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 anti-fibrotic 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.

^99mTc-(HE)₃-(GPO)₉ incorporates glycine-proline-hydroxyproline (GPO) repeats and can hybridize with denatured single- or double-stranded collagen. MI was induced in mice by ligation of the left anterior descending artery; sham-operated animals served as controls. At 2 weeks post-MI, animals underwent myocardial perfusion imaging or contrast-enhanced CT to detect the infarct zone, followed by SPECT/CT imaging using ^99mTc-(HE)₃(GPO)₉ or a control scrambled tracer. Tracer uptake was quantified in vivo and ex vivo with gamma counting and autoradiography. Different aspects of fibrosis were examined by tissue analysis, along with autoradiography with a matrix metalloproteinase-targeted radiotracer, ^99mTc-RYM1. Tracer binding was also assessed in human cardiac tissue through ex vivo autoradiography.

^99mTc-(HE)₃-(GPO)₉ SPECT/CT revealed significantly higher tracer uptake in the infarct zone of MI mice compared to 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 (R = 0.81, P < 0.01). The scrambled tracer exhibited minimal cardiac uptake, demonstrating the specificity of ^99mTc-(HE)₃-(GPO)₉ signal. Denatured collagen staining and ^99mTc-RYM1 autoradiography showed similar patterns as ex vivo ^99mTc-(HE)₃-(GPO)₉ autoradiography, while the ratio of denatured collagen to procollagen in the infarct zone significantly increased from day 3 to 2 weeks post-MI. Finally, ^99mTc-(HE)₃-(GPO)₉ bound to human fibrotic (but not normal) cardiac tissue.

^99mTc-(HE)₃-(GPO)₉ enables non-invasive detection of denatured collagen as a marker of collagen remodeling in vivo, offering a promising tool for assessing fibrotic remodeling after MI. Collagen, procollagen, and denatured collagen, along with MMP activation, exhibit distinct patterns, and their combined imaging may provide a comprehensive molecular fingerprint of cardiac fibrosis, advancing personalized management of cardiomyopathy.