CAIX-targeting radiotracers for hypoxia imaging in head and neck cancer models

Hypoxia-induced carbonic anhydrase IX (CAIX) expression is a prognostic marker in solid tumors. In recent years many radiotracers have been developed, but a fair comparison of these compounds is not possible because of the diversity in tumor models and other experimental parameters. In this study we performed a direct in vivo comparison of three promising CAIX targeting radiotracers in xenografted head and neck cancer models. The biodistribution of [(111)In]In-DOTA-ZCAIX:2 was directly compared with [(111)In]In-DTPA-G250-F(ab′)(2) and [(111)In] In-DTPA-G250 in female BALB/C nu/nu mice bearing two HNSCC xenografts with different levels of CAIX expression. In vivo biodistribution was quantified by means of microSPECT/CT scans and ex vivo biodistribution was determined with the use of a γ-counter. Tumors were snap frozen and sections were stained for CAIX expression, vessels, hypoxia (pimonidazole) and tumor blood perfusion. Tracer uptake was significantly higher in SSCNij153 tumors compared to SCCNij185 tumors for [(111)In]In-DOTA-HE3-ZCAIX:2: 0.32 ± 0.03 versus 0.18 ± 0.01%ID/g,(p = 0.003) 4 h p.i., for [(111)In]In-DTPA-girentuximab-F(ab′)(2): 3.0 ± 0.5%ID/g and 1.2 ± 0.1%ID/g (p = 0.03), 24 h p.i. and for [(111)In]In-DTPA-girentuximab: 30 ± 2.1%ID/g and 7.0 ± 1.0%ID/g (p = 0.0002) 72 h p.i. SPECT imaging with both [(111)In]In-DTPA-girentuximab-F(ab′)(2) and [(111)In]In-DTPA-girentuximab showed a clear difference in tracer distribution between the two tumor models. The whole IgG, i.e. [(111)In]In-DTPA-girentuximab, showed the highest tumor-to-muscle ratio. We showed that different CAIX-targeting radiotracers can discriminate a low CAIX-expressing tumor from a high CAIX-expressing head and neck cancer xenografts model. In these hypoxic head and neck xenograft models [(111)In]In-DTPA-girentuximab showed the most promising results.