Chemistry and Modern Techniques of Characterization of Co-Crystals

Abstract

Co-crystals are multicomponent molecular materials held together through non-covalent interactions that have recently attracted the attention of supramolecular scientists. They are the monophasic homogeneous materials where a naturally occurring pharmaceutical active ingredient (API) and a pharmaceutically acceptable co-crystal former are bonded together in a 1:1 via non-covalent forces such as H-bonds, π–π, and van der Waals forces. Co-crystallization is a promising research field, especially for the pharmaceutical industry, due to the enormous potential of improved solubility and bioavailability. Co-crystals are not the only multicomponent molecular materials, as there are many other forms of multicomponent molecular solids such as salts, hydrates, solvates, and eutectics. The formation of co-crystals can roughly be predicted by the value of ∆pKa, that is, if the ∆pKa is more than 3, then this monophasic homogeneous material usually falls in the category of salts, whereas if the ∆pKa is less than 2, then co-crystals are usually observed. A number of methods are available for the co-crystal formation, broadly classified into two classes established on state of formation, that is, solution-based and solid-based co-crystal formation. Similarly, a number of techniques are available for the characterization of co-crystals such as Fourier transforms-infrared spectroscopy, single-crystal and powder X-ray diffraction, etc. In this chapter, we will discuss the available methods for co-crystallization and its characterization.

1. Introduction

Co-crystals are crystalline complexes containing neutral molecular components combined by non-covalent forces forming a crystal framework [1]. Co-crystals are also defined as the combination of ionic or molecular pharmaceutical active ingredient (API) and co-crystal former that are solid at room temperature [2]. A comparison can also be drawn about the selection of salt. In this instance, the pK_a controversy concerns the selection the acid-base duos that combine to form salts. Study displays that a pK_a variance of minimum two digits (in base and acid) is essential to generate a salt that is solid in H₂O. The formation of co-crystals can roughly be predicted by the value of ∆pK_a; that is, if the ∆pK_a is more than 3, then this monophasic homogeneous material usually falls in the category of salts, whereas if the ∆pK_a is less than 2, then co-crystals are usually observed [3]. Accordingly, solvates hydrates, clathrates, or inclusion compounds (host and guest molecules), and pseudopolymorphs are excluded from the definition of co-crystals because all the mentioned species contain a liquid or gas component under ambient conditions, whereas a co-crystal contains only the solid constituents at room temperature. A general representation of various solid-state systems is given in Figure 1 [1].

Figure 1.

Common solid-state systems and their respective components.

Synthesis of co-crystals is a formidable task due to numerous constrictions like the nature of a solvent, the reactants, (1:1) equivalent of the co-crystal former and API, stirring, pH, heat, sort of glassware, etc., which are the effective variables associated with the mechanism of co-crystallization. The question here is how the neutral molecules interact to form a single state of same union (co-crystal) rather than a collection of untainted crystals [4]. The answer to this question is that the force responsible for co-crystal formation is electrostatic in nature; that is, it involves the attraction affinity of negative charge for positive charge. Intermolecular forces like X⋅⋅⋅X interactions (X = F, Cl, Br, I), hydrogen bonding, π⋅⋅⋅π stacking, and van der Waal interactions are involved for the development of co-crystals [5]. Thus, non-covalent interactions are the key factor in designing of co-crystal systems.

Many methods are available for the co-crystal production in which grinding is the pioneer one that was used the first time in 1893 when equal molarities of p-benzoquinone and hydroquinone produced quinhydrone co-crystals [6]. In the interim, numerous new well-defined co-crystals have been formed by both wet grinding and neat methods.

In this chapter, co-crystallization techniques and co-crystallization characterization techniques such as SC-XRD and powder X-ray diffractometry (P-XRD) which are two techniques extensively used for the structure determination of solids having single large crystal and solid in the powder form, respectively, density functional theory (DFT), spectroscopic analysis (UV/IR/FTIR), and morphological (PXRD, SEM, SXRD) and thermal analysis (DSC, TGA) are reviewed.

2. Techniques of co-crystallization

Various methods for co-crystal formation are currently employed, which may be broadly categorized into two classes established on the state of formation: solution-based and solid-based methods [7]. The solution-based techniques utilize large amounts of solvents, which require the separation of solvents after crystallization; on the other hand, solid-based route requires no or limited amount of a solvent [8]. The solution co-crystallization has several advantages over solid-state processes comprising better regulation of crystal properties, higher purity, and greater industrial scale. Solution co-crystallization can completely remove impurities from the crystallized product, helping to restrain challenges to achieve selective polymorphic crystallization. It is also a useful strategy for manufacturing co-crystals on an industrial level as the tools essential for extensive fabrication have been previously widely used in the pharmaceutical, food, and agrochemical manufacturing units. Solution co-crystallization has potential applications in various steps of co-crystal production, from initial screening to scale-up for commercial fabrication [9]. Though the solution co-crystallization is beneficial, solid-state co-crystallization is important from the green chemistry perspective.

2.1 Solution-based co-crystallization of nutraceuticals

Nutraceuticals which are a novel category of mixtures with recognized best-ever protection that can be used as attainable contenders are naturally arising for crystallization in the pharmaceutical industry. In 1989, Nutraceuticals, a terminology devised by DeFelice could be demarcated as a diet (or portion of a diet) that offers health or medical assistance, containing the preclusion or cure of a syndrome. Common types of nutraceuticals comprise polyphenols (e.g., phenolic acids, coumarins, stilbenes, and flavonoids) and vitamins [10]. There exist well-recognized approaches for the preparation of co-crystals, which result in crystals with desirable properties that help to evaluate crystal habit and other characteristics [11]. Isothermal ternary phase diagrams (Figure 2) show co-crystallization stability area when different components of the co-crystal are dissolved in solvents having similar and dissimilar solubility [12].

Figure 2.

Ternary phase diagrams with unlike solubility of components C₁ plus C₂ in the solvent S. Area “a” is for solution; “b” is for the component C₁ and solvent; “c” is for the component C₁ and co-crystal; “d” is for the component C₂ and solvent; “e” is for the component C₂ and co-crystal, and “f” is for the co-crystal.

2.1.1 Evaporative co-crystallization

Co-crystallization is an ordinary evaporation technique for spawning co-crystals, archetypally implemented for procurement of single-crystal co-crystals appropriate for diffraction studies to explicate the co-crystal structure. The procedure embroils the cloud seeding and development of a co-crystal from a solution of equally API and co-crystal former in a solvent, with super-saturation as long as by elimination of the solvent from the solution through vaporization. Distinct co-crystals, or the main part of the crystal sample, are garnered prior the solution vanish to aridness to certify the retrieval of an unsoiled crystal(s). A low degree of desiccation is customarily sought to make sure the development of a large number of minor crystals is disproportionate to an insignificant number of larger crystals. As crystal structure identification is a necessary step in the discovery of novel co-crystal forms, evaporative co-crystallization is evident in the majority of the co-crystal related research papers, and there are countless examples of it in the literature [13].

2.1.2 Cooling co-crystallization

It is performed by decreasing the temperature of the solution. A mixture of components and a solvent is heated to obtain a clear saturated solution and then, the temperature is decreased to get a supersaturated solution, and finally, co-crystals are precipitated out [14]. Cooling of crystals technique was used to formulate co-crystals of nicotinamide:carbamazepine using ethyl alcohol in an excess to create an accessible solution co-crystallization approach. Solvent miscellany, desupersaturation kinetics, and requirement of the thermodynamically stable co-crystal functioning series were used in the design of the procedure, which was displayed throughout to have at 1 L gauge the 90% yield. An analogous methodology was used by Holaň et al. in the formation of citric acid:agomelatine co-crystals, and the effect of freezing level and seed quantity on the crystal proportion dissemination in the concluding yield was evaluated [15].

2.1.3 Reaction co-crystallization

This method is based on the above Figure 2 for components having different solubility in a solvent. A component C₁ is added to the solution of a component C₂ near its saturation to obtain co-crystals [16]. The reaction co-crystallization is used to harvest co-crystals of carbamazepine:saccharin by mixing different starting solutions of both of the preparatory components. The technique was systematized by the ternary stage illustration and demonstrated a resilient functioning series for the co-crystal development and validated the predictable connection in induction period and supersaturation. Nicotinamide:carbamazepine co-crystal production was also implemented by the reaction co-crystallization using open-air conditions [17].

2.1.4 Isothermal slurry conversion

In this methodology, slurry is obtained with the conversion time depending on various factors such as nucleation, growth kinetics, relative concentration of co-former and API, and the resultant solubility [18]. This methodology implicates the formation of the mixture of the API and co-crystal former, generally in a stoichiometric ratio, in a solvent with a compact portion of deposit incessantly enduring in excess of a solvent. Technique, the addition of the API to a mixture of co-crystal former in a solvent, can be also adjusted in functional terms. However, this is a solution-based process, which does not entail the production of an immaculate (completely dissolved) preliminary solution, as is the situation of prior approaches pronounced above. The degree at which the slurry transformation transpires will diverge centered on the solubility, driving force, the comparative amount of the API and co-crystal former, and the cloud seeding and evolutionary kinetics of the system. A kinetic description of the isothermal slurry transformation of arbitrary co-crystals is inadequate. Zhang et al. scrutinized the adaptation time for theophylline to transform into a stoichiometric ratio glutaric acid co-crystal. While the slurry transformation process usually required a more quantity of starting components and will experience some material damage owing to enduring solubility in the solvent, it is considered as one of the best auspicious screening methodologies owing to its extraordinary proficiency [19].

2.2 Solid state co-crystallization

These methods of co-crystallization involve limited or no use of solvent and are therefore regarded as environment friendly, green, and economically viable. Following procedures are adopted in these methods.

2.2.1 Grinding

Grinding may be either neat grinding, that is, dry grinding or solvent-assisted grinding [20]. In dry or neat grinding method, stoichiometric amounts of selected solids for co-crystallization are mechanically (using mechanical force to create supramolecular synthons, that is, involving mechanochemistry, which is considered as eco-friendly route as it avoids the use of solvents) or manually mixed at high pressure while avoiding their melting [21]. A major disadvantage of the dry grinding is the absence of heating stage involved in co-crystallization, which is shown by many studies to be important in the co-crystallization process [22]. On the other hand in the liquid-assisted milling the above shortcoming is overtaken through adding little amount of liquid to the solids for the co-crystallization. The little quantity of solvent performs as a catalyst for the co-crystal development; better results have been obtained through this method [23]. Two carboxylic acid:sulfathiazole co-crystals were formed by milling stoichiometric proportions of sulfathiazole and the needed carboxylic acid in a Retsch blender grind at a frequency of 25 Hz and a temperature not exceeding 37°C for 90 min and it was done [22]. Compared with solution-based approaches, solid milling is associated with higher performance because no product is lost due to its solubility in a solvent. Problems with dry milling can comprise impossibility to form a co-crystal, inadequate modifications to the co-crystal, and crystalline flaws with the presence of possible several amorphous compounds [24].

2.2.2 Contact formation

Co-crystals have been prepared through mixing without using any mechanical force; however, this process requires controlled conditions of temperature and pressure. Various factors affect the formation of co-crystals such as pre-milling, moisture, and size of premixed materials [25, 26]. The extemporaneous production of co-crystals by intermixing an uncontaminated target molecule and co-crystal former using organized conditions has been stated. During co-crystallization, no mechanical forces are employed in this technique. However, in various circumstances, slight milling of untainted constituents separately afore intermixing has been completed. The impact on the co-crystallization rate of pre-milling of the elementary constituents carbamazepine and nicotinamide was described by Rodriguez-Hornedo et al. It was shown that the co-crystallization rate can be increased by using milled components as compared to unmilled components (12 vs. 80 days, correspondingly). Furthermore, the greater co-crystallization degree has been described for the indistinguishable arrangement at maximum temperatures and comparative dampness, nevertheless of the mechanical stimulation [20]. The formation of an isoniazid benzoic acid co-crystal via spontaneous co-crystallization was reported as well [24]. We found that a higher frequency of pre-grinding of uncontaminated components significantly improved the reaction rate [27].

2.2.3 Twin screw extrusion

In twin screw extrusion (TSE) technique, the mixing of materials is carried out in a device known as a twin screw extruder in which milling of the starting materials is carried out below their melting points. Proficient intermixing and high surface interaction and consequently enhanced co-crystal formation are achieved without using the solvent provided by this method [25, 26]. The two co-/counter-rotating screws are accompanied by twin screw units in a solitary barrel. Screw activity offers immediate intermixing and mobility of components sideways the span of the barrel. The materialization of four model co-crystals consumes a 16 mm TSE with four manageable temperature regions as reported by Daurio et al. Theophylline:citric acid, carbamazepine:saccharin, nicotinamide:cinnamic acid, and caffeine:oxalic acid co-crystals were formed by slight grinds of the dry fine particles of both starting components via the extruder. The effect of heat on the alteration to a co-crystal was specific to the co-crystal arrangement described, with no ostensive temperature effect on the co-crystal of carbamazepine:saccharin and robust temperature effect on the co-crystal of cinnamic acid: nicotinamide. In a different investigation, the same authors reported a saccharin:AMG-157 co-crystal formed from twin screw extrusion and solution crystallization. Co-crystals from TSE were presented to have augmented bulk density, surface region, and movement characteristics compared to those prepared from solution crystallization.

2.2.4 Hot melt extrusion (HME)

Hot melt extrusion is a comparatively modern addition to the co-crystal grounding options. Using a warmed screw extruder, this special technique achieves simultaneous softening and combining of the API and co-crystal former. Usually, the initial components are combined in the definite stoichiometric ratio and supplied to the warmed extruder. The complete intermixing of the starting components in the stoichiometric ratio occurs by melting. The co-crystal nucleates amenably in the melt and the uncontaminated co-crystal extrudate is separated from the extruder constantly. The benefits of the technique are as follows: the purging using organic solvents, rapid working times, improved adaptation compared with the solution-based techniques, reduced leftover, and the tools offering itself well to the constant drug manufacturing [8]. Co-crystal formation of carbamazepine:cinnamic acid by a single screw and twin screw extruder is reported by Moradiya et al. Co-crystals prepared by the twin screw extruder presented improved suspension characteristics as compared with those gained via single screw/solution-based procedures. Extrusion melt as an incessant engineering performance was applied to harvest indomethacin:saccharin co-crystals. Their studies demonstrated the three critical process parameters (CPP): temperature profile, screw speed, and forage rate, which are desired in the engineering of good-quality co-crystals. Furthermore, the rate of dissolution of co-crystals is not affected by temperature. However, as was established on Rietveld investigation the co-crystal uniform crystallinity is affected by temperature. It has been shown that in the conditions of unevenness in the crystal-form quality because of the process temperature, the dissolution degree will be affected successively. Moreover, the dissolution profile can be affected by the unit size of co-crystals.

This method helps in avoiding solvents and saves time due to the increased conversion to co-crystals. Besides other benefits of HME, this method falls within the requirement of US FDA practice analytical expertise for controlling, analyzing, and designing the engineering of pharmaceuticals. For example, drugs such as Rezulin, Kaletra, and Norvir have been prepared through this method and got FDA approval [28]. Other benefits include scaling-up of production to an industrial scale and being more economical because it is a continuous process.

2.3 Miscellaneous methods of co-crystal preparation

2.3.1 Heat-induced co-crystallization

Co-crystallization method using heat is a novel method to form co-crystals. It has several advantages compared with the solvent evaporation method such as it does not need any organic solvent and can be used without drug coformer solubility determination that is a time-consuming and laborious work [29].

2.3.2 Spray drying method

Spray drying is an instant and incessant method for solid manufacturing, generating dry fine particles from precursor via a warm air torrent [30, 31]. For drug-coformer different solubility systems, where an uncontaminated co-crystal cannot be designed by the solvent evaporation technique, co-crystallization via spray drying process can be used as an alternative process. Theophylline: nicotinamide, carbamazepine:glutaric acid, caffeine:glutaric acid, and succinic acid:urea co-crystals, as instances of different schemes, their untainted co-crystals cannot be produced by the solvent evaporation process; however, when spray drying process is employed, it efficaciously produces uncontaminated co-crystals. Patil et al. reported the co-crystallization via the spray drying process using carbamazepine and nicotinamide as drug and coformer models respectively in a stoichiometric ratio. Accidentally, nicotinamide:carbamazepine co-crystal produced by the spray drying methodology has an analogous characteristic with a co-crystal produced by the liquid facilitating grinding [32].

2.3.3 Supercritical fluid technology

Materials that have pressure and temperature greater than their critical state (P_c and T_c) are supercritical fluids. The leading objective of supercritical fluid technology is to regulate cloud seeding and crystal development courses. The most conspicuous supercritical fluid used in the pharmaceutical arena is carbon dioxide, with critical temperature and pressure of 31.0°C and 7.39 atm, respectively, because of its non-toxicity, non-flammability, and low-cost properties [22, 33]. Numerous approaches of employing the supercritical fluid carbon dioxide to produce co-crystals are as follows: (1) where CO₂ is used as an anti-solvent known as gas anti-solvent crystallization (GAS). As an example, the SAS (supercritical anti-solvent crystallization) and AAS (atomization and anti-solvent crystallization) methods of supercritical fluid technology were successfully used in the formation of the indomethacin:saccharin co-crystal, (2) where the carbon dioxide is used as a liquid and molecular movement enhancer called co-crystallization with the supercritical solvent (CSS), (3) where CO₂ is utilized as a solvent and molecular movement enhancer called supercritical anti-solvent crystallization (SAS), (4) where CO₂ is used as corsage enhancer or anti-solvent called atomization and anti-solvent crystallization (AAS), (5) where CO₂ is used as corsage enhancer or anti-solvent called supercritical fluid improved atomization (SEA), and (6) where CO₂ is used as a solvent known as rapid expansion of supercritical solutions (RESS) [34].

2.3.4 Laser irradiation

A current technique for co-crystallization described by Titapiwatanakun et al. uses carbon dioxide irradiation treatment for the production of a malonic acid:caffeine (1:2) and oxalic acid:caffeine (1:2) co-crystals. The energy given to the unit throughout irradiation spawns a quick escalation in heat in a short period, triggering the softening of the crystalline compound, monitored by material intermixing, and next prompts recrystallization upon freezing. In this scheme, requirements for a co-crystal former component that can be used for this process are that a co-crystal former is necessarily sublimable, to accelerate a cloud seeding method by the vapor phase [35].

2.3.5 Electrochemically induced co-crystallization

Urbanus et al. reported the perspective of using co-crystallization in combination with electrochemistry to exclude in situ products of acids in formation of a co-crystal system of 3-nitrobenzamide and cinnamic acid. Their results show that electrochemistry is used to attain neutral carboxylic acids by locally altering the pH and inducing limited dynamics of co-crystallization [36].

2.3.6 Acoustic resonant mixing

Acoustic resonant mixing (RAM) is used to combine the API and co-crystal former in the existence of a solvent to produce a co-crystal without any milling media. The supply of mechanical energy into a wet powder mixture is performed acoustically, promoting the complete intermixing of the constituents. Utilizing a laboratory RAM resonant acoustic blender operating at 80−100G and 60 Hz, a series of carbamazepine co-crystals have been efficaciously formed. The co-crystal product has been separated at a wide array of gauges from the lab, 100 mg, 1.5 g, and 22 g, and the equipment seems to hold itself up to scale [37].

2.3.7 Freeze-drying

Freeze drying, in principle recognized as lyophilization, has primarily been employed as dispensing scheme for a range of products, including foodstuff and drugs. The procedure functions to sublimate the component directly from the solid phase to the gas phase by decreasing the ambient pressure to cause the freezing of H₂O within the component also known as sublimation. It has also recently emerged as a viable process for preparing novel solid-state forms of co-crystals. Eddleston et al. reported the preparation of new theophylline:oxalic acid co-crystals by lyophilization. Co-crystallization occurs through an amorphous state that is produced as solvent sublimes throughout lyophilization [38].

2.3.8 Electrospray technology

Electrospray is a method that uses an electric field to create and charge droplets at the same time. In this method, the solutes (substances to be dissolved) are carried by a solution that flows out of a capillary nozzle at a high potential and is subjected to an electric field, which causes the droplets of solution to elongate and create a jet. After the solution jet has been dried, the resulting particles can be collected using a powder collector that has already been loaded with material. Co-crystals of carbamazepine and itraconazole with a variety of coformers were reported as a possible outcome of this procedure by Patil et al. [39].

2.4 Nano co-crystallization

Pharmaceutical nano co-crystals can considerably enhance the delivery qualities of ailing decipherable medicines, which have long posed a considerable issue for the pharmaceutical business. The investigation is into the production of nano co-crystals with high commercial value. Methods such as precipitation, media grinding, and high-pressure homogenization can be used to manufacture nanocrystals on a large scale. When delivered orally, intravenously, pulmonaryly, ocularly, or topically, nanocrystals exhibited the promising therapeutic utility. In addition, nanostructured medicinal molecules can be targeted to specific locations. Usually formation process for the nano-crystals comprising top-down flow process is a ball grinding technique that uses shear forces to produce high-pressure homogeneous nano-size particles. Precipitation is one of the bottom-up procedures that includes the growth [40].

3. Co-crystal characterization

The determination of physicochemical properties of co-crystals is an important part of the research, and therefore, the combination of techniques is employed for the co-crystal characterization, as no single method can fully elucidate their structure and properties [41]. The following characterization techniques for the structure determination are outlined and represented diagrammatically in Figure 3 [30].

Figure 3.

Techniques for characterization of co-crystals.

3.1 Analyzing structure through XRD

Powder-XRD and SC-XRD are two techniques extensively used for the structure determination of solids having single large crystal and solid in the powder form, respectively. The SC-XRD elucidates the structural properties such as lattice parameters, unit cell, space group, and intermolecular interactions and is the most reliable technique [31]. However, the drawback of the method is that obtaining single crystals is a difficult task and PXRD cannot differentiate between co-crystal and additional solid states such as polymorph, solvate, and hydrate [33].

3.2 Thermal analysis

Differential scanning calorimetry (DSC) is a frequently used technique for gaining thermal information such as enthalpy of melting and other melting data and is used for screening of co-crystals [22]. The type of solid formed can be identified from DSC thermogram. For example, a physical mixture has two endotherms indicating the melting points of components with no intermolecular forces between them [42], and a eutectic system has a particular endotherm having latent heat, not more than the corresponding components [43], while a co-crystal has a particular endotherm having latent heat lesser, higher, or in the middle of the parent components [44].

3.3 Spectroscopic methods

Various spectroscopic techniques such as Raman, nuclear magnetic reso¬nance (NMR), Fourier transform (FT-IR) infra-red spectroscopies, terahertz (THz) spectroscopic imaging, and various advanced applications of these methods are in practice to investigate the structure of co-crystals and find out the intermolecular forces between the components of co-crystals [45, 46].

3.4 Morphological analysis

The morphology of co-crystals is characterized by techniques such as fluorescence microscopy, scanning electron microscopy (SEM), and polarized optical microscopy [26, 47].

4. Conclusions

Co-crystallization is an effective tool and suggests one of the best auspicious routes to modify the physicochemical characteristics of the merging components. There are several methods for preparing co-crystals, extending from ordinary lab-size production to possibly large-scale unremitting fabrication. In this chapter, we have described the established and emerging approaches to co-crystal prepara-tion. Moreover, insights into the mechanisms of formation and techniques of characterization of co-crystals have also been presented.

Conflicts of interest

No conflicts of interest were declared by the authors.