AUTHOR=Cao Qi , Lu Xin , Azad Babak Behnam , Pomper Martin , Smith Mark , He Jiang , Pi Liya , Ren Bin , Ying Zhekang , Sichani Babak Saboury , Morris Michael , Dilsizian Vasken 

TITLE=cis-4-[18F]fluoro-L-proline Molecular Imaging Experimental Liver Fibrosis

JOURNAL=Frontiers in Molecular Biosciences

VOLUME=7

YEAR=2020

URL=https://www.frontiersin.org/journals/molecular-biosciences/articles/10.3389/fmolb.2020.00090

DOI=10.3389/fmolb.2020.00090

ISSN=2296-889X

ABSTRACT=<p><bold>Introduction:</bold> Early-stage liver fibrosis is potentially reversible, but difficult to diagnose. Clinical management would be enhanced by the development of a non-invasive imaging technique able to identify hepatic injury early, before end-stage fibrosis ensues. The analog of the amino acid proline, <italic>cis</italic>-4-[<sup>18</sup>F]fluoro-L-proline ([<sup>18</sup>F]fluoro-proline), which targets collagenogenesis in hepatic stellate cells (HSC), was used to detect fibrosis.</p><p><bold>Methods:</bold> Acute steatohepatitis was induced in experimental animals by liquid ethanol diet for 8 weeks, intra-gastric binge feedings every 10th day along with lipopolysaccharide (LPS) injection. The control animals received control diet for 8 weeks and an equivalent volume of saline on the same schedule as the acute steatohepatitis model. First, <italic>in vitro</italic> cellular experiments were carried out to assess [<sup>3</sup>H]proline uptake by HSC, hepatocytes and Kupffer cells derived from rats with acute steatohepatitis (<italic>n</italic> = 14) and controls (<italic>n</italic> = 14). Next, <italic>ex vivo</italic> liver experiments were done to investigate unlabeled proline-mediated collagen synthesis and its associated proline transporter expression in acute steatohepatitis (<italic>n</italic> = 5) and controls (<italic>n</italic> = 5). Last, <italic>in vivo</italic> dynamic and static [<sup>18</sup>F]fluoro-proline micro-PET/CT imaging was performed in animal models of acute steatohepatitis (n = 7) and control (n = 7) mice.</p><p><bold>Results:</bold> [<sup>3</sup>H]proline uptake was 5-fold higher in the HSCs of steatohepatitis rats than controls after incubation of up to 60 min. There was an excellent correlation between [<sup>3</sup>H]proline uptake and liver collagen expression (<italic>r</italic>-value &gt; 0.90, <italic>p</italic> &lt; 0.05). Subsequent liver tissue studies demonstrated 2–3-fold higher proline transporter expression in acute steatohepatitis animals than in controls, and proline-related collagen synthesis was blocked by this transporter inhibitor. <italic>In vivo</italic> micro-PET/CT studies with [<sup>18</sup>F]fluoro-proline showed 2–3-fold higher uptake in the livers of acute steatohepatitis mice than in controls. There was an excellent correlation between [<sup>18</sup>F]fluoro-proline uptake and liver collagen expression in the livers of acute steatohepatitis mice (<italic>r</italic>-value = 0.97, <italic>p</italic> &lt; 0.001).</p><p><bold>Conclusion:</bold> [<sup>18</sup>F]fluoro-proline localizes in the liver and correlates with collagenogenesis in acute steatohepatitis with a signal intensity that is sufficiently high to allow imaging with micro-PET/CT. Thus, [<sup>18</sup>F]fluoro-proline could serve as a PET imaging biomarker for detecting early-stage liver fibrosis.</p>