Crystal Engineering of Pharmaceutical Co-crystals: Application of Methyl Paraben as Molecular Hook

Assessing Structures and Solution Behaviors of Molecular and Ionic Cocrystals with a Common Bioactive Molecule: 2,4-Pyridinedicarboxylic Acid with Tranexamic Acid and Nicotinamide

Cocrystals of 2,4-pyridinedicarboxylic acid (PDA) with either nicotinamide (NTD) or tranexamic acid (TXA) as (PDA)·(NTD) and 2(PDA)·(TXA), respectively, are reported, with the former being a molecular cocrystal and the latter being an ionic cocrystal. Single-crystal structure analyses showed that PDA and its coformers are sustained by neutral and ionic hydrogen bonds. Suspensions of (PDA)·(NTD) resulted in complete conversion to PDA monohydrate after 48 h, while 2(PDA)·(TXA) was thermodynamically stable at a lower pH and showed a 2-fold increase in the PDA concentration, relative to pure PDA monohydrate under similar conditions. Thermal characterization of 2(PDA)·(TXA) displayed a lower melting point and a lower heat of fusion, relative to the pure components. Powder dissolution studies were evaluated for PDA, (PDA)·(NTD), and 2(PDA)·(TXA) and the corresponding physical mixtures. The percent of PDA dissolved rapidly reached near 100% for most cases; however, for 2(PDA)·(TXA), complete dissolution was not achieved, and the amount of PDA dissolved decreased to 85% after 3 h. Instability of 2(PDA)·(TXA) was likely a result of a high solution pH during dissolution, and our results confirm that the solution pH plays a key role in determining the solution behavior and phase stability of the cocrystals.

Multi-component Crystal Strategy for Improving Water Solubility and Antifungal Activity of Climbazole

PURPOSE

The primary problem with climbazole (CLB), a broad-spectrum imidazole antifungal drug, is its low water solubility. In order to increase its water solubility and antifungal activity, three new multi-component crystals were synthesized in this work, and the intermolecular interactions were systematically studied. This work helps to optimize the CLB product formulation and extend its application prospects.

METHODS

In this work, three novel multi-component crystals, CLB-malonic acid (CLB-MA) salt, CLB-succinic acid (CLB-SA) cocrystal and CLB-adipic acid (CLB-AA) cocrystal, were successfully synthesized. And the crystal structure, thermodynamic properties, solubility, dissolution, hygroscopicity, and antifungal activity of the three multi-component crystals were fully characterized by single-crystal X-ray diffraction (SCXRD), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), dynamic water vapor adsorption (DVS) and powder dissolution tests, etc. The molecular interactions and molecular stacking in multi-component crystals were studied by Hirshfeld surface (HS), molecular surface electrostatic potential (MEP), interaction region indication (IRI) and atom and molecule (AIM) techniques.

RESULTS

The results show that the three multi-component crystals have good moisture resistance stability, and their water solubility is 6-22 times that of pure CLB. Meanwhile, the measurement of the minimum inhibitory concentration (MIC) proves that the cocrystal/salt has a stronger antifungal activity than climbazole. Quantum chemistry calculations of crystal structure visualized and quantified the interactions that exist in multi-component crystals, and explored the microscopic mechanisms underlying the different performance of multi-component crystals.

Optimizing Drug Development: Harnessing the Sustainability of Pharmaceutical Cocrystals

Environmental impacts of the industrial revolution necessitate adoption of sustainable practices in all areas of development. The pharmaceutical industry faces increasing pressure to minimize its ecological footprint due to its significant contribution to environmental pollution. Over the past two decades, pharmaceutical cocrystals have received immense popularity due to their ability to optimize the critical attributes of active pharmaceutical ingredients and presented an avenue to bring improved drug products to the market. This review explores the potential of pharmaceutical cocrystals as an ecofriendly alternative to traditional solid forms, offering a sustainable approach to drug development. From reducing the number of required doses to improving the stability of actives, from eliminating synthetic operations to using pharmaceutically approved chemicals, from the use of continuous and solvent-free manufacturing methods to leveraging published data on the safety and toxicology, the cocrystallization approach contributes to sustainability of drug development. The latest trends suggest a promising role of pharmaceutical cocrystals in bringing novel and improved medicines to the market, which has been further fuelled by the recent guidance from the major regulatory agencies.

Selective Targeting of Lung Cancer Cells with Methylparaben-Tethered-Quinidine Cocrystals in 3D Spheroid Models

The development and design of pharmaceutical cocrystals for various biological applications has garnered significant interest. In this study, we have established methodologies for the growth of the methylparaben-quinidine cocrystal (MP-QU), which exhibits a well-defined order that favors structure-property correlation. To confirm the cocrystal formation, we subjected the cocrystals to various physicochemical analyses such as powder X-ray diffraction (PXRD), single-crystal X-ray diffraction (SCXRD), Raman, and IR spectroscopy. The results of the XRD pattern comparisons indicated no polymorphisms, and density functional theory (DFT) studies in both gaseous and liquid phases revealed enhanced stability. Our in silico docking studies demonstrated the cocrystal's high-affinity binding towards cancer-specific epidermal growth factor receptor (EGFR), Janus kinase (JAK), and other receptors. Furthermore, in vitro testing against three-dimensional (3D) spheroids of lung cancer (A549) and normal fibroblast cells (L929) demonstrated the cocrystal's higher anticancer potential, supported by cell viability measurements and live/dead assays. Interestingly, the cocrystal showed selectivity between cancerous and normal 3D spheroids. We found that the MP-QU cocrystal inhibited migration and invadopodia formation of cancer spheroids in a favorable 3D microenvironment.

Polymorphism of Carbamazepine Pharmaceutical Cocrystal: Structural Analysis and Solubility Performance

Polymorphism is a common phenomenon among single- and multicomponent molecular crystals that has a significant impact on the contemporary drug development process. A new polymorphic form of the drug carbamazepine (CBZ) cocrystal with methylparaben (MePRB) in a 1:1 molar ratio as well as the drug's channel-like cocrystal containing highly disordered coformer molecules have been obtained and characterized in this work using various analytical methods, including thermal analysis, Raman spectroscopy, and single-crystal and high-resolution synchrotron powder X-ray diffraction. Structural analysis of the solid forms revealed a close resemblance between novel form II and previously reported form I of the [CBZ + MePRB] (1:1) cocrystal in terms of hydrogen bond networks and overall packing arrangements. The channel-like cocrystal was found to belong to a distinct family of isostructural CBZ cocrystals with coformers of similar size and shape. Form I and form II of the 1:1 cocrystal appeared to be related by a monotropic relationship, with form II being proven to be the thermodynamically more stable phase. The dissolution performance of both polymorphs in aqueous media was significantly enhanced when compared with parent CBZ. However, considering the superior thermodynamic stability and consistent dissolution profile, the discovered form II of the [CBZ + MePRB] (1:1) cocrystal seems a more promising and reliable solid form for further pharmaceutical development.

Accurate location of hydrogen atoms in hydrogen bonds of tizoxanide from the combination of experimental and theoretical models

To obtain detailed information about the position of hydrogen atoms in hydrogen bonds, HBs, of crystalline organic molecular compounds is not an easy task. In this work we propose a combination of ssNMR experimental data with theoretical procedures to get such information. Furthermore, the combination of experimental and theoretical models provides us with well-defined grounds to analyse the strength of π-stacking interactions between layers of hydrogen bonded molecules. Two different theoretical models were considered, both approaches being quite different. The first one is a solid-state model, so that the periodicity of a crystalline system underlies calculations of the electronic energy, the electronic density and NMR parameters. The other one is a molecular model in which molecules are taken as isolated monomers, dimers and tetramers. These two models were applied to the tizoxanide, TIZ, molecular crystal though it can widely be applied to any other molecular crystal. By the application of the quantum molecular model it was possible to learn about the way the intermolecular HBs affect the position of hydrogen atoms that belong to HBs in TIZ. This molecule has two intermolecular HBs that stabilize the structure of a basic dimer, but it also has an intramolecular HB in each monomer whose position should be optimized together with the other ones. We found that by doing this it is possible to obtain reliable results of calculations of NMR spectroscopic parameters. Working with the solid-state model we found that any local variation of the TIZ crystalline structure is correlated with the variation of the values of the NMR parameters of each nucleus. The excellent agreement between experimental and calculated chemical shifts leads to the conclusion that the N10-H10 bond distance should be (1.00 ± 0.02) Å.

Multicomponent crystals of an artemisinin derivative and cinchona alkaloids for use as antimalarial drugs

Multicomponent crystals of an artemisinin derivative and cinchona alkaloids were produced, combining two major types of antimalaria drugs with unique hydrogen bond interactions. These salts demonstrate a new category of antimalarial pharmaceuticals.

Engineering and manufacturing of pharmaceutical co-crystals: a review of solvent-free manufacturing technologies

Design and synthesis of pharmaceutical cocrystals have received great interest in recent years. Cocrystallization of drug substances offers a tremendous opportunity for the development of new drug products with superior physical and pharmacological properties such as solubility, stability, hydroscopicity, dissolution rates and bioavailability. It is now possible to engineer and develop cocrystals via 'green chemistry' and environmentally friendly approaches such as solid-state synthesis in the absence of organic solvents. In addition, significant efforts have been directed towards computational screening, cocrystal manufacturing in a continuous manner and real-time monitoring for quality purposes by using various analytical tools. Pharmaceutical cocrystals are not fully exploited yet and there is a lot of ground to cover before they can be successfully utilized as medical products.

Crystal engineering of a zwitterionic drug to neutral cocrystals: a general solution for floxacins

The transformation of zwitterionic Sparfloxacin (SPX) to the neutral form is achieved by cocrystallization. Neutral forms of drugs are important for higher membrane permeability, while zwitterions are more soluble in water. The twin advantages of higher solubility/dissolution rate and good stability of neutral SPX are achieved in a molecular cocrystal compared to its zwitterionic SPX hydrate. The amine-phenol supramolecular synthon drives cocrystal formation, with the paraben ester acting as a "proton migrator" for the ionic to neutral transformation.

Biological evaluation of dinuclear copper complex/dichloroacetic acid cocrystal against human breast cancer: design, synthesis, characterization, DFT studies and cytotoxicity assays

Binuclear copper(ii) cocrystal “[Cu₂(valdien)₂⋯2Cl₂CHCOOH],” 1 was synthesized from H₂valdien scaffold and anticancer drug pharmacophore “dichloroacetic acid” embedded with two Cu(ii) connected via a hydrogen bonded network.

Pharmaceutical cocrystals: along the path to improved medicines

Cocrystals, a long known but understudied class of crystalline solids, have attracted interest from crystal engineers and pharmaceutical scientists in the past decade and are now an integral part of the preformulation stage of drug development.

The use of a selective saturation pulse to suppress t1 noise in two-dimensional (1)H fast magic angle spinning solid-state NMR spectroscopy

A selective saturation pulse at fast magic angle spinning (MAS) frequencies (60+kHz) suppresses t1 noise in the indirect dimension of two-dimensional (1)H MAS NMR spectra. The method is applied to a synthetic nucleoside with an intense methyl (1)H signal due to triisopropylsilyl (TIPS) protecting groups. Enhanced performance in terms of suppressing the methyl signal while minimising the loss of signal intensity of nearby resonances of interest relies on reducing spin diffusion--this is quantified by comparing two-dimensional (1)H NOESY-like spin diffusion spectra recorded at 30-70 kHz MAS. For a saturation pulse centred at the methyl resonance, the effect of changing the nutation frequency at different MAS frequencies as well as the effect of changing the pulse duration is investigated. By applying a pulse of duration 30 ms and nutation frequency 725 Hz at 70 kHz MAS, a good compromise of significant suppression of the methyl resonance combined with the signal intensity of resonances greater than 5 ppm away from the methyl resonance being largely unaffected is achieved. The effectiveness of using a selective saturation pulse is demonstrated for both homonuclear (1)H-(1)H double quantum (DQ)/single quantum (SQ) MAS and (14)N-(1)H heteronuclear multiple quantum coherence (HMQC) two-dimensional solid-state NMR experiments.

Control of Assembly of Dihydropyridyl and Pyridyl Molecules via Directed Hydrogen Bonding

The crystallization of two dihydropyridyl molecules, 1,4-bis(4-(3,5-dicyano-2,6-dipyridyl)dihydropyridyl)benzene ([C40H24N10]·2DMF, 1·2DMF; DMF = dimethylformamide) and 1,4-bis(4-(3,5-dicyano-2,6-dipyridyl)dihydropyridyl)phenylbenzene ([C46H28N10]·2DMF, 3·2DMF), and their respective oxidized pyridyl analogues, 1,4-bis(4-(3,5-dicyano-2,6-dipyridyl)pyridyl)benzene ([C40H20N10], 2) and 1,4-bis(4-(3,5-dicyano-2,6-dipyridyl)pyridyl)phenylbenzene ([C46H24N10]·DMF, 4·DMF), has been achieved under solvothermal conditions. The dihydropyridyl molecules are converted to their pyridyl products via in situ oxidative dehydrogenation in solution. The structures of the four molecules have been fully characterized by single crystal and powder X-ray diffraction. The oxidized pyridyl products, 2 and 4, are more elongated due to aromatization of the dihydropyridyl rings at each end of their parent molecules 1 and 3, respectively. The solid-state supramolecular structures of the pyridyl molecules are distinct from the dihydropyridyl molecules in terms of their hierarchical assembly via hydrogen bonding due to the loss of primary N-H hydrogen bond donors in the two electron oxidized tectons. Overall, the geometrically shorter molecules 1 and 3 display close-packed structures, whereas the more extended 2 and 4 assemble into more open supramolecular systems.

NMR crystallography of ezetimibe co-crystals

An efficient, simplified protocol for solvent-drop assisted co-crystal preparation of ezetimibe (a drug for the treatment of primary hypercholesterolemia) with both imidazole and l-proline has been derived. The structures of the white powders were successfully solved via "NMR crystallography" combining solid-state NMR, powder X-ray diffraction and DFT chemical shift computations. Detailed insights into the likely crystallization mechanism were obtained from competition experiments, where efficient co-crystallization was feasible using ezetimibe monohydrate as precursor indicating that the crystal water acts as "molecular catalyst". It was also found that co-crystallization of imidazole is favored over l-proline, thus suggesting a clear preference of neutral hydrogen bonds compared to charge-assisted motifs.

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.

A robust binary supramolecular organic framework (SOF) with high CO2 adsorption and selectivity

A robust binary hydrogen-bonded supramolecular organic framework (SOF-7) has been synthesized by solvothermal reaction of 1,4-bis-(4-(3,5-dicyano-2,6-dipyridyl)dihydropyridyl)benzene (1) and 5,5'-bis-(azanediyl)-oxalyl-diisophthalic acid (2). Single crystal X-ray diffraction analysis shows that SOF-7 comprises 2 and 1,4-bis-(4-(3,5-dicyano-2,6-dipyridyl)pyridyl)benzene (3); the latter formed in situ from the oxidative dehydrogenation of 1. SOF-7 shows a three-dimensional four-fold interpenetrated structure with complementary O-H···N hydrogen bonds to form channels that are decorated with cyano and amide groups. SOF-7 exhibits excellent thermal stability and solvent and moisture durability as well as permanent porosity. The activated desolvated material SOF-7a shows high CO2 adsorption capacity and selectivity compared with other porous organic materials assembled solely through hydrogen bonding.

The solid-state continuum: a perspective on the interrelationships between different solid-state forms in drug substance and drug product

Objective

The objective of the review is to provide an overview of the nomenclature used in the solid-state continuum and relate these to the development of drug substances and drug products.

Key findings

The importance of a rational approach to solid-state form selection, including integrated decision making (ensuring equal weight is given to the needs of the drug substance and the drug product), is vital for the effective development of a drug candidate. For example, how do secondary processing considerations influence the selection of drug substance solid-state form and resulting formulation, and how can drug substance solid-state form be used to optimise secondary processing? Further, the potential use of ‘crystal’ engineering to optimise stability, purity and optical resolutions, and the linked regulatory requirements, will be discussed.

Summary

The nomenclature used in the solid-state continuum, which contains a large number of different crystalline and non-crystalline forms, for example, amorphous systems, was reviewed. Further, the significant role of the drug substance within the solid oral dose form from a physicochemical perspective was covered.

pH-Switchable ampholytic supramolecular copolymers

β-sheet-encoded anionic and cationic dendritic peptide amphiphiles form supramolecular copolymers when self-assembled in a 1:1 feed ratio of the monomers. These ampholytic materials have been designed for on-off polymerization in response to pH triggers. The cooperative supramolecular self-assembly process is switched on at a physiologically relevant pH value and can be switched off by increasing or decreasing the pH value.

Crystal engineering of stable temozolomide cocrystals

The antitumor prodrug temozolomide (TMZ) decomposes in aqueous medium of pH≥7 but is relatively stable under acidic conditions. Pure TMZ is obtained as a white powder but turns pink and then brown, which is indicative of chemical degradation. Pharmaceutical cocrystals of TMZ were engineered with safe coformers such as oxalic acid, succinic acid, salicylic acid, d,l-malic acid, and d,l-tartaric acid, to stabilize the drug as a cocrystal. All cocrystals were characterized by powder X-ray diffraction (PXRD), single crystal X-ray diffraction, and FT-IR as well as FT-Raman spectroscopy. Temozolomide cocrystals with organic acids (pK(a) 2-6) were found to be more stable than the reference drug under physiological conditions. The half-life (T(1/2)) of TMZ-oxalic and TMZ-salicylic acid measured by UV/Vis spectroscopy in pH 7 buffer is two times longer than that of TMZ (3.5 h and 3.6 h vs. 1.7 h); TMZ-succinic acid, TMZ-tartaric acid, and TMZ-malic acid also exhibited a longer half-life (2.3, 2.5, and 2.8 h, respectively). Stability studies at 40 °C and 75 % relative humidity (ICH conditions) showed that hydrolytic degradation of temozolomide in the solid state started after one week, as determined by PXRD, whereas its cocrystals with succinic acid and oxalic acid were intact at 28 weeks, thus confirming the greater stability of cocrystals compared to the reference drug. The intrinsic dissolution rate (IDR) profile of TMZ-oxalic acid and TMZ-succinic acid cocrystals in buffer of pH 7 is comparable to that of temozolomide. Among the temozolomide cocrystals examined, those with succinic acid and oxalic acid exhibited both an improved stability and a comparable dissolution rate to the reference drug.