Imaging carbonic anhydrase IX as a method for monitoring hypoxia-related radioresistance in preclinical head and neck cancer models

BACKGROUND AND PURPOSE: Tumor hypoxia is an important cause of radioresistance and is associated with poor outcome. SPECT (Single-photon emission computed tomography) imaging enables visualizing tumor characteristics. We investigated the SPECT-radiotracer [(111)In]-girentuximab-F(ab’)(2) to image Carbonic Anhydrase IX (CAIX), an enzyme upregulated under hypoxic conditions. MATERIALS AND METHODS: Athymic mice with subcutaneous FaDu or SCCNij202 head and neck squamous cell carcinoma (HNSCC) xenografts were treated with atovaquone or were housed in a hypoxic chamber (8% O(2)). Next, [(111)In]-girentuximab-F(ab’)(2) was injected and 24 h later mice were euthanized for ex vivo biodistribution, autoradiography of the tumor, and immunohistochemical staining of the tumor. Tumor sections were analyzed for hypoxia, CAIX expression, vessels, and perfusion. Also, the effect of atovaquone on microSPECT scans was determined in the FaDu model. RESULTS: Atovaquone decreased CAIX expression by 69% (p = 0.017) compared with control tumors in FaDu, while in the SCCNij202 tumors no difference was observed. Hypoxic breathing did not increase CAIX expression or hypoxia staining in either tumor model, but did affect the necrotic tumor fraction. Ex vivo tracer uptake in the atovaquone treated group did not differ significantly from the control group, despite the difference in CAIX expression. Furthermore, SPECT imaging with [(111)In]-girentuximab-F(ab’)(2) did not discriminate atovaquone-treated versus control tumors. CONCLUSION: Atovaquone decreased CAIX expression only in the FaDu tumor model. [(111)In]-girentuximab-F(ab’)(2) specifically targets CAIX-expressing areas in HNSCC xenografts, but differences in vessel density and necrosis most likely affected tracer uptake in the tumors and therefore complicated quantification of changes in CAIX expression.