Structure of the caffeine-pyrogallol complex: revisiting a pioneering structural analysis of a model pharmaceutical cocrystal

The 1967 attempt of structural analysis of the solid-state complex of caffeine and pyrogallol was a pioneering structural investigation in the supramolecular chemistry of caffeine, of what today would easily be considered an archetype of a model pharmaceutical cocrystal. Re-investigating this historically important system demonstrates that this long overlooked complex is most likely a tetrahydrate with a different structure and composition than initially proposed, and provides the crystal structure of the anhydrous cocrystal.

Utilizing "Latent" Carbon: Repositioning Hydrogen-Bonded Synthons and Assemblies via Halogen Bonding to π-Systems

Molecules bearing carboxylic acid, amide, and hydroxyl groups are ubiquitous in crystal engineering, where robust hydrogen-bonded synthons centred on these functionalities enable reliable crystal structure design. We now show that halogen bonding to the carbon π-system of such molecules, traditionally ignored in crystal engineering, permits the recognition and directional assembly of the resulting hydrogen-bonded structural subunits, leaving the archetypal hydrogen-bonded ring, ladder, and chain homosynthons intact, but repositioned in space. When applied to heteromolecular synthons, this enables rearranging more complex hydrogen-bonded motifs and the evolution of binary cocrystals into ternary ones through "latent" carbon-based recognition sites, demonstrating a rational approach to build higher-order solid-state supramolecular assemblies.

Frontiers of molecular crystal structure prediction for pharmaceuticals and functional organic materials

The reliability of organic molecular crystal structure prediction has improved tremendously in recent years. Crystal structure predictions for small, mostly rigid molecules are quickly becoming routine. Structure predictions for larger, highly flexible molecules are more challenging, but their crystal structures can also now be predicted with increasing rates of success. These advances are ushering in a new era where crystal structure prediction drives the experimental discovery of new solid forms. After briefly discussing the computational methods that enable successful crystal structure prediction, this perspective presents case studies from the literature that demonstrate how state-of-the-art crystal structure prediction can transform how scientists approach problems involving the organic solid state. Applications to pharmaceuticals, porous organic materials, photomechanical crystals, organic semi-conductors, and nuclear magnetic resonance crystallography are included. Finally, efforts to improve our understanding of which predicted crystal structures can actually be produced experimentally and other outstanding challenges are discussed.

The synthesis and characterization of a series of cocrystals of an isoniazid derivative with butan-2-one and propan-2-one

Four cocrystals containing N'-(butan-2-ylidene)pyridine-4-carbohydrazide (izbt) and one cocrystal containing N'-isopropylideneisonicotinohydrazide (izact) were synthesized by reacting isoniazid with either butan-2-one (for the former) or acetone (for the latter). The coformers used to synthesize the izbt cocrystals were 2,4-dihydroxybenzoic acid, 2,5-dihydroxybenzoic acid, 2-chloro-4-nitrobenzoic acid and 1-naphthoic acid. 1-Naphthoic acid was also used with izact to form a cocrystal. The 1:1 cocrystals are: N'-(butan-2-ylidene)pyridine-4-carbohydrazide-1-naphthoic acid (izbt-1nta), C10H13N3O·C11H8O2, N'-(butan-2-ylidene)pyridine-4-carbohydrazide-2,4-dihydroxybenzoic acid (izbt-2,4-dhba), C10H13N3O·C7H6O4, N'-(propan-2-ylidene)pyridine-4-carbohydrazide-1-naphthoic acid (izact-1nta), C9H11N3O·C11H8O2, N'-(butan-2-ylidene)pyridine-4-carbohydrazide-2-chloro-4-nitrobenzoic acid (izbt-2c4n), C10H13N3O·C7H4ClNO4, and N'-(butan-2-ylidene)pyridine-4-carbohydrazide-2,5-dihydroxybenzoic acid (izbt-2,5-dhba), C10H13N3O·C7H6O4. The cocrystals containing izbt were compared to those containing the same (or similar) coformers with izact that have been reported either here or in the Cambridge Structural Database (CSD). Most of the cocrystals showed different packing despite having the same hydrogen-bonding motifs. The cocrystals were characterized by single-crystal X-ray diffraction (SC-XRD), powder X-ray diffraction (PXRD) and differential scanning calorimetry (DSC).

Using Prenucleation Aggregation of Caffeine-Benzoic Acid as a Rapid Indication of Co-crystallization from Solutions

Co-crystal design is a convenient way to remedy the poor biopharmaceutical properties of drugs. Most studies focus on experimental co-crystal screening or computational prediction, but hardly any work has been done toward fast, efficient, and reliable prediction of solution crystallization for co-crystal formation. Here, we study the caffeine-benzoic acid co-crystal system, due to its reported difficulty to crystallize from the solution phase. With this work, we investigate whether there is a link between prenucleation aggregation in solution and co-crystal formation and how to harness this for crystallization prediction. ¹H and ¹³C NMR spectroscopy is used to study the prenucleation interaction between caffeine and benzoic acid in methanol, acetone, and acetonitrile as examples of common solvents. In this system, crystallization from methanol leads to no co-crystallization, from acetone to concomitant crystallization of co-crystal and caffeine, and from acetonitrile to pure co-crystal formation from solution. Strong heteromeric dimers were found to exist in all three solvents. Ternary phase diagrams were defined and a solution-accessible co-crystal region was found for all solvents. For this system, the prenucleation clusters found in solution could be linked to the crystallization of the co-crystal. Crystallization from DMSO did not yield the co-crystal and there were no detectable prenucleation aggregates. NMR spectroscopy to probe dimers in solution can thus be used as a fast, reliable, and promising tool to predict co-crystallization from specific solvents and to screen for suitable solvents for manufacturing and scale-up.

Cocrystals; basic concepts, properties and formation strategies

Cocrystallization is an old technique and remains the focus of several research groups working in the field of Chemistry and Pharmacy. This technique is basically in field for improving physicochemical properties of material which can be active pharmaceutical ingredients (APIs) or other chemicals with poor profile. So this review article has been presented in order to combine various concepts for scientists working in the field of chemistry, pharmacy or crystal engineering, also it was attempt to elaborate concepts belonging to crystal designing, their structures and applications. A handsome efforts have been made to bring scientists together working in different fields and to make chemistry easier for a pharmacist and pharmacy for chemists pertaining to cocrystals. Various aspects of chemicals being used as co-formers have been explored which predict the formation of co-crystals or molecular salts and even inorganic cocrystals.

Seeking Rules Governing Mixed Molecular Crystallization

Mixed crystals result when components of the structure are randomly replaced by analogues in ratios that can be varied continuously over certain ranges. Mixed crystals are useful because their properties can be adjusted by increments, simply by altering the ratio of components. Unfortunately, no clear rules exist to predict when two compounds are similar enough to form mixed crystals containing substantial amounts of both. To gain further understanding, we have used single-crystal X-ray diffraction, computational methods, and other tools to study mixed crystallizations within a selected set of structurally related compounds. This work has allowed us to begin to clarify the rules governing the phenomenon by showing that mixed crystals can have compositions and properties that vary continuously over wide ranges, even when the individual components do not normally crystallize in the same way. Moreover, close agreement of the results of our experiments and computational modeling demonstrates that reliable predictions about mixed crystallization can be made, despite the complexity of the phenomenon.

The crystal structure of ethyl 2,3,5-trifluoro-4-(4-oxo-3,4-dihydropyridin-1(2H)-yl)benzoate, C14H12F3NO3

C14H12F3NO3, monoclinic, P21/n (no. 14), a = 14.4583(12) Å, b = 6.6553(5) Å, c = 14.8395(11) Å, β = 113.733(3)°, V = 1307.16(18) Å3, Z = 4, Rgt (F) = 0.0479, wRref (F 2) = 0.1235, T = 170 K.

Global analysis of the energy landscapes of molecular crystal structures by applying the threshold algorithm

Polymorphism in molecular crystals has important consequences for the control of materials properties and our understanding of crystallization. Computational methods, including crystal structure prediction, have provided important insight into polymorphism, but have usually been limited to assessing the relative energies of structures. We describe the implementation of the Monte Carlo threshold algorithm as a method to provide an estimate of the energy barriers separating crystal structures. By sampling the local energy minima accessible from multiple starting structures, the simulations yield a global picture of the crystal energy landscapes and provide valuable information on the depth of the energy minima associated with crystal structures. We present results from applying the threshold algorithm to four polymorphic organic molecular crystals, examine the influence of applying space group symmetry constraints during the simulations, and discuss the relationship between the structure of the energy landscape and the intermolecular interactions present in the crystals.

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.

Fluorobenzoic acid coformers to improve the solubility and permeability of the BCS class IV drug naftopidil

Crystalline salts of the low solubility and low permeability drug naftopidil were investigated with mono-, di-, tri-, and tetrafluorobenzoic acids as coformers to show that 245TFBA (2,4,5-trifluorobenzoic acid) is the optimal salt with faster dissolution and high permeability, thereby opening the study of fluorinated coformers in pharmaceutical cocrystals and salts.

Sustainable solid form screening: mechanochemical control over nucleobase hydrogen-bonded organic framework polymorphism

The choice is yours! Liquid-assisted grinding can be used to control HOF polymorphism.

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...

Improving chemical stability of resveratrol in hot melt extrusion based on formation of eutectic with nicotinamide

Hot melt extrusion (HME) is a technique applied in the preparation of pharmaceutical amorphous solid dispersions (ASD). Notably it is important to prevent thermal degradation of heat-sensitive drugs during HME. In this study, we present a new strategy to improve chemical stability of pharmaceutical compounds during HME through the formation of eutectics with small molecules. Resveratrol (RES) was selected as the model compound because it is a heat-liable natural product with a very high melting point of 267 °C. When heated at its melting point for 3 min, it degrades by 40%. RES can co-crystallize with nicotinamide (NIC) in solution, however, it can only form a eutectic with NIC during heating. HPMCAS was selected as the polymer matrix and the drug loading of RES was fixed as 20% (weight ratio). The lowest extrusion temperature that can result to RES-HPMCAS ASD is 215 °C. At this temperature, RES shows 7.36% degradation during extrusion. Replacement of 21.4% HPMCAS with NIC decreased the melting temperature of NIC and thus lowered the minimal extrusion temperature to 155 °C. This effectively prevented thermal degradation of RES without negatively affecting non-sink dissolution. The only extra cost for this method is stricter storage conditions (low temperature and low humidity) due to the low glass transition temperature of NIC. Similar strategy may be applied to other heat-liable drugs in similar ways. This study demonstrates the use of eutectic formation for preventing thermal degradation of drug during extrusion of ASD.

Use of additives to regulate solute aggregation and direct conformational polymorph nucleation of pimelic acid

Understanding the nucleation pathway and achieving regulation to produce the desired crystals are mutually beneficial. The authors previously proposed a nucleation pathway of conformational polymorphs in which solvation and solute self-assembly could affect the result of the conformational rearrangement and further nucleation outcomes. Based on this, herein α,ω-alkanedi-carb-oxy-lic acids (DAn, where n represents the number of carbon atoms in the molecule, n = 2-6, 8-11) were designed as homologous additives to interfere with the self-assembly of pimelic acid (DA7) to further induce the form II compound, which differs from form I only in conformation. Interestingly, longer-chain additives (DA6-11) have a stronger form II-inducing ability than short-chain ones (DA2-4). In addition, an apparent gradient of the degree of interference with solute self-assembly, consistent with form II-inducing ability, was detected by infrared and nuclear magnetic resonance spectroscopy. The calculated molecular electrostatic potential charges also clearly indicate that additive-solute electrostatic interactions gradually increase with increasing carbon chain length of the additives, reaching a maximum value with DA6-11. This novel use of additives demonstrates a direct link between solute aggregation and conformational polymorph nucleation.

Mechanochemical reactivity inhibited, prohibited and reversed by liquid additives: examples from crystal-form screens

We demonstrate that liquid additives can exert inhibitive or prohibitive effects on the mechanochemical formation of multi-component molecular crystals, and report that certain additives unexpectedly prompt the dismantling of such solids into physical mixtures of their constituents. Computational methods were employed in an attempt to identify possible reasons for these previously unrecognised effects of liquid additives on mechanochemical transformations.

Structure prediction of crystals, surfaces and nanoparticles

We review the current techniques used in the prediction of crystal structures and their surfaces and of the structures of nanoparticles. The main classes of search algorithm and energy function are summarized, and we discuss the growing role of methods based on machine learning. We illustrate the current status of the field with examples taken from metallic, inorganic and organic systems. This article is part of a discussion meeting issue 'Dynamic in situ microscopy relating structure and function'.

Co-crystal Prediction by Artificial Neural Networks*

A significant amount of attention has been given to the design and synthesis of co-crystals by both industry and academia because of its potential to change a molecule's physicochemical properties. Yet, difficulties arise when searching for adequate combinations of molecules (or coformers) to form co-crystals, hampering the efficient exploration of the target's solid-state landscape. This paper reports on the application of a data-driven co-crystal prediction method based on two types of artificial neural network models and co-crystal data present in the Cambridge Structural Database. The models accept pairs of coformers and predict whether a co-crystal is likely to form. By combining the output of multiple models of both types, our approach shows to have excellent performance on the proposed co-crystal training and validation sets, and has an estimated accuracy of 80 % for molecules for which previous co-crystallization data is unavailable.

Simultaneous automated image analysis and Raman spectroscopy of powders at an individual particle level

Solid form diversity of raw materials can be critical for the performance of the final drug product. In this study, Raman spectroscopy, image analysis and combined Raman and image analysis were utilized to characterize the solid form composition of a particulate raw material. Raman spectroscopy provides chemical information and is complementary to the physical information provided by image analysis. To demonstrate this approach, binary mixtures of two solid forms of carbamazepine with a distinct shape, an anhydrate (prism shaped) and a dihydrate (needle shaped), were characterized at an individual particle level. Partial least squares discriminant analysis classification models were developed and tested with known, gravimetrically mixed test samples, followed by analysis of unknown, commercially supplied carbamazepine raw material samples. Classification of several thousands of particles was performed, and it was observed that with the known binary mixtures, the minimum number of particles needed for the combined Raman spectroscopy - image analysis classification model was approximately 100 particles per solid form. The carbamazepine anhydrate and dihydrate particles were detected and classified with a classification error of 1 % using the combined model. Further, this approach allowed the identification of raw material solid form impurity in unknown raw material samples. Simultaneous automated image analysis and Raman spectroscopy of powders at an individual particle level has its potential in accurate detection of low amounts of unwanted solid forms in particulate raw material samples.

Rich polymorphism in nicotinamide revealed by melt crystallization and crystal structure prediction

Overprediction is a major limitation of current crystal structure prediction (CSP) methods. It is difficult to determine whether computer-predicted polymorphic structures are artefacts of the calculation model or are polymorphs that have not yet been found. Here, we reported the well-known vitamin nicotinamide (NIC) to be a highly polymorphic compound with nine solved single-crystal structures determined by performing melt crystallization. A CSP calculation successfully identifies all six Z' = 1 and 2 experimental structures, five of which defy 66 years of attempts at being explored using solution crystallization. Our study demonstrates that when combined with our strategy for cultivating single crystals from melt microdroplets, melt crystallization has turned out to be an efficient tool for exploring polymorphic landscapes to better understand polymorphic crystallization and to more effectively test the accuracy of theoretical predictions, especially in regions inaccessible by solution crystallization.

Multifidelity Statistical Machine Learning for Molecular Crystal Structure Prediction

The prediction of crystal structures from first-principles requires highly accurate energies for large numbers of putative crystal structures. High accuracy of solid state density functional theory (DFT) calculations is often required, but hundreds or more structures can be present in the low energy region of interest, so that the associated computational costs are prohibitive. Here, we apply statistical machine learning to predict expensive hybrid functional DFT (PBE0) calculations using a multifidelity approach to re-evaluate the energies of crystal structures predicted with an inexpensive force field. The method uses an autoregressive Gaussian process, making use of less expensive GGA DFT (PBE) calculations to bridge the gap between the force field and PBE0 energies. The method is benchmarked on the crystal structure landscapes of three small, hydrogen-bonded organic molecules and shown to produce accurate predictions of energies and crystal structure ranking using small numbers of the most expensive calculations; the PBE0 energies can be predicted with errors of less than 1 kJ mol^-1 with between 4.2 and 6.8% of the cost of the full calculations. As the model that we have developed is probabilistic, we discuss how the uncertainties in predicted energies impact the assessment of the energetic ranking of crystal structures.

Challenging the Ostwald rule of stages in mechanochemical cocrystallisation

Mechanochemistry provides an efficient, but still poorly understood route to synthesize and screen for polymorphs of organic solids. We present a hitherto unexplored effect of the milling assembly on the polymorphic outcome of mechanochemical cocrystallisation, tentatively related to the efficiency of mechanical energy transfer to the milled sample. Previous work on mechanochemical cocrystallisation has established that introducing liquid or polymer additives to milling systems can be used to direct polymorphic behavior, leading to extensive studies how the amount and nature of grinding additive affect reaction outcome and polymorphism. Here, focusing on a model pharmaceutical cocrystal of nicotinamide and adipic acid, we demonstrate that changes to the choice of milling media (i.e. number and material of milling balls) and/or the choice of milling assembly (i.e. jar material) can be used to direct polymorphism of mechanochemical cocrystallisation, enabling the selective synthesis, and even reversible and repeatable interconversion of cocrystal polymorphs. While real-time mechanistic studies of mechanochemical transformations of metal–organic materials have previously suggested that reactions follow a path described by Ostwald's rule of stages, i.e. from metastable to increasingly more stable product structures, the herein presented systematic study presents an exception to that rule, revealing that modification of energy input in the mechanochemical system, combined with a small energy difference between polymorphs, permits the selective synthesis of either the more stable room temperature form, or the new metastable high-temperature form, of the target cocrystal.

The choice of milling assembly (jar and ball material, number and size of balls) can be used to direct polymorphism in mechanochemical cocrystallisation, enabling the selective synthesis, and even reversible interconversion of cocrystal polymorphs.

ROY Reclaims Its Crown: New Ways To Increase Polymorphic Diversity

Chemical compounds that exist in multiple crystalline forms are said to exhibit polymorphism. Polymorphs have the same composition, but their structures and properties can vary markedly. In many fields, conditions for crystallizing compounds of interest are screened exhaustively to generate as many polymorphs as possible, from which the most advantageous form can be selected. We report new ways to search for polymorphs and increase polymorphic diversity, based on crystallization induced by suitably designed mixed-crystal seeds. The potential of the strategy has been demonstrated by using it to produce new polymorphs of the benchmark compound ROY as single crystals structurally characterized by X-ray diffraction. This allows ROY to reclaim its crown as the most polymorphic compound in the Cambridge Structural Database. More generally, the methods promise to become valuable tools for polymorphic screening in all fields where crystalline solids are used.

Disappearing Polymorphs in Metal-Organic Framework Chemistry: Unexpected Stabilization of a Layered Polymorph over an Interpenetrated Three-Dimensional Structure in Mercury Imidazolate

The "disappearing polymorph" phenomenon is well established in organic solids, and has had a profound effect in pharmaceutical materials science. The first example of this effect in metal-containing systems in general, and in coordination-network solids in particular, is here reported. Specifically, attempts to mechanochemically synthesize a known interpenetrated diamondoid (dia) mercury(II) imidazolate metal-organic framework (MOF) yielded a novel, more stable polymorph based on square-grid (sql) layers. Simultaneously, the dia-form was found to be highly elusive, observed only as a short-lived intermediate in monitoring solvent-free synthesis and not at all from solution. The destabilization of a dense dia-framework relative to a lower dimensionality one is in contrast to the behavior of other imidazolate MOFs, with periodic density functional theory (DFT) calculations showing that it arises from weak interactions, including structure-stabilizing agostic C-H⋅⋅⋅Hg contacts. While providing a new link between MOFs and crystal engineering of organic solids, these findings highlight a possible role for agostic interactions in directing topology and stability of MOF polymorphs.

Emerging role of primary heterogeneous nucleation in pharmaceutical crystallization

Crystallization is an important and difficult to control unit operation in the pharmaceutical industry. Crystallization can control molecular (i.e., polymorphism) and particulate (i.e., particle size and crystal habit) properties of active pharmaceutical ingredient (API). Moreover, these molecular and particulate properties govern the manufacturability, stability, and biopharmaceutical performance of the API and drug product. Nucleation is a key step and primary heterogeneous nucleation is a common mode of nucleation during crystallization. Hence, it is important to understand the parameters affecting primary heterogeneous nucleation, to achieve desirable properties in crystalline APIs. Primary heterogeneous crystallization has usually been linked to the surface characteristics like topography and functionality of the heteronucleant. The review outlines recent findings in the primary heterogeneous crystallization with specific emphasis on its pharmaceutical applications including regulatory considerations. Molecular-level mechanisms governing heteronucleation and subsequent outcome in terms of molecular as well as particulate-level properties of API have also been discussed. Moreover, general guidance for the selection of heteronucleant has also been included. Heterogeneous crystallization is a promising tool for efficient crystallization of API having properties for optimal pharmaceutical performance.

Control and prediction of the organic solid state: a challenge to theory and experiment†

The ability of theoretical chemists to quantitatively model the weak forces between organic molecules is being exploited to predict their crystal structures and estimate their physical properties. Evolving crystal structure prediction methods are increasingly being used to aid the design of organic functional materials and provide information about thermodynamically plausible polymorphs of speciality organic materials to aid, for example, pharmaceutical development. However, the increasingly sophisticated experimental studies for detecting the range of organic solid-state behaviours provide many challenges for improving quantitative theories that form the basis for the computer modelling. It is challenging to calculate the relative thermodynamic stability of different organic crystal structures, let alone understand the kinetic effects that determine which polymorphs can be observed and are practically important. However, collaborations between experiment and theory are reaching the stage of devising experiments to target the first crystallization of new polymorphs or create novel organic molecular materials.

Crystal structure of caffeinium triiodide – caffeine (1/1), C16H21I3N8O4

C16H21I3N8O4, monoclinic, P21/n (no. 14), a = 14.7257(7) Å, b = 10.5712(5) Å, c = 16.7501(8) Å, β = 114.408(2)°, V = 2374.4(2) Å3, Z = 4, R gt(F) = 0.0254, wR ref(F 2) = 0.0760, T = 290(2) K.

Evaluating the Energetic Driving Force for Cocrystal Formation

We present a periodic density functional theory study of the stability of 350 organic cocrystals relative to their pure single-component structures, the largest study of cocrystals yet performed with high-level computational methods. Our calculations demonstrate that cocrystals are on average 8 kJ mol^-1 more stable than their constituent single-component structures and are very rarely (<5% of cases) less stable; cocrystallization is almost always a thermodynamically favorable process. We consider the variation in stability between different categories of systems-hydrogen-bonded, halogen-bonded, and weakly bound cocrystals-finding that, contrary to chemical intuition, the presence of hydrogen or halogen bond interactions is not necessarily a good predictor of stability. Finally, we investigate the correlation of the relative stability with simple chemical descriptors: changes in packing efficiency and hydrogen bond strength. We find some broad qualitative agreement with chemical intuition-more densely packed cocrystals with stronger hydrogen bonding tend to be more stable-but the relationship is weak, suggesting that such simple descriptors do not capture the complex balance of interactions driving cocrystallization. Our conclusions suggest that while cocrystallization is often a thermodynamically favorable process, it remains difficult to formulate general rules to guide synthesis, highlighting the continued importance of high-level computation in predicting and rationalizing such systems.

Towards medicinal mechanochemistry: evolution of milling from pharmaceutical solid form screening to the synthesis of active pharmaceutical ingredients (APIs)

This overview highlights the emergent area of mechanochemical reactions for making active pharmaceutical ingredients (APIs), and covers the latest advances in the recently established area of mechanochemical screening and synthesis of pharmaceutical solid forms, specifically polymorphs, cocrystals, salts and salt cocrystals. We also provide an overview of the most recent developments in pharmaceutical uses of mechanochemistry, including real-time reaction monitoring, techniques for polymorph control and approaches for continuous manufacture using twin screw extrusion, and more. Most importantly, we show how the overlap of previously unrelated areas of mechanochemical screening for API solid forms, organic synthesis by milling, and mechanochemical screening for molecular recognition, enables the emergence of a new research discipline in which different aspects of pharmaceutical and medicinal chemistry are addressed through mechanochemistry rather than through conventional solution-based routes. The emergence of such medicinal mechanochemistry is likely to have a strong impact on future pharmaceutical and medicinal chemistry, as it offers not only access to materials and reactivity that are sometimes difficult or even impossible to access from solution, but can also provide a general answer to the demands of the pharmaceutical industry for cleaner, safer and efficient synthetic solutions.

Serendipitous isolation of a disappearing conformational polymorph of succinic acid challenges computational polymorph prediction

A new polymorph of succinic acid is an unusually challenging test for metastable polymorph prediction.

Is zeroth order crystal structure prediction (CSP_0) coming to maturity? What should we aim for in an ideal crystal structure prediction code?

Crystal structure prediction based on searching for the global minimum in the lattice energy (CSP_0) is growing in use for guiding the discovery of new materials, for example, new functional materials, new phases of interest to planetary scientists and new polymorphs relevant to pharmaceutical development. This Faraday Discussion can assess the progress of CSP_0 over the range of types of materials to which CSP is currently and could be applied, which depends on our ability to model the variety of interatomic forces in crystals. The basic hypothesis, that the outcome of crystallisation is determined by thermodynamics, needs examining by considering methods of modelling relative thermodynamic stability not only as a function of pressure and temperature, but also of size, solvent and the presence of heterogeneous templates or impurities (CSP_thd). Given that many important materials persist, and indeed may be formed, when they are not the most thermodynamically stable structure, we need to define what would be required of an ideal CSP code (CSP_aim).

Tandem In Situ Monitoring for Quantitative Assessment of Mechanochemical Reactions Involving Structurally Unknown Phases

We report herein quantitative in situ monitoring by simultaneous PXRD and Raman spectroscopy of the mechanochemical reaction between benzoic acid and nicotinamide, affording a rich polymorphic system with four new cocrystal polymorphs, multiple phase transformations, and a variety of reaction pathways. After observing polymorphs by in situ monitoring, we were able to isolate and characterize three of the four polymorphs, most of which are not accessible from solution. Relative stabilities among the isolated polymorphs at ambient conditions were established by slurry experiments. Using two complementary methods for in situ monitoring enabled quantitative assessment and kinetic analysis of each studied mechanochemical reaction, even when involving unknown crystal structures, and short-lived intermediates. In situ Raman monitoring was introduced here also as a standalone laboratory technique for quantitative assessment of mechanochemical reactions and understanding of mechanochemical reactivity. Our results provide an important step toward a complete and high-throughput quantitative approach to mechanochemical reaction kinetics and mechanisms, necessary for the development of the mechanistic framework of milling reactions.

Pharmaceutical cocrystals, salts and multicomponent systems; intermolecular interactions and property based design

As small molecule drugs become harder to develop and less cost effective for patient use, efficient strategies for their property improvement become increasingly important to global health initiatives. Improvements in the physical properties of Active Pharmaceutical Ingredients (APIs), without changes in the covalent chemistry, have long been possible through the application of binary component solids. This was first achieved through the use of pharmaceutical salts, within the last 10-15years with cocrystals and more recently coamorphous systems have also been consciously applied to this problem. In order to rationally discover the best multicomponent phase for drug development, intermolecular interactions need to be considered at all stages of the process. This review highlights the current thinking in this area and the state of the art in: pharmaceutical multicomponent phase design, the intermolecular interactions in these phases, the implications of these interactions on the material properties and the pharmacokinetics in a patient.

Screening for new pharmaceutical solid forms using mechanochemistry: A practical guide

Within the pharmaceutical industry, and elsewhere, the screening for new solid forms is a mandatory exercise for both existing and new chemical entities. This contribution focuses on mechanochemistry as a versatile approach for discovering new and alternative solid forms. Whilst a series of recently published extensive reviews exist which focus on mechanistic aspects and potential areas of development, in this review we focus on particular practical aspects of mechanochemistry in order to allow full optimisation of the approach in searches for new solid forms including polymorphs, salts and cocrystals as well as their solvated/hydrated analogues. As a consequence of the apparent experimental simplicity of the method (compared to more traditional protocols e.g. solvent-based methods), the high efficiency and range of conditions available in a mechanochemical screen, mechanochemistry should not be considered simply as an alternative method when other screening methods are not successful, but rather as a key strategy in any fully effective solid form screen providing reduced effort and time as well as the potential of requiring reduced amounts of material.

Crystal alignment of caffeine deposited onto single crystal surfaces via hot-wall epitaxy

Crystal growth of caffeine on single crystalline surfaces yields needle or bird-like shaped crystals depending on surface chemistry and symmetry.

Defined crystal growth is highly demanded for technological applications but also fundamental research. Within this work, the crystal growth of the asymmetric molecule caffeine was studied on single crystalline surfaces of muscovite mica, sodium chloride and potassium chloride. While elongated needle-like crystals grow on muscovite mica and sodium chloride, smaller individual “bird-like” structures were observed on potassium chloride. Depending on the surface type and temperature, the disk-shaped caffeine molecules prefer either an edge-on or flat-on orientation with respect to the surface, but in each case, a defined crystallographic relation between the surface and caffeine crystallites was determined by using the X-ray pole figure technique. On muscovite mica and sodium chloride, needle-like crystallites with edge-on oriented molecules aligned mainly with the unit cell c-axis (which coincides with the long needle axis) along the [1–10]_mica, [100]_mica, [110]_mica and [110]_NaCl, [1–10]_NaCl directions, respectively. Crystals consisting of flat-on oriented molecules on KCl showed also defined alignments with respect to the substrate, but due to the altered molecule–substrate contact, the b-axis aligned along [110]_KCl and [1–10]_KCl. Growth at elevated temperatures enabled changes in the crystal growth whereby more defined structures formed on NaCl. On KCl, the bird-like structures remained very similar, while caffeine on the mica surface at elevated temperatures resulted in even additional texture forming with the caffeine molecules now also favoring a flat-on orientation with respect to the surface. The systematic variation of various system parameters demonstrates how sensitive the growth behavior of caffeine on this variety of substrates is.

In silico screening of dicarboxylic acids for cocrystallization with phenylpiperazine derivatives based on both cocrystallization propensity and solubility advantage

In silico screening was performed to search for binary solids in which a phenylpiperazine-derivative drug was cocrystallized with a dicarboxylic acid. The phenylpiperazine derivative could be any of 61 such drugs, while the dicarboxylic acid could be any of nine such acids. The uniqueness of this approach was that two criteria had to be fulfilled simultaneously, namely a high propensity for cocrystallization and a sufficient solubility advantage. Using the mixing enthalpies of selected pairs of crystal formers with high affinities for one another permitted the classification of candidates with a high probability of cocrystallization. Further modeling of the solubility advantage allowed the identification of many binary solids that potentially exhibit significantly enhanced solubility in water. Based on the computed values for the mixing enthalpies and solubility advantage factors, it was concluded that dicarboxylic acids are both excellent coformers for cocrystallization with phenylpiperazines and very good solubility enhancers; indeed, the use of dicarboxylic acids as coformers would allow the degree of dissolution to be tuned for many of the studied drugs. The observed similarities of the cocrystallization landscapes of the studied drugs and excipients were also explored.

Generation of crystal structures using known crystal structures as analogues

This analysis attempts to answer the question of whether similar molecules crystallize in a similar manner. An analysis of structures in the Cambridge Structural Database shows that the answer is yes - sometimes they do, particularly for single-component structures. However, one does need to define what we mean by similar in both cases. Building on this observation we then demonstrate how this correlation between shape similarity and packing similarity can be used to generate potential lattices for molecules with no known crystal structure. Simple intermolecular interaction potentials can be used to minimize these potential lattices. Finally we discuss the many limitations of this approach.

Rationalization of the Color Properties of Fluorescein in the Solid State: A Combined Computational and Experimental Study

Fluorescein is known to exist in three tautomeric forms defined as quinoid, zwitterionic, and lactoid. In the solid state, the quinoid and zwitterionic forms give rise to red and yellow materials, respectively. The lactoid form has not been crystallized pure, although its cocrystal and solvate forms exhibit colors ranging from yellow to green. An explanation for the observed colors of the crystals is found using a combination of UV/Vis spectroscopy and plane-wave DFT calculations. The role of cocrystal coformers in modifying crystal color is also established. Several new crystal structures are determined using a combination of X-ray and electron diffraction, solid-state NMR spectroscopy, and crystal structure prediction (CSP). The protocol presented herein may be used to predict color properties of materials prior to their synthesis.

Can computed crystal energy landscapes help understand pharmaceutical solids?

Computational crystal structure prediction (CSP) methods can now be applied to the smaller pharmaceutical molecules currently in drug development. We review the recent uses of computed crystal energy landscapes for pharmaceuticals, concentrating on examples where they have been used in collaboration with industrial-style experimental solid form screening. There is a strong complementarity in aiding experiment to find and characterise practically important solid forms and understanding the nature of the solid form landscape.