Theoretical and Practical Approaches for Prediction of Drug–Polymer Miscibility and Solubility

Experimental, Thermodynamic, and Molecular Modeling Evaluation of Amorphous Simvastatin-Poly(vinylpyrrolidone) Solid Dispersions

A crucial step for the selection of proper amorphous solid dispersion (ASD) matrix carriers is the in-depth assessment of drug/polymer physicochemical properties. In this context, the present study extends the work of previously published attempts by evaluating the formation of simvastatin (SIM)-poly(vinylpyrrolidone) (PVP) ASDs with the aid of thermodynamic and molecular modeling. Specifically, the implementation of both Flory-Huggins lattice theory and molecular dynamics (MD) simulations was able to predict the miscibility between the two components (a finding that was experimentally verified via differential scanning calorimetry (DSC) and hot stage polarized microscopy), while a complete temperature-concentration phase-transition profile was constructed, leading to the identification of the thermodynamically metastable and unstable ASD zones. Furthermore, as in the case of previously published reports, the analysis of the ASDs via Fourier transform infrared spectroscopy did not clarify the type and extent of observed molecular interactions. Hence, in the present study, a computer-based MD simulation model was developed for the first time in order to gain an insight into the properties of the observed interactions. MD amorphous assemblies of SIM, PVP, and their mixtures were initially developed, and the calculated glass transition temperatures were in close agreement with experimentally obtained results, indicating that the developed models could be considered as realistic representations of the actual systems. Furthermore, molecular interactions evaluation via radial distribution function and radius of gyration analysis revealed that increasing SIM content results in a significant PVP chain shrinkage, which eventually leads to SIM-SIM amorphous intermolecular interactions, leading to the formation of amorphous drug zones. Finally, MD-based results were experimentally verified via DSC.

The design and development of high drug loading amorphous solid dispersion for hot-melt extrusion platform

Amorphous solid dispersion (ASD) is a formulation strategy extensively used to enhance the bioavailability of poorly water soluble drugs. Despite this, they are limited by various factors such as limited drug loading, poor stability, drug-excipient miscibility and the choice of process platforms. In this work, we have developed a strategy for the manufacture of high drug loaded ASD (HDASD) using hot-melt extrusion (HME) based platform. Three drug-polymer combinations, indomethacin-Eudragit®E, naproxen-Eudragit®E and ibuprofen-Eudragit®E, were used as the model systems. The design spaces were predicted through Flory-Huggins based theory, and the selected HDASDs at pre-defined conditions were manufactured using HME and quench-cooled melt methods. These HDASD systems were also extensively characterised via small angle/wide angle x-ray scattering, differential scanning calorimetry, Infrared and Raman spectroscopy and atomic force microscopy. It was verified that HDASDs were successfully produced via HME platform at the pre-defined conditions, with maximum drug loadings of 0.65, 0.70 and 0.60 w/w for drug indomethacin, ibuprofen and naproxen respectively. Enhanced physical stability was further confirmed by high humidity (95%RH) storage stability studies. Through this work, we have demonstrated that by the implementation of predictive thermodynamic modelling, HDASD formulation design can be integrated into the HME process design to ensure the desired quality of the final dosage form.

Automation Potential of a New, Rapid, Microscopy-Based Method for Screening Drug-Polymer Solubility

For the pharmaceutical industry, the preformulation screening of the compatibility of drug and polymeric excipients can often be time-consuming because of the use of trial-and-error approaches. This is also the case for selecting highly effective polymeric excipients for forming molecular dispersions in order to improve the dissolution and subsequent bio-availability of a poorly soluble drug. Previously, we developed a new thermal imaging-based rapid screening method, thermal analysis by structure characterization (TASC), which can rapidly detect the melting point depression of a crystalline drug in the presence of a polymeric material. In this study, we used melting point depression as an indicator of drug solubility in a polymer and further explored the potential of using the TASC method to rapidly screen and identify polymers in which a drug is likely to have high solubility. Here, we used a data bank of 5 model drugs and 10 different pharmaceutical grade polymers to validate the screening potential of TASC. The data indicated that TASC could provide significant improvement in the screening speed and reduce the materials used without compromising the sensitivity of detection. It should be highlighted that the current method is a screening method rather than a method that provides absolute measurement of the degree of solubility of a drug in a polymer. The results of this study confirmed that the TASC results of each drug-polymer pair could be used in data matrices to indicate the presence of significant interaction and solubility of the drug in the polymer. This forms the foundation for automating the screening process using artificial intelligence.

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.

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.

High-Throughput Synthesis and Screening of Rapidly Degrading Polyanhydride Nanoparticles

Combinatorial techniques can accelerate the discovery and development of polymeric nanodelivery devices by pairing high-throughput synthesis with rapid materials characterization. Biodegradable polyanhydrides demonstrate tunable release, high cellular internalization, and dose sparing properties when used as nanodelivery devices. This nanoparticle platform shows promising potential for small molecule drug delivery, but the pace of understanding and rational design of these nanomedicines is limited by the low throughput of conventional characterization. This study reports the use of a high-throughput method to synthesize libraries of a newly synthesized, rapidly eroding polyanhydride copolymer based on 1,8-bis(p-carboxyphenoxy)-3,6-dioxaoctane (CPTEG) and sebacic acid (SA) monomers. The high-throughput method enabled efficient screening of copolymer microstructure, revealing weak block-type and alternating architectures. The high-throughput method was adapted to synthesize nanoparticle libraries encapsulating hydrophobic model drugs. Drug release from these nanoparticles was rapid, with a majority of the payload released within 3 days. Drug release was dramatically slowed at acidic pH, which could be useful for oral drug delivery. Rhodamine B (RhoB) release kinetics generally followed patterns of polymer erosion kinetics, while Coomassie brilliant blue (CBB) released the fastest from the slowest degrading polymer chemistry and vice versa. These differences in trends between copolymer chemistry and release kinetics were hypothesized to arise from differences in mixing thermodynamics. A high-throughput method was developed to synthesize polymer-drug film libraries and characterize mixing thermodynamics by melting point depression. Rhodamine B had a negative χ for all copolymers with <30 mol % CPTEG tested, indicating a tendency toward miscibility. By contrast, CBB χ increased, eventually becoming positive near 15:85 CPTEG:SA, with increasing CPTEG content. This indicates an increasing tendency toward phase separation in CPTEG-rich copolymers. These in vitro results screening polymer-drug interactions showed good agreement with in silico predictions from Hansen solubility parameter estimation and were able to explain the observed differences in model drug release trends.

Preparation and Characterization of PEG4000 Palmitate/PEG8000 Palmitate-Solid Dispersion Containing the Poorly Water-Soluble Drug Andrographolide

Solid dispersion (SD) is the effective approach to improve the dissolution rate and bioavailability of class II drugs with low water solubility and high tissue permeability in the Biopharmaceutics Classification System. This study investigated the effects of polyethylene glycol (PEG) molecular weight in carrier material PEG palmitate on the properties of andrographolide (AG)-SD. We prepared SDs containing the poorly water-soluble drug AG by the freeze-drying method. The SDs were manufactured from two different polymers, PEG4000 palmitate and PEG8000 palmitate. The physicochemical properties of the AG-SDs were characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, differential scanning calorimetry, powder X-ray diffraction, scanning electron microscopy, dissolution testing, and so on. We found that AG-PEG4000 palmitate-SD and AG-PEG8000 palmitate-SD were similar in the surface morphology, specific surface area, and pore volume. Compared with the AG-PEG4000 palmitate-SD, the intermolecular interaction between PEG8000 palmitate and AG was stronger, and the thermal stability of AG-PEG8000 palmitate-SD was better. In the meanwhile, the AG relative crystallinity was lower and the AG dissolution rate was faster in AG-PEG8000 palmitate-SD. The results demonstrate that the increasing PEG molecular weight in the PEG palmitate can improve the compatibility between the poorly water-soluble drug and carrier material, which is beneficial to improve the SD thermal stability and increases the dissolution rate of poorly water-soluble drug in the SD.

Non-uniform drug distribution matrix system (NUDDMat) for zero-order release of drugs with different solubility

A decrease in the drug release rate over time typically affects the performance of hydrophilic matrices for oral prolonged release. To address such an issue, a Non-Uniform Drug Distribution Matrix (NUDDMat) based on hypromellose was proposed and demonstrated to yield zero-order release. The system consisted of 5 overlaid layers, applied by powder layering, having drug concentration decreasing from the inside towards the outside of the matrix according to a descending staircase function. In the present study, manufacturing and performance of the described delivery platform were evaluated using drug tracers having different water solubility. Lansoprazole, acetaminophen and losartan potassium were selected as slightly (SST), moderately (MST) and highly (HST) soluble tracers. By halving the thickness of the external layer, which contained no drug, linear release of HST and MST was obtained. The release behavior of the NUDDMat system loaded with a drug having pH-independent solubility was shown to be consistent in pH 1.2, 4.5 and 6.8 media. Based on these results, feasibility of the NUDDMat platform by powder layering was demonstrated using drugs having different physico-technological characteristics. Moreover, its ability to generate zero-order release was proved in the case of drugs with water solubility in a relatively wide range.

Predicting Density of Amorphous Solid Materials Using Molecular Dynamics Simulation

The true density of an amorphous solid is an important parameter for studying and modeling materials behavior. Experimental measurements of density using helium pycnometry are standard but may be prevented if the material is prone to rapid recrystallization, or preparation of gram quantities of reproducible pure component amorphous materials proves impossible. The density of an amorphous solid can be approximated by assuming it to be 95% of its respective crystallographic density; however, this can be inaccurate or impossible if the crystal structure is unknown. Molecular dynamic simulations were used to predict the density of 20 amorphous solid materials. The calculated density values for 10 amorphous solids were compared with densities that were experimentally determined using helium pycnometry. In these cases, the amorphous densities calculated using molecular dynamics had an average percent error of - 0.7% relative to the measured values, with a maximum error of - 3.48%. In contrast, comparisons of amorphous density approximated from crystallographic structures with pycnometrically measured values resulted in an average percent error of + 3.7%, with a maximum error of + 9.42%. These data suggest that the density of an amorphous solid can be accurately predicted using molecular dynamic simulations and allowed reliable calculation of density for the remaining 10 materials for which pycnometry could not be done.

Injection moulded controlled release amorphous solid dispersions: Synchronized drug and polymer release for robust performance

A study has been carried out to investigate controlled release performance of caplet shaped injection moulded (IM) amorphous solid dispersion (ASD) tablets based on the model drug AZD0837 and polyethylene oxide (PEO). The physical/chemical storage stability and release robustness of the IM tablets were characterized and compared to that of conventional extended release (ER) hydrophilic matrix tablets of the same raw materials and compositions manufactured via direct compression (DC). To gain an improved understanding of the release mechanisms, the dissolution of both the polymer and the drug were studied. Under conditions where the amount of dissolution media was limited, the controlled release ASD IM tablets demonstrated complete and synchronized release of both PEO and AZD0837 whereas the release of AZD0837 was found to be slower and incomplete from conventional direct compressed ER hydrophilic matrix tablets. The results clearly indicated that AZD0837 remained amorphous throughout the dissolution process and was maintained in a supersaturated state and hence kept stable with the aid of the polymeric carrier when released in a synchronized manner. In addition, it was found that the IM tablets were robust to variation in hydrodynamics of the dissolution environment and PEO molecular weight.

A novel drug–drug coamorphous system without molecular interactions: improve the physicochemical properties of tadalafil and repaglinide

Tadalafil and repaglinide, categorized as BCS class II drugs, have low oral bioavailabilities due to their poorly aqueous solubilities and dissolutions. The aim of this study was to enhance the dissolution of tadalafil and repaglinide by co-amorphization technology and evaluate the storage and compression stability of such coamorphous system. Based on Flory–Huggins interaction parameter (χ ≤ 0) and Hansen solubility parameter (δ_t ≤ 7 MPa^0.5) calculations, tadalafil and repaglinide was predicted to be well miscible with each other. Coamorphous tadalafil–repaglinide (molar ratio, 1 : 1) was prepared by solvent-evaporation method and characterized with respect to its thermal properties, possible molecular interactions. A single T_g (73.1 °C) observed in DSC and disappearance of crystallinity in PXRD indicated the formation of coamorphous system. Principal component analysis of FTIR in combination with Raman spectroscopy and Ss ¹³C NMR suggested the absence of intermolecular interactions in coamorphous tadalafil–repaglinide. In comparison to pure crystalline forms and their physical mixtures, both drugs in coamorphous system exhibited significant increases in intrinsic dissolution rate (1.5–3-fold) and could maintain supersaturated level for at least 4 hours in non-sink dissolution. In addition, the coamorphous tadalafil–repaglinide showed improved stability compared to the pure amorphous forms under long-term stability and accelerated storage conditions as well as under high compressing pressure. In conclusion, this study showed that co-amorphization technique is a promising approach for improving the dissolution rate of poorly water-soluble drugs and for stabilizing amorphous drugs.

The coamorphous tadalafil–repaglinide (molar ratio, 1 : 1) prepared by solvent-evaporation method significantly improve the physicochemical properties of tadalafil and repaglinide.

Rivaroxaban polymeric amorphous solid dispersions: Moisture-induced thermodynamic phase behavior and intermolecular interactions

The present study evaluates the physical stability and intermolecular interactions of Rivaroxaban (RXB) amorphous solid dispersions (ASDs) in polymeric carriers via thermodynamic modelling and molecular simulations. Specifically, the Flory-Huggins (FH) lattice solution theory was used to construct thermodynamic phase diagrams of RXB ASDs in four commonly used polymeric carriers (i.e. copovidone, coPVP, povidone, PVP, Soluplus, SOL and hypromellose acetate succinate, HPMCAS), which were stored under 0%, 60% and 75% relative humidity (RH) conditions. In order to verify the phase boundaries predicted by FH modelling (i.e. truly amorphous zone, amorphous-amorphous demixing zones and amorphous-API recrystallization zones), samples of ASDs were examined via polarized light microscopy after storage for up to six months at various RH conditions. Results showed a good agreement between the theoretical and the experimental approaches (i.e. coPVP and PVP resulted in less physically-stable ASDs compared to SOL and HPMCAS) indicating that the proposed FH-based modelling may be a useful tool in predicting long-term physical stability in high humidity conditions. In addition, molecular dynamics (MD) simulations were employed in order to interpret the observed differences in physical stability. Results, which were verified via differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR), suggested the formation of similar intermolecular interactions in all cases, indicating that the interaction with moisture water plays a more crucial role in ASD physical stability compared to the formation of intermolecular interactions between ASD components.

Inhomogeneous Distribution of Components in Solid Protein Pharmaceuticals: Origins, Consequences, Analysis, and Resolutions

Successful development of stable solid protein formulations usually requires the addition of one or several excipients to achieve optimal stability. In these products, there is a potential risk of an inhomogeneous distribution of the various ingredients, specifically the ratio of protein and stabilizer may vary. Such inhomogeneity can be detrimental for stability but is mostly neglected in literature. In the past, it was challenging to analyze inhomogeneous component distribution, but recent advances in analytical techniques have revealed new options to investigate this phenomenon. This paper aims to review fundamental aspects of the inhomogeneous distribution of components of freeze-dried and spray-dried protein formulations. Four key topics will be presented and discussed, including the sources of component inhomogeneity, its consequences on protein stability, the analytical methods to reveal component inhomogeneity, and possible solutions to prevent or mitigate inhomogeneity.

Use of Terahertz-Raman Spectroscopy to Determine Solubility of the Crystalline Active Pharmaceutical Ingredient in Polymeric Matrices during Hot Melt Extrusion

Polymer-based amorphous solid dispersions (ASDs) comprise one of the most promising formulation strategies devised to improve the oral bioavailability of poorly water-soluble drugs. Exploitation of such systems in marketed products has been limited because of poor understanding of physical stability. The internal disordered structure and increased free energy provide a thermodynamic driving force for phase separation and recrystallization, which can compromise therapeutic efficacy and limit product shelf life. A primary concern in the development of stable ASDs is the solubility of the drug in the polymeric carrier, but there is a scarcity of reliable analytical techniques for its determination. In this work, terahertz (THz) Raman spectroscopy was introduced as a novel empirical approach to determine the saturated solubility of crystalline active pharmaceutical ingredient (API) in polymeric matrices directly during hot melt extrusion. The solubility of a model compound, paracetamol, in two polymer systems, Affinisol 15LV (HPMC) and Plasdone S630 (copovidone), was determined by monitoring the API structural phase transitions from crystalline to amorphous as an excess of crystalline drug dissolved in the polymeric matrix. THz-Raman results enabled construction of solubility phase diagrams and highlighted significant differences in the solubilization capacity of the two polymer systems. The maximum stable API-load was 20 wt % for Affinisol 15LV and 40 wt % for Plasdone S630. Differential scanning calorimetry and XRPD studies corroborated these results. This approach has demonstrated a novel capability to provide real-time API-polymer phase equilibria data in a manufacturing relevant environment and promising potential to predict solid-state solubility and physical stability of ASDs.

Predicting physical stability of solid dispersions by machine learning techniques

Amorphous solid dispersion (SD) is an effective solubilization technique for water-insoluble drugs. However, physical stability issue of solid dispersions still heavily hindered the development of this technique. Traditional stability experiments need to be tested at least three to six months, which is time-consuming and unpredictable. In this research, a novel prediction model for physical stability of solid dispersion formulations was developed by machine learning techniques. 646 stability data points were collected and described by over 20 molecular descriptors. All data was classified into the training set (60%), validation set (20%), and testing set (20%) by the improved maximum dissimilarity algorithm (MD-FIS). Eight machine learning approaches were compared and random forest (RF) model achieved the best prediction accuracy (82.5%). Moreover, the RF models revealed the contribution of each input parameter, which provided us the theoretical guidance for solid dispersion formulations. Furthermore, the prediction model was confirmed by physical stability experiments of 17β-estradiol (ED)-PVP solid dispersions and the molecular mechanism was investigated by molecular modeling technique. In conclusion, an intelligent model was developed for the prediction of physical stability of solid dispersions, which benefit the rational formulation design of this technique. The integrated experimental, theoretical, modeling and data-driven AI methodology is also able to be used for future formulation development of other dosage forms.

The Investigation of Flory-Huggins Interaction Parameters for Amorphous Solid Dispersion Across the Entire Temperature and Composition Range

Amorphous solid dispersion (ASD) is one of the most promising enabling formulations featuring significant water solubility and bioavailability enhancements for biopharmaceutical classification system (BCS) class II and IV drugs. An accurate thermodynamic understanding of the ASD should be established for the ease of development of stable formulation with desired product performances. In this study, we report a first experimental approach combined with classic Flory–Huggins (F–H) modelling to understand the performances of ASD across the entire temperature and drug composition range. At low temperature and drug loading, water (moisture) was induced into the system to increase the mobility and accelerate the amorphous drug-amorphous polymer phase separation (AAPS). The binodal line indicating the boundary between one phase and AAPS of felodipine, PVPK15 and water ternary system was successfully measured, and the corresponding F–H interaction parameters (χ) for FD-PVPK15 binary system were derived. By combining dissolution/melting depression with AAPS approach, the relationship between temperature and drug loading with χ (Φ, T) for FD-PVPK15 system was modelled across the entire range as χ = 1.72 − 852/T + 5.17·Φ − 7.85·Φ². This empirical equation can provide better understanding and prediction for the miscibility and stability of drug-polymer ASD at all conditions.

Design and Evaluation of a Poly(Lactide-co-Glycolide)-Based In Situ Film-Forming System for Topical Delivery of Trolamine Salicylate

Trolamine salicylate (TS) is a topical anti-inflammatory analgesic used to treat small joint pain. The topical route is preferred over the oral one owing to gastrointestinal side effects. In this study, a poly(lactide-co-glycolide) (PLGA)-based in situ bio-adhesive film-forming system for the transdermal delivery of TS was designed and evaluated. Therefore, varying amounts (0%, 5%, 10%, 20%, and 25% (w/w)) of PLGA (EXPANSORB® DLG 50-2A, 50-5A, 50-8A, and 75-5A), ethyl 2-cyanoacrylate, poly (ethylene glycol) 400, and 1% of TS were dissolved together in acetone to form the bio-adhesive polymeric solution. In vitro drug permeation studies were performed on a vertical Franz diffusion cell and dermatomed porcine ear skin to evaluate the distinct formulations. The bio-adhesive polymeric solutions were prepared successfully and formed a thin film upon application in situ. A significantly higher amount of TS was delivered from a formulation containing 20% PLGA (45 ± 4 µg/cm2) and compared to PLGA-free counterpart (0.6 ± 0.2 µg/cm2). Furthermore, the addition of PLGA to the polymer film facilitated an early onset of TS delivery across dermatomed porcine skin. The optimized formulation also enhanced the delivery of TS into and across the skin.

Analytical and Computational Methods for the Estimation of Drug-Polymer Solubility and Miscibility in Solid Dispersions Development

The development of stable solid dispersion formulations that maintain desired improvement of drug dissolution rate during the entire shelf life requires the analysis of drug-polymer solubility and miscibility. Only if the drug concentration is below the solubility limit in the polymer, the physical stability of solid dispersions is guaranteed without risk for drug (re)crystallization. If the drug concentration is above the solubility, but below the miscibility limit, the system is stabilized through intimate drug-polymer mixing, with additional kinetic stabilization if stored sufficiently below the mixture glass transition temperature. Therefore, it is of particular importance to assess the drug-polymer solubility and miscibility, to select suitable formulation (a type of polymer and drug loading), manufacturing process, and storage conditions, with the aim to ensure physical stability during the product shelf life. Drug-polymer solubility and miscibility can be assessed using analytical methods, which can detect whether the system is single-phase or not. Thermodynamic modeling enables a mechanistic understanding of drug-polymer solubility and miscibility and identification of formulation compositions with the expected formation of the stable single-phase system. Advance molecular modeling and simulation techniques enable getting insight into interactions between the drug and polymer at the molecular level, which determine whether the single-phase system formation will occur or not.

Modelling phase separation in amorphous solid dispersions

Much work has been devoted to analysing thermodynamic models for solid dispersions with a view to identifying regions in the phase diagram where amorphous phase separation or drug recrystallization can occur. However, detailed partial differential equation non-equilibrium models that track the evolution of solid dispersions in time and space are lacking. Hence theoretical predictions for the timescale over which phase separation occurs in a solid dispersion are not available. In this paper, we address some of these deficiencies by (i) constructing a general multicomponent diffusion model for a dissolving solid dispersion; (ii) specializing the model to a binary drug/polymer system in storage; (iii) deriving an effective concentration dependent drug diffusion coefficient for the binary system, thereby obtaining a theoretical prediction for the timescale over which phase separation occurs; (iv) calculating the phase diagram for the Felodipine/HPMCAS system; and (iv) presenting a detailed numerical investigation of the Felodipine/HPMCAS system assuming a Flory-Huggins activity coefficient. The numerical simulations exhibit numerous interesting phenomena, such as the formation of polymer droplets and strings, Ostwald ripening/coarsening, phase inversion, and droplet-to-string transitions. A numerical simulation of the fabrication process for a solid dispersion in a hot melt extruder was also presented. STATEMENT OF SIGNIFICANCE: Solid dispersions are products that contain mixtures of drug and other materials e.g. polymer. These are liable to separate-out over time - a phenomenon known as phase separation. This means that it is possible the product differs both compositionally and structurally between the time of manufacture and the time it is taken by the patient, leading to poor bioavailability and so ultimately the shelf-life of the product has to be reduced. Theoretical predictions for the timescale over which phase separation occurs are not currently available. Also lacking are detailed partial differential equation non-equilibrium models that track the evolution of solid dispersions in time and space. This study addresses these issues, before presenting a detailed investigation of a particular drug-polymer system.

Continuous, simultaneous cocrystallization and formulation of Theophylline and 4-Aminobenzoic acid pharmaceutical cocrystals using twin screw melt granulation

In this work a cocrystal of Theophylline and 4Aminobenzoic acid was successfully produced and formulated using a hydrophilic binder with a novel continuous melt granulation approach. This melt granulation was followed with direct compression to generate oral solid dosage forms. The study revealed that the processing temperature, molecular weight of the binder and binder concentration were the most effective parameters for the production and formulation of high purity cocrystals. Superior tableting performance was observed for melt granulated cocrystals as compared with extruded cocrystals and pure theophylline. Moreover the prepared THP-4ABA melt granulated cocrystals were stable for 14 days (50 °C and 75% RH).

Synergistic effect of Soluplus and hyaluronic acid on the supersaturation maintenance of lovastatin: The facilitated in vitro-in vivo performance and improved physical stability

The objective of present study was to explore whether polysaccharide could be employed as potential crystal growth inhibitor and provides synergistic effect on the supersaturation maintaining of lovastatin (LOV) in combination of nucleation inhibitor. Soluplus (SOL) and hyaluronic acid (HA) were selected as the most effective nucleation and crystal growth inhibitor respectively. The interaction between SOL and HA was elucidated via characterizing the particle size, zeta potential, surface hydrophobicity, solvent relaxation time (T₂) and FT-IR. The supersaturated drug solution was spray dried into amorphous solid dispersion, then, the in vitro release, moisture uptake and physical stability were investigated. The synergistic effect between SOL and HA was dependent on drug concentration, drug/carrier and SOL/HA weight ratio, which facilitated both in vitro and in vivo performance. It was disclosed that HA could insert into SOL structure providing both electrostatic and steric stabilization. In conclusion, the combination of nucleation and crystal growth inhibitors is a promising approach for supersaturated drug delivery system.

Thermodynamic Principles for the Design of Polymers for Drug Formulations

Polymers play an essential role in drug formulation and production of medical devices, implants, and diagnostics. Following drug discovery, an appropriate formulation is selected to enable drug delivery. This task can be exceedingly challenging owing to the large number of potential delivery methods and formulation and process variables that can interact in complex ways. This evolving solubility challenge has inspired an increasing emphasis on the developability of drug candidates in early discovery as well as various advanced drug solubilization strategies. Among the latter, formulation approaches that lead to prolonged drug supersaturation to maximize the driving force for sustained intestinal absorption of an oral product, or to allow sufficient time for injection after reconstitution of a parenteral lyophile formulation, have attracted increasing interest. Although several kinetic and thermodynamic components are involved in stabilizing amorphous dispersions, it is generally assumed that maximum physical stability, defined in terms of inhibition of drug crystallization, requires that the drug and excipient remain intimately mixed. Phase separation of the drug from its excipient may be the first step that ultimately leads to crystallization. We discuss the role of advanced thermodynamics using two examples: ASD and vitamin E-stabilized ultrahigh-molecular weight polyethylene implants.

Synergetic effect of nucleation and crystal growth inhibitor on in vitro-in vivo performance of supersaturable lacidipine solid dispersion

Limited supersaturation maintaining duration is the main challenge for amorphous solid dispersion design. Nucleation or crystal growth inhibitors may function in different ways but the combination use of nucleation and crystal growth inhibitors in supersaturated system is rarely explored. Thus, using Lacidipine (LCDP) as a Biopharmaceutical Classification System (BCS) II model drug, the aim of this study was to explore whether the combination use of nucleation and crystal growth inhibitors could provide a synergistic effect on the in vitro-in vivo performance of poorly water-soluble drugs. First of all, based on compatibility screening using solubility parameter (Δδ) and crystallization inhibition efficiency as criteria, soluplus (SOL) and gum arabic (GA) were selected as the most effective nucleation and crystal growth inhibitor respectively. Thereafter, the supersaturated drug solutions were spray dried and characterized. The in vitro release, physical stability as well as pharmacokinetic behavior were investigated. It was found that the combination use of SOL and GA did not present remarkable advantage in prolonging the supersaturation time in solution state. However, their synergistic effect in equilibrium solubility and dissolution enhancement was noticed at SOL/GA ratio 3:1, with 5-7 times higher dissolution rate observed for LCDP/SOL/GA based formulation compared with that of LCDP/SOL, which was maintained even after three months accelerated stability test under non-sink condition. Moreover, compared to the LCDP/SOL formulation, approximately 2.8 and 2.5-fold increase in the maximum plasma concentration (C_max) and the area under the plasma-time curve (AUC_0-∞) was achieved with LCDP/SOL/GA based formulation. Possible mechanism of the synergistic effect was elucidated, indicating GA may penetrate into SOL particles providing both electrostatic and steric stabilization. In conclusion, the combination use of screened nucleation and crystal growth inhibitors might be an efficient approach to design supersaturated drug delivery system.

Fluorescence-based techniques to assess the miscibility and physical stability of a drug–lipid complex

The objective of this study was to evaluate the feasibility of using fluorescence-based techniques to assess the miscibility and physical stability of a drug–lipid complex pharmaceutical dosage form under a solvent-free condition. An indomethacin–phospholipid complex (IDM–DPC) was used as model complex for this study. The miscibility of indomethacin within the phospholipid was assessed by fluorescence spectroscopy, fluorescence microscopy, and infrared spectroscopy. The miscibility limit of the complex system was determined by fluorescence to be 20%–30% drug loading content, showing good correlation with infrared spectroscopy. The physical stability of the IDM–DPC stored at 40 °C was evaluated by fluorescence microscopy. Indomethacin formulated in the lipid complex with an indomethacin loading not more than 30% remained in an amorphous state within a period of 21 days, whereas the samples with a drug loading over 30% started to crystallize earlier with increasing drug content. IDM–DPC having higher miscibilities were found to be more resistant to recrystallization under heating, thus having better physical stability. Fluorescence-based techniques showed convenience and promise in characterizing drug–lipid miscibility and predicting storage stability under a solvent-free condition.

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.

Assessing Physical Stability Risk Using the Amorphous Classification System (ACS) Based on Simple Thermal Analysis

The purpose of this study is to develop a classification system utilizing milligram amounts of the compound for physical stability ranking of amorphous pharmaceuticals, which can be used as an early risk assessment tool for amorphous solid dispersion formulations. Simple thermal analysis utilizing a differential scanning calorimeter is used to characterize amorphous pharmaceuticals with respect to their molecular mobility and configurational entropy. Molecular mobility and configurational entropy are considered as two critical factors in determining the physical stability of amorphous phases. Theoretical arguments and numerical simulations suggest that the fragility strength parameter is a good indicator of the molecular mobility below T_g, and the heat capacity change at T_g is a good indicator of the configurational entropy. Using these two indicators, 40 structurally diverse pharmaceuticals with known physical stability were analyzed. Four classes of compounds are defined with class I being the most stable and class IV the least stable. The proposed amorphous classification system and methodology for estimating molecular mobility and configurational entropy provides an easily accessible framework to conduct early risk assessments related to physical stability challenges in developing amorphous formulations.

Miniaturized Measurement of Drug-Polymer Interactions via Viscosity Increase for Polymer Selection in Amorphous Solid Dispersions

Drug-polymer interactions have a substantial impact on stability and performance of amorphous solid dispersions (ASD) but are difficult to analyze. Whereas there are many screening methods described for polymer selection based for example on glass forming ability, drug-polymer miscibility, supersaturation, or inhibition of recrystallization, the distinct detection of physico-chemical interactions mostly lacks miniaturized techniques. This work presents an interaction screening assessing the relative viscosity increase between highly concentrated polymer solutions with and without the model drug ketoconazole (KTZ). The fluorescent molecular rotor 9-(2-carboxy-2-cyanovinyl)julolidine was added to the solutions in a miniaturized setup in μL-scale. Due to its environment-sensitive emission behavior, the integrated fluorescence intensity can be used as a viscosity dye within this screening approach (FluViSc). Differences in relative viscosity increases through addition of KTZ were proposed to rank polymers regarding KTZ-polymer interactions. Absolute viscosities were measured with a cone-plate rheometer as a complimentary method and supported the results acquired by the FluViSc. Solid-state nuclear magnetic resonance (ss-NMR) relaxation time measurements and Raman spectroscopy were utilized to investigate drug-polymer interactions at a molecular level. Whereas Raman spectroscopy was not suited to reveal KTZ-polymer interactions, ss-NMR relaxation time measurements differentiated between the selected polymeric carriers hydroxypropylmethylcellulose acetate succinate (HPMCAS) and polyvinylpyrrolidone vinyl acetate 60:40 (PVP-VA64). Interactions were detected for HPMCAS/KTZ ASD while there was no hint for interactions between KTZ and PVP-VA64. These results were in correlation with the FluViSc. The findings were correlated with the dissolution performance of ASD and found to be predictive for supersaturation and inhibition of precipitation during dissolution.

Data Analytics Approach for Rational Design of Nanomedicines with Programmable Drug Release

Drug delivery vehicles can improve the functional efficacy of existing antimicrobial therapies by improving biodistribution and targeting. A critical property of such nanomedicine formulations is their ability to control the release kinetics of their payloads. The combination of (and interactions among) polymer, drug, and nanoparticle properties gives rise to nonlinear behavioral relationships and large data space. These factors complicate both first-principles modeling and screening of nanomedicine formulations. Predictive analytics may offer a more efficient approach toward the rational design of nanomedicines by identifying key descriptors and correlating them to nanoparticle release behavior. In this work, antibiotic release kinetics data were generated from polyanhydride nanoparticle formulations with varying copolymer compositions, encapsulated drug type, and drug loading. Four antibiotics, doxycycline, rifampicin, chloramphenicol, and pyrazinamide, were used. Linear manifold learning methods were used to relate drug release properties with polymer, drug, and nanoparticle properties, and key descriptors were identified that are highly correlated with release properties. However, these linear methods could not predict release behavior. Nonlinear multivariate modeling based on graph theory was then used to deconvolute the governing relationships between these properties, and predictive models were generated to rapidly screen lead nanomedicine formulations with desirable release properties with minimal nanoparticle characterization. Release kinetics predictions of two drugs containing atoms not included in the model showed good agreement with experimental results, validating the model and indicating its potential to virtually explore new polymer and drug pairs not included in the training data set. The models were shown to be robust after the inclusion of these new formulations, in that the new inclusions did not significantly change model regression. This approach provides the first step toward the development of a framework that can be used to rationally design nanomedicine formulations by selecting the appropriate carrier for a drug payload to program desirable release kinetics.

Processability of AquaSolve™ LG polymer by hot-melt extrusion: Effects of pressurized CO2 on physicomechanical properties and API stability

The objective of this study was to investigate the processability of AquaSolve™ hydroxypropyl methylcellulose acetate succinate L grade (HPMCAS LG) via hot-melt extrusion and to examine the effect of pressurized carbon dioxide (P-CO₂) on the physicomechanical properties of efavirenz (EFA)-loaded extrudates. To optimize the process parameters and formulations, various physical mixtures of EFA (30%, 40%, and 50%, w/w) and HPMCAS LG (70%, 60%, and 50%, w/w), respectively, were extruded using a co-rotating twin-screw extruder with a standard screw configuration, with P-CO₂ injected into zone 8 of the extruder. Thermal characterization of the extrudates was performed using differential scanning calorimetry and thermogravimetric analysis. Scanning electron microscopy was employed to study the morphology and porosity of the formulations. Notably, the macroscopic morphology changed to a foam-like structure by P-CO₂ injection resulting in an increased specific surface area, porosity, and dissolution rate. Thus, HPMCAS LG extrusion, coupled with P-CO₂ injection, yielded faster dissolving extrudates. Stability studies indicated that HPMCAS LG was able to physically and chemically stabilize the amorphous state of high-dose EFA. Furthermore, the milling efficiency of the extrudates produced with P-CO₂ injection improved because of their increased porosity.

Effects of Processing on a Sustained Release Formulation Prepared by Twin-Screw Dry Granulation

Dry granulation is an indispensable process used to improve the flow property of moisture-sensitive materials. Considering the limitations of currently available dry granulation techniques, it is necessary to develop a novel technique. In this study, a twin-screw dry granulation (TSDG) technology was successfully applied to produce a sustained-release dry granule formulation, which was subsequently compressed into sustained-release tablets. Based on a preliminary study, theophylline was selected as model drug, Klucel^™ EF, Ethocel^™, and magnesium stearate were selected as excipients. A Resolution V Irregular Fraction Design was applied to determine the effect of different processing parameters (screw speed, feeding rate, barrel temperature, and screw configuration) on product properties (flow properties, particle size distribution, and dissolution time). A reliable model was achieved by combining the data obtained, and processing parameters were automatically optimized to attain the setting goal. In general, TSDG was demonstrated to be an alternative method for the preparation of dry granules. The continuous processing nature, simplicity of operation, and ease of optimization made TSDG competitive compared with other conventional dry granulation techniques.

Formulation and performance of Irbesartan nanocrystalline suspension and granulated or bead-layered dried powders - Part I

Nanocrystalline suspensions offer a promising approach to improve the dissolution rate of BCS Class II/IV drugs and hence oral bioavailability. Irbesartan (crystalline Form B), a poorly soluble drug substance was chosen as a model compound for the study. The objectives of the study were to formulate Irbesartan nanocrystalline suspension via media milling, study the effects of process and formulation variables on particle size reduction, and evaluate bead layering or spray granulation as drying processes. A Design of Experiment approach was utilized to understand the impact of formulation variables on particle size reduction via media milling. Drug concentration and type of stabilizer were found to be significant in particle size reduction. Optimized Irbesartan nanocrystalline suspension (i.e. at 10% w/w with 1% w/w poloxamer 407) showed superior in vitro dissolution profile compared to unmilled suspension. Optimized Irbesartan nanocrystalline suspension was converted into dried powders either by bead layering (with microcrystalline cellulose) or by spray granulation (either with mannitol or microcrystalline cellulose). DSC and PXRD studies revealed that Irbesartan remained crystalline post drying. Microcrystalline cellulose beads layered with Irbesartan nanocrystals showed about 65% drug dissolution within the first 10 min of dissolution study. Mannitol granules containing Irbesartan nanocrystals were fast dissolving (i.e. >90% drug dissolution within 10 min) compared to microcrystalline cellulose granules (i.e. approx. 46% drug dissolution within 10 min). Irbesartan nanocrystalline suspension had the fastest dissolution rates (i.e. >90% drug dissolution in two minutes) followed by mannitol-based granules containing dried Irbesartan nanocrystals (i.e. >90% drug dissolution in ten minutes).

Effect of Carrier Lipophilicity and Preparation Method on the Properties of Andrographolide⁻Solid Dispersion

Solid dispersion (SD) is a useful approach to improve the dissolution rate and bioavailability of poorly water-soluble drugs. This work investigated the effects of carrier material lipophilicity and preparation method on the properties of andrographolide (AG)–SD. The SDs of AG and the carrier materials, polyethylene glycol (PEG) and PEG grafted with carbon chains of different length (grafted PEG), have been prepared by spray-drying and vacuum-drying methods. In AG–SDs prepared by the different preparation methods with the same polymer as carrier material, the intermolecular interaction, 5% weight-loss temperature, the melting temperature (T_m), surface morphology, crystallinity, and dissolution behavior have significant differences. In the AG–SDs prepared by the same spray-drying method with different grafted PEG as carrier material, T_m, surface morphology, crystallinity, and dissolution behavior had little difference. In the AG–SDs prepared by the same vacuum-drying method with different grafted PEG as carrier material, the crystallinity and T_m decreased, and the dissolution rate of AG increased with the increase of grafted PEG lipophilicity. The preparation method has an important effect on the properties of SD. The increase of carrier material lipophilicity is beneficial to the thermal stability of SD, the decrease of crystallinity and the increase of dissolution rate of a poorly water-soluble drug in the SD.