Carbonic anhydrase IX is a promising target for the search for new antitumor compounds with improved properties. Using the molecular hybridization approach, on the basis of structures of a selective carbonic anhydrase IX inhibitor 3 and an activator of apoptosis 2 (1), a series of 1-substituted isatin-5-sulfonamides 5a-5u were designed and synthesized. The study of the inhibitory activity of isatin-5-sulfonamides showed the ability to inhibit I, II, IX, XII isoforms at nano- and micromolar concentrations. Docking of compounds 5e and 5k into the active site of II and IX carbonic anhydrase isoforms showed the coordination of sulfonamidate anions with zinc cations, as well as a number of additional hydrophobic interactions. The trifluoromethylthio derivative 5r suppressed the growth of tumor cells at low micromolar concentrations, maintaining activity on resistant lines and under hypoxic conditions. Immunoblotting of MCF7 cells treated with the 5r revealed its antiestrogenic activity and ability to activate apoptosis in tumor cells.