The Role of Drug–Polymer Hydrogen Bonding Interactions on the Molecular Mobility and Physical Stability of Nifedipine Solid Dispersions

Amorphous Drug-Polymer Salts

Amorphous formulations provide a general approach to improving the solubility and bioavailability of drugs. Amorphous medicines for global health should resist crystallization under the stressful tropical conditions (high temperature and humidity) and often require high drug loading. We discuss the recent progress in employing drug–polymer salts to meet these goals. Through local salt formation, an ultra-thin polyelectrolyte coating can form on the surface of amorphous drugs, immobilizing interfacial molecules and inhibiting fast crystal growth at the surface. The coated particles show improved wetting and dissolution. By forming an amorphous drug–polymer salt throughout the bulk, stability can be vastly enhanced against crystallization under tropical conditions without sacrificing the dissolution rate. Examples of these approaches are given, along with suggestions for future work.

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.

The Role of Hydrogen Bonding in Paracetamol-Solvent and Paracetamol-Hydrogel Matrix Interactions

The photophysical and photochemical properties of antipyretic drug - paracetamol (PAR) and its two analogs with different substituents (acetanilide (ACT) and N-ethylaniline (NEA)) in 14 solvents of different polarity were investigated by the use of steady-state spectroscopic technique and quantum-chemical calculations. As expected, the results show that the spectroscopic behavior of PAR, ACT, and NEA is highly dependent on the nature of the solute-solvent interactions (non-specific (dipole-dipole) and specific (hydrogen bonding)). To characterize these interactions, the multiparameter regression analysis proposed by Catalán was used. In order to obtain a deeper insight into the electronic and optical properties of the studied molecules, the difference of the dipole moments of a molecule in the ground and excited state were determined using the theory proposed by Lippert, Mataga, McRae, Bakhshiev, Bilot, and Kawski. Additionally, the influence of the solute polarizability on the determined dipole moments was discussed. The results of the solvatochromic studies were related to the observations of the release kinetics of PAR, ACT, and NEA from polyurethane hydrogels. The release kinetics was analyzed using the Korsmayer-Peppas and Hopfenberg models. Finally, the influence of the functional groups of the investigated compounds on the release time from the hydrogel matrix was analyzed.

Drug Release and Nanodroplet Formation from Amorphous Solid Dispersions: Insight into the Roles of Drug Physicochemical Properties and Polymer Selection

Dissolution of amorphous solid dispersions (ASD) can lead to the formation of amorphous drug-rich nano species (nanodroplets) via liquid-liquid phase separation or glass-liquid phase separation when the drug concentration exceeds the amorphous solubility. These nanodroplets have been shown to be beneficial for ASD performance both in vitro and in vivo. Thus, understanding the generation and stability of nanodroplets from ASD formulations is important. In this study, the impacts of polymer selection and active pharmaceutical ingredient (API) physicochemical properties (wet glass transition temperature (T_g) and log P) on nanodroplet release were studied. Six APIs with different physicochemical properties were formulated as ASDs with two polymers, polyvinylpyrrolidone/vinyl acetate (PVPVA) and hydroxypropyl methylcellulose acetate succinate (HPMCAS). Their release performance was evaluated using both powder and surface normalized dissolution of compacts. In general, HPMCAS-based dispersions resulted in higher drug release compared to PVPVA-based dispersions. The two polymers also exhibited different trends in nanodroplet formation as a function of drug loading (DL). PVPVA ASDs exhibited a "falling-off-the-cliff" effect, with a dramatic decline in release performance with a small increase in drug loading, while HPMCAS ASDs exhibited a negative "slope" in the release rate as a function of drug loading. For both polymers, low T_g compounds achieved higher levels of nanodroplet formation compared to high T_g compounds. The nanodroplets generated from ASD dissolution were also monitored with dynamic light scattering, and HPMCAS was found to be more effective at stabilizing nanodroplets against size increase. Insights from this study may be used to guide formulation design and selection of excipients based on API physicochemical properties.

The molecular design of drug-ionic liquids for transdermal drug delivery: Mechanistic study of counterions structure on complex formation and skin permeation

Though ionic liquids (ILs) as novel enhancers had garnered wide attention, detailed studies elucidating molecular design of drug-ILs were missing and mechanisms of their formation and skin permeation were still lacking. Herein, we systematically investigated effects of counterions structures on formation and skin permeation of drug-ILs. Firstly, effects of counterions on formation of drug-ILs were dependent on polarizability, molecular weight (M.W.) and polar surface area of counterions. It was caused by strong charge assisted hydrogen bond and van der Waals interactions revealed through FT-IR, X-ray photoelectron spectroscopy and molecular docking, which undermined ionic interactions and reduced total interaction strength, thereby produced lower lattice energy. Then, skin permeability of drug-ILs had a good parabola relationship with M.W., polarizability and log P of counterions. The underlying mechanism was the increased drug miscibility with stratum corneum, which caused conformational disorder and phase transition of lipid bilayers characterized by ATR-FTIR, DSC and confocal laser scanning microscopy. Finally, the drug-ILs proved to be non-irritating using in vivo skin erythema analysis. In conclusion, the quantitative structure-activity relationship models based on counterions structure to predict formation and skin permeation of drug-ILs were developed, which provided basic theory for design of drug-ILs with high permeation-enhancing efficiency.

Bottlebrush Polymer Excipients Enhance Drug Solubility: Influence of End-Group Hydrophilicity and Thermoresponsiveness

Bottlebrush polymers have great potential as vehicles to noncovalently sequester, stabilize, and deliver hydrophobic small molecule actives. To this end, we synthesized a poly(N-isopropylacrylamide-stat-N,N-dimethylacrylamide) bottlebrush copolymer using ring-opening metathesis polymerization and developed a facile method to control the thermoresponsive properties using postpolymerization modification. Six increasingly hydrophilic end-groups were installed, yielding cloud point temperature control over a range of 22-42 °C. Solubility enhancement of the antiseizure medication, phenytoin, increased significantly with the hydrophilicity of the end-group moiety. Notably, carboxylated bottlebrush copolymers solubilized formulations with higher drug loadings than linear copolymers because they exist as unimolecular nanoparticles with a synthetically defined density of polymer chains that are more stable in solution. This work provides the first investigation of bottlebrush polymers for hydrophobic noncovalent sequestration and solubilization of pharmaceuticals.

Advantages of introducing an effective crystalline inhibitor in curcumin amorphous solid dispersions formulated by Eudragit E100

OBJECTIVES

This paper was to elucidate the advantages of using an effective crystalline inhibitor, which was hydroxypropyl methylcellulose E5 (HPMC), in inhibiting crystallisation for curcumin amorphous solid dispersion (Cur ASDs) formulated by Eudragit E100 (E100).

METHODS

Physical characterisation such as differential scanning calorimetry and powder X-ray diffraction revealed the solid state during the formation of dispersion and clarified the compatibility between Cur and excipient.

KEY FINDINGS

The liquidity of excipient and the change of Tg in Cur ASDs demonstrated that the addition of HPMC can reduce molecule motion of the whole system, improve Tg of Cur ASDs and inhibit crystallisation of Cur ASDs. The water uptake experiment and molecular dynamic modelling further confirmed the effective solution and matrix crystallisation inhibition role of HPMC.

CONCLUSIONS

The elucidation of HPMC as auxiliary excipient on inhibiting crystallisation for Cur ASDs will bring huge value in designing Cur ASDs in the future.

Enhanced Drug Loading in the Drug-in-Adhesive Transdermal Patch Utilizing a Drug-Ionic Liquid Strategy: Insight into the Role of Ionic Hydrogen Bonding

Though pharmaceutical polymers were widely used in inhibiting drug recrystallization via strong intermolecular hydrogen and ionic bonds, the improved drug stability was achieved at the cost of the drug release rate or amount in the drug-in-adhesive transdermal patch. To overcame the difficulty, this study aimed to increase drug loading utilizing a novel drug-ionic liquid (drug-IL) strategy and illustrate the underlying molecular mechanism. Here, naproxen (NPX) and triamylamine (TAA) were chosen as the model drug and corresponding counterion, respectively. In addiiton, carboxylic pressure-sensitive adhesive (PSA) was chosen as the model polymer. The drug-IL (NPX-TAA) was synthesized and characterized by differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FT-IR), and proton nuclear magnetic resonance. The miscibility between NPX-TAA and PSA was assessed using microscopy study, X-ray diffraction, fluorescence spectroscopy, and solubility parameter calculation. In addition, molecular mechanisms of crystallization inhibition were revealed by FT-IR, Raman spectroscopy, DSC, X-ray photoelectron spectroscopy (XPS), and molecular docking. Finally, the release pattern of the high load patch of NPX-TAA was evaluated using in vitro drug release and verified by a skin permeation experiment. The results showed that drug loading in PSA was increased by 5.0 times, which was caused by the synergistic effect of strong ionic hydrogen bonding (the decreased intensity and blue shift of the O-H peak of COOH in PSA) formed between NPX-TAA and PSA-COO^- and normal hydrogen bonding (red shift of the C═O peak in PSA) formed between NPX-TAA and the carbonyl group of PSA. In addition, -NH⁺ of TAA was confirmed as the molecular basis of ionic hydrogen bonding through new peak appearance (binding energy: 400.0 eV) in XPS spectra. Moreover, high drug release percent (80.8 ± 1.8%) was achieved even at high drug loading compared with the control group (72.4 ± 2.2%). Thus, this study introduced an effective drug-IL method to enhance drug loading capacity and illustrated the brand-new action mechanism, which provided a powerful instrument for the development of a high drug loading-high release patch.

Effect of polymeric excipients on nucleation and crystal growth kinetics of amorphous fluconazole

Three chemically distinct polymeric excipients show significantly different effects on the nucleation and crystal growth kinetics of amorphous fluconazole, a classical antifungal drug.

Impact of the Chain Length and Topology of the Acetylated Oligosaccharide on the Crystallization Tendency of Naproxen from Amorphous Binary Mixtures

The impact of the chain length or dispersity of polymers in controlling the crystallization of amorphous active pharmaceutical ingredients (APIs) has been discussed for a long time. However, because of the weak control of these parameters in the majority of macromolecules used in pharmaceutical formulations, the abovementioned topic is poorly understood. Herein, four acetylated oligosaccharides, maltose (acMAL), raffinose (acRAF), stachyose (acSTA), and α-cyclodextrin (ac-α-CD) of growing chain lengths and different topologies (linear vs cyclic), mimicking the growing backbone of the polymer, were selected to probe the influence of these structural factors on the crystallization of naproxen (NAP)—an API that does not vitrify regardless of the cooling rate applied in our experiment. It was found that in equimolar systems composed of NAP and linear acetylated oligosaccharides, the progress and activation barrier for crystallization are dependent on the molecular weight of the excipient despite the fact that results of Fourier transform infrared studies indicated that there is no difference in the interaction pattern between measured samples. On the other hand, complementary dielectric, calorimetric, and X-ray diffraction data clearly demonstrated that NAP mixed with ac-α-CD (cyclic saccharide) does not tend to crystallize even in the system with a much higher content of APIs. To explain this interesting finding, we have carried out further density functional theory computations, which revealed that incorporation of NAP into the cavity of ac-α-CD is hardly possible because this state is of much higher energy (up to 80 kJ/mol) with respect to the one where the API is located outside of the saccharide torus. Hence, although at the moment, it is very difficult to explain the much stronger impact of the cyclic saccharide on the suppression of crystallization and enhanced stability of NAP with respect to the linear carbohydrates, our studies clearly showed that the chain length and the topology of the excipient play a significant role in controlling the crystallization of this API.

The Influence of the Strength of Drug-Polymer Interactions on the Dissolution of Amorphous Solid Dispersions

In an earlier report, ionic interactions between ketoconazole (KTZ), a weakly basic drug, and poly(acrylic acid) (PAA), an anionic polymer, resulted in a dramatic decrease in molecular mobility as well as reduced crystallization propensity of amorphous solid dispersion (ASD) in the solid state. On the other hand, weaker dipole-dipole interactions between KTZ and polyvinylpyrrolidone (PVP) resulted in ASDs with higher crystallization propensity (Mistry Mol Pharm., 2015, 12 (9), 3339-3350). In this work, we investigated the behavior of the ketoconazole (KTZ) solid dispersions in aqueous media. In vitro dissolution tests showed that the PAA ASD maintained the level of supersaturation for a longer duration than the PVP ASD at low polymer contents (4-20% w/w polymer). Additionally, the PAA ASDs were more resistant to drug crystallization in aqueous medium when measured with synchrotron X-ray diffractometry. Two-dimensional ¹H nuclear Overhauser effect spectroscopy (NOESY) NMR cross peaks between ketoconazole and PAA confirmed the existence of drug-polymer interactions in D₂O. The interaction was accompanied by a reduced drug diffusivity as monitored by 2D diffusion ordered spectroscopy (DOSY) NMR and enthalpy-driven when characterized by isothermal titration calorimetry (ITC). On the other hand, drug-polymer interactions were not detected between ketoconazole and PVP in aqueous solution, with NOESY, DOSY, or ITC. The results suggest that interactions that stabilize ASDs in the solid state can also be relevant and important in sustaining supersaturation in solution.

Prediction of the physical stability of amorphous solid dispersions: relationship of aging and phase separation with the thermodynamic and kinetic models along with characterization techniques

Introduction: Solid dispersion has been considered to be one of the most promising methods for improving the solubility and bioavailability of insoluble drugs. However, the physical stability of solid dispersions (SDs), including its aging and recrystallization, or phase separation, has always been one of the most challenging problems in the process of formulation development and storage.Areas covered: The high energy state of SDs is one of the primary reasons for the poor physical stability. The factors affecting the physical stability of SDs have been described from the perspective of thermodynamics and kinetics, and the corresponding theoretical model is put forward. We briefly summarize several commonly used techniques to characterize the thermodynamic and kinetic properties of SDs. Specific measures to improve the physical stability of SDs have been proposed from the perspective of prescription screening, process parameters, and storage conditions.Expert opinion: The separation of the drug from the polymer, the formation, and migration of drug crystals will cause the SDs to shift toward the direction of energy reduction, which is the intrinsic cause of instability. Furthermore, computational simulation can be used for efficient and rapid screening suitable for the excipients to improve the physical stability of SDs.

Molecular Mobility and Crystal Growth in Amorphous Binary Drug Delivery Systems: Effects of Low-Concentration Poly(Ethylene Oxide)

Polymer additives have been widely reported to affect the crystallization of amorphous drugs, while the underlying mechanism is poorly understood. The present study aims to investigate the relationship between the crystal growth and the molecular mobility of amorphous nifedipine (NIF) in the presence and absence of low-concentration poly(ethylene oxide) (PEO). The addition of 3% w/w PEO yields approximately a 5-fold increase in the crystal growth rate of NIF in the glassy matrix and a 10-fold increase in the supercooled liquid. Broadband dielectric spectroscopy is performed to investigate the molecular mobility of amorphous pure NIF system and NIF doped with low-concentration PEO. With 3% w/w PEO, the structural relaxation time τ_α of amorphous NIF significantly decreases, indicating an increase in the global molecular mobility. However, the increase of the molecular mobility is insufficient to explain the 5- to 10-fold increase of the crystal growth rate at the same τ_α scale. Moreover, we compare the accelerating effect of PEO in NIF-PEO systems to other PEO-doped systems. The accelerating effect of low-concentration PEO on the crystal growth of amorphous drugs is found to be independent of the Flory-Huggins interaction, T_g of the drug, or the increase of the global molecular mobility. These findings suggest that an in-depth understanding regarding the effects of polymer additives on the crystallization of drugs should consider the localized mobility of the host molecules near the crystal-liquid interface.

In situ solid-state NMR characterization of pharmaceutical materials: An example of drug-polymer thermal mixing

Pharmaceutical amorphous solid dispersions, a multicomponent system prepared by dispersing drug substances into polymeric matrix via thermal and mechanical processes, represent a major platform to deliver the poorly water-soluble drug. Microscopic properties of drug-polymer contacts play mechanistic roles in manipulating long-term physical stability as well as dissolution profiles. Although solid-state nuclear magnetic resonance has been utilized as an indispensable tool to probe structural details, previous studies are limited to ex situ characterizations. Our work provides likely the first documented example to investigate comelting of ketoconazole and polyacrylic acid, as a model system, in an in situ manner. Their physical mixture is melted and mixed in the solid-state nuclear magnetic resonance rotor under magic angle spinning at up to approximately 400 K. Critical structural events of molecular miscibility and interaction have been successfully identified. These results design and evaluate the instrumental and experimental protocols for real-time characterizations of the comelting of pharmaceutical materials.

Physical stability and release properties of lumefantrine amorphous solid dispersion granules prepared by a simple solvent evaporation approach

Amorphous solid dispersions (ASDs) of lumefantrine, which has low aqueous solubility, have been shown to improve bioavailability relative to crystalline formulations. Herein, the crystallization tendency and release properties of a variety of lumefantrine ASD granules, formed on a blend of microcrystalline cellulose and anhydrous lactose, prepared using a simple solvent evaporation method, were evaluated. Several polymers, a majority of which contained acidic moieties, and different drug loadings were assessed. Crystallinity as a function of time following exposure to stress storage conditions of 40 °C and 75% relative humidity was monitored for the various dispersions. Release testing was performed and ASD characteristics were further evaluated using infrared and X-ray photoelectron spectroscopy (XPS). A large difference in stability to crystallization was observed between the various ASDs, most notably depending on polymer chemistry. This could be largely rationalized based on the extent of drug-polymer interactions, specifically the degree of lumefantrine-polymer salt formation, which could be readily assessed with XPS spectroscopy. Lumefantrine release from the ASDs also varied considerably, whereby the best polymer for promoting physical stability did not lead to the highest extent of drug release. Several formulations led to concentrations above the amorphous solubility of lumefantrine, with the formation of nano-sized drug-rich aggregates. A balance between the ability of a given polymer to promote physical stability and drug release may need to be sought.

Molecular Dynamics and Physical Stability of Ibuprofen in Binary Mixtures with an Acetylated Derivative of Maltose

In this paper, we explore the strategy increasingly used to improve the bioavailability of poorly water-soluble crystalline drugs by formulating their amorphous solid dispersions. We focus on the potential application of a low molecular weight excipient octaacetyl-maltose (acMAL) to prepare physically stable amorphous solid dispersions with ibuprofen (IBU) aimed at enhancing water solubility of the drug compared to that of its crystalline counterpart. We thoroughly investigate global and local molecular dynamics, thermal properties, and physical stability of the IBU+acMAL binary systems by using broadband dielectric spectroscopy and differential scanning calorimetry as well as test their water solubility and dissolution rate. The obtained results are extensively discussed by analyzing several factors considered to affect the physical stability of amorphous systems, including those related to the global mobility, such as plasticization/antiplasticization effects, the activation energy, fragility parameter, and the number of dynamically correlated molecules as well as specific intermolecular interactions like hydrogen bonds, supporting the latter by density functional theory calculations. The observations made for the IBU+acMAL binary systems and drawn recommendations give a better insight into our understanding of molecular mechanisms governing the physical stability of amorphous solid dispersions.

Molecular Mechanism of Crystalline-to-Amorphous Conversion of Pharmaceutical Solids from 19F Magic Angle Spinning NMR

Crystalline and amorphous materials usually possess distinct physicochemical properties due to major variations in long-range and local molecular packings. Enhanced fundamental knowledge of the molecular details of crystalline-to-amorphous interconversions is necessary to correlate the intermolecular structure to material properties and functions. While crystal structures can be readily obtained by X-ray crystallography, the microstructure of amorphous materials has rarely been explored due to a lack of high-resolution techniques capable of probing local molecular structures. Moreover, there is increasing interest in understanding the molecular nature of amorphous solids in pharmaceutical sciences due to the widespread utilization of amorphous active pharmaceutical ingredients (APIs) in pharmaceutical development for solubility and bioavailability enhancement. In this study, we explore multidimensional ¹³C and ¹⁹F magic angle spinning (MAS) NMR spectroscopy to study the molecular packing of amorphous posaconazole (POSA) in conjunction with the crystalline counterpart. Utilizing methods integrating homonuclear and heteronuclear ¹H, ¹³C, and ¹⁹F correlation spectroscopy and atomic ¹⁹F-to-¹³C distance measurements, we identified the major differences in molecular packing between crystalline and amorphous POSA. The intermolecular "head-to-head" interaction along the molecule's major axis, as well as the "head-to-tail" molecular packing perpendicular to the major axis in POSA crystals, was recapitulated by MAS NMR. Furthermore, critical intermolecular distances in the crystal lattice were determined. Most importantly, the head-to-tail contact of two neighboring molecules was found to be preserved in amorphous POSA, suggesting localized molecular order, whereas crucial interactions for head-to-head packing are absent in the amorphous form resulting in long-range disorder. Our study, likely one of the first documented examples, provides molecular-level structural details to understand the molecular mechanism of crystalline-to-amorphous conversion of fluorine-containing drug substances occurring in drug processing and development and establish a high-resolution experimental protocol for investigating amorphous materials.

Probing the Molecular-Level Interactions in an Active Pharmaceutical Ingredient (API) - Polymer Dispersion and the Resulting Impact on Drug Product Formulation

PURPOSE

An investigation of underlying mechanisms of API-polymer interaction patterns has the potential to provide valuable insights for selecting appropriate formulations with superior physical stability and processability.

MATERIALS AND METHODS

In this study, copovidone was used as a polymeric carrier for several model compounds including clotrimazole, nifedipine, and posaconazole. The varied chemical structures conferred the ability for the model compounds to form distinct interactions with copovidone. Rheology and nuclear magnetic resonance (NMR) were combined to investigate the molecular pattern and relative strength of active pharmaceutical ingredient (API)-polymer interactions. In addition, the impact of the interactions on formulation processability via hot melt extrusion (HME) and physical stability were evaluated.

RESULTS

The rheological response of an API-polymer system was found to be highly sensitive to API-polymer interaction, depending both on API chemistry and API-polymer miscibility. In the systems studied, dispersed API induced a stronger plasticizer effect on the polymer matrix compared to crystalline/aggregated API. Correspondingly, the processing torque via HME showed a proportional relationship with the maximum complex viscosity of the API-polymer system. In order to quantitatively evaluate the relative strength of the API-polymer interaction, homogeneously dispersed API-polymer amorphous samples were prepared by HME at an elevated temperature. DSC, XRD, and rheology were employed to confirm the amorphous integrity and homogeneity of the resultant extrudates. Subsequently, the homogeneously dispersed API-polymer amorphous dispersions were interrogated by rheology and NMR to provide a qualitative and quantitative assessment of the nature of the API-polymer interaction, both macroscopically and microscopically. Rheological master curves of frequency sweeps of the extrudates exhibited a strong dependence on the API chemistry and revealed a rank ordering of the relative strength of API-copovidone interactions, in the order of posaconazole > nifedipine > clotrimazole. NMR data provided the means to precisely map the API-polymer interaction pattern and identify the specific sites of interaction from a molecular perspective. Finally, the impact of API-polymer interactions on the physical stability of the resultant extrudates was studied.

CONCLUSION

Qualitative and quantitative evaluation of the relative strength of the API-polymer interaction was successfully accomplished by utilizing combined rheology and NMR. Graphical Abstract.

Physicochemical Properties of Poly-Vinyl Polymers and Their Influence on Ketoprofen Amorphous Solid Dispersion Performance: A Polymer Selection Case Study

When developing an amorphous solid dispersion (ASD), a prudent choice of polymer is critical to several aspects of ASD performance including: processability, solid state stability and dissolution rate. However, there is little guidance available to formulators to aid judicious polymer selection and a “trial and error” approach is often taken. This study aims to facilitate rational polymer selection and formulation design by generating ASDs using a range of poly-vinyl polymers and ketoprofen as a model active pharmaceutical ingredient (API) and evaluating several aspects of their performance. The molecular weight of the polymer and the ratio of vinyl pyrrolidone to vinyl acetate in the polymer were found to influence the relative humidity at which the relative humidity induced glass transition occurred, as well as the extent of ketoprofen supersaturation achieved during dynamic solubility testing. Interestingly, ASD tablets containing polymers with the vinyl pyrrolidone functional group exhibited higher tensile strengths than those without. This points towards the binder functionality of vinyl pyrrolidone. In conclusion, the physicochemical properties of poly-vinyl polymers greatly influence ketoprofen ASD performance and due regard should be paid to these properties in order to develop an ASD with the desired attributes.

Mechanistic insights of the controlled release capacity of polar functional group in transdermal drug delivery system: the relationship of hydrogen bonding strength and controlled release capacity

BACKGROUND

Hydrogen bonding interaction was considered to play a critical role in controlling drug release from transdermal patch. However, the quantitative evaluation of hydrogen bonding strength between drug and polar functional group was rarely reported, and the relationship between hydrogen bonding strength and controlled release capacity of pressure sensitive adhesive (PSA) was not well understood. The present study shed light on this relationship.

METHODS

Acrylate PSAs with amide group were synthesized by a free radical-initiated solution polymerization. Six drugs, i.e., etodolac, ketoprofen, gemfibrozil, zolmitriptan, propranolol and lidocaine, were selected as model drugs. In vitro drug release and skin permeation experiments and in vivo pharmacokinetic experiment were performed. Partial correlation analysis, fourier-transform infrared spectroscopy and molecular simulation were conducted to provide molecular details of drug-PSA interactions. Mechanical test, rheology study, and modulated differential scanning calorimetry study were performed to scrutinize the free volume and molecular mobility of PSAs.

RESULTS

Release rate of all six drugs from amide PSAs decreased with the increase of amide group concentrations; however, only zolmitriptan and propranolol showed decreased skin permeation rate. It was found that drug release was controlled by amide group through hydrogen bonding, and controlled release extent was positively correlated with hydrogen bonding strength.

CONCLUSION

From these results, we concluded that drugs with strong hydrogen bond forming ability and high skin permeation were suitable to use amide PSAs to regulate their release rate from patch.

Hypromellose acetate succinate based amorphous solid dispersions via hot melt extrusion: Effect of drug physicochemical properties

In this study, the impact of drug and hydroxypropyl methylcellulose acetate succinate (HPMCAS) grades physicochemical properties on extrusion process, dissolution and stability of the hot melt extruded amorphous solid dispersions (ASDs) of nifedipine and efavirenz was investigated. Incorporation of drugs affected the extrusion temperature required for solid dispersion preparation. Differential scanning calorimetry and powder X-ray diffraction studies confirmed the amorphous conversion of the drugs in the prepared formulations. The amorphous nature of ASDs was unchanged after 3 months of stability testing at 40 °C and 75% relative humidity. The dissolution efficiency of the ASDs was dependent on the log P of the drug. The inhibitory effect of HPMCAS on drug precipitation was dependent on the hydrophobic interactions between drug and polymer, polymer grade, and dose of the drug. The dissolution efficiency and dissolution rate of the ASDs were dependent on the log P of the drug and solubility and hydrophilicity of the polymer grade respectively. The inhibitory effect of HPMCAS on drug precipitation was dependent on the hydrophobic interactions between drug and polymer, polymer grade, and the dissolution dose of the drug.

Does the molecular mobility and flexibility of the saccharide ring affect the glass-forming ability of naproxen in binary mixtures?

In this paper, we studied the impact of saccharides having a similar backbone but differing in the degree of freedom, local molecular mobility, flexibility of the ring and intermolecular interactions on the glass-forming ability (GFA) of naproxen (NAP) in binary mixtures. For this purpose, a series of methyl and acetyl derivatives of glucose (GLS) and anhydroglucose (anhGLS), as well as neat anhGLS have been used to produce homogeneous solid dispersions (SDs) of varying molar concentration of examined active pharmaceutical ingredient (API). Systematic measurements with the use of Differential Scanning Calorimetry (DSC) and Broadband Dielectric Spectroscopy (BDS) enabled us to determine the phase transitions, homogeneity and molecular mobility of the investigated binary mixtures as well as the impact of excipient on the crystallization tendency of NAP. It turned out that acetylated glucose (acGLS), one of the most mobile and flexible saccharides of all examined herein materials, is the best excipient enhancing the GFA of studied API. Although, it should be noted that upon storage at room temperature, we observed the recrystallization of NAP from binary mixtures. Interestingly, API always crystallized to the initial polymorphic form, as shown by X-ray diffraction (XRD) investigations. Finally, since additional measurements with the use of Fourier Transform Infrared (FTIR) Spectroscopy clearly indicated that there are no significant differences in the intermolecular interactions in the systems composed of NAP and all examined saccharides, one can postulate that the mobility and ring flexibility of the matrix have, , the most important impact on the crystallization tendency of NAP upon cooling. Consequently, it seems that in some cases, more mobile/flexible matrices can be a much better choice to enhance the glass-forming ability of studied pharmaceutical.

Dicarboxylic acid as a linker to improve the content of amorphous drug in drug-in-polymer film: Effects of molecular mobility, electrical conductivity and intermolecular interactions

Amorphous solid dispersion (ASD) is a well-established approach to improve the dissolution rate of the drugs with low water solubility. However, the application of the ASD was hindered by the low drug content and high risk of re-crystallization of drugs. The purpose of this research was to develop an ASD film with high content of amorphous olanzapine (OLN) for oral delivery. To overcome the high crystallization tendency of OLN in polyvinyl alcohol (PVA) films, three dicarboxylic acids (succinic acid (Suc), fumaric acid (Fum) and malic acid (Mal)) were introduced in the drug-in-polymer system as linkers between the drug and the polymer. The influence of the linkers on the re-crystallization of OLN in PVA films was evaluated by polarized light microscopy (PLM) and x-ray diffraction (XRD). Then, the possible mechanisms of crystallization inhibition were discussed based on the results of dielectric spectroscopy (DES), differential scanning calorimetry (DSC), attenuated total reflectance Fourier transform infrared (ATR-FTIR), Raman spectroscopy and molecular modeling. Finally, the effect of the linkers on the in vitro dissolution of the OLN-in-PVA films was studied in simulant saliva, and the in vivo performance of the optimal formulation was evaluated in rats. The results showed that OLN-in-PVA film have lower molecular mobility, lower electrical conductivity and stronger intermolecular interactions with the existence of Mal, which led to a better crystallization inhibition of OLN in PVA films. The re-crystallization of OLN in PVA films decreased the dissolution rate of OLN in simulant saliva. The in vivo performance of the optimal formulation was similar with that of OLN solution in rats. This study introduced a novel strategy to reduce the risk of drug re-crystallization in ASD, and also provided a deeper insight into the mechanisms of crystallization inhibition in ASD. The results will improve the judicious selection of excipients in pharmaceutical formulations.

Amorphous solid dispersion formation via solvent granulation - A case study with ritonavir and lopinavir

Herein, we evaluate the potential of using a simple solvent granulation process to prepare a binary drug amorphous solid dispersion (ASD) containing two anti-HIV drugs, ritonavir and lopinavir. The drugs were granulated onto a mixture of lactose and microcrystalline cellulose, followed by drying to remove the solvent. The resultant granules were characterized and each drug was found to be X-ray amorphous. No crystallization was observed following storage for 1 month under accelerated stability conditions (40 °C and 75% relative humidity). The dissolution behavior of the compacted granules was compared with the marketed formulation. The dissolution rate of ritonavir was found to be significantly retarded relative to the commercial product when the two drugs were co-granulated. However, comparable release could be achieved when each drug was individually granulated, followed by combination and compaction. The solvent granulation approach may be a viable method to make ASDs of low dose drugs with low crystallization tendencies.

Mechanistic Insights of the Critical Role of Hydrogen Donor in Controlling Drug Release From Acrylate Adhesive

In the present study, a pyrrolidone adhesive and an amide adhesive were synthesized, and their molecular mechanisms of controlled drug release were described. Using zolmitriptan as model drug, in vitro drug release and skin permeation experiments were performed. Adhesive properties were evaluated using modulated differential scanning calorimetry and rheology study. Free volume of polymer was directly obtained by positron annihilation lifetime spectroscopy. Intermolecular interactions between drugs and adhesives were determined by FTIR spectroscopic analysis and molecular simulation. Release percent (24 h) of zolmitriptan from pyrrolidone adhesive was about 55.8 ± 3.1% (w/w), while from amide adhesive, the release percent (24 h) was about 40.1 ± 1.6% (w/w). The free volume sizes of pyrrolidone adhesive and amide adhesive were about 2309.6 ų and 2854.5 ų, respectively, which were much larger than molecular volume of zolmitriptan (about 285.7 ų). Thus, the polymer networks might not hinder drug diffusion from the view of free volume. Comparing chemical structures of pyrrolidone group and primary amide group, the main difference was that primary amide group of amide adhesive possessed 2 hydrogen donors. It was proved that hydrogen bonding between zolmitriptan and hydrogen donor of primary amide group played a critical role in controlling drug release.

Preparation of Amorphous Composite Particles of Drugs with Ursodeoxycholic Acid as Preclinical Formulations

We studied the possibility of using ursodeoxycholic acid (UDCA) as an excipient to create an amorphous composite that can be administered to animals in preclinical studies of experimental drugs. Three UDCA-based amorphous samples composed of nifedipine (NIF), indomethacin (IND), and naproxen (NAP) were found by screening. The UDCA-based formulations were adjudged amorphous by solid-state analysis using X-ray powder diffraction and differential scanning calorimetry. In addition, amorphous samples of NIF-UDCA, IND-UDCA, and NAP-UDCA did not crystallize while in 1% methyl cellulose (MC) solution for 120 min, although an amorphous solid dispersion of NIF-poly(vinylpyrrolidone) (PVP) crystallized rapidly. The low hygroscopicity of UDCA helps NIF maintain an amorphous state in 1% MC solution. The UDCA-based amorphous composites can be administered as suspended formulations to animals in preclinical studies.

Like Dissolves Like? A Comprehensive Evaluation of Partial Solubility Parameters to Predict Polymer-Drug Compatibility in Ultrahigh Drug-Loaded Polymer Micelles

Despite decades of research, our understanding of the molecular interactions between drugs and polymers in drug-loaded polymer micelles does not extend much beyond concepts such as "like-dissolves-like" or hydrophilic/hydrophobic. However, polymer-drug compatibility strongly affects formulation properties and therefore the translation of a formulation into the clinics. Specific interactions such as hydrogen-bonding, π-π stacking, or coordination interactions can be utilized to increase drug loading. This is commonly based on trial and error and eventually leads to an optimized drug carrier. Unfortunately, due to the unique characteristics of each drug, the deduction of advanced general concepts remains challenging. Furthermore, the introduction of complex moieties or specifically modified polymers hampers systematic investigations regarding polymer-drug compatibility as well as clinical translation. In this study, we reduced the complexity to isolate the crucial factors determining drug loading. Therefore, the compatibility of 18 different amphiphilic polymers for five different hydrophobic drugs was determined empirically. Subsequently, the obtained specificities were compared to theoretical compatibilities derived from either the Flory-Huggins interaction parameters or the Hansen solubility parameters. In general, the Flory-Huggins interaction parameters were less suited to correctly estimate the experimental drug solubilization compared to the Hansen solubility parameters. The latter were able to correctly predict some trend regarding good and poor solubilizers, yet the overall predictive strength of Hansen solubility parameters is clearly unsatisfactory.

Micro-scale solubility assessments and prediction models for active pharmaceutical ingredients in polymeric matrices

The number of models for assessing the solubility of active pharmaceutical ingredients (APIs) in polymeric matrices on the one hand and the extent of available associated data on the other hand has been rising steadily in the past few years. However, according to our knowledge an overview on the methods used for prediction and the respective experimental data is missing. Therefore, we compiled experimental data, the techniques used for their determination and the models used for estimating the solubility. Our focus was on polymers commonly used in spray drying and hot-melt extrusion to form amorphous solid dispersions (ASDs), namely polyvinylpyrrolidone grades (PVP), polyvinyl acetate (PVAc), vinylpyrrolidone-vinyl acetate copolymer (copovidone, COP), polyvinyl caprolactam-polyvinyl acetate-polyethylene glycol graft polymer (Soluplus®, SOL), different types of methacrylate copolymers (PMMA), polyethylene glycol grades (PEG) and hydroxypropyl-methylcellulose grades (HPMC). The literature data were further supplemented by our own results. The final data set included 37 APIs and two sugar derivatives. The majority of the prediction models was constituted by the melting point depression method, dissolution endpoint measurements, indirect solubility determination by T_g and the use of low molecular weight analogues. We observed that the API solubility depended more on the working group which conducted the experiments than on the measuring technique used. Furthermore, this compilation should assist researchers in choosing a prediction method suited for their investigations. Furthermore, a statistical assessment using recursive feature elimination was performed to identify descriptors of molecules, which are connected to the API solubility in polymeric matrices. It is capable of predicting the criterium 20% API soluble at 100 °C (Yes/No) for an unknown compound with a balanced accuracy of 71%. The identified 8 descriptors to be connected to API solubility in polymeric matrices were the number of hydrogen bonding donors, three descriptors related to the hydrophobicity of the molecule, glass transition temperature, fractional negative polar van der Waals surface area, out-of-plane potential energy and the fraction of rotatable bonds. Finally, in addition to our own model, the data set should help researchers in training their own solubility prediction models.

Effect of Polymer Hydrophobicity on the Stability of Amorphous Solid Dispersions and Supersaturated Solutions of a Hydrophobic Pharmaceutical

Amorphous solid dispersions of pharmaceuticals often show improved solubility over crystalline forms. However, the crystallization of amorphous solid dispersions during storage, or from elevated supersaturation once dissolved, compromise the solubility advantage of delivery in the amorphous phase. To combat this phenomenon, polymer additives are often included in solid dispersions to inhibit crystallization; however, the optimal properties for polymer to stabilize against crystallization are not fully understood, and furthermore, it is not known how inhibition of precipitation from solution is related to the propensity of a polymer to inhibit crystallization from the amorphous phase. Here, polymers of varied hydrophobicity are employed as crystallization inhibitors in supersaturated solutions and amorphous solid dispersions of the BCS Class II pharmaceutical ethenzamide to investigate the chemical features of polymer that lead to long-term stability for a hydrophobic pharmaceutical. A postpolymerization functionalization strategy was employed to alter the hydrophobicity of poly( N-hydroxyethyl acrylamide) without changing physical properties such as number-average chain length. It was found that supersaturation maintenance for ethenzamide is improved by increasing the hydrophobicity of dissolved polymer in aqueous solution. Furthermore, amorphous solid dispersions of ethenzamide containing a more hydrophobic polymer showed superior stability compared to those containing a less hydrophobic polymer. This trend of increasing polymer hydrophobicity leading to improved amorphous stability is interpreted by parsing the effects of water absorption in amorphous solid dispersions using intermolecular interaction strengths derived from global structural analysis. By comparing the structure-function relationships, which dictate stability in solution and amorphous solid dispersions, the effect of hydrophobicity can be broadly understood for the design of polymers to impart stability throughout the application of amorphous solid dispersions.

A supramolecular synthon approach to design amorphous solid dispersions with exceptional physical stability

A supramolecular synthon approach was exploited to design amorphous solid dispersions (ASDs) of drugs containing an amino aromatic nitrogen moiety and a polyacrylic acid polymer.

Curcumin-loaded self-nanomicellizing solid dispersion system: part I: development, optimization, characterization, and oral bioavailability

Curcumin (CUR) is considered as one of the most bioactive molecules ever discovered from nature due to its proven anti-inflammatory and antioxidant in both preclinical and clinical studies. Despite its proven safety and efficacy, the clinical translation of CUR into a useful therapeutic agent is still limited due to its poor oral bioavailability. To overcome its limitation and enhance oral bioavailability by improving its aqueous solubility, stability, and intestinal permeability, a novel CUR formulation (NCF) was developed using the self-nanomicellizing solid dispersion strategy. From the initial screening of polymers for their potential to improve the solubility and stability, Soluplus (SOL) was selected. The optimized NCF demonstrated over 20,000-fold improvement in aqueous solubility as a result of amorphization, hydrogen bonding interaction, and micellization determined using differential scanning calorimetry, X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, nuclear magnetic resonance, dynamic light scattering, and transmission electron microscopy. Moreover, the greater stabilizing effect in alkaline pH and light was observed. Furthermore, significant enhancement of dissolution and permeability of CUR across everted sacs of rat small intestine were noticed. Pharmacokinetic studies demonstrated that the oral bioavailability of CUR was increased 117 and 17-fold in case of NCF and physical mixture of CUR and SOL compared to CUR suspension. These results suggest NCF identified as a promising new approach for repositioning of CUR for pharmaceutical application by enhancing the oral bioavailability of CUR. The findings herein stimulate further in vivo evaluations and clinical tests of NCF.