Purpose: Hypoxia is commonly observed in regions of primary tumors and metastases, and is associated with resistance to treatment, more aggressive tumor phenotypes and poor prognosis. Reliable and validated imaging biomarkers of hypoxia are needed for pre-clinical studies and clinical use. Expression of cell-surface carbonic anhydrases IX and XII (CAIX and CAXII) in tumor cells has been associated with tumor hypoxia. CAIX and CAXII specific antibodies conjugated to fluorescent dye were evaluated for the non-invasive detection of hypoxia in vivo.
Procedures: Human breast cancer cell lines (MCF10A, DCIS, MCF7, ZR-75.1 and MDA-mb231) were characterized for CAIX and CAXII expression by real-time RT-PCR and immunocytochemistry (ICC) under normoxic and hypoxic conditions. Immunohistochemical (IHC) staining of CAIX, CAXII and the commercially available exogenous hypoxia marker, pimonidazole, was performed using sections of ZR-75.1 and MDA-mb-231 orthotopic breast cancer xenograft tumors from nude mice. In vivo fluorescence imaging of ZR-75.1 tumors in animals housed at varied levels of oxygen was used to quantify the relative uptake of the CAIX and CAXII agents and a commercially available sulfonamide-based agent. Corresponding tumor sections were IHC stained for CAIX, CAXII and pimonidazole.
Results: CAIX mRNA expression was significantly higher (p < 0.05) in hypoxia for all cell lines, which was in agreement with protein expression by ICC. CAXII expression was mixed, with a modest hypoxia-related increase in two cell lines (p < 0.05) and no change in others. Quantified IHC staining of ZR-75.1 and MDA-mb-231 tumor sections showed that CAIX and CAXII expression was elevated in regions with pimonidazole staining, but CAXII levels were lower than CAIX. Tumor uptake of the CAIX targeted agent, and IHC staining of CAIX and pimonidazole in corresponding tumor sections were correlated, and co-registered, and shown to be significantly elevated by level of oxygenation (p < 0.001): hypoxia > normoxia > hyperoxia. However, the CAXII and sulfonamide agents were not significantly correlated with hypoxia.
Conclusion: These studies suggest that the fluorescently labeled CAIX-specific agent is a more robust indicator of hypoxia in vivo compared to the CAXII-specific agent or the agent specific to the CA active site.
Keywords: Breast cancer; Carbonic anhydrase IX; Carbonic anhydrase XII; Non-invasive imaging of hypoxia; Targeted agents.