ImmunoPET imaging of human CD8⁺ T cells with novel ⁶⁸Ga-labeled nanobody companion diagnostic agents.

ABSTRACT:

Background

Although immunotherapy has revolutionized treatment strategies for some types of cancers, most patients failed to respond or obtain long-term benefit. Tumor-infiltrating CD8⁺ T lymphocytes are closely related to the treatment outcome and prognosis of patients. Therefore, noninvasive elucidation of both systemic and tumor-infiltrating CD8⁺ T lymphocytes is of extraordinary significance for patients during cancer immunotherapy. Herein, a panel of ⁶⁸Ga-labeled Nanobodies were designed and investigated to track human CD8⁺ T cells in vivo through immuno-positron emission tomography (immunoPET).

Results

Among the screened Nanobodies, SNA006a showed the highest binding affinity and specificity to both human CD8 protein and CD8⁺ cells in vitro, with the equilibrium dissociation constant (K_D) of 6.4?×?10^-10 M and 4.6?×?10^-10 M, respectively. ⁶⁸Ga-NOTA-SNA006 was obtained with high radiochemical yield and purity, and stayed stable for at least 1 h both in vitro and in vivo. Biodistribution and Micro-PET/CT imaging studies revealed that all tracers specifically concentrated in the CD8⁺ tumors with low accumulation in CD8^- tumors and normal organs except the kidneys, where the tracer was excreted and reabsorbed. Notably, the high uptake of ⁶⁸Ga-NOTA-SNA006a in CD8⁺ tumors was rapid and persistent, which reached 24.41?±?1.00% ID/g at 1.5 h after intravenous injection, resulting in excellent target-to-background ratios (TBRs). More specifically, the tumor-to-muscle, tumor-to-liver, and CD8⁺ to CD8^- tumor was 28.10?±?3.68, 5.26?±?0.86, and 19.58?±?2.70 at 1.5 h, respectively. Furthermore, in the humanized PBMC-NSG and HSC-NPG mouse models, ⁶⁸Ga-NOTA-SNA006a accumulated in both CD8⁺ tumors and specific tissues such as liver, spleen and lung where human CD8 antigen was overexpressed or CD8⁺ T cells located during immunoPET imaging.

Conclusions

⁶⁸Ga-NOTA-SNA006a, a novel Nanobody tracer targeting human CD8 antigen, was developed with high radiochemical purity and high affinity. Compared with other candidates, the long retention time, low background, excellent TBRs of ⁶⁸Ga-NOTA-SNA006a make it precisely track the human CD8⁺ T cells in mice models, showing great potential for immunotherapy monitoring and efficacy evaluation.

SUBMITTER: Zhao H 

PROVIDER: S-EPMC7871532 | biostudies-literature | 2021 Feb

REPOSITORIES: biostudies-literature