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-T1GIT therapies in predinical models. 'I'herefore, 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 (Cu-64) and zirconium-89 (Zr-89). 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 Zr-89-labeled probe demonstrated superior contrast than the Cu-64 probe due to Zr-89's longer half-life matching the TIGIT antibody's pharmacokinetics. The Zr-89 probe was used to quantify TIGIT expression on TILs in B16 melanoma in immunocompctent mice and confirmed by ex vivo flow cytometry. Conclusions: This study develops and validates novel TIGIT-specific Cu-64 and Zr-89 PET probes for quantifying TIGIT expression on TILs for diagnosis of patient selection for anti-TIGIT therapies.

Automated summary

This study developed novel PET probes for TIGIT, demonstrating high specificity and immunoreactivity for TIGIT. In mouse models, these probes showed potential applications in diagnosis and monitoring of therapeutic responses in tumors.