From Ostrich to Mouse to Human: Translation of the Functional Liver PET Tracer [⁶⁸Ga]Ga-TEoS-DAZA

The aim of this study was to investigate the dynamic biodistribution of [⁶⁸Ga]Ga-TEoS-DAZA, a functional liver PET tracer, in 2 preclinical models (ostrich embryos and mice) and in a healthy human liver donor to identify similarities and differences among the 3 species, which are relevant in translational nuclear medicine. Furthermore, the molecular pathway and metabolism of [⁶⁸Ga]Ga-TEoS-DAZA was investigated. Methods: The dynamic biodistribution of [⁶⁸Ga]Ga-TEoS-DAZA was determined via PET/CT in ostrich embryos, in healthy mice (C57BL/6), and in a healthy human liver donor. Hepatocyte transporter binding studies were performed in transfected HEK293t cells. Metabolite analysis was performed in samples from ostrich embryos and a healthy liver donor. Blocking studies against cyclosporine A were performed in ostrich embryos. Results: The biodistribution of [⁶⁸Ga]Ga-TEoS-DAZA was comparable in ostrich embryos, healthy mice (C57BL/6), and the healthy donor. In all 3 species, the tracer showed specific uptake in liver tissue (30-40 %IA at time of peak) and subsequent biliary excretion, whereas less than 5 %IA activity was excreted renally. The hepatic transit time in mice was significantly faster than in ostrich embryos and human, with mice exhibiting a much shorter time-to-peak (1.7 min) than the other 2 species (15-21 min) and rapid clearance of the tracer from the liver into the intestines. [⁶⁸Ga]Ga-TEoS-DAZA is a substrate for OATP1B3, with tracer uptake into the liver being hampered in the presence of cyclosporine A. Tissue samples revealed an as yet unknown radiometabolite of [⁶⁸Ga]Ga-TEoS-DAZA, indicating hepatic metabolism. Conclusion: [⁶⁸Ga]Ga-TEoS-DAZA was shown to be a suitable hepatobiliary tracer using both mice and ostrich embryos as preclinical models; however, there were limits in translatability in both models because of a distinctly faster hepatic uptake and biliary excretion (mice) or a slower biliary excretion (ostrich embryo) compared with that in the human.