Noninvasive in vivo deoxycytidine kinase (dCK)-PET identifies tumor-draining lymph nodes upon immune checkpoint inhibitor therapy.

Efficient application of immunotherapy necessitates advanced whole-body imaging techniques to monitor sites of immune cell activation. Deoxycytidine kinase (dCK), a key enzyme in the deoxynucleotide salvage pathway, is upregulated in proliferating immune cells and can be targeted by the radiotracers [(18)F]FAC (preclinical) and [(18)F]CFA (clinical), allowing for noninvasive monitoring of immune activation in lymphatic organs via positron emission tomography (PET). In this study, we aimed to assess the efficacy of [(18)F]FAC in detecting immune activation upon immune checkpoint inhibitor therapy (CIT). In vitro, activated T cells and macrophages exhibited significantly higher [(18)F]FAC uptake compared to their naïve counterparts. In vivo, preclinical [(18)F]FAC-PET/MRI revealed a CIT-induced significant increase in [(18)F]FAC uptake in tumor-draining lymph nodes (TDLNs) compared to contralateral lymph nodes, independent of tumor responsiveness. This phenomenon was absent in TDLNs of sham-treated experimental mice. Ex vivo cell sorting further confirmed elevated [(18)F]FAC uptake in T cells from TDLNs following CIT. Consistently, [(18)F]CFA-PET/CT imaging in metastatic melanoma patients demonstrated CIT-induced enhanced regional LN uptake. Together, these findings establish a strong correlation between CIT-induced immune activation and [(18)F]FAC/[(18)F]CFA uptake, underscoring the critical role of TDLNs in cancer immuotherapy. The radiotracers [(18)F]FAC and [(18)F]CFA provide valuable tools for noninvasive monitoring of immune cell activation, potentially unveiling tumor-microenvironment-related resistance mechanisms and advancing the utility of PET imaging in immunotherapy monitoring and patient stratification.