The Overlapping Thermodynamic Dissociation Constants of the Antidepressant Vortioxetine Using UV–VIS Multiwavelength pH-Titration Data

Potentiometric and spectrophotometric pH-titrations of the antidepressant drug Vortioxetine were compared for dissociation constants determinations. Vortioxetine is an atypical antidepressant, i.e., it is a serotonin modulator and stimulator. Depressive disorders are common mental health conditions that are thought to be caused by an imbalance in serotonin and norepinephrine in addition to multiple situational, cognitive, and medical factors. A chemometrics approach to the nonlinear regression of the pH-spectra (REACTLAB, SQUAD84) and pH-titration (ESAB) were used to determine the two overlapping dissociation constants. A sparingly soluble neutral base LH of Vortioxetine hydrobromide was protonated to form the two still-soluble cations $$ {\text{LH}}_{2}^{ + } $$ and $$ {\text{LH}}_{ 3}^{{ 2 { + }}} $$ in pure water. In the range of pH (5–10), the two dissociation constants could be reliably estimated from small changes in the spectra of 9.2 × 10−5 mol·dm−3 Vortioxetine. Although the change of pH affected changes in the chromophore to a small extent, two thermodynamic dissociation constants were estimated: $$ {\text{p}}K_{\text{a1}}^{\text{T}} $$  = 7.22 and $$ {\text{p}}K_{\text{a2}}^{\text{T}} $$  = 8.67 at 25 °C and $$ {\text{p}}K_{\text{a1}}^{\text{T}} $$  = 7.27 and $$ {\text{p}}K_{\text{a2}}^{\text{T}} $$  = 8.79 at 37 °C. The graph of molar absorption coefficients of variously protonated species as a function of wavelength shows that the spectra of species $$ {\text{LH}}_{2}^{ + } $$ and LH vary in color, while protonation of the chromophore $$ {\text{LH}}_{2}^{ + } $$ to $$ {\text{LH}}_{ 3}^{{ 2 { + }}} $$ has less influence on the chromophores of the Vortioxetine hydrobromide molecule. Two thermodynamic dissociation constants of 3 × 10−4 mol·dm−3 Vortioxetine were determined by regression analysis of the potentiometric titration curves, $$ {\text{p}}K_{\text{a1}}^{\text{T}} $$  = 7.08 and $$ {\text{p}}K_{\text{a2}}^{\text{T}} $$  = 8.50 at 25 °C and $$ {\text{p}}K_{\text{a1}}^{\text{T}} $$  = 7.33 and $$ {\text{p}}K_{\text{a2}}^{\text{T}} $$  = 8.76 at 37 °C. A prediction of the dissociation constants of Vortioxetine was carried out using the MARVIN and ACD/Percepta programs and only two dissociation constants were proposed theoretically.