Diagnosis of ventricular assist device (LVAD) thrombosis using fibrin-specific 99mTc imaging agent

Objectives To develop a high avidity anti-fibrin 99mTc probe to localize and quantify thrombus within the high shear environment of titanium axial-flow pumps.

Methods Monomeric bifunctional ligands with a fibrin-specific peptide, a short spacer, and technetium chelating amino acid sequence (F1A) were covalently inter-coupled via a 4-arm-PEG2000-tetramer to form F4A and each was radiolabeled with 99mTc using the IsoLink procedure.

Results LVAD titanium (1mm) housing attenuated the gamma rays by 20%. 99mTc-F1A bound to fibrin with a Kd~4.5 nM, whereas 99mTc-F4A bound more avidly and was not displaced by F1A competition at 120,000:1. 99mTc-F1A binding to fibrin clots was severely impaired by plasma (p<0.05), but no plasma interference was detected for 99mTc-F4A (p>0.05). Pharmacokinetic studies revealed a faster beta-elimination half-life for 99mTc-F4A (124±41min) versus 99mTc-F1A (176±26min). 99mTc-F1A and 99mTc-F4A were both cleared into urine rapidly with negligible liver or spleen accumulation. In vivo studies in mice (n=4/treatment) with carotid thrombus demonstrated an ROI signal intensity (normalized for dose and animal weight) with 99mTc-F4A (0.93±0.11), which was competitively inhibited by 35% (p<0.05) with 3:1 blockade using unlabeled F4A. Using a mock flow-loop (200ml, 50:50, PBS: plasma) with LVAD operating at 10,000 RPM (6 L/min), inline clots exposed to 99mTc-F4A accumulated 0.745±0.04% of the radioactivity dosed (250 µCi) in 2 min compared with clots targeted with 99mTc-F1A (0.16±0.11%), which remained at background levels.

Conclusions A novel, fibrin-specific 99mTc small tetrameric molecular imaging agent (99mTc-F4A) was developed to detect, localize, and quantify intra-LVAD thrombosis noninvasively under high-shear conditions and improve medical and surgical management of LVAD patients.