Study of microSPECT-CT imaging of mice orthotopic glioma with ⁹⁹Tc^m-Galacto-RGD₂

Objective: To investigate the synthesis, in vivo biodistribution of ⁹⁹Tc^m-HYNIC-PEG₄-E[PEG4-c(RGDfk)]₂(⁹⁹Tc^m-Galacto-RGD₂), and its potential usage for targeted imaging of mice orthotopic glioma. Methods: ⁹⁹Tc^m-Galacto-RGD₂ was synthesized straightforward and its radiochemical purity and stability and distribution in mice were analyzed. MicroSPECT-CT imaging was done in a mice orthotopic glioma model, which had been set up with U87MG cells, after administration of ⁹⁹Tc^m-Galacto-RGD₂. Region of interest (ROI) of glioma was drawn on SPECT-CT section images to quantify tumor uptake (%ID/cm³). Glioma was harvested for pathological examination. Linear-regression was used to analyze the relationship between integrin α_Ⅴ β₃ and tumor uptake(%ID/cm³). Results: The radiochemical purity of ⁹⁹Tc^m-Galacto-RGD₂ was (97.7 ± 0.8)% and stable in vitro. Hynic-Galacto-RGD₂ could specifically bind to integrin α_Ⅴβ₃ of tumor cells with a IC₅₀ of (18 ± 3)nmol/L. After tail vein injection, ⁹⁹Tc^m-Galacto-RGD₂ was rapidly discharged from the blood, liver, kidneys and had a relative low concentration in normal brain tissue. MicroSPECT-CT imaging demonstrated that, after 60 min of injection, this drug was well uptaken by glioma tumor than that after 30 min (t = 7.13, P < 0.05), and the tumor to normal brain tissue (T/B) uptake ratio of ⁹⁹Tc^m-Galacto-RGD₂ was 13.92 ± 3.43. Injection of HYNIC-Galacto-RGD₂ 2 min prior to ⁹⁹Tc^m-Galacto-RGD₂ injection extensively reduced the uptake of radioactive drug in tumor tissue(t = 11.36, P < 0.05). Bland-Altman analysis showed that tumor volume based on SPECT-CT imaging measurement had almost same value with the tumor reference volume (95% CI = -11.94%-11.92%). In addition, the tumor uptake of ⁹⁹Tc^m-Galacto-RGD₂ and cellular integrin α_Ⅴβ₃ expression level had a linear relationship (R² = 0.896). Conclusions: Stable ⁹⁹Tc^m-Galacto-RGD₂ can be synthesized easily and is applicable for microSPECT-CT imaging analysis of orthotopic glioma in mice together with the evaluation of integrin α_Ⅴβ₃ level in tumor. (authors)