Measuring fibrosis: is seeing really believing?

Radiotracers for Imaging of Fibrosis: Advances during the Last Two Decades and Future Directions

Fibrosis accompanies various pathologies, and there is thus an unmet medical need for non-invasive, sensitive, and quantitative methods for the assessment of fibrotic processes. Currently, needle biopsy with subsequent histological analysis is routinely used for the diagnosis along with morphological imaging techniques, such as computed tomography (CT), magnetic resonance imaging (MRI), and ultrasound (US). However, none of these imaging techniques are sufficiently sensitive and accurate to detect minor changes in fibrosis. More importantly, they do not provide information on fibrotic activity on the molecular level, which is critical for fundamental understanding of the underlying biology and disease course. Molecular imaging technology using positron emission tomography (PET) offers the possibility of imaging not only physiological real-time activity, but also high-sensitivity and accurate quantification. This diagnostic tool is well established in oncology and has exhibited exponential development during the last two decades. However, PET diagnostics has only recently been widely applied in the area of fibrosis. This review presents the progress of development of radiopharmaceuticals for non-invasive detection of fibrotic processes, including the fibrotic scar itself, the deposition of new fibrotic components (fibrogenesis), or the degradation of existing fibrosis (fibrolysis).

Synthesis and preclinical evaluation of 68Ga-labeled collagelin analogs for imaging and quantification of fibrosis

OBJECTIVES

Fibrosis affecting functionality of vital organs such as liver, lung, heart, and kidney, is involved in many chronic diseases. Positron emission tomography (PET) would not only provide precise localization and extent of the affected tissue but also allow the accurate quantification of the fibrotic process for the subsequent prognosis.

METHODS

A cyclic peptide c[CPGRVMHGLHLGDDEGPC] conjugated either to 2-(4,7-bis(2-(tert-butoxy)-2-oxoethyl)-1,4,7-triazonan-1-yl)acetic acid (NOTA(tBu)2) or 4-(4,7-bis(2-(tert-butoxy)-2-oxoethyl)-1,4,7-triazacyclononan-1-yl)-5-(tert-butoxy)-5-oxopentanoic acid (NODAGA(tBu)3) via polyethylene glycol link (PEG2) was synthesized and labeled with (68)Ga. Non-specific organ distribution, blood clearance, and excretion were investigated ex vivo in healthy rats. The binding specificity of the radioligands was assessed in vitro using autoradiography on cryosections of dog fibrotic heart tissue.

RESULTS

The yield of NOTA-PEG2-c[CPGRVMHGLHLGDDEGPC] and NODAGA-PEG2-c[CPGRVMHGLHLGDDEGPC] was 56% and 41%, respectively. Non-decay-corrected radiochemical yield was 80 ± 5% with radiochemical purity of 95 ± 4%. Pharmacokinetic studies in healthy male Sprague-Dawley rats showed fast blood clearance and renal excretion. Lower uptake in liver, spleen, and kidney was found for [[(68)Ga]Ga-NOTA](+1)-PEG2-c[CPGRVMHGLHLGDDEGPC] as compared to [[(68)Ga]Ga-NODAGA](0)-PEG2-c[CPGRVMHGLHLGDDEGPC]. Histologic evaluation of the left ventricle (LV) myocardium from a dog with severe mitral regurgitation (MR), revealed mild to moderate perivascular and subendocardial, and mild diffuse interstitial fibrosis. The tracer binding to the cryosections of the tissue was specific with the equilibrium Kd of 2.3 ± 0.8 μM and 2.1 ± 0.9 μM, respectively for [(68)Ga]Ga-NO2A-Col and [(68)Ga]Ga-NODAGA-Col.

CONCLUSIONS

Two novel peptide based agents for the imaging of fibrosis by PET were developed. Moderation of the biodistribution could be achieved by variation of the charge on the complex moiety of the agents. The combination of the fast clearance from non-target organs as well as organs of interest such as lung, heart, and liver and binding specificity to the target tissue suggests the potential of the analogs for the imaging of fibrosis.