Evaluation of the Pka Values and Ionization Sequence of Bumetanide Using 1H and 13C NMR and UV Spectroscopy

Abstract

Non-thiazide loop diuretics such as bumetanide and furosemide are well-established medicaments, with varying substituents that alter their chemical properties in ways that affect biological parameters, including biodistribution, efficacy, and safety. However, literature data supporting the assignment of pK_a values and ionization sequence of bumetanide are limited. The present study summarizes available literature data and then characterizes nuclear magnetic resonance (NMR) and ultraviolet (UV) spectral changes over a range of pH values to delineate the apparent sequence of deprotonations. Three macroscopic pK_a values, 1.44, 3.74, and 10.37, were determined in water via pH titration experiments monitored by ¹H NMR. Both ¹H and ¹³C analyses support assignment of the lowest pK_a value to the anilinium ion, as described for another diuretic, piretanide. However, UV data suggest that initial deprotonation of the benzoic acid competes to some extent with deprotonation of the anilinium ion, such that both pathways may operate simultaneously in an aqueous environment. Thus, the observed pK_a values likely represent the macroscopic averages of the two competing deprotonation sequences. Calculated pK_a values for bumetanide, piretanide, and model compounds are contrary to the deprotonation sequence we and others have determined experimentally, which is not surprising given the complicated factors driving the acid–base properties of these molecules. Drug Dev Res 72: 416–426, 2011. © 2011 Wiley-Liss, Inc.