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Poor vascularization of solid tumors leads to inadequate nutrient and oxygen supplies which forces tumor cells to reprogram their metabolism. Consequently, the tumor cell's environment becomes acidic and hypoxic. This triggers signaling cascades involving e.g. heterodimeric hypoxia-inducible factor (HIF). Activation of this hypoxia-induced transcriptional program is crucial for the tumor cell to survive its hostile microenvironment and its ability to metastasize. One of the genes HIF upregulated is carbonic anhydrase (CA)-IX (CAIX, gene G250/MN-encoded transmembrane protein). CA-IX catalyzes carbon dioxide (CO₂) thereby generating a proton (H⁺) and bicarbonate (HCO₃⁻), the latter of which is transported back into the cell and utilized to help safeguard intracellular pH (pHi) stability. Except for the stomach and the gallbladder, CA-IX expression is negligible in normal tissues. In contrast, a broad range of tumors …