Discovery of Glycine Hydrazide Pore-occluding CFTR Inhibitors : Mechanism, Structure–Activity Analysis, and In Vivo Efficacy

The cystic fibrosis transmembrane conductance regulator (CFTR) protein is a cAMP-regulated epithelial Cl⁻ channel that, when defective, causes cystic fibrosis. Screening of a collection of 100,000 diverse small molecules revealed four novel chemical classes of CFTR inhibitors with K_i < 10 μM, one of which (glycine hydrazides) had many active structural analogues. Analysis of a series of synthesized glycine hydrazide analogues revealed maximal inhibitory potency for N-(2-naphthalenyl) and 3,5-dibromo-2,4-dihydroxyphenyl substituents. The compound N-(2-naphthalenyl)-[(3,5-dibromo-2,4-dihydroxyphenyl)methylene]glycine hydrazide (GlyH-101) reversibly inhibited CFTR Cl⁻ conductance in <1 min. Whole-cell current measurements revealed voltage-dependent CFTR block by GlyH-101 with strong inward rectification, producing an increase in apparent inhibitory constant K_i from 1.4 μM at +60 mV to 5.6 μM at −60 mV. Apparent potency was reduced by lowering extracellular Cl⁻ concentration. Patch-clamp experiments indicated fast channel closures within bursts of channel openings, reducing mean channel open time from 264 to 13 ms (−60 mV holding potential, 5 μM GlyH-101). GlyH-101 inhibitory potency was independent of pH from 6.5–8.0, where it exists predominantly as a monovalent anion with solubility ∼1 mM in water. Topical GlyH-101 (10 μM) in mice rapidly and reversibly inhibited forskolin-induced hyperpolarization in nasal potential differences. In a closed-loop model of cholera, intraluminal GlyH-101 (2.5 μg) reduced by ∼80% cholera toxin–induced intestinal fluid secretion. Compared with the thiazolidinone CFTR inhibitor CFTR_inh-172, GlyH-101 has substantially greater water solubility and rapidity of action, and a novel inhibition mechanism involving occlusion near the external pore entrance. Glycine hydrazides may be useful as probes of CFTR pore structure, in creating animal models of CF, and as antidiarrheals in enterotoxic-mediated secretory diarrheas.