Granzyme B‑Targeting Quenched Activity-Based Probes for Assessing Tumor Response to Immunotherapy

Molecular imaging of immune activation holds tremendous
potential
for the development of novel immunotherapies. In particular, chemical
probes capable of detecting immune responses before changes in tumor
size occur can guide early therapeutic strategies. Here, we present
a quenched activity-based probe targeting granzyme B as a biomarker
of antitumor immunity. Through optimization of the peptide recognition
element and functional chemical warhead, we have developed an optical
imaging probe, Cy5-IEPCyaPhP-QSY21, which rapidly reacts
with granzyme B at substoichiometric concentrations and enables efficient,
selective labeling of the active enzyme in a complex proteome. With
high specificity and minimal background signal, this probe produces
granzyme B-induced near-infrared fluorescence signals in the tumors
of living mice shortly after injection. Both in vivo and ex vivo fluorescence
signals correlate with granzyme B expression and activity, and the
population of CD8+ cells in tumor tissues. Moreover, it demonstrates
the potential to track tumor response to immunotherapy. Thus, this
study offers a chemical tool for assessing immune-mediated anticancer
activity using noninvasive optical imaging.