Solubility Determination from Clear Points upon Solvent Addition

Solubility of Organic Salts in Solvent-Antisolvent Mixtures: A Combined Experimental and Molecular Dynamics Simulations Approach

We combine molecular dynamics simulations with experiments to estimate solubilities of an organic salt in complex growth environments. We predict the solubility by simulations of the growth and dissolution of ions at the crystal surface kink sites at different solution concentrations. Thereby, the solubility is identified as the solution's salt concentration, where the energy of the ion pair dissolved in solution equals the energy of the ion pair crystallized at the kink sites. The simulation methodology is demonstrated for the case of anhydrous sodium acetate crystallized from various solvent-antisolvent mixtures. To validate the predicted solubilities, we have measured the solubilities of sodium acetate in-house, using an experimental setup and measurement protocol that guarantees moisture-free conditions, which is key for a hygroscopic compound like sodium acetate. We observe excellent agreement between the experimental and the computationally evaluated solubilities for sodium acetate in different solvent-antisolvent mixtures. Given the agreement and the rich data the simulations produce, we can use them to complement experimental tasks, which in turn will reduce time and capital in the design of complicated industrial crystallization processes of organic salts.

Direct Image Feature Extraction and Multivariate Analysis for Crystallization Process Characterization

Small-scale crystallization experiments (1-8 mL) are widely used during early-stage crystallization process development to obtain initial information on solubility, metastable zone width, as well as attainable nucleation and/or growth kinetics in a material-efficient manner. Digital imaging is used to monitor these experiments either providing qualitative information or for object detection coupled with size and shape characterization. In this study, a novel approach for the routine characterization of image data from such crystallization experiments is presented employing methodologies for direct image feature extraction. A total of 80 image features were extracted based on simple image statistics, histogram parametrization, and a series of targeted image transformations to assess local grayscale characteristics. These features were utilized for applications of clear/cloud point detection and crystal suspension density prediction. Compared to commonly used transmission-based methods (mean absolute error 8.99 mg/mL), the image-based detection method is significantly more accurate for clear and cloud point detection with a mean absolute error of 0.42 mg/mL against a manually assessed ground truth. Extracted image features were further used as part of a partial least-squares regression (PLSR) model to successfully predict crystal suspension densities up to 40 mg/mL (R ² > 0.81, Q ² > 0.83). These quantitative measurements reliably provide crucial information on composition and kinetics for early parameter estimation and process modeling. The image analysis methodologies have a great potential to be translated to other imaging techniques for process monitoring of key physical parameters to accelerate the development and control of particle/crystallization processes.

An industrial perspective on co-crystals: Screening, identification and development of the less utilised solid form in drug discovery and development

Active pharmaceutical ingredients are commonly marketed as a solid form due to ease of transport, storage and administration. In the design of a drug formulation, the selection of the solid form is incredibly important and is traditionally based on what polymorphs, hydrates or salts are available for that compound. Co-crystals, another potential solid form available, are currently not as readily considered as a viable solid form for the development process. Even though co-crystals are gaining an ever-increasing level of interest within the pharmaceutical community, their acceptance and application is still not as standard as other solid forms such as the ubiquitous pharmaceutical salt and stabilised amorphous formulations. Presented in this chapter is information that would allow for a co-crystal screen to be planned and conducted as well as scaled up using solution and mechanochemistry based methods commonly employed in both the literature and industry. Also presented are methods for identifying the formation of a co-crystal using a variety of analytical techniques as well as the importance of confirming the formation of co-crystals from a legal perspective and demonstrating the legal precedent by looking at co-crystalline products already on the market. The benefits of co-crystals have been well established, and presented in this chapter are a selection of examples which best exemplify their potential. The goal of this chapter is to increase the understanding of co-crystals and how they may be successfully exploited in early stage development.

Automated solubility screening platform using computer vision

Solubility screening is an essential, routine process that is often labor intensive. Robotic platforms have been developed to automate some aspects of the manual labor involved. However, many of the existing systems rely on traditional analytic techniques such as high-performance liquid chromatography, which require pre-calibration for each compound and can be resource consuming. In addition, automation is not typically end-to-end, requiring user intervention to move vials, establish analytical methods for each compound and interpret the raw data. We developed a closed-loop, flexible robotic system with integrated solid and liquid dosing capabilities that relies on computer vision and iterative feedback to successfully measure caffeine solubility in multiple solvents. After initial researcher input (<2 min), the system ran autonomously, screening five different solvent systems (20-80 min each). The resulting solubility values matched those obtained using traditional manual techniques.

•

We demonstrate a modular, closed-loop robotic platform for solubility screening

•

Automated solvent titration is informed by computer vision and turbidity monitoring

•

No human intervention or HPLC analysis is required during the experimental loop

•

Solubility values obtained by the system match those obtained via traditional methods

Solubility Measurements and Correlation of MBQ-167 in Neat and Binary Solvent Mixtures

MBQ-167 is a novel, small-molecule dual inhibitor of Rac and Cdc42, small GTPases that are involved in cytoskeletal organization, cell cycle progression, and cell migration. In an in vivo mouse model, MBQ-167 has been shown to significantly reduce mammary tumor growth and metastasis and is currently undergoing preclinical studies for the treatment of metastatic cancer. To date, no solubility data have been reported for this compound. For this reason, the present study aims to determine the solubility of this compound in eight neat solvents (acetonitrile, 1-butanol, 2-butanol, ethanol, ethyl acetate, methanol, 1-propanol, and 2-propanol) and two binary solvent mixtures [ethyl acetate (2) + heptane (3) and ethanol (2) + water (3)] between the temperatures of 278.15 and 333.15 K. The results obtained employing the polythermal method show that the solubility of MBQ-167 increases with an increase in temperature in all neat solvents used within this study. Moreover, in the two binary solvent mixtures, the solubility of this compound increases with increasing temperature and decreases with an increasing mass fraction of the antisolvent (heptane or water). The experimental solubility data were correlated using the modified Apelblat and λh model equations. The predicted solubility data acquired from the Apelblat and λh model equations correlate well with the experimental solubility data as indicated by the low ARD % (≤1.8304 and ≤6.5366, respectively). No solvent-mediated polymorphic phase transitions were observed while performing the solubility studies, and no other solid forms were detected after the recrystallization in the solvents and solvent mixtures. The solubility data determined here can offer pathways to develop pharmaceutical crystallization processes that can further the translation of MBQ-167 into a clinical setting.

Ternary Phase Diagram Development and Production of Niclosamide-Urea Co-Crystal by Spray Drying

In this work, a ternary phase diagram was developed for a Niclosamide-urea co-crystal (NCS-UR) in isopropanol (IPA) using a combination of slurry and solvent addition methods. The ternary phase diagram showed that solubility of Niclosamide and urea differed by an order of magnitude in IPA, leading to an incongruently saturating system. Spray drying was explored as a method to generate NCS-UR. Co-crystals with small, uniform particle size were successfully prepared by spray drying from equimolar solutions with yield up to 73%. Co-crystals were phase pure by X-ray powder diffraction (XRPD) and differential scanning calorimetry (DSC) from all conditions explored. Somewhat similar particles were obtained at inlet temperature of 70 °C (mean size of 2.0 μm) compared to 85 °C (2.8-3.4 μm). Based on the TPD, isolating phase pure co-crystal through solution crystallization in IPA would require excess urea. However, spray drying did not require excess co-former. The in-vitro solubility of NCS-UR was compared to anhydrous NCS in biorelevant media. NCS-UR did not give improvement in solubility at 1 h or 24 h. Overall, this work showed that spray drying is a feasible process for preparing phase pure co-crystals for an incongruently saturating system and simultaneously generating micron size particles.

Formulation of Tioconazole and Melaleuca alternifolia Essential Oil Pickering Emulsions for Onychomycosis Topical Treatment

Onychomycosis is a disease that affects many adults, whose treatment includes both oral and topical therapies with low cure rates. The topical therapy is less effective but causes fewer side effects. This is why the development of an effective, easy to apply formulation for topical treatment is of high importance. We have used a nanotechnological approach to formulate Pickering emulsions (PEs) with well-defined properties to achieve site-specific delivery for antifungal drug combination of tioconazole and Melaleuca alternifolia essential oil. Silica nanoparticles with tailored size and partially hydrophobic surface have been synthesized and used for the stabilization of PEs. In vitro diffusion studies have been performed to evaluate the drug delivery properties of PEs. Ethanolic solution (ES) and conventional emulsions (CE) have been used as reference drug formulations. The examination of the antifungal effect of PEs has been performed on Candida albicans and Trichophyton rubrum as main pathogens. In vitro microbiological experimental results suggest that PEs are better candidates for onychomycosis topical treatment than CE or ES of the examined drugs. The used drugs have shown a significant synergistic effect, and the combination with an effective drug delivery system can result in a promising drug form for the topical treatment of onychomycosis.

In the Context of Polymorphism: Accurate Measurement, and Validation of Solubility Data

Solubility measurements for polymorphic compounds are often accompanied by solvent-mediated phase transformations. In this study, solubility measurements from undersaturated solutions are employed to investigate the solubility of the two most stable polymorphs of flufenamic acid (FFA forms I and III), tolfenamic acid (TA forms I and II), and the only known form of niflumic acid (NA). The solubility was measured from 278.15 to 333.15 K in four alcohols of a homologous series (methanol, ethanol, 1-propanol, n-butanol) using the polythermal method. It was established that the solubility of these compounds increases with increasing temperature. The solubility curves of FFA forms I and III intersect at ~315.15 K (42 °C) in all four solvents, which represents the transition temperature of the enantiotropic pair. In the case of TA, the solubility of form II could not be reliably obtained in any of the solvents because of the fast solvent-mediated phase transformation. The solubility of the only known form of NA was also determined, and no other polymorphs of NA were observed. The experimental solubility data of FFA (forms I and III), TA (form I), and NA in these four solvents was correlated using the modified Apelblat and λh model equations. The correlated and experimentally determined solubility data obtained serves to (i) guide the accurate determination of the solubility for polymorphic compounds, (ii) assess the role of the solvent in mediating transformations, and (iii) provide a route to engineer advanced crystallization processes for these pharmaceutical compounds.

Solubility Determination and Correlation of Warfarin Sodium 2‑Propanol Solvate in Pure, Binary, and Ternary Solvent Mixtures

The solubility of warfarin sodium isopropanol solvate (WS·IPA), a widely used anticoagulant, was determined at temperatures ranging from 278.15 to 333.15 K in four pure solvents (acetone, ethanol, IPA, and water), five binary solvent mixtures (IPA + acetone, IPA + ethanol, IPA + water, IPA + heptane, and IPA + hexane), and five ternary solvent mixtures (IPA + acetone + heptane, IPA + acetone + hexane, IPA + ethanol + heptane, IPA + ethanol + hexane, and IPA + water + heptane) using the polythermal method. It was demonstrated that the solubility of WS·IPA increases with increasing temperature in the pure solvents and at constant solvent composition in the solvent mixtures. In addition, the solubility of WS·IPA in IPA increases with increasing content of acetone, ethanol, and water, which act as cosolvents, and decreases with increasing content of heptane and hexane, which act as antisolvents. The experimental solubility data of WS·IPA in pure solvents and binary and ternary solvent mixtures were correlated using the modified Apelblat and λh model equations. The correlated solubility data agree with the experimental data based on the relative deviation and the average relative deviation (ARD %) values. Thus, the correlated and experimentally derived solubility data of WS·IPA provide a pathway to engineer advanced pharmaceutical crystallization processes for WS·IPA.

Measurement and Correlation of the Solubility of 5-Fluorouracil in Pure and Binary Solvents

The solubility of 5-ffuorouracil (5-FU), a widely used chemotherapeutic agent to treat solid tumors, which include colorectal, head and neck, breast, and lung cancer, was determined at temperatures ranging from 278.15 to 333.15 K in 11 pure solvents and binary water + ethanol solvent mixtures using the polythermal method. It was demonstrated that the solubility of 5-FU increases with increasing temperature in the pure solvents and at constant solvent composition in the solvent mixtures. Moreover, the solubility of 5-FU in the solvent mixtures exceeds its solubility in pure water and ethanol. The experimental solubility data of 5-FU in the pure solvents and solvent mixtures were correlated using the modified Apelblat and λh model equations. The predicted solubility data obtained agree with the experimental data based on the calculated relative deviation (RD) and the average relative deviation (ARD%) values. The selected solvents are categorized as either Class 2 or 3 (less toxic and lower risk to human health) solvents, and hence the correlated and experimentally derived solubility data of 5-FU presented provide a pathway to develop and engineer enhanced pharmaceutical processes and products based on this compound.