Design, synthesis and evaluation of 18F-labeled cationic carbonic anhydrase IX inhibitors for PET imaging

Abstract

Carbonic anhydrase IX (CA-IX) is a marker for tumor hypoxia, and its expression is negatively correlated with patient survival. CA-IX represents a potential target for eliminating hypoxic cancers. We synthesized fluorinated cationic sulfonamide inhibitors 1-3 designed to target CA-IX. The binding affinity for CA-IX ranged from 0.22 to 0.96 μM. We evaluated compound 2 as a diagnostic PET imaging agent. Compound 2 was radiolabeled with ¹⁸F in 10 ± 4% decay-corrected radiochemical yield with 85.1 ± 70.3 GBq/μmol specific activity and >98% radiochemical purity. ¹⁸F-labeled 2 was stable in mouse plasma at 37 °C after 1 h incubation. PET/CT imaging was conducted at 1 h post-injection in a human colorectal cancer xenograft model. ¹⁸F-labeled 2 cleared through hepatobiliary and renal pathways. Tumor uptake was approximately 0.41 ± 0.06% ID/g, with a tumor-to-muscle ratio of 1.99 ± 0.25. Subsequently, tumor xenografts were visualized with moderate contrast. This study demonstrates the use of a cationic motif for conferring isoform selectively for CA-IX imaging agents.

Keywords: Carbonic anhydrase IX; fluorine-18; molecular imaging; positron emission tomography.

Figure 1.

Reported sulfonamide derivatives that are CA-IX selective agents. (A) Trimeric AmBF₃-(AEBS)₃ and AmBF₃-(ABS)₃, (B) Ga-DOTA-AEBSA, and (C) cationic sulfonamide derivatives.

Figure 2.

Chemical structures of three cationic carbonic anhydrase IX inhibitors evaluated in this study.

Scheme 1.

Synthesis of cationic carbonic anhydrase IX inhibitors 1–3.

Scheme 2.

(A) Synthesis of the radiolabeling precursor 8. (B) Synthesis of ¹⁸F-labeled 2.

Figure 3.

HPLC chromatograms of ¹⁸F-labeled 2 from (A) QC sample, or plasma sample after being incubated at 37 °C for (B) 5 min, (C) 15 min, or (D) 60 min.

Figure 4.

Biodistribution of ¹⁸F-labeled 2 at 1 h post-injection in HT-29 tumor-bearing mice. Values (%ID/g) are presented as mean ± standard deviation (n = 5).

Figure 5.

Representative PET and CT images acquired at 1 h post-injection with ¹⁸F-labeled 2 in HT-29 colorectal cancer xenograft-bearing mice. Arrow indicates location of tumor.

Figure 6.

Time activity curves for ¹⁸F-labeled 2 using ROIs drawn around high activity organs (A) heart, liver, and kidneys, and low activity organs (B) tumor, bone, muscle, and muscle.