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Selective Laser Sintering 3-Dimensional Printing as a Single Step Process to Prepare Amorphous Solid Dispersion Dosage Forms for Improved Solubility and Dissolution Rate

  • Author Footnotes
    c Daniel A. Davis Jr. and Rishi Thakkar contributed equally to this manuscript.
    Daniel A. Davis Jr.
    Correspondence
    Corresponding author. Department of Molecular Pharmaceutics and Drug Delivery, The Division of Molecular Pharmaceutics and Drug Delivery, College of Pharmacy, The University of Texas at Austin, Austin, TX 78712, USA.
    Footnotes
    c Daniel A. Davis Jr. and Rishi Thakkar contributed equally to this manuscript.
    Affiliations
    Department of Molecular Pharmaceutics and Drug Delivery, College of Pharmacy, The University of Texas at Austin, Austin, TX 78712, USA
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  • Author Footnotes
    c Daniel A. Davis Jr. and Rishi Thakkar contributed equally to this manuscript.
    Rishi Thakkar
    Footnotes
    c Daniel A. Davis Jr. and Rishi Thakkar contributed equally to this manuscript.
    Affiliations
    Department of Molecular Pharmaceutics and Drug Delivery, College of Pharmacy, The University of Texas at Austin, Austin, TX 78712, USA
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  • Yongchao Su
    Affiliations
    Pharmaceutical Sciences, Merck & Co, Inc, Rahway, NJ 07065, USA
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  • Robert O. Williams III
    Affiliations
    Department of Molecular Pharmaceutics and Drug Delivery, College of Pharmacy, The University of Texas at Austin, Austin, TX 78712, USA
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  • Mohammed Maniruzzaman
    Affiliations
    Department of Molecular Pharmaceutics and Drug Delivery, College of Pharmacy, The University of Texas at Austin, Austin, TX 78712, USA
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  • Author Footnotes
    c Daniel A. Davis Jr. and Rishi Thakkar contributed equally to this manuscript.
Published:November 20, 2020DOI:https://doi.org/10.1016/j.xphs.2020.11.012

      Abstract

      This study reports the development of ritonavir-copovidone amorphous solid dispersions (ASDs) and dosage forms thereof using selective laser sintering (SLS) 3-dimensional (3-D) printing in a single step, circumventing the post-processing steps required in common techniques employed to make ASDs. For this study, different drug loads of ritonavir with copovidone were processed at varying processing conditions to understand the impact, range, and correlation of these parameters for successful ASD formation. Further, ASDs characterized using conventional and advanced solid-state techniques including wide-angle X-ray scattering (WAXS), solid-state nuclear magnetic resonance (ssNMR), revealed the full conversion of the crystalline drug to its amorphous form as a function of laser-assisted selective fusion in a layer-by-layer manner. It was observed that an optimum combination of the powder flow properties, surface temperature, chamber temperature, laser speed, and hatch spacing was crucial for successful ASD formation, any deviations resulted in print failures or only partial amorphous conversion. Moreover, a 21-fold increase in solubility was demonstrated by the SLS 3-D printed tablets. The results confirmed that SLS 3-D printing can be used as a single-step platform for creating ASD-based pharmaceutical dosage forms with a solubility advantage.

      Keywords

      Introduction

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      ElectronLaserDensity(Jmm3)=LaserPower(w)LS×HS×LT
      (1)


      Hatch spacing or hatch distance (HS) is defined as the minimum distance required between the center of one laser beam to the center of the next laser beam.
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      In-process measurement and monitoring of a polymer laser sintering powder bed with fringe projection.
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      The capability for SLS-3DP to provide patient-tailored medications and fixed-dose combinations has been established but, applying the SLS-3DP process to improve the solubility of poorly water-soluble drugs by forming an amorphous solid dispersion has not been investigated. We hypothesize that optimizing printing parameters, not previously investigated (e.g., hatch spacing), in addition to understanding the energy requirement needed to render a composition amorphous when mixing is absent, can make SLS-3DP a viable 1-step process to create final pharmaceutical dosage forms in the amorphous state.

      Materials and Methods

       Materials

      Ritonavir was purchased from Nexconn Pharmatechs Ltd. (Shenzhen, China). Polyvinylpyrrolidone-vinyl acetate copolymer, Kollidon® Va64, with an average molecular weight of 65,000 g/mol, was donated by BASF Corporation (Florham Park, NJ). FujiSil (Colloidal Silicon Dioxide) was donated by Fuji chemical industries USA, Inc. Candurin® gold sheen was donated from EMD Performance Materials (Philadelphia, PA). The desktop SLS sintratec kit was purchased and self-assembled from Sintratec AG (Brugg, Switzerland). Glass number 50 capillaries (2.0 mm) were purchased from Hampton Research Corp (Aliso Viejo, CA). Monohydrate and dihydrate sodium phosphate salts were purchased from Fisher Scientific (Pittsburg, PA). The bio-relevant dissolution media fasted state simulated intestinal fluid (FaSSIF) powder was purchased from Biorelevant.com LTD (Surrey, United Kingdom). HPLC grade acetonitrile and methanol were purchased from Fisher Scientific (Pittsburg, PA); all other chemicals and reagents were ACS grade or higher.

       Pre-Formulation and Printing Parameters

       Modulated Differential Scanning Calorimetry

      Modulated differential scanning calorimetry (mDSC) was conducted on a Q20 DSC unit (TA Instruments, New Castle, DE). 8–10 mg samples were weighed into Tzero pans using a Sartorius 3.6P microbalance (Göttingen, Germany). mDSC determined the glass transition temperature (Tg) and melting point (TM) with a heating rate of 3 °C/min from 35 to 180 °C, modulation temperature of 0.3 °C every 50 s, and nitrogen sample purge flow of 50 mL/min.
      • Ellenberger D.J.
      • Miller D.A.
      • Kucera S.U.
      • Williams R.O.
      Generation of a weakly acidic amorphous solid dispersion of the weak base ritonavir with equivalent in vitro and in vivo performance to Norvir tablet.

       Powder Blends

      A limitation of the manufacturer's design of the Sintratec printer is the large powder volume necessary to print small research scale batches, typically a minimum of 200-g batches are necessary to print. One benefit is that this powder can be recycled, as the printing process impacts only the print area specified by the stereolithography (.STL) file. Compositions of Ritonavir:Kollidon® VA64 (Va-64):candurin (10:87:3) are hereafter referred to as F1. The physical mixture blend was prepared by geometrically diluting ritonavir within Kollidon® Va64 and candurin using a mortar and pestle. The physical mixtures were passed through the 12-inch diameter, no. 170 sieve to break down any agglomerates present before the processing. To conserve the active pharmaceutical ingredient (API) after all printing parameters were explored for the F1 composition the necessary amounts of compositional components were added to the remaining F1 blend to create the F2, and subsequently, F3 blends are shown in Table 1.
      Table 1Percentages of Components Used Within Different Formulations Under Investigation.
      ComponentF1 (%)F2 (%)F3 (%)
      Ritonavir102020
      Va64877776
      Candurin333
      Silicon Dioxide
      Silicon Dioxide was added as a glidant to improve flow properties at increased ritonavir concentrations.
      001
      a Silicon Dioxide was added as a glidant to improve flow properties at increased ritonavir concentrations.

       Selective Laser Sintering 3-Dimensional Printing

      Powder blends were passed through the 12-inch diameter, no. 170 sieve to break down any agglomerates present before the processing, this sieved physical mixture was introduced to the reservoir chamber of the sintratec kit (Sintratec, Switzerland), which is an SLS based 3D printer. During the process, the equipment sweeps an appropriate amount of the physical mixture from the reservoir chamber and spreads the powder in the print chamber. The chamber temperature was set below the surface temperature to ensure the proper transfer of the physical mixture to the print chamber. A chamber temperature close to the surface temperature or higher than the surface temperature can lead to agglomeration and fusion of the blend in the reservoir chamber, which is an undesired phenomenon and can lead to print failure. The software controls the IR lamp located above the print chamber, which maintains the surface temperature. For all the formulations, the surface temperature was set at least 20 °C below the temperature required for the melt fusion of ritonavir with copovidone to form the ASD. Once the print layer reaches the set surface temperature, the laser fuses the components in the physical blend by scanning the appropriate surface at a certain scan speed and a pattern complying with the hatch spacing set in the printing parameters which leads to the formation of an ASD. The printing parameters were controlled using sintratec central software (see, Table 2). After printing the tablets, they were collected, weighed, and stored for further characterization.
      Table 2Different Formulations With Their Corresponding Printing Parameters Were Explored to Create Amorphous SLS-3DP Tablets.
      Formulation Key
      All formulations have 3% of candurin.
      % RTVL.S.S.T. (°C)C.T. (°C)H.S.Comments
      F1–P1-10102511090125Tablet could not be made
      F1–P2-1010251109025Tablet could not be made (everything melts)
      F1–P3-1010251009025Tablet had crystallinity present
      F1–P4-1010501059025Selected for 10%
      F2–P1-20 (without SiO2)20251109025Print failure (Poor flow), 1% silicon dioxide added to improve the flow
      F3–P2S-2020251109025Print failure (Everything melts)
      F3–P5S-2020501109025Print failure (Tablet sinters to surrounding powder)
      F3–P6S-2020501059025Print successful
      F3–P7S-2020751059025Print successful
      F3–P8S-20201001059025Print successful
      LS is laser speed (mm/s), ST is the surface temperature (°C), CT is chamber temperature (°C), HS is hatch spacing.
      a All formulations have 3% of candurin.

       Hot-Melt Extrusion

      The exact material composition used for 3-D printing of ASDs was processed using a co-rotating twin-screw extruder with 16 mm outer diameter (OD) (Nano-16 Twin screw extruder, Leistritz, Nuremberg, Germany). These physical blends were introduced to the extruder using a calibrated volumetric feeder (Brabender twin screw feeder with stirring agitators, Brabender Technologie, Duisburg, Germany). The extrusion was performed at a barrel temperature of 130 °C and a screw speed of 50 RPM. The extruded filaments were collected from the 2.5 mm diameter die when the extruder reached an equilibrium state (i.e., the torque of the system stabilized to 680 ± 30 Gm). The collected filaments were milled using an IKA tube mill control (IKA-Werke, Staufen, Germany) and were further subjected to solid-state characterization techniques described in the later sections of this manuscript.

       Final Composition Characterization and Performance Evaluation

       Powder X-Ray Diffraction

      Powder X-Ray Diffraction (pXRD) to evaluate crystallinity was conducted on a Rigaku MiniFlex600 (Rigaku, The Woodlands, TX). The instrument is equipped with a Cu-kα radiation source utilizing a current of 15 mA and voltage set at 40 kV. Using aluminum sample holders, a two-theta range of 10–35°was scanned at 2.0°/min using a step size of 0.02° while samples were rotated. Data were analyzed using MDI JADE 9 software (Materials Data Inc., Livermore, CA).

       Fourier-Transform Infrared Spectroscopy

      The amorphous nature of the printed tablets was evaluated by attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) on a Nicolet™ IS 50 Spectrometer (Thermo Scientific, Waltham, MA). Analysis conditions scanned from 700 to 4000 cm−1 using a resolution of 4 cm−1 with 64 scans and a germanium crystal, with constant torque applied to the sample. Omnic™ analysis software evaluated results.
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       Wide Angle X-Ray Scattering

      The advanced characterization wide angle X-Ray Scatterings (WAXS) technique evaluated trace crystallinity within the printed tablets. WAXS results were used in combination with pXRD and DSC results to confirm the amorphous nature of the printed tablets. A SAXSLab instrument (WAXSLab, Northampton, MA) equipped with a microfocus Cu K-alpha rotating anode X-ray source operated at 50 kV and 0.6 mA. The PILATUS3 R 300 K detector (DECTRIS Ltd., Philadelphia, PA) contains 3 detecting modules with an area of 83.8 × 106.6 mm2 and a pixel size of 172 × 172 μm2. Ganesha instrument software (SAXSLab, Northampton, MA) controlled the instrument; the distance between the detector and sample ranged from 0.95 to 1.45 m. Samples were packed in 2 mm glass capillaries (Hampton Research, Aliso Viejo, CA) and analyzed using an acquisition time of 300 s with a 2 mm off-centered beam stop mask. A correction was made for the 2 mm sample thickness. A correction was also made for background radiation and incident beam strength. SAXSGUi software (SAXSLab, Northampton, MA) analyzed the data.
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       Solid-State Nuclear Magnetic Resonance

      A 400 MHz Bruker Avance III HD spectrometer (Bruker Corporation, Billerica, MA) in the Biopharmaceutical NMR laboratory (BNL) at Pharmaceutical Sciences, MRL (Merck & Co., INC., West Point, PA) was utilized for solid-state nuclear magnetic resonance (ssNMR) measurements. Using a Bruker 4 mm double resonance 1H/13C magic angle spinning (MAS) probe, One-dimensional (1D) 13C cross polarization and two-dimensional (2D) 1H–13C heteronuclear correlation (HETCOR) spectra were acquired at 298 K, 12 kHz MAS, and processed utilizing Bruker TopSpin Software. In all experiments, CP utilized a linear ramped power level of 80–100 kHz, 2 ms contact time. During the acquisition, a high-power SPINAL64 proton decoupling at a field strength of 80 kHz was utilized. Additionally, 2D experiments between 1H and 13C used a contact time of 2 ms. 1H–13C CP based saturation recovery experiments through 13C observation determined the 1H spin-lattice relaxation time in the laboratory frame, T1. The determined T1 relaxation time gives insight into the compositions' miscibility and domain size.
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      The domain size can be calculated using the relaxation time, t, using the spin diffusion equation below (Eq. (2)):
      L=6Dst
      (2)


      Where L is the magnetization diffusion length that describes domain size and Ds is the spin diffusion speed. A constant of 8.0 × 10 −12 cm2/s is commonly used for rigid systems.
      • Brettmann B.K.
      • Myerson A.S.
      • Trout B.L.
      Solid-state nuclear magnetic resonance study of the physical stability of electrospun drug and polymer solid solutions.

       Method of Analysis

      Sample purity, potency, and in vitro dissolution was analyzed using a Dionex UltiMate 3000 high-pressure liquid chromatography (HPLC) system (Thermo Scientific, Sunnyvale, CA) equipped with Chromeleion 7 software for data acquisition and analysis. The HPLC method was adapted from a previously established protocol by Ellenberger et al. The aqueous phase, mobile phase A, consists of HPLC grade water. The organic phase, mobile phase B, consists of HPLC grade acetonitrile. During the analysis, the system is held isocratic (30% A, 70% acetonitrile). The flow rate was set to 1.5 mL/min with 5-min run time and a 10 μL injection volume.
      • Ellenberger D.J.
      • Miller D.A.
      • Kucera S.U.
      • Williams R.O.
      Generation of a weakly acidic amorphous solid dispersion of the weak base ritonavir with equivalent in vitro and in vivo performance to Norvir tablet.
      An ultiMate RS Variable Wavelength detector was set to 240 nm.
      • Purohit H.S.
      • Taylor L.S.
      Phase behavior of ritonavir amorphous solid dispersions during hydration and dissolution.
      The column used for separation was a 25cmx4.6 mm, 5 μm particle size, stainless steel C-18 column (Nucleosil®100-5C18 (Suppleco series), Millipore Sigma, Burlington, Massachusetts).

       Non-Sink pH-Shift Dissolution

      The final tablets printed floated when dropped into the dissolution media. The tablet weights used for the dissolution study have been mentioned in section Tablet Performance. The 10% ritonavir tablets disintegrated faster than the 20% ritonavir tablets. A small volume pH-shift dissolution using 0.01 M HCL to mimic the stomach and pH 6.8 FaSSIF to mimic the small intestine was used for the weakly basic drug ritonavir. Dissolution was performed on an SR8 Plus dissolution tester (Hanson Research Corp., Chatsworth, CA) equipped with mini-paddles. The dissolution apparatus uses 150 mL glass vessels, operated at 37 °C and 100 RPM. Tablets (n = 3) were dropped into 90 mL of 0.01 N HCL, and at 30 min 60 mL of FaSSIF (2.24 g/L SIF in 0.1 M sodium phosphate buffer) is used for the pH shift transition, bringing the total volume to 150 mL. 1 mL samples were taken and replaced with an equivalent amount of media at 5, 10, 15, 25, 35, 45, 60, 90, 120, 180, 240, and 360 min. The samples were filtered through a 0.22 μm, 13 mm PTFE syringe filter, and diluted 1:1 with methanol. The final pH was recorded after the 360-min time point. HPLC analysis determined ritonavir content at each time point.
      • Jermain S.V.
      • Lowinger M.B.
      • Ellenberger D.J.
      • Miller D.A.
      • Su Y.
      • Williams R.O.
      In vitro and in vivo behaviors of KinetiSol and spray-dried amorphous solid dispersions of a weakly basic drug and ionic polymer.

      Results

       Pre-Formulation Characterization

      As previously stated, the composition's physicochemical properties influence the printing parameter selection. Knowledge of the composition's melting point depression and glass transition temperature allows the surface temperature selection to be at or below the glass transition and below the melting point of the composition. mDSC results indicated that RTV's melting point in the physical mixture is 122 °C, Fig. 1.
      Figure thumbnail gr1
      Fig. 1mDSC plots of the compositions (a) overlay of ritonavir (RTV), physical mixture (PM): RTV + Va64+candurin, F1–P4-10, F3–P7S-20, F1–P4-10 (HME), F3–P7S-20 (HME) DSC (b) F1–P3-10 DSC depicting trace crystallinity (c) DSC of 0.6 mg RTV depicting the sensitivity of the equipment even at low drug loads.
      In addition to ritonavir's shear and thermal sensitivity, ritonavir has inherently poor flow properties, which have been demonstrated in the previous literature.
      • Mahajan N.M.
      • Malghade A.D.
      • Dumore N.G.
      • Thenge R.R.
      Design and development of crystallo-co-agglomerates of ritonavir for the improvement of physicochemical properties.
      A number 170 sieve (88 μm) was used, which removed/broke down large agglomerates from the powder blend before placing it within the SLS-3DP chamber. The Various formulations investigated to create an ASD printed tablet are shown in Table 1. As the drug load increased, silicon dioxide was added to the formulation (F3) to improve the flow properties for successful printing.
      • Majerová D.
      • Kulaviak L.
      • Růžička M.
      • Štěpánek F.
      • Zámostný P.
      Effect of colloidal silica on rheological properties of common pharmaceutical excipients.
      Increased ritonavir (Carr's index >40) drug load in formulation F2 decreased the formulation flow properties to the extent that caused printing failure. The addition of 1% glidant, silicon dioxide, improved the composition's flow sufficiently to resume printing.

       SLS-3DP Printing

      Different iterations of processing parameter's influence on RTV tablets were carried out, highlighting their respective impact on the SLS-3DP process (Table 2). The hatch spacing in formulation F1–P1-10 was too large to allow tablet sintering. In the case of F1–P2-10, when the hatch spacing was decreased, the printing time increased to the extent that the entire composition within the chamber fused, due to prolonged exposure at the selected surface temperature. As a result, in formulation F1–P3-10, the surface temperature was decreased to allow for the increased printing time; consequently, the tablet successfully printed but exhibited trace crystallinity. Finally, in formulation F1–P4-10, surface temperature and laser speed were increased to supply enough energy while minimizing exposure to elevated temperatures. F1–P4-10 completely converted crystalline ritonavir to the amorphous phase, exhibited no degradation, and produced a smooth tablet. The tablets with 10% drug load had a diameter of 12 ± 0.13 mm and a height of 5 ± 0.17 mm, and the tablets with 20% drug load had a diameter of 10 ± 0.24 mm and a height of 4 ± 0.33 mm, the dimensions of the tablets were changed to keep the drug dose similar in both formulations. The 20% tablet weight was found to be 267.4 ± 9.5 mg, and that of 10% tablet was found to be 501.3 ± 1.98 mg. The standard deviation in the weight and dimensions of the 10% tablets was smaller as compared to the 20% tablets; this can be attributed to the better flow properties of the 10% RTV blend as compared to the 20% blend, which could be the reason for the inconsistencies in printing, and in extension of the aforementioned parameters. This is merely a visual speculation and no statistical analysis was performed to investigate this observation further.
      Increasing the RTV drug load in formulations F2 and F3 impacted the SLS process; therefore, changes in the formulation were made to compensate for decreased compositional flow properties of the physical mixture. The F2–P1-20 composition was considered un-processable due to the reduced flow properties. The addition of silicon dioxide in the F3 formulation enabled successful printing at an increased drug load. Considering how sensitive ritonavir is to processing conditions, purity was tested for all formulations with no degradation observed.

       ASD Tablet Characterization

      Basic and advanced solid-state characterization techniques evaluated the amorphous nature and interactions of SLS-3DP RTV tablets.
      • Ma X.
      • Williams R.O.
      Characterization of amorphous solid dispersions: an update.
      For mDSC analysis, printed tablets were crushed using a mortar and pestle and resultantly assessed to evaluate crystallinity
      • Dedroog S.
      • Pas T.
      • Vergauwen B.
      • Huygens C.
      • Van den Mooter G.
      Solid-state analysis of amorphous solid dispersions: why DSC and XRPD may not be regarded as stand-alone techniques.
      (Fig. 1). F1–P3-10, in Fig. 1, exhibits a small endotherm, which can be attributed to trace crystallinity from the SLS-3DP process. On the other hand, the absence of an endotherm in F1–P4-10 indicates RTV was converted to its amorphous form (Fig. 1).
      • Bochmann E.S.
      • Neumann D.
      • Gryczke A.
      • Wagner K.G.
      Micro-scale prediction method for API-solubility in polymeric matrices and process model for forming amorphous solid dispersion by hot-melt extrusion.
      The selected formulation with a 20% drug load (F3–P7S-20) also showed an absence of an endotherm, which may suggest the successful conversion of the RTV in the formulation to an amorphous form (Fig. 1).
      • Bochmann E.S.
      • Neumann D.
      • Gryczke A.
      • Wagner K.G.
      Micro-scale prediction method for API-solubility in polymeric matrices and process model for forming amorphous solid dispersion by hot-melt extrusion.
      Additionally, the compositions that did not exhibit an endotherm but exhibited a single glass transition (≈75 °C) between the glass transition temperature of the individual components (e.g., 50 °C for RTV and 108 °C for Va64), suggest miscibility as per the Gordon-Taylor theory.
      • Maniruzzaman M.
      • Morgan D.J.
      • Mendham A.P.
      • Pang J.
      • Snowden M.J.
      • Douroumis D.
      Drug–polymer intermolecular interactions in hot-melt extruded solid dispersions.
      Similar observations were made for F3–P6S-20, although F3–P8S-20 observed the presence of a small endothermic peak of RTV. The observation made for F1–P3-10 hints towards the importance of surface temperature and that for F3–P8S-20 towards the importance of laser speed in the development of ASDs using SLS-3-D printing. These parameters and their impact have been discussed in-depth in later sections of this manuscript.
      Powder X-ray diffraction further corroborated the mDSC results. Candurin has a unique diffraction profile across the two-theta region analyzed, with the most intense Bragg's peak being at 19.8 and 25.2 two-theta (2θ) degrees.
      • Davis Jr., D.A.
      • Miller D.
      • Williams R.O.
      Thermally conductive excipient expands KinetSol® processing capabilities.
      These specific peaks associated with candurin are also found in the F1–P4-10 and F3–P7-20 samples that were previously shown amorphous by mDSC. The region highlighted in Fig. 2 depicts the overlap between the amorphous sample and candurin. If the presence of candurin is disregarded, a traditional broad halo would be present.
      Figure thumbnail gr2
      Fig. 2PXRD overlay of candurin, ritonavir (RTV), Kollidon Va64, physical mixture, F1–P4-10, F3–P7S-20, F1–P4-10 (HME), and F3–P7S-20 (HME). The highlighted region on the left and the right depicts the recurring candurin peak at 19.8 and 25.2 two-theta (2θ), and the highlighted region in the center depicts the absence of intense characteristic peaks of RTV in the ASDs. Scans are translated along the y-axis to aid in differentiation.
      Fourier transform infrared spectroscopy analysis can evaluate amorphous nature and interaction occurring within the composition by observing a shift in the peak to a lower wavenumber and an increase in full width at half maximum (FWHM) for equivalent crystalline to amorphous peaks, respectively.
      • Chen Y.
      • Wang S.
      • Wang S.
      • Liu C.
      • Su C.
      • Hageman M.
      • et al.
      Initial drug dissolution from amorphous solid dispersions controlled by polymer dissolution and drug-polymer interaction.
      ,
      • Sun M.
      • Wu C.
      • Fu Q.
      • Di D.
      • Kuang X.
      • Wang C.
      • et al.
      Solvent-shift strategy to identify suitable polymers to inhibit humidity-induced solid-state crystallization of lacidipine amorphous solid dispersions.
      In Fig. 3, crystalline peaks associated with ritonavir are no longer present in the amorphous F1–P4-10 and F3–P7S-20 samples. Characteristic peaks of ritonavir can be observed at 3340 cm−1 (O–H), 3110 cm−1 (N–H), 1612 cm−1, 1662 cm−1, 1704 cm−1 (C O), and 1092 cm−1 (C–O) in the pure drug sample, and that of Va64 at 1681 cm−1, and 1740 cm−1 are distinct in the pure and physical mixture samples in Fig. 3.
      • Sinha S.
      • Ali M.
      • Baboota S.
      • Ahuja A.
      • Kumar A.
      • Ali J.
      • et al.
      Solid dispersion as an approach for bioavailability enhancement of poorly water-soluble drug ritonavir.
      Figure thumbnail gr3
      Fig. 3FT-IR overlay of F1–P4-10, F3–P7S-20, and components within the compositional matrix. Scans were transposed to aid in differentiation.
      These functional groups and their intermolecular interactions are crucial for maintaining the crystalline structure of the API. In an ASD, these drug-drug intermolecular interactions are broken down, and new interactions are established, usually between the drug and the polymeric carrier. These new interactions not only signify the successful production of an ASD but also play a vital role in stabilizing the drug in its amorphous state. This is where ASDs fundamentally differ from pure amorphous drug species. Weak intermolecular interactions between two species can be identified by observing the different phenomenon in FT-IR spectra. One indicator is the disappearance or reduced intensity of functional ‘stretch’ peaks as observed for ‘O–H’ and the ‘N–H’ peaks (i.e., visible in the pure drug and physical mixtures but absent in the ASDs manufactured using both SLS and HME).
      • Thakkar R.
      • Ashour E.A.
      • Shukla A.
      • Wang R.
      • Chambliss W.G.
      • Bandari S.
      • et al.
      A comparison between lab-scale and hot-melt-extruder-based anti-inflammatory ointment manufacturing.
      Another noticeable change is the peak shifts which usually represent a change in confirmation/steric environment as seen for the ‘C–O’ stretch peaks of RTV shifting to a lower wavenumber in the ASDs.
      • Kandori H.
      • Maeda A.
      FTIR spectroscopy reveals microscopic structural changes of the Protein around the rhodopsin chromophore upon Photoisomerization.
      Finally, the formation of hybrid peaks represents the mixing of two components at a molecular level, which is expected in an ASD and can be observed by the hybrid complex formed in the ASDs at 1667 cm−1 (shift from 1612 cm−1, 1662 cm−1 for the drug and 1681 cm−1 for the polymer) and 1735 cm−1 (shift from 1704 cm−1 for the drug and 1740 cm−1 for the polymer) which represent the carbonyl and ester groups, whereas these peaks, although superimposed, are distinct in the physical mixtures tested.
      • Saboo S.
      • Kestur U.S.
      • Flaherty D.P.
      • Taylor L.S.
      Congruent release of drug and polymer from amorphous solid dispersions: insights into the role of drug-polymer hydrogen bonding, surface crystallization, and glass transition.
      Moreover, amine (-NH) peaks for the RTV can be seen at 3110 cm−1 in the pure drug and the physical mixture samples, whereas these peaks are not visible anymore in the HME and the SLS formulations. These observations suggest the possible involvement of the amine group in the drug-polymer interactions in the ASDs. These observations can be interpreted as weak intermolecular interactions between the ester groups in the polymer and the amine groups in the drug, which are key for stabilizing an ASD.
      The advanced solid-state characterization technique, wide-angle X-ray scattering (WAXS), is a sensitive technique, able to detect trace crystallinity to 0.5% API in composition.
      • Ma X.
      • Müller F.
      • Huang S.
      • Lowinger M.
      • Liu X.
      • Schooler R.
      • et al.
      Influence of carbamazepine dihydrate on the preparation of amorphous solid dispersions by hot melt extrusion.
      This advanced characterization technique was used to ensure crystallinity associated with ritonavir was not present when the drug load increased to 20%. Candurin has a unique profile that is unaffected by different processing methods. Highlighted in blue are peaks that are associated with candurin in the F3–P7S-20 sample (Fig. 4). Peaks attributed to ritonavir are not present, indicating an amorphous tablet at 20% drug load.
      Figure thumbnail gr4
      Fig. 4WAXS pattern overlay of amorphous F3–P7S-20, with candurin. The regions shaded in blue denotes peaks present in the F3–P7S-20 sample corresponding to candurin.
      The SLS-3DP processed composition, F3–P7S-20, and a reference composition containing the F3 formulation processed by HME were compared at a molecular level by ssNMR analysis. This advanced characterization technique was utilized to assess and compare the amorphous nature, interactions, domain sizes, and miscibility between two equivalent compositions formulated by two different processes, SLS-3DP, and HME.
      • Pham T.N.
      • Watson S.A.
      • Edwards A.J.
      • Chavda M.
      • Clawson J.S.
      • Strohmeier M.
      • et al.
      Analysis of amorphous solid dispersions using 2D solid-state NMR and 1H T1 relaxation measurements.
      1D CP-MAS evaluated potential differences in amorphous nature between the SLS-3DP and HME processed formulations, Fig. 5. Amorphous forms are identified by broader peaks,
      • Pham T.N.
      • Watson S.A.
      • Edwards A.J.
      • Chavda M.
      • Clawson J.S.
      • Strohmeier M.
      • et al.
      Analysis of amorphous solid dispersions using 2D solid-state NMR and 1H T1 relaxation measurements.
      in both the HME and SLS spectra there is an absence of sharper peaks indicative of crystallinity and existence of broader peaks corresponding to an amorphous composition.
      Figure thumbnail gr5
      Fig. 51D13C CP-MAS spectra of crystalline RTV, physical mixtures, HME, and SLS samples. The SLS spectrum corresponds to F3–P7S-20. The HME spectrum corresponds to the same composition utilized by the SLS process, F3, but processed using HME to act as a reference standard.
      Furthermore, 2D HETCOR experiments were leveraged to probe further potential molecular differences between SLS-3DP and HME samples by extending the analysis to an additional 1H dimension. The proton and carbon chemcial shifts associated with the SLS-3DP sample and the HME comparator are nearly identical when compared to one another, Fig. 6.
      Figure thumbnail gr6
      Fig. 613C–1H HETCOR spectra of HME and SLS samples. The SLS spectrum corresponds to F3–P7S-20. The HME spectrum corresponds to the same composition utilized by the SLS process, F3, but processed using HME to act as a reference standard.
      The 13C-detected saturation pulse sequence determined the 1H spin-lattice of ritonavir and copovidone in the laboratory time frame, T1. For the 20% drug load, spectra measurements took greater than one month to complete; therefore, data for the rotating frame, T1ρ, was not obtained. However, the miscibility of the compositions can still be claimed solely based on the T1 data at the 100-nm resolution reported.
      • Brettmann B.K.
      • Myerson A.S.
      • Trout B.L.
      Solid-state nuclear magnetic resonance study of the physical stability of electrospun drug and polymer solid solutions.
      The relaxation times with their corresponding domain sizes and miscibility are shown in Table 3.
      Table 31H spin-Lattice Relaxation Measurement Evaluating the Miscibility and Domain Size of Identical Formulations Processed by SLS-3DP and HME.
      SampleComponentT1 (s)△T1 (s)Domain Size (nm)Miscibility
      F3-HMERTV5.90 ± 0.100.20168Miscible
      Va646.10 ± 0.61171
      F3–P7S-20RTV2.25 ± 0.100.0104Miscible
      Va642.22 ± 0.07103
      In an amorphous sample, relaxation time values converge to similar values and act as excellent indicators to determine the scale of blend miscibility.
      • Paudel A.
      • Geppi M.
      • Mooter G.V.
      Structural and dynamic properties of amorphous solid dispersions: the role of solid-state nuclear magnetic resonance spectroscopy and relaxometry.
      The greater the discrepancy between relaxation times points to a heterogeneous ASD.
      • Brettmann B.K.
      • Myerson A.S.
      • Trout B.L.
      Solid-state nuclear magnetic resonance study of the physical stability of electrospun drug and polymer solid solutions.
      Table 3 shows that both formulation processes produce highly miscible systems at the T1 domain but exhibit significant differences in domain sizes from the relaxation data. The △T1 (s) of the SLS formulation is lower compared to that of the HME formulation, suggesting a higher degree of miscibility. The underlying laser-based fusion and processing parameters controlled in a layer-by-layer manner in SLS might have played a pivotal role in this observed molecular phenomenon.

       Tablet Performance

      Final Pharmaceutical dosage forms (e.g., SLS-3DP Tablets) dissolution profiles were compared to the physical mixture powder. F1–P4-10 tablet weights were: 503.86, 502.78, 520.64 mg F3–7S-20 tablet weights were: 280.02, 248.6, 269.72 mg. Physical mixture weights were: 500.23, 500.51, and 502.45 mg. The benefit of the printed amorphous solid dispersion was evaluated by a small volume pH-shift dissolution with bio-relevant media to mimic gastrointestinal transit of orally administered tablets. A 10-fold concentration increase was seen with F1–P4-10 amorphous tablets compared to the physical mixture in the acidic phase before the pH shift. A 21-fold increase in the RTV concentration was seen with F1–P4-10 amorphous tablets compared to the physical mixture at pH 6.8 near the 3-h time point. Additionally, the reduction in concentration at 240 min is the recrystallization of supersaturated ritonavir, a phenomenon common in ASDs. Similar results were observed for F3–P7S-20 tablets (Fig. 7). An improved dissolution profile was seen for F1–P4-10 at both pH 2 and pH 6.8 media, which further supports the results observed from the ssNMR data depicting that the tablets were ASDs of the drug.
      • Jermain S.V.
      • Lowinger M.B.
      • Ellenberger D.J.
      • Miller D.A.
      • Su Y.
      • Williams R.O.
      In vitro and in vivo behaviors of KinetiSol and spray-dried amorphous solid dispersions of a weakly basic drug and ionic polymer.
      ,
      • Indulkar A.S.
      • Lou X.
      • Zhang G.G.Z.
      • Taylor L.S.
      Insights into the dissolution Mechanism of ritonavir–copovidone amorphous solid dispersions: importance of congruent release for enhanced performance.
      Figure thumbnail gr7
      Fig. 7In vitro drug release profiles of F1–P4-10 tablets, F3–P7S-20 tablets, ritonavir: Va64: Candurin (10:87:3) physical mixture powder.

      Discussion

      Ritonavir's low water solubility and thermal/shear sensitivity make it challenging to formulate in an ASD.
      • Mahajan N.M.
      • Malghade A.D.
      • Dumore N.G.
      • Thenge R.R.
      Design and development of crystallo-co-agglomerates of ritonavir for the improvement of physicochemical properties.
      Moreover, Ritonavir's crystalline form is a fluffy white powder that exhibits poor flow properties.
      • Mahajan N.M.
      • Malghade A.D.
      • Dumore N.G.
      • Thenge R.R.
      Design and development of crystallo-co-agglomerates of ritonavir for the improvement of physicochemical properties.
      One of the limitations of the SLS-3DP process is processing compositions that have poor flow properties or agglomerates present.
      • Tan Y.
      • Zheng J.
      • Gao W.
      • Jiang S.
      • Feng Y.
      The effect of powder flowability in the selective laser sintering process.
      The reduced flow properties were observed to leave streaks within the powder bed, disturbing the uniformity and compromising the integrity of the printing process. In the case of ritonavir, as the drug load increased to 20%, a glidant was necessary to enable successful printing. Hausner's ratio is a means of assessing compositions flow properties, the larger the discrepancy between the bulk and tapped densities predicts poor flow properties. Ritonavir has been reported to have a Hausner's ratio of 1.52,
      • Mahajan N.M.
      • Malghade A.D.
      • Dumore N.G.
      • Thenge R.R.
      Design and development of crystallo-co-agglomerates of ritonavir for the improvement of physicochemical properties.
      therefore, placing it in the worst flow classification category (i.e., exceedingly poor). Previously, ibuprofen has been classified as having poor or very poor flow characteristics (i.e. Hausner's ratio of 1.47 ± 0.08),
      • Liu L.X.
      • Marziano I.
      • Bentham A.C.
      • Litster J.D.
      • White E.T.
      • Howes T.
      Effect of particle properties on the flowability of ibuprofen powders.
      when incorporated into the SLS-3DP process limitations attributed to low flowability have not been reported.
      • Fina F.
      • Goyanes A.
      • Gaisford S.
      • Basit A.W.
      Selective laser sintering (SLS) 3D printing of medicines.
      Therefore, for the first time, for instances where compositions exhibit poor flow properties, the addition of a glidant can make previously unprintable formulations printable. The second concern with an SLS manufacturing process is the thermal degradation of the processed powder blend,
      • DePalma K.
      • Walluk M.R.
      • Murtaugh A.
      • Hilton J.
      • McConky S.
      • Hilton B.
      Assessment of 3D printing using fused deposition modeling and selective laser sintering for a circular economy.
      especially the drug in this case. Embodying ritonavir, as the model drug within SLS-3DP, highlights the robustness of the process to formulate challenging ASDs successfully. Considering ritonavir's thermal sensitivity,
      • LaFountaine J.S.
      • Jermain S.V.
      • Prasad L.K.
      • Brough C.
      • Miller D.A.
      • Lubda D.
      • et al.
      Enabling thermal processing of ritonavir–polyvinyl alcohol amorphous solid dispersions by KinetiSol® Dispersing.
      ,
      • Ellenberger D.J.
      • Miller D.A.
      • Kucera S.U.
      • Williams R.O.
      Generation of a weakly acidic amorphous solid dispersion of the weak base ritonavir with equivalent in vitro and in vivo performance to Norvir tablet.
      minimal exposure of the composition to the SLS laser decreases the probability of RTV degradation from excess exposure to elevated temperature.
      • Titapiwatanakun V.
      • Tankul J.
      • Basit A.W.
      • Gaisford S.
      Laser irradiation to produce amorphous pharmaceuticals.
      The total energy absorbed by the composition is a function of the laser density and the composition's ability to absorb the wavelength emitted by the laser (Eq. (1)). Although thermal degradation of components has been reported in several studies exploring the application of SLS for non-pharmaceutical processes, none of the previous studies have reported the effects of the temperature and laser beam on the stability of the processed drugs as pointed out by Awad et al. (2020) in a recent review on the application of SLS in pharmaceutical processing.
      • Awad A.
      • Fina F.
      • Goyanes A.
      • Gaisford S.
      • Basit A.W.
      3D printing: principles and pharmaceutical applications of selective laser sintering.
      This is the first study to have processed APIs prone to thermal degradation using an SLS process, whilst circumventing any degradation by finetuning the processing parameters such as surface temperature and laser speed.
      Further, the effects of hatch spacing in SLS-3DP to produce successfully printed tablets have not previously been reported; from Equation (1), we know the electron laser density is inversely proportional to the laser speed, hatch spacing, and layer thickness.
      • Southon N.
      • Stavroulakis P.
      • Goodridge R.
      • Leach R.
      In-process measurement and monitoring of a polymer laser sintering powder bed with fringe projection.
      ,
      • Negarestani R.
      • Li L.
      Laser machining of fibre-reinforced polymeric composite materials.
      With all other laser parameters equal, F1–P3-10 decreased the surface temperature and hatch spacing when compared to F1–P1-10. The F1–P1-10 formulation failed to print due to insufficient energy to the system; therefore, decreasing the surface temperature would require increased energy input from the laser. F1–P3-10 successfully overcame this increased energy requirement and printed a 3DP-SLS tablet, with trace crystallinity, by decreasing the hatch spacing from 125 mm to 25 mm, illustrating the increased electron density achieved by reducing hatch spacing. Furthermore, to overcome the trace crystallinity present within the F1–P3-10, additional energy input is required. To minimize the potential degradation of RTV, the electron laser density was not increased; instead, the surface temperature was raised to supply sufficient energy to the system, creating an amorphous RTV ASD without trace crystallinity.
      In these examples, different energy thresholds are required to print a tablet versus converting a tablet to an amorphous state. The total energy applied to the system is a function of the electron laser density and the ability of the composition to absorb a percentage of the energy emitted by the laser. Each composition will have a different electron laser density necessary to overcome each threshold dependent on the composition's capacity to absorb at the wavelength emitted by the laser. For example, Fina et al. used the same printing parameters for all polymers and drug loadings; moreover, adjusting printing parameters to cater to each composition potentially could have overcome trace crystallinity issues.
      • Fina F.
      • Goyanes A.
      • Gaisford S.
      • Basit A.W.
      Selective laser sintering (SLS) 3D printing of medicines.
      ,
      • Allahham N.
      • Fina F.
      • Marcuta C.
      • Kraschew L.
      • Mohr W.
      • Gaisford S.
      • et al.
      Selective laser sintering 3D printing of orally disintegrating Printlets containing ondansetron.
      In this example, F1–P3-10 had sufficient energy to overcome the threshold to sinter the composition and produce a tablet but not enough energy to convert the tablet to the amorphous form. F1–P4-10 used a higher surface temperature to overcome the second energy threshold needed to melt the crystalline drug; this resulted in an amorphous tablet. The composition's capacity to absorb energy the laser emits allows the F3 formulations to convert to the amorphous phase when higher laser speeds are used (e.g., less electron laser density).
      A fundamental understanding of the interplay between printing parameters, chamber and surface temperature, and the composition's unique capacity to absorb a fraction of the laser's energy in combination with the compositions glass transition and melting point offers various parameters to create an SLS-3DP ASD. Selecting a surface temperature that is too close to the composition's glass transition temperature or melting point in combination with printing parameters that are too slow, leads to a print failure where the entire printing bed melts together, as was the case in F1–P2-10 and F3–P2S-20. On the other hand, increasing the laser speed, hatch spacing, or decreasing the surface temperature enough, results in trace crystallinity (e.g., F1–P3-10) and if decreased enough print failure (e.g., F1–P1-10). Lastly, if the hatch spacing is too large, the spacing in which the laser travels may be insufficient to sinter all the powder, leaving trace crystallinity within the tablet, as observed in F1–P1-10.
      To highlight the innovation of the RTV ASD printed tablet, an extensive analysis of prior publications is required to appreciate the novelty of this study. The University College of London (UCL) has published extensively on the capabilities to integrate SLS-3DP tablets into personalized patient medication.
      • Fina F.
      • Goyanes A.
      • Gaisford S.
      • Basit A.W.
      Selective laser sintering (SLS) 3D printing of medicines.
      ,
      • Awad A.
      • Fina F.
      • Goyanes A.
      • Gaisford S.
      • Basit A.W.
      3D printing: principles and pharmaceutical applications of selective laser sintering.
      • Fina F.
      • Goyanes A.
      • Madla C.M.
      • Awad A.
      • Trenfield S.J.
      • Kuek J.
      • et al.
      3D printing of drug-loaded gyroid lattices using selective laser sintering.
      • Awad A.
      • Fina F.
      • Trenfield S.J.
      • Patel P.
      • Goyanes A.
      • Gaisford S.
      • et al.
      3D printed pellets (Miniprintlets): a novel, multi-drug, controlled release platform technology.
      ,
      • Trenfield S.J.
      • Goyanes A.
      • Telford R.
      • Wilsdon D.
      • Rowland M.
      • Gaisford S.
      • et al.
      3D printed drug products: non-destructive dose verification using a rapid point-and-shoot approach.
      They have successfully demonstrated the robustness of the SLS-3DP process to reproducibly print tablets with different polymers, and the ability to print different tablet structures to modify single and multidrug release.
      • Awad A.
      • Fina F.
      • Trenfield S.J.
      • Patel P.
      • Goyanes A.
      • Gaisford S.
      • et al.
      3D printed pellets (Miniprintlets): a novel, multi-drug, controlled release platform technology.
      Predominately, studies have utilized acetaminophen (APAP), a BCS Class III drug.
      • Fina F.
      • Goyanes A.
      • Gaisford S.
      • Basit A.W.
      Selective laser sintering (SLS) 3D printing of medicines.
      To date, the benefit of incorporating a poorly water-soluble drug (e.g., BCS Class II or IV) into the SLS-3DP process to improve bioavailability by producing a fully amorphous tablet that demonstrates superior dissolution performance, has not been demonstrated.
      Previously Fina et al. characterized two different polymers, Kollicoat IR and Eudragit L 100-55, with varying APAP drug loads. Kollicoat IR 5% and 20% APAP formulations appeared amorphous by DSC but crystalline by pXRD, whereas 35% APAP and Eudragit formulations show a small melting endotherm by DSC but amorphous by pXRD, representing a lack of agreement between characterization techniques. Additionally, in the pXRD diffractogram, an undefined small peak is present in all compositions.
      • Fina F.
      • Goyanes A.
      • Gaisford S.
      • Basit A.W.
      Selective laser sintering (SLS) 3D printing of medicines.
      In a later publication, Trenfield et al. confirmed the small discrepancies mentioned in the inaugural APAP: L 100-55 study; that is, crystallinity on the surface was attributed to un-sintered powder while the tablet core was predominately amorphous.
      • Trenfield S.J.
      • Goyanes A.
      • Telford R.
      • Wilsdon D.
      • Rowland M.
      • Gaisford S.
      • et al.
      3D printed drug products: non-destructive dose verification using a rapid point-and-shoot approach.
      Though predominately amorphous, trace crystallinity is still observed with Raman confocal microscopy, and lack of detection of 3% candurin's unique diffraction profile questions the sensitivity and the ability to detect crystallinity. To not limit the present study by sensitivity, advanced characterization techniques were adapted for a more sensitive analysis; this capability was leveraged to unquestionably confirm the SLS-3DP process: completely converted RTV into an ASD, eliminated trace crystallinity, improved the dissolution profile, and created an ASD with similar molecular miscibility to other manufacturing techniques.
      The ability to detect crystalline RTV by mDSC in both the physical mixture and the 0.6 mg of pure RTV (Fig. 1) confirmed the instrument's capability to detect the 10% RTV drug load. In addition to using pXRD to validate mDSC results, WAXS experiments were utilized for their sensitivity to detect trace crystallinity in samples (i.e., 0.5% crystallinity). For the 20% RTV drug load sample, F3–P7S-20, the WAXS data does not detect any peaks representative of crystalline RTV, the peaks present correspond to candurin. Agreement between characterization techniques increases the confidence that the tablets are completely amorphous; for final validation, ssNMR not only assessed the tablets' amorphous nature but also compared the domain sizes of an SLS-3DP ASD to that of an identical ASD processed by HME.
      ssNMR has exceptional abilities to discern subtle differences in the dynamic and structural properties of ASDs prepared by different processes.
      • Paudel A.
      • Geppi M.
      • Mooter G.V.
      Structural and dynamic properties of amorphous solid dispersions: the role of solid-state nuclear magnetic resonance spectroscopy and relaxometry.
      For example, using ss-NMR, Chiang et al. were able to discern different levels of ASD miscibility between the same composition processed by spray drying, fast evaporation, and lyophilization.
      • Chiang P.-C.
      • Cui Y.
      • Ran Y.
      • Lubach J.
      • Chou K.
      • Bao L.
      • et al.
      In vitro and in vivo evaluation of amorphous solid dispersions generated by different bench-scale processes, using griseofulvin as a model compound.
      Therefore, we can establish the robustness of a new processing technology at a molecular level by comparing it to an industry-standard; in this study, identical compositions were processed by the innovative SLS-3DP process and compared to HME, the industry standard. To understand the relevance, Norvir is a marketed product that uses HME to formulate 15% ritonavir and copovidone into an ASD.
      • Ellenberger D.J.
      • Miller D.A.
      • Kucera S.U.
      • Williams R.O.
      Generation of a weakly acidic amorphous solid dispersion of the weak base ritonavir with equivalent in vitro and in vivo performance to Norvir tablet.
      From the 1H relaxation behaviors of the laboratory frame, Table 3, both the SLS-3DP and HME systems produced molecularly miscible compositions, attributed to the high agreement between ritonavir and copovidone relaxation times.
      • Brettmann B.K.
      • Myerson A.S.
      • Trout B.L.
      Solid-state nuclear magnetic resonance study of the physical stability of electrospun drug and polymer solid solutions.
      Previously, the importance of the formation of molecular miscible systems was shown by Hanada et al., where molecularly miscible systems exhibit greater physical stability and were found to be less prone to crystallization. HME is an extensively studied processing method where its ability to form miscible systems has been well documented,
      • Kulkarni C.
      • Kelly A.L.
      • Gough T.
      • Jadhav V.
      • Singh K.K.
      • Paradkar A.
      Application of hot melt extrusion for improving bioavailability of artemisinin a thermolabile drug.
      ,
      • Maniruzzaman M.
      • Morgan D.J.
      • Mendham A.P.
      • Pang J.
      • Snowden M.J.
      • Douroumis D.
      Drug–polymer intermolecular interactions in hot-melt extruded solid dispersions.
      ,
      • Hanada M.
      • Jermain S.V.
      • Lu X.
      • Su Y.
      • Williams R.O.
      Predicting physical stability of ternary amorphous solid dispersions using specific mechanical energy in a hot melt extrusion process.
      contrarily, this is the first time the ability of SLS-3DP has been shown to create a molecularly miscible ASD. The relaxation time in the laboratory frame, T1, is measured in the order of 1 s and characterize domain sizes on the length scale of 20–100 nm. Processes that exhibit a lower degree of mixing (i.e., melt quenching) or slower evaporation (i.e., film casting) achieve phase separation at larger domain sizes; Purohit et al. demonstrated phase separation occurred at a larger domain size for ASD samples prepared by film casting compared to References.
      • Purohit H.S.
      • Ormes J.D.
      • Saboo S.
      • Su Y.
      • Lamm M.S.
      • Mann A.K.P.
      • et al.
      Insights into nano- and micron-scale phase separation in amorphous solid dispersions using fluorescence-based techniques in combination with solid state nuclear magnetic resonance spectroscopy.
      Despite both processing techniques producing miscible ASDs, the SLS-3DP process achieved a smaller ritonavir and copovidone domain size than the HME comparator (i.e., 104 vs. 168 nm), suggesting a higher degree of mixing occurring within the SLS-3DP process compared to HME. This may indicate that SLS formed an amorphous system where drug and polymer exhibited stronger interactions (as suggested by the FT-IR results); these interactions may translate to better storage stability of the dosage form, although a stability test was beyond the scope of this research and should be considered for further research on this topic. The underlying laser-based fusion and processing parameters controlled in a layer-by-layer manner in SLS might have played a pivotal role in this observed molecular phenomenon. In future studies, more quantitative ssNMR experiments are needed to further elucidate the molecular interaction between the drug and polymer in solid-state.
      1D13C CP-MAS and2D13C–1H HECTOR spectra are show in Figs. 4 and 5, respectively. These spectra are utilized to elucidate the subtle molecular differences between SLS-3DP and HME processed samples; which, upon examination, both samples exhibited identical molecular structures. Additionally, the 1D13C CP-MAS spectra detect trace crystallinity at the highest sensitivity, neither the HME nor SLS-3DP sample exhibited crystallinity. A similar analysis was performed by Jermain et al., where two identical compositions were processed by spray drying and Kinetisol® processing; both ASD compositions, upon 1D and 2D investigation, exhibited identical molecular details and similar domain sizes.
      • Jermain S.V.
      • Lowinger M.B.
      • Ellenberger D.J.
      • Miller D.A.
      • Su Y.
      • Williams R.O.
      In vitro and in vivo behaviors of KinetiSol and spray-dried amorphous solid dispersions of a weakly basic drug and ionic polymer.
      In their study, though the compositions were similar at a molecular level this did not ensure similar bioavailability results, differences in ASD performance were attributed to the physical properties unique to the particles.
      • Jermain S.V.
      • Lowinger M.B.
      • Ellenberger D.J.
      • Miller D.A.
      • Su Y.
      • Williams R.O.
      In vitro and in vivo behaviors of KinetiSol and spray-dried amorphous solid dispersions of a weakly basic drug and ionic polymer.
      Therefore, even though SLS-3DP produced a miscible system with smaller domain sizes than the HME ASD, bioequivalence cannot be assumed; SLS-3DP will produce unique particles that are dependent on the process and influence the resultant dissolution and bioavailability of ritonavir.
      The dissolution profile for F1–P4-10 is not unique to the composition but also highly dependent upon the printing parameters. Previously, Fina et al. demonstrated the dependence of laser speed and % drug release; as the laser speed is increased, the tablet releases the drug faster. A similar pattern was observed for formulations with a 20% RTV drug load. F3–P6S-20 and F3–P7S-20 otherwise identical were processed at different laser speeds of 50 mm/s and 75 mm/s, respectively. The reasoning for this difference in performance could be related to the time the laser spends to sinter a particular region of the design. The more the time spent, the slower the drug release from the formulation. Knowing the F3–P7S-20 composition forms a molecularly miscible ASD, the change in dissolution profiles is associated with changes in the physical property of the tablet particles, as seen previously in Jermain et al., with spray-dried and kinetiSol® particles.
      • Jermain S.V.
      • Lowinger M.B.
      • Ellenberger D.J.
      • Miller D.A.
      • Su Y.
      • Williams R.O.
      In vitro and in vivo behaviors of KinetiSol and spray-dried amorphous solid dispersions of a weakly basic drug and ionic polymer.
      Therefore, the laser printing parameters (i.e., laser speed) may be changing the physical characteristics of the particles in the printed tablet, thereby modifying the dissolution profile. However, the various laser speeds were found to be within the range of parameters required to form an ASD, which can be observed from the previously discussed solid-state characterization and the similar solubility enhancement observed for the two formulations under discussion. To sum up the discussion, this study outlined an SLS process for 3-D printing of ASDs in a single step, whilst highlighting the importance and range of hatch spacing, laser speed, and surface and chamber temperatures required which has never been reported before. Further, we confirmed this claim by conventional, and advanced characterization techniques using ASDs produced by HME as reference standards. Furthermore, the study assessed the performance of the dosage form demonstrating a notable solubility advantage over the pure drug. Future studies with different drugs to further bolster these claims are underway.

      Conclusion

      Until now, it has not been evident of the potential benefit SLS-3DP can have towards poorly water-soluble drugs, besides printing tablets and controlling drug release. This new application of SLS-3DP details the ability to convert poorly water-soluble drugs to their amorphous state for solubility and bioavailability improvement. Intentional experimental design has highlighted the importance of hatch spacing, and surface temperature concerning the composition's melting point plays in the ability of SLS-3DP to create a fully amorphous product. The reproduction of the methods from prior arts failed to produce an amorphous solid dispersion. Printing parameters not previously investigated (e.g., hatch spacing) were determined to be critical printing parameters to create an amorphous solid dispersion. Before adjusting the hatch spacing, it was not possible to print an SLS-3DP amorphous solid dispersion by adjusting the laser speed, chamber temperature, and surface temperature. For the first time, an amorphous solid dispersion using 3DP-SLS was created that improved the solubility of a poorly soluble API.

      Conflicts of Interest & Disclosures

      The authors declare no conflict of interest. Davis, Thakkar, Maniruzzaman, and Williams are co-inventors on related intellectual property.

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