First-in-class -PET radiotracers for GPR39: A platform for noninvasive target engagement imaging in vivo

Disrupted zinc homeostasis is a hallmark of Alzheimer's disease. As a key zinc-sensing receptor, GPR39, a G protein-coupled receptor implicated in neurodegeneration, vascular regulation, and neuropsychiatric disorders, represents an emerging target for molecular imaging. However, the lack of suitable positron emission tomography (PET) radiotracers has limited the in vivo characterization of GPR39. Quantitative assessment of GPR39 in vivo could improve our understanding of its role in physiology and disease and facilitate the development and evaluation of targeted therapeutics. Here, we introduce [18F]GPR39 2.1 as a PET radiotracer for specific imaging of GPR39. [18F]GPR39 2.1 exhibited excellent serum stability with negligible radiometabolite formation. In vitro cell uptake studies demonstrated a 55-70% reduction in tracer uptake following pharmacological blockade. PET imaging showed rapid brain uptake, efficient blood-brain barrier penetration, and gradual washout. Blockade studies in rats demonstrated significant reductions in brain uptake of up to approximately 90% (p<0.0001) across multiple brain regions. GPR39 specificity was further validated in GPR39 knockout mice, which showed 76% (p<0.002) and 90% (p<0.0002) reductions in brain uptake by PET imaging and biodistribution, respectively. Moreover, 5xFAD transgenic mice exhibited significantly lower brain uptake by PET imaging (40%, p<0.0002) and biodistribution (77%, p<0.002), indicating sensitivity of [18F]GPR39 2.1 to disease-associated alterations in GPR39. Biodistribution studies demonstrated favorable pharmacokinetics, while autoradiography further confirmed GPR39-specific binding, consistent with the PET and biodistribution findings. Collectively, these results establish [18F]GPR39 2.1 as a stable, brain-penetrant, and highly specific PET radiotracer for noninvasive imaging of GPR39 in vivo. This work provides a translational framework for investigating GPR39 biology and supports the development of GPR39 as an imaging and therapeutic target in neurological and systemic diseases associated with GPR39 dysregulation.