BS43 A new collagen III-specific MRI imaging probe to assess cardiac fibrosis

Heart failure (HF) has reached epidemic proportions, affecting approximately 64 million people globally and is the main cause of death and disability.1 Myocardial fibrosis, characterised by changes in the amount and/or distribution of collagen I and III, impairs cardiac function and relates to adverse outcomes of HF.2 3 Clinically, we rely on indirect or surrogate measurements of collagen in the myocardium and current targeted molecular imaging probes are limited to collagen I. Here, we report the discovery of a peptide selective for collagen III and a strategy to develop an imaging probe with superior properties for in vivo molecular magnetic resonance imaging (MRI) applications. A small peptide was screened and selected from a library of peptides with potential to bind to collagen identified based on protein-protein interaction studies. The peptide was conjugated to a DOTA-chelator and labelled with Europium [Eu(III)] for in vitro binding assays; gallium (68Ga) for in vivo PET/CT biodistribution; and gadolinium [Gd(III)] for in vivo MRI studies. The probe was further modified to increase the number of Gd(III) per peptide (from one to four) to amplify and prolong the MRI signal. The probe was validated using a surgical mouse model of myocardial infarction (MI). In vivo MRI was performed at days 10 and 21 post-MI (n=7). Imaging findings were validated with tissue analysis. A negative control probe, carrying a scrambled peptide sequence was used. All MRI experiments were performed at a 3 Tesla clinical MRI scanner. In vitro binding assays showed that the probe has a good affinity towards collagen III (Kd= 5.2±1.3µM) that is in the ideal range for a molecular imaging probe.4 Lack of binding of the scrambled probe (negative control) proved the specificity our probe (figure 1A). In vivo PET/CT biodistribution showed favourable pharmacokinetics with fast blood clearance and no unspecific binding (figure 1B). In vivo cardiac MRI showed selective late gadolinium enhancement (LGE) of the fibrotic scar at day 10 which decreased by day 21. This observation is expected as collagen III naturally gets replaced by collagen I at the later stages of cardiac fibrosis. The imaging data are validated histologically showing co-localisation of the MRI signal with collagen III (green colour) at day 10 and reduction of collagen III at day 21 (figure 2). Importantly, no enhancement was observed using the negative control probe and a clinically approved non-collagen targeting probe (Gadovist). We have developed a new molecular imaging probe specific for collagen type III. Using this probe, we have successfully imaged - previously undetectable - collagen III in cardiac fibrosis. This approach may enable early detection and characterisation of cardiac fibrosis in vivo allowing staging of disease and monitoring of therapies