Crystal Engineering of Pharmaceutical Cocrystals in the Discovery and Development of Improved Drugs

The subject of crystal engineering started in the 1970s with the study of topochemical reactions in the solid state. A broad chemical definition of crystal engineering was published in 1989, and the supramolecular synthon concept was proposed in 1995 followed by heterosynthons and their potential applications for the design of pharmaceutical cocrystals in 2004. This review traces the development of supramolecular synthons as robust and recurring hydrogen bond patterns for the design and construction of supramolecular architectures, notably, pharmaceutical cocrystals beginning in the early 2000s to the present time. The ability of a cocrystal between an active pharmaceutical ingredient (API) and a pharmaceutically acceptable coformer to systematically tune the physicochemical properties of a drug (i.e., solubility, permeability, hydration, color, compaction, tableting, bioavailability) without changing its molecular structure is the hallmark of the pharmaceutical cocrystals platform, as a bridge between drug discovery and pharmaceutical development. With the design of cocrystals via heterosynthons and prototype case studies to improve drug solubility in place (2000-2015), the period between 2015 to the present time has witnessed the launch of several salt-cocrystal drugs with improved efficacy and high bioavailability. This review on the design, synthesis, and applications of pharmaceutical cocrystals to afford improved drug products and drug substances will interest researchers in crystal engineering, supramolecular chemistry, medicinal chemistry, process development, and pharmaceutical and materials sciences. The scale-up of drug cocrystals and salts using continuous manufacturing technologies provides high-value pharmaceuticals with economic and environmental benefits.

A study to discover novel pharmaceutical cocrystals of pelubiprofen with a machine learning approach compared

Pelubiprofen (PF), a biopharmaceutical classification system (BCS) class II non-steroidal anti-inflammatory drug, has been on the market only in its crystalline form. To discover the first cocrystal form(s) of the...

Polymorphism and distinct physicochemical properties of the phloretin–nicotinamide cocrystal

Two novel polymorphs of a cocrystal involving phloretin and nicotinamide were identified and found where the polymorphs show distinct crystal structures, photoluminescence, and dissolution rates.

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.

Solution Cocrystallization: A Scalable Approach for Cocrystal Production

With an increasing interest in cocrystals due to various advantages, demand for large-scale cocrystallization techniques is rising. Solution cocrystallization is a solvent-based approach that utilizes several single-component crystallization concepts as well as equipment for generating cocrystals. Solution-based techniques can produce cocrystals with reasonable control on purity, size distribution, morphology, and polymorphic form. Many of them also offer a scalable solution for the industrial production of cocrystals. However, the complexity of the thermodynamic landscape and the kinetics of cocrystallization offers fresh challenges which are not encountered in single component crystallization. This review focuses on the recent developments in different solution cocrystallization techniques for the production of pharmaceutically relevant cocrystals. The review consists of two sections. The first section describes the various solution cocrystallization methods, highlighting their benefits and limitations. The second section emphasizes the challenges in developing these techniques to an industrial scale and identifies the major thrust areas where further research is required.

Heteroleptic Cu(ii) saccharin complexes: intriguing coordination modes and properties

A new Cu(ii) O,S-coordinated saccharinate complex is reported, showing crystallochromism and NIR emission in the solid state, in silico ligand exchange reactions with N/S-donor ligands in the presence of model molecules mimicking biological targets.

A cocrystal of L-ascorbic acid with picolinic acid: the role of O-H...O, N-H...O and C-H...O hydrogen bonds and L-ascorbic acid conformation in structure stabilization

A new 1:1 cocrystal (L-Asc-Pic) of L-ascorbic acid (vitamin C) with picolinic acid was prepared as a powder and as single crystals. The crystal structure was solved and refined from single-crystal X-ray diffraction (SCXRD) data collected at 293 (2) and 100 (2) K. The samples of the L-Asc-Pic cocrystal were characterized by elemental (HCNS) analysis and titrimetric methods, TG/DTG/DSC, and IR and Raman spectroscopy. The asymmetric unit comprises a picolinic acid zwitterion and an L-ascorbic acid molecule. The stabilization energy of intermolecular interactions involving hydrogen bonds, the vibrational spectrum and the energies of the frontier molecular orbitals were calculated using the GAUSSIAN09 and the CrystalExplorer17 programs. The charge distribution on the atoms of the L-Asc-Pic cocrystal, L-ascorbic acid itself and its 12 known cocrystals (structures from Version 5.40 of the Cambridge Structural Database) were calculated by the methods of Mulliken, Voronoi and Hirshfeld charge analyses (ADF) at the bp86/TZ2P+ level of theory. The total effective charges and conformations of the L-ascorbic acid molecules in the new and previously reported cocrystals were compared with those of the two symmetry-independent molecules in the crystals of L-ascorbic acid. A correlation between molecular conformation and its effective charge is discussed.

Hot stage microscopy and its applications in pharmaceutical characterization

Hot stage microscopy (HSM) is a thermal analysis technique that combines the best properties of thermal analysis and microscopy. HSM is rapidly gaining interest in pharmaceuticals as well as in other fields as a regular characterization technique. In pharmaceuticals HSM is used to support differential scanning calorimetry (DSC) and thermo-gravimetric analysis (TGA) observations and to detect small changes in the sample that may be missed by DSC and TGA during a thermal experiment. Study of various physical and chemical properties such sample morphology, crystalline nature, polymorphism, desolvation, miscibility, melting, solid state transitions and incompatibility between various pharmaceutical compounds can be carried out using HSM. HSM is also widely used to screen cocrystals, excipients and polymers for solid dispersions. With the advancements in research methodologies, it is now possible to use HSM in conjunction with other characterization techniques such as Fourier transform infrared spectroscopy (FTIR), DSC, Raman spectroscopy, scanning electron microscopy (SEM) which may have additional benefits over traditional characterization techniques for rapid and comprehensive solid state characterization.

MCR-ALS analysis of IR spectroscopy and XRD for the investigation of ibuprofen - nicotinamide cocrystal formation

To improve aqueous solubility, a poorly water-soluble active ingredient is classically combined with a conformer to form cocrystals. Hot melt extrusion is one preparation method for the formation of cocrystal solids. The aim of our study was to determine the optimal temperature conditions for the formation of ibuprofen and nicotinamide cocrystals using real-time infrared (IR) and X-ray diffraction (XRD) measurements. IR spectra and XRD patterns were subjected to multivariate curve resolution alternating least squares (MCR-ALS) analysis and decomposed into several components. Each component was descriptive of a specific step in the formation of the cocrystal. Cocrystal formation was followed by a separation phase between amorphous ibuprofen and crystalline nicotinamide. Our results suggest that, when using the hot melt exclusion method, careful consideration should be made towards optimizing processing temperatures in order to prevent amorphization and promote control over the process of cocrystal formation.

Feasibility of electrospray deposition for rapid screening of the cocrystal formation and single step, continuous production of pharmaceutical nanococrystals

OBJECTIVES

This study employed electrospray deposition (ESD) for simultaneous synthesis and particle engineering of cocrystals.

SIGNIFICANCE

Exploring new methods for the efficient production of cocrystals with desired particle properties is an essential demand.

METHODS

The possibility of cocrystal formation by ESD was examined for indomethacin-saccharin, indomethacin-nicotinamide, naproxen-nicotinamide, and naproxen-iso-nicotinamide cocrystals. Solutions of the drug and coformer at stoichiometric ratios were sprayed to a high electric field which caused rapid evaporation of the solvent and the formation of fine particles. The phase purity, size, and morphology of products were compared with reference cocrystals. Experiments were performed to evaluate the effects of stoichiometric ratio, concentration and solvent type on the cocrystal formation. Physical stability and dissolution properties of the electrosprayed cocrystals were also compared with reference cocrystals.

RESULTS

ESD was found to be an efficient and rapid method to produce cocrystals for all studied systems other than indomethacin-nicotinamide. Pure cocrystals only formed at a specific drug:coformer ratio. The solvent type has a weak effect on the cocrystal formation and morphology. Electrosprayed cocrystals exhibited nano to micrometer sizes with distinct morphologies with comparable physical stability with reference cocrystals. Nanococrystals of indomethacin-saccharin with a mean size of 219 nm displayed a threefold higher dissolution rate than solvent evaporated cocrystal.

CONCLUSION

ESD successfully was utilized to produce pure cocrystals of poorly soluble drugs with different morphologies and sizes ranging from nano to micrometer sizes in one step. This study highlighted the usefulness of ESD for simultaneous preparation and particle engineering of pharmaceutical cocrystals.

Tuning mechanical behaviour by controlling the structure of a series of theophylline co-crystals

Six new co-crystals of theophylline with some substituted carboxylic acids, amides and one active pharmaceutical ingredient (API) have been synthesized.

Selection of effective cocrystals former for dissolution rate improvement of active pharmaceutical ingredients based on lipoaffinity index

New theoretical screening procedure was proposed for appropriate selection of potential cocrystal formers possessing the ability of enhancing dissolution rates of drugs. The procedure relies on the training set comprising 102 positive and 17 negative cases of cocrystals found in the literature. Despite the fact that the only available data were of qualitative character, performed statistical analysis using binary classification allowed to formulate quantitative criterions. Among considered 3679 molecular descriptors the relative value of lipoaffinity index, expressed as the difference between values calculated for active compound and excipient, has been found as the most appropriate measure suited for discrimination of positive and negative cases. Assuming 5% precision, the applied classification criterion led to inclusion of 70% positive cases in the final prediction. Since lipoaffinity index is a molecular descriptor computed using only 2D information about a chemical structure, its estimation is straightforward and computationally inexpensive. The inclusion of an additional criterion quantifying the cocrystallization probability leads to the following conjunction criterions H_mix<-0.18 and ΔLA>3.61, allowing for identification of dissolution rate enhancers. The screening procedure was applied for finding the most promising coformers of such drugs as Iloperidone, Ritonavir, Carbamazepine and Enthenzamide.

A comprehensive spectroscopic study of the polymorphs of diflunisal and their phase transformations

Understanding phase transitions in pharmaceutical materials is of vital importance for drug manufacturing, processing and storage. In this paper we have carried out comprehensive high-resolution spectroscopic studies on the polymorphs of the non-steroidal anti-inflammatory drug diflunisal that has four known polymorphs, forms I-IV (FI-FIV), three of which have known crystal structures. Phase transformations during milling, heating, melt-quenching and exposure to high relative humidity were investigated using Raman and terahertz spectroscopy in combination with differential scanning calorimetry and X-ray powder diffraction. The observed phase transformations indicate the stability order FIII>FI>FII, FIV. Furthermore, crystallization experiments from the gas phase and from solution by fast evaporation of different solvents were carried out. Fast evaporation of an ethanolic solution below 70°C was identified as a reliable and convenient method to obtain the somewhat elusive FII in bulk quantities.

Thermodynamic Investigation of Carbamazepine-Saccharin Co-Crystal Polymorphs

Polymorphism in active pharmaceutical ingredients can be regarded as critical for the potential that crystal form can have on the quality, efficacy, and safety of the final drug product. The current contribution aims to characterize thermodynamic interrelationship of a dimorphic co-crystal, FI and FII, involving carbamazepine (CBZ) and saccharin (SAC) molecules. Supramolecular synthesis of CBZ-SAC FI and FII has been performed using thermokinetic methods and systematically characterized by differential scanning calorimetry, powder X-ray diffraction, solubility, and slurry measurements. According to the heat of fusion rule by Burger and Ramberger, FI (ΔH_fus = 121.1 J/g; melting point, 172.5°C) and FII (ΔH_fus = 110.3 J/g; melting point, 164.7°C) are monotropically related. The solubility and van't Hoff plot results suggest FI stable and FII metastable forms. This study reveals that CBZ-SAC co-crystal phases, FI or FII, could be stable to heat-induced stresses; however, FII converts to FI during solution-mediated transformation.

Studies on Aspirin Crystals Generated by a Modified Vapor Diffusion Method

The objectives of the current investigation were (1) to study the influence of selected two different non-solvents (diethylether and dichloromethane) on the drug crystal formation of a model drug, aspirin (ASP-I) by the modified vapor diffusion method and (2) to characterize and compare the generated crystals (ASP-II and ASP-III) using different analytical techniques with that of unprocessed ASP-I. When compared to the classical vapor diffusion method which consumes about 15 days to generate drug crystals, the modified method needs only 12 h to get the same. Fourier transform-infrared spectroscopy (FT-IR) reveals that the internal structures of ASP-II and ASP-III crystals were identical when compared with ASP-I. Although the drug crystals showed a close similarity in X-ray diffraction patterns, the difference in the relative intensities of some of the diffraction peaks (especially at 2θ values of around 7.7 and 15.5) could be attributed to the crystal habit or crystal size modification. Similarly, the differential scanning calorimetry (DSC) study speculates that only the crystal habit modifications might occur but without involving any change in internal structure of the generated drug polymorphic form I. This is further substantiated from the scanning electron microscopy (SEM) pictures that indicated the formation of platy shape for the ASP-II crystals and needle shape for the ASP-III crystals. In addition, the observed slow dissolution of ASP crystals should indicate polymorph form I formation. Thus, the modified vapor diffusion method could routinely be used to screen and legally secure all possible forms of other drug entities too.

Direct co-crystal assembly from synthesis to co-crystallization

The ternary phase diagram for the direct assembly of a 2 : 1 co-crystal of benzoic acid–sodium benzoate was illustrated. Benzoic acid acted as a reactant and underwent neutralization to produce sodium benzoate. The excessive amount of benzoic acid as an API can interact with the product, sodium benzoate as another API, to undertake API–API co-crystallization by cooling.

Polymorphism in anti-hyperammonemic agent N-carbamoyl-l-glutamic acid

Solid form screen of anti-hyperammonemic drug carglumic acid (CGA) resulted in two polymorphs, Form-I and Form-II. The crystal structure of Form-I is sustained by an acid catemer synthon, whereas Form-II has an acid–amide heterosynthon. Slurry grinding, thermal stress, stability measurements, and DVS analysis confirm the thermodynamic stability of Form-I.

Drug–drug salt forms of ciprofloxacin with diflunisal and indoprofen

Two drug–drug salt forms of ciprofloxacin (CIP) with diflunisal (CIP/DIF) and indoprofen (CIP/INDP/H₂O) were synthesized and characterized by PXRD, FTIR, DSC and TGA. Crystal structure determination revealed the transferability of the robust synthon 1 in the two novel salt forms.

Crystal engineering of homochiral molecular organization of naproxen in cocrystals and their thermal phase transformation studies

Crystal engineering principles were used to produce the homochiral R- and S-chains of naproxen (NPX) by cocrystallization with bipyridine (BPY) and piperazine (PIZ).

Tautomeric preference in polymorphs and pseudopolymorphs of succinylsulfathiazole: fast evaporation screening and thermal studies

We employ a fast evaporation crystallization method to identify two new solvates from acetone and tetrahydrofuran of succinylsulfathiazole (SST), as well as determined single crystal structures for previous unsolved forms. SST adopts exclusively the imidine form in all its crystalline forms.

Anti-solvent co-crystallization of carbamazepine and saccharin

The co-crystal approach has been investigated extensively over the past decade as one of the most promising methods to enhance the dissolution properties of insoluble drug substances. Co-crystal powders are typically produced by mechanical grinding (neat or wet) or a solution method (evaporation or cooling). In this study, high-purity carbamazepine-saccharin (CBZ-SAC) co-crystals were manufactured by a novel method, anti-solvent addition. Among various solvents, methanol was found to perform well with water as the anti-solvent for the co-crystallization of CBZ and SAC. When water was added to the methanol solution of CBZ and SAC at room temperature under agitation, nucleation of CBZ-SAC co-crystals occurred within 2-3 min. Co-crystallization was complete after 30 min, giving a solid yield as high as 84.5% on a CBZ basis. The effects of initial concentrations, focusing on the SAC/CBZ ratio, were examined to establish optimal conditions. The whole anti-solvent co-crystallization process was monitored at-line via ATR-FTIR analysis of regularly sampled solutions. The nucleation and crystal growth of CBZ-SAC co-crystals were detected by a significant increase in absorption in the range of 2400-2260 cm(-1), associated with the formation of hydrogen bonds between the carbonyl group in CBZ and the N-H of SAC. When CBZ hydrates were formed as impurities during anti-solvent co-crystallization, the hydrogen bonding between methanol and water was reduced greatly, primarily due to the incorporation of water molecules into the CBZ crystal lattice. In conclusion, an anti-solvent approach can be used to produce highly pure CBZ-SAC co-crystal powders with a high solid yield.

Cocrystallization for successful drug delivery

INTRODUCTION

Cocrystallization is an effective crystal engineering approach for modifying the crystal structure and properties of drugs. The number of examples of solving drug formulation and manufacture problems by cocrystallization is rapidly growing. An updated review that systematically examines the cocrystal research in the context of drug delivery is timely and valuable.

AREAS COVERED

Topics covered in this review include nature of cocrystal, impact of cocrystallization on key pharmaceutical properties (both enhancement and deterioration), cocrystal preparation method and future directions in this field. The focus of this review is on the crystal engineering and pharmaceutical literature in the last 5 years. However, classical literature is also examined when relevant.

EXPERT OPINION

The most effective cocrystal research relies on both in-depth understanding of structure-property relationship and efficient preparation of desired cocrystals. The future cocrystal research will see growth in the areas of designing ternary and higher-order structures, cocrystals between neutral and ionic species, cocrystal polymorphism, cocrystal glasses and thermodynamics of cocrystallization.

Pharmaceutical cocrystals and poorly soluble drugs

In recent years cocrystal formation has emerged as a viable strategy towards improving the solubility and bioavailability of poorly soluble drugs. In this review the success of numerous pharmaceutical cocrystals for the improvement of the solubility and dissolution rates of poorly soluble drugs is demonstrated using various examples taken from the literature. The role of crystal engineering principles in the selection of appropriate coformers and the nature of the supramolecular synthons present within the crystals are described. Evidence for improved animal pharmacokinetic data is given for several systems. A summary is provided of our current understanding of the relationship between cocrystal structure and solution phase interactions on solubility as well as those factors that influence overall cocrystal thermodynamic stability.