Carbonic Anhydrase-II Inhibition. What are the True Enzyme–Inhibitory Properties of the Sulfamide Cognate of Topiramate?

The marketed drug topiramate (1) is a moderate inhibitor of carbonic anhydrase-II (CA-II) (Ki or Kd = 0.3–0.6 µM), whereas sulfamide cognate 2 is a comparatively weak inhibitor (Ki or Kd = 25–650 µM). From an X-ray cocrystal structure of 2·CA-II, Winum et al. (J. Med. Chem. 2006, 49, 7024) proposed that an adverse steric interaction between the C8 methyl group in 2 and Ala-65 of CA-II is responsible for the diminished CA-II inhibitory potency of 2. We performed a straightforward test of this Ala-65 effect by synthesizing and examining ligand 3, which lacks the offending (pro-S or C8) methyl substituent in 2. We also prepared and evaluated related sulfamides 5, 7, and 9. In a CA-II inhibition assay (4-nitrophenyl acetate), the Ki for 3 was ∼300 µM, indicating very weak inhibition, close to that for 2 (4NPA, Ki = 340 µM). In a CA-II binding assay (ThermoFluor), the Kd for 3 was >57 µM, indicating very weak binding, lower than the affinity of 2 (Kd = 25 µM). Our results draw into question the proposed steric...