Development of a quaternary ammonium poly (amidoamine) dendrimer-based drug carrier for the solubility enhancement and sustained release of furosemide

Furosemide (FRSD) is a loop diuretic that has been categorized as a class IV drug according to the Biopharmaceutics Classification System (BCS). It is used in the treatment of congestive heart failure and edema. Owing to low solubility and permeability, its oral bioavailability is very poor. In this study, two types of poly (amidoamine) dendrimer-based drug carriers (generation G2 and G3) were synthesized to increase the bioavailability of FRSD through solubility enhancement and sustained release. The developed dendrimers enhanced the solubility of FRSD 58- and 109-fold, respectively, compared with pure FRSD. In vitro studies demonstrated that the maximum time taken to release 95% of the drug from G2 and G3 was 420-510 min, respectively, whereas for pure FRSD the maximum time was only 90 min. Such a delayed release is strong evidence for sustained drug release. Cytotoxicity studies using Vero and HBL 100 cell lines through an MTT assay revealed increased cell viability, indicating reduced cytotoxicity and improved bioavailability. Therefore, the present dendrimer-based drug carriers are proven to be prominent, benign, biocompatible, and efficient for poorly soluble drugs, such as FRSD. Therefore, they could be convenient choices for real-time applications of drug delivery.

Structural features, dissolution performance and anthelmintic efficacy of multicomponent solid forms of fenbendazole with maleic and oxalic acids

Two new multicomponent crystalline phases of fenbendazole (FNB), a benzimidazole anthelmintic agent, with maleic and oxalic acids have been prepared, and their structural and physicochemical properties carefully investigated.

Quantitative Monitoring of Cocrystal Polymorphisms in Model Tablets Using Transmission Low-Frequency Raman Spectroscopy

Cocrystallization is a technique for improving the physical properties of active pharmaceutical ingredients. However, cocrystals can transform into more stable polymorphs as well as dissociate to original materials. Therefore, an analytical technique is required to determine the polymorphic transformation quickly and accurately in tablets. The purpose of this study is to develop a method to monitor cocrystal polymorphs in model tablets using transmission low-frequency Raman spectroscopy. The tablets, consisting of only metastable polymorphs of caffeine-glutaric acid cocrystals, were stored under various relative humidity levels. The composition of the cocrystal polymorphs were calculated from a calibration curve relating the actual composition to the predicted values calculated by partial least squares regression processing of low-frequency Raman spectra. The metastable form gradually converted to a stable form, and polymorphic phase transformation occurred with increasing relative humidity. Ninety-six percent of the metastable form converted into a stable form stored at 25 °C after 3 h at 95% RH. In conclusion, transmission low-frequency Raman spectroscopy can be used to quantitatively monitor cocrystal polymorphs. This technique is one of the candidate techniques to quantifiably evaluate the physico-chemical stability of cocrystal polymorphs in tablets.

Non-Covalent Reactions Supporting Antiviral Development

Viruses are the current big enemy of the world's healthcare systems. As the small infector causes various deadly diseases, from influenza and HIV to COVID-19, the virus continues to evolve from one type to its mutants. Therefore, the development of antivirals demands tremendous attention and resources for drug researchers around the world. Active pharmaceutical ingredients (API) development includes discovering new drug compounds and developing existing ones. However, to innovate a new antiviral takes a very long time to test its safety and effectiveness, from structure modeling to synthesis, and then requires various stages of clinical trials. Meanwhile, developing the existing API can be more efficient because it reduces many development stages. One approach in this effort is to modify the solid structures to improve their physicochemical properties and enhance their activity. This review discusses antiviral multicomponent systems under the research phase and has been marketed. The discussion includes the types of antivirals, their counterpart compound, screening, manufacturing methods, multicomponent systems yielded, characterization methods, physicochemical properties, and their effects on their pharmacological activities. It is hoped that the opportunities and challenges of solid antiviral drug modifications can be drawn in this review as important information for further antiviral development.

Inhibition of Liquid-Liquid Phase Separation for Breaking the Solubility Barrier of Amorphous Solid Dispersions to Improve Oral Absorption of Naftopidil

Amorphous solid dispersion (ASD) is one of the most promising technologies for improving the oral absorption of poorly soluble compounds. In this study, naftopidil (NFT) ASDs were prepared using vinylpyrrolidone-vinyl acetate copolymer (PVPVA), hydroxypropyl methylcellulose acetate succinate (HPMCAS), and poly(methacrylic acid-co-methyl methacrylate) L100-55 (Eudragit) to improve the dissolution and oral absorption behaviors of NFT. During the dissolution process of ASD, liquid-liquid phase separation (LLPS) may occur when certain requirements are met for providing a maximum quasi-stable concentration achievable by amorphization. The occurrence of LLPS was confirmed in the presence of PVPVA and HPMCAS; however, Eudragit inhibited LLPS owing to its molecular interaction with NFT. Although the dissolution behavior of the Eudragit ASD was found to be markedly poorer than that of other ASDs, it offered the best oral absorption in rats. The findings of the current study highlight the possibility for improving the oral absorption of poorly soluble drugs by this ASD, which should be eliminated from candidate formulations based on the conventional in vitro tests.

Pharmaceutical Salts of Piroxicam and Meloxicam with Organic Counterions

Piroxicam (PRM) and meloxicam (MEL) are two nonsteroidal anti-inflammatory drugs, belonging to the Biopharmaceutics Classification System Class II drugs. In this study, six novel pharmaceutical salts of PRM and MEL with three basic organic counterions, that is, 4-aminopyridine (4AP), 4-dimethylaminopyridine (4DMP), and piperazine (PPZ), were prepared by both slurrying and slow evaporation. These salts were characterized by single-crystal and powder X-ray diffraction, thermal analysis, and Fourier transform infrared spectroscopy. All six salts, especially MEL-4DMP and MEL-4AP, showed a significantly improved apparent solubility and dissolution rate in sodium phosphate solution compared with the pure APIs. Notably, PRM-4AP and PRM-4DMP salts exhibited enhanced fluorescence, and the PRM-PPZ salt showed weaker fluorescence compared with that of pure PRM due to different luminescence mechanisms.

Conformational Trimorphism in an Ionic Cocrystal of Hesperetin

We report the existence of conformational polymorphism in an ionic cocrystal (ICC) of the nutraceutical compound hesperetin (HES) in which its tetraethylammonium (TEA⁺) salt serves as a coformer. Three polymorphs, HESTEA-α, HESTEA-β and HESTEA-γ, were characterized by single-crystal X-ray diffraction (SCXRD). Each polymorph was found to be sustained by phenol···phenolate supramolecular heterosynthons that self-assemble with phenol···phenol supramolecular homosynthons into C ₃ ²(7) H-bonded motifs. Conformational variability in HES moieties and different relative orientations of the H-bonded motifs resulted in distinct crystal packing patterns: HESTEA-α and HESTEA-β exhibit H-bonded sheets; HESTEA-γ is sustained by bilayers of H-bonded tapes. All three polymorphs were found to be stable upon exposure to humidity under accelerated stability conditions for 2 weeks. Under competitive slurry conditions, HESTEA-α was observed to transform to the β or γ forms. Solvent selection impacted the relationship between HESTEA-β (favored in EtOH) and HESTEA-γ (favored in MeOH). A mixture of the β and γ forms was found to be present following H₂O slurry.

Crystal Engineering of Ionic Cocrystals Sustained by Azolium···Azole Heterosynthons

Crystal engineering of multi-component molecular crystals, cocrystals, is a subject of growing interest, thanks in part to the potential utility of pharmaceutical cocrystals as drug substances with improved properties. Whereas molecular cocrystals (MCCs) are quite well studied from a design perspective, ionic cocrystals (ICCs) remain relatively underexplored despite there being several recently FDA-approved drug products based upon ICCs. Successful cocrystal design strategies typically depend on strong and directional noncovalent interactions between coformers, as exemplified by hydrogen bonds. Understanding of the hierarchy of such interactions is key to successful outcomes in cocrystal design. We herein address the crystal engineering of ICCs comprising azole functional groups, particularly imidazoles and triazoles, which are commonly encountered in biologically active molecules. Specifically, azoles were studied for their propensity to serve as coformers with strong organic (trifluoroacetic acid and p-toluenesulfonic acid) and inorganic (hydrochloric acid, hydrobromic acid and nitric acid) acids to gain insight into the hierarchy of NH+···N (azolium-azole) supramolecular heterosynthons. Accordingly, we combined data mining of the Cambridge Structural Database (CSD) with the structural characterization of 16 new ICCs (11 imidazoles, 4 triazoles, one imidazole-triazole). Analysis of the new ICCs and 66 relevant hits archived in the CSD revealed that supramolecular synthons between identical azole rings (A+B−A) are much more commonly encountered, 71, than supramolecular synthons between different azole rings (A+B−C), 11. The average NH+···N distance found in the new ICCs reported herein is 2.697(3) Å and binding energy calculations suggested that hydrogen bond strengths range from 31–46 kJ mol−1. The azolium-triazole ICC (A+B−C) was obtained via mechanochemistry and differed from the other ICCs studied as there was no NH+···N hydrogen bonding. That the CNC angles in imidazoles and 1,2,4-triazoles are sensitive to protonation, the cationic forms having larger (approximately 4.4 degrees) values than comparable neutral rings, was used as a parameter to distinguish between protonated and neutral azole rings. Our results indicate that ICCs based upon azolium-azole supramolecular heterosynthons are viable targets, which has implications for the development of new azole drug substances with improved properties.

Solubility and Thermodynamic Properties of Febuxostat in Various (PEG 400 + Water) Mixtures

The solubility of the poorly soluble medicine febuxostat (FXT) (3) in various {polyethylene glycol 400 (PEG 400) (1) + water (H₂O) (2)} mixtures has been examined at 298.2-318.2 K and 101.1 kPa. FXT solubility was measured using an isothermal method and correlated with "van't Hoff, Apelblat, Buchowski-Ksiazczak λh, Yalkowsky-Roseman, Jouyban-Acree, and Jouyban-Acree-van't Hoff models". FXT mole fraction solubility was enhanced via an increase in temperature and PEG 400 mass fraction in {(PEG 400 (1) + H₂O (2)} mixtures. Neat PEG 400 showed the highest mole fraction solubility of FXT (3.11 × 10^-2 at 318.2 K), while neat H₂O had the lowest (1.91 × 10^-7 at 298.2 K). The overall error value was less than 6.0% for each computational model, indicating good correlations. Based on the positive values of apparent standard enthalpies (46.72-70.30 kJ mol^-1) and apparent standard entropies (106.4-118.5 J mol^-1 K^-1), the dissolution of FXT was "endothermic and entropy-driven" in all {PEG 400 (1) + H₂O (2)} mixtures examined. The main mechanism for FXT solvation in {PEG 400 (1) + H₂O (2)} mixtures was discovered to be an enthalpy-driven process. In comparison to FXT-H₂O, FXT-PEG 400 showed the strongest molecular interactions. In conclusion, these results suggested that PEG 400 has considerable potential for solubilizing a poorly soluble FXT in H₂O.

An Artificial Gut/Absorption Simulator: Simultaneous Evaluation of Desupersaturation and Absorption from Ketoconazole Supersaturated Solutions

For supersaturating formulations of BCS-II compounds, which by definition have high intestinal permeability, a closed USP apparatus does not provide the necessary absorptive conditions during dissolution. To address this, an artificial gut simulator (AGS) has been constructed consisting of a 2.5 mL donor compartment in which a hollow fiber-based absorption module is suspended. Drug from donor diffuses across the hollow fiber membrane to be absorbed by the continuously flowing intraluminal receiver fluid. The membrane surface area and intraluminal fluid flow rate are tuned to obtain the physiologically observed absorption rate constant for a weakly basic, poorly water-soluble model compound, ketoconazole (KTZ). Supersaturated solutions of KTZ were generated in the donor in pH 6.5 phosphate buffer by the pH-shift method in the absence (closed system, control) and presence (open system, biorelevant) of an optimally or suboptimally tuned absorption module. Drug concentrations in the donor and intraluminal fluids were determined by in-line UV spectroscopy. The presence of an absorptive sink reduced the supersaturated solution's crystallization propensity, more so in the case of the optimally tuned AGS. This study demonstrates the significance of simulating absorption of drug at a physiological rate during dissolution studies, especially to predict the performance of formulations of BCS-II drugs.

Comparing and Quantifying the Efficiency of Cocrystal Screening Methods for Praziquantel

Pharmaceutical cocrystals are highly interesting due to their effect on physicochemical properties and their role in separation technologies, particularly for chiral molecules. Detection of new cocrystals is a challenge, and robust screening methods are required. As numerous techniques exist that differ in their crystallization mechanisms, their efficiencies depend on the coformers investigated. The most important parameters characterizing the methods are the (a) screenable coformer fraction, (b) coformer success rate, (c) ability to give several cocrystals per successful coformer, (d) identification of new stable phases, and (e) experimental convenience. Based on these parameters, we compare and quantify the performance of three methods: liquid-assisted grinding, solvent evaporation, and saturation temperature measurements of mixtures. These methods were used to screen 30 molecules, predicted by a network-based link prediction algorithm (described in Cryst. Growth Des. 2021, 21(6), 3428-3437) as potential coformers for the target molecule praziquantel. The solvent evaporation method presented more drawbacks than advantages, liquid-assisted grinding emerged as the most successful and the quickest, while saturation temperature measurements provided equally good results in a slower route yielding additional solubility information relevant for future screenings, single-crystal growth, and cocrystal production processes. Seventeen cocrystals were found, with 14 showing stability and 12 structures resolved.

A novel cocrystal of metformin hydrochloride with citric acid: Systematic synthesis and computational simulation

Pharmaceutical cocrystals have matured into an effective technique for tuning the physicochemical and mechanical properties of drugs in solid form simultaneously. Herein, in order to provide a novel cocrystal form of oral medicine metformin hydrochloride (MH), citric acid (CA) was selected as an efficient ligand after screening a variety of inorganic and organic acids. Thus, based on the principle of crystal engineering, we report a novel cocrystal: metformin hydrochloride - citric acid (MHCA) after the systematic screening, which was experimentally proved to be constituted with 1:1 stoichiometry. Compared with pure MH, MHCA has been proved higher solubility in water, methanol, and ethanol from 283.15 to 313.15 K. Through single-crystal X-ray diffraction (SC-XRD), the particular molecular structure of MHCA has been determined as the orthorhombic system and Pbca space group. Besides, the binding model of MH-CA was built for investigating the binding energy and stability between two components at 278, 298, and 318 K, which were found to be essential for the prediction and analysis of cocrystals. The contribution of different intermolecular interactions and the strength of molecular packing in the cocrystal also have been investigated by Hirshfeld surface analysis. It was found that the cocrystal structure was mainly stabilized by intermolecular hydrogen bonds existing as N-H···O between components, which indicated that the diffusion-combination trend of molecules enhanced the regular array of cocrystal. The results revealed that the molecules of MH and CA formed supramolecular cocrystals mainly induced by hydrogen bonds after passive contacts, such as co-crystallization or grind.

The Relevance of Crystal Forms in the Pharmaceutical Field: Sword of Damocles or Innovation Tools?

This review is aimed to provide to an “educated but non-expert” readership and an overview of the scientific, commercial, and ethical importance of investigating the crystalline forms (polymorphs, hydrates, and co-crystals) of active pharmaceutical ingredients (API). The existence of multiple crystal forms of an API is relevant not only for the selection of the best solid material to carry through the various stages of drug development, including the choice of dosage and of excipients suitable for drug development and marketing, but also in terms of intellectual property protection and/or extension. This is because the physico-chemical properties, such as solubility, dissolution rate, thermal stability, processability, etc., of the solid API may depend, sometimes dramatically, on the crystal form, with important implications on the drug’s ultimate efficacy. This review will recount how the scientific community and the pharmaceutical industry learned from the catastrophic consequences of the appearance of new, more stable, and unsuspected crystal forms. The relevant aspects of hydrates, the most common pharmaceutical solid solvates, and of co-crystals, the association of two or more solid components in the same crystalline materials, will also be discussed. Examples will be provided of how to tackle multiple crystal forms with screening protocols and theoretical approaches, and ultimately how to turn into discovery and innovation the purposed preparation of new crystalline forms of an API.

Amide-Driven Secondary Building Unit Structural Transformations between Zn(II) Coordination Polymers

The behavior of coordination polymers (CPs) against external stimuli has witnessed remarkable attention, especially when the resulting CPs present reversible molecular arrays. Accordingly, CPs with these characteristics can lead to differences in their properties owing to these structural differences, being promising for their use as potential molecular switches with diverse applications. Herein, we have synthesized four Zn(II) CPs bearing α-acetamidocinnamic acid (HACA) and 4,4'-bipyridine (4,4'-bipy). The reaction between Zn(OAc)₂·2H₂O, HACA, and 4,4'-bipy yields {[Zn(ACA)₂(4,4'-bipy)]·EtOH} _n (1), which was used for the formation of three CPs through dissolution-recrystallization structural transformations (DRSTs): {[Zn(ACA)₂(4,4'-bipy)]·2MeOH} _n (2), {[Zn₂(μ-ACA)₂(ACA)₂(4,4'-bipy)]·2H₂O} _n (3), and {[Zn₃(μ-ACA)₆(4,4'-bipy)]·0.75CHCl₃} _n (4). The study of the four crystal structures revealed that their secondary building units (SBUs) comprise monomeric, dimeric, and trimeric arrangements linked by 4,4'-bipy ligands. The fundamental role of the utilized solvent and/or temperature, as well as their effect on the orientation of the amide moieties driving the formation of the different SBUs is discussed. Furthermore, the reversibility and interconversion between the four CPs have been assayed. Finally, their solid-state photoluminescence has evinced that the effect of the amide moieties not only predetermine a different SBU but also lead to a different emission in 4 compared with 1-3.

DFT–Assisted Structure Determination from Powder X-ray Diffraction Data of a New Zonisamide/ϵ-Caprolactam Cocrystal

The crystal structure of a new zonisamide cocrystal, an anticonvulsant drug used to treat the symptoms of epilepsy and Parkinson’s disease, with ϵ-caprolactam is reported herein. The structure has been solved by direct space methodologies from powder X-ray diffraction data. The refinement of the structure was conducted by the Rietveld method assisted by the dispersion-corrected density-functional theory (D-DFT) calculations and periodic boundary conditions. Further analysis of the structure reveals several H-bonded synthons and self–assembled dimers that have been further analyzed by DFT calculations and other computational tools such as molecular electrostatic potential (MEP) surfaces and the quantum theory of “atom-in-molecules” (QTAIM).

Synchronization of Cocrystal Dissolution and Drug Precipitation to Sustain Drug Supersaturation

A graphical analysis of both drug and coformer concentrations contributed by dissolving cocrystals is presented in the context of a simplified cocrystal phase diagram. The conceptual basis and analysis identify parameters that control cocrystal dissolution-drug supersaturation-precipitation (DSP) behavior. The important effects of coformer concentration, cocrystal dose, and cocrystal solubility on drug supersaturation levels are demonstrated and quantified by the DSP_index. While the studies presented rely on high and nonstoichiometric coformer concentrations contributed by the dissolving cocrystals, the concepts and findings can answer the question of whether and how much coformer should be added to cocrystal dissolution media or formulations.

Crystal Engineering of Ionic Cocrystals Sustained by the Phenol-Phenolate Supramolecular Heterosynthon

Although crystal engineering strategies are generally well explored in the context of multicomponent crystals (cocrystals) formed by neutral coformers (molecular cocrystals), cocrystals comprised of one or more salts (ionic cocrystals, ICCs) are understudied. We herein address the design, preparation, and structural characterization of ICCs formed by phenolic moieties, a common group in natural products and drug molecules. Organic and inorganic bases were reacted with the following phenolic coformers: phenol, resorcinol, phloroglucinol, 4-methoxyphenol, and 4-isopropylphenol. Nine ICCs were crystallized, each of them sustained by the phenol-phenolate supramolecular heterosynthon (PhOH···PhO^-). Such ICCs are of potential utility, and there are numerous examples of phenolic compounds that are biologically active, some of which suffer from low aqueous solubility. The propensity to form ICCs sustained by the PhOH···PhO^- supramolecular heterosynthon was evaluated through a combination of Cambridge Structural Database (CSD) mining, structural characterization of nine novel ICCs, and calculation of interaction energies. Our analysis of these 9 ICCs and the 41 relevant entries archived in the CSD revealed that phenol groups can reliably form ICCs through charge-assisted PhOH···PhO^- interactions. This conclusion is supported by hydrogen-bond strength calculations derived from CrystalExplorer that reveal the PhOH···PhO^- interaction to be around 3 times stronger than the phenol-phenol hydrogen bond. The PhOH···PhO^- supramolecular heterosynthon could therefore enable crystal engineering studies of a large number of phenolic pharmaceutical and nutraceutical compounds with their conjugate bases.

Two Novel Co-Crystals of Naproxen: Comparison of Stability, Solubility and Intermolecular Interaction

Two novel co-crystals of naproxen (NPX) were designed and prepared at a stoichiometric ratio of 1:1, namely, naproxen–caprolactam (NPX–CPL) and naproxen–oxymatrine (NPX–OMT). The characteristics of the co-crystals were evaluated in terms of stability and solubility studies. In terms of solubility, in four kinds of solvent systems with different pH, the solubility of NPX–OMT was significantly improved compared with that of NPX, whereas the NPX–CPL showed advantages in acidic solvent systems, indicating that the co-crystals can be applied to concoct preparations depending on therapeutic purposes. Furthermore, the experimental results of the thermal analysis showed that the co-crystal NPX–OMT had better thermal stability than the co-crystal NPX–CPL. Finally, as a complement to the single crystal X-ray diffraction (SC XRD) method, the theoretical calculation based on density functional theory (DFT) was also used to reveal the intermolecular interaction of the co-crystals at the molecular level and visually display the difference between them.

Levofloxacin Cocrystal/Salt with Phthalimide and Caffeic Acid as Promising Solid-State Approach to Improve Antimicrobial Efficiency

To overcome the issue of multidrug resistant (MDR) microbes, the exploration of ways to improve the antimicrobial efficiency of existing antibiotics is one of the promising approaches. In search of synthons with higher efficiency, in current investigations, cocrystal and amorphous salt of levofloxacin hemihydrate (LEV) were developed with phthalimide (PTH) and caffeic acid (CFA). New materials were characterized with the help of FT-IR, Raman spectroscopy, powder X-ray diffraction (PXRD), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Shifting, attenuation, appearance/disappearance and broadening of bands were observed in the FT-IR and Raman spectra of the materials as evidence of the required product. The PXRD diffraction pattern observed for LEV-PTH indicated cocrystal while halo diffractogram of LEV-CFA revealed amorphous nature. DSC/TG analysis confirmed the hydrated nature of the cocrystal/salt. The dissolution rate and antimicrobial activity against selected strains, K.pneumonia, E. coli and S. typhi of parent drug and the new material were compared. The zone of inhibition (ZI) observed for 5 µg LEV-PTH was 30.4 + 0.36 (K. pneumonia), 26.33 + 0.35 (E. coli) and 30.03 + 0.25 mm (S. typhi) while LEV-CFA salt (5 µg) against the same strains inhibited 33.96 ± 0.25, 31.66 ± 0.35 and 27.93 ± 0.40 mm, respectively. These novel formulations enhance the dissolution rate as well as antibacterial efficiency and are expected to be potent against MDR bacterial strains.

Polymorphism in Ionic Cocrystals Comprising Lithium Salts and l-Proline

The occurrence of polymorphism in ionic cocrystals formed by two lithium salts, lithium salicylate (LIS) and lithium 4-methoxybenzoate (L4M), and l-proline (PRO) has been investigated. The previously reported monoclinic form of the 1:1 cocrystal of LIS and PRO, LISPRO(α), and a new thermodynamically stable orthorhombic polymorph, LISPRO(β), were prepared and characterized. The two polymorphs form square grid, sql, topology coordination networks and differ mainly in the conformation of the salicylate ions and positioning of the sql nets. LISPRO(α) was observed to transform to LISPRO(β) under slurry conditions. The 1:1 ionic cocrystal of L4M and PRO (L4MPRO) was found to form three polymorphs. Apart from the previously reported orthorhombic crystal form, L4MPRO(α), two new monoclinic crystal forms, L4MPRO(β) and L4MPRO(γ), were obtained by modifying crystallization conditions. The new polymorphs were found to be metastable, undergoing transformations to L4MPRO(α) upon exposure to humidity. Experimental conditions that induce transformations between the polymorphs of LISPRO and L4MPRO are detailed, and the structural differences between the polymorphs are discussed in the broader context of polymorphism.

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.

Intermolecular Interactions of 3,5-bis(4-Methoxyphenyl)-4,5-dihydro-1H-pyrazole-1-carbothioamide in a Cocrystal with 1,3-bis(4-Methoxyphenyl)prop-2-en-1-one and Dimethylformamide Solvate

Two new multicomponent crystals consisting of 3,5-bis(4-methoxyphenyl)-4,5-dihydro-1H-pyrazole-1-carbothioamide (1) with 1,3-bis(4-methoxyphenyl)prop-2-en-1-one (2) and with dimethylformamide (DMF), both in 1:1 ratio, prepared and structurally characterized. The occurrence of 1 in different crystal structures enabled a comparison of hydrogen bonding contacts between the two structures as well as with the known structure of pure 1. The backbone of molecule 1 was similar in the structures but the orientation of the methoxy groups varied. Molecule 1 was involved in various combinations of the possible hydrogen bonding contacts, including N–H…O=C, N–H…OMe, and N–H…S. Both N–H hydrogens in the cocrystal (1–2) and the solvate (1-DMF) participated in hydrogen bonding but only one hydrogen atom took part in the structure of pure 1. The S atom accepted contacts in both the structures of pure 1 and cocrystal 1–2 but not in that of the 1-DMF solvate. The oxygen atoms of both methoxy groups acted as acceptors in the structure of pure 1, whereas one oxygen was involved in the 1-DMF solvate and none in cocrystal 1–2.

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.

Solid-State and Particle Size Control of Pharmaceutical Cocrystals using Atomization-Based Techniques

Poor bioavailability and aqueous solubility represent a major constraint during the development of new API molecules and can influence the impact of new medicines or halt their approval to the market. Cocrystals offer a novel and competitive advantage over other conventional methods with respect towards the substantial improvement in solubility profiles relative to the single-API crystals. Furthermore, the production of such cocrystals through atomization-based methods allow for greater control, with respect to particle size reduction, to further increase the solubility of the API. Such atomization-based methods include supercritical fluid methods, conventional spray drying and electrohydrodynamic atomization/electrospraying. The influence of process parameters such as solution flow rates, pressure and solution concentration, in controlling the solid-state and final particle size are discussed in this review with respect to atomization-based methods. For the last decade, literature has been attempting to catch-up with new regulatory rulings regarding the classification of cocrystals, due in part to data sparsity. In recent years, there has been an increase in cocrystal publications, specifically employing atomization-based methods. This review considers the benefits to employing atomization-based methods for the generation of pharmaceutical cocrystals, examines the most recent regulatory changes regarding cocrystals and provides an outlook towards the future of this field.

Differences in Coformer Interactions of the 2,4-Diaminopyrimidines Pyrimethamine and Trimethoprim

The identification and study of supramolecular synthons is a fundamental task in the design of pharmaceutical cocrystals. The malaria drug pyrimethamine (pyr) and the antibiotic trimethoprim (tmp) are both 2,4-diaminopyrimidine derivatives, providing the same C-NH₂/N=C/C-NH₂ and C-NH₂/N=C interaction sites. In this article, we analyze and compare the synthons observed in the crystal structures of tmp and pyr cocrystals and molecular salts with sulfamethazine (smz), α-ketoglutaric acid (keto), oxalic acid (ox), sebacic acid (seb), and azeliac acid (az). We show that the same coformer interacts with different binding sites of the 2,4-diaminopyrimidine ring in the respective tmp and pyr cocrystals or binds at the same site but gives H bonding patterns with different graph set notions. Pyr·smz·CH₃OH is the first crystal structure in which the interaction of the sulfa drug at the C-NH₂/N=C/C-NH₂ site with three parallel NH₂···N, N···NH_sulfonamide, and NH₂···O=S H bonds is observed. The main synthon in (tmp⁺)(keto^-).0.5H₂O and (tmp⁺)₂(ox^2-)·2CH₃OH is the motif of fused R ₂ ¹(6) and R ₁ ²(5) rings instead of the R ₂ ²(8) motif typically observed in tmp⁺ and pyr⁺ carboxylates. Tmp/az is a rare example of cocrystal-salt polymorphism where the two solid-state forms have the same composition, stoichiometry, and main synthon. Theoretical calculations were performed to understand the order of stability, which is tmp·az cocrystal > (tmp⁺)(az^-) salt. Finally, two three-component tmp/sulfa drug/carboxylate cocrystals with a unique ternary synthon are described.

Supramolecular Synthon Promiscuity in Phosphoric Acid-Dihydrogen Phosphate Ionic Cocrystals

Approximately 80% of active pharmaceutical ingredients (APIs) studied as lead candidates in drug development exhibit low aqueous solubility, which typically results in such APIs being poorly absorbed and exhibiting low bioavailability. Salts of ionizable APIs and, more recently, pharmaceutical cocrystals can address low solubility and other relevant physicochemical properties. Pharmaceutical cocrystals are amenable to design through crystal engineering because supramolecular synthons, especially those sustained by hydrogen bonds, can be anticipated through computational modeling or Cambridge Structural Database (CSD) mining. In this contribution, we report a combined experimental and CSD study on a class of cocrystals that, although present in approved drug substances, remains understudied from a crystal engineering perspective: ionic cocrystals composed of dihydrogen phosphate (DHP) salts and phosphoric acid (PA). Ten novel DHP:PA ionic cocrystals were prepared from nine organic bases (4,4'-bipyridine, 5-aminoquinoline, 4,4'-azopyridine, 1,4-diazabicyclo[2.2.2]octane, piperazine, 1,2-bis(4-pyridyl)ethane, 1,2-bis(4-pyridyl)xylene, 1,2-di(4-pyridyl)-1,2-ethanediol, and isoquinoline-5-carboxylic acid) and one anticonvulsant API, lamotrigine. From the resulting crystal structures and a CSD search of previously reported DHP:PA ionic cocrystals, 46 distinct hydrogen bonding motifs (HBMs) have been identified between DHP anions, PA molecules, and, in some cases, water molecules. Our results indicate that although DHP:PA ionic cocrystals are a challenge from a crystal engineering perspective, they are formed reliably and, given that phosphoric acid is a pharmaceutically acceptable coformer, this makes them relevant to pharmaceutical science.

Cold photo-carving of halogen-bonded co-crystals of a dye and a volatile co-former using visible light

The formation of co-crystals by the assembly of molecules with complementary molecular recognition functionalities is a popular strategy to design or improve a range of solid-state properties, including those relevant for pharmaceuticals, photo- or thermoresponsive materials and organic electronics. Here, we report halogen-bonded co-crystals of a fluorinated azobenzene derivative with a volatile component-either dioxane or pyrazine-that can be cut, carved or engraved with low-power visible light. This cold photo-carving process is enabled by the co-crystallization of a light-absorbing azo dye with a volatile component, which gives rise to materials that can be selectively disassembled with micrometre precision using low-power, non-burning laser irradiation or a commercial confocal microscope. The ability to shape co-crystals in three dimensions using laser powers of 0.5-20 mW-substantially lower than those used for metals, ceramics or polymers-is rationalized by photo-carving that targets the disruption of weak supramolecular interactions, rather than the covalent bonds or ionic structures targeted by conventional laser beam or focused ion beam machining processes.

Three-Dimensional Supramolecular Architectures with MnII Ions Assembled from Hydrogen Bonding Interactions: Crystal Structures and Antiferromagnetic Properties

Two novel cocrystal Mn^II compounds were successfully synthesized. The composition of two kinds crystals correspond to [Mn(hfac)₂L^a ₂·Mn(hfac)₂L^a(H₂O)·Mn(hfac)₂(H₂O)₂] (1) and [Mn(hfac)₂L^b ₂·Mn(hfac)₂(H₂O)₂·0.5(C₆H₁₄)] (2) [L^a = 1,3-bis(1'-oxyl-3'-oxido-4',4',5',5'-tetramethyl-4,5-dihydro-1H-imidazol-2-y1)benzene; L^b = 1-(1'-oxyl-4',4',5',5'-tetramethylimidazolin-2-yl)-3-(1'-oxyl-3'-oxo-4',4',5',5'-tetramethylimidazolin-2-yl)benzene; hfac = hexafluoroacetylacetonato). Surprisingly, the compounds were not polymeric or clusters but, more interestingly, different ratio biradical-metal coordination compound cocrystals. The extensive intramolecular H-bonds are the cause of formation of the cocrystal structures by assembly in the two manganese(II) derivatives; and another factor is the halogen bonds between CF₃ of hfac groups. Furthermore, three-dimensional supramolecular architectures were formed. The magnetic susceptibility of both compounds showed strong antiferromagnetic interactions involving the coordinated radical unit and the metal and lesser contribution from ferromagnetic interactions between the radical units. For compound 1, a good fit was obtained for g _Mn = 2.08, g _rad = 2.00 (fixed), J ₁ = -294.3 cm^-1, J ₂ = 6.2 cm^-1 and J ₃ = 10.8 cm^-1. A reasonable fit for compound 2 was obtained for g _Mn = 2.04, g _rad = 2.00 (fixed), J ₁' = -273.4 cm^-1 and J ₂' = 8.6 cm^-1.

Nanocrystals based pulmonary inhalation delivery system: advance and challenge

Pulmonary inhalation administration is an ideal approach to locally treat lung disease and to achieve systemic administration for other diseases. However, the complex nature of the structural characteristics of the lungs often results in the difficulty in the development of lung inhalation preparations. Nanocrystals technology provides a potential formulation strategy for the pulmonary delivery of poorly soluble drugs, owing to the decreased particle size of drug, which is a potential approach to overcome the physiological barrier existing in the lungs and significantly increased bioavailability of drugs. The pulmonary inhalation administration has attracted considerable attentions in recent years. This review discusses the barriers for pulmonary drug delivery and the recent advance of the nanocrystals in pulmonary inhalation delivery. The presence of nanocrystals opens up new prospects for the development of novel pulmonary delivery system. The particle size control, physical instability, potential cytotoxicity, and clearance mechanism of inhaled nanocrystals based formulations are the major considerations in formulation development.

Local Anesthetic Ropivacaine Exhibits Therapeutic Effects in Cancers

Despite the significant progress in cancer treatment, new anticancer therapeutics drugs with new structures and/or mechanisms are still in urgent need to tackle many key challenges. Drug repurposing is a feasible strategy in discovering new drugs among the approved drugs by defining new indications. Recently, ropivacaine, a local anesthetic that has been applied in clinical practice for several decades, has been found to possess inhibitory activity and sensitizing effects when combined with conventional chemotherapeutics toward cancer cells. While its full applications and the exact targets remain to be revealed, it has been indicated that its anticancer potency was mediated by multiple mechanisms, such as modulating sodium channel, inducing mitochondria-associated apoptosis, cell cycle arrest, inhibiting autophagy, and/or regulating other key players in cancer cells, which can be termed as multi-targets/functions that require more in-depth studies. In this review, we attempted to summarize the research past decade of using ropivacaine in suppressing cancer growth and sensitizing anticancer drugs both in-vitro and in-vivo, and tried to interpret the underlying action modes. The information gained in these findings may inspire multidisciplinary efforts to develop/discover more novel anticancer agents via drug repurposing.

Molecular Structural, Hydrogen Bonding Interactions, and Chemical Reactivity Studies of Ezetimibe-L-Proline Cocrystal Using Spectroscopic and Quantum Chemical Approach

Ezetimibe (EZT) being an anticholesterol drug is frequently used for the reduction of elevated blood cholesterol levels. With the purpose of improving the physicochemical properties of EZT, in the present study, cocrystals of ezetimibe with L-proline have been studied. Theoretical geometry optimization of EZT-L-proline cocrystal, energies, and structure–activity relationship was carried out at the DFT level of theory using B3LYP functional complemented by 6-311++G(d,p) basis set. To better understand the role of hydrogen bonding, two different models (EZT + L-proline and EZT + 2L-proline) of EZT-L-proline cocrystal were studied. Spectral techniques (FTIR and FT-Raman) combined with quantum chemical methodologies were successfully implemented for the detailed vibrational assignment of fundamental modes. It is a zwitterionic cocrystal hydrogen bonded with the OH group of EZT and the COO− group of L-proline. The existence and strength of hydrogen bonds were examined by a natural bond orbital analysis (NBO) supported by the quantum theory of atoms in molecule (QTAIM). Chemical reactivity was reflected by the HOMO–LUMO analysis. A smaller energy gap in the cocrystal in comparison to API shows that a cocrystal is softer and chemically more reactive. MEPS and Fukui functions revealed the reactive sites of cocrystals. The calculated binding energy of the cocrystal from counterpoise method was −11.44 kcal/mol (EZT + L-proline) and −26.19 kcal/mol (EZT + 2L-proline). The comparative study between EZT-L-proline and EZT suggest that cocrystals can be better used as an alternative to comprehend the effect of hydrogen bonding in biomolecules and enhance the pharmacological properties of active pharmaceutical ingredients (APIs).

Simultaneous improvement of physical stability, dissolution, bioavailability, and antithrombus efficacy of Aspirin and Ligustrazine through cocrystallization

A novel 1:1 cocrystal between two cardiovascular drugs, aspirin (ASA) and ligustrazine (tetramethylpyrazine, TMP) has been synthesized and characterized. The structure of this drug-drug cocrystal, ASA-TMP, was determined using single crystal X-ray crystallography. The ASA-TMP cocrystal exhibits a significantly reduced sublimation tendency than TMP. Importantly, cocrystallization simultaneously improves bioavailability of both parent drugs. This suggests the possibility of developing a more effective antithrombosis drug therapy given the synergistic pharmacological effects of the two parent drugs.