Pharmaceutical salts of ethionamide with GRAS counter ion donors to enhance the solubility

Calculating pH-solubility profile and pHmax for monoprotic salts of poorly water-soluble weak bases

Pharmaceutical salts are a commonly used strategy to improve the bioavailability of poorly water-soluble active pharmaceutical ingredients (APIs). The selected salt form is expected to have high solubility to obtain optimal supersaturation and sufficient physical stability for adequate shelf life. In this paper, we aim to develop equations to describe critical parameters of salts, including pH-solubility profile and pHmax. The equations serve as a valuable tool to aid in the calculation of salt solubility at pH below the pHmax in the presence of common counter ions. This provides the knowledge to assess the risks of pre-selection of the salt formers without the necessity of salt synthesis. The solubilities calculated by this model demonstrate good agreement with experimental solubility results reported in the literature. Compared to the conventional approaches for salt solubility and pHmax calculation, our model stands out, especially for poorly water-soluble bases with low pKa values, which benefit the most from salt formation. Moreover, the equations are used to support the concept that salt selection should focus on finding salt forms with sufficient solubility, rather than the most soluble salt, as excessively high solubility could be detrimental to physical stability.

Supramolecular Organization in Salts of Riluzole with Dihydroxybenzoic Acids—The Key Role of the Mutual Arrangement of OH Groups

Intermolecular interactions, in particular hydrogen bonds, play a key role in crystal engineering. The ability to form hydrogen bonds of various types and strengths causes competition between supramolecular synthons in pharmaceutical multicomponent crystals. In this work, we investigate the influence of positional isomerism on the packing arrangements and the network of hydrogen bonds in multicomponent crystals of the drug riluzole with hydroxyl derivatives of salicylic acid. The supramolecular organization of the riluzole salt containing 2,6-dihydroxybenzoic acid differs from that of the solid forms with 2,4- and 2,5-dihydroxybenzoic acids. Because the second OH group is not at position 6 in the latter crystals, intermolecular charge-assisted hydrogen bonds are formed. According to periodic DFT calculations, the enthalpy of these H-bonds exceeds 30 kJ·mol−1. The positional isomerism appears to have little effect on the enthalpy of the primary supramolecular synthon (65–70 kJ·mol−1), but it does result in the formation of a two-dimensional network of hydrogen bonds and an increase in the overall lattice energy. According to the results of the present study, 2,6-dihydroxybenzoic acid can be treated as a promising counterion for the design of pharmaceutical multicomponent crystals.

New Co-Crystals/Salts of Gallic Acid and Substituted Pyridines: An Effect of Ortho-Substituents on the Formation of an Acid–Pyridine Heterosynthon

Co-crystallization of gallic acid with pyridines and their polyaromatic analogue, quinoline, ortho-substituted by various proton-donating groups able to form hydrogen bonds, produced the only reported co-crystal of gallic acid with an ortho-substituted pyridine, 2-hydroxypyridine, as its preferred pyridone-2 tautomer, and four new crystalline products of gallic acid. These co-crystals, or gallate salts depending on the choice of the pyridine-containing compound, as predicted by the pKa rule, were identified by X-ray diffraction to feature the popular acid–pyridine heterosynthon found in most of the two-component systems of gallic acid that lack ortho-substituents in the pyridine-containing compound. This single-point heterosynthon is, however, modified by one or two proton-donating ortho-substituents, which sometimes may transform into the proton acceptors in an adopted tautomer or zwitterion, to produce its two- or other multi-point variants, including a very rare four-point heterosynthon. The hydrogen bonds they form with the gallic acid species in the appropriate co-crystals/salts strongly favors the formation of the acid–pyridine heterosynthon over the acid–acid homosynthon. In the competitive conditions of multi-component systems, such a modification might be used to reduce supramolecular-synthon-based polymorphism to produce new pharmaceuticals and other crystalline materials with designed properties.

Novel pharmaceutical salts of cephalexin with organic counterions: structural analysis and properties

Three novel hydrated pharmaceutical salts of cephalexin with acidic organic counterions were successfully obtained and thoroughly characterized by various analytical techniques.

Salt Solubility and Disproportionation - Uses and Limitations of Equations for pHmax and the In-silico Prediction of pHmax

A multiphasic mass action equilibrium model was used to study the phase properties near the critical pH ('pH_max') in an acid-base transformation of a solid drug salt into its corresponding solid free base form in pure water slurries. The goal of this study was to better define the characteristics of disproportionation of pharmaceutical salts, objectively (i) to classify salts as μ-type (microclimate stable) or δ-type (disproportionation prone) based on the relationship between the calculated pH_max and the calculated pH of the saturated salt solution, (ii) to compare the distribution of μ/δ-type salts to predictions from the disproportionation potential equation introduced by Merritt et al.,²⁰ (iii) to determine if the intrinsic solubility of the free base, S₀, can be predicted from the measured μ-type salt solubility as a means of estimating the value of pH_max, (iv) to determine S₀ directly from the measured δ-type salt solubility, and (v) to address some of the limitations of the equations commonly used to calculate pH_max. When the salt solubility is measured for a basic API (pK_a of which is known), but the experimental value of S₀ is unavailable, a potentially useful simple screen for disproportionation is still possible, since pH_max can be estimated from a 'μ-predicted' (objective iii) or 'δ-measured' S₀ (objective iv). Twelve model weak base API were selected in the study. For each API, 2-17 different salt forms with reported salt solubilities in distilled water were sourced from the literature. In all, 73 salt solubility values based on 29 different salt-forming acids comprise the studied set. All the corresponding free base solubility values were available. The pK_a values for all the acids and bases studied are generally well known. For each API salt, an acid-base titration simulation was performed, anchored to the measured salt solubility value, using the general mass action analysis program pDISOL-X. The log S-pH profiles were drawn out by analytic continuity from pH 0 to 13, as described in detail previously.²⁴ Potentially useful in-silico models were developed that correlate pS₀ to linear functions of the salt solubility in water, pS_w, the partition coefficient of the salt-forming acid (log P_OCT^acid) and the melting point (mp) of the drug salt, thereby enabling the derivation of the approximate pH_max value from the predicted pS₀.

Investigations on the Solubility of Vortioxetine Based on X-ray Structural Data and Crystal Contacts

Investigation on the solid-state pharmaceutical chemistry has been known as an intriguing strategy to not only modify the physicochemical properties of drugs but also expand the solid form landscape. Vortioxetine (VOT) is an effective but poorly soluble antidepressant. To improve the solubility of vortioxetine and expand possible solid forms, in this paper, four novel solid forms of vortioxetine with dihydroxybenzoic acids (VOT-23BA, VOT-24BA-TOL, VOT-25BA, and VOT-26BA, 23BA = 2,3-dihydroxybenzoic acid, 24BA = 2,4-dihydroxybenzoic acid, 25BA = 2,5-dihydroxybenzoic acid, 26BA = 2,6-dihydroxybenzoic acid, and TOL = toluene) were synthesized first by a solvent evaporation method and then characterized by single-crystal X-ray diffraction (SCXRD), thermal, and XRD techniques. VOT-24BA-TOL, VOT-25BA, and VOT-26BA, showed similar [2+2] tetrameric R 4 4 (12) hydrogen bonds by acid-piperazine heterosynthon. In the VOT-23BA-H2O salt, the VOT cation and 23BA anion interacted through protonated piperazine-hydroxyl N-H···O hydrogen bonds, not protonated piperazine-deprotonated carboxylic acid N-H···O hydrogen bonds. Solubility studies were carried out in purified water and it was found that the VOT-23BA-H2O, VOT-25BA, and VOT-26BA salts exhibited an increase in water compared to pure VOT. The solubility of the stabilized salt formations followed the order of VOT-25BA > VOT-26BA > VOT-23BA-H2O in purified water.

Application of the solubility parameter concept to assist with oral delivery of poorly water-soluble drugs – a PEARRL review

Objectives

Solubility parameters have been used for decades in various scientific fields including pharmaceutics. It is, however, still a field of active research both on a conceptual and experimental level. This work addresses the need to review solubility parameter applications in pharmaceutics of poorly water-soluble drugs.

Key findings

An overview of the different experimental and calculation methods to determine solubility parameters is provided, which covers from classical to modern approaches. In the pharmaceutical field, solubility parameters are primarily used to guide organic solvent selection, cocrystals and salt screening, lipid-based delivery, solid dispersions and nano- or microparticulate drug delivery systems. Solubility parameters have been applied for a quantitative assessment of mixtures, or they are simply used to rank excipients for a given drug.

Summary

In particular, partial solubility parameters hold great promise for aiding the development of poorly soluble drug delivery systems. This is particularly true in early-stage development, where compound availability and resources are limited. The experimental determination of solubility parameters has its merits despite being rather labour-intensive because further data can be used to continuously improve in silico predictions. Such improvements will ensure that solubility parameters will also in future guide scientists in finding suitable drug formulations.

Characterization of systems with amino-acids and oligosaccharides as modifiers of biopharmaceutical properties of furosemide

Furosemide is the most commonly prescribed diuretic drug in spite of its suboptimal biopharmaceutical properties. In this work, the addition of different amino-acids was studied with the aim of selecting an enhancer of the furosemide solubility. The best results were obtained with arginine. Also, binary (furosemide:arginine) and ternary (furosemide:arginine:β-cyclodextrin and furosemide:arginine:maltodextrin) systems were prepared by the kneading method and they were compared with their corresponding physical mixtures. These new systems were characterized by Fourier transform infrared and Raman spectroscopy, X-ray powder diffractometry, scanning electron microscopy, thermogravimetric analysis, and differential scanning calorimetry. In addition, dissolution studies were performed in simulated gastric fluid. The best results in relation to improving biopharmaceutical properties were obtained with a binary combination of furosemide and arginine, demonstrating that this system could result in a suitable candidate for the development of a promising pharmaceutical formulation of the drug.

Two novel cocrystals of lamotrigine with isomeric bipyridines and in situ monitoring of the cocrystallization

Crystal engineering strategy was applied to develop new solid forms of lamotrigine. Two novel cocrystals of lamotrigine forming with 4,4'-bipyridine (2:1) and 2,2'-bipyridine cocrystal (1:1.5) were successfully obtained by neat grinding and liquid assisted grinding. The novel cocrystals were fully characterized and confirmed by X-ray diffraction, thermal and spectroscopic analysis. DXRxi Raman microscope was also used to identify the cocrystals. The factors such as solvent and the structure of coformers which influenced the cocrystal formation were discussed. Furthermore, the novel cocrystals were both obtained by slurry crystallization. Process analytical technologies including focused beam reflectance measurement and attenuated total reflectance Fourier Transform Infrared were applied to investigate the cocrystallization process and the mechanism. HPLC analysis showed that the dissolution rate and the solubility of the two novel cocrystals were both improved.

Improving solubility and avoiding hygroscopicity of tetrahydroberberine by forming hydrochloride salts by introducing solvents: [HTHB]Cl, [HTHB]Cl·CH3OH and [HTHB]Cl·CH3COOH

Improving the solubility of tetrahydroberberine by forming hydrochloride salts and avoiding the hygroscopicity.