PET Imaging of T Cells: Target Identification and Feasibility Assessment.

Imaging T cells using positron emission tomography (PET) would be highly useful for diagnosis and monitoring in immunology and oncology patients. There are, however, no obvious targets that can be used to develop imaging agents for this purpose. We evaluated several potential target proteins with selective expression in T cells, and for which lead molecules were available: protein kinase C isozyme θ (PKC θ), lymphocyte-specific protein tyrosine kinase (Lck), zeta-chain-associated protein kinase 70 (ZAP70), and interleukin-2-inducible T-cell kinase (Itk). Ultimately, we focused on Itk and identified a tool molecule with properties suitable for in vivo imaging of T cells: (5aR)-5,5-difluoro-5a-methyl-N-(1-((S)-3-(methylsulfonyl)phenyl)(tetrahydro-2H-pyran-4-yl)methyl)-1H-pyrazol-4-yl)-1,4,4a,5,5a,6-hexahydrocyclopropa[f]indazole-3-carboxamide (23). Although it does not have the optimal profile for clinical use, this molecule indicates that it might be possible to develop Itk-selective PET ligands for imaging the distribution of T cells in patients.