Supramolecular chemistry and crystal engineering

Perspectives on weak interactions in complex materials at different length scales

Nanocomposite materials consist of nanometer-sized quantum objects such as atoms, molecules, voids or nanoparticles embedded in a host material. These quantum objects can be exploited as a super-structure, which can be designed to create material properties targeted for specific applications. For electromagnetism, such targeted properties include field enhancements around the bandgap of a semiconductor used for solar cells, directional decay in topological insulators, high kinetic inductance in superconducting circuits, and many more. Despite very different application areas, all of these properties are united by the common aim of exploiting collective interaction effects between quantum objects. The literature on the topic spreads over very many different disciplines and scientific communities. In this review, we present a cross-disciplinary overview of different approaches for the creation, analysis and theoretical description of nanocomposites with applications related to electromagnetic properties.

Dehydroepiandrosterone Cocrystals with Improved Solubility and Bioavailability

Dehydroepiandrosterone (DHEA) is an FDA-approved food supplement used as an assisted reproductive sex hormone. The bioavailability is severely limited by its poor solubility (23 µg/mL). Herein, we aimed to modulate its solubility through cocrystallization. Eight cocrystals of DHEA with pyrocatechol (CAT), hydroquinone (HQ), resorcinol (RES), phloroglucinol (PG), 1,5-dihydroxy naphthalene (DHN), p-hydroxybenzoic acid (PHBA), gallic acid (GA), and 5-hydroxyisophthalic acid (5HIPA) were designed and synthesized. Some basic characterization tools, including powder X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry, and Fourier transform infrared spectroscopy, were also applied in our work for basic analyses of cocrystals. It is indicated that DHEA-GA exhibits its superiority in dissolution and pharmacokinetic behaviors. While the area under the curve values of DHEA-GA is improved at the ratio of 2.2, the corresponding bioavailability of DHEA is expected to be accordingly increased.

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.

Mechanical Force Induced Formation of Extrinsic Micropores in Coordination Polymers

Mechanical force can be employed not only to efficiently synthesize new materials under environmentally friendly conditions but also to change the macroscopic and microscopic properties of materials. Although coordination polymers (CPs) are attractive functional materials because they possess high structural designability and diversity, mechanical force-induced structural and functional changes of CPs are challenging issues. In this study, two one-dimensional CPs, one a densely packed nonporous CP [Cu₂(bza)₄(pyr)] (1) and the other a porous CP [Cu₂(1-nap)₄(pyr)] (2) (bza = benzoate, 1-nap = 1-naphthoate and pyr = pyrimidine), were subjected to ball-milling to assess the effect of mechanical force on their porosities. Ball mill treatments were found to induce an amorphization and cause a 30 fold enhancement of the CO₂ adsorption amount at 195 K and P/P₀ ∼ 1 for 1 and a slightly decreased CO₂ adsorption amount for 2. The results of thorough characterization studies suggest that the formation of extrinsic micropores in addition to extrinsic mesopores/macropores between particles takes place by ball milling.

Contributions of secondary alcohol-ketone O-H...O=C and furan-acetate Csp2-H...OOC synthons to the supramolecular packings of two bioactive molecules

The crystal structures of rubescin D (1, C₂₆H₃₀O₅) and monadelphin A (2, C₃₀H₃₆O₁₁), bioactive molecules of the vilasinin and gedunin classes of limonoids, respectively, are reported for the first time and the synthons affecting their crystal packings are analyzed on the basis of their occurrences in molecules in the Cambridge Structural Database that share the same moieties. Rubescin D, 1, crystallizes in the space group P2₁ and its molecular structure consists of three six-membered rings A, C and D having, respectively, envelope, twist-boat and half-chair conformations, and three five-membered rings with half-chair (B and E) and planar conformations (F). Many synthons found in the crystal packing of 1 are in agreement with expectations derived from molecules displaying the same moieties. However, the secondary alcohol-ketone O-H...O=C synthon, which has a low occurrence (2.9%), contributes much to the layered packing, while the furan-ketone Csp²-H...O=C and secondary alcohol-epoxide O-H...OC₂ synthons usually found in these compounds (occurrences of 20.6 and 17.6%, respectively) are missing. The packing of 1 is close to that of ceramicine B (3), but is completely different from that of TS3 (4), suggesting that the absence of the epoxide group in 3 would have favoured the furan-secondary alcohol Csp²-H...OH synthon and that the missing hydroxy group in 4, a strong hydrogen-bond donor, would have favoured the involvement of water molecules in the crystal packing. The molecular structure of monadelphin A, 2, consists of four six-membered fused rings (A, B, C and D) and one five-membered ring (E); they have twist-boat (A and C), chair (B), screw-boat (D) and planar (E) conformations. The molecule crystallizes in the space group P2₁2₁2₁ with the contribution of many synthons usually found in compounds having the same moieties. However, the secondary alcohol-acetate O-H...OOC and secondary alcohol-ketone O-H...O=C synthons (occurrences of 16.7% each in these compounds) are missing. The furan-acetate Csp²-H...OOC synthon not observed in these compounds greatly contributes to the layered packing of 2. The layered packing is very close to those of 7-oxogedunin (5) and 6-dehydro-7-deacetoxy-7-oxogedunin (6), which both crystallize in the space group P2₁.

A novel crystalline molecular salt of sulfamethoxazole and amantadine hybridizing antiviral-antibacterial dual drugs with optimal in vitro/vivo pharmaceutical properties

In order to exploit the advantages to the full of multidrug salification strategy in amending the pharmaceutical properties of drugs both in vitro and in vivo, and further to open up a new way for its applications in bacteria-virus mixed cross-infection drugs, a novel dual-drug crystalline molecular salt hybridizing antibacterial drug sulfamethoxazole (SFM) with antiviral ingredient amantadine (ATE), namely SFM-ATE, is successfully designed and synthesized via multidrug salification strategy oriented by proton exchange reaction. The crystal structure of the firstly obtained molecular salt is precisely identified by employing single-crystal X-ray diffraction and multiple other techniques. The results show that, in the crystal lattice of molecular salt SFM-ATE, the classical hydrogen bonds together with charge-assisted hydrogen bonds contribute to two- dimensional networks, between which the hydrophobic interaction plays an important role. The relevant in vitro/vivo pharmaceutical properties of the dual-drug molecular salt are carried out through a comparative investigation of theoretical and experimental methods. It has been found that SFM displays concurrent improvements over the bulk drug in its permeability and dissolution after forming the molecular salt, which is supported by the molecular electrostatic potential calculation and Hirshfeld surface analysis. Encouragingly, the perfected in vitro biopharmaceutical properties can effectually turn into the in vivo pharmacokinetic preponderances with the expedited peak plasma concentration, lengthened half-life and enhanced bioavailability. Better yet, the antibacterial activities of SFM from the molecular salt get stronger with enlargement in inhibition areas and reduction in values of minimum inhibitory concentrations against the tested bacterial strains. Consequently, the present contribution not only supplies an opportunity for widening applications for classical sulfa drugs via dual-drug salification strategy, but also offers an alternative approach in dealing with viral-bacterial coinfection even other complex diseases by drugs' hybridization at the molecular level.

Synergistic effect of various intermolecular interactions on self-assembly and optoelectronic behaviour in co-crystals/salts of tetrabromoterephthalic acid: a report on their structure, theoretical study and Hirshfeld surface analysis

Significance of Br···O and Br···π interactions in self-assembly in presence of hydrogen bonding and π···π interactions and the importance of charge separation, Br···π and π···π interactions on opto-electrical properties have been established.

Investigating the solid-state assembly of pharmaceutically-relevant N,N-dimethyl-O-thiocarbamates in the absence of labile hydrogen bonds

There are many active pharmaceutical ingredients that lack N–H, O–H and S–H hydrogen-bond donor functional groups.

A crystalline solid adduct of sulfathiazole–amantadine: the first dual-drug molecular salt containing both antiviral and antibacterial ingredients

The physicochemical properties and bioactivity of a newly synthesized dual-drug molecular salt, sulfathiazole–amantadine, are theoretically and experimentally studied.

Insight into the Structure and Properties of Novel Imidazole-Based Salts of Salicylic Acid

The preparation of new active pharmaceutical ingredient (API) multicomponent crystal forms, especially co-crystals and salts, is being considered as a reliable strategy to improve API solubility and bioavailability. In this study, three novel imidazole-based salts of the poorly water-soluble salicylic acid (SA) are reported exhibiting a remarkable improvement in solubility and dissolution rate properties. All structures were solved by powder X-ray diffraction. Multiple complementary techniques were used to solve co-crystal/salt ambiguities: density functional theory calculations, Raman and ¹H/¹³C solid-state NMR spectroscopies. In all molecular salts, the crystal packing interactions are based on a common charged assisted ⁺N-H_(SA)⋯O⁻_(co-former) hydrogen bond interaction. The presence of an extra methyl group in different positions of the co-former, induced different supramolecular arrangements, yielding salts with different physicochemical properties. All salts present much higher solubility and dissolution rate than pure SA. The most promising results were obtained for the salts with imidazole and 1-methylimidazole co-formers.

What if Cocrystallization Fails for Neutral Molecules? Screening Offered Eutectics as Alternate Pharmaceutical Materials: Leflunomide-a Case Study

Background: The manuscript is aimed to optimize the biopharmaceutical parameters of a poorly soluble, neutral anti-rheumatic drug ‘leflunomide’ by preparing its non-covalent derivatives (NCDs). For this various monocarboxylic acids- (adipic acid, picolinic acid) and dicarboxylic acids (maleic acid, malonic acid, sorbic acid), as well as pyridine carboxamide derivatives (nicotinamide, isonicotinamide), are used as coformers. Methods: The novel solid forms were rationally prepared and systematically characterized. Further, these solid forms were subjected to equilibrium solubility and intrinsic dissolution rate (IDR) analysis in three aqueous media (pH 1.2, pH 4.5 and pH 6.8). In vivo plasma studies in male Wistar rats were done to assess the effect on area under the curve (AUC) and the maximum concentration (Cmax) of leflunomide in prepared solid forms. Results: These NCD were primarily characterized to be eutectics rather than cocrystals as expected. The stoichiometry was established by phase diagrams. The negative value of heat of mixing indicated them to be of cluster type. In addition, leflunomide in eutectics showed approximately 9 folds increase in solubility up to 4 hours. Besides this, approximately 4 folds enhancement in the in IDR was also observed. Maximum increase in bioavailability indicated by enhanced values of AUC and Cmax (490.29 μg h-1 mL-1 and 31.42 μg mL-1, respectively) for leflunomide-maleic acid eutectic in comparison to pure LEF (AUC: 193.20 μg h-1 mL-1 and Cmax: 12.09 μg mL-1). Conclusion: The unsuccessful cocrystallization experiments were found be the latent eutectics. The evaluation of these novel eutectics of poorly soluble drug exhibited possibility to further amplify the scope of accessible material phase options other than pure active pharmaceutical ingredient (API) without disturbing the structural integrity.

Continuous manufacturing of co-crystals: challenges and prospects

The last decade has witnessed extensive growth in the field of co-crystallization for mitigating the solubility and dissolution-related issues of poorly soluble drugs. This is largely because co-crystals can modify the physicochemical properties of drugs without any covalent modification in the drug molecules. The US Food and Drug Administration (FDA) now considers drug products that are designed to contain a new co-crystal, analogous to new polymorph of the active pharmaceutical ingredient (API). This positive change in regulatory perspective coupled with successful commercialization of valsartan-sacubitril co-crystal (Entresto, Novartis) has now brought co-crystals into focus, in both industries as well as academia. Co-crystal prediction, screening, and synthesis have been reported in literature; however, co-crystal production at a larger scale needs further investigations. With this aim, the article describes various continuous methods for co-crystal production, along with in-line monitoring during co-crystal production, emphasizing on process analytical technology (PAT). In addition, the scale-up issues of continuous and batch co-crystallization and other suitable techniques for pharmaceutical scale up are detailed. Quality control aspects and regulatory viewpoint crucial for commercial success are elaborated in the future perspective.

Supramolecular synthesis and characterization of crystalline solids obtained from the reaction of 5-fluorocytosine with nitro compounds

In this manuscript we introduce a broad solid-state characterization of 5-fluorocytosine (5-FC) solid forms obtained with picric (PA) and 3,5-dinitrosalicylic (DNSA) nitro acids.

Pharmaceutical cocrystals: walking the talk

Pharmaceutical cocrystals belong to a sub-class of cocrystals wherein one of the components is a drug molecule (or an active pharmaceutical ingredient, API) and the second is a benign food or drug grade additive (generally regarded as safe, GRAS). The two components are hydrogen-bonded in a fixed stoichiometric ratio in the crystal lattice. In the past decade, pharmaceutical cocrystals have demonstrated significant promise in their ability to modify the physicochemical and pharmacokinetic properties of drug substances, such as the solubility and dissolution rate, bioavailability, particle morphology and size, tableting and compaction, melting point, physical form, biochemical and hydration stability, and permeability. In this feature review, we highlight some prominent examples of drug cocrystals which exhibit variable hardness/softness and elasticity/plasticity depending on coformer selection, improvement of solubility and permeability in the same cocrystal, increase of the melting point for solid formulation, enhanced color performance, photostability and hydration stability, and a longer half-life. Cocrystals of flavanoids and polyphenols can make improved pharmaceuticals and also extend to the larger class of nutraceuticals. The application of crystal engineering to assemble ternary cocrystals expands this field to drug-drug cocrystals which may be useful in multi-drug resistance, mitigating side effects of drugs, or attenuating/enhancing drug action synergistically by rational selection. The advent of new techniques for structural characterization beyond the standard X-ray diffraction will provide a better understanding of drug phases which are at the borderline of crystalline-amorphous nature and even newer opportunities in the future.

Pharmaceutical Cocrystals: Regulatory and Strategic Aspects, Design and Development

Cocrystal is a concept of the supramolecular chemistry which is gaining the extensive interest of researchers from pharmaceutical and chemical sciences and of drug regulatory agencies. The prominent reason of which is its ability to modify physicochemical properties of active pharmaceutical ingredients. During the development of the pharmaceutical product, formulators have to optimize the physicochemical properties of active pharmaceutical ingredients. Pharmaceutical cocrystals can be employed to improve vital physicochemical characteristics of a drug, including solubility, dissolution, bioavailability and stability of pharmaceutical compounds while maintaining its therapeutic activity. It is advantageous being a green synthesis approach for production of pharmaceutical compounds. The formation polymorphic forms, solvates, hydrates and salts of cocrystals during the synthesis reported in the literature which can be a potential issue in the development of pharmaceutical cocrystals. The approaches like hydrogen bonding rules, solubility parameters, screening through the CSD database or thermodynamic characteristics can be utilized for the rational design of cocrystals and selection of coformers for synthesis multi-component cocrystals. Considering the significance of pharmaceutical cocrystals pharmaceutical regulatory authorities in the United States and Europe issued guidance documents which may be helpful for pharmaceutical product registration in these regions. In this article, we deal with the design, synthesis, strategic aspects and characteristics of cocrystals along perspectives on its regulatory and intellectual property considerations.

Thermodynamic entropy of organic oxidation in the water environment: experimental evaluation compared to semi-empirical calculation

Residual organic matters in the secondary effluent are usually less biodegradable in terms of the total organic carbon content, and when discharged into a receiving water body, their further decomposition most likely mainly occurs due to chemical oxidation. Using this scenario, a semi-empirical method was previously developed to calculate the thermodynamic entropy of organic oxidation to quantitatively evaluate the impact of organic discharge on the water environment. In this study, the relationship between the entropy increase (ΔS_C) and excess organic mass (ΔTOC) was experimentally verified via combustion heat measurement using typical organic chemicals and mixtures. For individual organic chemicals, a linear relationship was detected between ΔS_C and ΔTOC with the same proportionality coefficient, 54.0 kJ/g, determined in the previous semi-empirical relationship. For the organic mixtures, a linear relationship was also identified; however, the proportionality coefficient was 69.2 kJ/g, indicating an approximately 28 % increase in the oxidation heat required to decompose the same organic mass. This increase in energy can likely be attributed to the synergistic effects of hydrogen bonding, hydrophobic interactions, π-π interactions, and van der Waals interactions between functional groups of different organic compounds. Intermolecular interactions may result in 17-32 % more dissociation energy for organic mixtures compared to the organic components' chemical structures. Because organics discharged into a water body are always a mixture of organic compounds, the proportionality coefficient obtained using organic mixtures should be adopted to modify the previously proposed semi-empirical equation.

Binary and ternary cocrystals of sulfa drug acetazolamide with pyridine carboxamides and cyclic amides

A novel design strategy for cocrystals of a sulfonamide drug with pyridine carboxamides and cyclic amides is developed based on synthon identification as well as size and shape match of coformers. Binary adducts of acetazolamide (ACZ) with lactams (valerolactam and caprolactam, VLM, CPR), cyclic amides (2-pyridone, labeled as 2HP and its derivatives MeHP, OMeHP) and pyridine amides (nicotinamide and picolinamide, NAM, PAM) were obtained by manual grinding, and their single crystals by solution crystallization. The heterosynthons in the binary cocrystals of ACZ with these coformers suggested a ternary combination for ACZ with pyridone and nicotinamide. Novel supramolecular synthons of ACZ with lactams and pyridine carboxamides are reported together with binary and ternary cocrystals for a sulfonamide drug. This crystal engineering study resulted in the first ternary cocrystal of acetazolamide with amide coformers, ACZ-NAM-2HP (1:1:1).

The crystal design of polar one-dimensional hydrogen-bonded copper coordination complexes

Polar crystals exhibiting second-order harmonic generation (SHG) were designed by adjusting the intermolecular interactions of mononuclear Cu(ii) complexes in which one H2O, two pyridines (py), and two p-substituted benzoate (p-RBA) ligands (R = F, Cl, Br, I, CH3, and OCH3) were coordinated to a Cu(ii) ion, forming a penta-coordinated asymmetric [Cu(ii)(p-RBA)2(py)2(H2O)] mononuclear structure with a permanent dipole moment along the direction of the Cu-OH2 coordination axis. Each asymmetric [Cu(ii)(p-RBA)2(py)2(H2O)] complex formed a polar one-dimensional hydrogen-bonded chain, [Cu(ii)(p-RBA)2(py)2(H2O)]∞, between the non-coordinated carboxylate oxygen atom of the p-RBA ligand and the hydrogen atom of the H2O molecule. The formation of a polar crystal depended on the arrangement of polar hydrogen-bonded chains; the parallel arrangement of each polar chain resulted in a polar crystal. The chemical design of the R group in the p-RBA ligand enabled tuning of the magnitude of the interchain interactions and crystal polarity; polar crystals were obtained using p-RBA ligands with R = Cl, Br, I, and OCH3. In contrast, apolar crystals were grown from complexes containing p-RBA ligands with R = F and CH3. In all crystals, a polar two-dimensional (2D) layer constructed from the parallel polar [Cu(ii)(p-RBA)2(py)2(H2O)]∞ chain arrangement was formed based on weak van der Waals C-H...(-)O- interactions between the hydrogen atom of py and the carboxylate oxygen atom of the p-RBA ligand. Weak interlayer halogen (X)...π and multipoint C-H...π interactions played important roles in forming parallel arrangements of polar 2D layers and polar crystals, but there were no effective intermolecular interactions between the polar 2D layers in apolar [Cu(ii)(p-FBA)2(py)2(H2O)] and [Cu(ii)(p-CH3BA)2(py)2(H2O)] crystals. The magnitudes of the interlayer interactions in the polar crystals were larger than those in the apolar ones because of the effective intermolecular interactions. The SHG intensities of the four polar crystals were approximately 0.7 times larger than that of sucrose.

Persistence of C–H⋯π(chelate ring) interactions in the crystal structures of Pd(S2COR)2. The utility of Pd(S2COR)2as precursors for palladium sulphide materials

The influence of C–H⋯π(PdS₂C) interactions in the molecular packing of Pd(S₂COR)₂increases as the steric bulk ofRincreases.

New forms of old drugs: improving without changing

OBJECTIVES

In a short approach, we want to present the improvements that have recently been done in the world of new solid forms of known active pharmaceutical ingredients (APIs). The different strategies will be addressed, and successful examples will be given.

KEY FINDINGS

This overview presents a possible step to overcome the 10-15 years of hard work involved in launching a new drug in the market: the use of new forms of well-known APIs, and improve their efficiency by enhancing their bioavailability and pharmacokinetics. It discusses some of the latest progresses.

SUMMARY

We want to present, in a brief overview, what recently has been done to improve the discovery of innovative methods of using well-known APIs, and improve their efficiency. Multicomponent crystal forms have shown to be the most promising achievements to accomplish these aims, by altering API physico-chemical properties, such as solubility, thermal stability, shelf life, dissolution rate and compressibility. API-ionic liquids (ILs) and their advantages will be briefly referred. An outline of what has recently been achieved in metal drug coordination and in drug storage and delivery using bio-inspired metal-organic frameworks (BioMOFs) will also be addressed.

Modulating the solubility of sulfacetamide by means of cocrystals

The antibacterial drug sulfacetamide was screened with pharmaceutically acceptable coformers to discover solid forms with low solubility. Cocrystals with INIC and CAF exhibited 0.64 and 0.68 times the IDR of the parent drug. SACT–CAF cocrystal with lower solubility and good stability is a potential candidate to increase the drug residence time at the site of action for improved therapeutic efficacy.

On the role of DMSO-O(lone pair)⋯π(arene), DMSO-S(lone pair)⋯π(arene) and SO⋯π(arene) interactions in the crystal structures of dimethyl sulphoxide (DMSO) solvates

DMSO-O(lone pair)⋯π(arene), DMSO-S(lone pair)⋯π(arene) and SO⋯π(arene) interactions are found in DMSO solvates.

New cocrystals of ezetimibe with l-proline and imidazole

Two new cocrystals of ezetimibe were identified and scale-up. Ezetimibe–proline cocrystal showed improved apparent solubility and physical stability.

Supramolecular pyridyl urea gels as soft matter with antibacterial properties against MRSA and/or E. coli

The development of a family of twelve aryl pyridyl ureas, their crystallography and the ability of a number of these to form hydrogen bonding supramolecular gels with antimicrobial properties are described.

Isolation of two different Ni2Zn complexes with an unprecedented cocrystal formed by one of them and a "coordination positional isomer" of the other

A new homometallic trinuclear Ni(II) complex [(NiL)2Ni(NCS)2] (1) and three heterometallic trinuclear Ni(II)-Zn(II)-Ni(II) complexes [(NiL)2Zn(NCS)2] (2), [(NiL)2Zn(NCS)2(CH3OH)2]·2CH3OH (3) and {[(NiL)2Zn(NCS)2(CH3OH)2]} {[(NiL)2Zn(NCS)2]} (4) have been synthesized by using the "complex as ligand" approach with the "metalloligand" [NiL] (H2L = N,N'-bis(salicylidene)-1,3-propanediamine) and thiocyanate in different ratios. All the complexes have been structurally and magnetically characterized. In the isomorphous complexes 1 and 2, the two terminal square planar Ni atoms and the central octahedral nickel atom (in 1) or zinc atom (in 2) are arranged in a bent structure where two cis κN-SCN(-) thiocyanate ions are coordinated to the central atom. The chemical composition of 3 is very similar to that of 2 but, in 3, the central Zn atom is tetrahedral and the κN-SCN(-) thiocyanate ions occupy an axial position of each terminal nickel atom (which now are octahedral with the sixth position occupied by a methanol molecule). Complex 4 consists of two closely related trinuclear units 4A and 4B. In 4A, the coordination environments of the metals are identical to those of 3 whereas 4B is a "coordination position isomer" of complex 2 with the central square pyramidal Zn and one of the terminal square pyramidal Ni atoms coordinated by two κN-SCN(-) thiocyanate ions. Complex 4 is a unique example of a cocrystal formed by two similar trinuclear units (4A and 4B) where 4A is identical to an existing complex (3) and 4B is a "coordination position isomer" of another existing complex (2).

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.