Noninvasive interrogation of CD8+ T cell effector function for monitoring early tumor responses to immunotherapy

Abstract

Accurately identifying patients who respond to immunotherapy remains clinically challenging. A noninvasive method that can longitudinally capture information about immune cell function and assist in the early assessment of tumor responses is highly desirable for precision immunotherapy. Here, we show that PET imaging using a granzyme B–targeted radiotracer named 68 Ga-grazytracer, could noninvasively and effectively predict tumor responses to immune checkpoint inhibitors and adoptive T cell transfer therapy in multiple tumor models. 68 Ga-grazytracer was designed and selected from several radiotracers based on non-aldehyde peptidomimetics, and exhibited excellent in vivo metabolic stability and favorable targeting efficiency to granzyme B secreted by effector CD8 + T cells during immune responses. 68 Ga-grazytracer permitted more sensitive discrimination of responders and nonresponders than did 18 F-fluorodeoxyglucose, distinguishing between tumor pseudoprogression and true progression upon immune checkpoint blockade therapy in mouse models with varying immunogenicity. In a preliminary clinical trial with 5 patients, no adverse events were observed after 68 Ga-grazytracer injection, and clinical responses in cancer patients undergoing immunotherapy were favorably correlated with 68 Ga-grazytracer PET results. These results highlight the potential of 68 Ga-grazytracer PET to enhance the clinical effectiveness of granzyme B secretion–related immunotherapies by supporting early response assessment and precise patient stratification in a noninvasive and longitudinal manner.

Imaging of Activated T Cells

“…The nucleoside analogue undergoes phosphorylation by deoxyguanosine kinase, in particular when there is lower concentrations of the substrate in the mitochondria and the cytoplasm (8). Namavari et al first synthesized the tracer 2′-deoxy-2′- 18 Ffluoro-9-β-D-arabinofuranosylguanine ( 18 F-AraG) and demonstrated uptake in activated T cells (9), however Levi et al were the first to evaluate immune checkpoint blockade (ICB)related changes in immune response using the tracer. Longitudinal 18 F-AraG imaging in a murine sarcoma virusmoloney leukemia virus induced rhabdomyosarcoma model from week 1 to week 3 followed the course of immune activation with stable increase in tumor and draining lymph nodes signal overtime.…”

“…Longitudinal 18 F-AraG imaging in a murine sarcoma virusmoloney leukemia virus induced rhabdomyosarcoma model from week 1 to week 3 followed the course of immune activation with stable increase in tumor and draining lymph nodes signal overtime. 18 Ffluorodeoxyglucose ( 18 FDG) tumor signal, however, followed the pattern of tumor growth starting from initial increase to decline from week 1 to week 3. Furthermore, 18 F-AraG PET imaging in MC38-bearing mice showed a significant difference in tumor uptake between the baseline and only 2 days following initiation of treatment with anti-PD-1 therapy, with a nonsignificant increase in non-responders.…”

“…18 Ffluorodeoxyglucose ( 18 FDG) tumor signal, however, followed the pattern of tumor growth starting from initial increase to decline from week 1 to week 3. Furthermore, 18 F-AraG PET imaging in MC38-bearing mice showed a significant difference in tumor uptake between the baseline and only 2 days following initiation of treatment with anti-PD-1 therapy, with a nonsignificant increase in non-responders. Overall, the tracer predicted response to anti-PD-1 therapy early in the course of the disease (10).…”

Functional imaging of immune cell subpopulations in the tumor microenvironment: clinical implications

“…Therefore, there is a clear clinical need for a noninvasive approach that longitudinally interrogates immune effector responses in the TME. The PET imaging described by Zhou et al offers such an approach ( 1 ).…”

“…Additional PET approaches for tracing activated T cells and quantifying the number of T cells have used [ 18 F]F-AraG ( 3 ) and 89 Zr-Df-IAB22M2C ( 4 ), respectively. Attempts to develop PET imaging of granzyme B have been ongoing ( 5 , 6 ), but the study by Zhou et al is the first to demonstrate its clinical utility ( 1 ).…”