Quantifying Molecular Mixing and Heterogeneity in Pharmaceutical Dispersions at Sub-100 nm Resolution by Spin Diffusion NMR

Application of atomic force microscopy in the development of amorphous solid dispersion

Development of Amorphous Solid Dispersion (ASD) requires an in-depth characterization at different stages due to its structural and functional complexity. Various tools are conventionally used to investigate the processing, stability, and functionality of ASDs. However, many subtle features remain poorly understood due to lack of nano-scale characterization tools in routine practice. Atomic force microscopy (AFM) is a type of scanning probe microscopy, used for high resolution imaging and measuring features at the nano-scale. In recent years AFM has been used increasingly as a characterization tool in different areas of the development of ASD, including drug-polymer miscibility, localized characterization of the phase separated domains, lateral molecular diffusivity on ASD surface, crystallinity and crystallization kinetics in ASD, phase behavior of ASD during dissolution, and conformation of polymer during dissolution. In this review, we have highlighted the current applications of AFM in capturing critical aspects of stability and dissolution behavior of ASD. Potential areas of future development in this domain have been discussed.

Miscibility of amorphous solid dispersions: A rheological and solid-state NMR spectroscopy study

Miscibility is critical in the prediction of stability against crystallization of amorphous solid dispersions (ASDs) in the solid state. However, currently available approaches for its determination are limited by both theoretical and practical considerations. Recently, a rheological approach guided by the polymer overlap concentration (c*) has been proposed for miscibility quantification of ASDs [J. Pharm. Sci., 112 (2023) 204-212] and shown to be useful in predicting both accelerated and long term physical stability in the absence of moisture. However, this approach can only be performed at high temperatures (slightly above the melting temperature, Tm, of drugs), and little is known about the difference in miscibility between high and low temperatures (e.g., below the glass transition temperature, Tg). Here we compare the miscibility of nifedipine (NIF)/polyvinylpyrrolidone (PVP) ASDs as determined by the rheological approach at 175°C (∼3°C above Tm of NIF) and solid state NMR (ssNMR) 1H T1 and T1ρ relaxation times at -20°C (∼66°C below Tg of NIF). Our results indicate agreement between the two methods. For low molecular weight (Mw) PVP, T1ρ measurements are more consistent with the rheological approach, while T1 measurements are closer for relatively high Mw PVP. Our findings support the use of the c* based rheological approach for inferring miscibility of deeply cooled ASDs.

Selective Methods Promote Protein Solid-State NMR

Solid-state nuclear magnetic resonance (ssNMR) is indispensable for studying the structures, dynamics, and interactions of insoluble proteins in native or native-like environments. While ssNMR includes numerous nonselective techniques for general analysis, it also provides various selective methods that allow for the extraction of precise details about proteins. This perspective highlights three key aspects of selective methods: selective signals of protein segments, selective recoupling, and site-specific insights into proteins. These methods leverage protein topology, labeling strategies, and the tailored manipulation of spin interactions through radio frequency (RF) pulses, significantly promoting the field of protein ssNMR. With ongoing advancements in higher magnetic fields and faster magic angle spinning (MAS), there remains an ongoing need to enhance the selectivity and efficiency of selective ssNMR methods, facilitating deeper atomic-level insights into complex biological systems.

COSMOPharm: Drug-Polymer Compatibility of Pharmaceutical Amorphous Solid Dispersions from COSMO-SAC

The quantum mechanics-aided COSMO-SAC activity coefficient model is applied and systematically examined for predicting the thermodynamic compatibility of drugs and polymers. The drug-polymer compatibility is a key aspect in the rational selection of optimal polymeric carriers for pharmaceutical amorphous solid dispersions (ASD) that enhance drug bioavailability. The drug-polymer compatibility is evaluated in terms of both solubility and miscibility, calculated using standard thermodynamic equilibrium relations based on the activity coefficients predicted by COSMO-SAC. As inherent to COSMO-SAC, our approach relies only on quantum-mechanically derived σ-profiles of the considered molecular species and involves no parameter fitting to experimental data. All σ-profiles used were determined in this work, with those of the polymers being derived from their shorter oligomers by replicating the properties of their central monomer unit(s). Quantitatively, COSMO-SAC achieved an overall average absolute deviation of 13% in weight fraction drug solubility predictions compared to experimental data. Qualitatively, COSMO-SAC correctly categorized different polymer types in terms of their compatibility with drugs and provided meaningful estimations of the amorphous-amorphous phase separation. Furthermore, we analyzed the sensitivity of the COSMO-SAC results for ASD to different model configurations and σ-profiles of polymers. In general, while the free volume and dispersion terms exerted a limited effect on predictions, the structures of oligomers used to produce σ-profiles of polymers appeared to be more important, especially in the case of strongly interacting polymers. Explanations for these observations are provided. COSMO-SAC proved to be an efficient method for compatibility prediction and polymer screening in ASD, particularly in terms of its performance-cost ratio, as it relies only on first-principles calculations for the considered molecular species. The open-source nature of both COSMO-SAC and the Python-based tool COSMOPharm, developed in this work for predicting the API-polymer thermodynamic compatibility, invites interested readers to explore and utilize this method for further research or assistance in the design of pharmaceutical formulations.

A Numerical simulation method for analyzing 1H spin diffusion NMR for Multicomponent and multiphase polymer systems

A numerical simulation method, namely, SDNMR-WEBFIT, is reported for simulating proton spin diffusion NMR based on the Levenberg-Marquardt algorithm and a pseudo-2D diffusion model. This method is used for the precise quantification of dynamics heterogeneity of the interphase within multiphase polymer systems. The numerical simulation method provides measurements of spin-lattice relaxation time (T1), proton density (ρH), lamellar thickness (d), and spin diffusion coefficient (D) for each component. The pseudo-2D diffusion model is employed to simulate the proton spin diffusion build-up/decay curves, simultaneously calculating the lateral fraction of island-like structures (x-ratio). Such approach was successfully applied to various polymer systems, such as semi-crystalline polymer (Poly(ε-caprolactone), PCL), block copolymers (Styrene-butadiene-styrene triblock copolymer, SBS), and plasticized semi-polymers (Polvinyl alcohol, PVA).

Probing the Molecular and Macroscopic Structure of Solid Solutions by Dynamic Nuclear Polarization (DNP) Enhanced 13C and 15N Solid-State NMR Spectroscopy

Crystallization is a widely used purification technique in the manufacture of active pharmaceutical ingredients (APIs) and precursor molecules. However, when impurities and desired compounds have similar molecular structures, separation by crystallization may become challenging. In such cases, some impurities may form crystalline solid solutions with the desired product during recrystallization. Understanding the molecular structure of these recrystallized solid solutions is crucial to devise methods for effective purification. Unfortunately, there are limited analytical techniques that provide insights into the molecular structure or spatial distribution of impurities that are incorporated within recrystallized products. In this study, we investigated model solid solutions formed by recrystallizing salicylic acid (SA) in the presence of anthranilic acid (AA). These two molecules are known to form crystalline solid solutions due to their similar molecular structures. To overcome challenges associated with the long 1H longitudinal relaxation times (T1(1H)) of SA and AA, we employed dynamic nuclear polarization (DNP) and 15N isotope enrichment to enable solid-state NMR experiments. Results of solid-state NMR experiments and DFT calculations revealed that SA and AA are homogeneously alloyed as a solid solution. Heteronuclear correlation (HETCOR) experiments and plane-wave DFT structural models provide further evidence of the molecular-level interactions between SA and AA. This research provides valuable insights into the molecular structure of recrystallized solid solutions, contributing to the development of effective purification strategies and an understanding of the physicochemical properties of solid solutions.

Evaluation of Printability of PVA-Based Tablets from Powder and Assessment of Critical Rheological Parameters

Fused deposition modeling (FDM) is a rather new technology in the production of personalized dosage forms. The melting and printing of polymer-active pharmaceutical ingredient (API)-mixtures can be used to produce oral dosage forms with different dosage as well as release behavior. This process is utilized to increase the bioavailability of pharmaceutically relevant active ingredients that are poorly soluble in physiological medium by transforming them into solid amorphous dispersions (ASD). The release from such ASDs is expected to be faster and higher compared to the raw materials and thus enhance bioavailability. Printing directly from powder while forming ASDs from loperamide in Polyvinylalcohol was realized. Different techniques such as a change in infill and the incorporation of sorbitol as a plastisizer to change release patterns as well as a non-destructive way for the determination of API distribution were shown. By measuring the melt viscosities of the mixtures printed, a rheological model for the printer used is proposed.

Phase homogeneity in ternary amorphous solid dispersions and its impact on solubility, dissolution and supersaturation - Influence of processing and hydroxypropyl cellulose grade

As performance of ternary amorphous solid dispersions (ASDs) depends on the solid-state characteristics and polymer mixing, a comprehensive understanding of synergistic interactions between the polymers in regard of dissolution enhancement of poorly soluble drugs and subsequent supersaturation stabilization is necessary. By choosing hot-melt extrusion (HME) and vacuum compression molding (VCM) as preparation techniques, we manipulated the phase behavior of ternary efavirenz (EFV) ASDs, comprising of either hydroxypropyl cellulose (HPC)-SSL or HPC-UL in combination with Eudragit® L 100-55 (EL 100-55) (50:50 polymer ratio), leading to single-phased (HME) and heterogeneous ASDs (VCM). Due to higher kinetic solid-state solubility of EFV in HPC polymers compared to EL 100-55, we visualized higher drug distribution into HPC-rich phases of the phase-separated ternary VCM ASDs via confocal Raman microscopy. Additionally, we observed differences in the extent of phase-separation in dependence on the selected HPC grade. As HPC-UL exhibited decisive lower melt viscosity than HPC-SSL, formation of partially miscible phases between HPC-UL and EL 100-55 was facilitated. Consequently, as homogeneously mixed polymer phases were required for optimal extent of solubility improvement, the manufacturing-dependent differences in dissolution performances were smaller using HPC-UL, instead of HPC-SSL, i.e. using HPC-UL was less demanding on shear stress provided by the process.

Measurement of spin-lattice relaxation times in multiphase polymer systems

Solid-state Nuclear Magnetic Resonance (NMR) has emerged as a pivotal technique for unraveling the microstructure and dynamics of intricate polymer and biological materials. Within this context, site-specific proton spin-lattice relaxation times in the laboratory frame (T1) and rotating frame (T1ρ) have become indispensable tools for investigating phase separation structures and molecular dynamics in multiphase polymer systems. Notably, the site-specific measurement of proton T1 and T1ρ is usually achieved via 13C detection in polymers, where 1H polarization is typically transferred to 13C via cross polarization (CP). Nevertheless, CP relies on the 1H-13C heteronuclear dipolar couplings, and thus it does not work well for the mobile components. In this study, via the integration of CP and RINEPT (refocused insensitive nuclei enhanced by polarization transfer), we propose a robust approach for the measurement of site-specific proton T1 and T1ρ in multiphase polymers. It overcomes the limitation of CP on transferring 1H polarization to 13C in mobile components, and thus enables simultaneous determination of site-specific proton T1 and T1ρ in rigid and mobile components in multiphase polymers in a single experiment. Such experiment can also be used for dynamics-based spectral editing due to the dynamic selectivity of CP- and RINEPT-based polarization transfer process. The proposed experiments are well demonstrated on three typical multiphase polymer systems, poly(methyl methacrylate)/polybutadiene (PMMA/PB) polymer blend, polyurethane (PU) and polystyrene-polybutadiene-polystyrene (SBS) elastomers. We envisage the proposed experiments can be a universal avenue for structural and dynamic elucidation of multiphase polymers containing both rigid and mobile components.

Exploiting solid-state dynamic nuclear polarization NMR spectroscopy to establish the spatial distribution of polymorphic phases in a solid material

Solid-state DNP NMR can enhance the ability to detect minor amounts of solid phases within heterogenous materials. Here we demonstrate that NMR contrast based on the transport of DNP-enhanced polarization can be exploited in the challenging case of early detection of a small amount of a minor polymorphic phase within a major polymorph, and we show that this approach can yield quantitative information on the spatial distribution of the two polymorphs. We focus on the detection of a minor amount (<4%) of polymorph III of m-aminobenzoic acid within a powder sample of polymorph I at natural isotopic abundance. Based on proposed models of the spatial distribution of the two polymorphs, simulations of 1H spin diffusion allow NMR data to be calculated for each model as a function of particle size and the relative amounts of the polymorphs. A comparison between simulated and experimental NMR data allows the model(s) best representing the spatial distribution of the polymorphs in the system to be established.

Multiscale X-ray imaging and characterisation of pharmaceutical dosage forms

A correlative, multiscale imaging methodology for visualising and quantifying the morphology of solid dosage forms by combining ptychographic X-ray computed nanotomography (PXCT) and scanning small- and wide-angle X-ray scattering (S/WAXS) is presented. The methodology presents a workflow for multiscale analysis, where structures are characterised from the nanometre to millimetre regime. Here, the method is demonstrated by characterising a hot-melt extruded, partly crystalline, solid dispersion of carbamazepine in ethyl cellulose. Characterisation of the morphology and solid-state phase of the drug in solid dosage forms is central as this affects the performance of the final formulation. The 3D morphology was visualised at a resolution of 80 nm over an extended volume through PXCT, revealing an oriented structure of crystalline drug domains aligned in the direction of extrusion. Scanning S/WAXS, showed that the nanostructure is similar over the cross section of the extruded filament, with minor radial changes in domain sizes and degree of orientation. The polymorphic forms of carbamazepine were qualified with WAXS, showing a heterogeneous distribution of the metastable forms I and II. This demonstrates the methodology for multiscale structural characterization and imaging to enable a better understanding of the relationships between morphology, performance, and processing conditions of solid dosage forms.

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.

Effect of Buffer Salts on Physical Stability of Lyophilized and Spray-Dried Protein Formulations Containing Bovine Serum Albumin and Trehalose

This study examined the effect of buffer salts on the physical stability of spray-dried and lyophilized formulations of a model protein, bovine serum albumin (BSA). BSA formulations with various buffers were dried by either lyophilization or spray drying. The protein powders were then characterized using solid-state Fourier transform infrared spectroscopy (ssFTIR), powder X-ray diffraction (PXRD), size exclusion chromatography (SEC), solid-state hydrogen/deuterium exchange with mass spectrometry (ssHDX-MS), and solid-state nuclear magnetic resonance spectroscopy (ssNMR). Particle characterizations such as Brunauer-Emmett-Teller (BET) surface area, particle size distribution, and particle morphology were also performed. Results from conventional techniques such as ssFTIR did not exhibit correlations with the physical stability of studied formulations. Deconvoluted peak areas of deuterated samples from the ssHDX-MS study showed a satisfactory correlation with the loss of the monomeric peak area measured by SEC (R2 of 0.8722 for spray-dried formulations and 0.8428 for lyophilized formulations) in the 90-day accelerated stability study conducted at 40°C. mDSC and PXRD was unable to measure phase separation in the samples right after drying. In contrast, ssNMR successfully detected the occurrence of phase separation between the succinic buffer component and protein in the lyophilized formulation, which results in a distribution of microenvironmental acidity and the subsequent loss of long-term stability. Moreover, our results suggested that buffer salts have less impact on physical stability for the spray-dried formulations than the lyophilized solids.

Solid-state NMR spectroscopy of roasted and ground coffee samples: Evidences for phase heterogeneity and prospects of applications in food screening

The advancement in the use of spectroscopic techniques to investigate coffee samples is of high interest especially considering the widespread problems with coffee adulteration and counterfeiting. In this work, the use of solid-state nuclear magnetic resonance (NMR) is investigated as a means to probe the various chemically-distinct phases existent in roasted coffee samples and to detect the occurrence of counterfeiting or adulterations in coffee blends. Routine solid-state ¹H and ¹³C NMR spectra allowed the distinction between different coffee types (Arabica/Robusta) and the evaluation of the presence of these components in coffee blends. On the other hand, the use of more specialized solid-state NMR experiments revealed the existence of phases with different molecular mobilities (e.g., associated with lipids or carbohydrates). The results illustrate the usefulness of solid-state NMR spectroscopy to examine molecular mobilities and interactions and to aid in the quality control of coffee-related products.

Influence of Co-amorphization on the Physical Stability and Dissolution Performance of an Anthelmintic Drug Flubendazole

In this work, the co-amorphization approach was applied to flubendazole (FluBZ), resulting in the formation of two novel solid forms of FluBZ with l-phenylalanine (Phe) and l-tryptophan (Trp). A variety of physicochemical techniques have been used to describe new systems, including powder X-ray diffraction, thermal methods, infrared spectroscopy, and scanning electron microscopy. Co-amorphization has been shown to suppress crystallization tendency and considerably increase the shelf-life storage of amorphous flubendazole solid across a wide range of relative humidities. The dissolution behavior of the amorphous forms in biorelevant media at pH = 1.6, pH = 6.5, and 37 °C has been studied in terms of C_max (maximum FluBZ concentration), T_max (time to attain peak drug concentration), and AUC (concentration area under the curve during dissolution). At pH = 6.5, a continuous supersaturation and the highest AUC value of all examined systems were observed for the FluBZ-Phe (1:1) system. The phase solubility diagrams revealed that the reason for the better dissolution performance of FluBZ-Phe (1:1) at pH = 6.5 is a complexation between the components in a solution. This work highlights the applicability of co-amorphous systems in improving the physical stability and dissolution performance of drug compounds with poor biopharmaceutical characteristics.

Ultrafast Magic Angle Spinning Solid-State NMR Spectroscopy: Advances in Methodology and Applications

Solid-state NMR spectroscopy is one of the most commonly used techniques to study the atomic-resolution structure and dynamics of various chemical, biological, material, and pharmaceutical systems spanning multiple forms, including crystalline, liquid crystalline, fibrous, and amorphous states. Despite the unique advantages of solid-state NMR spectroscopy, its poor spectral resolution and sensitivity have severely limited the scope of this technique. Fortunately, the recent developments in probe technology that mechanically rotate the sample fast (100 kHz and above) to obtain "solution-like" NMR spectra of solids with higher resolution and sensitivity have opened numerous avenues for the development of novel NMR techniques and their applications to study a plethora of solids including globular and membrane-associated proteins, self-assembled protein aggregates such as amyloid fibers, RNA, viral assemblies, polymorphic pharmaceuticals, metal-organic framework, bone materials, and inorganic materials. While the ultrafast-MAS continues to be developed, the minute sample quantity and radio frequency requirements, shorter recycle delays enabling fast data acquisition, the feasibility of employing proton detection, enhancement in proton spectral resolution and polarization transfer efficiency, and high sensitivity per unit sample are some of the remarkable benefits of the ultrafast-MAS technology as demonstrated by the reported studies in the literature. Although the very low sample volume and very high RF power could be limitations for some of the systems, the advantages have spurred solid-state NMR investigation into increasingly complex biological and material systems. As ultrafast-MAS NMR techniques are increasingly used in multidisciplinary research areas, further development of instrumentation, probes, and advanced methods are pursued in parallel to overcome the limitations and challenges for widespread applications. This review article is focused on providing timely comprehensive coverage of the major developments on instrumentation, theory, techniques, applications, limitations, and future scope of ultrafast-MAS technology.

Formulation and Processing Strategies which Underpin Susceptibility to Matrix Crystallization in Amorphous Solid Dispersions

Through matrix crystallization, an amorphous solid may transform directly into its more stable crystalline state, reducing the driving force for dissolution. Herein, the mechanism of matrix crystallization in an amorphous solid dispersion (ASD) was probed. ASDs of bicalutamide/copovidone were prepared by solvent evaporation and hot melt extrusion, and sized by mortar and pestle or cryomilling techniques, modulating the level of mechanical activation experienced by the sample. Drug loading (DL) of the binary ASD was varied from 5-50%, and ternary systems were formulated at 30% DL with two surfactants (sodium dodecyl sulfate, Vitamin E TPGS). Imaging of partially dissolved or crystallized compacts by scanning electron microscopy with energy-dispersive X-ray analysis and confocal fluorescence microscopy was performed to investigate pathways of hydration, phase separation, and crystallization. Monitoring drug and polymer release of ASD powder under non-sink conditions provided insight into supersaturation and desupersaturation profiles. Systems at the greatest risk of matrix crystallization had high DLs, underwent mechanical activation, and/or contained surfactant. Systems having greatest resistance to matrix crystallization had rapid and congruent drug and polymer release. This study has implications for formulation and process design of ASDs and risk assessment of matrix crystallization.

Ultrafast magic angle spinning NMR characterization of pharmaceutical solid polymorphism: A posaconazole example

Protons represent the most NMR-sensitive nucleus in pharmaceutical compounds. Therefore, proton-detected solid-state NMR techniques under fast magic angle spinning are among the few solutions to overcome the challenge of low sensitivity to analyze natural abundant drug substances and products. In this study, we report the structural characterization of crystal polymorphs of a commercial drug molecule, posaconazole, with a relatively large molecular weight of 700.8 g·mol^-1 and at the natural abundance. The enhanced sensitivity and resolution at 100 kHz MAS enables the exploration of the distinct intermolecular packing in posaconazole forms I, III, and γ. These results demonstrate that proton-detected homo- and heteronuclear correlation methods can probe the structural details of pharmaceutical polymorphism.

Amorphous Solid Dispersions: Role of the Polymer and Its Importance in Physical Stability and In Vitro Performance

Amorphous solid dispersions stabilized by one or more polymer(s) have been widely used for delivering amorphous drugs with poor water solubilities, and they have gained great market success. Polymer selection is important for preparing robust amorphous solid dispersions, and considerations should be given as to how the critical attributes of a polymer can enhance the physical stability, and the in vitro and in vivo performances of a drug. This article provides a comprehensive overview for recent developments in the understanding the role of polymers in amorphous solid dispersions from the aspects of nucleation, crystal growth, overall crystallization, miscibility, phase separation, dissolution, and supersaturation. The critical properties of polymers affecting the physical stability and the in vitro performance of amorphous solid dispersions are also highlighted. Moreover, a perspective regarding the current research gaps and novel research directions for better understanding the role of the polymer is provided. This review will provide guidance for the rational design of polymer-based amorphous pharmaceutical solids with desired physicochemical properties from the perspective of physical stability and in vitro performance.

19F Solid-state NMR characterization of pharmaceutical solids

Solid-state NMR has been increasingly recognized as a high-resolution and versatile spectroscopic tool to characterize drug substances and products. However, the analysis of pharmaceutical materials is often carried out at natural isotopic abundance and a relatively low drug loading in multi-component systems and therefore suffers from challenges of low sensitivity. The fact that fluorinated therapeutics are well represented in pipeline drugs and commercial products offers an excellent opportunity to utilize fluorine as a molecular probe for pharmaceutical analysis. We aim to review recent advancements of ¹⁹F magic angle spinning NMR methods in modern drug research and development. Applications to polymorph screening at the micromolar level, structural elucidation, and investigation of molecular interactions at the Ångström to submicron resolution in drug delivery, stability, and quality will be discussed.

Design, optimization, and in vitro characterization of idebenone-loaded PLGA microspheres for LHON treatment

Biodegradable poly(lactic-co-glycolic acid) microspheres (PLGA MSs) are attractive delivery systems for site-specific maintained release of therapeutic active substances into the intravitreal chamber. The design, development, and characterization of idebenone-loaded PLGA microspheres by means of an oil-in-water emulsion/solvent evaporation method enabled the obtention of appropriate production yield, encapsulation efficiency and loading values. MSs revealed spherical shape, with a size range of 10-25 μm and a smooth and non-porous surface. Fourier-transform infrared spectroscopy (FTIR) spectra demonstrated no chemical interactions between idebenone and polymers. Solid-state nuclear magnetic resonance (NMR), X-ray diffractometry, differential scanning calorimetry (DSC) and thermogravimetry (TGA) analyses indicated that microencapsulation led to drug amorphization. In vitro release profiles were fitted to a biexponential kinetic profile. Idebenone-loaded PLGA MSs showed no cytotoxic effects in an organotypic tissue model. Results suggest that PLGA MSs could be an alternative intraocular system for long-term idebenone administration, showing potential therapeutic advantages as a new therapeutic approach to the Leber's Hereditary Optic Neuropathy (LHON) treatment by intravitreal administration.

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.

Understanding the Impact of Protein-Excipient Interactions on Physical Stability of Spray-Dried Protein Solids

Mannitol, leucine, and trehalose have been widely used in spray-dried formulations, especially for inhalation formulations. The individual contribution of these excipients on protein physical stability in spray-dried solids was studied here using bovine serum albumin (BSA) as a model protein. The spray-dried solids were characterized with scanning electron microscopy, powder X-ray diffraction, and solid-state Fourier-transform infrared spectroscopy to analyze particle morphology, crystallinity, and secondary structure change, respectively. Advanced solid-state characterizations were conducted with solid-state hydrogen-deuterium exchange (ssHDX) and solid-state nuclear magnetic resonance (ssNMR) to explore protein conformation and molecular interactions in the context of the system physical stability. Trehalose remained amorphous after spray-drying and was miscible with BSA, forming hydrogen bonds to maintain protein conformation, whereby this system showed the least monomer loss in the stability study. As indicated by ssNMR, both crystalline and amorphous forms of mannitol existed in the spray-dried BSA-mannitol solids, which explained its partial stabilizing effect on BSA. Leucine showed the strongest crystallization tendency after spray-drying and did not provide a stabilizing effect due to substantial immiscibility and phase separation with BSA as a result of crystal formation. This work showed novel applications of ssNMR in examining protein conformation and protein-excipient interaction in dry formulations. Overall, our results demonstrate the pivotal role of advanced solid-state characterization techniques in understanding the physical stability of spray-dried protein solids.

High Sensitivity Detection of a Solubility Limiting Surface Transformation of Drug Particles by DNP SENS

We investigate the presence of a surface species for the active pharmaceutical ingredient (API) AZD9496 with dynamic nuclear polarization surface enhanced nuclear spectroscopy (DNP SENS). We show that using DNP we can elucidate the presence of an amorphous form of the API at the surface of crystalline particles of the salt form. The amorphous form of the API has distinguishable ¹³C chemical shifts when compared to the salt form under various acidic conditions. The predominant form in frozen particles of AZD9496 is the salt, and we provide evidence to suggest that the amorphous layer at the surface is mainly made up of the dissociated free form.