PET imaging of TIGIT expression on tumor-infiltrating lymphocytes

Abstract

Purpose: Therapeutic checkpoint inhibitors on tumor-infiltrating lymphocytes (TILs) 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 (NK) 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 pre-clinical models. Therefore, quantifying TIGIT expression in tumors is necessary for determining if a patient may respond to anti-TIGIT therapy. Positron emission tomography (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.