Effect of Chelate Type and Radioisotope on the Imaging Efficacy of 4 Fibrin-Specific PET Probes.
Thrombus formation plays a major role in cardiovascular diseases, but noninvasive thrombus imaging is still challenging. Fibrin is a major component of both arterial and venous thrombi and represents an ideal candidate for imaging of thrombosis. Recently, we showed that (64)Cu-DOTA-labeled PET probes based on fibrin-specific peptides are suitable for thrombus imaging in vivo; however, the metabolic stability of these probes was limited. Here, we describe 4 new probes using either (64)Cu or aluminum fluoride (Al(18)F) chelated to 2 NOTA derivatives.


METHODS
Probes were synthesized using a known fibrin-specific peptide conjugated to either NODAGA (FBP8, FBP10) or NOTA-monoamide (FBP9, FBP11) as chelators, followed by labeling with (64)Cu (FBP8 and FBP9) or Al(18)F (FBP10 and FBP11). PET imaging efficacy, pharmacokinetics, biodistribution, and metabolic stability were assessed in a rat model of arterial thrombosis.


RESULTS
All probes had similar nanomolar affinity (435-760 nM) for the soluble fibrin fragment DD(E). PET imaging allowed clear visualization of thrombus by all probes, with a 5-fold or higher thrombus-to-background ratio. Compared with the previous DOTA derivative, the new (64)Cu probes FBP8 and FBP9 showed substantially improved metabolic stability (>85% intact in blood at 4 h after injection), resulting in high uptake at the target site (0.5-0.8 percentage injected dose per gram) that persisted over 5 h, producing increasingly greater target-to-background ratios. The thrombus uptake was 5- to 20-fold higher than the uptake in the contralateral artery, blood, muscle, lungs, bone, spleen, large intestine, and heart at 2 h after injection and 10- to 40-fold higher at 5 h. The Al(18)F derivatives FBP10 and FBP11 were less stable, in particular the NODAGA conjugate (FBP10, <30% intact in blood at 4 h after injection), which showed high bone uptake and low thrombus-to-background ratios that decreased over time. The high thrombus-to-contralateral ratios for all probes were confirmed by ex vivo biodistribution and autoradiography. The uptake in the liver (<0.5 percentage injected dose per gram), kidneys, and blood were similar for all tracers, and they all showed predominant renal clearance.


CONCLUSIONS
FBP8, FBP9, and FBP11 showed excellent metabolic stability and high thrombus-to-background ratios and represent promising candidates for imaging of thrombosis in vivo.
Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, Massachusetts.
Francesco
F
Blasi
Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, Massachusetts.
Bruno L
BL
Oliveira
Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, Massachusetts.
Tyson A
TA
Rietz
Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, Massachusetts.
Nicholas J
NJ
Rotile
Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, Massachusetts.
Helen
H
Day
Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, Massachusetts.
Richard J
RJ
Looby
Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, Massachusetts.
Ilknur
I
Ay
Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, Massachusetts caravan@nmr.mgh.harvard.edu.
Peter
P
Caravan
D000818:Animals; D002614:Chelating Agents / Q000737:chemistry*; D004355:Drug Stability; D005337:Fibrin / Q000378:metabolism*; D006571:Heterocyclic Compounds / Q000737:chemistry; D006573:Heterocyclic Compounds, 1-Ring; D008297:Male; D010455:Peptides / Q000737:chemistry* / Q000493:pharmacokinetics; D049268:Positron-Emission Tomography / Q000379:methods*; D011868:Radioisotopes; D051381:Rats; D017208:Rats, Wistar; D013927:Thrombosis / Q000000981:diagnostic imaging
D002614:Chelating Agents; D006571:Heterocyclic Compounds; D006573:Heterocyclic Compounds, 1-Ring; D010455:Peptides; D011868:Radioisotopes; C048993:1,4,7-triazacyclononane-N,N',N''-triacetic acid; D005337:Fibrin
64Cu; Al18F; fibrin; positron emission tomography; thrombosis