Development of the ^99mTc-Hydroxamamide Complex as a Probe Targeting Carbonic Anhydrase IX

Area of Science:

• Radiochemistry
• Molecular Imaging
• Cancer Biology

Background:

• Carbonic anhydrase IX (CA-IX) is a tumor-specific biomarker expressed in hypoxic tumor regions.
• Hypoxia is associated with tumor progression and resistance to cancer therapies.
• Targeting CA-IX offers a potential strategy for tumor-specific imaging.

Purpose of the Study:

• To design and synthesize novel technetium-99m ([^99mTc]) labeled hydroxamamide ([^99mTc]Ham) and methyl-substituted-hydroxamamide ([^99mTc]MHam) complexes.
• To evaluate the CA-IX binding affinity and specificity of these novel radiotracers.
• To assess the potential of these compounds for in vivo imaging of CA-IX-expressing tumors.

Main Methods:

• Synthesis of [^99mTc]Ham and [^99mTc]MHam complexes incorporating bivalent sulfonamide (SA) and ureidosulfonamide (UR) ligands.
• In vitro cell binding assays using CA-IX high-expressing (HT-29) and low-expressing cells.
• Assessment of CA-IX specificity using acetazolamide, a CA inhibitor.
• In vivo biodistribution studies in HT-29 tumor-bearing mice.

Main Results:

• [^99mTc]Ham complexes with bivalent UR ligands ([^99mTc]URB2A, [^99mTc]URB2B) exhibited higher uptake in HT-29 cells compared to SA-based complexes.
• The uptake of these complexes was reduced by acetazolamide, confirming CA-IX-specific binding.
• [^99mTc]URB2A demonstrated moderate tumor uptake in vivo, but the tumor-to-blood ratio was insufficient for effective imaging.

Conclusions:

• Novel [^99mTc]Ham and [^99mTc]MHam complexes show specific binding to CA-IX in vitro.
• The bivalent ureidosulfonamide ligand demonstrated superior performance compared to sulfonamide.
• Further structural modifications are necessary to develop effective CA-IX-targeting imaging probes for clinical application.