The Combined Use of Imaging Approaches to Assess Drug Release from Multicomponent Solid Dispersions
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Imaging methods were used as tools to provide an understanding of phenomena that occur during dissolution experiments, and ultimately to select the best ratio of two polymers in a matrix in terms of enhancement of the dissolution rate and prevention of crystallization during dissolution.
Magnetic resonance imaging, ATR-FTIR spectroscopic imaging and Raman mapping have been used to study the release mechanism of a poorly water soluble drug, aprepitant, from multicomponent amorphous solid dispersions. Solid dispersions were prepared based on the combination of two selected polymers - Soluplus, as a solubilizer, and PVP, as a dissolution enhancer. Formulations were prepared in a ratio of Soluplus:PVP 1:10, 1:5, 1:3, and 1:1, in order to obtain favorable properties of the polymer carrier.
The crystallization of aprepitant during dissolution has occurred to a varying degree in the polymer ratios 1:10, 1:5, and 1:3, but the increasing presence of Soluplus in the formulation delayed the onset of crystallization. The Soluplus:PVP 1:1 solid dispersion proved to be the best matrix studied, combining the abilities of both polymers in a synergistic manner.
Aprepitant dissolution rate has been significantly enhanced. This study highlights the benefits of combining imaging methods in order to understand the release process.
KEY WORDSamorphous solid dispersion confocal Raman spectroscopy crystallisation FT-IR spectroscopic imaging magnetic resonance imaging
Attenuated total reflection-Fourier transform infrared spectroscopy
Biopharmaceutics classification system
Differential scanning calorimetry
Dynamic vapour sorption
Fourier transform infrared spectroscopy
High performance liquid chromatography
Magnetic resonance imaging
Multi slice multi echo
Modulated temperature differential scanning calorimetry
United States pharmacopeia
Amorphous solid dispersions are widely used to enhance dissolution rates and absorptions of orally administered formulations of poorly water-soluble drugs. The formation of an amorphous solid dispersion involves the combination of two or more chemically distinct components – typically a poorly soluble, hydrophobic drug and a readily soluble, hydrophilic polymer – into a single matrix (1, 2, 3). Amorphous solid dispersions can be formed either by mixing the components in the molten state, followed by cooling (e.g. hot-melt extrusion process), or by dissolving them in a common solvent, followed by rapid evaporation (e.g. spray drying process). Depending upon the nature of the components and their ratio in the matrix, pharmaceutical formulations based on amorphous solid dispersions can suffer from thermodynamic instability, resulting in unexpected crystallization of the drug in the solid state during storage or during dissolution. Consequently, this causes a reduction in the amount of the drug bioavailability (4,5).
The dissolution of tablets formed from amorphous solid dispersions is a relatively complex process where several phenomena occurr simultaneously (6). These include the hydration, water ingress, swelling and erosion of the polymer matrix, as well as the diffusion of the drug across the swollen gel layer and into the bulk solution. As the local drug:polymer:solvent ratio varies in different regions of the dissolving tablet, due to different diffusion coefficients of each component, the drug can reach a locally supersaturated state that leads to crystallization. The crystallization (nucleation and crystal growth) is influenced by multiple factors such as the degree of supersaturation, the viscosity of the polymer gel, and the interfacial energy between the crystal nuclei and the solvent (7). In this context, the polymer plays an important role as it can keep the drug in the supersaturated state and therefore inhibit or delay crystallization (5) through a combination of viscosity and surface-energy effects.
From a design of formulation perspective, the selection of the most suitable polymers must reflect processability during the preparation of amorphous solid dispersions, stability of the amorphous form of drug during storage, and the ability to control drug release and inhibit crystallization during dissolution (8, 9, 10). Often, a single polymer will not guarantee all the above-mentioned properties simultaneously. For example, although polymers with a strong affinity towards the drug molecules via hydrogen bonding or hydrophobic interactions could be effective at preventing crystallization (11, 12, 13, 14, 15, 16), they might at the same time restrict the ingress of water into the tablet, resulting in sub-optimal release profiles.
In our recent work (9,17,18), we have shown that both Soluplus (an amphiphilic polymer) and polyvinylpyrrolidone (PVP) were able to form stable amorphous solid dispersions with aprepitant at a drug:polymer ratio of 1:3 by weight. However, neither polymer alone could provide an ideal drug release profile. While Soluplus was able to suppress crystallization, the release rate was limited by the slow diffusion of water into the matrix. On the other hand, the release of aprepitant from a PVP matrix was way too fast, resulting in crystallization of the drug. Therefore, it was suggested that a combination of polymers with different (even opposite) properties in a mixed matrix could result in the favorable characteristics from each of the components in the final formulation (19,20).
In order to rationalize the selection of polymers for the mixed-matrix formulations, it is important to understand the underlying mechanism of drug release and the molecular interactions between individual components during dissolution. To this end, it is beneficial to combine standard USP-type dissolution tests with chemically specific, spectroscopic imaging-based analytical approaches such as attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopic imaging (21, 22, 23, 24), magnetic resonance imaging (MRI) (25,26), UV imaging (27,28) and Raman imaging (29,30). These techniques allow the visualization of dynamic physico-chemical processes within a tablet under dissolution conditions, making it possible to elucidate phenomena that could not be easily identifiable from the USP release curve. Since each imaging method is based on different physical principles with a corresponding difference in the chemical, spatial and temporal resolution, their combination may be necessary to reveal a full picture of the dissolution process (17,31,32).
Materials and Methods
The drug aprepitant was kindly provided by Zentiva, k.s. (Prague, Czech Republic). Aprepitant is a poorly water-soluble drug (category II) according to the Biopharmaceutics Classification System (BCS) criteria. Two different polymers were used as matrix materials for the amorphous solid dispersions. Polyvinylpyrrolidone K30 (PVP), obtained from BASF (Germany), is a water soluble polymer with a molecular weight of 30 000 g/mol. Soluplus (polyvinyl caprolactam–polyvinyl acetate–polyethylene glycol graft copolymer), obtained from BASF (Germany), is an amphiphilic, solubility enhancing excipient with an average molecular weight of 118 000 g/mol.
Preparation of Solid Dispersions
Amorphous solid dispersions were prepared by spray drying. The drug:polymer ratio in the solid dispersions was fixed at 1:3 by weight, where the polymer matrix was composed of systematically varying Soluplus:PVP ratios ranging from 1:0, 1:1, 1:3, 1:5, 1:10 to 0:1. To prepare the amorphous dispersion, aprepitant (1.0 g) was dissolved in ethanol (150 ml), the solution was mixed at 40°C for 15 min, and the required amount of the polymers was added until complete dissolution to achieve an overall drug:polymer ratio of 1:3 w/w. The solution was spray dried using the Mini Spray Dryer B-290 (Büchi, Switzerland) with an inert nitrogen loop. The spray-dried particles were subsequently compressed to tablets (140 mg, round flat shape, 7 mm in diameter) at a compression force 5 kN. In addition to a formulation where both polymers and the drug were spray dried together, tablets compressed from a physical blend of spray dried amorphous solid dispersion made of aprepitant:PVP in ratio 1:3, and admixed spray dried particles of pure Soluplus, were formed.
Differential Scanning Calorimetry
The glass transition temperatures of the solid dispersions were measured by modulated temperature differential scanning calorimetry (MTDSC) immediately after preparation. DSC measurements were performed on a TA Instruments, Discovery DSC apparatus. The samples were weighed in aluminum pans (40 μl), covered and measured in a nitrogen flow. Investigations were performed in a temperature range of 0 to 300°C with a heating rate of 5°C/min (amplitude = 0.8°C, period = 60 s). The average weight of the sample was approximately 4–5 mg.
Magnetic Resonance Imaging
The Magnetic Resonance Imaging (MRI) Desktop System Icon (Bruker BioSpin, Germany) was used to observe the water ingress into tablets and structural changes in the gel layer during dissolution. The MRI analysis was based on multi-slice-multi-echo (MSME) sequences with echo time 25 ms, repetition time 1500 ms, number of averages 2, number of repetitions 1. The images were weighted by relaxation times T1. The resolution of the images was 128 × 128 pixels for a field of view 1.8 × 1.8 cm. The slice thickness was 1 mm. The first scan was used to localize the position of the tablet in the flow cell and choose the number, position and thickness of slices. The dissolution medium was water at a flow rate of 5 ml/min and room temperature. The scans were taken every 8 min. Further details of the experimental set-up can be found in (33).
Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) Spectroscopy and Spectroscopic Imaging
FTIR spectra for all of the pure material and formulations studied in this investigation were measured using an Alpha-P spectrometer (Bruker, UK) in ATR mode, fitted with a diamond crystal. Spectra were recorded across the range of 4000–600 cm−1, using a spectral resolution of 8 cm−1 and 32 co-added scans.
To collect ATR-FTIR spectroscopic images of the dissolving tablet compacts, an ATR accessory (Pike, USA) fitted with a zinc selenide (ZnSe) crystal was employed. This ATR accessory was placed in an IMAC sampling compartment that was attached to an FTIR spectrometer (Equinox 55, Bruker) and the imaging data was recorded using a focal plane array (FPA) detector. Spectra in the mid-IR region between 4000 and 900 cm−1 was recorded for all of the dissolution experiments using a spectral resolution of 8 cm−1 and 32 co-added scans. The FPA detector was setup to record an array size of 96 × 96 pixels, meaning that 9216 individual FTIR spectra were recorded in a single experiment. This resulted in spectroscopic images with dimensions of approximately 7.75 × 6.05 mm2 and a spatial resolution of 100–150 μm (34).
The Specific Integration Ranges of Spectral Bands Used to Generate ATR-FTIR Spectroscopic Images for the Different Components of Interest in this Investigation
Spectral band peak/cm−1
Dissolution Methodology Under ATR-FTIR Spectroscopic Imaging
The dissolution experiments were setup by positioning the tablet compact in the centre of the 20 mm ZnSe crystal. A custom designed Perspex flow cell (35) placed above the tablet and a rubber O-ring was used to form a seal between the ZnSe crystal and the flow cell. Furthermore, the Perspex flow cell was used to provide sufficient pressure from the top of the tablet tablet (3 mm in diameter) that allowed contact to be achieved that allowed contact to be achieved between the sample and the ATR crystal. Good contact is needed for collection of reliable spectral information when using the ATR sampling methodology since the penetration of the evanescent IR beam is typically up to 10 μm beyond the surface of the crystal.
The dissolution medium (distilled water) was pumped through the flow cell at a rate of 5 ml/min. It should be realised that when using this setup the tablet compact is sandwiched between the flow cell and the measuring surface of the ZnSe crystal, meaning that the dissolution medium only contacts the side, not the top or bottom, surface of the tablet.
The spectra of pure components were measured by the inVia Reflex confocal Raman microscope (Renishaw, UK) at an excitation wavelength of 785 nm (operated at laser power of 50 mW) with an integration time of 0.5 s. The spectral range was 1800–730 cm−1 and the spectra were obtained from amorphous spray dried powder or the initial crystalline drug.
In situ dissolution behavior of amorphous solid dispersions was studied using the inVia Reflex confocal Raman microscope (Renishaw, UK) with a specifically designed cells enabling the measurement of dissolution under water in stagnant conditions or under flow (for details of the flow cell, see Supplementary Information 1). The dissolution medium was distilled water at room temperature and a flow rate of 5 ml/min. The x-y surface area scans were used to measure the possible crystallization of aprepitant during dissolution every 5 min. By focusing on the surface of the tablet at each time interval (the tablet swells or dissolves upon dissolution, therefore the z-coordinate of the imaging area must be adjusted), x-y surface area scans (20 × 20 μm2), with a 2 μm step size using a 48× immersion objective were performed. Subsequently, the spectral data sets were background and cosmic rays corrected, processed by the intensity of unique bands of amorphous (1007 cm−1) and crystalline (1043 cm−1) aprepitant, and converted to false-color images by the software Wire 4.1.
USP Dissolution Testing
In addition to imaging-based dissolution experiments (MRI, ATR-FTIR spectroscopic imaging, Raman mapping), standard in vitro dissolution testing was performed according to the United States Pharmacopeia (USP) type I method. The dissolution tests were conducted using a Sotax semi-automated system (Sotax AT7) with a validated analytical method (HPLC Waters 2695 Alliance with UV detection). The samples were filtered using a single filter with a pore size of 40 μm. The dissolution profile was measured with baskets at 100 rpm in 150 ml of distilled water at room temperature. The concentration of aprepitant in the solution was determined at sampling intervals ranging from 30 to 120 min for a period of up to 510 min.
Results and Discussion
Solid State Characterization of the Solid Dispersions
The amorphous nature of the formulations prepared as described in section 2.2, where the drug is molecularly dispersed in the polymer matrix, was confirmed by DSC analysis. The glass transition temperatures of the spray-dried materials with varying composition of the polymer matrix were 138.0, 135.4, 134.0, and 127.9°C for a Soluplus:PVP ratio of 1:10, 1:5, 1:3, and 1:1, respectively. Usually, the Tg curve obeys the Gordon–Taylor equation. Tg decreases from the pure PVP with Tg 159.5°C to pure polymer Soluplus (Tg 68.2°C) depending on the ratio of both polymers in carrier. Tg of amorphous pure Aprepitant is 93.3°C. It should be realized that the aprepitant:polymer ratio was kept constant in all cases, i.e. the above ratios refer to the composition of the polymer matrix only, which represents 75% of the formulation on a mass basis. As reported in our previous work (18), the glass transition temperatures of solid dispersions with pure polymers were 142.9 and 57.8°C in the case of aprepitant:PVP and aprepitant:Soluplus, respectively.
Magnetic Resonance Imaging (MRI)
In contrast, the polymer carrier with a Soluplus:PVP ratio 1:1 shows a substantially lower rate of tablet hydration (Fig. 2b). No gel layer is formed around the tablet after contact with water within the 140 min time frame of the experiment (Fig. 3d). It seems that water penetrates to the tablet preferentially through cracks formed during dissolution (upper right-hand corner of the tablet in Fig. 2b). The lower hydration rate of the tablet can be attributed to lower hygroscopicity of Soluplus, which represents a higer proportion of the polymer matrix in this case. Crucially, MRI did not indicate any crystallization processes during the dissolution of tablets made with a Soluplus:PVP ratio 1:1 (Fig. 3d).
ATR-FTIR Spectroscopic Imaging
ATR-FTIR spectroscopic imaging has a significant advantage of providing chemically specific information about the solid dispersion components. It simultaneously measures thousands of infrared spectra and provides spatially resolved quantitative information about the concentration of the individual components in the measured area. A potential disadvantage of ATR-FTIR spectroscopic imaging is the need to physically press the tablet against the ATR crystal, which may influence the natural dissolution mechanisms by constraining water ingress and gel layer formation to just the outside surface of the tablet.
It can be expected that the depletion rate of PVP from the mixed polymer matrix would depend on the ratio of the two polymers. The depletion process depends on the rate of hydration, polymer chain disentanglement and diffusion of each polymer through the composite matrix.
For the Soluplus:PVP ratio 1:5, the crystalline phase occurs more symmetrically in a circular region corresponding to the periphery of the original tablet (Fig. 6a). The intensity is lower (despite identical drug load in the tablet), meaning that crystallisation was partially suppressed. Increasing the Soluplus:PVP ratio to 1:3 results in a further delay of the onset of crystallization, to a point that the crystalline phase no longer forms a hollow circular ring, but is restricted to a central area of the original tablet. Finally, the highest Soluplus:PVP ratio 1:1 was able to suppress crystal formation altogether beyond 180 min, which is again complementary to the observations made using MRI (Figs. 2b and 3d).
The formation of solid particles in the gel layer was observed by MRI in the previous section and identified by ATR-FTIR spectroscopic imaging as the crystalline form of aprepitant. However, the spatial localization of the crystallization event in the case of the Soluplus:PVP ratio 1:10 is indicative of a possible heterogeneous nucleation, which could be influenced by the physical presence of the ATR crystal. Heterogeneous nucleation typically occurs at lower supersaturation levels than homogeneous nucleation. Therefore, the crystallization of aprepitant could be initiated and observed sooner in the ATR-FTIR spectroscopic images because of the experimental setup in which the tablet is in direct contact with the ATR crystal that may provide nucleation points. This is in contrast to a tablet in an unrestricted environment such as the MRI dissolution cell (cf. Fig. 2). Furthermore, the spatial resolution of ATR-FTIR images is 100–150 μm for the specific optical configuration used in this study, so any initial crystallization sites forming that are significantly smaller than this size may not be resolved.
The experimental setup of the dissolution cell in Section 2.8 makes it possible to observe any structural changes (particularly drug crystallization) at the free tablet surface in contact with the dissolution medium. The surface of the tablet is not limited for water penetration or dissolution of drug, and there is no foreign surface for preferred heterogeneous nucleation. Differentiation of the amorphous and crystalline form of drug was achieved based on their individual Raman spectra. Unique bands used for the differentiation of the amorphous drug, crystalline drug, Soluplus and PVP were 1005, 1047, 1450 and 935 cm−1, respectively, similarly to those described in (18).
From the formulation perspective, the inhibition of aprepitant crystallization is possible even with admixed Soluplus. However, the amount of admixed Soluplus in the final formulation would have to be significantly higher than the amount of Soluplus in the solid dispersion carrier in order to achieve comparable inhibition of crystallization (for details of the effect of admixed Soluplus, see Supplementary Information 2).
USP Dissolution Testing
The multicomponent carriers of the solid dispersions, namely the ratio 1:10, 1:5, and 1:3, show a slow drug dissolution rate that is very similar to dissolution from the pure Soluplus matrix. However, after approx. 360 min, the dissolution rate of aprepitant was observed to slow down even further. The most ideal dissolution profile for Aprepitant was obtained for the combination Soluplus:PVP 1:1, where the dissolution rate of drug is considerably enhanced. Dissolution rate during the first 60 min is as fast as that from pure PVP, but this trend continues throughout the duration of the experiment and does not slow down even after 480 min. In this combination, the favorable properties of both polymers are manifested. The solubilisation effect of Soluplus inhibits drug precipitation in solution, while PVP improves the dissolution rate by its fast dissolution from the carrier.
The combination of three spectroscopic imaging methods MRI, ATR-FTIR spectroscopic imaging, and Raman mapping were succesfully employed to understand the mechanism of drug release from multicomponent amorphous solid dispersions. Each approach was found to complement each other and reveal important information about the tablet dissolution process. Specifically, MRI provides information about the rate of dissolution medium penetration into the tablet and the kinetics of swelling and erosion of the gel layer. ATR-FTIR spectroscopic imaging makes it possible to distinguish and characterise the individual components that make up the tablet formulation, including the structural form of the API and individual excipients. It provides information about the evolution of concentration profiles within the tablet and reveals the diffusion rate of the individual components through the tablet matrix. Raman imaging provies information about the local composition and various phase transitions (e.g. crystallization) that may occur on the surface of the tablet in contact with the dissolution medium. Finaly, a USP dissolution test provides standardized quantitative information about the rate of drug release. These techniques together provide an explanation to the phenomenon of drug crystallization during dissolution and show a global picture about the different water penetration and polymer dissolution rates that none of the techniques alone could conclusively determine.
In the specific case of aprepitant release from a mixed-matrix tablet, the Soluplus:PVP ratio 1:1 in the amorphous solid dispersion has been identified by the in vitro spectroscopic imaging approaches and dissolution tests as the best matix combining the favorable properties of both polymers for aprepitant dissolution. The drug dissolution rate has been significantly enhanced, and at the same time the drug has not precipitated during dissolution.
Crystallization was succesfully detected by each imaging technique. More specifically, MRI was able to evaluate a newly formed solid phase in gel layer while the spectroscopic imaging methods (ATR-FTIR spectroscopic imaging and Raman mapping) determined the crystallization due to the structural changes of an amorphous to a crystalline state, manifested and characterized in their respective spectra.
Prepare amorphous solid dispersions with pure polymers and their mixtures, characterize their solid state behaviour and stability by standard solid-state characterisation methods (XRD, DSC, DVS). Determine the maximum amount of API in the matrix that still forms stable amorphous solid dispersion.
Carry out dissolution tests from the amorphous solid dispersions and note any “unusual” behaviour such as a change in rate of the dissolution curve or a decrease of concentration in time, which could signal drug crystallisation or other drug release inhibiting phenomena.
Use MRI to determine the rate of dissolution medium penetration into the tablet. Using mass balance and the USP release curve, determine if the matrix hydration rate is the rate-limiting step. If so, consider a change of formulation to enhance the rate of penetration.
Use ATR-FTIR spectroscopic imaging to observe the concentration profiles of the API and individual excipients in the hydrated tablet matrix. Use the ATR-FTIR spectra to identify interactions between the API and excipients that might influence the API diffusion rate and/or its tendency to crystallise.
If there is a suspicion of API crystallisation during dissolution, use Raman mapping with surface x-y scans to identify the presence of the crystalline phase and evaluate the influence of formulation variables on the timing and extent of API crystallisation at the surface of the tablet.
It should be realised that each API and formulation has its specific behaviour and the time pressure of formulation development in the industrial context may not always allow a full and rigorous analysis to be performed. Nevertheless, we hope to have shown that the combination of standard USP dissolution tests with several complementary spectroscopic imaging methods is a powerful approach that can reveal the mechanisms and phenomena that govern drug release from amorphous solid dispersions.
ACKNOWLEDGMENTS AND DISCLOSURES
Financial support from the Specific university research MSMT (20-SVV/2016) is gratefully ackonwledged. F.S. would like to acknowledge support from the Agency for Healthcare Research of the Czech Republic (project no. 16-34342A). K.P. would like to acknowledge support from the Czechoslovak Microscopy Society (CSMS).
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