Cocrystal solubility product analysis - Dual concentration-pH mass action model not dependent on explicit solubility equations

A comparison of USP 2 and µDISS Profiler™ apparatus for studying dissolution phenomena of ibuprofen and its salts

BACKGROUND

Pharmaceutical salts of poorly soluble drugs typically dissolve faster than their corresponding free acid or base, resulting in supersaturation under some circumstances. The key questions relevant to drug bioavailability "does the salt invoke the supersaturated state?" and, if so, "does precipitation occur?" remain. To answer these questions, different types of dissolution equipment are often used at different stages of the development process.

AIM

To compare the dissolution behaviour of ibuprofen and its sodium and lysine salts in the USP 2 apparatus and the µDISS Profiler™ apparatus. The dissolution, supersaturation of the salt forms and precipitation to the free acid of ibuprofen were characterized along with the dissolution of the free acid form.

METHODS

Media containing different concentrations of the salt-forming counterions - sodium and lysine - were used to investigate the influence of the type of dissolution apparatus used for the study on dissolution, supersaturation and precipitation behaviour.

KEY RESULTS

Supersaturation was observed for both the sodium and lysinate salts of ibuprofen in all USP 2 apparatus and µDISS Profiler™ experiments. However, precipitation tended to be far greater in the µDISS Profiler™ than in the USP 2 apparatus. The difference was most pronounced in pH 4.5 acetate buffer, in which precipitation was observed exclusively in experiments with the µDISS Profiler™.

CONCLUSION

Choice of dissolution apparatus can affect the dissolution/supersaturation/precipitation characteristics of pharmaceutical salts. This has to be carefully taken into account when investigating salts over different stages of pharmaceutical research and development.

New Lidocaine-Based Pharmaceutical Cocrystals: Preparation, Characterization, and Influence of the Racemic vs. Enantiopure Coformer on the Physico-Chemical Properties

This study describes the preparation, characterization, and influence of the enantiopure vs. racemic coformer on the physico-chemical properties of a pharmaceutical cocrystal. For that purpose, two new 1:1 cocrystals, namely lidocaine:dl-menthol and lidocaine:d-menthol, were prepared. The menthol racemate-based cocrystal was evaluated by means of X-ray diffraction, infrared spectroscopy, Raman, thermal analysis, and solubility experiments. The results were exhaustively compared with the first menthol-based pharmaceutical cocrystal, i.e., lidocaine:l-menthol, discovered in our group 12 years ago. Furthermore, the stable lidocaine/dl-menthol phase diagram has been screened, thoroughly evaluated, and compared to the enantiopure phase diagram. Thus, it has been proven that the racemic vs. enantiopure coformer leads to increased solubility and improved dissolution of lidocaine due to the low stable form induced by menthol molecular disorder in the lidocaine:dl-menthol cocrystal. To date, the 1:1 lidocaine:dl-menthol cocrystal is the third menthol-based pharmaceutical cocrystal, after the 1:1 lidocaine:l-menthol and the 1:2 lopinavir:l-menthol cocrystals reported in 2010 and 2022, respectively. Overall, this study shows promising potential for designing new materials with both improved characteristics and functional properties in the fields of pharmaceutical sciences and crystal engineering.

Characterizing the Physicochemical Properties of Two Weakly Basic Drugs and the Precipitates Obtained from Biorelevant Media

Generally, some weakly basic insoluble drugs will undergo precipitate and redissolution after emptying from the stomach to the small intestinal, resulting in the limited ability to predict the absorption characteristics of compounds in advance. Absorption is determined by the solubility and permeability of compounds, which are related to physicochemical properties, while knowledge about the absorption of redissolved precipitate is poorly documented. Considering that biorelevant media have been widely used to simulate gastrointestinal fluids, sufficient precipitates can be obtained in biorelevant media in vitro. Herein, the purpose of this manuscript is to evaluate the physicochemical properties of precipitates obtained from biorelevant media and active pharmaceutical ingredients (API), and then to explore the potential absorption difference between API and precipitates. Precipitates can be formed by the interaction between compounds and intestinal fluid contents, leading to changes in the crystal structure, melting point, and melting process. However, the newly formed crystals have some advantageous properties compared with the API, such as the improved dissolved rate and the increased intrinsic dissolution rate. Additionally, the permeability of some precipitates obtained from biorelevant media was different from API. Meanwhile, the permeability of rivaroxaban and Drug-A was decreased by 1.92-fold and 3.53-fold, respectively, when the experiments were performed in a biorelevant medium instead of a traditional medium. Therefore, the absorption of precipitate may differ from that of API, and the permeability assay in traditional medium may be overestimated. Based on the research results, it is crucial to understand the physicochemical properties of precipitates and API, which can be used as the departure point to improve the prediction performance of absorption.

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.

Is equilibrium slurry pH a good surrogate for solid surface pH during drug dissolution?

The purpose of the present study was to investigate the suitability of equilibrium slurry pH (pH_eq) as a surrogate of solid surface pH during drug dissolution (pH₀). A comprehensive calculation scheme for pH_eq and pH₀ was formalized based on the principle of charge neutrality (equilibrium charge neutrality for pH_eq and charge flux neutrality for pH₀). The formalized scheme was then used to investigate the validity of pH₀ ≈ pH_eq approximation. The approximation of pH₀ ≈ pH_eq was suggested to be accurate for small molecules (ca. MW = 150) in high concentration buffer media (ca. buffer capacity = 30 mM/ΔpH). In addition, it is valid provided no precipitation of its free form for salts (vice versa for free forms) in both the slurry pH measurement and at the dissolving drug surface. The formalized calculation scheme is simple and applicable to free and salt form drugs in unbuffered and buffered media including bicarbonate buffer. The computational expense is very small so that it is applicable to various computer simulations such as biopharmaceutics modeling and simulation.

Disproportionation of Pharmaceutical Salts: pHmax and Phase-Solubility/pH Variance

A multiphasic mass action equilibrium model is used to show that the critical pH in the acid-base disproportionation of a solid salt into its corresponding solid free-base form in aqueous suspensions, widely known as "pH_max", is incompletely interpreted. It is shown that the traditional thermodynamic model does not predict the invariance of pH and solubility during the salt-to-free-base conversion process in an alkalimetric titration. Rather, the conversion entails a range of pH and solubility values, depending on the amount of added excess salt above that needed to form a saturated solution. A more precise definition is proposed for pH_max (pH at the maximum solubility of a eutectic mixture), and three new terms are introduced: pH_min (pH at the minimum solubility of the eutectic mixture), pH_δ (disproportionation invariant pH within the eutectic, i.e., the equilibrium pH of a spontaneously disproportionating salt slurry), and pH_γ (Gibbs pH at which disproportionation yields equimolar amounts of excess salt and excess free-base solids within the eutectic). Two test compounds with reported multiple salts and the free-base solubility values were selected to illustrate the expanded concepts, the bases WR-122455 and RPR-127963. Also, dibasic calcium phosphate was selected as an ionizable test excipient. The salts are designated in the study as μ-type, when they are thermodynamically stable with respect to spontaneous disproportionation in pure water (e.g., WR-122455 salts), and δ-type, when they are predicted to spontaneously disproportionate in pure water (e.g., RPR-127963 salts). In an alkalimetric titration, when an acidified suspension of a salt of a basic molecule is titrated with a strong base (e.g., NaOH), the passage across the eutectic domain (bounded by pH_max and pH_min) is often characterized by (a) minimum in ionic strength either at pH_max (μ-type salt) or pH_δ (δ-type salt) and (b) maximum buffer capacity at pH_γ. When the alkalimetric titration is performed with a large excess of added salt, a wide eutectic domain forms: pH_max and pH_δ remain invariant, but pH_min and pH_γ shift substantially in pH. The acid-base mass action model described here can be useful in predicting the stability of salt formulations in mixtures with excipients that can act as pH modifiers.

Identification of a previously unreported co-crystal form of acetazolamide: a combination of multiple experimental and virtual screening methods

In this work, we demonstrate an approach of trying multiple methods in a more comprehensive search for co-crystals of acetazolamide.

Affinity prediction computations and mechanosynthesis of carbamazepine based cocrystals

A combination of the excess enthalpy with the fusion entropy of the pure coformer is suggested to be of interest for coformers screening in order to form a multicomponent system with a given API (cocrystal/co-amorphous).

Crystal structure of a 1:1 salt of 4-amino-benzoic acid (vitamin B10) with pyrazinoic acid

The title 1:1 salt, C7H8NO2 +·C5H3N2O2 - (systematic name: 4-carb-oxy-anilinium pyrazine-2-carboxyl-ate), was synthesized successfully by slow evaporation of a saturated solution from water-ethanol (1:1 v/v) mixture and characterized by X-ray diffraction (SCXRD, PXRD) and calorimetry (DSC). The crystal structure of the salt was solved and refined at 150 and 293 K. The salt crystallizes with one mol-ecule of 4-amino-benzoic acid (PABA) and one mol-ecule of pyrazinoic acid (POA) in the asymmetric unit. In the crystal, the PABA and POA mol-ecules are associated via COOH⋯Narom heterosynthons, which are connected by N-H⋯O hydrogen bonds, creating zigzag chains. The chains are further linked by N-H⋯O hydrogen bonds and π-π stacking inter-actions along the b axis [centroid-to-centroid distances = 3.7377 (13) and 3.8034 (13) Å at 150 and 293 K, respectively] to form a layered three-dimensional structure.

Practical guidelines for the characterization and quality control of pure drug nanoparticles and nano-cocrystals in the pharmaceutical industry

The number of poorly soluble drug candidates is increasing, and this is also seen in the research interest towards drug nanoparticles and (nano-)cocrystals; improved solubility is the most important application of these nanosystems. In order to confirm the functionality of these nanoparticles throughout their lifecycle, repeatability of the formulation processes, functional performance of the formed systems in pre-determined way and system stability, a thorough physicochemical understanding with the aid of necessary analytical techniques is needed. Even very minor deviations in for example particle size or size deviation in nanoscale can alter the product bioavailability, and the effect is even more dramatic with the smallest particle size fractions. Also, small particle size sets special requirements for the analytical techniques. In this review most important physicochemical properties of drug nanocrystals and nano-cocrystals are presented, suitable analytical techniques, their pros and cons, are described with the extra input on practical point of view.