Effects of film coating thickness and drug layer uniformity on in vitro drug release from sustained-release coated pellets: A case study using terahertz pulsed imaging

Film Coating of Phosphorylated Mandua Starch on Matrix Tablets for pH-Sensitive Release of Mesalamine

Chemically modified mandua starch was successfully synthesized and applied to coat mesalamine-loaded matrix tablets. The coating material was an aqueous dispersion of mandua starch modified by sodium trimetaphosphate and sodium tripolyphosphate. To investigate the colon-targeting release competence, chemically modified mandua starch film-coated mesalamine tablets were produced using the wet granulation method followed by dip coating. The effect of the coating on the colon-targeted release of the resultant delivery system was inspected in healthy human volunteers and rabbits using roentgenography. The results show that drug release was controlled when the coating level was 10% w/w. The release percentage in the upper gastric phase (pH 1.2, simulated gastric fluid) was less than 6% and reached up to 59.51% w/w after 14 h in simulated colonic fluid. In addition to in vivo roentgenographic studies in healthy rabbits, human volunteer studies proved the colon targeting efficiency of the formulation. These results clearly demonstrated that chemically modified mandua starch has high effectiveness as a novel aqueous coating material for controlled release or colon targeting.

Imaging techniques for studying solid dosage formulation: Principles and applications

Classic methods for evaluating the disintegration and dissolution kinetics of solid dosage forms are no longer sufficient to meet the growing demands in the pharmaceutical field. Hence, scientists have turned to imaging techniques and computer technology to develop innovative visualization methods. These methods allow for a visual understanding of the disintegration or dissolution process and offer valuable insights into the drug release kinetics. This article aims to provide an overview of the commonly used imaging techniques and their applications in studying the disintegration or dissolution of solid dosage forms. Therefore, imaging presents a novel and alternative approach to understanding the mechanisms of disintegration and dissolution in the formulation study of solid dosages.

Application of Process Analytical Technology for Pharmaceutical Coating: Challenges, Pitfalls, and Trends

Coating process is a critical unit operation for manufacturing solid oral dosage forms. For a long time, the coating weight gain has been discerned as the most important, if not only, characteristic describing the coating quality. As the introduction of quality by design (QbD) and advancement of process analytical technology (PAT), nowadays more techniques are available to analyze other quality attributes which have been overlooked but have substantial impacts on the performance of coated products. The techniques that permit rapid and non-destructive measurements are of particular importance to improve process operation and product quality. This article reviews the analytical techniques that have been and potentially could be used as PAT tools for characterizing the quality of pharmaceutical coating product. By identifying the challenges and pitfalls encountered during PAT application, the review aims at fostering the adoption of PAT for paving the way to enhanced quality and efficiency of the coating processes.

Review of Terahertz Pulsed Imaging for Pharmaceutical Film Coating Analysis

Terahertz pulsed imaging (TPI) was introduced approximately fifteen years ago and has attracted a lot of interest in the pharmaceutical industry as a fast, non-destructive modality for quantifying film coatings on pharmaceutical dosage forms. In this topical review, we look back at the use of TPI for analysing pharmaceutical film coatings, highlighting the main contributions made and outlining the key challenges ahead.

Real time MRI to elucidate the functionality of coating films intended for modified release

Polymer films based on mixtures of ethyl cellulose (EC) and hydroxypropyl cellulose (HPC) have been widely used to coat pellets and tablets to modify the release profile of drugs. For three different EC/HPC films we used ¹H and ¹⁹F MRI in combination with a designed release cell to monitor the drug, polymer and water in 5 dimensional (5D) datasets; three spatial, one diffusion or relaxation and a temporal dimension, in real time. We observed that the water inflow through the films correlated with the initiation of the dissolution of the drug in the tablet beneath the film. Leaching of the pore forming HPC further accelerated water penetration and resulted in a drug release onset after a hydrostatic pressure was generated below the film indicated by positional changes of the film. For the more permeable film, both water ingress and drug egress showed a large variability of release over the film surface indicating the heterogeneity of the system. Furthermore, the ¹H diffusion dataset revealed the formation of a gel layer of HPC at the film surface. We conclude that the setup presented provides a significant level of details, which are not achieved with traditional methods.

Recognition of Pharmacological Bi-Heterocyclic Compounds by Using Terahertz Time Domain Spectroscopy and Chemometrics

In this study, we presented the concept and implementation of a fully functional system for the recognition of bi-heterocyclic compounds. We have conducted research into the application of machine learning methods to correctly recognize compounds based on THz spectra, and we have described the process of selecting optimal parameters for the kernel support vector machine (KSVM) with an additional `unknown' class. The chemical compounds used in the study contain a target molecule, used in pharmacy to combat inflammatory states formed in living organisms. Ready-made medical products with similar properties are commonly referred to as non-steroidal anti-inflammatory drugs (NSAIDs) once authorised on the pharmaceutical market. It was crucial to clearly determine whether the tested sample is a chemical compound known to researchers or is a completely new structure which should be additionally tested using other spectrometric methods. Our approach allows us to achieve 100% accuracy of the classification of the tested chemical compounds in the time of several milliseconds counted for 30 samples of the test set. It fits perfectly into the concept of rapid recognition of bi-heterocyclic compounds without the need to analyse the percentage composition of compound components, assuming that the sample is classified in a known group. The method allows us to minimize testing costs and significant reduction of the time of analysis.

Understanding the release performance of pellets with hydrophobic inclusions in sustained-release coating

The aim of this study was to evaluate how the addition of hydrophobic inclusions to sustained-release pellet film coat can affect drug release. Sustained-release formulations, in particular multiparticulate systems are gaining popularity because they are able to reduce the dosing frequency of drugs that require multiple dosing. In addition, the risk of dose dumping is low and local gastrointestinal irritation is minimised. Metformin-loaded pellets, prepared via extrusion-spheronisation, were coated with ethyl cellulose (EC)-based film coat, with and without hydrophobic inclusions to a series of coat thickness. Stearic acid 50 (SA) and hydrogenated castor oil (HCO) were the hydrophobic inclusions used. Drug release was investigated using the USP dissolution apparatus 2 and an ultraviolet imager. Release kinetics were analysed using the zero-order model. The physical properties of the pellets were characterised before and after dissolution. The addition of hydrophobic inclusions to EC-based film coat slowed down drug release, with SA slowing down drug release more than HCO. The influence of hydrophobic inclusions on drug release was clearly observable when the pellets were coated to 10% weight gain. It was postulated that the hydrophobic inclusions acted as physical barriers to increase the tortuosity of the diffusional path through the pellet film coat. The use of hydrophobic inclusions to control the rate of drug dissolution was shown to be promising. This could translate into potential cost and time savings with less materials and time used.

Bio-based Large Tablet Controlled-Release Urea: Synthesis, Characterization, and Controlled-Released Mechanisms

To improve nitrogen (N) use efficiency and minimize environmental pollution caused by fertilizer overuse, novel bio-based large tablet controlled-release urea (LTCRU) was prepared using bio-based coating materials to coat large tablet urea (LTU) derived from urea prills (U). Nano fumed silica (NFS) was added to the bio-based coating materials to improve the slow-release properties. The surface area of the LTU and U was measured by three-dimensional scanning. In comparison to U, LTU had a smaller surface area/weight ratio, which can reduce the coating materials. Scanning electron microscopy analysis showed that the addition of NFS in bio-based coating materials reduced the porosity of the coating shells of LTCRUs and, thus, enhanced the N release longevity of the controlled-released fertilizer. Dependent upon the pores on the coating shells of LTCRU, two N release patterns were revealed. Because of the good release characteristics, the novel LTCRU shows great potential to support sustainable agricultural production.

Determination of plane stress state using terahertz time-domain spectroscopy

THz wave has been increasingly applied in engineering practice. One of its outstanding advantages is the penetrability through certain optically opaque materials, whose interior properties could be therefore obtained. In this report, we develop an experimental method to determine the plane stress state of optically opaque materials based on the stress-optical law using terahertz time-domain spectroscopy (THz-TDS). In this method, two polarizers are combined into the conventional THz-TDS system to sense and adjust the polarization state of THz waves and a theoretical model is established to describe the relationship between phase delay of the received THz wave and the plane stress applied on the specimen. Three stress parameters that represent the plane stress state are finally determined through an error function of THz wave phase-delay. Experiments were conducted on polytetrafluoroethylene (PTFE) specimen and a reasonably good agreement was found with measurement using traditional strain gauges. The presented results validate the effectiveness of the proposed method. The proposed method could be further used in nondestructive tests for a wide range of optically opaque materials.

Application of terahertz pulse imaging as PAT tool for non-destructive evaluation of film-coated tablets under different manufacturing conditions

Film-coated tablets (FCTs) are a popular solid dosage form in pharmaceutical industry. Manufacturing conditions during the film-coating process affect the properties of the film layer, which might result in critical quality problems. Here, we analyzed the properties of the film layer using a non-destructive approach with terahertz pulsed imaging (TPI). Hydrophilic tablets that become distended upon water absorption were used as core tablets and coated with film under different manufacturing conditions. TPI-derived parameters such as film thickness (FT), film surface reflectance (FSR), and interface density difference (IDD) between the film layer and core tablet were affected by manufacturing conditions and influenced critical quality attributes of FCTs. Relative standard deviation of FSR within tablets correlated well with surface roughness. Tensile strength could be predicted in a non-destructive manner using the multivariate regression equation to estimate the core tablet density by film layer density and IDD. The absolute value of IDD (Lateral) correlated with the risk of cracking on the lateral film layer when stored in a high-humidity environment. Further, in-process control was proposed for this value during the film-coating process, which will enable a feedback control system to be applied to process parameters and reduced risk of cracking without a stability test.

Antimicrobial films containing microparticles for the enhancement of long-term sustained release

Coated packagings with thin films containing antimicrobial agents are an alternative technology to ensure the protection of products against microbial contaminations. Indeed, they allow lowering the antimicrobial concentration in the bulk of the product while meeting the safety requirements and the growing consumer demand for low preservative concentrations. Microencapsulation is a suitable way for controlling active agent release and providing a long-term activity. This work aims at combining both technical solutions with coatings containing antimicrobial microparticles for the achievement of long-term sustained release. Polyethylene surfaces were functionalized with microparticles of poly(methyl methacrylate) (PMMA) loaded with phenylethyl alcohol (PEA) as antimicrobial agent by the dip coating process using a polyurethane binder. The release of PEA into water from coated polyethylene surfaces and from PMMA microparticles was investigated to assess the sustained release and its mechanisms. Films with various thicknesses of 400-1000 µm containing antimicrobial microparticles demonstrated unusual long-term release longer than 3 months. The diffusion of the antimicrobial agent through PMMA was the rate limiting step of the sustained release. PEA release increased as the contact area of the protruding microparticles with the external medium increased and the thickness of the film decreased. Such antimicrobial agents encapsulated inside thin coatings are promising with regards to antimicrobial preservation of products along their full shelf-life.

In-line spatial filtering velocimetry for particle size and film thickness determination in fluidized-bed pellet coating processes

A spatial filtering velocimetry (SFV) probe was applied to monitor the increase in particle size during pellet Wurster coating processes in-line. Accuracy of the in-line obtained pellet sizes was proven by at-line performed digital image analysis (DIA). Regarding particle growth, high conformity between both analytical methods (SFV/DIA) was examined for different coating processes. The influence of ring buffer size and the process of filling the buffer were investigated. With buffer sizes of 30,000-50,000 particles best results were obtained in this study. Investigated process parameters, such as inlet air volume and spray rate, had different effects on the impact of the SFV probe. While the particle rate (the number of particles detected by the SVF probe per second) was highly dependent on the inlet air volume, different spray rates of up to ・}1 g/min did not affect the detected particle growth. Artefacts and delays in SFV particle sizing appeared especially at the beginning of the coating processes. The slope of the particle growth during the final spraying period was therefore used to determine coating thickness.

Model drug as pore former for controlled release of water-soluble metoprolol succinate from ethylcellulose-coated pellets without lag phase: opportunities and challenges

The objective of the present study was to evaluate the feasibility of using model drug metoprolol succinate (MS) as a pore former to modify the initial lag phase (i.e., a slow or non-release phase in the first 1-2 h) associated with the drug release from coated pellets. MS-layered cores with high drug-layering efficiency (97% w/w) were first prepared by spraying a highly concentrated drug aqueous solution (60% w/w, 70°C) on non-pareils without using other binders. The presence of MS in ethylcellulose (EC) coating solution significantly improved the coating process by reducing pellets sticking, which often occurs during organic coating. There may be a maximum physical compatibility of MS with EC, and the physical state of the drug in the functional coating layer of EC/MS (80:20) was simultaneously crystalline and non-crystalline (amorphous or solid molecule solution). The lag phase associated with hydroxypropylcellulose (HPC) as a pore former was not observed when MS was used as a pore former. The drug release from EC/MS-coated pellets was pH independent, inversely proportional to the coating levels, and directly related to the pore former levels. The functional coating layer with MS as a pore former was not completely stabilized without curing. Curing at 60°C for 1 day could substantially improve the stability of EC/MS-coated pellets. The physical state of the drug in the free film of EC/MS (85:15) changed partially from amorphous to crystal when cured at 60°C for 1 day, which should be attributed to the incompatibility of the drug with EC.

Evaluation of the coat quality of sustained release pellets by individual pellet dissolution methodology

This study explored the application of 400-DS dissolution apparatus 7 for individual pellet dissolution methodology by a design of experiment approach and compared its capability with that of the USP dissolution apparatus 1 and 2 for differentiating the coat quality of sustained release pellets. Drug loaded pellets were prepared by extrusion-spheronization from powder blends comprising 50%, w/w metformin, 25%, w/w microcrystalline cellulose and 25%, w/w lactose, and then coated with ethyl cellulose to produce sustained release pellets with 8% and 10%, w/w coat weight gains. Various pellet properties were investigated, including cumulative drug release behaviours of ensemble and individual pellets. When USP dissolution apparatus 1 and 2 were used for drug release study of the sustained release pellets prepared, floating and clumping of pellets were observed and confounded the release profiles of the ensemble pellets. Hence, the release profiles obtained did not characterize the actual drug release from individual pellet and the applicability of USP dissolution apparatus 1 and 2 to evaluate the coat quality of sustained release pellets was limited. The cumulative release profile of individual pellet using the 400-DS dissolution apparatus 7 was found to be more precise at distinguishing differences in the applied coat quality. The dip speed and dip interval of the reciprocating holder were critical operational parameters of 400-DS dissolution apparatus 7 that affected the drug release rate of a sustained release pellet during the individual dissolution study. The individual dissolution methodology using the 400-DS dissolution apparatus 7 is a promising technique to evaluate the individual pellet coat quality without the influence of confounding factors such as pellet floating and clumping observed during drug release test with dissolution apparatus 1 and 2, as well as to facilitate the elucidation of the actual drug release mechanism conferred by the applied sustained release coat onto the pellets.

Non-destructive evaluation of polymer coating structures on pharmaceutical pellets using full-field optical coherence tomography

Full-field optical coherence tomography (FF-OCT) using a conventional light-emitting diode and a complementary metal-oxide semiconductor camera has been developed for characterising coatings on small pellet samples. A set of en-face images covering an area of 700 × 700 μm(2) was taken over a depth range of 166 μm. The three-dimensional structural information, such as the coating thickness and uniformity, was subsequently obtained by analysis of the recorded en-face images. Drug-loaded pharmaceutical sustained-release pellets with two coating layers and of a sub-millimetre diameter were studied to demonstrate the usefulness of the developed system. We have shown that both coatings can be clearly resolved and the thickness was determined to be 40 and 50 μm for the outer and inner coating layers, respectively. It was also found that the outer coating layer is relatively uniform, whereas the inner coating layer has many particle-like features. X-ray computed microtomography measurements carried out on the same pellet sample confirmed all these findings. The presented FF-OCT approach is inexpensive and has better spatial resolution compared with other non-destructive analysis techniques such as terahertz pulsed imaging, and is thus considered advantageous for the quantitative analysis of thin coatings on small pellet samples.

Terahertz pulsed imaging as an advanced characterisation tool for film coatings--a review

Solid dosage forms are the pharmaceutical drug delivery systems of choice for oral drug delivery. These solid dosage forms are often coated to modify the physico-chemical properties of the active pharmaceutical ingredients (APIs), in particular to alter release kinetics. Since the product performance of coated dosage forms is a function of their critical coating attributes, including coating thickness, uniformity, and density, more advanced quality control techniques than weight gain are required. A recently introduced non-destructive method to quantitatively characterise coating quality is terahertz pulsed imaging (TPI). The ability of terahertz radiation to penetrate many pharmaceutical materials enables structural features of coated solid dosage forms to be probed at depth, which is not readily achievable with other established imaging techniques, e.g. near-infrared (NIR) and Raman spectroscopy. In this review TPI is introduced and various applications of the technique in pharmaceutical coating analysis are discussed. These include evaluation of coating thickness, uniformity, surface morphology, density, defects and buried structures as well as correlation between TPI measurements and drug release performance, coating process monitoring and scale up. Furthermore, challenges and limitations of the technique are discussed.

Comparing and evaluating the efficacy of the TOR18FG Leeds test X-ray phantom for T-rays

The commercially available X-ray fluoroscopy quality assurance phantom, the Leeds test object TOR18FG, was found to be suitable to assess T-ray image quality in the range (0.1-0.4) THz at a depth of 0.5 cm. Previous to this only custom made phantoms, made especially for the T-ray region, assessed T-ray spatial resolution. However, if sub-wavelength techniques are used, the Leeds test phantom may be implemented to measure the T-ray systems spatial resolution, allowing us to directly compare X-ray and T-ray spatial resolution. The systems compared include a Gulmay Orthovoltage machine (X-ray), the On Board Imager (OBI) of a Varian linear accelerator (X-ray), a two-colour system (T-ray) and Terahertz Time Domain Spectroscopy (THz-TDS) system. X-rays were found to have a spatial resolution of 1.25 lp/mm using the On Board Imager of a Varian Linear Accelerator whilst T-rays imaged using a broadband source imaged through a spatial pinhole had a spatial resolution of 0.56 lp/mm. The TOR18FG background material was found to block, 90% and 99% of the broadband T-rays emitted from a THz-TDS photo-conductive emitter, at 0.4 THz and 0.53 THz respectively. Contrast sensitivity was found to be 3% for 25 cm × 25 cm X-ray field at 65 kV, whilst this value could not be established for T-rays using the TOR18FG. All contrast circles were found to be the same for T-rays i.e. all 40% at 0.1 THz. Images of the same leaf were taken with diagnostic X-rays and both broadband and continuous wave (CW) T-ray systems. T-rays proved superior in providing image contrast, for a hydrated leaf, over X-rays.

Applying terahertz technology for nondestructive detection of crack initiation in a film-coated layer on a swelling tablet

Here, we describe a nondestructive approach using terahertz wave to detect crack initiation in a film-coated layer on a drug tablet. During scale-up and scale-down of the film coating process, differences in film density and gaps between the film-coated layer and the uncoated tablet were generated due to differences in film coating process parameters, such as the tablet-filling rate in the coating machine, spray pressure, and gas-liquid ratio etc. Tablets using the PEO/PEG formulation were employed as uncoated tablets. We found that heat and humidity caused tablets to swell, thereby breaking the film-coated layer. Using our novel approach with terahertz wave nondestructively detect film surface density (FSD) and interface density differences (IDDs) between the film-coated layer and an uncoated tablet. We also found that a reduced FSD and IDD between the film-coated layer and uncoated tablet increased the risk of crack initiation in the film-coated layer, thereby enabling us to nondestructively predict initiation of cracks in the film-coated layer. Using this method, crack initiation can be nondestructively assessed in swelling tablets after the film coating process without conducting accelerated stability tests, and film coating process parameters during scale-up and scale-down studies can be appropriately established.

Comparative study of the uniformity of coating thickness of pellets coated with a conventional Wurster chamber and a swirl generator-equipped Wurster chamber

This study evaluated the performance of two bottom-spray coaters and the effect of pellet-size variability on coating uniformity. A conventional Wurster chamber was used for the first group of trials, and a Wurster chamber with a novel swirl-flow generator design was used for the second. The results confirmed that when using a conventional Wurster coating chamber, pellets with a smaller diameter receive significantly less coating material compared to those with larger diameters. The swirl generator-equipped Wurster chamber achieved close to uniform coating thickness regardless of pellet size. The ratio (M(S)) of the mass of dye deposited in the coating layer to pellet surface area indicates that coating was much more evenly distributed using the swirl-flow coater. Coating thickness was also analyzed using SEM micrographs and the results were in close agreement with the M(S) factor values. Inter-particle coating mass variation was also lower in case of swirl-flow coater. The results of this study show that a swirl-flow coater is suitable for coating particles of variable size. They also showed an improvement in coating process yield when using the swirl-flow coater.