Aspartame as a co-former in co-amorphous systems

Nucleotides as new co-formers in co-amorphous systems: Enhanced dissolution rate, water solubility and physical stability

Developing co-amorphous systems is an attractive strategy to improve the dissolution rate of poorly water-soluble drugs. Various co-formers have been investigated. However, previous studies revealed that it is a challenge to develop satisfied acidic co-formers, e.g., acidic amino acids showed much poorer co-former properties than neutral and basic amino acids. Only a few acidic co-formers have been reported, such as aspartic acid, glutamic acid, and some other organic acids. Thus, this study aims to explore the possibility of adenosine monophosphate and adenosine diphosphate used as acidic co-formers. Mebendazole, celecoxib and tadalafil were used as the model drugs. The drug-co-former co-amorphous systems were prepared via ball milling and confirmed using XRPD. The dissolution study suggested that the solubility and dissolution rate of the drug-co-formers systems were increased significantly compared to the corresponding crystalline and amorphous drugs. The stability study revealed that using the two nucleotides as co-formers enhanced the physical stability of pure amorphous drugs. Molecular interactions were observed in MEB-co-former and TAD-co-former systems and positively affected the pharmaceutical performance of the investigated co-amorphous systems. In conclusion, the two nucleotides could be promising potential acidic co-formers for co-amorphous systems.

Amorphous solid dispersions: Stability mechanism, design strategy and key production technique of hot melt extrusion

Solid dispersion (SD) system has been used as an effective formulation strategy to increase in vitro and in vivo performances of poorly water-soluble drugs, such as solubility/dissolution, stability and bioavailability. This review provides a comprehensive SD classification and identifies the most popular amorphous solid dispersions (ASDs). Meanwhile, this review further puts forward the systematic design strategy of satisfactory ASDs in terms of drug properties, carrier selection, preparation methods and stabilization mechanisms. In addition, hot melt extrusion (HME) as the continuous manufacturing technique is described including the principle and structure of HME instrument, key process parameters and production application, in order to guide the scale-up of ASDs and develop more ASD products to the market in pharmaceutical industry.

Dissolution changes in drug-amino acid/biotin co-amorphous systems: Decreased/increased dissolution during storage without recrystallization

Co-amorphous systems have been proven to be a promising strategy to address the poor water solubility of poorly water-soluble drugs. Generally, the initial dissolution behaviors after co-amorphous system preparation and the potential recrystallization during storage are used to evaluate the performance of co-amorphous systems. However, this study reveals that decreased dissolution and unexpected increased dissolution were observed during storage though the co-amorphous systems maintained amorphous form. Three drugs (valsartan, tadalafil, mebendazole) and three co-formers (arginine, tryptophan, biotin) were used to prepare co-amorphous systems and the samples were stored for different times. After stored for 80 d, most of the co-amorphous systems maintained amorphous form, however, decreased and increased intrinsic dissolution rates (IDRs) were both observed in these non-recrystallized co-amorphous systems. The moisture changes of the systems during storage and the possible drug-co-former molecular interactions showed no effect on the dissolution changes, while phase separation might play a role in it. In conclusion, more attention should be paid to the dissolution changes of co-amorphous systems during storage. Focusing on the initial dissolution behaviors after sample preparation and the physical recrystallization during storage is not enough for the development of co-amorphous systems in future.

Ternary Solid Dispersions: A Review of the Preparation, Characterization, Mechanism of Drug Release, and Physical Stability

The prevalence of active pharmaceutical ingredients (APIs) with low water solubility has experienced a significant increase in recent years. These APIs present challenges in formulation, particularly for oral dosage forms, despite their considerable therapeutic potential. Therefore, the improvement of solubility has become a major concern for pharmaceutical enterprises to increase the bioavailability of APIs. A promising formulation approach that can effectively improve the dissolution profile and the bioavailability of poorly water-soluble drugs is the utilization of amorphous systems. Numerous formulation methods have been developed to enhance poorly water-soluble drugs through amorphization systems, including co-amorphous formulations, amorphous solid dispersions (ASDs), and the use of mesoporous silica as a carrier. Furthermore, the successful enhancement of certain drugs with poor aqueous solubility through amorphization has led to their incorporation into various commercially available preparations, such as ASDs, where the crystalline structure of APIs is transformed into an amorphous state within a hydrophilic matrix. A novel approach, known as ternary solid dispersions (TSDs), has emerged to address the solubility and bioavailability challenges associated with amorphous drugs. Meanwhile, the introduction of a third component in the ASD and co-amorphous systems has demonstrated the potential to improve performance in terms of solubility, physical stability, and processability. This comprehensive review discusses the preparation and characterization of poorly water-soluble drugs in ternary solid dispersions and their mechanisms of drug release and physical stability.

Characterizing the Impact of Chitosan on the Nucleation and Crystal Growth of Ritonavir from Supersaturated Solutions

The addition of polymeric materials is often used to delay nucleation or crystal growth and maintain the high supersaturation of amorphous drugs. Therefore, this study aimed to investigate the impact of chitosan on the supersaturation behavior of drugs with a low recrystallization tendency and elucidate the mechanism of its crystallization inhibition in an aqueous solution. It was carried out using ritonavir (RTV) as a model of poorly water-soluble drugs categorized as class III of Taylor's classification, while chitosan was used as a polymer, and hypromellose (HPMC) was used for comparison. The inhibition of the nucleation and crystal growth of RTV by chitosan was examined by measuring the induction time. The interactions of RTV with chitosan and HPMC were evaluated by NMR measurements, FT-IR, and an in silico analysis. The results showed that the solubilities of amorphous RTV with and without HPMC were quite similar, while the amorphous solubility was significantly increased by the chitosan addition due to the solubilization effect. In the absence of the polymer, RTV started to precipitate after 30 min, indicating that it is a slow crystallizer. Chitosan and HPMC effectively inhibited the nucleation of RTV, as reflected by a 48-64-fold enhancement in the induction time. Furthermore, NMR, FT-IR, and in silico analysis demonstrated that the hydrogen bond interaction between the amine group of RTV and a proton of chitosan, as well as the carbonyl group of RTV and a proton of HPMC, was observed. This indicated that the hydrogen bond interaction between RTV and chitosan as well as HPMC can contribute to the crystallization inhibition and maintenance of RTV in a supersaturated state. Therefore, the addition of chitosan can delay nucleation, which is crucial for stabilizing supersaturated drug solutions, specifically for a drug with a low crystallization tendency.

Development of Co-Amorphous Loratadine–Citric Acid Orodispersible Drug Formulations

This study aimed at the preparation and characterization of co-amorphous loratadine–citric acid orally disintegrating dosage forms (ODx). A co-amorphous loratadine–citric acid was prepared by solvent evaporation method in three different molecular ratios. DSC, FTIR, and dissolution studies have been conducted for the binary system. The co-amorphous system was used to obtain oral lyophilizates and orally disintegrating tablets by direct compression. Diameter, thickness, hardness, disintegration time, uniformity of mass, and dissolution was determined for the dosage forms. DSC curves showed a lack of sharp endothermic peaks for the binary systems. FTIR spectra presented a hypsochromic modification of the characteristic peaks. Dissolution studies indicated a five-fold increase in the dissolved amount compared to pure loratadine in water. Disintegration times of direct compression ODx varied in the range of 34–41 s and for freeze-dried ODx in the range of 8–9 s. Friability was under 1% in all cases. The dissolution of loratadine in buffer solution at pH = 1 was almost complete. In conclusion binary systems of loratadine and citric acid enhance solubility and combined with the orally disintegrating pharmaceutical form also increase patient compliance.

Inhibition of Crystal Nucleation and Growth in Aqueous Drug Solutions: Impact of Different Polymers on the Supersaturation Profiles of Amorphous Drugs-The Case of Alpha-Mangostin

The polymer used in supersaturated solutions plays a critical role in maintaining supersaturation levels of amorphous drugs. The prevention of drug crystallization in the supersaturated solutions by adding polymers depends on their ability to inhibit nucleation and crystal growth of drugs. This showed that understanding the mechanism of nucleation inhibition by polymers is necessary to develop the drug formulation in supersaturated solutions. Therefore, this study aims to evaluate the impact of water-soluble polymers on the supersaturation behavior of drugs and elucidate the mechanism of maintaining the supersaturation levels in an aqueous solution. It was carried out using alpha-mangostin (AM) as a model of the poorly water-soluble drug, while hypromellose (HPMC), polyvinylpyrrolidone (PVP), and eudragit were used as polymers. Their ability to inhibit the nucleation and crystal growth of AM was also evaluated. The supersaturation profiles of AM were measured in biorelevant dissolution media, while the crystal growth rate of AM was evaluated from the decrease in dissolved drug concentration by determining the induction time for AM nucleation. The interaction of AM with each polymer was evaluated and predicted by FT-IR, NMR measurement, and an in silico study, respectively. Based on observation, the PVP effectively maintained AM in a supersaturated state for the long term while eudragit conserved for 15 min. Meanwhile, an inhibitory effect of HPMC on the AM crystal nucleation was not observed. It was also \]-+discovered that the effectiveness of the various polymers depends on the interaction between the polymer and the drug. FT-IR and in silico studies demonstrated that the interaction of PVP-AM had the best polymer compared to eudragit and HPMC. NMR analysis suggested that the interaction between the methyl group from PVP with the carbonyl group of AM occurred in the PVP solution. The viscosity measurement revealed that the inhibition of nucleation and crystal growth of AM was not caused by increasing the viscosity. These results indicated that polymer-AM interactions could contribute to the crystallization inhibition and maintenance of AM in a supersaturated state. Therefore, an investigation of the mechanism of drug nucleation inhibition by polymers is recommended in the selection of crystallization inhibitors and a planned strategy to develop supersaturated formulations of drugs.

Co-amorphous Systems of Sinomenine with Platensimycin or Sulfasalazine: Physical Stability and Excipient-Adjusted Release Behavior

There is strong interest to develop affordable treatments for the infection-associated rheumatoid arthritis (RA). Here, we present a drug-drug co-amorphous strategy against RA and the associated bacterial infection by the preparation and characterization of two co-amorphous systems of sinomenine (SIN) with platensimycin (PTM) or sulfasalazine (SULF), two potent antibiotics. Both of them were comprehensively characterized using powder X-ray diffraction, temperature-modulated differential scanning calorimetry, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The co-amorphous forms of SIN-PTM and SIN-SULF exhibited high T_gs at 139.10 ± 1.0 and 153.3 ± 0.2 °C, respectively. After 6 months of accelerated tests and 1 month of drug-excipient compatibility experiments, two co-amorphous systems displayed satisfactory physical stability. The formation of salt and strong intermolecular interactions between SIN and PTM or SULF, as well as the decreased molecular mobility in co-amorphous systems, may be the intrinsic mechanisms underlying the excellent physical stability of both co-amorphous systems. In dissolution tests, two co-amorphous systems displayed distinct reduced SIN-accumulative releases (below 20% after 6 h of release experiments), which may lead to its poor therapeutic effect. Hence, we demonstrated a controlled release strategy for SIN by the addition of a small percentage of polymers and a small-molecule surfactant to these two co-amorphous samples as convenient drug excipients, which may also be used to improve the unsatisfactory dissolution behaviors of the previously reported SIN co-amorphous systems. Several hydrogen bonding interactions between SIN and PTM or SULF could be identified in NMR experiments in DMSO-d₆, which may be underlying reasons of decreased dissolution behaviors of both co-amorphous forms. These drug-drug co-amorphous systems could be a potential strategy for the treatment of infection-associated RA.

Mechanical Activation by Ball Milling as a Strategy to Prepare Highly Soluble Pharmaceutical Formulations in the Form of Co-Amorphous, Co-Crystals, or Polymorphs

Almost half of orally administered active pharmaceutical ingredients (APIs) have low solubility, which affects their bioavailability. In the last two decades, several alternatives have been proposed to modify the crystalline structure of APIs to improve their solubility; these strategies consist of inducing supramolecular structural changes in the active pharmaceutical ingredients, such as the amorphization and preparation of co-crystals or polymorphs. Since many APIs are thermosensitive, non-thermal emerging alternative techniques, such as mechanical activation by milling, have become increasingly common as a preparation method for drug formulations. This review summarizes the recent research in preparing pharmaceutical formulations (co-amorphous, co-crystals, and polymorphs) through ball milling to enhance the physicochemical properties of active pharmaceutical ingredients. This report includes detailed experimental milling conditions (instrumentation, temperature, time, solvent, etc.), as well as solubility, bioavailability, structural, and thermal stability data. The results and description of characterization techniques to determine the structural modifications resulting from transforming a pure crystalline API into a co-crystal, polymorph, or co-amorphous system are presented. Additionally, the characterization methodologies and results of intermolecular interactions induced by mechanical activation are discussed to explain the properties of the pharmaceutical formulations obtained after the ball milling process.

Amorphous and Co-Amorphous Olanzapine Stability in Formulations Intended for Wet Granulation and Pelletization

The preparation of amorphous and co-amorphous systems (CAMs) effectively addresses the solubility and bioavailability issues of poorly water-soluble chemical entities. However, stress conditions imposed during common pharmaceutical processing (e.g., tableting) may cause the recrystallization of the systems, warranting close stability monitoring throughout production. This work aimed at assessing the water and heat stability of amorphous olanzapine (OLZ) and OLZ-CAMs when subject to wet granulation and pelletization. Starting materials and products were characterized using calorimetry, diffractometry and spectroscopy, and their performance behavior was evaluated by dissolution testing. The results indicated that amorphous OLZ was reconverted back to a crystalline state after exposure to water and heat; conversely, OLZ-CAMs stabilized with saccharin (SAC), a sulfonic acid, did not show any significant loss of the amorphous content, confirming the higher stability of OLZ in the CAM. Besides resistance under the processing conditions of the dosage forms considered, OLZ-CAMs presented a higher solubility and dissolution rate than the respective crystalline counterpart. Furthermore, in situ co-amorphization of OLZ and SAC during granule production with high fractions of water unveils the possibility of reducing production steps and associated costs.

Co-amorphous systems using epigallocatechin-3-gallate as a co-former: stability, in vitro dissolution, in vivo bioavailability and underlying molecular mechanisms

Co-amorphous strategy has been extensively investigated to improve the dissolution of hydrophobic drugs. Here, epigallocatechin-3-gallate (EGCG) was exploited as a co-former in co-amorphous systems based on its unique structure including phenyl rings, phenolic hydroxyl groups and the galloyl moiety. Two model BCS class II drugs, simvastatin (SIM) and nifedipine (NIF), were selected to be co-amorphized with EGCG. All drug-EGCG systems at three molar ratios became amorphous by the means of spray drying and showed high physically stable either under dry condition and 75% RH at 40 °C or under dry conditions at 25 °C. The optimal feed molar ratios of both EGCG based co-amorphous systems fabricated were determined to be three, under which the significant increases were obtained in the maximum apparent concentrations of 4.90-fold for SIM at 1 h and 106.03-fold for NIF at 0.25 h compared to crystalline drugs by non-sink dissolution studies. The underlying molecular mechanisms of two co-amorphous systems formation were involved in molecular miscibility, hydrogen bonds and π-π stacking interactions unraveled by means of DSC, FTIR and molecular dynamics simulations. More to the point, oral pharmacokinetic studies in rats demonstrated that co-amorphous SIM-EGCG and NIF-EGCG systems at 1:3 have a significant increase in C_max of 1.81- and 5.69-fold, and AUC _{0-24 h} of 1.62- and 4.57-fold compared with those of corresponding crystalline drugs, respectively. In conclusion, EGCG is proved to be a promising co-former in co-amorphous systems.

Prediction and Preparation of Coamorphous Phases of a Bislactam

The effectiveness of a partial least squares-discriminant analysis coamorphous prediction model was tested using coamorphous screening data for a promising coamorphous former, the dimer of N-vinyl(caprolactam) (bisVCap) with a range of active pharmaceutical ingredients. The prediction model predicted 71% of the systems correctly. An experimental coamorphous screen was performed with this coformer with 13 different active pharmaceutical ingredients, and the results were compared to the predictions from the model. A total of 85% of the systems were correctly predicted. Stability assessments of three coamorphous systems showed that the prediction model score did not strongly correlate with the stability of the coamorphous material. The model performed well with small-molecule coformers, such as bisVCap, despite the difference in structure and properties compared to the amino-acid-based model training set.

A Comparative Study of the Pharmaceutical Properties between Amorphous Drugs Loaded-Mesoporous Silica and Pure Amorphous Drugs Prepared by Solvent Evaporation

The formulation of poorly water-soluble drugs is one of the main challenges in the pharmaceutical industry, especially in the development of oral dosage forms. Meanwhile, there is an increase in the number of poorly soluble drugs that have been discovered as new chemical entities. It was also reported that the physical transformation of a drug from a crystalline form into an amorphous state could be used to increase its solubility. Therefore, this study aims to evaluate the pharmaceutical properties of amorphous drug loaded-mesoporous silica (MPS) and pure amorphous drugs. Ritonavir (RTV) was used as a model of a poorly water-soluble drug due to its low recrystallization tendency. RTV loaded-MPS (RTV/MPS) and RTV amorphous were prepared using the solvent evaporation method. Based on observation, a halo pattern in the powder X-ray diffraction pattern and a single glass transition (T_g) in the modulated differential scanning calorimetry (MDSC) curve was discovered in RTV amorphous, indicating its amorphization. The T_g was not detected in RTV/MPS, which showed that the loading RTV was completed. The solid-state NMR and FT-IR spectroscopy also showed the interaction between RTV and the surface of MPS in the mesopores. The high supersaturation of RTV was not achieved for both RTV/MPS and the amorphous state due to its strong interaction with the surface of MPS and was not properly dispersed in the medium, respectively. In the dissolution test, the molecular dispersion of RTV within MPS caused rapid dissolution at the beginning, while the amorphous showed a low rate due to its agglomeration. The stability examination showed that the loading process significantly improved the physical and chemical stability of RTV amorphous. These results indicated that the pharmaceutical properties of amorphous drugs could be improved by loaded-MPS.

Mechanistic insight into gel-induced aggregation of amorphous curcumin during dissolution process

Amorphous curcumin (CUR) exhibited a decreased dissolution rate in comparison with the crystalline counterpart due to its gel formation during dissolution. The main purpose of the present study is to explore the mechanism of such gelation phenomenon. It was found that the dissolution of amorphous CUR and gel properties were influenced by the temperature and pH of the media. The formed gels were characterized by TPA, SEM, DSC, XRPD, FTIR and PLM. The results indicated that the gelation process led to the formation of a porous structure in which water molecules infiltrate, and entered into its supercooled liquid state with high viscosity when contacting aqueous media, accompanied by decreased T_g and crystalline transformation. In addition, mixing with hydrophilic excipients (such as hydrophilic silica) accelerated the gel formation of amorphous CUR, while the addition of hydrophobic excipients (such as hydrophobic silica and magnesium stearate) could effectively weaken and even eliminate the gelation, hence significantly improving its dissolution. Furthermore, according to contact angle measurement and fluorescence microscope observation, hydrophilic excipients were found to be able to accelerate water entering into the interior of amorphous CUR, hence facilitating the gelation, while hydrophobic excipients would hinder water infiltration into the powder and thus achieve degelation. In conclusion, it is important to recognize that the gelation potential of some amorphous materials should be considered in developing robust amorphous drug product of high quality and performance.

Recent Technologies for Amorphization of Poorly Water-Soluble Drugs

Amorphization technology has been the subject of continuous attention in the pharmaceutical industry, as a means to enhance the solubility of poorly water-soluble drugs. Being in a high energy state, amorphous formulations generally display significantly increased apparent solubility as compared to their crystalline counterparts, which may allow them to generate a supersaturated state in the gastrointestinal tract and in turn, improve the bioavailability. Conventionally, hydrophilic polymers have been used as carriers, in which the amorphous drugs were dispersed and stabilized to form polymeric amorphous solid dispersions. However, the technique had its limitations, some of which include the need for a large number of carriers, the tendency to recrystallize during storage, and the possibility of thermal decomposition of the drug during preparation. Therefore, emerging amorphization technologies have focused on the investigation of novel amorphous-stabilizing carriers and preparation methods that can improve the drug loading and the degree of amorphization. This review highlights the recent pharmaceutical approaches utilizing drug amorphization, such as co-amorphous systems, mesoporous particle-based techniques, and in situ amorphization. Recent updates on these technologies in the last five years are discussed with a focus on their characteristics and commercial potential.

Prediction of the Synergistic Glass Transition Temperature of Coamorphous Molecular Glasses Using Activity Coefficient Models

The glass transition temperature (T_g) of a binary miscible mixture of molecular glasses, termed a coamorphous glass, is often synergistically increased over that expected for an athermal mixture due to the strong interactions between the two components. This synergistic interaction is particularly important for the formulation of coamorphous pharmaceuticals since the molecular interactions and resulting T_g strongly impact stability against crystallization, dissolution kinetics, and bioavailability. Current models that describe the composition dependence of T_g for binary systems, including the Gordon-Taylor, Fox, Kwei, and Braun-Kovacs equations, fail to describe the behavior of coamorphous pharmaceuticals using parameters consistent with experimental ΔC_P and Δα. Here, we develop a robust thermodynamic approach extending the Couchman and Karasz method through the use of activity coefficient models, including the two-parameter Margules, non-random-two-liquid (NRTL), and three-suffix Redlich-Kister models. We find that the models, using experimental values of ΔC_P and fitting parameters related to the binary interactions, successfully describe observed synergistic elevations and inflections in the T_g versus composition response of coamorphous pharmaceuticals. Moreover, the predictions from the NRTL model are improved when the association-NRTL version of that model is used. Results are reported and discussed for four different coamorphous systems: indomethacin-glibenclamide, indomethacin-arginine, acetaminophen-indomethacin, and fenretinide-cholic acid.

Considerations for the selection of co-formers in the preparation of co-amorphous formulations

Co-amorphous drug delivery systems are evolving as a credible alternative to amorphous solid dispersions technology. In Co-amorphous systems (CAMs), a drug is stabilized in amorphous form using small molecular weight compounds called as co-formers. A wide variety of small molecular weight co-formers have been leveraged in the preparation of CAMs. The stability and supersaturation potential of prepared co-amorphous phases largely depend on the type of co-former employed in the CAMs. However, the rationality behind the co-former selection in co-amorphous systems is poorly understood and scarcely compiled in the literature. There are various facets to the rational selection of co-former for CAMs. In this context, the present review compiles various factors affecting the co-former selection. The factors have been broadly classified under Thermodynamic, Kinetic and Pharmacokinetic-Pharmacologically relevant parameters. In particular, the importance of Glass transition, Miscibility, Liquid-Liquid phase separation (LLPS), Crystallization inhibition has been deliberated in detail.

Effect of drug-coformer interactions on drug dissolution from a coamorphous in mesoporous silica

In this study, the molecular state of ritonavir (RTN)-saccharin (SAC) coamorphous incorporated into mesoporous silica by solvent evaporation and the effect of SAC on the RTN dissolution from mesopores were investigated. The amorphization of RTN-SAC was confirmed as a halo pattern in powder X-ray diffraction measurements and a single glass transition event in the modulated differential scanning calorimetry (MDSC) curve. ¹³C solid-state NMR spectroscopy revealed a hydrogen bond between the thiazole nitrogen of RTN and the amine proton of SAC. The glass transition of the RTN-SAC coamorphous in mesoporous silica was not found in the MDSC curve, indicating that RTN and SAC were monomolecularly incorporated into the mesopores. Solid-state NMR measurements suggested that the co-incorporation of SAC into the mesopores decreased the local mobility of the thiazole group of RTN via hydrogen bond formation. The RTN-SAC 1:1 coamorphous in mesoporous silica retained the X-ray halo-patterns after 30 d of storage, even under high temperature and humidity conditions. In the dissolution test, the RTN-SAC 1:1 coamorphous in mesoporous silica maintained RTN supersaturation for a longer time than the RTN amorphous in mesoporous silica. This study demonstrated that the drug-coformer interaction within mesoporous silica can significantly improve drug dissolution.

Co-Amorphous Drug Formulations in Numbers: Recent Advances in Co-Amorphous Drug Formulations with Focus on Co-Formability, Molar Ratio, Preparation Methods, Physical Stability, In Vitro and In Vivo Performance, and New Formulation Strategies

Co-amorphous drug delivery systems (CAMS) are characterized by the combination of two or more (initially crystalline) low molecular weight components that form a homogeneous single-phase amorphous system. Over the past decades, CAMS have been widely investigated as a promising approach to address the challenge of low water solubility of many active pharmaceutical ingredients. Most of the studies on CAMS were performed on a case-by-case basis, and only a few systematic studies are available. A quantitative analysis of the literature on CAMS under certain aspects highlights not only which aspects have been of great interest, but also which future developments are necessary to expand this research field. This review provides a comprehensive updated overview on the current published work on CAMS using a quantitative approach, focusing on three critical quality attributes of CAMS, i.e., co-formability, physical stability, and dissolution performance. Specifically, co-formability, molar ratio of drug and co-former, preparation methods, physical stability, and in vitro and in vivo performance were covered. For each aspect, a quantitative assessment on the current status was performed, allowing both recent advances and remaining research gaps to be identified. Furthermore, novel research aspects such as the design of ternary CAMS are discussed.

Indomethacin co-amorphous drug-drug systems with improved solubility, supersaturation, dissolution rate and physical stability

In this study, new co-amorphous drug systems were designed using a pharmacologically relevant combination to improve the solubility and dissolution of indomethacin. Combinations of indomethacin-paracetamol (IND-PAR) as an anti-inflammatory/pain killer, and indomethacin-nicotinamide (IND-NCT) for prevention of gastric ulcers caused by IND, were developed for co-amorphization. The effect of PAR and NCT on the solubility, supersaturation, and dissolution of the poorly soluble counterpart, IND, was investigated. PAR and NCT were found to enhance the solubility and supersaturation of IND in biorelevant medium (FaSSIF) and in FaSSIF blank. Differential scanning calorimetry (DSC) showed capability of IND-PAR and IND-NCT binary mixtures to form eutectic mixture. Powder X-ray diffraction and DSC indicated the formation of a homogenous co-amorphous system with single T_g value. Hydrogen bonding between IND and each of PAR and NCT were found to stabilize the co-amorphous systems as supported by FTIR studies. The intrinsic dissolution rate under sink conditions was improved over that of plain amorphous IND both in FaSSIF and FaSSIF blank. IND-PAR 2:1 and IND-NCT 1:1 were extremely stable and remained amorphous for 7 months at 25 °C, while all co-amorphous formulations were stable at least up to one month at 40 °C under dry condition. The present work demonstrates an improved approach to combine IND-PAR and IND-NCT as promising co-amorphous systems for potential therapeutical applications.

Insights into co-amorphous systems in therapeutic drug delivery

Co-amorphous (CAM) systems are promising drug-delivery systems in the arena of therapeutic drug delivery, addressing the poor aqueous solubility of drugs by enhancing solubility and thereby improving the oral bioavailability and therapeutic effect of the drug. A CAM system is a single-phase homogeneous blend of two or more low molecular weight molecules that can be drug–drug or drug–co-former, stabilized via intermolecular interactions, adding the benefit of thermodynamic stability. This review covers the fundamentals of CAM systems and recent advances in formulation development. In particular, we strive to address the theoretical, molecular, technical and biopharmaceutical aspects, advantages over polymeric amorphous solid dispersions, mechanisms of stabilization of amorphous forms, insights into unexplored in silico tools in excipient selection and regulatory viewpoints.

Co-amorphous formation of piroxicam-citric acid to generate supersaturation and improve skin permeation

The objective of this study was to prepare a co-amorphous formulation of piroxicam (PIR), a non-steroidal anti-inflammatory drug, and citric acid (CA), and evaluate its skin permeation ability. A spray-drying method was employed to prepare the co-amorphous formulation and its physical properties were characterized. X-ray powder diffraction and thermal analysis confirmed a homogeneous amorphous state, and the infrared spectra revealed intermolecular interactions between PIR and CA, suggesting formation of a co-amorphous formulation of PIR and CA. The PIR-CA co-amorphous formulation exhibited no crystallization for 60 days at 4/25/40°C with silica gel. The PIR-CA co-amorphous formulation increased the solubility of PIR in polyethylene glycol 400 compared with that of the pure drug, and physical mixture (PM) of PIR and CA, confirming a supersaturated state in the formulation. The PIR-CA co-amorphous formulation demonstrated higher skin permeation than PIR alone or PM of PIR and CA, and the flux value was consistent with the degree of saturation. Thus, the increase in the skin permeation of PIR from the PIR-CA co-amorphous formulation directly depended on the increased thermodynamic activity by supersaturation in the absence of interactions between the drug and co-former in the vehicle.

Predictive identification of co-formers in co-amorphous systems

This work aims to understand the properties of co-formers that form co-amorphous pharmaceutical materials and to predict co-amorphous system formation. A partial least square - discriminant analysis (PLS-DA) was performed using known co-amorphous systems described by 36 variables based on the properties of the co-former and the binding energy of the system. The PLS-DA investigated the propensity to form co-amorphous material of the active pharmaceutical ingredients: mebendazole, carvedilol, indomethacin, simvastatin, carbamazepine and furosemide in combination with 20 amino acid co-formers. The variables that were found to favour the propensity to form co-amorphous systems appear to be a relatively large value for average molecular weight and the sum of the difference between hydrogen bond donors and hydrogen bond acceptors for both components, and a relatively small or negative value for excess enthalpy of mixing, excess enthalpy of hydrogen bonding and the difference in the Hansen parameter for hydrogen bonding of the coformer and the active pharmaceutical ingredient (API). To test the predictive power of this model, 29 potential co-formers were used to form either co-amorphous or crystalline two-component materials with mebendazole. Of these 29 two-component systems, the co-amorphous nature of a total of 26 materials was correctly predicted by the model, giving a predictive hit rate of 90 %.

Comparison of co-former performance in co-amorphous formulations: Single amino acids, amino acid physical mixtures, amino acid salts and dipeptides as co-formers

Co-amorphous systems have been shown to be a potential approach to address the poor water solubility challenge of many drugs. Various low molecular weight molecules, especially amino acids, have been used as potential co-formers. In this study, the differences between various combinations of amino acid-based systems, i.e. the single amino acids glutamic acid (Glu) and arginine (Arg), their physical mixture, glutamic acid-arginine crystalline and amorphous salts, and the corresponding dipeptides (GluArg, ArgGlu), were investigated. Mebendazole (Meb) was used as the model drug. Pure Meb and Meb-co-former mixtures were ball milled to prepare the co-amorphous samples. The shortest amorphization time upon ball milling (30 min) was found for Meb mixtures with the dipeptides and the Glu·Arg amorphous salt. All other samples became amorphous upon milling for 90 min, except Meb-Glu, where Glu remained partially crystalline. Both, single-phase (Meb-Glu-Arg ternary mixtures) and two-phase amorphous systems (Meb-Arg, Meb-GluArg, Meb-ArgGlu) were obtained for different Meb-co-former mixtures after milling. Whilst all co-formers improved the dissolution rate of Meb in a similar fashion (dissolution rate increased by 3.5 to 5.7-fold with respect to crystalline Meb), the highest stability improvement was observed for Meb-dipeptide systems. Interestingly, even though being a two-phase amorphous system, dipeptides were superior to the other co-formers as they possessed higher physical stability.

Involvement of metabolism-permeability in enhancing the oral bioavailability of curcumin in excipient-free solid dispersions co-formed with piperine

Curcumin (CUR) has gained increasing interest worldwide due to multiple biological activities. However, the therapeutic application remains limited because of its low aqueous solubility, intestinal metabolism and poor membrane permeability. In present study, an excipient-free CUR solid dispersion co-formed with piperine (PIP), the absorption enhancer involving metabolism-permeability, was successfully prepared by melting and quench cooling (co-amorphous CUR-PIP). The co-amorphous CUR-PIP exhibited superior performance in non-sink dissolution compared with crystalline and amorphous CUR, and showed physically stable at least 3 months, attributing to the strong molecular interactions between CUR and PIP as evaluated by FTIR spectra. Furthermore, the combination of PIP with CUR in the co-amorphous formulation could inhibit the glucuronidation of CUR, as exhibited in the in vitro assay of rat intestinal microsomes. The co-amorphous CUR-PIP would also exhibit higher gastrointestinal membrane permeability of CUR, as confirmed by P_app of CUR in Caco-2 model. After administration of co-amorphous CUR-PIP, the AUC of CUR significantly increased by 2.16- and 1.92-fold those in crystalline and amorphous CUR, respectively. This study demonstrates that the developed co-amorphous CUR-PIP can enhance the bioavailability of CUR by increasing its dissolution, inhibiting metabolic processes, and facilitating membrane permeability.

Transformations between Co-Amorphous and Co-Crystal Systems and Their Influence on the Formation and Physical Stability of Co-Amorphous Systems

The formation of co-amorphous and co-crystal systems are attractive formulation strategies for poorly water-soluble drugs. Intermolecular interactions between the drug and the coformer(s) play an important role in the formation of both systems, making the investigation of transformations between the two systems specifically interesting. The aim of this study thus was to investigate the transformation between the two systems and its influence on the formation and physical stability of co-amorphous systems. Carbamazepine (CBZ) along with benzoic acid, maleic acid, succinic acid, tartaric acid, saccharin, and nicotinamide were used as materials. First, CBZ- co-former co-crystals were prepared. Then the co-crystals and CBZ- co-former physical mixtures were ball milled to investigate the possible co-amorphization process. The XRPD and DSC results showed that CBZ and coformers tended to maintain (co-crystals as the starting material) or form co-crystals (physical mixtures as the starting material), rather than to form co-amorphous systems. Next, co-amorphization from CBZ- co-former physical mixtures via quench cooling was studied. While co-amorphous systems were obtained, the physical stability of these was very low, and the samples recrystallized to either co-crystal forms or the individual components. In conclusion, a possible transformation between the two systems was confirmed, but the resulting co-amorphous systems were highly unstable.

Dipeptides as co-formers in co-amorphous systems

Drug-amino acid co-amorphous systems have become increasingly well-investigated systems to improve dissolution rate of poorly water-soluble drugs. In this study, dipeptides were investigated as co-formers for co-amorphous systems based on the hypothesis that dipeptides might combine the inherent properties of the two included amino acids. Co-amorphization of the model drug mebendazole was investigated with five dipeptides, tryptophan-phenylalanine, phenylalanine-tryptophan, aspartic acid-tyrosine, histidine-glycine and proline-tryptophan. The dipeptides were chosen to investigate whether the side chains (nonpolar, polar, basic and acidic), and the sequence of amino acids (tryptophan-phenylalanine versus phenylalanine-tryptophan) have an influence on the performance of dipeptides as co-formers. All mebendazole-dipeptide systems became amorphous after ball milling for only 30 min, while this generally was not the case for the single amino acids or physical mixtures of the amino acids forming the dipeptides. Dissolution studies showed that the dissolution rate of mebendazole from most co-amorphous systems was increased significantly compared to crystalline and amorphous mebendazole. However, no clear trend for the drug dissolution enhancement was observed within the different co-amorphous drug-dipeptide systems. The stability study revealed that co-amorphous mebendazole-dipeptide systems showed higher physical stability compared to amorphous mebendazole. In conclusion, dipeptides are shown to be promising co-formers for co-amorphous systems.

21st century developments in the understanding and control of molecular solids

This highlight article surveys some of the key recent advances in crystallization techniques, polymorphism, co-crystals, amorphous materials and crystal engineering.