Liquid Co-crystals of Dual-Active Phenothiazine–NSAID Drugs: Synthesis, Spectroscopic, and Thermal Characterization

Four aliphatic phenothiazine cations (promazinium, promethazinium, chlorpromazinium, and triflupromazinium) were each paired with docusate anions and three different NSAID anions (ibuprofen, salicylate, and naproxen) to form fifteen glassy materials and one solid. The compounds were prepared via the metathesis reaction between the corresponding phenothiazine hydrochloride salts and sodium docusate or sodium NSAID salts and were obtained as liquid co-crystals with various degrees of ionization. The self-diffusion coefficients of several derivatives in 0.06 M DMSO-d6 solutions were determined using DOSY NMR spectroscopy. The influence of the size, shape of the compounds, and intermolecular forces has been investigated by using the four promazine and the four ibuprofen co-crystals. The ion pairs (or aggregates) were found to be maintained in six out of the seven compounds examined. All fifteen glassy compounds showed reversible glass transitions in the −25 to 10 °C range with the docusate derivatives exhibiting the highest thermal stability (Tonset values being at least 40 °C higher than those of the corresponding phenothiazine hydrochlorides).


■ INTRODUCTION
Phenothiazine drugs, thiazine-based drugs used in pharma since 1950s, are known to have multiple biological effects (e.g., anti-psychotic, antimalarial, antimicrobial, tranquilizers, antiinflammatory, anti-psychotropic, and antitumor) 1 with the aliphatic derivatives (e.g., promazine, chlorpromazine, triflupromazine, and promethazine) having a moderate biological activity while the piperidine derivatives (e.g., thioridazine) having a strong biological activity, and the piperazine derivatives (e.g., trifluperazine and fluphenazine) having weak biological activity (Table 1). 1 As most pharmaceuticals, these drugs are administered as solid-state inorganic salts (i.e., hydrochlorides), and they present all the solid-state disadvantages. 2−4 The existence of polymorphic structures (i.e., different crystalline structures with different or no biological activity) is a major issue for the pharmaceutical industry as it can lead to potential legal ramifications; 5 multiple court cases addressed the polymorphic transformation of pharmaceuticals showing that, in USA, many solid forms of pharmaceuticals are different drugs altogether. 6 Research is currently focused on minimizing and/or eliminating this issue; numerous directions are investigated with the conversion of solid-state drugs into (a) liquid-state salts (aka ionic liquids, ILs, or salts that melt below 100°C) and (b) co-crystals being two major approaches. 7 Conversion of pharmaceuticals into a liquid state was proven by numerous researchers 8−12 as a viable route for eliminating the solid-state disadvantages. For example, Hough et al. showed that Zantac (ranitidine hydrochloride), a drug included in court cases due to its polymorphic conversion, can readily convert into ranitidine docusate, a liquid form of the ranitidine drug obtained by pairing the ranitidine cation with the docusate anion. 13 Moreover, there is also an increased interest in bringing these liquid-state pharmaceuticals to market; for example, lidocaine etodolac is a dual-active liquid-state drug that completed phase III clinical trials with randomized testing being conducted on groups of up to 3000 patients. 14 The ionic liquid strategy can be applied to organic compounds that contain ionizable groups; the most common groups used in ionic liquid synthesis are Bronsted precursors such as carboxylic acids (R−CO 2 H) and carboxylates (R− CO 2 − M + ) as anion sources and amines (1°R−NH 2 , 2°R pharmaceutical activity of the constituent active ingredients from liquid co-crystals will not be affected while their physical properties (e.g., solubility) will be improved. 17,18 Phenothiazine drugs are sold as hydrochloride salts. Therefore, they are ideal cation precursors in the synthesis of dual-active PILs. Currently, 19 there are only three reports on liquid-state phenothiazine drugs (SciFinder search, "phenothiazine ionic liquids"); in a first report from 2007, Rogers et al. 20 reported on the synthesis of two double-salt ionic liquids where the anion is either the docusate or salicylate anion while the cation is a 1:1 mixture of promethazine and ephedrine cations held together through an intramolecular hydrogen bond. Second, in 2010, Dumitriu et al. 21 synthesized new liquid-state imidazolium-functionalized phenothiazine derivatives. Finally, Oh et al. 22 used N-substituted phenothiazines as pendant groups in polymeric ionic liquids. However, there is no report on investigating the synthesis of liquid-state dualfunctional phenothiazine drugs. Because phenothiazine drugs do not have an analgesic effect, they are administered together with analgesics. For example, a combination of the prochlorperazine phenothiazine drug and an analgesic is required for the target therapy. 23 Here, we explore the possibility of converting aliphatic phenothiazine drugs into liquid-state dual-functional drugs by combining aliphatic Sulfonic acids: very strong acids (pK a ∼ −7); e.g., methanesulfonic acid: pK a = −1.85; ** pK a of the conjugated acid.   Table 1) to potentially form dual functioning PILs with improved properties. The anion precursors were chosen based on the ΔpK a between the phenothiazine hydrochloride salts and the anion-conjugated acids 24−26 while sodium docusate was chosen due to its known properties as a transdermal penetration enhancer and its ability to form liquid-state compounds when paired with various cations. The anion/ cation molar ratio was kept the same as in the commercially available phenothiazine hydrochloride salts, allowing us to hypothesize that the biological properties of the cations will remain unaffected.
Sixteen new compounds were synthesized through a metathesis reaction between the cation (phenothiazine hydrochlorides) and anion precursors (sodium docusate or NSAID carboxylates) ( Figure 2) as follows: a mixture of equimolar amounts of anion and cation precursors in acetone was stirred for 24 h at room temperature. The inorganic byproduct formed (NaCl) was filtered through a 0.2 μm syringe filter, and the removal of volatiles via rotary evaporation allowed the formation of fifteen highly viscous compounds and a solid ([Prm][Sal]) in high yields (>89%) and purity (Table S1, Supporting Information). All the obtained compounds were characterized using Fourier transform infrared (FT-IR) and nuclear magnetic resonance spectroscopy [one-dimensional (1D) and two-dimensional (2D) NMR].
FT-IR Spectroscopy. FT-IR spectra ranging between 650 and 3800 cm −1 (resolution of 1 cm −1 ) were obtained for neat samples using a Perkin-Elmer attenuated total reflection (ATR)-IR Instrument. These spectra revealed that the position of the carboxylate stretches from the anions (labeled A in sodium salts and A′ in phenothiazine derivatives, Figure  3A1,A2) is shifted from ∼1550 cm −1 in sodium salts to ∼1730 cm −1 in the phenothiazine derivatives. For example, in the case of ibuprofen derivatives, the carboxylate stretch A appears at 1545 cm − Figure 3B1−B4). Moreover, the N−H stretch from the cation present in all the phenothiazine hydrochlorides between 2300 and 2600 cm −1 seems to also be present in the synthesized compounds but with a lower intensity, further confirming the sharing of a hydrogen between the cation and anion ( Figure S1, Supporting Information). The IR results can be explained by a decrease in the electron density around the carbonyl group from ibuprofen anions due to a decrease in the degree of ionization in the synthesized compounds. Similar results were observed for all the phenothiazine compounds synthesized (the full FT-IR spectra are shown in Figures S2− S5, Supporting Information). These observations are consistent with a "liquid co-crystal" behavior of all the synthesized compounds, where a hydrogen is shared between the cation and the anion (refer to Figure 2A2).
NMR Spectroscopy. The purity and formation of the compounds in a 1:1 stoichiometry were determined using NMR spectroscopy (purity: Table S1, Supporting Information; full NMR spectra: Figures S6−S21, Supporting Information). Furthermore, evidence of the co-crystal behavior of the synthesized compounds was seen through 1D and 2D NMR characterization. NMR spectra of 0.06 M DMSO-d 6 solutions were recorded using a Bruker 500 MHz spectrometer at 25°C. NMR samples were prepared as follows: a pre-weighted amount of the compound was dissolved in 0.75 mL of DMSOd 6 to obtain solutions of 0.06 M concentration. To ensure complete solvation of each compound, the samples were sonicated for 5 min before spectrum collection.  Figure 4A1) and on the chemical shifts of the methyl and methine groups from [Prm] derivatives ( Figure  4A2) was investigated. Figure 4B1       free acid, H[Ibu] ( Figure 5). A higher influence was obtained on the methine group: this group (highlighted in green in Figure 5), a quartet at 3. 25  However, no significant influence is seen for the adjacent methyl group (highlighted in purple in Figure 5) . This is most likely due to a higher distance between this group and the basic site. The more downfield chemical shift values for the methine group in the synthesized phenothiazine ibuprofen compounds is consistent with a decrease in the electron density around the carboxylate anion, which further confirms their lower degree of ionization when compared to Na[Ibu] and therefore their existence as co-crystals.
Diffusion-Ordered (DOSY) NMR Spectroscopy. 1 H DOSY NMR is a powerful technique used to investigate the existence of individual species in a solution; in the case of ionic compounds, it can be used to determine if the cation and anion remain associated in solutions. 27 This can provide information on the transport properties and helps with the structural characterization of the compounds. The self-diffusion coefficients for the ion pair or the cation and anion present in the 0.06 M DMSO-d 6 Figure S24A Thermal Characterization. The thermal behavior of the synthesized compounds was further investigated. Decomposition temperatures (T onset and T endset ) were determined using thermogravimetric analysis (TGA) while phase transitions (glass transitions, T g , solid−solid transitions, T transition , and/or melting points, T m ) were determined using differential scanning calorimetry (DSC). The obtained T onset and T endset values are shown in Figure 8 and Table 3, the T g values are shown in Table 3, while the full TGA and DSC graphs are shown in Figures S30−S37 from Supporting Information.
TGA. All the synthesized compounds but [Pro][Sal] underwent a one-step decomposition. The highest thermal stability was obtained for the docusate derivatives, where T onset (°C) values were at least 40°C higher than those of the   Cation Influence on Thermal Stability (Figure 9). In the case of [Ibu] (cyan line in Figure 9) and [Doc] (red line in Figure 9)  (black line in Figure 9) and [Nap] (magenta line in Figure 9

■ CONCLUSIONS
Phenothiazine drugs are known to have multiple pharmacological effects but no known analgesic effect. Therefore, these drugs are administered in combination with NSAIDs. Combining the two types of drugs into one single, liquidstate, compound would potentially aid with the drug administration while addressing other inherent solid-state issues such as polymorphism. The ΔpK a difference between the pK a of the conjugated acids and the acids used in this study (>5) suggests that a liquid-state behavior is indeed expected. In our search for new phenothiazine ionic liquids, we have found that aliphatic phenothiazine drugs (cation precursors) can be paired with various NSAIDs (anion precursors) to form dual functional PILs with various degrees of proton transfer between the cation and anion (i.e., various degree of ionization), species also known as liquid co-crystals. According to various reports, co-crystal pharmaceuticals will keep the pharmacological properties of the component drugs. Therefore, the new liquid co-crystals reported here will not only benefit from dual functionality but will also show potential This information is the first step toward providing an insight into the transport properties of these compounds (i.e., transdermal delivery): PILs have been shown to permeate through a skin-mimicking membrane faster than their corresponding ionic salts. Therefore, investigating the transdermal delivery of these compounds is currently on-going in our laboratory.
Characterization. NMR Spectroscopy. All the NMR spectra ( 1 H, 13 C, 19 F, and the 1 H− 15 N HSQC) were acquired as follows: 0.06 M solutions of the investigated compounds in DMSO-d 6 were prepared, and the spectra were recorded at 25°C using a Bruker 500 MHz spectrometer. FT-IR Spectroscopy. FT-IR spectra ranging between 650 and 3800 cm −1 were obtained from neat aliquots of each API-IL by using a Perkin Elmer ATR-IR Instrument. The resolution for the spectra was of 1 cm −1 .
TGA. TGA experiments were performed on a Thermal Analysis Discovery TGA550 instrument. Samples (2−15 mg) were placed in 70 μL platinum pans and were analyzed as follows: the samples were heated under a flow of nitrogen from 25 to 800°C in a dynamic heating regime by using a 5°C/min heating ramp and an isotherm of 30 min at 75°C.   DSC Analyses. DSC data were collected using a Thermal Analysis Discovery DSC250 instrument under a continuous flow of nitrogen. DSC samples were prepared by placing 2−15 mg of compound into sealed T Zero pans covered with hermetic Al lids and sealed using a T-Zero press. The data collection involved three cycles each consisting of four steps: (1) heating to 90°C at a 5°C/min heating rate; (2) a 5 min isotherm at 90°C; (3) cooling at a 5°C/min rate to −90°C; and (4) a 5 min isotherm at −90°C.
Syntheses. General Synthesis of Phenothiazine Compounds. Phenothiazine hydrochloride was suspended in anhydrous acetone, and an equimolar amount of anion precursor was added. The resulting suspension was stirred for 20−24 h at room temperature. The inorganic salt formed (NaCl) was removed by filtration through a 0.2 μm filter, and the solvent from the obtained solution was removed using a