Carbonic Anhydrase IX Targeting Mn(II)-Based Magnetic Resonance Molecular Imaging Probe for Hypoxia Tumors

Physiological hypoxic conditions in the tumor microenvironmentand consequential overexpression of carbonic anhydrase IX (CA IX)are two characteristics shared by numerous types of solid malignanttumors. Early detection with hypoxia assessment is crucial to improvethe prognosis and therapy outcomes of hypoxia tumors. Herein, usingacetazolamide (AZA) as a CA IX-targeting moiety, we design and synthesizean Mn(II)-based MR imaging probe (named AZA-TA-Mn) incorporatingAZA and two Mn(II) chelates of Mn-TyEDTA on a rigid triazine (TA)scaffold. The per Mn relaxivity of AZA-TA-Mn is 2-foldhigher than its monomeric Mn-TyEDTA, which allows it for low-doseimaging of hypoxic tumors. In a xenograft mice model of esophagealsquamous cell carcinoma (ESCC), a low dosage of AZA-TA-Mn (0.05 mmol/kg) can selectively produce prolonged and stronger contrastenhancement in the tumor compared to the non-specific Gd-DTPA (0.1mmol/kg). A competition study of co-injection of free AZA and Mn(II)probes confirms the in vivo tumor selectivity of AZA-TA-Mn, resulting in a more than 2.5-fold decreased tumor-to-muscle contrast-to-noiseratio (Delta CNR) at 60 min post-injection. MR imaging results werefurther supported by the quantitative analysis of Mn tissue levels,as the co-injection of free AZA resulted in significantly reducedMn accumulation in tumor tissues. Finally, immunofluorescence stainingof tissue sections confirms the positive correlation between the tumoraccumulation of AZA-TA-Mn and CA IX overexpression. Hence,using CA IX as the hypoxia biomarker, our results illustrate a practicalstrategy for the development of novel imaging probes for hypoxic tumors.