A Search for the Protonation Model with Thermodynamic Dissociation Constants and (Extra)-Thermodynamics of Nilotinib Hydrochloride (TASIGNA)

Abstract

Nilotinib hydrochloride (AMN107, TASIGNA, Novartis) is used to treat adults with chronic myeloid leukemia (CML), a type of leukemia. It is a novel, orally active BCR-ABL tyrosine kinase inhibitor derived from aminopyrimidine that is 30 times more effective against CML cells than is Imatinib. The nonlinear regression of the A versus pH spectra with REACTLAB and SQUAD84 and of the pH-titration curve with ESAB determined the four close and consecutive dissociation constants in the 11 steps of the newly proposed procedure. Prediction of pK_a performed by MARVIN, PALLAS and ACD/Percepta determined the protonation sites. The sparingly soluble nilotinib hydrochloride denoted as L forms four water-soluble LH⁺, ${\text{LH}}_{2}^{2 + }$, ${\text{LH}}_{3}^{3 + }$, and ${\text{LH}}_{4}^{4 + }$ cations. Although the adjusted pH has less effect on the absorbance changes in the chromophore, four thermodynamic dissociation constants were reliably determined: ${\text{p}}K_{\text{a1}}^{\text{T}}$ = 3.60 ± 0.04, ${\text{p}}K_{\text{a2}}^{\text{T}}$ = 4.42 ± 0.07, ${\text{p}}K_{\text{a3}}^{\text{T}}$ = 4.71 ± 0.04, and ${\text{p}}K_{\text{a4}}^{\text{T}}$ = 4.84 ± 0.03 at 25 °C and ${\text{p}}K_{\text{a1}}^{\text{T}}$ = 3.61 ± 0.11, ${\text{p}}K_{\text{a2}}^{\text{T}}$ = 4.29 ± 0.18, ${\text{p}}K_{\text{a3}}^{\text{T}}$ = 4.49 ± 0.02, and ${\text{p}}K_{\text{a4}}^{\text{T}}$ = 5.05 ± 0.03 at 37 °C, and by regression analysis of potentiometric titration curves with ESAB, ${\text{p}}K_{\text{a1}}^{\text{T}}$ = 3.74 ± 0.01, ${\text{p}}K_{\text{a2}}^{\text{T}}$ = 4.05 ± 0.01, ${\text{p}}K_{\text{a3}}^{\text{T}}$ = 4.25 ± 0.01, and ${\text{p}}K_{\text{a4}}^{\text{T}}$ = 4.91 ± 0.20 at 25 °C and ${\text{p}}K_{\text{a1}}^{\text{T}}$ = 3.63 ± 0.03, ${\text{p}}K_{\text{a2}}^{\text{T}}$ = 3.96 ± 0.03, ${\text{p}}K_{\text{a3}}^{\text{T}}$ = 4.18 ± 0.03, and ${\text{p}}K_{4 1}^{\text{T}}$ = 4.81 ± 0.05 at 37 °C. Positive enthalpy values ΔH° at 25 °C showed that the dissociation process is endothermic and is accompanied by heat absorption. Inasmuch as the entropy values of the dissociation process ΔS° at 25 °C and 37 °C were negative, the dissociation process is reversible.

Graphical Abstract