ had two. In spite of this limitation, the Fab worked well in experimental studies, and was selected for testing in human subjects. In parallel with the evolution from intact antibody to Fab fragment, Dr. Khaw changed radiolabels from ¹³¹I for the intact antibody to ¹¹¹In-DTPA for the (Fab′)₂, and subsequently to ^99mTc-DTPA coupled to human antimyosin Fab for studies in patients with acute infarction.3 Further fragmentation of antimyosin, to single chain sFv (molecular weight ~ 28 kDa, radiolabeled with ^99mTc) was tested in mice and in a canine model of infarction.4 The single chain fragment had similar immunoreactivity to the Fab. The half-time of sFv blood clearance was reduced from 2.8 hours for the Fab to 0.54 hours for the sFv, while achieving similar uptake in the experimental infarct. Infarcts were clearly visible 1 hour after sFv injection, while 3 hours was required for imaging with the Fab. …