Direct block of the cystic fibrosis transmembrane conductance regulator Cl- channel by niflumic acid

Niflumic acid is widely used to inhibit Ca2+-activated Cl- channels. However, the chemical structure of niflumic acid resembles that of diphenylamine-2-carboxylate, a drug that inhibits the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel. To investigate how niflumic acid inhibits the CFTR Cl- channel, we studied recombinant wildtype human CFTR in excised inside-out membrane patches. When added to the intracellular solution, niflumic acid caused a concentration- and voltage-dependent decrease of CFTR Cl- current with half-maximal inhibitory concentration (K-i) of 253 mM and Hill co-efficient of similar to 1, at - 50 mV. Niflumic acid inhibition of single CFTR Cl- channels was characterised by a very fast, flickery block that decreased dramatically current amplitude without altering open-probability. Consistent with these data, spectral analysis of CFTR Cl- currents suggested that channel block by niflumic acid was described by the closed <----> open <----> blocked kinetic scheme with blocker on rate (k(on)) = 13.9 x 10(6) M-1 s(-1), off rate (k(off)) = 3348 s(-1) and dissociation constant (K-d) = 241 muM, at - 50 mV. Based on these data, we tested the effects of niflumic acid on transepithelial Cl- secretion and cyst growth using type I MDCK epithelial cells. Niflumic acid (200 muM) inhibited cAMP-stimulated, bumetanide-sensitive short-circuit current by 55%. Moreover, the drug potently retarded cyst growth. We conclude that niflumic acid is an open-channel blocker of CFTR that inhibits Cl- permeation by plugging the channel pore. It or related agents might be of value in the development of new therapies for autosomal dominant polycystic kidney disease.