Phloxine B interacts with the cystic fibrosis transmembrane conductance regulator at multiple sites to modulate channel activity.

The fluorescein derivative phloxine B is a potent modulator of the cystic fibrosis transmembrane conductance regulator (CFTR). Low micromolar concentrations of phloxine B stimulate CFTR Cl(-) currents, whereas higher concentrations of the drug inhibit CFTR. In this study, we investigated the mechanism of action of phloxine B. Phloxine B (1 microm) stimulated wild-type CFTR and the most common cystic fibrosis mutation, DeltaF508, by increasing the open probability of phosphorylated CFTR Cl(-) channels. At each concentration of ATP tested, the drug slowed the rate of channel closure without altering the opening rate. Based on the effects of fluorescein derivatives on transport ATPases, these data suggest that phloxine B might stimulate CFTR by binding to the ATP-binding site of the second nucleotide-binding domain (NBD2) to slow the dissociation of ATP from NBD1. Channel block by phloxine B (40 microm) was voltage-dependent, enhanced when external Cl(-) concentration was reduced and unaffected by ATP (5 mm), suggesting that phloxine B inhibits CFTR by occluding the pore. We conclude that phloxine B interacts directly with CFTR at multiple sites to modulate channel activity. It or related agents might be of value in the development of new treatments for diseases caused by the malfunction of CFTR.