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

Carbonic anhydrase IX (CA-IX) is regarded as a favorable target for in vivo imaging because of its specific expression in hypoxic regions of tumors. Hypoxia assists tumor propagation and growth and is resistant to chemotherapy and radiotherapy. Here, we designed and synthesized [^99mTc]hydroxamamide ([^99mTc]Ham) and [^99mTc]methyl-substituted-hydroxamamide ([^99mTc]MHam) complexes including a bivalent CA-IX ligand, sulfonamide (SA), and ureidosulfonamide (UR). In a cell binding assay, [^99mTc]Ham complexes with bivalent SA ([^99mTc]SAB2A and [^99mTc]SAB2B) and UR ([^99mTc]URB2A and [^99mTc]URB2B) showed significantly greater uptake into CA-IX high-expressing (HT-29) cells than that into CA-IX low-expressing cells. Since the binding affinity of [^99mTc]URB2A and [^99mTc]URB2B for CA-IX was significantly higher than that of [^99mTc]SAB2A and [^99mTc]SAB2B, we additionally synthesized [^99mTc]MURB2 (a [^99mTc]MHam complex with bivalent UR) and evaluated the CA-IX-specific binding affinity of [^99mTc]URB2A, [^99mTc]URB2B, and [^99mTc]MURB2. Their uptake into HT-29 cells was reduced by the addition of a CA inhibitor, acetazolamide, suggesting their CA-IX-specific binding affinity. A biodistribution study in HT-29 tumor-bearing mice was carried out using [^99mTc]URB2A and [^99mTc]MURB2 with the highest specificity for HT-29 cells. [^99mTc]URB2A showed moderate tumor uptake and reduction by coinjection with acetazolamide; however, the tumor/blood ratio was insufficient for in vivo imaging. These results provided key information for the design of novel Ham-based imaging probes targeting CA-IX.