Studies of
monoclonal antibody-based imaging agents show that blood clearance is inversely proportional to molecular size, i.e., Fab or Fab' > F(ab')2 >
IgG.
Indium-111-antimyosin Fab-
DTPA is a highly specific and sensitive marker for myocardial
necrosis. An improvement on current antibody diagnostic imaging may result from the use of smaller labeled fragments. We report the first in vivo targeting of acute
myocardial infarction with a novel recombinant
single-chain Fv (sFv) antimyosin
protein. The sFv (MW = 27,594) is approximately one-half the size of the Fab and is comprised of the heavy and light chain variable regions from the
myosin-specific murine
monoclonal antibody R11D10 which were joined by a 15-amino-acid linker and expressed as a fusion
protein (sFv) in E. coli. The binding affinity of the sFv for
cardiac myosin was similar to the affinity observed for the
Fab fragment. Technetium-99m labeling of the sFv was accomplished by the attachment of a cleavable,
ester-linked bifunctional
chelator (RP-1). Comparative studies in mice showed
99mTc-sFv-RP-1 cleared significantly faster (p < 0.001) than
99mTc-Fab'-RP-1 and 111In-Fab-DTPA antimyosin fragments. Furthermore, measurement of
99mTc-sFv-RP-1 blood clearance in a canine model of acute
myocardial infarction gave a mean T1/2 of 0.54 +/- 0.13 hr versus 2.80 +/- 0.57 and 2.58 +/- 0.64 hr for Fab-
DTPA and Fab'-RP-1 (p < 0.05), respectively. Despite its comparatively rapid clearance, 99mTc sFv-RP-1 had similar uptake in the
infarct compared to the Fab'-RP-1. In addition,
infarct visualization was more rapid with the sFv. Thus, these data demonstrate antimyosin sFv possesses characteristics necessary for rapid imaging of myocardial
necrosis.