Carbonic anhydrase inhibitors: Thioxolone versus sulfonamides for obtaining isozyme-selective inhibitors?

Inhibition of 13 mammalian isoforms of the metalloenzyme carbonic anhydrase (CA, EC 4.2.1.1), CA I–XV, with thioxolone (6-hydroxy-1,3-benzoxathiol-2-one) and two sulfonamides was investigated. Thioxolone was inefficient for generating isozyme-selective inhibitors, since except for CA I which is inhibited in the nanomolar range (KI of 91 nM), the remaining 12 isoforms were inhibited with a very flat profile (KIs in the range of only 4.93–9.04 μM). In contrast to thioxolone, 3,5-dichloro-4-hydroxybenzenesulfonamide as well as the clinically used heterocyclic sulfonamide acetazolamide showed KIs in the range of 58 nM–78.6 μM and 2.5 nM–200 μM, respectively, against the 13 investigated mammalian CAs. The sulfonamide zinc-binding group is thus superior to the thiol one for generating CA inhibitors with a varied and sometimes isozyme-selective inhibition profile against the mammalian enzymes.