CD11b-Based Pre-Targeted SPECT/CT Imaging Allows for the Detection of Inflammation in Aortic Aneurysm

Purpose: To investigate the feasibility of a pre-targeted imaging strategy based on the cycloaddition between 1,2,4,5-terazine (Tz) and trans-cyclooctene (TCO) for evaluating CD11b expression in inflammatory aortic aneurysm (AA) using single photon emission computed tomography/computed tomography (SPECT/CT). Methods: C57BL/6J mice were fed beta-aminopropionitrile (1 g/kg/day) for 4 weeks to establish AA models. Anti-CD11b-TCO was synthesized and Tc-99m-HYNIC-PEG(11)-Tz was designed for pre-targeted SPECT/CT. The affinity and specificity of the probe for the inflammatory cell line Raw-264.7 were investigated. Then, anti-CD11b-TCO pre-targeted and Tc-99m-HYNIC-PEG(11)-Tz based SPECT/CT were performed to detect in vivo inflammation in AA. Finally, ex vivo aortic breast-specific gamma imaging (BSGI), Western blot assays, and immunohistochemical CD11b staining were performed to confirm the in vivo findings of SPECT/CT. Results: In the AA models, 65.22% (15/23) had aortic lesions, including 43.48% (10/23) AA lesions. The anti-CD(11)b-TCO presented with a high TCO coupling ratio (7.43), and the Tc-99m-HYNIC-PEG(11)-Tz showed high radio-purity (>95%), good in vitro stability and a rapid clearance rate. Additionally, anti-CD11b-TCO and Tc-99m-HYNIC-PEG(11)-Tz presented high click rate (similar to 89%). The in vitro clicked compound, Tc-99m-HYNIC-PEG(11)-Tz/TCO-anti-CD11b, showed high affinity and specificity for Raw-264.7 cells. (TcHYNIC)-Tc-99m-PEG(11)-Tz/TCO-anti-CD11b pre-targeting SPECT/CT successfully demonstrated inflammatory AA with a high AA-to-background ratio in AA mice, compared to AA mice that were injected with Tc-99m-HYNIC-Tz/TCO-IgG (8.13 versus 3.71, P < 0.001) and control mice injected with Tc-99m-HYNIC-Tz/TCO-anti-CD11b (8.13 versus 3.66, P < 0.001). This result was confirmed by ex vivo BSGI performed immediately after SPECT/CT and immunohistochemical CD11b staining. Conclusion: SPECT/CT imaging using the anti-CD11b-TCO/Tz-PEG(11)-HYNIC-Tc-99m based pre-targeting imaging strategy allows for the detection of inflammation in progressive AA.