Carbonic anhydrase IX (CA IX) is a suitable target for various anticancer strategies. It is a cell surface protein that is present in human tumors, but not in the corresponding normal tissues. Expression of CA IX is induced by hypoxia and correlates with cancer prognosis in many tumor types. Moreover, CA IX is functionally implicated in cancer progression as a pro-survival factor protecting cancer cells against hypoxia and acidosis via its capability to regulate pH and cell adhesion. Cancer-related distribution of CA IX allows for targeting cancer cells by antibodies binding to its extracellular domain, whereas functional involvement of CA IX opens the possibility to hit cancer cells by blocking their adaptation to physiologic stresses via inhibition of CA IX enzyme activity. The latter strategy is recently receiving considerable attention and great efforts are made to produce CA IX-selective inhibitor derivatives with anticancer effects. On the other hand, targeting CA IX-expressing cells by immunotherapy has reached clinical trials and is close to application in treatment of renal cell carcinoma patients. Nevertheless, development and characterization of new CA IX-specific antibodies is still ongoing. Here we describe a mouse monoclonal antibody VII/20 directed to catalytic domain of CA IX. We show that upon binding to CA IX, the VII/20 MAb undergoes efficient receptor-mediated internalization, which is a process regulating abundance and signaling of cell surface proteins and has considerable impact on immunotherapy. We evaluated biological properties of the MAb and demonstrated its capacity to elicit anti-cancer effect in mouse xenograft model of colorectal carcinoma. Thus, the VII/20 MAb might serve as a tool for preclinical studies of immunotherapeutic strategies against non-RCC tumors. These have not been explored so far and include broad spectrum of cancer types, treatment of which might benefit from CA IX-mediated targeting.

G250 (Girentuximab) is a chimeric IgG1 monoclonal antibody (MAb) currently being evaluated as an immunotherapy for kidney cancer. It targets carbonic anhydrase protein (CA Ⅸ), a transmembrane carbonic anhydrase (CA) isoform, which is regulated by VHL/HIF pathway and hence expressed in the majority of renal cell carcinomas (RCCs) as well as in hypoxic non‑RCC tumours. CA Ⅸ functions in pH regulation and cell migration/invasion, and supports tumour cell survival in hypoxia and/or acidosis. It contains a highly active extracellular catalytic domain (CA) extended N-terminally with a proteoglycan-like region and C-terminally with short transmembrane and intracellular regions. Here we characterize the binding and internalization properties of G250, as well as its therapeutic effects in animal model, and discuss the impact of G250‑mediated immunotherapy in non‑RCC tumours. We demonstrated that G250 MAb recognizes a conformational epitope in the CA domain, detects the soluble CA Ⅸ ectodomain (ECD), but not the splicing variant, and does not cross-react with CA Ⅰ, Ⅱ, and Ⅻ isoforms. We showed that G250 internalizes via clathrin-coated vesicles, escapes degradation in lysosomes and enters the recycling pathway via the perinuclear compartment. This results in long intracellular persistence and enables consecutive internalization cycles. Moreover, the recycled antibody maintains an intact Fc portion potentially capable of continuous induction of antibody-dependent cell-mediated cytotoxicity (ADCC) response, thus explaining its therapeutic efficacy. Finally, we showed that G250 treatment is effective against HT-29 colorectal carcinoma xenografts that differ from RCC by more heterogeneous, hypoxia-related expression of CA Ⅸ. These results suggest potential therapeutic usefulness of the G250 MAb in non-RCC tumours.

BackgroundCarbonic anhydrase IX (CA IX) is a tumor-associated, highly active, transmembrane carbonic anhydrase isoform regulated by hypoxia and implicated in pH control and adhesion-migration-invasion. CA IX ectodomain (ECD) is shed from the tumor cell surface to serum/plasma of patients, where it can signify cancer prognosis. We previously showed that the CA IX ECD release is mediated by disintegrin and metalloproteinase ADAM17. Here we investigated the CA IX ECD shedding in tumor cells undergoing apoptosis in response to cytotoxic drugs, including cycloheximide and doxorubicin.MethodsPresence of cell surface CA IX was correlated to the extent of apoptosis by flow cytometry in cell lines with natural or ectopic CA IX expression. CA IX ECD level was assessed by ELISA using CA IX-specific monoclonal antibodies. Effect of recombinant CA IX ECD on the activation of molecular pathways was evaluated using the cell-based dual-luciferase reporter assay.ResultsWe found a significantly lower occurrence of apoptosis in the CA IX-positive cell subpopulation than in the CA IX-negative one. We also demonstrated that the cell-surface CA IX level dropped during the death progress due to an increased ECD shedding, which required a functional ADAM17. Inhibitors of metalloproteinases reduced CA IX ECD shedding, but not apoptosis. The CA IX ECD release induced by cytotoxic drugs was connected to elevated expression of CA IX in the surviving fraction of cells. Moreover, an externally added recombinant CA IX ECD activated a pathway driven by the Nanog transcription factor implicated in epithelial-mesenchymal transition and stemness.ConclusionsThese findings imply that the increased level of the circulating CA IX ECD might be useful as an indicator of an effective antitumor chemotherapy. Conversely, elevated CA IX ECD might generate unwanted effects through autocrine/paracrine signaling potentially contributing to resistance and tumor progression.