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 β-aminopropionitrile (1 g/kg/day) for 4 weeks to establish AA models. Anti-CD11b-TCO was synthesized and ^99mTc-HYNIC-PEG₁₁-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 ^99mTc-HYNIC-PEG₁₁-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-CD11b-TCO presented with a high TCO coupling ratio (7.43), and the ^99mTc-HYNIC-PEG₁₁-Tz showed high radio-purity (> 95%), good in vitro stability and a rapid clearance rate. Additionally, anti-CD11b-TCO and ^99mTc-HYNIC-PEG₁₁-Tz presented high click rate (∼ 89%). The in vitro clicked compound, ^99mTc-HYNIC-PEG₁₁-Tz/TCO-anti-CD11b, showed high affinity and specificity for Raw-264.7 cells. ^99mTc-HYNIC-PEG₁₁-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 ^99mTc-HYNIC-Tz/TCO-IgG (8.13 versus 3.71, P < 0.001) and control mice injected with ^99mTc-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-PEG₁₁-HYNIC-^99mTc based pre-targeting imaging strategy allows for the detection of inflammation in progressive AA.
Keywords: aortic aneurysm, inflammation, SPECT/CT, CD11b, macrophage