Multiwavelength UV-Metric and pH-Metric Determination of the Multiple Dissociation Constants of the Lesinurad

Abstract

Potentiometric and spectrophotometric pH-titrations of the lesinurad for three consecutive dissociation constants determination were compared. Lesinurad is a selective inhibitor of uric acid reabsorption as part of a combination of medicines to treat high levels of uric acid in blood, also called hyperuricemia. Nonlinear regression of the pH-spectra with REACTLAB and SQUAD84 and of the pH-titration curve with ESAB determined three multiple close dissociation constants.The protonation scheme of lesinurad was suggested. A sparingly soluble anion L(-)of lesinurad was protonated to the still soluble species LH, LH(2)(+)and LH(3)(2+)in pure water. Three consecutive thermodynamic dissociation constants were estimated pK(a1)(T) = 2.09, pK(a2)(T) = 4.25, pK(a3)(T) = 6.58 at 25 degrees C and pK(a1)(T) = 1.96, pK(a2)(T) = 4.16, pK(a3)(T) = 6.32 at 37 degrees C by UV-metric spectra analysis. The graph of molar absorption coefficients shows that the spectrum of species LH2+ and LH vary in colour, while protonation of chromophore LH2+ to LH32+ has less influence on chromophores in the lesinurad molecule. Three multiple thermodynamic dissociation constants of 1 x 10(-4)M lesinurad were determined by the pH-metric analysis pk(a1 )(T)= 2.39, pk(a2)(T) = 3.47, pk(a3)(T) = 6.17 at 25 degrees C and pK(a1)(T) = 2.08, pk(a2 )(T)= 3.29, pk(a3)(T) = 6.03 at 37 degrees C. The values of enthalpy Delta H-0(pK(a1) )= 19.19 kJ mol(-1), Delta H-0(pK(a2))= 13.29 kJ mol(-1), Delta H-0(pK(a3)) = 38.39 kJ mol(-1), show the dissociation process is endothermic. The positive values of Delta G(0)(pK(a1)) = 11.93 kJ mol(-1), Delta G(0)(pK(a2)) = 24.26 kJ mol (1), Delta G(0)(pK(a3)) = 37.56 kJ mol(.) (1) at 25 degrees C indicate that the dissociation process of pK(a2) is not spontaneous, which was confirmed by its value of entropy Delta S-0(pK(a1) ) = 24.37J mol(-1), Delta S-0(pK(a2)) = -36.79J mol(-1), Delta S-0(pK(a3)) = 2.79J mol(-1). Three macro-dissociation constants of lesinurad and protonation locations were predicted by MARVIN and ACD/Percepta.