Rapid InfarctImagingwith a Technetium-99m- LabeledAntimyosin Recombinant Single-Chain Fv: Evaluationin a Canine Model of Acute Myocardial Infarction

Studiesof monodonalantibody-basedimagingagents show that bloodclearanceis inverselyproportional to molecular size, i.e., Fabor Fab' > F(ab')@ > lgG. Indium-i11-antimyosin Fab-DTPAis a highly specific and sensitivemarker for myo cardialnecrosis.An improvement on currentantibodydiag nostic imaging may result from the use of smaller labeled fragments.We report the first in vivo targetingof acute myocardialinfarction with a novel recombinantsingle-chain Fv (sFv) antimyosinprotein.The sFv (MW = 27,594) is approximatelyone-halfthe size of the Fab and is comprised of theheavyandlightchainvariableregionsfromthemyosin specificmunnemonoclonal antibodyAuDiO which were joined by a 15-amino-addlinker and expressed as a fusion protein (sFv) in E. coil. The binding affinity of the sFv for cardiac myosin was similar to the affinity observed for the Fab fragment.Technetium-99m labelingof the sFv was ac complished by the attachment of a cleavable, ester-linked bifunctional chelator (RP-1). Comparative studies in mice showed @Tc-sFv-RP-1 cleared significantly faster (p < 0.001)than @Tc-Fab'-RP-1 and 111In-Fab-DTPA antimyosin fragments. Furthermore, measurement of @“Tc-sFv-RP-1 blood clearance in a canine model of acute myocardial infarc tion gave a meanT112 of 0.54 ±0.13 hr versus 2.80 ±0.57 and 2.58 ±0.64 hr for Fab-DTPAand Fab'-RP-l (p < 0.05), respectively. Despiteitscomparatively rapidclearance, @“Tc sFv-RP-1had similaruptakein the infarctcomparedto the Fab'-RP-l. In addition,infarctvisualization was morerapid with the sFv. Thus, these data demonstrateantimyosinsFv possessescharacteristicsnecessaryfor rapid imagingof myocardialnecrosis. J NucIMed 1993;34:234-241