Effect of Drug–Polymer Interactions through Hypromellose Acetate Succinate Substituents on the Physical Stability on Solid Dispersions Studied by Fourier-Transform Infrared and Solid-State Nuclear Magnetic Resonance

Comparative analysis of drug-salt-polymer interactions by experiment and molecular simulation improves biopharmaceutical performance

The propensity of poorly water-soluble drugs to aggregate at supersaturation impedes their bioavailability. Supersaturated amorphous drug-salt-polymer systems provide an emergent approach to this problem. However, the effects of polymers on drug-drug interactions in aqueous phase are largely unexplored and it is unclear how to choose an optimal salt-polymer combination for a particular drug. Here, we describe a comparative experimental and computational characterization of amorphous solid dispersions containing the drug celecoxib, and a polymer, polyvinylpyrrolidone vinyl acetate (PVP-VA) or hydroxypropyl methylcellulose acetate succinate, with or without Na+/K+ salts. Classical models for drug-polymer interactions fail to identify the best drug-salt-polymer combination. In contrast, more stable drug-polymer interaction energies computed from molecular dynamics simulations correlate with prolonged stability of supersaturated amorphous drug-salt-polymer systems, along with better dissolution and pharmacokinetic profiles. The celecoxib-salt-PVP-VA formulations exhibit excellent biopharmaceutical performance, offering the prospect of a low-dosage regimen for this widely used anti-inflammatory, thereby increasing cost-effectiveness, and reducing side-effects.

Advances in the development of amorphous solid dispersions: The role of polymeric carriers

Amorphous solid dispersion (ASD) is one of the most effective approaches for delivering poorly soluble drugs. In ASDs, polymeric materials serve as the carriers in which the drugs are dispersed at the molecular level. To prepare the solid dispersions, there are many polymers with various physicochemical and thermochemical characteristics available for use in ASD formulations. Polymer selection is of great importance because it influences the stability, solubility and dissolution rates, manufacturing process, and bioavailability of the ASD. This review article provides a comprehensive overview of ASDs from the perspectives of physicochemical characteristics of polymers, formulation designs and preparation methods. Furthermore, considerations of safety and regulatory requirements along with the studies recommended for characterizing and evaluating polymeric carriers are briefly discussed.

Formation of a Stable Co-Amorphous System for a Brick Dust Molecule by Utilizing Sodium Taurocholate with High Glass Transition Temperature

Brick dust molecules are usually poorly soluble in water and lipoidal components, making it difficult to formulate them in dosage forms that provide efficient pharmacological effects. A co-amorphous system is an effective strategy to resolve these issues. However, their glass transition temperatures (Tg) are relatively lower than those of polymeric amorphous solid dispersions, suggesting the instability of the co-amorphous system. This study aimed to formulate a stable co-amorphous system for brick dust molecules by utilizing sodium taurocholate (NaTC) with a higher Tg. A novel neuropeptide Y5 receptor antagonist (AntiY5R) and NaTC with Tg of 155 °C were used as the brick dust model and coformer, respectively. Ball milling formed a co-amorphous system for AntiY5R and NaTC (AntiY5R-NaTC) at various molar ratios. Deviation from the theoretical Tg value and peak shifts in Fourier-transform infrared spectroscopy indicated intermolecular interactions between AntiY5R and NaTC. AntiY5R-NaTC at equal molar ratios resulting in an 8.5-fold increase in AntiY5R solubility over its crystalline form. The co-amorphous system remained amorphous for 1 month at 25 °C and 40 °C. These results suggest that the co-amorphous system formed by utilizing NaTC as a coformer could stably maintain the amorphous state and enhance the solubility of brick dust molecules.

Physical aging of hydroxypropyl methylcellulose acetate succinate via enthalpy recovery

Amorphous solid dispersions (ASDs) utilize the kinetic stability of the amorphous state to stabilize drug molecules within a glassy polymer matrix. Therefore, understanding the glassy-state stability of the polymer excipient is critical to ASD design and performance. Here, we investigated the physical aging of hydroxypropyl methylcellulose acetate succinate (HPMCAS), a commonly used polymer in ASD formulations. We found that HPMCAS exhibited conventional physical aging behavior when annealed near the glass transition temperature (T_g). In this scenario, structural recovery was facilitated by α-relaxation dynamics. However, when annealed well below T_g, a sub-α-relaxation process facilitated low-temperature physical aging in HPMCAS. Nevertheless, the physical aging rate exhibited no significant change up to 40 K below T_g, below which it exhibited a near monotonic decrease with decreasing temperature. Finally, infrared spectroscopy was employed to assess any effect of physical aging on the chemical structure of HPMCAS, which is known to be susceptible to degradation at temperatures 30 K above its T_g. Our results provide critical insights necessary to understand better the link between the stability of ASDs and physical aging of the glassy polymer matrix.

New Development in Understanding Drug-Polymer Interactions in Pharmaceutical Amorphous Solid Dispersions from Solid-State Nuclear Magnetic Resonance

Pharmaceutical amorphous solid dispersions (ASDs) represent a widely used technology to increase the bioavailability of active pharmaceutical ingredients (APIs). ASDs are based on an amorphous API dispersed in a polymer, and their stability is driven by the presence of strong intermolecular interactions between these two species (e.g., hydrogen bond, electrostatic interactions, etc.). The understanding of these interactions at the atomic level is therefore crucial, and solid-state nuclear magnetic resonance (NMR) has demonstrated itself as a very powerful technique for probing API-polymer interactions. Other reviews have also reported exciting approaches to study the structures and dynamic properties of ASDs and largely focused on the study of API-polymer miscibility and on the identification of API-polymer interactions. Considering the increased use of NMR in the field, the aim of this Review is to specifically highlight recent experimental strategies used to identify API-polymer interactions and report promising recent examples using one-dimensional (1D) and two-dimensional (2D) experiments by exploiting the following emerging approaches of very-high magnetic field and ultrafast magic angle spinning (MAS). A range of different ASDs spanning APIs and polymers with varied structural motifs is targeted to illustrate new ways to understand the mechanism of stability of ASDs to enable the design of new dispersions.

Hydroxypropyl methylcellulose acetate succinate as an exceptional polymer for amorphous solid dispersion formulations: A review from bench to clinic

Amorphous solid dispersions (ASDs) are a proven system for achieving a supersaturated state of drug, in which the concentration of drug is greater than its crystalline solubility. The usage of Hydroxypropyl Methylcellulose Acetate Succinate (HPMCAS) in the development of ASDs has grown significantly, as evidenced by the fact that majority of commercially approved ASD formulations are based on HPMCAS. HPMCAS has been widely utilized as a solubility enhancer and precipitation inhibitor or stabilizer to achieve supersaturation and inhibit crystallization of drugs in the gastrointestinal tract. The characteristics of HPMCAS ASDs such as less hygroscopic, strong drug-polymer hydrophobic interactions, high solubilization efficiency, greater potential to generate, maintain drug supersaturation and crystallization inhibition outperform other polymeric carriers in ASD development. Furthermore, combining HPMCAS with other polymers or surfactants as ternary ASDs could be a viable approach for enhancing oral absorption of poorly soluble drugs. This review discusses the concepts of supersaturation maintenance or precipitation inhibition of HPMCAS in the ASD formulations. In addition, the mechanisms underlying for improved dissolution performance, oral bioavailability and stability of HPMCAS ASDs are explored.

Revealing the roles of polymers in supersaturation stabilization from the perspective of crystallization behaviors: A case of nimodipine

Formulating drugs into amorphous solid dispersions (ASDs) represents an attractive means to enhance the aqueous solubility of drugs. Furthermore, water-soluble polymers have proven highly advantageous for stabilizing supersaturated solutions of ASDs. However, the performance and mechanism of various polymers in stabilizing supersaturated drug solutions have not been well-studied. The aim of this study was to investigate the effects of different commercial polymers on the dissolution behaviors and supersaturation stabilization of the ASDs and to further explore the mechanism of polymer mediated supersaturation maintenance by studying the crystallization behaviors of the ASDs. In this study, nimodipine (NMD) was used as a model drug because of its poor water-solubility and fast crystallization rate in aqueous solution, and three polymers polyvinylpyrrolidone (PVP), vinylpyrrolidone-vinyl acetate copolymer (PVP VA), and polyvinyl caprolactam-polyvinyl acetate-polyethylene glycol graft co-polymer (Soluplus) was selected as the drug carriers to form the ASDs with NMD. Solid-state characterizations of the ASDs confirmed the amorphous state of the ASD systems. ASD_{PVP VA} demonstrated superior supersaturation maintenance in dissolution experiments compared to the other two ASD systems. Among the polymers tested, PVP VA most efficiently maintained dissolution of NMD and prevented its crystallization from the supersaturated solution. The ability of PVP VA to most-effectively maintain supersaturation of the drug was manifested by inhibition of crystal nucleation rather than inhibition of crystal growth following nucleation. These results suggest that nucleation inhibition was instrumental in enabling the polymer-mediated supersaturation maintenance, at least with NMD.

Hot-melt extruded hydroxypropyl methylcellulose acetate succinate based amorphous solid dispersions: Impact of polymeric combinations on supersaturation kinetics and dissolution performance

Nucleation inhibition and maintenance of drug supersaturation over a prolonged period are desirable for improving oral absorption of amorphous solid dispersions. The present study investigates the impact of binary and ternary amorphous solid dispersions on the supersaturation kinetics of nifedipine using the polymers hydroxypropylmethylcellulose acetate succinate (HPMCAS) LG, and HG, Eudragit® RSPO, Eudragit® FS100, Kollidon® VA64 and Plasdone™ K-29/32. The amorphous solubility, nucleation induction time, and particle size analysis of nifedipine in a supersaturated solution were performed with and without the presence of polymers, alone or in combination. The HPMCAS-HG and HPMCAS-HG + LG combinations showed the highest nifedipine amorphous solubility of 169.47, 149.151 µg/mL, respectively and delay in nucleation induction time up to 120 min compared to other polymeric combinations. The solid dispersions prepared via hot melt extrusion showed the transformation of crystalline nifedipine to amorphous form. The in-vitro non-sink dissolution study revealed that although the binary nifedipine/HPMCAS-LG system had shown the greater supersaturation concentration of 66.1 µg/mL but could not maintain a supersaturation level up to 360 min. A synergistic effect emerged for ternary nifedipine/HPMCAS-LG/HPMCAS-HG, and nifedipine/HPMCAS-LG/Eudragit®FS100 systems maintained the supersaturation level with enhanced dissolution performance, demonstrating the potential of polymeric combinations for improved amorphous solid dispersion performance.

Computational approach to elucidate the formation and stabilization mechanism of amorphous formulation using molecular dynamics simulation and fragment molecular orbital calculation

α-Glycosyl rutin (Rutin-G) consists of a flavonol skeleton and sugar groups and is a promising additive for amorphous formulations. In our previous study, experimental approaches suggested an interaction between the model drug carbamazepine (CBZ) and flavonol skeleton of Rutin-G that stabilizes amorphous formulations. In the present study, the formation and stabilization mechanisms of CBZ/Rutin-G amorphous formulation were investigated using a computational approach. The CBZ/Rutin-G amorphous formulation was obtained via molecular dynamics (MD) simulation, which mimicked the melt-quenching method. Root mean square deviation analysis revealed that the translational motion of CBZ during the cooling process was suppressed by adding Rutin-G. Monitoring the atomic distance during the cooling process revealed that hydrogen bonds via carboxamide oxygen of CBZ with hydroxyl hydrogen of Rutin-G were preferentially formed with flavonol skeletons than sugar groups. The simulated amorphous formulation was then calculated using fragment molecular orbital (FMO) method. The quantitative evaluation of multiple interactions revealed that the hydrogen bond energy was higher in CBZ-sugar groups than in CBZ-flavonol skeleton, while the π-type of interaction energy was higher in CBZ-flavonol skeleton than in CBZ-sugar groups. The computational approach combining MD simulation and FMO calculation provides information on various interactions that are difficult to detect using experimental approaches, which helps understand the formation and stabilization mechanism of amorphous formulations.

Application of Multivariate Methods to Evaluate Differential Material Attributes of HPMC from Different Sources

The aim of the present study is to achieve differential material attributes (DMAs) of hydroxypropyl methylcellulose (HPMC) with different viscosity grades (K4M, K15M, and K100M) from different manufacturers (Anhui Shanhe and Dow Chemical). Two kinds of multivariate methods, principal component analysis (PCA) and orthogonal partial least squares discriminant analysis (OPLS-DA), were adopted. The physicochemical properties of HPMC were systematically investigated via various techniques (e.g., SEM, particle size detection, and SeDeM characterization). Data from 33 characterization variables were applied to the multivariate methods. The PCA and OPLS-DA results indicated the differences between the HPMC from two manufacturers by the common variables that include the tablet hardness (HD), tensile strength (TS), bulk density, interparticle porosity, Carr index, cohesion index, Hausner ratio, flowability, and the width of the particle size distribution (span). Interestingly, these variables showed a certain correlation with each other, supporting the characterization results. Except for these different variables of the HPMC obtained by multivariate analysis results, distinguishable shapes and surface morphologies also appeared between different sources. To sum up, the powder properties (particle size, surface topography, dimension, flowability, and compressibility) and the tablet properties (HD and TS) were recognized as the DMAs of HPMC samples. This work provided the multivariate methods for the physicochemical characterization of HPMC, with potential in the quality control and formulation development.

Variable-Temperature NMR Analysis of the Thermodynamics of Polymer Partitioning between Aqueous and Drug-Rich Phases and Its Significance for Amorphous Formulations

We previously reported that the polymers used in amorphous solid dispersion (ASD) formulations, such as polyvinylpyrrolidone (PVP), polyvinylpyrrolidone/vinyl acetate (PVP-VA), and hypromellose (HPMC), distribute into the drug-rich phase of ibuprofen (IBP) formed by liquid-liquid phase separation, resulting in a reduction in the maximum drug supersaturation in the aqueous phase. Herein, the mechanism underlying the partitioning of the polymer into the drug-rich phase was investigated from a thermodynamic perspective. The dissolved IBP concentration in the aqueous phase and the amount of polymer distributed into the IBP-rich phase were quantitatively analyzed in IBP-supersaturated solutions containing different polymers using variable-temperature solution-state nuclear magnetic resonance (NMR) spectroscopy. The polymer weight ratio in the IBP-rich phase increased at higher temperatures, leading to a more notable reduction of IBP amorphous solubility. Among the polymers, the amorphous solubility reduction was the greatest for the PVP-VA solution at lower temperatures, while HPMC reduced the amorphous solubility to the greatest extent at higher temperatures. The change in the order of polymer impact on the amorphous solubility resulted from the differences in the temperature dependency of polymer partitioning. The van't Hoff plot of the polymer partition coefficient revealed that both enthalpy and entropy changes for polymer transfer into the IBP-rich phase from the aqueous phase (ΔH_{aqueous→IBP-rich} and ΔS_{aqueous→IBP-rich}) gave positive values for most of the measured temperature range, indicating that polymer partitioning into the IBP-rich phase was an endothermic but entropically favorable process. The polymer transfer into the IBP-rich phase was more endothermic for HPMC than for PVP and PVP-VA. The solid-state NMR analysis of the IBP/polymer ASD implied that the newly formed IBP/polymer interactions in the IBP-rich phase upon polymer incorporation were weaker for HPMC, providing a rationale for the larger positive transfer enthalpy for HPMC. The change in Gibbs free energy for polymer transfer (ΔG_{aqueous→IBP-rich}) showed negative values across the experimental temperature range, decreasing with an increase in temperature, indicating that the distribution of the polymer into the IBP-rich phase is favored at higher temperatures. Moreover, ΔG_{aqueous→IBP-rich} for HPMC showed the greatest decrease with the temperature, likely reflecting the temperature-induced dehydration of HPMC in the aqueous phase. This study contributes fundamental insights into the phenomenon of polymer partitioning into drug-rich phases, furthering the understanding of achievable supersaturation levels and ultimately providing information on polymer selection for ASD formulations.

Synthesis and release behavior of layered double hydroxides-carbamazepine composites

Carbamazepine (CBZ) was incorporated into layered double hydroxides (LDH) to be used as a controlled drug system in solid tumors. CBZ has a formal charge of zero, so its incorporation in the anionic clay implies a challenge. Aiming to overcome this problem, CBZ was loaded into LDH with sodium cholate (SC), a surfactant with negative charge and, for comparison, without SC by the reconstruction method. Surprisingly, it was found that both resultant nanocomposites had similar CBZ encapsulation efficiency, around 75%, and the LDH-CBZ system without SC showed a better performance in relation to the release kinetics of CBZ in simulated body fluid (pH 7.4) and acetate buffer simulating the cellular cytoplasm (pH 4.8) than the system with SC. The CBZ dimensions were measured with Chem3D and, according to the basal spacing obtained from X-ray patterns, it can be arranged in the LDH-CBZ system as a monolayer with the long axis parallel to the LDH layers. Fourier transform infrared spectroscopy and solid state NMR measurements confirmed the presence of the drug, and thermogravimetric analyses showed an enhanced thermal stability for CBZ. These results have interesting implications since they increase the spectrum of LDH application as a controlled drug system to a large number of nonionic drugs, without the addition of other components.

Drug-Polymer Interactions in Acetaminophen/Hydroxypropylmethylcellulose Acetyl Succinate Amorphous Solid Dispersions Revealed by Multidimensional Multinuclear Solid-State NMR Spectroscopy

The bioavailability of insoluble crystalline active pharmaceutical ingredients (APIs) can be enhanced by formulation as amorphous solid dispersions (ASDs). One of the key factors of ASD stabilization is the formation of drug-polymer interactions at the molecular level. Here, we used a range of multidimensional and multinuclear nuclear magnetic resonance (NMR) experiments to identify these interactions in amorphous acetaminophen (paracetamol)/hydroxypropylmethylcellulose acetyl succinate (HPMC-AS) ASDs at various drug loadings. At low drug loading (<20 wt %), we showed that 1H-13C through-space heteronuclear correlation experiments identify proximity between aromatic protons in acetaminophen with cellulose backbone protons in HPMC-AS. We also show that 14N-1H heteronuclear multiple quantum coherence (HMQC) experiments are a powerful approach in probing spatial interactions in amorphous materials and establish the presence of hydrogen bonds (H-bond) between the amide nitrogen of acetaminophen with the cellulose ring methyl protons in these ASDs. In contrast, at higher drug loading (40 wt %), no acetaminophen/HPMC-AS spatial proximity was identified and domains of recrystallization of amorphous acetaminophen into its crystalline form I, the most thermodynamically stable polymorph, and form II are identified. These results provide atomic scale understanding of the interactions in the acetaminophen/HPMC-AS ASD occurring via H-bond interactions.

Effect of Chemical Penetration Enhancer-Adhesive Interaction on Drug Release from Transdermal Patch: Mechanism Study Based on FT-IR Spectroscopy, 13C NMR Spectroscopy, and Molecular Simulation

Chemical penetration enhancers (CPEs) are commonly added into transdermal patches to impart improved skin permeation of drug. However, significant unexplained variability in drug release kinetics in transdermal patches is possible as a result of the addition of CPEs; investigations into the underlying mechanisms are still limited. In the present study, a diverse set of CPEs was employed to draw broad conclusions. Solubility parameters of CPEs and acrylate pressure-sensitive adhesive were calculated by molecular dynamics simulation and Fedors group contribution method to evaluate drug-adhesive miscibility. CPE-adhesive interaction was characterized by FT-IR study, ¹³C NMR spectroscopy, and molecular docking simulation. Results showed that release enhancement ratio (ER_R) of CPEs for zolmitriptan was rank ordered as isopropyl myristate > azone > Plurol Oleique^® CC497 > Span^® 80 > N-methylpyrrolidone > Transcutol^® P. It was found that solubility parameter difference (Δδ) between CPE and adhesive was negatively related with ER_R. It was proved that hydrogen bonding between CPE and adhesive would increase drug release rate, but only if the CPE showed good miscibility with adhesive. CPE like isopropyl myristate, which had good miscibility with adhesive, could decrease drug-adhesive interaction leading to the release of drug from adhesive.

Stabilization mechanism of amorphous carbamazepine by transglycosylated rutin, a non-polymeric amorphous additive with a high glass transition temperature

α-Glycosyl rutin (Rutin-G), composed of a flavonol skeleton and sugar groups, is a promising non-polymeric additive for stabilizing amorphous drug formulations. In this study, the mechanism of the stabilization of the amorphous state of carbamazepine (CBZ) by Rutin-G was investigated. In comparison with hypromellose (HPMC), which is commonly used as a crystallization inhibitor for amorphous drugs, Rutin-G significantly stabilized amorphous CBZ. Moreover, the dissolution rate and the resultant supersaturation level of CBZ were significantly improved in the CBZ/Rutin-G spray-dried samples (SPDs) owing to the rapid dissolution property of Rutin-G. Differential scanning calorimetry measurement demonstrated a high glass transition temperature (T_g) of 186.4°C corresponding to Rutin-G. The CBZ/Rutin-G SPDs with CBZ weight ratios up to 80% showed single glass transitions, indicating the homogeneity of CBZ and Rutin-G. A solid-state NMR experiment using ¹³C- and ¹⁵N-labeled CBZ demonstrated the interaction between the flavonol skeleton of Rutin-G and the amide group of CBZ. A ¹H-¹³C two-dimensional heteronuclear correlation NMR experiment and quantum mechanical calculations confirmed the presence of a possible hydrogen bond between the amino proton in CBZ and the carbonyl oxygen in the flavonol skeleton of Rutin-G. This specific hydrogen bond could contribute to the strong interaction between CBZ and Rutin-G, resulting in the high stability of amorphous CBZ in the CBZ/Rutin-G SPD. Hence, Rutin-G, a non-polymeric amorphous additive with high T_g, high miscibility with drugs, and rapid and pH-independent dissolution properties could be useful in the preparation of amorphous formulations.

Applicability of an Experimental Grade of Hydroxypropyl Methylcellulose Acetate Succinate as a Carrier for Formation of Solid Dispersion with Indomethacin

The transformation of a crystalline drug into an amorphous form is a promising way to enhance the oral bioavailability of poorly water-soluble drugs. Blending of a carrier, such as a hydrophilic polymer, with an amorphous drug is a widely used method to produce a solid dispersion and inhibit crystallization. This study investigates an experimental grade of hydroxypropyl methylcellulose acetate succinate, HPMCAS-MX (MX), as a solid dispersion carrier. Enhancement of thermal stability and reduction of the glass transition temperature (Tg) of MX compared with those of the conventional grade were evaluated through thermogravimetric analysis and differential scanning calorimetry (DSC). The formation of a homogeneous amorphous solid dispersion between MX and indomethacin was confirmed by X-ray powder diffraction analysis, DSC, and Raman mapping. It was observed that 10–30% MX did not act as an anti-plasticizer, but the utilization of >40% MX caused an increase in Tg and reduction of molecular mobility. This could be explained by a change in intermolecular interactions, inferred from infrared spectroscopy combined with principal component analysis. HPMCAS-MX exhibited similar performance to that of conventional-grade, HPMCAS-MG. Although HPMCAS-MX has thermal properties different from those of conventional-grade HPMCAS-MG, it retains its ability as a solid dispersion carrier.

Characterizing Drug-Polymer Interactions in Aqueous Solution with Analytical Ultracentrifugation

We present a new approach for characterizing drug-polymer interactions in aqueous media, using sedimentation velocity analytical ultracentrifugation (AUC). We investigated the potential interaction of ketoconazole (KTZ), a poorly water-soluble drug, with polyacrylic acid (PAA) and a polyvinyl caprolactam-polyvinyl acetate-polyethylene glycol graft copolymer (Soluplus) in aqueous buffers. The effect of the polymer on the sedimentation coefficient of the drug was the observable metric. The drug alone, when subjected to AUC, exhibited a very narrow sedimentation peak at 0.2 Svedberg (S), in agreement with the expectation for a monomeric drug with a molar mass < 1000 Dalton. Conversely, the neat polymers showed broad profiles with higher sedimentation coefficients, reflecting their larger more heterogeneous size distributions. The sedimentation profiles of the drug-polymer mixtures were expectedly different from the profile of the neat drug. With KTZ-Soluplus, a complete shift to faster sedimentation times (indicative of an interaction) was observed, while with KTZ-PAA, a split peak indicated the existence of the drug in both free and interacting states. The sedimentation profile of carbamazepine, a second model drug, in the presence of hydroxypropyl methyl cellulose acetate succinate (HPMCAS, another polymer) revealed multiple "populations" of drug-polymer species, very similar to the sedimentation profile of neat HPMCAS. The interactions probed by AUC were compared with the results from isothermal titration calorimetry. In vitro dissolution tests performed on amorphous solid dispersions prepared with the same drug-polymer pairs suggested that the interactions may play a role in prolonging drug supersaturation. The results show the possibility of characterizing drug-polymer interactions in aqueous solution with high hydrodynamic resolution, addressing a major challenge frequently encountered in the mechanistic investigations of the dissolution behavior of amorphous solid dispersions.

Solid state nuclear magnetic resonance studies of hydroxypropylmethylcellulose acetyl succinate polymer, a useful carrier in pharmaceutical solid dispersions

Hydroxypropylmethylcellulose (HPMC) acetyl succinate (HPMC-AS) is a key polymer used for the enablement of amorphous solid dispersions (ASDs) in oral solid dosage forms. Choice of the appropriate grade within the material is often made empirically by the manufacturer of small-scale formulations, followed by extensive real time stability. A key factor in understanding and predicting the performance of an ASD is related to the presence of hydrogen (or other) bonds between the polymer and active pharmaceutical ingredient (API), which will increase stability over the parameters captured by miscibility and predicted by the Gordon-Taylor equation. Solid state nuclear magnetic resonance (NMR) is particularly well equipped to probe spatial proximities, for example, between polymer and API; however, in the case of HPMC-AS, these interactions have been sometimes difficult to identity as the carbon-13 NMR spectra assignment is yet to be firmly established. Using feedstock, selectively substituted HPMC polymers, and NMR editing experiments, we propose here a comprehensive understanding of the chemical structure of HPMC-AS and a definitive spectral assignment of the ¹³ C NMR spectra of this polymer. The NMR data also capture the molar ratios of the acetate and succinate moieties present in HPMC-AS of various grades without the need for post treatment required by chromatography methods commonly use in pharmacopoeia. This knowledge will allow the prediction and measurement of interactions between polymers and APIs and therefore a rational choice of polymer grade to enhance the solid state stability of ASDs.

Degrees of order: A comparison of nanocrystal and amorphous solids for poorly soluble drugs

Poor aqueous solubility is currently a prevalent issue in the development of small molecule pharmaceuticals. Several methods are possible for improving the solubility, dissolution rate and bioavailability of Biopharmaceutics Classification System (BCS) class II and class IV drugs. Two solid state approaches, which rely on reductions in order, and can theoretically be applied to all molecules without any specific chemical prerequisites (compared with e.g. ionizable or co-former groups, or sufficient lipophilicity), are the use of the amorphous form and nanocrystals. Research involving these two approaches is relatively extensive and commercial products are now available based on these technologies. Nevertheless, their formulation remains more challenging than with conventional dosage forms. This article describes these two technologies from both theoretical and practical perspectives by briefly discussing the physicochemical backgrounds behind these approaches, as well as the resulting practical implications, both positive and negative. Case studies demonstrating the benefits and challenges of these two techniques are presented.

Selective detection of active pharmaceutical ingredients in tablet formulations using solid-state NMR spectroscopy

Atomic-level characterization of active pharmaceutical ingredients (API) is crucial in pharmaceutical industry because APIs play an important role in physicochemical properties of drug formulations. However, the analysis of targeted APIs in intact tablet formulations is less straightforward due to the coexistence of excipients as major components and different APIs at dilute concentrations (often below 10 ​wt% loading). Although solid-state (ss) NMR spectroscopy is widely used to investigate short-range order, polymorphism, and pseudo-polymorphism in neat pharmaceutical compounds, the analysis of complex drug formulations is often limited by overlapped signals that originate from structurally different APIs and excipients. In particular, such examples are frequently encountered in the analysis of ¹H ssNMR spectra of pharmaceutical formulations. While the high-resolution in ¹H ssNMR spectra can be attained by, for example, high magnetic fields accompanied by fast magic-angle spinning (MAS) approaches, the spectral complexity associated with the mixtures of compounds hinders the accurate determination of chemical shifts and through-space proximities. Here we propose a fast MAS (70 ​kHz) NMR experiment for the selective detection of ¹H signals associated with an API from a severely overlapped NMR spectrum of a tablet formulation. Spectral simplification is achieved by combining (i) symmetry-based dipolar recoupling (SR4₁²) rotational-echo saturation-pulse double-resonance (RESPDOR) with phase-modulate (PM) saturation pulses, (ii) radio frequency-driven recoupling (RFDR), and (iii) double-quantum excitation using Back-to-Back (BaBa) pulse sequence elements. First, ¹H sites in close proximities to ¹⁴N nuclei of an API are excited using a PM-S-RESPDOR sequence, and simultaneously, the other unwanted ¹H signals of excipients are suppressed. Then, ¹H magnetization transfer to adjacent ¹H sites in the API is achieved by spin diffusion process using a RFDR sequence, which polarizes to ¹H sites within the crystalline API regions of the drug formulation. Next, a PM-S-RESPDOR-RFDR sequence is combined with a Back-to-Back (BaBa) sequence to elucidate local-structures and ¹H-¹H proximities of the API in a dosage form. The PM-S-RESPDOR-RFDR-BaBa experiment is employed in one- (1D) and two-dimensional (2D) versions to selectively detect the ¹H ssNMR spectrum of l-cysteine (10.6 ​wt% or 0.11 ​mg) in a commercial formulation, and compared with the spectra of neat l-cysteine recorded using a standard BaBa experiment. The 2D ¹H double-quantum-single-quantum (DQ-SQ) spectrum of the API (l-cysteine)-detected pharmaceutical tablet is in good agreement with the 2D ¹H DQ-SQ spectrum obtained from the pure API molecule. Furthermore, the sensitivity and robustness of the experiment is examined by selectively detecting ¹H{¹⁴N} signals in an amino acid salt, l-histidine^.H₂O^.HCl.

Solid-state properties, solubility, stability and dissolution behaviour of co-amorphous solid dispersions of baicalin

Formation of different baicalin co-former based solid dispersion formulations, which demonstrate improved solubility and dissolution performance.