Targeting Cytotoxic T-Cell Effector Function with Granzyme B PET: Translational Advances Toward Predicting Immunotherapy Outcomes.

The clinical success of cancer immunotherapy has highlighted the urgent need for biomarkers capable of capturing dynamic and functional anti-tumor immune activity. Granzyme B (GzmB), a serine protease released by activated cytotoxic CD8⁺ T lymphocytes and natural killer cells during target-cell killing, represents a proximal and mechanistically linked marker of immune effector function. Positron emission tomography (PET) tracers targeting GzmB enable non-invasive, whole-body assessment of cytotoxic activity in vivo, potentially overcoming the spatial and temporal limitations of tissue biopsy and conventional imaging. Preclinical investigations using ⁶⁸Ga- and 18F-labeled peptide-based tracers, including [⁶⁸Ga]Ga-NOTA-GZP and [18F]AlF-mNOTA-GZP, consistently demonstrate early discrimination between responders and non-responders to immune checkpoint blockade and combination therapies, often preceding measurable tumor shrinkage or metabolic changes on [18F]FDG PET. Emerging first-in-human studies further support the feasibility, safety, and predictive potential of GzmB PET across multiple tumor types. Despite promising results, several challenges remain, including optimal imaging timing, signal heterogeneity related to non-canonical GzmB sources, and the need for standardized quantitative thresholds and multicenter validation. Overall, GzmB PET represents a biologically grounded and translationally advanced approach for early response assessment, patient stratification, and potential theranostic applications in precision immuno-oncology.