Functional Suitability Evaluation of Radioimmunoconjugates of Nanobodies against PD-L1 and HER2/neu for Tumor Theranostics

The aim of the study was to evaluate the applicability of radioimmunoconjugates (RIC) of nanobodies against PD-L1 and HER2/neu for the diagnosis and therapy of malignant tumors.

Materials and Methods. The nanobodies to human biomarkers PD-L1 and HER2/neu were conjugated with radionuclides ⁶⁸Ga and ¹⁷⁷Lu using the chelating agent DOTA. RIC biodistribution was studied in experimental models on F1 (DBA/2xBALB/c) mice, which were inoculated with genetically modified CT26 murine carcinoma cells expressing human PD-L1 or HER2/neu. RICs containing the isotope ⁶⁸Ga and intended for tumor detection were administered to animals intravenously at a dose of 1.0–1.2 MBq. In 0.5, 1.5, and 4 h the radioactivity accumulation in specific tumors carrying human biomarkers was assessed using direct dosimetry compared with control tumors. RICs containing ¹⁷⁷Lu were administered for therapy at a dose of 0.8–1.6 MBq, and studied similarly for 96 h after administration.

Results. After RICs containing ⁶⁸Ga were administered to mice, the greatest difference between specific and control tumors was observed in 1.5 h. At the same time, there was a multiple excess of the specific tumor radioactivity over the blood and muscle tissue radioactivity, which should provide high contrast imaging. After administering RICs containing ¹⁷⁷Lu to mice, the radioactivity in specific tumors persisted for 48 h producing a long-term effect of the radioisotope on the tumor. With the introduction of all RICs, there was a rapid elimination of radioactivity from the blood through urine, which was associated with the use of nanobodies with a molecular weight of 13 kDa and not containing sites of interaction with Fc receptors. There were found the significant accumulation and long-term retention of radioactivity in the kidneys. The work revealed a dependence of the radioactivity biodistribution on the isotope used: the time-normalized radioactivity of some organs and tissues, including tumors, after the administration of RICs containing ⁶⁸Ga appeared to be higher than after administering the RICs of the same specificity, but containing ¹⁷⁷Lu radioisotopes.

Conclusion. The findings indicated the functional suitability of the pairs of RIC nanobodies with ⁶⁸Ga and ¹⁷⁷Lu radioisotopes for theranostics of malignant tumors expressing PD-L1 and HER2/neu biomarkers.

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