A novel indium-labeled dual carbonic anhydrase 9-targeted probe as a potential SPECT imaging radiotracer for detection of hypoxic colorectal cancer cells.

Tumor hypoxia is a common feature in colorectal cancer (CRC), and is associated with resistance to radiotherapy and chemotherapy. Thus, a specifically targeted probe for the detection of hypoxic CRC cells is urgently needed. Carbonic anhydrase 9 (CA9) is considered to be a specific marker for hypoxic CRC diagnosis. Here, a nuclear imaging Indium-111 (In)-labeled dual CA9-targeted probe was synthesized and evaluated for CA9 detection in in vitro, in vivo, and in human samples. The CA9-targeted peptide (CA9tp) and CA9 inhibitor acetazolamide (AAZ) were combined to form a dual CA9-targeted probe (AAZ-CA9tp) using an automatic microwave peptide synthesizer, which then was conjugated with 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) for radioisotope (In) labeling (In-DOTA-AAZ-CA9tp). The assays for cell binding, stability, and toxicity were conducted in hypoxic CRC HCT15 cells. The analyses for imaging and biodistribution were performed in an HCT15 xenograft mouse model. The binding and distribution of In-DOTA-AAZ-CA9tp were detected in human CRC samples using microautoradiography. AAZ-CA9tp possessed good CA9-targeting ability in hypoxic HCT15 cells. The dual CA9-targeted radiotracer showed high serum stability, high surface binding, and high affinity in vitro. After exposure of In-DOTA-AAZ-CA9tp to the HCT15-bearing xenograft mice, the levels of In-DOTA-AAZ-CA9tp were markedly and specifically increased in the hypoxic tumor tissues compared to control mice. In-DOTA-AAZ-CA9tp also targeted the areas of CA9 overexpression in human colorectal tumor tissue sections. The results of this study suggest that the novel In-DOTA-AAZ-CA9tp nuclear imaging agent may be a useful tool for the detection of hypoxic CRC cells in clinical practice.