Chronic liver damage usually leads to fibrosis, which can progress to cirrhosis. Cirrhosis is the primary cause of hepatic failure, hepatocellular cancer, and liver-related death in the United States. Currently, liver biopsy is the only method to assess fibrosis, but it is prone to sampling error, is invasive, and is risky. Importantly, there exists no method that permits non-invasive measurement of the progression of fibrosis, i.e., fibrogenesis. This lack has precluded individual risk assessment and clinical studies to test antifibrotic agents. Our preliminary data show that molecular imaging of hepatic fibrogenesis is possible. We have developed trimerized small molecule ligands that, when linked to a 99mTc-radiotracer, specifically target activated hepatic stellate cells (via the PDGF-? receptor) and activated cholangiocytes (via integrin av?6). In vitro and in vivo studies in rodent models of liver fibrosis confirmed their high affinity (in the low nanomolar/picomolar) range and their specificity to detect hepatic fibrogenesis well above the background signal. In this proposal we plan to: 1) improve the affinity, biodistribution, and clearance of trimerized av?6 and PDGF- ? receptor targeting ligands for 99mTc-based SPECT/CT imaging by use of adamantane backbones with longer and more rigid linkers, 2) optimize in vivo biodistribution and clearance by addition of an inert bulking domain, 3) resynthesis of the targeting ligands as 18F derivatives for PET/CT imaging and validation of their ability to quantify fibrogenesis non-invasively, 4) validate the targeting constructs in two in vivo models of hepatic fibrogenesis before and after treatment with antifibrotic agents, 5) reduce the high nonspecific background signal in liver, and 6) combine microSPECT and microPET with MRI to improve anatomical co-registration. Completion of the Specific Aims will result in one or more optimized radiotracers, and an optimized workflow for SPECT, PET, CT, and MRI imaging. As such, it will create the first integrated system for detection and quantitation of hepatic fibrogenesis, laying the foundation for rapid translation of the technology to the clinic. This technology will permit: 1) individual risk assessment of fibrosis progression, and 2) the rapid quantitative testing of antifibrotic agents and their combinations in small groups of individualized patients by measuring the extent of signal suppression after a single or only a few doses of these agents. Public Health Relevance: Chronic liver disease often leads to liver fibrosis (formation of scar tissue), which in turn can progress to cirrhosis and end-stage liver failure. Invasive biopsy is presently the only way to assess patients for the progression of fibrosis (i.e., fibrogenesis) and the effectiveness of antifibrotic drugs. We have developed two targeted small molecule ligands specific for cell surface markers unregulated during fibrogenesis and show in preliminary studies that these agents perform well as SPECT diagnostic agents in vitro and in vivo. In this study, we describe a systematic set of experiments aimed at optimizing fibrogenesis-specific radiotracers for SPECT/MRI and PET/MRI imaging, with the goal being their rapid translation to the clinic.
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