Modeling of adhesion in tablet compression--I. Atomic force microscopy and molecular simulation

Virtual screening of drug materials for pharmaceutical tablet manufacturability with reference to sticking

The manufacturing of pharmaceutical solid dosage forms, such as tablets involves a large number of successive processing operations including crystallisation of the drug substance, granulation, drying, milling, mixing of the formulation, and compaction. Each step is fraught with manufacturing problems. Undesired adhesion of powders to the surface of the compaction tooling, known as sticking, is a frequent and highly disruptive problem that occurs at the very end of the process chain when the tablet is formed. As an alternative to the mechanistic approaches to address sticking, we introduce two different machine learning strategies to predict sticking directly from the chemical formula of the drug substance, represented by molecular descriptors. An empirical database for sticking behaviour was developed and used to train the machine learning (ML) algorithms to predict sticking properties from molecular descriptors. The ML model has successfully classified sticking/non-sticking behaviour of powders with 100% separation. Predictions were made for materials in the handbook of Pharmaceutical Excipients and a subset of molecules included in the ChemBL database, demonstrating the potential use of machine learning approaches to screen for sticking propensity early at drug discovery and development stages. This is the first-time molecular descriptors and machine learning were used to predict and screen for sticking behaviour. The method has potential to transform the development of medicines by providing manufacturability information at drug screening stage and is potentially applicable to other manufacturing problems controlled by the chemistry of the drug substance.

Sticking Detection by Repeated Compactions on a Single Tablet

"Sticking" during tablet manufacture is the term used to describe the accumulation of adhered tablet material on the punch over the course of several compaction cycles. The occurrence of sticking can affect tablet weight, image, and structural integrity and halt manufacturing operations. The earlier the risk of sticking is detected during R&D, the more options are available for mitigation and the less potential there is for significant delays and costs. The detection osf sticking, however, during the early stages of drug development is challenging due to the limitations of available material quantity. In this work, single tablet multi-compaction (STMC) and a highly sensitive laser reflection sensor are used to detect the propensity of sticking with ibuprofen powder blends. STMC can differentiate the various formulations and replicates the trends of sticking at different punch speeds. The results demonstrate the potential for STMC to be used as an extremely material sparing (requiring very few tablets) methodology for the assessment of sticking during early-stage development.

Understanding the role of magnesium stearate in lowering punch sticking propensity of drugs during compression

The sticking of active pharmaceutical ingredient (API) to the surfaces of compaction tooling, frequently referred to as punch sticking, causes costly downtime or product failures in commercial tablet manufacturing. Magnesium stearate (MgSt) is a common tablet lubricant known to ameliorate the sticking problem, even though there exist exceptions. The mechanism by which MgSt lowers punch sticking propensity (PSP) by covering API surface is sensible but not yet experimentally proven. This work was aimed at elucidating the link between PSP and surface area coverage (SAC) of tablets by MgSt, in relation to some key formulation properties and process parameters, namely MgSt concentration, API loading, API particle size, and mixing conditions. The study was conducted using two model APIs with known high PSPs, tafamidis (TAF) and ertugliflozin-pyroglutamic acid (ERT). Results showed that PSP decreases exponentially with increasing SAC by MgSt. The composition of material stuck to punch face was also explored to better understand the onset of punch sticking and the impact of possible MgSt-effected punch conditioning event.

Tools shaping drug discovery and development

Spectroscopic, scattering, and imaging methods play an important role in advancing the study of pharmaceutical and biopharmaceutical therapies. The tools more familiar to scientists within industry and beyond, such as nuclear magnetic resonance and fluorescence spectroscopy, serve two functions: as simple high-throughput techniques for identification and purity analysis, and as potential tools for measuring dynamics and structures of complex biological systems, from proteins and nucleic acids to membranes and nanoparticle delivery systems. With the expansion of commercial small-angle x-ray scattering instruments into the laboratory setting and the accessibility of industrial researchers to small-angle neutron scattering facilities, scattering methods are now used more frequently in the industrial research setting, and probe-less time-resolved small-angle scattering experiments are now able to be conducted to truly probe the mechanism of reactions and the location of individual components in complex model or biological systems. The availability of atomic force microscopes in the past several decades enables measurements that are, in some ways, complementary to the spectroscopic techniques, and wholly orthogonal in others, such as those related to nanomechanics. As therapies have advanced from small molecules to protein biologics and now messenger RNA vaccines, the depth of biophysical knowledge must continue to serve in drug discovery and development to ensure quality of the drug, and the characterization toolbox must be opened up to adapt traditional spectroscopic methods and adopt new techniques for unraveling the complexities of the new modalities. The overview of the biophysical methods in this review is meant to showcase the uses of multiple techniques for different modalities and present recent applications for tackling particularly challenging situations in drug development that can be solved with the aid of fluorescence spectroscopy, nuclear magnetic resonance spectroscopy, atomic force microscopy, and small-angle scattering.

Modeling of Adhesion in Tablet Compression at the Molecular Level Using Thermal Analysis and Molecular Simulations

The molecular basis of adhesion leading to sticking was investigated by exploring the correlation between thermal analysis and molecular simulations. It is hypothesized that intermolecular interactions between a drug molecule and a punch face are the first step in the adhesion process and the rank order of adhesion during tablet compression should correspond to the rank order of the energies of these interactions. In the present study, the sticking propensity was investigated using ibuprofen, flurbiprofen, and ketoprofen as model substances. At the intermolecular level, a thermal analysis model was proposed as an experimental technique to estimate the work of adhesion between ibuprofen, flurbiprofen, and ketoprofen in a DSC aluminum pan. The linear relationship was established between the enthalpy of vaporization and sample mass to demonstrate the accuracy of the instruments used. The threshold mass for ibuprofen, flurbiprofen, and ketoprofen was determined to be 107, 112, and 222 μg, respectively, after three replicate measurements consistent with the experimental results. Ketoprofen showed a 2-fold higher threshold mass compared to ibuprofen and flurbiprofen, which predicts that ketoprofen should have the highest sticking propensity. Computationally, the rank order of the work of adhesion between ibuprofen, flurbiprofen, and ketoprofen with the metal surface was simulated to be -75.91, 44.75, and -96.91 kcal/mol, respectively, using Materials Studio. The rank order of the interaction between the drug molecule and the iron superlattice decreases in the order ketoprofen > ibuprofen > flurbiprofen. The results indicate that the thermal model can be successfully implemented to assess the sticking propensity of a drug at the molecular level. Also, a new molecular simulation script was successfully applied to determine the interaction energy of the drug molecule upon contact with iron.

Predicting lubricants effect on tablet sticking using ketoprofen as model drug and evaluating sticking propensity using different metals and powder rheology

Tablet sticking occurrence is a persistent, costly, and time-consuming problem that needs to be resolved. Predicting the sticking tendency of a new formulation has been very difficult during the development batches because of short runs and limited data. A model formulation comprising ketoprofen and microcrystalline cellulose was used to predict the effect of magnesium stearate and sodium stearyl fumarate on the occurrence of tablet sticking relative to different punch metals. Lubricant amounts were varied from 0.0% to 2.0 %w/w. Five different metal coupons were used to represent punch metals. The sticking index (SI) of each formulation relative to each metal coupon was determined by measuring angle of internal friction and angle of wall friction by performing shear cell test and wall friction test, respectively. The SI was used to predict each formulation's sticking tendency rank order relative to metal coupon. Both lubricants show a decrease in the powder blend's sticking propensity with increased lubricant concentration. The predicted sticking propensity rank order was then validated by the compression study. The result suggests that the SI can be used to predict tablet sticking, such as by changing the composition of the formulation or changing the punch metal during tablet compression.

Impact of Electric Fields on the Nanoscale Behavior of Lipid Monolayers at the Surface of Graphite in Solution

The nanoscale organization and dynamics of lipid molecules in self-assembled membranes is central to the biological function of cells and in the technological development of synthetic lipid structures as well as in devices such as biosensors. Here, we explore the nanoscale molecular arrangement and dynamics of lipids assembled in monolayers at the surface of highly ordered pyrolytic graphite (HOPG), in different ionic solutions, and under electrical potentials. Using a combination of atomic force microscopy and fluorescence recovery after photobleaching, we show that HOPG is able to support fully formed and fluid lipid membranes, but mesoscale order and corrugations can be observed depending on the type of the lipid considered (1,2-dioleoyl- sn-glycero-3-phosphocholine, 1,2-dioleoyl- sn-glycero-3-phospho-l-serine (DOPS), and 1,2-dioleoyl-3-trimethylammoniumpropane) and the ion present (Na⁺, Ca²⁺, Cl^-). Interfacial solvation forces and ion-specific effects dominate over the electrostatic changes induced by moderate electric fields (±1.0 V vs Ag/AgCl reference electrode) with particularly marked effects in the presence of calcium, and for DOPS. Our results provide insights into the interplay between the molecular, ionic, and electrostatic interactions and the formation of dynamical ordered structures in fluid lipid membranes.

Direct compaction: An update of materials, trouble-shooting, and application

Direct compaction (DC) is the preferred choice for tablet manufacturing; however, only less than 20% of active pharmaceutical ingredients could be compacted via DC as its high requirement for functional properties of materials. Materials with improper functionalities could lead to serious troubles during DC manufacturing, such as content non-uniformity, sticking, and capping, all of which profoundly affect the properties of final products and, thus, severely restrict the practical application of DC. With undoubted importance, these seem to be unexpectedly ignored by reviewers but not researchers in terms of many original research articles published recently. Therefore, as an informative supplement and update, this review mainly focused on trouble-shooting and application situation of DC, together with several newly reported materials.

Integration of active pharmaceutical ingredient solid form selection and particle engineering into drug product design

This review seeks to offer a broad perspective that encompasses an understanding of the drug product attributes affected by active pharmaceutical ingredient (API) physical properties, their link to solid form selection and the role of particle engineering. While the crucial role of active pharmaceutical ingredient (API) solid form selection is universally acknowledged in the pharmaceutical industry, the value of increasing effort to understanding the link between solid form, API physical properties and drug product formulation and manufacture is now also being recognised.

A truly holistic strategy for drug product development should focus on connecting solid form selection, particle engineering and formulation design to both exploit opportunities to access simpler manufacturing operations and prevent failures. Modelling and predictive tools that assist in establishing these links early in product development are discussed. In addition, the potential for differences between the ingoing API physical properties and those in the final product caused by drug product processing is considered. The focus of this review is on oral solid dosage forms and dry powder inhaler products for lung delivery.

Image analysis quantification of sticking and picking events of pharmaceutical powders compressed on a rotary tablet press simulator

PURPOSE

The aim of this work was to develop a quantification method based on image analysis, able to characterize sticking during pharmaceutical tableting. Relationship between image analysis features and relevant mechanical parameters recorded on an instrumented tablet press simulator were investigated.

METHODS

Image analysis, based on gray levels co-occurrence matrices (GLCM), generated textural features of the tablet surface. The tableting simulator (Stylcam® 200R, Medelpharm), instrumented with force and displacement transducers, provided accurate records. The tablet defects and compaction process parameters were studied using three pharmaceutical powders (Fast-Flo® lactose, anhydrous Emcompress® and Avicel® PH200 microcrystalline cellulose), five compression pressures (60 to 250 MPa), five lubricating levels, and three types of punches (standard steel, amorphous hard carbon and anti-sticking punches).

RESULTS

Texture parameters made it possible to quantify with precision tablets’ aspect. The selected parameter IC2 (Information on Correlation 2) plotted versus the ratio between the ejection shear stress (Esh) and the compression pressure (Cp) let appear a relevant knowledge space where it was possible to identify a normal and a degraded tableting mode. A positive link between those two parameters was shown.

CONCLUSION

Since the Esh/Cp ratio was related to image analysis results, it proved to be an interesting defect tag.

Stuck in traffic: Patterns of powder adhesion

The adhesion of fine particles to surfaces is important for applications ranging from drug delivery to fouling of solar cells. In this letter, we show that powder adhesion can occur in unexpected patterns, concentrating particular grain types in some locations and clearing them from others, and we propose a straightforward traffic model that appears to reproduce many of the behaviors seen. The model predicts different patterns depending on inter-particle cohesion, and we find in both experiment and model that adhesion occurs in three distinct stages.

Partial least square projections to latent structures analysis (PLS) in evaluating and predicting drug release from starch acetate matrix tablets

The aim of this study was to evaluate whether or not starch acetate can act as a release-controlling excipient with physicochemically different drugs, using multivariate data analyses (PLS) for the modelling of drug release from starch acetate tablets. In addition, variables contributing to drug release at certain points of time were studied. Physicochemical properties of drugs were calculated by the VolSurf method. Nine different formulations were produced with six different model drugs. In vitro dissolution studies were carried out for the determination of the drug release profiles for each formulation. PLS analysis was used to evaluate the properties dominating the drug release. Drug release profiles varied widely, depending on the formulation, process, and physicochemical properties of drugs. PLS models were developed to describe the release phenomena. It was observed that at the beginning of the dissolution process, formulation and process variables played a major role in the drug release. Later in the dissolution process, the molecular properties of drugs became the dominant variables. Starch acetate was able to act as a release-controlling excipient with different drugs. Multivariate data analyses were found to be a powerful tool for the evaluation and prediction of drug-release characteristics from the tableted starch acetate matrix.

What can we learn from atomic force microscopy adhesion measurements with single drug particles?

Frequently solid dosage form formulation manufacture and delivery depend critically on the control and exploitation of interparticulate interactions. Traditional approaches to understand such interactions rely on indirect assessments of adhesion or consider the behaviour of large numbers of particles. In recent years, the possibility of characterizing and perhaps quantifying forces of adhesion between individual micron and sub-micron sized particles has become viable using the atomic force microscope. This has significant potential in formulation development, particularly in the optimization of inhalation and other solid-dosage form based therapies. However, before a widespread acceptance of this approach by pharmaceutical scientists and industry can proceed a number of issues remain to be considered. These include how can single particle events be mapped on to bulk behaviour, the need to understand the sometimes wide variations in adhesion data observed and can formulations be compared quantitatively and perhaps be screened by this approach?

Mechanism of glidants: Investigation of the effect of different colloidal silicon dioxide types on powder flow by atomic force and scanning electron microscopy

The effect of hydrophilic and hydrophobic colloidal silicon dioxide types (CSD) on the flow characteristics of microcrystalline cellulose (MCC) under different mixing conditions was macroscopically measured using the angle of repose method, the bulk and tapped densities. CSD ameliorated the flow characteristics in general, but hydrophobic CSD was more effective compared to the hydrophilic types under gentle mixing conditions. The macroscopic effect was explained on the particle level by scanning electron (SEM) and atomic force microscopy (AFM) studies. The CSD distribution on the MCC surface was more uniform for the hydrophobic type and was independent from the mixing conditions used in this study. From the cumulative adhesion force distributions of the mixtures, determined by AFM, the mean and the standard deviation of the adhesion force were calculated. The means were 44.8 nN for MCC alone, 25.2 and 28.3 nN for mixtures containing the two hydrophilic types, and 13.8 N for the hydrophobic CSD under gentle mixing conditions in a Turbula mixer. Stronger mixing in a plowshare mixer led to a further reduction to 17.5 and 17.4 nN for the two hydrophilic types, while the hydrophobic CSD showing a value of 13.9 nN was unchanged. A linear correlation between the angle of repose and the adhesion force could be established, indicating that for routine measurements of the efficiency of a glidant the simple angle of repose method is sufficient.

Characterization of pharmaceutical solids by scanning probe microscopy

The force-displacement profiles of four well-characterized materials that represent both soft/hard and plastic/brittle materials have been obtained using a novel nanoindentation technique. Flat surfaces of acetaminophen, potassium chloride, sucrose, and sodium stearate were prepared by melting or recrystallization, and the melting points were measured. Topographic and the corresponding first derivative images were obtained both before and after indentation. The materials were indented using a 10 s loading time, followed by a 2 s hold, and ending with a 10 s unloading time thereby providing a unique force-displacement profile for each material. The loading profile of acetaminophen was discontinuous, whereas the profiles for the other three materials were smooth. The profiles were analyzed and the rank order of hardness was sucrose > acetaminophen > KCl > sodium stearate, which is consistent with the literature. The rank order of the stiffness, which is related to the modulus of elasticity, was sucrose > KCl > acetaminophen > sodium stearate. Given the flexibility and power of this approach, nanoindentation coupled with atomic force microscopy may be a useful means to characterize materials for evaluating tablet-processing conditions, perhaps at a preformulation stage.

Crystal structure and surface properties of an investigational drug—A case study

In this study we investigate the correlations between the single crystal structure, the crystal habitat and morphology, and surface energetics of an investigational pharmaceutical compound. Crystal structure of this investigational pharmaceutical solid has been solved from single crystal X-ray analysis. Crystallographic data are as follows: triclinic, P1 (no. 1), a = 6.1511 (8) A, b = 13.5004 (18) A, c = 17.417 (2) A, alpha = 68.259 (2) degrees, beta = 80.188 (2) degrees, gamma = 82.472 (2) degrees, V = 1320.2 (3) A(3), Z = 2. The external morphology of this crystalline solid was predicted by molecular modelling using attachment energies to be thin-plate like with a dominant face (001). The predicted morphology was confirmed by scanning electron micrographs (SEM) and the Miller Index of the dominant face was complemented by X-ray powder diffraction (XRPD) method. The microscopic layering structures of crystals and surface stability of the dominant faces were investigated using atomic force microscopy (AFM). Contact angle measurement showed that the surface of the dominant face is hydrophilic as predicted from crystal structure.

Modeling of adhesion in tablet compression. II. Compaction studies using a compaction simulator and an instrumented tablet press

Adhesion problems are usually not identified until prolonged compression runs are studied near the end of the drug development process. During tablet manufacturing, adhesion problems encountered are usually addressed by statistically designed experiments based on experience. It would be a significant benefit for the pharmaceutical industry if adhesion problems could be identified early in drug development based on molecular considerations of the drug substance and/or prototype formulations. Drug substance-punch face interactions were reported in the first of the articles in this series, and focused on the elucidation of adhesion problems in tablet compression. It was hypothesized that the intermolecular interactions between drug molecules and the punch face was the first step (or criterion) in the adhesion process, and that the rank order of adhesion during tablet compression should correspond with the rank order of these energies of interaction. That is, the interaction between the molecular structure of the drug and the metal surface determines the primary interaction event or relative potential for adhesion, while the mechanical processes and/or lubrication effects may subsequently impact the extent of adhesion. Molecular simulations and atomic force microscopy were used to establish the rank order of the work of adhesion of a series of profen compounds. The results predicted that the relative degree of drug substance-punch face adhesion should decrease in the order of ketoprofen > ibuprofen > flurbiprofen. In this study, the authors investigated whether the rank order of the work of adhesion established on the molecular level and interparticulate level holds true in the tableting environment by measuring tablet take-off force, ejection force, and visual observation of the punch surfaces for both pure drug compacts and formulated tablets. The compaction simulator was used for pure profen compacts, while the instrumented tablet press for formulated tablets. Due to the inability to extract the adhesion force component from the total ejection force measurement, tablet ejection force was not used as a criterion to judge the adhesion behavior of the model compounds. The criteria used for judgement of punch face adhesion were tablet take-off force and visual observation of the punch faces. The rank order of adhesion for both pure drug and formulated tablets was determined to follow the order of ketoprofen > ibuprofen > flurbiprofen. The effect of run time on adhesion behavior was also investigated. Therefore, the rank order of the punch-face adhesion tendencies for the series of profen compounds was determined, and found to agree with the data from the predictive methods reported in the first article.