Cl⁻-channel blockers in the thick ascending limb of the loop of Henle Structure activity relationship

Abstract

On the basis of our findings with diphenylamine-2-carboxylate [5] we have searched for compounds which possess an even higher affinity for the Cl⁻-channels in the basolateral membrane of the thick ascending limb of the loop of Henle. To quantitiy the inhibitory potency, we performed measurements of the equivalent short circuit current, corresponding to the secondary active transport of Cl⁻ [8] and measurements of the voltage across the basolateral membrane. A survey of 219 compounds reveals that relatively simple modifications in the structure of diphenylamine-2-carboxylate led to very potent blockers such as 5-nitro-2-(3-phenylpropylamino)-benzoate which inhibits the short circuit current half maximally (IC₅₀) at 8·10⁻⁸ mol/l. A comparison of the structural formula and the respective IC₅₀ values leads to several empirical conclusions: 1. The potent compounds are lipophilic due to the apolar residue (e.g. phenyl- or cycloalkyl group). Replacing this part of the molecule by an aliphatic chain (up to 4 C-atoms) leads to inactive compounds. 2. Most of the inhibitors are secondary amines. Linking other than with-NH- between the phenyl ring and the benzoic acid results in inactive compounds. Tertiary amines, such as in case of 2-(N,N-diphenylamine) benzoic acid or N-methylphenylaminebenzoic acid are poorly active. 3. The carboxylate group of the benzoate moiety must be in ortho position to the amino group. 4. Introduction of substituents into the benzoate moiety e.g.-NO₂ (in meta position to the carboxylate group), or by-Cl (in para position to the carboxylate group) results in an increase of inhibitory potency. 5. A-CH₂-,-C₂H₄-,-C₃H₆-) spacer between the amino bridge and the phenyl ring increases the affinity for the Cl⁻-channel by several orders of magnitude. The above described structure activity relationship renders it likely that these chloride channel blockers possess several sites of interaction: The negatively charged carboxylate group, the secondary amine group which probably carries a positive partial charge, and for the very potent agents (nos. 130, 143, 144, and 145) an additional negative partial charge at the respective-Cl or-NO₂ substituent. Finally, also an apolar interaction with an cycloalkyl or cycloaryl residue seems to be required, and this site of interaction has a defined spacing from the secondary amino nitrogen.