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PET Imaging of TIGIT Expression on Tumor-Infiltrating Lymphocytes

Purpose: Therapeutic checkpoint inhibitors on tumor-infiltrating lymphocytes (TIL) are being increasingly utilized in the clinic. The T-cell immunoreceptor with Ig and ITIM domains (TIGIT) is an inhibitory receptor expressed on T and natural killer cells. The TIGIT signaling pathway is an alternative target for checkpoint blockade to current PD-1/CTLA-4 strategies. Elevated TIGIT expression in the tumor microenvironment correlates with better therapeutic responses to anti-TIGIT therapies in preclinical models. Therefore, quantifying TIGIT expression in tumors is necessary for determining whether a patient may respond to anti-TIGIT therapy. PET imaging of TIGIT expression on TILs can therefore aid diagnosis and in monitoring therapeutic responses. Experimental Design: Antibody-based TIGIT imaging radiotracers were developed with the PET radionuclides copper-64 (64Cu) and zirconium-89 (89Zr). In vitro characterization of the imaging probes was followed by in vivo evaluation in both xenografts and syngeneic tumor models in mouse. Results: Two anti-TIGIT probes were developed and exhibited immunoreactivity of >72%, serum stability of >95%, and specificity for TIGIT with both mouse TIGIT-expressing HeLa cells and ex vivo–activated primary splenocytes. In vivo, the 89Zr-labeled probe demonstrated superior contrast than the 64Cu probe due to 89Zr's longer half-life matching the TIGIT antibody's pharmacokinetics. The 89Zr probe was used to quantify TIGIT expression on TILs in B16 melanoma in immunocompetent mice and confirmed by ex vivo flow cytometry. Conclusions: This study develops and validates novel TIGIT-specific 64Cu and 89Zr PET probes for quantifying TIGIT expression on TILs for diagnosis of patient selection for anti-TIGIT therapies.