gms | German Medical Science

Objective: Despite many data from previous cohort studies, the absolute risk of rupture of intracranial aneurysms (IAs) remains uncertain. Existing imaging modalities illustrate the lumen of unruptured IAs, whereas recent studies have indicated the relevance of inflammation and structural turnover in IA walls. We previously facilitated radiocarbon birth dating of the main molecular constituent in IAs, i.e. collagen I, to quantify collagen turnover in IAs ex vivo. Recent studies have reported the use of a new contrast agent for specific labeling of collagen I in the setting of lung fibrosis. Therefore we strived to develop a novel Positron Emission Tomography contrast agent to visualize immature/novel collagen I in human IAs.

Methods: We synthesized a cyclic peptide called collagelin conjugated to NODAGA as a chelator and radiolabeled this with 68Gallium (68Ga-NODAGA-Col). We performed stability tests by analyzing the radiolabeling agent with high-performance liquid chromatography (HPLC) after exposure to different pH, temperature conditions and human serum. Collagen I and albumin – as negative control – were incubated for 10, 30 and 60 minutes with 68Ga-NODAGA-Col. Rat tail tendon consisting predominantly of collagen I and rat brain tissue – as negative control – were also incubated for 10, 30 and 60 minutes with 68Ga-NODAGA-Col. Protein-bound radioactivity was measured by gamma counter. Immunoblotting was performed on rat tail tendon and brain with anti-collagen antibodies.

Results: HPLC analysis demonstrated stability of the collagen tracer under various pH, temperature conditions and in human serum. Collagen I had highly significant more counts per minute compared to albumin at each point in time (t-test: 10min: p=0.001, 30min: p=0.001, 60min: p=0.003) which prooves the specific binding of the collagen tracer. Rat tail tendon also demonstrated significantly higher radioactivity in comparison to rat brain at each point in time (t-test: 10min: p=0.002, 30min: p=0.03, 60min: p=0.02) confirming the specific binding. To rule out meaningful amounts of collagen I in rat brain, confocal microscopy revealed more fluorescence in rat tail tendon compared to rat brain after labeling collagen I by immunoblotting.

Conclusion: Our synthesized radiotracer selectively labels collagen I as a protein itself and in tissue. On the basis of these promising results binding specificity will further be investigated by autoradiography after incubating sections of rat tail tendon, cadaveric cerebral vessels and human IAs.