NPPB (Synonyms: HOE 144, Hoechst 144)

NPPB, 5-nitro-2-(3-phenylpropylamino)-benzoic acid, is a potent inhibitor of chloride channel with IC50 of 80 nM for the short circuit current.[1]
Chloride channel blockers possess several sites of interaction, including the negatively charged carboxylate group, the secondary amine group which probably carries a positive partial charge, and for the very potent agents like NPPB an additional negative partial charge at the -NO2 substituent. In addition, an apolar interaction with a cycloaryl residue is necessary, and this site of interaction has a specific spacing from the secondary amino nitrogen.[1]
NPPB was evaluated for the activity on the equivalent short circuit current, corresponding to the secondary active transport of Cl- and measurements of the voltage across the basolateral membrane. The result revealed that NPPB possessed a good potency with IC50 of 80 nM for inhibiting the short circuit current. Furthermore, NPPB was also tested for its activity on various anion channels. Adopting freshly-isolated cells from the rat portal vein, the effects of NPPB were investigated on evoked and spontaneous currents by use of conventional whole-cell recording and perforated-patch techniques. At a holding potential of -60 mV in potassium-free, caesium-containing solutions, NPPB (10 μM) inhibited Ca-sensitive chloride currents (ICI(Ca)) evoked by caffeine (10 mM) and by noradrenaline (10μM) by the extend of 58% and 96%, respectively. In addition, at a holding potential of -2 mV in potassium -containing solutions, NPPB (10 μM) inhibited charybdotoxin-sensitive potassium-currents (IBK(Ca)) induced by noradrenaline (10 μM) and acetylcholine (10 μM) by approximately 90%. NPPB's inhibitory effects of volume-activated taurine, glucose, and uridine influxes was studied. The IC50 for the inhibition of the volume- activated fluxes by NPPB was around 12 μM. [1-3]
References:
[1] Wangemann, Ph, et al. "Cl−-channel blockers in the thick ascending limb of the loop of Henle Structure activity relationship." Pflügers Archiv 407.2 (1986): S128-S141.
[2] Kirkup, A. J., G. Edwards, and A. H. Weston. "Investigation of the effects of 5‐nitro‐2‐(3‐phenylpropylamino)‐benzoic acid (NPPB) on membrane currents in rat portal vein." British journal of pharmacology 117.1 (1996): 175-183.
[3] Kirk, Kiaran, J. C. Ellory, and J. D. Young. "Transport of organic substrates via a volume-activated channel." Journal of Biological Chemistry 267.33 (1992): 23475-23478.