A Novel Drug-Drug Salt of Naproxen and Vortioxetine: Synthesis, Characterization, and Solubility Study

Naproxen (NAP) is an aromatic propionic acid nonsteroidal anti-infammatory drug, and vortioxetine (VOT) is a novel antidepressant drug. In this study, a new 1:1 drug-drug salt of NAP and VOT (namely, NAP-VOT) was designed and synthesized by liquid-assisted grinding and slow evaporation. Te obtained salt was characterized by single-crystal X-ray difraction, powder X-ray difraction (PXRD), and diferential scanning calorimetry (DSC). Single-crystal structure showed that NAP-VOT is a molecular salt. Te NAP and VOTmolecules in the salt were connected by N-H + · ·· O hydrogen bonds between the carbonyl oxygen of NAP and the piperazine group of VOT. In addition, solubility and dissolution rate experiments were performed in water and pH 6.86 phosphate bufers, and the result suggested that salt formation could increase the solubility and dissolution rate of NAP and VOT in water. Furthermore, this study provides a new research idea to solve the problem of drug-drug combination by achieving drug-drug association at the molecular level.

Research has shown that there is an interaction between pain and depression. Chronic pain often triggers depression, and mental depression can exacerbate physical pain, so drug combinations between depression and painkillers have become routine [21,22]. However, drug combinations are a physical mix and there is no guarantee of physical and chemical compatibility between the two APIs. Tis paper is the frst attempt to achieve a combination between antidepressants and anti-infammatory and analgesic drugs at the molecular level. Compared to fxed-dose combinations, drug-drug cocrystallization has greater advantages. It avoids the stability and chemical incompatibility problems between drug molecules and is more conducive to drug absorption.
Naproxen is a commonly used aromatic propionic acid nonsteroidal anti-infammatory drug that exerts antiinfammatory and analgesic efects by inhibiting the synthesis of prostaglandins [23][24][25][26]. Vortioxetine (VOT) is a novel antidepressant that is not only efective in improving the symptoms of depressed patients, but also in improving the mood of depressed patients, reducing anxiety, and improving sleep quality [27][28][29]. In the present work, a novel drug-drug salt comprised of NAP and VOT (namely NAP-VOT) with a 1 : 1 stoichiometric ratio was synthesized and characterized by single-crystal X-ray difraction data (SCXRD), powder X-ray difraction (PXRD), and diferential scanning calorimeter (DSC) measurements. In addition, the solubility and powder dissolution rate of the NAP-VOT were compared with that of the parent drug NAP and VOT.
Te molecular structures of NAP and VOT are displayed in Scheme 1.

Instrumentations and Materials.
All chemicals and reagents were obtained from a variety of business sources and utilized without further purifcation. Te powder X-ray difraction (PXRD) of NAP, VOT, and NAP-VOT salt were performed using a Bruker corporation D8 ADVANCE difractometer with a Cu-Kα radiation tube (λ �1.5418Å) at 40 mA and 40 kV. Diferential scanning calorimetry (DSC) measurements were performed on a Mettler-Toledo analyzer at a heating rate of 10°C/min in a nitrogen environment. On a Bruker Apex II CCD difractometer operating at 50 kV and 30 mA, single-crystal X-ray difraction measurements of the NAP-VOT was recorded using Mo Kα radiation (λ � 0.71073Å). Te SHELXS program was used to solve the crystal structure, which was then refned with the SHELXL program [30]. Te NAP-VOTsalt was analyzed and produced on the basis of its single-crystal data using CrystalExplorer 3.1 software. Tables 1 and 2 show the crystal structure parameters as well as hydrogen bond distances and angles.

Preparation of NAP-VOT.
NAP-VOT salt was synthesized using a slow evaporation method. NAP (10 mg, 43.4 mmol) and VOT (13 mg, 43.4 mmol) solutions were prepared in a 1 : 1 equimolar ratio using a mixture of solvents (5 mL, acetonitrile: water, v/v 80 : 20%). Te solutions were stirred at 50°C for 1 hour and left to evaporate slowly at room temperature. After 7-10 days, colorless needle crystals were obtained. Yield: 7.2 mg (31%). NAP-VOT salt was also prepared by the acetonitrile-assisted grinding method.

High-Performance Liquid Chromatography (HPLC)
Assay. Te concentration of NAP and VOT was determined by an Agilent 1260 Infnity II HPLC system. NAP and VOT were separated by an EC-C18 column (150 mm × 3.0 mm and 2.7 μm particle size). Mobile phase consisting of 60 : 40 (v/v) acetonitrile and potassium phosphate (pH 2.50, 0.01 M) was run at 0.6 mL/min. Te column temperature was kept at 35°C and both NAP and VOT were detected at 226 nm.

Solubility and Powder Dissolution
Studies. Solubility and powder dissolution tests were carried out in a roundbottomed fask containing water and pH 6.86 phosphate bufer medium at 37 ± 0.5°C. Excess sample was added to 25 mL mother liquor in a round bottom fask and stirred magnetically for 24 h. Ten, the suspension was taken and fltered through a 0.22 μm membrane and the concentration of the fltrate was determined and analyzed by HPLC (n � 3). Residual samples were also collected for PXRD and DSC testing, the aim of which was to determine the stability of the residual samples. Powder dissolution experiments were similar to the solubility test, the diference being that the samples were taken at a fxed point in time while an equal amount of fresh mother liquor is added to keep the volume of the solution constant.

Hirshfeld Surface Analyses.
Hirshfeld surface analysis provides a visual insight into the weak force interactions between molecules [33,34]. In order to compare the change in molecular weak forces after salt formation, the Hirshfeld surface analyses of NAP, VOT, and NAP-VOT were generated, and the Hirshfeld surfaces are shown in Figure 4. As can be seen from Figure 4, the H· · ·H weak interaction contribution values in NAP, VOT, and NAP-VOT account for 44.8%, 74.2%, and 58.5%, respectively. It is clear that the H· · ·H weakly interacting contribution values are dominant in the corresponding groups, where the contribution of H· · ·H weakly interacting NAP is smaller than that of VOT. Currently, the H· · ·H weak interaction contribution values of NAP-VOT are between NAP and VOT, indicating the NAP-VOT are structurally denser than NAP and looser VOT, which provide an opportunity to regulate the NAP and VOT solubility by creating favourable conditions. In addition, the intermolecular O---H hydrogen bonding interactions in NAP-VOT have a combined polar contact of 14.1%, which is better than that of pure VOT molecules (0%). Te enhanced polar weak interaction provides more polar bonding sites for the solvation efect, which might enhance the solubility of NAP-VOT.

PXRD Analyses.
Te PXRD patterns for NAP, VOT, and NAP-VOT are presented in Figure 5. NAP-VOT displayed major difraction peaks at 4.04°, 8 Te difractogram of NAP-VOT was distinguishable from NAP and VOT, indicating the formation of a new crystalline phase. In addition, the experimental PXRD pattern of NAP-VOT was in good agreement with the calculated profle, suggesting the high purity of the powder sample.

Solubility and Dissolution Studies.
Te solubility of NAP, VOT, and NAP-VOT in pure water and pH 6.86 bufer solution are shown in Table 3. Te solubility order was NAP-VOT > NAP > VOT in water and NAP > NAP-VOT > VOT in pH 6.86 bufer solution. Te NAP-VOT salt showed better  solubility in water when compared to NAP and VOT, while the solubility of NAP-VOT at pH 6.86 phosphate bufers was in between that of NAP and VOT. Te formation of the remaining materials after solubility studies was tested through PXRD and DSC, and the result of PXRD and DSC is shown in Figure 7. Te remaining solids showed that NAP and NAP-VOT were stable in water and pH 6.86 bufer solutions, but VOT was unstable in pH 6.86. Te powder dissolution profles of NAP-VOT were evaluated in water and pH 6.86 phosphate bufers, and the results are shown in Figure 8. Te result showed that the dissolution profle of NAP-VOT in pH 6.86 was lower than NAP but better than VOT. On the other hand, the dissolution rate of NAP-VOT in water is superior to that of NAP and VOT.

Conclusions
In conclusion, a novel drug-drug salt comprised of NAP and VOT (NAP-VOT) with a 1 : 1 stoichiometric ratio was synthesized to solve the absorption problem when coadministering drugs and to truly achieve drug combination at the molecular level. Te NAP and VOT molecules in the salt were connected by N-H + · · ·O hydrogen bonds between the carbonyl oxygen of NAP and the piperazine group of VOT. In addition, the physicochemical properties, solubility, and dissolution rates of the salt were systematically investigated. As expected, the solubility and dissolution rate of the NAP-VOTsalt signifcantly increased in comparison to NAP and VOT in water. Furthermore, this study is an important guide to address the problem of poor adherence to medication in drug combinations.

Data Availability
CCDC 2234586 contains the supplementary crystallographic data for this paper. Tese data can be obtained free of charge from Te Cambridge Crystallographic Data Centre via https://www.ccdc.cam.ac.uk/structures.

Conflicts of Interest
Te authors declare that they have no conficts of interest.

Authors' Contributions
Te manuscript was written and analyzed through contributions of all authors. All authors conceptualized and designed the study. Material preparation and experimental study were performed by Xianrui Zhang and Yuting Gao.
Te data analysis and software calculation were performed by Xianrui Zhang. Te funding support and frst draft of the manuscript were written by Xianrui Zhang and all authors commented on the previous version of the manuscript. All authors read and approved the fnal manuscript.