Exploring the effect of protein secondary structure on the solid state and physical stability of protein-based amorphous solid dispersions

Amorphous solid dispersions (ASDs) using proteins as carriers have emerged as a promising strategy for stabilizing amorphous drug molecules. Proteins possess diverse three-dimensional structures that significantly influence their own properties and may also impact the properties of ASDs. We prepared β-lactoglobulin (BLG) with different contents of β-sheet and α-helical secondary structures by initially dissolving BLG in different mixed solvents, containing different ratios of water, methanol/ethanol, and acetic acid, followed by spray drying of the solutions. Our findings revealed that an increase in α-helical content resulted in a decrease in the glass transition temperature (Tg) of the protein. Subsequently, we utilized the corresponding mixed solvents to dissolve both BLG and the model drug celecoxib (CEL), allowing the preparation of ASDs containing either β-sheet-rich or α-helix/random coil-rich BLG. Using spray drying, we successfully developed BLG-based ASDs with drug loadings ranging from 10 wt% to 90 wt%. At drug loadings below 40 wt%, samples prepared using both methods exhibited single-phase ASDs. However, heterogeneous systems formed when the drug loading exceeded 40 wt%. At higher drug loadings, physical stability assessments demonstrated that the α-helix/random coil-rich BLG structure exerted a more pronounced stabilizing effect on the drug-rich phase compared to the β-sheet-rich BLG. Overall, our results highlight the importance of considering protein secondary structure in the design of ASDs.

Investigating the influence of protein secondary structure on the dissolution behavior of β-lactoglobulin-based amorphous solid dispersions

Proteins acting as carriers in amorphous solid dispersions (ASDs) demonstrate a notable sensitivity to the spray drying process, potentially leading to changes in their conformation. The main aim of this study was to investigate the dissolution performance of ASDs based on proteins with different content of secondary structures, specifically β-sheet and α-helix structures. We prepared β-sheet-rich and α-helix-rich β-lactoglobulin (BLG), along with corresponding ASDs containing 10 wt% and 30 wt% drug loadings, through spray drying using celecoxib as the model drug. Circular dichroism and Fourier Transform Infrared Spectroscopy results revealed that even though changes in secondary structure were obtained in the spray-dried powders, the BLGs exhibited reversibility upon re-dissolving in phosphate buffer with varying pH levels. Both β-sheet-rich BLG and α-helix-rich BLG exhibited enhanced dissolution rates and higher solubility in the media with pH values far from the isoelectric point (pI) of BLG (pH 2, 7, 8, and 9) compared to the pH closer to the pI (pH 3, 4, 5, and 6). Notably, the release rate and solubility of the drug and BLG from both types of BLG-based ASDs at 10 wt% drug loading were largely dependent on the solubility of pure SD-BLGs. α-helix-rich BLG-ASDs consistently exhibited equivalent or superior performance to β-sheet-rich BLG-ASDs in terms of drug release rate and solubility, regardless of drug loading. Moreover, both types of BLG-based ASDs at 10 wt% drug loading exhibited faster release rates and higher solubility, for both the drug and BLG, compared to the ASDs at 30 wt% drug loading in pHs 2, 7, and 9 media.

Use of solid thermolytic salts to facilitate microwave-induced in situ amorphization

Moisture was frequently used as dielectric heating source in classical microwave-able systems to facilitate microwave-induced in situ amorphization, however such systems may face the potential of drug hydrolysis. In this study, solid thermolytic salts were proposed to function as moisture substitutes and their feasibility and impacts on microwave-induced in situ amorphization were investigated. It was found that NH4HCO3 was a promising solid alkaline salt to facilitate both microwave-induced in situ amorphization and in situ salt formation of acidic indomethacin (IND). Moreover, it could improve the chemical stability of the drug and the dissolution performance of compacts relative to classical moisture-based compacts upon microwaving. Further mechanistic study suggested that the in situ amorphization occurred prior to the in situ salt formation, especially in formulations with low drug loadings and high solid salt mass ratios. For compacts with low polymer ratios, in situ salt formation took place subsequently, where the previously amorphized IND within compacts could interact with the NH3 gas produced in situ by the decomposition of NH4HCO3 and form the ammonium IND salt. Microwaving time showed great impacts on the decomposition of NH4HCO3 and the in situ generation of water and NH3, which indirectly affected the amorphization and salt formation of IND. In comparison to the moisture-based systems, the NH4HCO3-based system showed a number of advantages, including the reduced potential of IND hydrolysis due to the absence of absorbed moisture, a wider category of applicable polymeric carriers other than hygroscopic polymers, and an increase in drug loading up to 50% (w/w).

Effect of Drug Loading in Mesoporous Silica on Amorphous Stability and Performance

The encapsulation of drugs within mesoporous silica (MS) has for several years been a subject of research. Previous studies proposed that drug loadings up to the monomolecular loading capacity (MLC) are the optimal choice for maintaining the drug in an amorphous form, whereas filling the pores above the monolayer and up to the pore filling capacity (PFC) may introduce some physical instabilities. The aim of this study was to assess the effect of drug loading in MS-based amorphous formulations on the stability of the amorphous form of the drug as well as the dissolution. In particular, the following drug loadings were investigated: below MLC, at MLC, between MLC and PFC and at PFC. The drug-loaded MS formulations were analyzed directly after preparation and after 18 months of storage under accelerated conditions (40 °C in both dry and humid conditions). The MLC and PFC for the drug celecoxib (CEL) on the MS ParteckSLC500 (SLC) were determined at 33.5 wt.% and 48.4 wt.%, respectively. This study found that SLC can effectively preserve the amorphous form of the drug for 18 months, provided that the loading is below the PFC (<48.4 wt.%) and no humidity is present. On the other hand, drug loading at the PFC showed recrystallization even when stored under dry conditions. Under humid conditions, however, all samples, regardless of drug loading, showed recrystallization upon storage. In terms of dissolution, all freshly prepared formulations showed supersaturation. For drug loadings below PFC, a degree of supersaturation (DS) around 15 was measured before precipitation was observed. For drug loadings at PFC, the DS was found to be lower and only 6-times compared to the crystalline solubility. Lastly, for those samples that remained amorphous during storage for 18 months, the release profiles were found to be the same as the freshly loaded samples, with similar Cmax, Tmax and dissolution rate.

Analysis of stabilization mechanisms in β-lactoglobulin-based amorphous solid dispersions by experimental and computational approaches

Our previous work shows that β-lactoglobulin-stabilized amorphous solid dispersion (ASD) loaded with 70 % indomethacin remains stable for more than 12 months. The stability is probably due to hydrogen bond networks spread throughout the ASD, facilitated by the indomethacin which has both hydrogen donors and acceptors. To investigate the stabilization mechanisms further, here we tested five other drug molecules, including two without any hydrogen bond donors. A combination of experimental techniques (differential scanning calorimetry, X-ray power diffraction) and molecular dynamics simulations was used to find the maximum drug loadings for ASDs with furosemide, griseofulvin, ibuprofen, ketoconazole and rifaximin. This approach revealed the underlying stabilization factors and the capacity of computer simulations to predict ASD stability. We searched the ASD models for crystalline patterns, and analyzed diffusivity of the drug molecules and hydrogen bond formation. ASDs loaded with rifaximin and ketoconazole remained stable for at least 12 months, even at 90 % drug loading, whereas stable drug loadings for furosemide, griseofulvin and ibuprofen were at a maximum of 70, 50 and 40 %, respectively. Steric confinement and hydrogen bonding to the proteins were the most important stabilization mechanisms at low drug loadings (≤ 40 %). Inter-drug hydrogen bond networks (including those with induced donors), ionic interactions, and a high Tg of the drug molecule were additional factors stabilizing the ASDs at drug loading greater than 40 %.

Mechanisms of Drug Solubility Enhancement Induced by β-Lactoglobulin-Based Amorphous Solid Dispersions

Protein-based amorphous solid dispersions (ASDs) have emerged as a promising approach for enhancing solubility in comparison to crystalline drugs. The dissolution behavior of protein-based amorphous solid dispersions (ASDs) was investigated in various pH media. ASDs of four poorly soluble model drugs with acidic (furosemide and indomethacin), basic (carvedilol), and neutral (celecoxib) properties were prepared by spray drying at 30 wt % drug loading with the protein β-lactoglobulin (BLG). The effect of spray-dried BLG (SD-BLG) solubility and protein binding ability with dissolved drugs in solution were investigated to retrieve the mechanisms governing the improvement of drug solubility from the BLG-based ASDs. Powder dissolution results showed that all ASDs obtained a higher maximum concentration (Cmax) compared to the respective pure crystalline drugs. It was found that the solubility increase of the drugs from the ASDs was to a large extent dependent on the solubility of the pure SD-BLG at the investigated pH values (low solubility at pH near the isoelectric point (pI) of BLG). Furthermore, drug-protein interactions in a solution were observed, in particular at pH values where the drugs were neutral. These drug-protein interactions also resulted, to some extent, in the stabilization of the drug in supersaturation.

Protein Based Amorphous Solid Dispersion: a Case Study Investigating Different Whey Proteins at High Drug Loading

PURPOSE

Whey protein isolate (WPI) has previously been shown to be a promising new excipient for the development of amorphous solid dispersions (ASD) at a high drug loading of 50% (w/w). Whilst WPI is a protein mixture, comprising mainly the three proteins β-lactoglobulin (BLG), α-lactalbumin (ALA), casein glycomacropeptides (CGMP), the individual contributions of these three proteins to the overall performance of whey protein based ASDs has still not been investigated. In addition, the limitations of the technology at even higher drug loadings (i.e., more than 50%) have not yet been explored. In this study, BLG, ALA, CGMP and WPI were each prepared as ASDs with the two poorly water-soluble drugs (Compound A and Compound B) at 50%, 60% and 70% drug loadings.

METHODS

Solid state characterization, dissolution rate and physical stability of the obtained samples were analyzed.

RESULTS

All the obtained samples were amorphous and showed faster dissolution rates compared to the respective pure crystalline drugs. However, the BLG based formulations-at least for Compound A-were outperforming the other ASDs in terms of stability, dissolution enhancement and solubility increase.

CONCLUSION

Overall, the study confirmed that the investigated whey proteins showed their potential in developing ASDs even at high drug loadings of up to 70%.

Enhanced dissolution rate of nimodipine through β-lactoglobulin based formulation

Amorphous solid dispersions (ASD) have been considered as one of the most effective strategies to increase solubility and dissolution rate of poorly water-soluble drugs. Carriers, in which the poorly water-soluble drug is dispersed, contribute a large extent to the solid-state properties, stabilities and dissolution performance of ASDs. This study investigated the solid-state properties, physical stability, and in vitro dissolution behaviour of nimodipine ASDs formulated with a traditional polymeric carrier, i.e., polyvinylpyrrolidone (PVP) and a novel carrier, i.e., β-lactoglobulin (BLG). The ASDs with both carriers were prepared using ball milling as preparative technique at 10 %, 17.5 %, 25 %, 30 % and 40 % drug loadings (DLs). All the formulations were found to be amorphous upon milling for 60 min based on X-ray powder diffraction measurements, however, the ASDs were found to be homogeneous unequivocally only at DLs below 25 %. After open storage at accelerated conditions (40 °C/75 % relative humidity), only the ASDs formulated with BLG at 10 % and 17.5 % DLs maintained the amorphous form. The dissolution study revealed that all the freshly prepared ASDs formulated with PVP and the ASDs formulated with BLG at or above 25 % DLs, showed a low drug release (<30 µg/mL in simulated gastric fluid, < 70 µg/mL in simulated intestinal fluid). Whilst the ASD formulated with BLG at 10 % DL exhibited a high drug release with a maximum concentration (C_max) of 251 µg/mL in simulated gastric fluid and 231 µg/mL in simulated intestinal fluid. Surprisingly, the ASD formulated with BLG at 17.5 % DL demonstrated an even higher drug release (C_max, 643 µg/mL in simulated gastric fluid, 332 µg/mL in simulated intestinal fluid), compared to the ASD of 10 % DL. These findings underline the importance of rationally investigating both carrier types and DL in the design of ASDs, in order to obtain a stable ASD with the desired enhanced dissolution rate of poorly water-soluble drugs.

The effects of surfactants on the performance of polymer-based microwave-induced in situ amorphization

Microwave-induced in situ amorphization is a novel technology for preparing amorphous solid dispersions (ASDs) to address the challenges of their long-term physical stability and downstream processing. To date, only few types of dielectric materials have been reported for microwave-induced in situ amorphization, which restricted the extensive research of this technology. This study aimed to investigate the feasibility and mechanisms of utilizing the non-ionic surfactants, i.e. Kollisolv P124, Kolliphor RH40, D-ɑ-tocopheryl polyethylene glycol succinate (TPGS), Tween (T) 60 (T60), T65, T80 and T85, as plasticizers to facilitate microwave-induced in situ amorphization. It was found that the successful application of surfactants could be related with their low T_m, low M_w and high HLB. Kolliphor RH40 was selected as a typical surfactant due to its excellent dielectric heating ability, plasticizing effect and solubilizing effect when facilitating amorphization. Then, the dissolution-mediated in situ amorphization mechanism was investigated and intuitively demonstrated. For the most promising formulation, i.e. microwaved systems with Korlliphor RH40 at 1.5 (w/w) plasticizer/polymer ratio, a complete and fast in vitro dissolution was observed relative to the untreated systems. In conclusion, non-ionic surfactants had the potential to facilitate microwave-induced in situ amorphization, which provided a new direction in the formulation designation for microwave-able systems.

Stabilizing Mechanisms of β-Lactoglobulin in Amorphous Solid Dispersions of Indomethacin

Proteins, and in particular whey proteins, have recently been introduced as a promising excipient class for stabilizing amorphous solid dispersions. However, despite the efficacy of the approach, the molecular mechanisms behind the stabilization of the drug in the amorphous form are not yet understood. To investigate these, we used experimental and computational techniques to study the impact of drug loading on the stability of protein-stabilized amorphous formulations. β-Lactoglobulin, a major component of whey, was chosen as a model protein and indomethacin as a model drug. Samples, prepared by either ball milling or spray drying, formed single-phase amorphous solid dispersions with one glass transition temperature at drug loadings lower than 40-50%; however, a second glass transition temperature appeared at drug loadings higher than 40-50%. Using molecular dynamics simulations, we found that a drug-rich phase occurred at a loading of 40-50% and higher, in agreement with the experimental data. The simulations revealed that the mechanisms of the indomethacin stabilization by β-lactoglobulin were a combination of (a) reduced mobility of the drug molecules in the first drug shell and (b) hydrogen-bond networks. These networks, formed mostly by glutamic and aspartic acids, are situated at the β-lactoglobulin surface, and dependent on the drug loading (>40%), propagated into the second and subsequent drug layers. The simulations indicate that the reduced mobility dominates at low (<40%) drug loadings, whereas hydrogen-bond networks dominate at loadings up to 75%. The computer simulation results agreed with the experimental physical stability data, which showed a significant stabilization effect up to a drug fraction of 70% under dry storage. However, under humid conditions, stabilization was only sufficient for drug loadings up to 50%, confirming the detrimental effect of humidity on the stability of protein-stabilized amorphous formulations.

Supersaturated amorphous solid dispersions of celecoxib prepared by in situ microwave irradiation

This study investigated the ability of in situ amorphization using microwave irradiation in order to prepare highly supersaturated ASDs, i.e. ASDs with drug loads higher than the saturation solubility in the polymer at ambient temperature. For this purpose, compacts containing the crystalline drug celecoxib (CCX) and polyvinylpyrrolidone (PVP), polyvinylpyrrolidone-vinyl acetate copolymer (PVP/VA), or polyvinyl acetate (PVAc), were prepared at drug loads between 30-90 % w/w. Sodium dihydrogen phosphate (NaH₂PO₄) monohydrate was included in all compacts, as a source of water, to facilitate the dielectric heating of the compacts upon dehydration during microwave irradiation. After processing, the samples were analysed towards their solid state using X-ray powder diffraction (XRPD) and modulated differential scanning calorimetry (mDSC). Complete amorphisation of CCX was achieved across all the investigated polymers and with a maximal drug load of 90, 80, and 50 % w/w in PVP, PVP/VA, and PVAc, respectively. These drug loads corresponded to a 2.3-, 2.4-, and 10.0-fold supersaturation in the investigated polymers at ambient temperature. However, dissolution experiments with the in situ prepared ASDs in fasted state simulated intestinal fluid (FaSSIF), showed a lower initial drug release (0-2 hours) compared to equivalent physical mixtures of crystalline CCX and polymers or crystalline CCX alone. The lower drug release rate was explained by the fusion of individual drug and polymer particles during microwave irradiation and, subsequently, a lack of disintegration of the monolithic ASDs. Nevertheless, supersaturation of CCX in FaSSIF was achieved with the in situ amorphised ASDs with PVP and PVP/VA, albeit only after 3-24 h. Overall, the present study confirmed that it is feasible to prepare supersaturated ASDs in situ. However, in the current experimental setup, the monolithic nature of the resulting ASDs is considered a limiting factor in the practical applicability of this preparation method, due to limited disintegration and the associated negative effect on the drug release.

Hyperthermia-Induced In Situ Drug Amorphization by Superparamagnetic Nanoparticles in Oral Dosage Forms

Superparamagnetic iron oxide nanoparticles (SPIONs) generate heat upon exposure to an alternating magnetic field (AMF), which has been studied for hyperthermia treatment and triggered drug release. This study introduces a novel application of magnetic hyperthermia to induce amorphization of a poorly aqueous soluble drug, celecoxib, in situ in tablets for oral administration. Poor aqueous solubility of many drug candidates is a major hurdle in oral drug development. A novel approach to overcome this challenge is in situ amorphization of crystalline drugs. This method facilitates amorphization by molecular dispersion of the drug in a polymeric network inside a tablet, circumventing the physical instability encountered during the manufacturing and storage of conventional amorphous solid dispersions. However, the current shortcomings of this approach include low drug loading, toxicity of excipients, and drug degradation. Here, doped SPIONs produced by flame spray pyrolysis are compacted with polyvinylpyrrolidone and celecoxib and exposed to an AMF in solid state. A design of experiments approach was used to investigate the effects of SPION composition (Zn_0.5Fe_2.5O₄ and Mn_0.5Fe_2.5O₄), doped SPION content (10-20 wt %), drug load (30-50 wt %), and duration of AMF (3-15 min) on the degree of drug amorphization. The degree of amorphization is strongly linked to the maximum tablet temperature achieved during the AMF exposure (r = 0.96), which depends on the SPION composition and content in the tablets. Complete amorphization is achieved with 20 wt % Mn_0.5Fe_2.5O₄ and 30 wt % celecoxib in the tablets that reached the maximum temperature of 165.2 °C after 15 min of AMF exposure. Furthermore, manganese ferrite exhibits no toxicity in human intestinal Caco-2 cell lines. The resulting maximum solubility of in situ amorphized celecoxib is 5 times higher than that of crystalline celecoxib in biorelevant intestinal fluid. This demonstrates the promising capability of SPIONs as enabling excipients to magnetically induce amorphization in situ in oral dosage forms.

Investigation into the role of the polymer in enhancing microwave-induced in situ amorphization

Microwave-induced in situ amorphization is an emerging technology to tackle the persistent stability issue of amorphous solid dispersions (ASDs) during manufacture and storage. The aim of this study was to introduce new effective polymeric carriers with diverse properties to microwave-induced in situ amorphization and to better understand their functions in relation to the final dissolution performance of microwaved tablets. Tablets composed of indomethacin (IND) and different polymers were compacted, stored at 75% relative humidity for at least 1 week and microwaved at 1000 W to induce amorphization. A series of polymers, polyvinylpyrrolidone/vinyl acetate copolymers (PVP/VA) of different monomer weight ratios displaying varyingproperties in functional groupratio, hygroscopicity, molecular weight (M_w), and glass transition temperature (T_g) of the polymer were used as model carriers. The results suggested that more than 90% of IND was amorphized after 20 mins microwaving in all 20% (w/w) drug loaded tablets except for IND:PVAc tablets presenting approx. 36% residual crystallinity. Among them, tablets composed of PVP/VA I-335 and PVP K30 achieved complete in situ amorphization upon microwaving. Further analysis indicated that the influencing factors, polymer M_w and T_g of moisture-plasticized polymer, played a major role in microwave-induced in situ amorphization. In in vitro dissolution study, ASDs containing PVP/VA I-535 with moderate hydrophilicity and 0.96 ± 1.92% IND residual crystallinity showed the most rapid and complete drug release among all formulations, presenting the most promising dissolution performance. Further study on the chemical stability of such formulation showed a statistically insignificant decrease of drug content after pre-conditioning and microwaving (P = 0.288 > 0.05).

Microwave-Induced in Situ Drug Amorphization Using a Mixture of Polyethylene Glycol and Polyvinylpyrrolidone

The use of a mixture of polyethylene glycol (PEG) and polyvinylpyrrolidone (PVP) was investigated for microwave-induced in situ amorphization of celecoxib (CCX) inside compacts. Such amorphization requires the presence of a dipolar excipient in the formulation to ensure heating of the compact by absorption of the microwaves. Previously, the hygroscopic nature of PVP was exploited for this purpose. By exposing PVP-based compacts for set time intervals at defined relative humidity, controlled water sorption into the compacts was achieved. In the present study, PEG was proposed as the microwave absorbing excipient instead of water, to avoid the water sorption step. However, it was found that PEG alone melted upon exposure to microwave radiation and caused the compact to deform. Furthermore, CCX was found to recrystallize upon cooling in PEG-based formulations. Hence, a mixture of PEG and PVP was used, where the presence of PVP preserved the physical shape of the compact, and the physical state of the amorphous solid dispersion. To study the impact of the polymer mixture, different compact compositions of CCX, PEG and PVP were prepared. When exposing the compacts to microwave radiation, it was found that the PEG:PVP ratio was critical for in situ amorphization and that complete amorphization was only achieved above a certain temperature threshold.

Enabling formulations of aprepitant: in vitro and in vivo comparison of nanocrystalline, amorphous and deep eutectic solvent based formulations

A deep eutectic solvent (DES) is a eutectic system consisting of hydrogen bond donor and acceptor has been suggested as a promising formulation strategy for poorly soluble drugs. A DES consisting of choline chloride and levulinic acid in a 1:2  molar ratio was used to formulate a liquid solution of the model drug aprepitant. This formulation was tested in vitro (drug release and permeability) and in vivo (rat model) and compared with the performance of amorphous aprepitant and the commercial aprepitant nanocrystalline formulation. In this study a DES formulation is compared for the first time directly to other established enabling formulations. The in vitro drug release study demonstrated that the DES formulation and the amorphous form both were able to induce an apparent supersaturation followed by subsequent drug precipitation. To mitigate the risk of precipitation, HPMC was predissolved in the dissolution medium, which successfully reduced the degree of precipitation. In line with the results from the release study, an in vitro permeation study showed superior permeation of the drug from the DES formulation and from the amorphous form compared to the nanocrystalline formulation. However, the promising in vitro findings could not be directly translated into an increased in vivo performance in rats compared to the nanocrystalline formulation. Whilst the DES formulation (34 ± 4%) showed a higher oral bioavailability compared to amorphous aprepitant (20 ± 4%), it was on par with the oral bioavailability obtained from the nanocrystalline formulation (36 ± 2%).

Utilizing Laser Activation of Photothermal Plasmonic Nanoparticles to Induce On-Demand Drug Amorphization inside a Tablet

Poor aqueous drug solubility represents a major challenge in oral drug delivery. A novel approach to overcome this challenge is drug amorphization inside a tablet, that is, on-demand drug amorphization. The amorphous form is a thermodynamically instable, disordered solid-state with increased dissolution rate and solubility compared to its crystalline counterpart. During on-demand drug amorphization, the drug molecularly disperses into a polymer to form an amorphous solid at elevated temperatures inside a tablet. This study investigates, for the first time, the utilization of photothermal plasmonic nanoparticles for on-demand drug amorphization as a new pharmaceutical application. For this, near-IR photothermal plasmonic nanoparticles were tableted together with a crystalline drug (celecoxib) and a polymer (polyvinylpyrrolidone). The tablets were subjected to a near-IR laser at different intensities and durations to study the rate of drug amorphization under each condition. During laser irradiation, the plasmonic nanoparticles homogeneously heated the tablet. The temperature was directly related to the rate and degree of amorphization. Exposure times as low as 180 s at 1.12 W cm^-2 laser intensity with only 0.25 wt % plasmonic nanoparticles and up to 50 wt % drug load resulted in complete drug amorphization. Therefore, near-IR photothermal plasmonic nanoparticles are promising excipients for on-demand drug amorphization with laser irradiation.

Microwave induced in situ amorphisation facilitated by crystalline hydrates

Amorphisation within the final dosage form, i.e. in situ amorphisation, seeks to circumvent the potential stability issues associated with poorly soluble drugs in amorphous solid dispersions (ASDs). Microwave irradiation has previously been shown to enable in situ preparation of ASDs, when a high amount of microwave absorbing water was introduced into the final dosage form by conditioning at high relative humidity. In this study, an alternative to this conditioning step was investigated by introducing crystal water in form of sodium dihydrogen phosphate (NaH₂PO₄) di-, and monohydrate, in compacts prepared with 30 % w/w celecoxib (CCX) in polyvinylpyrrolidone K12 (PVP). As controls, compacts prepared with NaH₂PO₄ anhydrate and without NaH₂PO₄ were included in the study. The quantification of amorphous CCX after microwave irradiation showed an increase in CCX amorphicity for compacts containing NaH₂PO₄ di-, and monohydrate with increasing irradiation time. Complete amorphisation of CCX in compacts containing NaH₂PO₄ di-, and monohydrate was observed after 6 min, while no appreciable amorphisation was observed for the control compacts containing NaH₂PO₄ anhydrate and without NaH₂PO₄. Modulated differential scanning calorimetric analysis revealed that a homogenous ASD was formed after 12 min and 6 min for compacts containing NaH₂PO₄ di-, and monohydrate, respectively. Thermal gravimetric analysis indicated that NaH₂PO₄ monohydrate showed higher dehydration rates compared to the dihydrate, which in turn resulted in higher compact temperatures, and overall increased the rate of amorphisation and reduced the microwave irradiation time necessary to achieve a homogenous ASD. The present results confirmed the suitability of NaH₂PO₄ di- and monohydrate as alternative sources of water, the primary microwave absorbing material, for in situ microwave amorphisation. The use of crystalline hydrates as water reservoirs for in situ amorphisation circumvents the time-consuming and highly impractical conditioning step previously reported in order to achieve complete amorphisation. Additionally, it allows for easier and more accurate adjustment of the compacts water content, which directly affects the temperature reached during microwave irradiation, and thus, the rate of amorphisation.

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 %.

The Use of Glycerol as an Enabling Excipient for Microwave-Induced In Situ Drug Amorphization

Microwave-induced in situ amorphization is a promising approach to circumvent stability and manufacturing issues associated with amorphous solid dispersions (ASD). Using in situ amorphization, the crystalline state of the drug is converted into its amorphous form inside the dosage form, e.g. a compact, upon exposure to microwave radiation. The study aimed to investigate the feasibility of using glycerol as an enabling excipient in compacts prepared from mixtures of indomethacin and Soluplus®. Additionally, the possibility to form a supersaturated ASD upon exposure to microwave radiation due to elevated temperatures was investigated. It was found that glycerol i) acts as a dielectric heating source absorbing the microwaves, ii) plasticizes the polymer Soluplus® and iii) increases the solubility of the drug indomethacin in the polymer Soluplus®. Additionally, it was found that fully amorphous ASDs could be achieved with drug loadings below -, and slightly above the saturation solubility of indomethacin in the Soluplus®/glycerol mixtures, after exposure to 20 min of microwave radiation. Hence, glycerol was a feasible excipient for the microwave-induced in situ amorphization and allowed the preparation of a, at room temperature, supersaturated ASD, due to the elevated temperatures obtained during exposure to microwave radiation.

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.

Formulation of co-amorphous systems from naproxen and naproxen sodium and in situ monitoring of physicochemical state changes during dissolution testing by Raman spectroscopy

Co-amorphous systems comprising low-molecular weight drugs and co-formers constitute an interesting approach to optimize pharmaceutical performance of drugs with low aqueous solubility. Within the different types of co-amorphous systems, the combination of a drug with its own salt may be an attractive formulation option due the absence of any inactive co-formers. The aim of this study was to investigate the possibility of forming a co-amorphous system from naproxen (NAP) and its sodium salt (NAP(Na)). Ball milling of NAP and NAP(Na) at equal molar ratio resulted in the formation of a co-amorphous system whilst NAP and NAP(Na) alone were crystalline following both, ball milling and melt quenching. Infrared spectroscopy and physical stability testing revealed that intermolecular interactions were able to maintain the ball milled NAP-NAP(Na) system amorphous for 2 months at 40 °C. Surprisingly, the dissolution rate of co-amorphous NAP-NAP(Na) was only intermediate between those of crystalline NAP and crystalline NAP(Na). In situ Raman spectroscopic measurements indicated an initial phase separation of the co-amorphous form to NAP and NAP(Na) followed by dissociation of sodium from NAP(Na) and crystallization to NAP. These findings contribute to the design of co-amorphous formulations with the combination of a drug and its own salt.

Microwave-Induced In Situ Amorphization: A New Strategy for Tackling the Stability Issue of Amorphous Solid Dispersions

The thermodynamically unstable nature of amorphous drugs has led to a persistent stability issue of amorphous solid dispersions (ASDs). Lately, microwave-induced in situ amorphization has been proposed as a promising solution to this problem, where the originally loaded crystalline drug is in situ amorphized within the final dosage form using a household microwave oven prior to oral administration. In addition to circumventing issues with physical stability, it can also simplify the problematic downstream processing of ASDs. In this review paper, we address the significance of exploring and developing this novel technology with an emphasis on systemically reviewing the currently available literature in this pharmaceutical arena and highlighting the underlying mechanisms involved in inducing in situ amorphization. Specifically, in order to achieve a high drug amorphicity, formulations should be composed of drugs with high solubility in polymers, as well as polymers with high hygroscopicity and good post-plasticized flexibility of chains. Furthermore, high microwave energy input is considered to be a desirable factor. Lastly, this review discusses challenges in the development of this technology including chemical stability, selection criteria for excipients and the dissolution performance of the microwave-induced ASDs.

Hot Melt Coating of Amorphous Carvedilol

The use of amorphous drug delivery systems is an attractive approach to improve the bioavailability of low molecular weight drug candidates that suffer from poor aqueous solubility. However, the pharmaceutical performance of many neat amorphous drugs is compromised by their tendency for recrystallization during storage and lumping upon dissolution, which may be improved by the application of coatings on amorphous surfaces. In this study, hot melt coating (HMC) as a solvent-free coating method was utilized to coat amorphous carvedilol (CRV) particles with tripalmitin containing 10% (w/w) and 20% (w/w) of polysorbate 65 (PS65) in a fluid bed coater. Lipid coated amorphous particles were assessed in terms of their physical stability during storage and their drug release during dynamic in vitro lipolysis. The release of CRV during in vitro lipolysis was shown to be mainly dependent on the PS65 concentration in the coating layer, with a PS65 concentration of 20% (w/w) resulting in an immediate release profile. The physical stability of the amorphous CRV core, however, was negatively affected by the lipid coating, resulting in the recrystallization of CRV at the interface between the crystalline lipid layer and the amorphous drug core. Our study demonstrated the feasibility of lipid spray coating of amorphous CRV as a strategy to modify the drug release from amorphous systems but at the same time highlights the importance of surface-mediated processes for the physical stability of the amorphous form.

Influence of the Polymer Glass Transition Temperature and Molecular Weight on Drug Amorphization Kinetics Using Ball Milling

In this study, the putative correlation between the molecular mobility of a polymer and the ball milling drug amorphization kinetics (i.e., time to reach full drug amorphization, t_a) was studied using different grades of dextran (Dex) and polyvinylpyrrolidone (PVP) and the two model drugs indomethacin (IND) and chloramphenicol (CAP). In general, IND had lower t_a values than CAP, indicating that IND amorphized faster than CAP in the presence of the polymers. In addition, an increase in polymer molecular weight (M_w) also led to an increase in t_a for all systems investigated up to a critical M_w for each polymer, which was in line with an increase of the glass transition temperature (T_g) up to the critical M_w of each polymer. Hence, the increase in t_a seemed to correlate well with the T_g/M_w of the polymers, which indicates that the polymers’ molecular mobility had an influence on the drug amorphization kinetics during ball milling.

The influence of drug and polymer particle size on the in situ amorphization using microwave irradiation

In this study, the impact of drug and polymer particle size on the in situ amorphization using microwave irradiation at a frequency of 2.45 GHz were investigated. Using ball milling and sieve fractioning, the crystalline drug celecoxib (CCX) and the polymer polyvinylpyrrolidone (PVP) were divided into two particle size fractions, i.e. small (<71 µm) and large (>71 µm) particles. Subsequently, compacts containing a drug load of 30% (w/w) crystalline CCX in PVP were prepared and subjected to microwave radiation for an accumulated duration of 600 sec in intervals of 60 sec as well as continuously for 600 sec. It was found that the compacts containing small CCX particles displayed faster rates of amorphization and a higher degree of amorphization during microwave irradiation as compared to the compacts containing large CCX particles. For compacts with small CCX particles, interval exposure to microwave radiation resulted in a maximum degree of amorphization of 24%, whilst a fully amorphous solid dispersion (100%) was achieved after 600 sec of continuous exposure to microwave radiation. By monitoring the temperature in the core of the compacts during exposure to microwave radiation using a fiber optic temperature probe, it was found that the total exposure time above the glass transition temperature (T_g) was shorter for the interval exposure method compared to continuous exposure to microwave radiation. Therefore, it is proposed that the in situ formation of an amorphous solid dispersion is governed by the dissolution of drug into the polymer, which most likely is accelerated above the T_g of the compacts. Hence, prolonging the exposure time above the T_g, and increasing the surface area of the drug by particle size reduction will increase the dissolution rate and thus, rate and degree of amorphization of CCX during exposure to microwave radiation.

Improving the drug load and in vitro performance of supersaturated self-nanoemulsifying drug delivery systems (super-SNEDDS) using polymeric precipitation inhibitors

In this study, the influence of the polymeric precipitation inhibitor (PPI) PVP/VA 64 (polyvinylpyrrolidone-co-vinyl acetate) on the physical stability and in vitro performance of supersaturated self-nanoemulsifying drug delivery systems (super-SNEDDS) containing the model drug simvastatin (SIM) was investigated. A heating-cooling cycle was employed to dissolve (i) the drug in the SNEDDS preconcentrate, generating super-SNEDDS, or (ii) the drug and PPI generating PPI super-SNEDDS, both containing drug loads of 200% and 250% (with regard to the equilibrium solubility of SIM in the blank SNEDDS). PPI super-SNEDDS were prepared at PPI concentrations of 1%, 10% and 20% (w/w), respectively. The formulations were characterized using polarized light microscopy, dynamic light scattering, rheological profiling and dynamic in vitro lipolysis. The physical stability of PPI super-SNEDDS correlated with an increase in viscosity due to the additionally dissolved PVP/VA 64. PPI super-SNEDDS with drug loads of 200% and 250% containing 20% (w/w) PPI showed no drug recrystallization after more than 6 months of storage at room temperature, whereas PPI-free super-SNEDDS (250% drug load) recrystallized within two hours after equilibration to room temperature. All formulations formed nanosized droplets after emulsification in Milli-Q water. The droplet size was not affected by the PPI, but increased slightly with increasing drug load (z-average of 47.3 ± 0.4 nm for SNEDDS with 200% drug load and 55.6 ± 1.3 nm for SNEDDS with 250% drug load). PPI super-SNEDDS with a drug load of 200% containing 20% (w/w) PVP/VA 64 showed an improved performance during dynamic in vitro lipolysis, maintaining a 2.5-fold higher degree of supersaturation after 15 min of digestion compared to PPI-free super-SNEDDS of the same drug load. In conclusion, the study demonstrated the feasibility of stabilizing higher drug loads and improving the in vitro performance of super-SNEDDS by incorporating PVP/VA 64 into the preconcentrate.

Impact of drug loading in mesoporous silica-amorphous formulations on the physical stability of drugs with high recrystallization tendency

In this study, a method is described to determine the monolayer loading capacity (MLC) of the drugs naproxen and ibuprofen, both having high recrystallization tendencies, in mesoporous silica (MS), a well known carrier that is able to stabilize the amorphous form of a drug. The stabilization has been suggested to be due to direct absorption of the drug molecules onto the MS surface, i.e. the drug monolayer. In addition, drug that is not in direct contact with MS surface can fill the pores up to its pore filling capacity (PFC) and is potentially stabilized by confinement due to the pore size being smaller than a crystal nuclei. For drugs with high recrystallization tendencies, any drug outside the pores crystallizes due to its poor physical stability. The drug monolayer does not contribute to the glass transition temperature (Tg ) in the DSC, however, the confined amorphous drug above MLC has a Tg and the heat capacity (ΔC p) over the Tg increases with an increasing fraction of confined amorphous drug. Hence, several drug loading values above the MLC were investigated towards the presence of a Tg and ΔC p using differential scanning calorimetry (DSC). A linear correlation between the amount of confined amorphous drug and its ΔC p was identified for the mixtures between the MLC and PFC. By subsequent extrapolation to zero ΔC p the experimental MLC could be determined. Using theoretical density functional theory (DFT) and ab initio Molecular Dynamics (AIMD), the binding energies for the monolayer suggested that the monolayer in fact is thermodynamically more favorable than the crystalline form, whereas the confined amorphous form is thermodynamically less favorable. Consequently, a physical stability study showed that the confined amorphous drugs above the MLC were thermodynamically unstable and consequently flowing out of the pores in order to crystallize, whereas the monolayer remained physically stable.

Tailor-made solvents for pharmaceutical use? Experimental and computational approach for determining solubility in deep eutectic solvents (DES)

A deep eutectic solvent (DES) is a mixture of two or more chemicals that interact via hydrogen bonding and has a melting point far below that of the individual components. DESs have been proposed as alternative solvents for poorly soluble active pharmaceutical ingredients (API). In this study, the solvation capacities of six deep eutectic solvents were compared to water and three conventional pharmaceutical solvents (PEG 300, ethanol and glycerol) for 11 APIs. The experimentally determined solubilities were compared to computational solubilities predicted by the Conductor-like Screening Model for Real Solvents (COSMO-RS). While the conventional pharmaceutical solvents PEG 300 and ethanol were the best solvents for the majority of the studied APIs, API-DES combinations were identified, which exceeded the API solubility found in the conventional pharmaceutical solvents. Furthermore, it was also possible to obtain high solubilities in the DESs relative to water, suggesting DESs to be potential solvents for poorly water soluble APIs. In addition, the relative increase in solubility found in the experimental data could be well predicted ab initio using COSMO-RS. Hence, COSMO-RS may in the future be used to reduce the experimental screening of potential DESs for a given API.

Direct Measurement of Amorphous Solubility

Amorphous materials exhibit distinct physicochemical properties compared to their respective crystalline counterparts. One of these properties, the increased solubility of amorphous materials, is exploited in the pharmaceutical industry as a way of increasing bioavailability of poorly water-soluble drugs. Despite the increasing interest in drug amorphization, the analytical physicochemical toolbox is lacking a reliable method for direct amorphous solubility assessment. Here, we show, for the first time, a direct approach to measure the amorphous solubility of diverse drugs by combining optics with fluidics, the single particle analysis (SPA) method. Moreover, a comparison was made to a theoretical estimation based on thermal analysis and to a standardized supersaturation and precipitation method. We have found a good level of agreement between the three methods. Importantly, the SPA method allowed for the first experimental measurement of the amorphous solubility for griseofulvin, a fast crystallizing drug, without the use of a crystallization inhibitor. In conclusion, the SPA approach enables rapid and straightforward determination of the supersaturation potential for amorphous materials of less than 0.1 mg, which could prove highly beneficial in the fields of materials science, analytical chemistry, physical chemistry, food science, pharmaceutical science, and others.

Design of Inhalable Solid Dosage Forms of Budesonide and Theophylline for Pulmonary Combination Therapy

Corticosteroid resistance poses a major challenge to effective treatment of chronic obstructive pulmonary diseases. However, corticosteroid resistance can be overcome by co-administration of theophylline. The aim of this study was to formulate the corticosteroid budesonide with theophylline into inhalable dry powders intended for pulmonary combination therapy. Four types of spray-dried powders were prepared: (i) budesonide and theophylline co-dissolved and processed using a 2-fluid nozzle spray drier, (ii) budesonide nanocrystals and dissolved theophylline co-dispersed and processed using a 2-fluid nozzle spray drier, (iii) dissolved budesonide and dissolved theophylline processed using a 3-fluid nozzle spray drier, and (iv) budesonide nanocrystals and dissolved theophylline processed using a 3-fluid nozzle spray drier. Spray drying from the solutions resulted in co-amorphous (i) and partially amorphous powders (iii), whereas spray drying of the nanosuspensions resulted in crystalline products (ii and iv). Even though budesonide was amorphous in (i) and (iii), it failed to exhibit any dissolution advantage over the unprocessed budesonide. In contrast, the dissolution of budesonide from its nanocrystalline formulations, i.e., (ii) and (iv), was significantly higher compared to a physical mixture or unprocessed budesonide. Furthermore, the spray-dried powders obtained from the 2-fluid nozzle spray drier, i.e., (i) and (ii), exhibited co-deposition of budesonide and theophylline at the same weight ratio in the aerodynamic assessment using the New Generation Impactor. In contrast, the depositions of budesonide and theophylline deviated from the starting weight ratio in the aerodynamic assessment of spray-dried powders obtained from the 3-fluid nozzle spray drier, i.e., (iii) and (iv). Based on these results, the powders spray-dried from the suspension by using the 2-fluid nozzle spray drier, i.e., (ii), offered the best formulation properties given the physically stable crystalline solid-state properties and the co-deposition profile.

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.

Characterising glass transition temperatures and glass dynamics in mesoporous silica-based amorphous drugs

Amorphous drug molecules bound to MPS surface are restricted in mobility, but they exhibit a primary glass transition temperature.

Co-former selection for co-amorphous drug-amino acid formulations

We have previously developed a fast screening method on the ability of twenty amino acids (AA) to form co-amorphous formulations with six drugs upon ball milling. In this work, the potential advantages in physical stability and dissolution rate of the 36 successful co-amorphous formulations, compared to the pure amorphous drug, were further investigated. The physical stability of the formulations at dry conditions was assessed by X-ray powder diffraction (XRPD) and their thermal behavior by differential scanning calorimetry (DSC). In addition, the intrinsic dissolution rate (IDR) of all formulations was determined in phosphate buffer (10 mM, pH 6.8). Finally, all the co-amorphous formulations were summarized into different groups, according to the outcome of the co-formability, physical stability and dissolution rate screenings, and guidelines could be drawn for selection of co-formers for a new given drug: (i) For acidic drugs, basic AAs (arginine, histidine, and lysine) are good co-formers with respect to the three critical quality attributes: co-formability, physical stability and dissolution. High glass transition temperatures (T_g), physical stability for 1-2 years, and accelerated IDR were observed. (ii) For basic and neutral drugs, non-polar AAs with aromatic groups such as tryptophan (TRP) and phenylalanine (PHE) should be explored as first choice. These combinations presented high T_gs, which generally translated into good physical stability. The IDR of TRP- and PHE-based formulations were usually superior to the IDR of the pure amorphous drugs; (iii) Non-polar AAs with aliphatic structures such as leucine, isoleucine, methionine and valine did not provide an increase in T_g or IDR compared to the pure amorphous drug, and appear to be less feasible AAs for co-amorphous formulations.

Whey proteins as stabilizers in amorphous solid dispersions

Whey proteins are extensively used as nutritional supplements but have so far not been investigated as co-formers for amorphous solid dispersions (ASD) to enhance the solubility and dissolution rate of poorly water soluble drugs. In this study, whey protein isolate (WPI) and whey protein hydrolysate (WPH) were each mixed with three poorly water soluble drugs (indomethacin: IND, carvedilol: CAR and furosemide: FUR) and prepared as ASDs at 50% (w/w) drug loading using vibrational ball milling. Subsequently, solid state characteristics, dissolution rate and physical stability of the obtained samples were analyzed. All ASDs showed a significant increase in their glass transition temperatures, as well as faster dissolution rates and higher apparent solubilities compared to both the respective pure crystalline and amorphous drugs. The saturation solubility of the drugs was increased in the presence of the whey proteins, and the investigated ASDs showed supersaturation by attaining higher drug concentrations compared to the respective saturation solubilities. Upon storage, ASDs containing IND were found to be physically stable for at least 27 months, whereas, ASDs containing CAR or FUR were stable for about 8 months and 17 months, respectively. This was a tremendous increase in physical stability compared to the pure amorphous drugs which recrystallized within less than one week. Overall, WPI and WPH proved to be promising co-formers and amorphous stabilizers in ASD formulations.

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.

Influence of preparation technique on co-amorphization of carvedilol with acidic amino acids

Basic amino acids (AAs) have successfully been used as co-formers with acidic drugs for the preparation of co-amorphous formulations using ball-milling (BM) and spray-drying (SD). In contrast, acidic AAs have been reported as poor co-formers for co-amorphous formulations, even for basic drugs, when using BM as a preparation technique. In this study the basic drug carvedilol (CAR) and the two acidic AAs, glutamic acid and aspartic acid, were used to explore the possibilities of producing co-amorphous formulations using BM, SD and liquid assisted grinding (LAG). X-ray powder diffraction, thermal analysis and Fourier-transform infrared spectroscopy were used to determine the solid state form of the various CAR-AA mixtures prepared. BM the CAR-AA mixtures for 60 min did not result in co-amorphization as XRPD revealed remaining crystallinity of both CAR and the AA. On the other hand, successful co-amorphous salt formation was obtained for all SD samples. Differential scanning calorimetry showed that all the SD CAR-AA mixtures had a single glass transition temperature of approximately 80 °C. The CAR-AA mixtures prepared by LAG showed some polymorphic conversion of CAR. Intrinsic dissolution testing showed the highest dissolution rate for all SD mixtures due to co-amorphous salt formation. Hence it was observed that of the three preparation techniques used, successful co-amorphous formulations of a basic drug with an acidic AA could only be prepared by SD.

In vitro and in vivo comparison between crystalline and co-amorphous salts of naproxen-arginine

Liquid-assisted grinding (LAG) and dry ball milling (DBM) have recently been used to obtain different physical forms of drug-amino acid salts with promising dissolution and physical stability properties. In this work, crystalline and co-amorphous naproxen-arginine mixtures were prepared using LAG and DBM, respectively, and compared with regard to their in vitro and in vivo performance. X-ray powder diffraction and Fourier-transformed infrared spectroscopy showed that LAG led to the formation of a crystalline salt, while DBM led to a co-amorphous salt. These results agreed with the differential scanning calorimetry profiles: a melting point of 230 °C was determined for the crystalline salt, while the co-amorphous formulation showed a single glass transition temperature at approx. 92 °C. Both solid state forms of the salt showed increased intrinsic dissolution rates (14.8 and 74.1-fold, respectively) and also higher solubility (25.3 and 29.8-fold, respectively) compared to the pure crystalline drug in vitro. Subsequently, the co-amorphous salt revealed an improved bioavailability in a pharmacokinetic study, showing a 1.5-fold increase in AUC_0-t and a 2.15-fold increase in c_max compared to the pure crystalline drug. In contrast, even though showing a better in vitro performance, the crystalline salt interestingly did not show an increase in bioavailability in comparison to pure crystalline naproxen.

The Influence of Polymers on the Supersaturation Potential of Poor and Good Glass Formers

The increasing number of poorly water-soluble drug candidates in pharmaceutical development is a major challenge. Enabling techniques such as amorphization of the crystalline drug can result in supersaturation with respect to the thermodynamically most stable form of the drug, thereby possibly increasing its bioavailability after oral administration. The ease with which such crystalline drugs can be amorphized is known as their glass forming ability (GFA) and is commonly described by the critical cooling rate. In this study, the supersaturation potential, i.e., the maximum apparent degree of supersaturation, of poor and good glass formers is investigated in the absence or presence of either hypromellose acetate succinate L-grade (HPMCAS-L) or vinylpyrrolidine-vinyl acetate copolymer (PVPVA64) in fasted state simulated intestinal fluid (FaSSIF). The GFA of cinnarizine, itraconazole, ketoconazole, naproxen, phenytoin, and probenecid was determined by melt quenching the crystalline drugs to determine their respective critical cooling rate. The inherent supersaturation potential of the drugs in FaSSIF was determined by a solvent shift method where the respective drugs were dissolved in dimethyl sulfoxide and then added to FaSSIF. This study showed that the poor glass formers naproxen, phenytoin, and probenecid could not supersaturate on their own, however for some drug:polymer combinations of naproxen and phenytoin, supersaturation of the drug was enabled by the polymer. In contrast, all of the good glass formers—cinnarizine, itraconazole, and ketoconazole—could supersaturate on their own. Furthermore, the maximum achievable concentration of the good glass formers was unaffected by the presence of a polymer.

Aspartame as a co-former in co-amorphous systems

Co-amorphous drug delivery systems are a promising approach to improve the dissolution rate and therefore potentially the oral bioavailability of poorly-water soluble drugs. Several low molecular weight excipients, for instance amino acids, have previously been shown to stabilize the amorphous form and increase the dissolution rate of drugs. In this study, the feasibility of aspartame, a methyl ester of the aspartic acid-phenylalanine dipeptide, as a co-former was investigated and compared with the respective single amino acids, both alone and in combination. The poorly water-soluble compounds mebendazole, tadalafil and piroxicam were chosen as model drugs. In contrast to the single amino acids or the physical mixture of both, all drug-aspartame mixtures became amorphous upon 90 min of ball milling. Only a single glass transition temperature (T_g) was detected by modulated differential scanning calorimetry, which indicates that a homogeneous single-phase co-amorphous system was obtained. Powder dissolution tests showed that the dissolution rates of the drugs from drug-aspartame co-amorphous samples were increased compared to crystalline drugs. Furthermore, supersaturation was observed for the mebendazole-aspartame and tadalafil-aspartame co-amorphous systems. In conclusion, aspartame has been shown to be a promising co-former in co-amorphous systems, superior to the single amino acids or their mixtures.

The Role of Glass Transition Temperatures in Coamorphous Drug-Amino Acid Formulations

The improved physical stability associated with coamorphous drug-amino acid (AA) formulations may indicate a decrease in mobility of the amorphous drug molecules, compared to the neat amorphous form of the drug. Since the characteristic glass transition temperatures ( T_gα and T_gβ) represent molecular mobility in amorphous systems, we aimed to characterize T_gα and T_gβ and to determine their role in physical stability as well as their potential usefulness to determine the presence of an excess component (either drug or AA) in coamorphous systems. Indomethacin (IND)-tryptophan (TRP) and carvedilol (CAR)-TRP were used as model coamorphous systems. The analytical techniques used were X-ray powder diffractometry (XRPD) to determine the solid-state form, dynamic mechanical analysis (DMA) to probe T_gα and T_gβ, and differential scanning calorimetry (DSC) to probe thermal behavior of the coamorphous systems. T_gα analysis showed a gradual monotonous increase in T_gα values with increasing AA concentration, and this increase in the T_gα value is not the cause of the improved physical stability. The T_gβ analysis for the IND-TRP sample with 10% drug had a T_gβ of 226.8 K, and samples with 20-90% drug had similar T_gβ values around 212.5 K. For CAR-TRP, samples with 10-40% drug had similar T_gβ values around 230.5 K, and samples with 50-90% drug had similar T_gβ values around 223.3 K. The similar T_gβ values in coamorphous systems at different drug ratios indicate that they in fact are the T_gβ of the component that is in excess to an ideal drug-AA coamorphous mixture. DSC and XRPD analysis showed that for IND-TRP, IND is in excess if the drug concentration is 30% or above and will eventually recrystallize. For CAR-TRP, CAR is in excess and recrystallizes when the drug concentration is 50% or above. We have proposed a means of estimating, on the basis of T_gβ, which drug to AA ratios will lead to optimally physically stable coamorphous systems that can be considered for further development.

Organic acids as co-formers for co-amorphous systems - Influence of variation in molar ratio on the physicochemical properties of the co-amorphous systems

Co-amorphous drug delivery systems are attracting increasing attention in the pharmaceutical field, due to their promising potential to improve the solubility and bioavailability of poorly water-soluble drugs. In this study, three organic acids, namely benzoic acid, malic acid and citric acid, were investigated as co-formers for the weakly basic drug carvedilol. It was hypothesised that the mono-, di- and triprotic nature of the organic acids could result in co-amorphous salt formation with carvedilol at the respective stoichiometric molar ratios, leading to different physicochemical properties of the co-amorphous samples. The carvedilol-organic acid samples were spray dried at molar ratios from 1:4 to 4:1 and amorphous products were obtained for all mixtures except for carvedilol-benzoic acid at a molar ratio of 1:4. A positive deviation of the glass transition temperature compared to the Gordon-Taylor equation was seen for all co-amorphous samples. Salt formation was confirmed by FTIR, but interestingly complete salt formation did not simply follow the molar ratio of the number of basic and acidic groups, most likely due to steric hindrance. As more than one molecule of carvedilol was found to be involved in most co-amorphous systems with the organic acids, this approach allows for a higher "drug loading" compared to other co-formers that usually form co-amorphous systems at a 1:1 M ratio. In addition, the large number of available organic acids offers various options for selecting co-formers.

Undesired co-amorphisation of indomethacin and arginine during combined storage at high humidity conditions

The use of co-amorphous systems for solubility enhancement of poorly water-soluble drugs has recently gained interest in the field of pharmaceutical technology. However, undesired co-amorphisation of a drug may lead to an alteration of the performance of the drug product, e.g. the previously observed co-amorphisation of indomethacin and arginine upon storage of tablets containing both components in an initially crystalline form at room temperature (RT) and 75% relative humidity (RH). Therefore, the aim of the present study was to further investigate this unintended co-amorphisation by storing plain crystalline γ-indomethacin and arginine as well as physical mixtures of both components at RT and three different RH levels (28, 58, and 75% RH). After storage for up to 101 days, their properties were analysed by X-ray powder diffraction, infrared spectroscopy, differential scanning calorimetry, thermogravimetric analysis, and HPLC. Results showed that the solid state of plain γ-indomethacin did not change during storage at all three storage conditions. In contrast, arginine was found to form a dihydrate upon storage at RT/58% RH and RT/75% RH. The physical mixtures, stored at RT/28% RH and RT/58% RH, remained crystalline and were chemically stable, while the formation of a co-amorphous salt between indomethacin and arginine as well as basic hydrolysis of indomethacin started already 1 day after exposure to RT/75% RH. Moreover, formation of a crystalline salt of indomethacin and arginine upon storage at RT/75% RH was observed. As neither of these instabilities occurred, if indomethacin was stored separately, the simultaneous effects of arginine and moisture on the solid state properties and chemical stability of indomethacin should be taken into account, if selecting arginine as excipient.

Influence of Solvent Composition on the Performance of Spray-Dried Co-Amorphous Formulations

Ball-milling is usually used to prepare co-amorphous drug–amino acid (AA) mixtures. In this study, co-amorphous drug–AA mixtures were produced using spray-drying, a scalable industrially preferred preparation method. The influence of the solvent type and solvent composition was investigated. Mixtures of indomethacin (IND) and each of the three AAs arginine, histidine, and lysine were ball-milled and spray-dried at a 1:1 molar ratio, respectively. Spray-drying was performed at different solvent ratios in (a) ethanol and water mixtures and (b) acetone and water mixtures. Different ratios of these solvents were chosen to study the effect of solvent mixtures on co-amorphous formulation. Residual crystallinity, thermal properties, salt/partial salt formation, and powder dissolution profiles of the IND–AA mixtures were investigated and compared to pure crystalline and amorphous IND. It was found that using spray-drying as a preparation method, all IND–AA mixtures could be successfully converted into the respective co-amorphous forms, irrespective of the type of solvent used, but depending on the solvent mixture ratios. Both ball-milled and spray-dried co-amorphous samples showed an enhanced dissolution rate and maintained supersaturation compared to the crystalline and amorphous IND itself. The spray-dried samples resulting in co-amorphous samples were stable for at least seven months of storage.