Molecular Imaging of Collagen Turnover in Myocardial Infarction

Cardiac fibrosis is a key contributor to cardiomyopathy after myocardial infarction (MI). Existing imaging techniques can detect established fibrotic changes; however, they lack sensitivity for ongoing collagen turnover-a dynamic process involving the denaturation of collagen triple helix. Molecular imaging of this process could enhance risk assessment and aid in the development of antifibrotic treatments. This study aimed to evaluate ^99mTc-(HE)₃-(GPO)₉, a radiotracer designed to target denatured collagen, as a biomarker of collagen turnover after MI. This tracer features a polyhistidine-glutamic acid [(HE)₃] N-terminal sequence for site-specific radiolabeling linked to a C-terminal-targeting moiety consisting of 9 glycine-proline-hydroxyproline repeats [(GPO)₉] via a flexible 3-glycine linker. Methods: MI was induced in mice by ligation of the left anterior descending artery; animals who underwent sham surgery served as controls. At 2 wk after MI, animals underwent myocardial perfusion imaging or contrast-enhanced CT to detect the infarct zone, followed by SPECT/CT imaging with ^99mTc-(HE)₃-(GPO)₉ or a control tracer with scrambled peptide. Tracer uptake was quantified in vivo and ex vivo with γ-counting and autoradiography. Different aspects of fibrosis were examined using tissue analysis, along with autoradiography with a matrix metalloproteinase-targeted radiotracer, ^99mTc-RYM1, at 3 d, 1 wk, and 2 wk after MI. Tracer binding was also assessed in human cardiac tissue using ex vivo autoradiography. Results: ^99mTc-(HE)₃-(GPO)₉ SPECT/CT revealed significantly higher tracer uptake in the infarct zone of MI mice compared with the remote zone and sham controls (P < 0.0001 for both). Tracer uptake was confirmed by autoradiography, which showed a strong correlation between SPECT and autoradiography (ρ = 0.81, P < 0.05). The control tracer exhibited minimal cardiac uptake, demonstrating the specificity of the ^99mTc-(HE)₃-(GPO)₉ signal. Denatured collagen staining and ^99mTc-RYM1 autoradiography showed patterns similar to that shown in ex vivo ^99mTc-(HE)₃-(GPO)₉ autoradiography, whereas the ratio of denatured collagen to procollagen in the infarct zone significantly increased from day 3 to 2 wk after MI. Finally, ^99mTc-(HE)₃-(GPO)₉ demonstrated binding to human fibrotic (but not normal) cardiac tissue. Conclusion: ^99mTc-(HE)₃-(GPO)₉ enabled noninvasive detection of denatured collagen after MI as a marker of collagen remodeling in vivo. In combination with other fibrosis imaging tracers, ^99mTc-(HE)₃-(GPO)₉ may provide a comprehensive molecular fingerprint of cardiac fibrosis, advancing personalized management of cardiomyopathy.