Dry powder inhaler: influence of humidity on topology and adhesion studied by AFM.

In the dry powder inhalers (DPIs), the adhesion results of the interactions between the active substance and the excipient. The carrier and the micronized drug particle morphologies are believed to affect the delivery of the drug. In this work, the couple studied was the lactose monohydrate and micronized zanamivir, used for the treatment of influenza. In a first approach, observations by scanning electron microscopy (SEM) have shown that the relative humidity (RH) greatly influenced the zanamivir amount fixed on the lactose monohydrate surface. This paper deals with the direct measurement in controlled atmosphere by atomic force microscopy (AFM) of the forces and the interaction ranges between a zanamivir probe and a lactose substrate. Selected zanamivir crystals were attached to the standard AFM probe. Different RH have been used in order to determine influent parameters permitting to identify the nature of adhesion forces between them. This study demonstrated that the increase of RH modified progressively the surface topology of the two components and increased the adhesion force.     

Characterization of a surface modified dry powder inhalation carrier prepared by "particle smoothing"

Atomic force microscopy (AFM) was used to investigate drug-carrier interactions between beclometasone dipropionate (BDP) and a series of untreated and modified lactose surfaces. This quantitative information was correlated with bulk characterization methods and an in-vitro study. Modified lactose surfaces were prepared using a proprietary process referred to as "particle smoothing" to obtain smooth carrier surfaces with or without the presence of magnesium stearate. The engineering of lactose carrier surfaces using the particle smoothing process resulted in significant differences in surface morphology when compared with the "as supplied" starting material. The energy of separation, between BDP and lactose samples, determined by AFM suggested similar lognormal distributions with a rank decrease in median separation energy (e(0.5)) (26.7, 20.6 and 7.7 microJ for untreated, particle-smoothed and particle-smoothed with magnesium stearate, respectively). A series of in-vitro twin stage impinger studies showed good correlation with the AFM separation energy measurements. The mean fine particle dose increased for the two processed lactose samples, with a significant increase for the lactose processed with magnesium stearate, 102.0+/-16 microg compared with 24.2+/-10.7 microg for the untreated lactose. Thus, the AFM presents as a possible pre-formulation tool for rapid characterization of particle interactions.     

Intrinsic adhesion force of lubricants to steel surface

The intrinsic adhesion forces of lubricants and other pharmaceutical materials to a steel surface were quantitatively compared using Atomic Force Microscopy (AFM). A steel sphere was attached to the tip of an AFM cantilever, and its adhesion forces to the substrate surfaces of magnesium stearate, sodium stearyl fumarate, lactose, 4-acetamidophenol, and naproxen were measured. Surface roughness varied by an order of magnitude among the materials. However, the results clearly showed that the two lubricants had about half the intrinsic adhesion force as lactose, 4-acetamidophenol, and naproxen. Differences in the intrinsic adhesion forces of the two lubricants were insignificant. The lubricant molecules were unable to cover the steel surface during AFM measurements. Intrinsic adhesion force can slightly be modified by surface treatment and compaction, and its tip-to-tip variation was not greater than its difference between lubricants and other pharmaceutical particles. This study provides a quantitative fundamental basis for understanding adhesion related issues.     

Surface roughness contribution to the adhesion force distribution of salmeterol xinafoate on lactose carriers by atomic force microscopy

Adhesion force distributions of silica spheres (5 and 20 microm) and salmeterol xinafoate (4 microm) particles with inhalation grade lactose surfaces and spin coated lactose films were determined by atomic force microscopy (AFM) to investigate the influence of surface roughness on the force distributions. The roughness of lactose particles and films was determined by both AFM and confocal microscopy (CM); the lactose particles showed RMS R(q) values between 0.93 and 2.2 microm. The adhesion force distributions for silica and SX probes were significantly different for the different lactose carriers and broad, e.g., the adhesion force distribution between a 5 microm silica sphere and lactose particles ranged from 5 to 105 nN. This contrasted with distributions on smooth spin coated lactose films (RMS R(q) of 0.28 nm) which were not significantly different and were narrow, e.g., the adhesion force distribution between a 5 microm silica sphere and spin coated lactose films was between 42 and 68 nN. In addition, no significant difference in adhesion force distribution occurred with silica probe size on the lactose carrier surface. The use of X-ray photoelectron spectroscopic analysis confirmed that the lactose surfaces were free of impurities that might contribute to variation in adhesion. Although the almost atomically flat films showed some adhesion variability, the surface roughness of the lactose particles was a major contributing factor to the broad distributions seen in this study.     

Investigations on the Mechanism of Magnesium Stearate to Modify Aerosol Performance in Dry Powder Inhaled Formulations

The potential of the force control agent magnesium stearate (MgSt) to enhance the aerosol performance of lactose-based dry powder inhaled (DPI) formulations was investigated in this study. The excipient-blends were investigated with analytical techniques including time-of-flight secondary ion mass spectrometry and single particle aerosol mass spectrometry (SPAMS), and particle size, morphology, and surface properties were evaluated. Excipient-blends were manufactured either by high-shear or low-shear blending lactose carrier with different amounts of MgSt in the range from 0% to 10% (w/w). Fluticasone propionate (FP) and salmeterol xinafoate (SX) used as model active pharmaceutical ingredients were added by low-shear mixing. The in vitro aerosol performance in terms of aerodynamic particle size distribution and fine particle fraction (FPF) of the FP and SX DPI formulations was evaluated with the Next Generation Impactor and also with SPAMS using a Breezhaler^® inhalation device. The distribution of MgSt on the lactose carrier in the blends was visualized and found to depend strongly on the blending method. This affected drug particle detachment from the carrier and thus impacted aerosol performance for FP and SX. Compared with blends without force control agent, low-shear blending of MgSt increases the FPF of the model drug SX, whereas high-shear blending significantly increased FPF of both SX and FP. The interactions between drug and carrier particles were substantially affected by the choice of blending technique of MgSt with lactose. This allows detailed control of aerosol performance of a DPI by an adequate choice of the blending technique. SPAMS successfully demonstrated that it is capable to distinguish changes in DPI formulations blended with different amounts of MgSt, and additional information in terms of dispersibility of fine particles could be generated.     

Comparative study of erythritol and lactose monohydrate as carriers for inhalation: atomic force microscopy and in vitro correlation.

The adhesion of micronised salbutamol sulphate to two carrier excipients, lactose monohydrate and erythritol, was investigated using the atomic force microscope (AFM) colloid probe technique and correlated with their respective physico-mechanical properties and aerosolisation performance. The particle size, morphology and moisture sorption properties of the carriers were similar thereby allowing direct comparison of functionality. AFM force measurements (n = 1024 force curves) were obtained between salbutamol sulphate drug probes (n = 4) and the excipients, as 63-90 microm sieve fractions and atomically smooth crystals. In general, significant differences in drug adhesion to lactose monohydrate and erythritol were observed (ANOVA, p<0.05), with erythritol exhibiting relatively greater adhesiveness. A linear relationship between drug probe adhesion to lactose monohydrate and drug probe adhesion to erythritol was established with salbutamol sulphate-lactose monohydrate adhesion being 60-70% of that of the erythritol system. In vitro analysis suggested good correlation with the adhesion measurements. The aerosolisation of salbutamol sulphate from erythritol carrier particles was significantly less (ANOVA, p<0.05) than from lactose monohydrate, with a fine particle dose (<6.4 microm) of 41.9 +/- 7.4 microg and 24.9 +/- 3.1 microg for the lactose monohydrate and erythritol carriers, respectively (n = 3).     

Influence of fine lactose and magnesium stearate on low dose dry powder inhaler formulations

The behaviour of dry powder blends for inhalation, depending on the amount of fine lactose particles smaller than 10microm and the presence of magnesium stearate (MgSt), was studied in this work. A laser light diffraction method was developed to determine accurately size and volume fraction of these fine lactose particles in coarse carrier lactose (x(50) approximately 220microm). A linear relationship between measured volume fraction undersize at 10microm Q(3)(10microm) and added fine lactose could be established. Aerodynamic particle size distribution analysis of lactose showed that the fine lactose was attached to the coarse particles. In the presence of MgSt this interaction was increased. Consequently, the number of free active sites on the carrier surface was reduced and the investigated drug (formoterol fumarate dihydrate) was more effectively delivered. Addition of fine lactose and MgSt improved the aerodynamic performance the drug, as determined by resulting fine particle fraction, by 3% (for each 1% of added fine lactose) and 10%, respectively. Stability tests indicated that added MgSt was the most relevant of the studied parameter to achieve a stable aerodynamic performance. Its ability to protect the moisture uptake into the system was considered as rational for this effect.     

Characterisation of adhesional properties of lactose carriers using atomic force microscopy

The atomic force microscopy (AFM) colloid probe technique was investigated as a method for the characterisation of adhesional properties of pharmaceutical powder surfaces. Lactose carriers used in dry powder inhaler (DPI) formulations were chosen for investigation since adhesion between the carrier surface and drug particles has been proposed to affect the dispersion of drug particles. Individual adhesion forces were determined by measuring the detachment forces in air between the colloid probe and the lactose particle surface. The colloid probe consisted of a silica sphere (10 microm diameter) attached to a V-shaped silicon nitride cantilever (spring constant, k=0.42 N/m). Adhesion forces were calculated from individual force-distance curves using Hooke's Law. Individual forces measured at various adhesion sites were observed to be reproducible and stable over 10 min (coefficient of variation, CV below 5%). The adhesion force distribution determined from measurements at multiple sites (n>50) on each sample followed a log-normal relationship (regression coefficient, r(2) ranged between 0.95 and 0.99). This enabled characterisation in terms of the geometric mean adhesion force and a geometric standard deviation (GSD). Significant differences (P<0.001) in adhesion force were observed between samples, ranging from 37.47+/-1.95 to 117.48+/-2.20 nN. This study demonstrates the suitability of AFM as sensitive technique for the characterisation of adhesional properties of pharmaceutical particles.     

Affinity scale between a carrier and a drug in DPI studied by atomic force microscopy

The dry powder inhalers (DPIs) consist, in the most cases, of ordered mixture where the particles adhesion results of interactions between the drug and the carrier. Generally, one step of production process is the micronization of the drug particles in order to reduce the size for ordered mixing optimization. But this operation is known to partially create an amorphous surface. In this case, surrounding storage conditions, like relative humidity (RH), are able to modify the percentage of amorphous drug surface. The aim of this study was to investigate surface reactivity, surface energy and direct force measurements by atomic force microscopy (AFM) between lactose (carrier) and zanamivir (drug) crystals references in various conditions of RH. Secondly, an amorphization of the drug surface was induced by humidity relative treatment in order to evaluate the consequences of the transition from crystal to amorphous phase. The study demonstrated that the amorphization of drug surface induces an increase of drug affinity with the carrier surface. Ex situ and in situ amorphization of zanamivir tend to reach the affinity measured between raw materials: carrier and micronized drug particles. AFM allowed adhesion force discrimination between the different forms of the drug particles and demonstrated the potential for investigating adhesion properties in DPI formulation.     

Predicting the behavior of novel sugar carriers for dry powder inhaler formulations via the use of a cohesive-adhesive force balance approach

The aim of this work was to utilize the recently developed cohesive-adhesive balance (CAB) technique for analyzing quantitative AFM measurements to compare the relative forces of interaction of micronized salbutamol sulfate particles and a selection of specifically grown sugar substrates (beta cyclodextrin, lactose, raffinose, trehalose and xylitol). The interfacial behavior was subsequently related to the in-vitro delivery performance of these sugars as carrier particles in dry powder inhalation (DPI) formulations. The CAB analysis indicated that the rank order of adhesion between salbutamol sulfate and the sugars was beta cyclodextrin < lactose < trehalose < raffinose < xylitol. The beta cyclodextrin was the only substrate with which salbutamol sulfate demonstrated a greater cohesive behavior. All other sugars exhibited an adhesive dominance. In-vitro deposition performance of the salbutamol sulfate based carrier DPI formulations showed that the rank order of the fine particle fraction (FPF) was beta cyclodextrin > lactose > raffinose > trehalose > xylitol. A linear correlation (R(2) = 0.9572) was observed between the FPF and cohesive-adhesive ratios of the AFM force measurements. The observed link between CAB analysis of the interactive forces and in-vitro performance of carrier based formulations suggested a fundamental understanding of the relative balance of the various forces of interaction within a dry powder formulation may provide a critical insight into the behavior of these formulations.     

Drug-excipient interactions resulting from powder mixing. III: Solid state properties and their effect on drug dissolution

Micronized prednisone was used to study the effect of powder mixing on drug-excipient interactions and their effect on in vitro dissolution from uncompacted, hand-filled capsules. Two powder formulations contained CaHPO4 X 2H2O (dibasic calcium phosphate dihydrate) as a filler and potato starch or sodium starch glycolate as a disintegrant. The third powder formulation contained pregelatinized starch as a disintegrant/filler. The lubricant in these formulations was magnesium stearate. When drug, CaHPO4 X 2H2O, and the disintegrant were thoroughly mixed and hand filled into capsules without compaction, only approximately 70% of the drug dissolved in 30 min. The incomplete dissolution of the drug was caused by the formation of agglomerates and the inclusion of the drug particles by these agglomerates. In contrast, when a mixture of drug and pregelatinized starch was used, complete dissolution of the drug was achieved after 30 min due to the absence of agglomeration and inclusion. Prolonged mixing of the formulation containing CaHPO4 X 2H2O with magnesium stearate resulted in a decrease in the dissolution rate. The total amount of the drug dissolved at the end of 30 min was reduced from 70 to 20%. The decrease in the rate of drug dissolution resulted from drug-excipient interactions which caused flaking of the magnesium stearate particles. The adhesion of these flakes to the drug particles and drug-excipient agglomerates resulted in hydrophobic coating which reduced water penetration. The rate of drug dissolution was not affected when drug and pregelatinized starch were mixed with magnesium stearate for a prolonged time due to the absence of magnesium stearate flaking and film formation.     

Qualitative proof of liquid dispersion and penetration-involved granule formation in a high shear mixer.

The origination of granules in the early seconds is an important aspect of high shear granulation. To elucidate these mechanisms, a substandard amount (1.5% w/w) of an aqueous hydroxypropyl cellulose solution was added to four different lactose mixtures: (1) lactose 100 M (d(4,3) approximately 170 microm), (2) lactose 200 M (d(4,3) approximately 50 microm), and (3, 4) 10% magnesium stearate/lactose 100 or 200 M. Between 1 and 15 s after binder addition samples were taken, which were immediately frozen in liquid nitrogen. The frozen sample was sieved into granular (> 280 microm) and non-granular-material (< 280 microm). The binder distribution in these fractions was determined. The observed binder distribution behaviour revealed that three different nucleation mechanisms can occur: (I) For lactose 100 M holds that all the binder is initially located in the granules. These granules are subsequently broken again. (II) The lactose 200 M granules also contain 100% of the added binder liquid. Contrary to lactose 100 M the lactose 200 M granules remain intact during the process. It is argued that in both cases liquid penetration is responsible for the accumulation of all liquid in the granules. A theoretical evaluation also confirmed that liquid penetration leads to the formation of the primary granules (III) No liquid penetration is possible in the hydrophobic magnesium stearate/lactose mixtures and the binder is completely dispersed in the non-granular material.     

Lactose as a carrier in dry powder formulations: the influence of surface characteristics on drug delivery.

The aim of the study was to investigate the interdependence of carrier particle size, surface treatment of the carrier, and inclusion of fines on the drug delivery from dry power inhaler formulations. Two size fractions (< 63 and 63-90 microm) of alpha-lactose monohydrate were subjected to treatment with 95% (v/v) ethanol to introduce small asperities or cavities onto the otherwise smooth surface without substantially changing the particle shape. After blending with albuterol sulfate [ALB; volume median diameter (VMD), 1.9 microm; geometric standard deviation (GSD), 1.5], the solvent-treated lactose produced a fine particle fraction (FPF; < 6.18 microm) and dispersibility of the drug that was significantly (ANOVA p < 0.01) lower than that which resulted from formulations containing untreated lactose of a similar size fraction, after aerosolization at 60 L min(-1) via a Rotahaler. The two size fractions of the treated lactose resulted in similar deposition profiles of ALB. The effects of such surface asperities or cavities of lactose were offset by introducing a small amount (5% w/w) of smaller-sized lactose (5-10 microm) to the powder formulations. The fine lactose increased the FPF and dispersibility of ALB to such a level that all lactose batches, regardless of particle size or whether solvent treated, produced a similar fraction of aerosolized ALB. The inclusion of recrystallized needle lactose (5-15 microm) was superior to micronized lactose in improving the aerosolization of ALB. The findings of this study indicate that the presence and characteristics of the finer fraction of lactose carrier particles dominate over the particle size and surface smoothness of the carrier particles in determining dispersion and deaggregation of drugs from dry powder formulations for inhalation.     

Single particle friction on blister packaging materials used in dry powder inhalers

Using atomic force microscopy (AFM) the adhesion and sliding friction behaviour of single lactose particles attached directly to AFM cantilevers has been studied. Measurements were made on the two sides of a blister packaging material used in dry powder inhalers (DPI). Although no significant differences in adhesion were observed, clear differences in particle friction were evident, where one side offers consistently greater friction across the range of loads studied here. The packaging samples were characterised by time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS) and found to have different surface chemistries. The observed difference in friction behaviour is discussed in the context of the differences seen in surface chemistry, topography and hardness. It is reasoned that in this case hardness has the largest influence, and on one sample soft surface layers are displaced by the particle. A clear relationship between friction and load was only observed with one of the three particles tested; this was attributed to multiple asperities being brought into contact, illustrating the important role of nanoscale contact geometry in determining friction behaviour.     

An Atomic Force Microscopy Study of the Effect of Nanoscale Contact Geometry and Surface Chemistry on the Adhesion of Pharmaceutical Particles

PURPOSE: To understand differences in particle adhesion observed with increasing humidity between samples of salbutamol sulfate prepared by two different methods. METHODS: Atomic force microscopy (AFM) force measurements were performed as a function of humidity (<10% to 65% RH) using two systems. The first system used clean AFM tips against compressed disks of micronized and solution enhanced dispersion by supercritical fluid (SEDS) salbutamol. The second system involved particles of both salbutamol samples mounted onto the apexes of AFM cantilevers, and force measurements being performed against a highly orientated pyrolytic graphite (HOPG) substrate. Following these measurements, the contact asperities of the tips were characterized. RESULTS: The first system showed a maximum in the observed adhesion at 22% relative humidity (RH) for the SEDS salbutamol compared to 44% RH for the micronized salbutamol. The second system showed a mix of peaks and continual increases in adhesion with humidity. The predicted Johnson-Kendall-Roberts forces were calculated and divided by the actual forces in order to produce a ratio. CONCLUSIONS: By relating the nature of the asperities to the force measurements, we propose a model in which adhesion scenarios range from single asperity nanometer-scale contact in which peaks in the adhesion were observed, to multiasperity contact where a continuous increase in adhesion was seen with humidity.     

A study of single drug particle adhesion interactions using atomic force microscopy

This paper aims to use Atomic Force Microscopy (AFM) to characterise the interaction forces between micronized salbutamol particles, an active ingredient frequently used in metered dose inhalers, and also to glass, lactose and a fluoropolymer. The methodology used involves challenging a salbutamol functionalized AFM tip to the surfaces of interest and measuring the force experienced by the cantilever as a function of tip-sample separation. Analysis of this force-distance data allows quantification of the particle-substrate adhesion. This study yields a ranking of adhesion as glass>lactose>salbutamol>polytetrafluoroethylene (PTFE). An increase in the interaction force between the salbutamol particle and PTFE on repeated contact due to tribocharging is also observed.     

Adhesion forces in interactive mixtures for dry powder inhalers--evaluation of a new measuring method.

Dry powder inhalers mostly contain carrier based formulations where micronized drug particles are adhered to coarse carrier particles. The performance of the dry powder inhaler depends on the inhaler device, the inhalation manoeuvre and the formulation. The most important factor influencing the behaviour of the formulation is the adhesion force acting between the active ingredient and the carrier particles, which can be measured using different methods, for example the centrifuge technique or atomic force microscopy. In this study the tensile strength method, usually applied to determine cohesion forces between powder particles of one material, is optimized for adhesion force measurements between powder particles of unlike materials. Adhesion force measurements between the carrier materials lactose or mannitol and the drug substance salbutamol sulphate using the tensile strength method and the atomic force microscopy show higher values with increasing relative humidity. Consequently, the fine particle fraction determined using the Next Generation Impactor decreases with increasing relative humidity as a result of the enhanced interparticle interactions.     

Surface Energy and Interparticle Force Correlation in Model pMDI Formulations

Purpose. To compare experimental measurements of particle cohesion and adhesion forces in a model propellant with theoretical measurements of the interfacial free energy of particulate interactions; with the aim of characterizing suspension stability of pressurized metered dose inhalers (pMDIs).Methods. Interparticulate forces of salbutamol sulfate, budesonide, and formoterol fumarate dihydrate were investigated by in situ atomic force microscopy (AFM) in a model propellant 2H,3H perfluoropentane. The surface thermodynamic properties were determined by contact angle (CA) and inverse gas chromatography (IGC). Experimental data were compared with theoretical work of adhesion/cohesion using a surface component approach (SCA), taking into account both dispersive and polar contributions of the surface free energy.Results. Results indicated that the measured forces of interaction between particles in model propellant could not be accounted for by theoretical treatment of the dispersive surface free energies via CA and IGC. A correlation between theoretical work of adhesion/cohesion and AFM measurements was observed upon the introduction of the polar interfacial interactions within the SCA model.Conclusions. It is suggested that the polar contributions of the surface free energy measurements of particles may play a crucial role in particle interaction within propellant-based systems. Together with the application of a SCA model, this approach may be capable of predicting suspension stability of pMDI formulations. Note: This revised article was published online in May 2005 and now shows a corrected title.     

Improvement of Inhalation Profile of DPI Formulations by Carrier Treatment with Magnesium Stearate

In the present study, the applicability of magnesium stearate-treated lactose carrier particles was evaluated to improve the inhalation performance of catalase microspheres. Bovine liver catalase microspheres with different stabilizers were prepared by spray drying. For the surface modification, lactose products were treated with 1 and 2% magnesium stearate by manual mixing and gently passed through a # 60 size mesh. The results revealed that magnesium stearate can be applied as a performance improver for catalase microsphere based DPI formulations.     

Towards screening of inhalation formulations: measuring interactions with atomic force microscopy

This review charts the progress of atomic force microscopy (AFM) to investigate particle interactions relevant to the performance of inhalers. AFM provides a unique opportunity to examine and quantify single particle behaviour of powdered drugs and excipients in a variety of environmental conditions. An introduction to AFM and particle interactions is given. Comparative experiments that rank adhesion between materials, and quantitative experiments that lead to the measurement of properties such as the work of adhesion and surface energy, are reviewed. The AFM has been widely used to investigate the effects of relative humidity and surface roughness on particle adhesion; these experiments are also reviewed. In the final section, the potential of this approach to screen formulations is discussed.