CA9 and cancer: Small-molecule CAIX inhibitors [21], but also small-molecule drug conjugates, antibody-drug conjugates, or cytokine-drug conjugates, targeting these enzymes were demonstrated to constitute an innovative antitumor strategy in the last decade [22]. In the present study, the cytotoxic effects of a sulphonamide CAIX inhibitor, derivative compound E (4-(2-((5-bromo-2-hydroxybenzyl) amino) ethyl) benzenesulphonamide), were investigated in the cancer cells with high CAIX expression.