Hypoxic tumor cells are resistant to radiotherapy and various chemotherapeutic agents. The pretherapeutic assessment of intratumoral hypoxia may allow selection of patients for intensified treatment regimens. Carbonic anhydrase IX (CAIX) is an endogenous hypoxia-related protein involved in pH regulation and is upregulated in many tumor types. Radionuclide imaging using a monoclonal antibody against CAIX, such as cG250, may allow noninvasive PET of hypoxia in these tumor types. The aims of this study were to investigate whether (89)Zr-labeled cG250-F(ab')(2) allowed visualization of tumor hypoxia using small-animal PET and whether the tracer showed spatial correlation to the microscopic distribution of CAIX-expressing cells in a human head and neck xenograft tumor model.
Methods: Athymic mice with subcutaneous human head and neck carcinoma xenografts (SCCNij3) were imaged with small-animal PET after injection of (89)Zr-cG250-F(ab')(2). PET images were parameterized in terms of standardized uptake values (SUVs). After injection with the nitroimidazole hypoxia marker pimonidazole and the perfusion marker Hoechst 33342, the animals were sacrificed, tumors excised, and CAIX- and pimonidazole-marked hypoxia and blood perfusion were analyzed immunohistochemically. (89)Zr-cG250-F(ab')(2) tumor uptake was analyzed by ex vivo activity counting and by autoradiography of tumor sections.
Results: As early as 4 h after administration, accumulation of (89)Zr-cG250-F(ab')(2) in the tumor had occurred and tumors were clearly visualized by PET, with reduced uptake by 24 h after injection. Pixel-by-pixel analysis showed a significant positive spatial correlation between CAIX expression and (89)Zr-cG250-F(ab')(2) localization (r = 0.57-0.74; P < 0.0001). Also, significant correlations were found between pimonidazole staining intensity and (89)Zr-cG250-F(ab')(2) activity concentration, although less strong (r = 0.46-0.68; P < 0.0001). Tumor maximum SUV correlated significantly with tumor uptake determined ex vivo (r = 0.93; P = 0.0067), as did fractions of CAIX and pimonidazole in tumor sections (r = 0.75; P = 0.03 and r = 0.78; P = 0.02, respectively).
Conclusion: (89)Zr-labeled cG250-F(ab')(2) small-animal PET showed rapid accumulation in a head and neck xenograft tumor model with good correlation to CAIX expression on a microscopic level.