Effect of hydrophobic coating on the behavior of a hygroscopic aerosol powder in an environment of controlled temperature and relative humidity

Advancements in Particle Engineering for Inhalation Delivery of Small Molecules and Biotherapeutics

Dry powder inhalation formulations have become increasingly popular for local and systemic delivery of small molecules and biotherapeutics. Powder formulations provide distinct advantages over liquid formulations such as elimination of cold chain due to room temperature stability, improved portability, and the potential for increasing patient adherence. To become a viable product, it is essential to develop formulations that are stable (physically, chemically and/or biologically) and inhalable over the shelf-life. Physical particulate properties such as particle size, morphology and density, as well as chemical properties can significantly impact aerosol performance of the powder. This review will cover these critical attributes that can be engineered to enhance the dispersibility of inhalation powder formulations. Challenges in particle engineering for biotherapeutics will be assessed, followed by formulation strategies for overcoming the hurdles. Finally, the review will discuss recent examples of successful dry powder biotherapeutic formulations for inhalation delivery that have been clinically assessed.

Optimizing Spray-Dried Porous Particles for High Dose Delivery with a Portable Dry Powder Inhaler

This manuscript critically reviews the design and delivery of spray-dried particles for the achievement of high total lung doses (TLD) with a portable dry powder inhaler. We introduce a new metric termed the product density, which is simply the TLD of a drug divided by the volume of the receptacle it is contained within. The product density is given by the product of three terms: the packing density (the mass of powder divided by the volume of the receptacle), the drug loading (the mass of drug divided by the mass of powder), and the aerosol performance (the TLD divided by the mass of drug). This manuscript discusses strategies for maximizing each of these terms. Spray drying at low drying rates with small amounts of a shell-forming excipient (low Peclet number) leads to the formation of higher density particles with high packing densities. This enables ultrahigh TLD (>100 mg of drug) to be achieved from a single receptacle. The emptying of powder from capsules is directly proportional to the mass of powder in the receptacle, requiring an inhaled volume of about 1 L for fill masses between 40 and 50 mg and up to 3.2 L for a fill mass of 150 mg.

Inhaled antibodies: Quality and performance considerations

The use of antibodies in the treatment of lung diseases is of increasing interest especially as the search for COVID-19 therapies has unfolded. Historically, the use of antibody therapy was based on multiple targets including receptors involved in local hyper-reactivity in asthma, viruses and micro-organisms involved in a variety of pulmonary infectious disease. Generally, protein therapeutics pose challenges with respect to formulation and delivery to retain activity and assure therapy. The specificity of antibodies amplifies the need for attention to molecular integrity not only in formulation but also during aerosol delivery for pulmonary administration. Drug product development can be viewed from considerations of route of administration, dosage form, quality, and performance measures. Nebulizers and dry powder inhalers have been used to deliver protein therapeutics and each has its advantages that should be matched to the needs of the drug and the disease. This review offers insight into quality and performance barriers and the opportunities that arise from meeting them effectively.

The effect of metal salts on aerosol performance of spray dried carrier-free formulations of levofloxacin

PURPOSE

Metal salts are used in formulation of dry powder inhalers (DPIs) for different purposes. Recently the role of these salts in production of small, dense but highly dispersible particles has emerged. In this study the effect of some such salts on dispersibility and respirability of spray dried levofloxacin formulations was evaluated in normal and reduced inhalation air flow or by increasing powder filling in capsules.

METHODS

levofloxacin was co-spray dried with different concentrations of common metal chlorides (NaCl, KCl, CaCl₂ and MgCl₂) either with or without leucine as dispersibility enhancer. Particle size, moisture, morphology, triboelectrification tendency and fine particle fraction (FPF) of resulting powders were evaluated. In addition, the effect of these salts and leucine on dispersibility of resulting powders in reduced air flow rate and increased capsule filling mass were evaluated.

RESULTS

Presence of higher tested concentrations of divalent cations increased water content, and reduced FPF significantly. Addition of leucine reduced water content and electrostatic charge, increased particle size and FPF and improved spray drying yield significantly. Lower concentrations of salts did not affect FPF of leucine containing powders significantly, but presence of 2.5% NaCl or MgCl₂ preserved the dispersibility in higher capsule fillings. A 2.5% concentration of NaCl in such formulations preserved dispersibility in lower air flows.

CONCLUSION

Higher amounts of divalent salts increases triboelectrification and moisture absorption, and reduces FPF. Lower concentrations of NaCl could not improve FPF of leucine containing formulations significantly, but preserves dispersibility in low air flows and high capsule fillings. Graphical abstract.

Fumaryl diketopiperazine based effervescent microparticles to escape macrophage phagocytosis for enhanced treatment of pneumonia via pulmonary delivery

The treatment of pulmonary infections with antibiotics administered via pulmonary delivery provides for higher local therapeutic efficacy rather than through systemic administration. Pneumonia is globally considered a major cause of death due to a lack of proper medication. The treatment of pneumonia with inhalable antibiotics (such as azithromycin (AZM)) can provide a maximum pulmonary therapeutic effect without significant systemic side effects. Compared to non-effervescent microparticles, effervescent microparticles can provide an active driving force to release loaded antibiotics for subsequent distribution deep into the lung by virtue of its smaller size. In this study, N-fumaroylated diketopiperazine (FDKP) was used as a carrier to prepare effervescent inhalable microparticles loaded with AZM (AZM@FDKP-E-MPs). This effervescent dry powder was characterized for both in vitro and in vivo deposition in the lung and the results obtained showed significant improvement in lung deposition and anti-bacterial efficiency, suggesting a strong potential application for pneumonia treatment.

Understanding the Impact of Relative Humidity and Coexisting Soluble Iron on the OH-Initiated Heterogeneous Oxidation of Organophosphate Flame Retardants

The current uncertainties in the reactivity and atmospheric persistence of particle-associated chemicals present a challenge for the prediction of long-range transport and deposition of emerging chemicals such as organophosphate flame retardants, which are ubiquitous in the global environment. Here, the OH-initiated heterogeneous oxidation kinetics of organophosphate flame retardants (OPFRs) coated on inert (NH₄)₂SO₄ and redox-active FeSO₄ particles were systematically determined as a function of relative humidity (RH). The derived reaction rate constants for the heterogeneous loss of tricresyl phosphate (TCP; k_TCP) and tris(2-butoxyethyl) phosphate (TBEP; k_TBEP) were in the range of (2.69-3.57) × 10^-12 and (3.06-5.55) × 10^-12 cm³ molecules^-1 s^-1, respectively, depending on the RH and coexisting Fe(II) content. The k_TCP (coated on (NH₄)₂SO₄) was relatively constant over the investigated RH range while k_TBEP was enhanced by up to 19% with increasing RH. For both OPFRs, the presence of Fe(II) enhanced their k by up to 53% over inert (NH₄)₂SO₄. These enhancement effects (RH and Fe(II)) were attributed to fundamental changes in the organic phase state (higher RH lowered particle viscosity) and Fenton-type chemistry which resulted in the formation of reactive oxygen species, respectively. Such findings serve to emphasize the importance of ambient RH, the phase state of particle-bound organics in general, and the presence of coexisting metallic species for an accurate description of the degradation kinetics and aging of particulate OPFRs in models used to evaluate their atmospheric persistence.

Investigation of Electrostatic Behavior of Dry Powder-Inhaled Model Formulations

The accumulation of electrostatic charge on drug particles and excipient powders arising from interparticulate collisions or contacts with other surfaces can lead to agglomeration and adhesion problems during the manufacturing process, filling, and delivery of dry powder inhaler (DPI) formulations. The objective of the study was to investigate the role of triboelectrification to better understand the influence of electrostatic charge on the performance of DPIs with 2 capsule-based dimensionally similar devices constructed with different materials. In addition, strategies to reduce electrostatic charge build up during the manufacturing process, and the processes involved in this phenomenon were investigated. Electrostatic charge measurements showed that there was a significant difference in electrostatic charge generated between tested formulations and devices. This affects particle detachment from carrier and thus significantly impacts aerosol performance. Conditioning fluticasone DPI capsules at defined temperature and humidity conditions reduced electrostatic charges acquired during manufacturing. Conditioning salmeterol DPI capsules at same conditions seemed disadvantageous for their aerosol performance because of increasing capillary forces and solid bridge formation caused by water absorption. Knowledge and understanding of the role of electrostatic forces in influencing DPI formulation performance was increased by these studies.

The influence of storage relative humidity on aerosolization of co-spray dried powders of hygroscopic kanamycin with the hydrophobic drug rifampicin

The purpose of this study was to investigate the influence of storage humidity on in vitro aerosolization and physicochemical properties of co-spray dried powders of kanamycin with rifampicin. The powders were stored for one-month in an open Petri dish at different relative humidities (RHs) (15%, 43%, and 75%) and 25 ± 2 °C. The in vitro aerosolization (fine particle fraction, FPF) of the powders was determined by a next generation impactor (NGI). The moisture content, particle morphology and crystallinity of the powders were determined by Karl Fischer titration, scanning electron microscopy, and X-ray powder diffractometry, respectively. At all RH, the FPF of hydrophobic rifampicin-only powder was unaffected but the FPF of hygroscopic kanamycin-only powder significantly decreased even at 43% RH. The kanamycin-only particles fused together, crystallized and formed hard cakes at 75% RH. The aerosolization of kanamycin and rifampicin in the combination powders remained unaffected at 15% and 43% RH, but aerosolization significantly decreased at 75% RH. Enrichment of the surface of the particles with hydrophobic rifampicin did not protect the combination powders from moisture uptake but it prevented particle agglomeration up to 43% RH. At 75% RH, the moisture uptake led to agglomeration of the particles of the combination powder particles and consequently an increase in aerodynamic diameter. Further studies are required to investigate how rifampicin enrichment prevents particle agglomeration, the possible mechanisms (e.g. particle interactions due to capillary forces or electrostatic forces) for the changes in the aerosolization and changes in surface composition during storage.

Moisture-Resistant Co-Spray-Dried Netilmicin with l-Leucine as Dry Powder Inhalation for the Treatment of Respiratory Infections

Netilmicin (NTM) is one of the first-line drugs for lower respiratory tract infections (LRTI) therapy, but its nephrotoxicity and ototoxicity caused by intravenous injection restrict its clinical application. Dry powder inhalation (DPI) is a popular local drug delivery system that is introduced as a solution. Due to the nature of NTM hygroscopicity that hinders its direct use through DPI, in this study, L-leucine (LL) was added into NTM dry powder to reduce its moisture absorption rate and improve its aerosolization performance. NTM DPIs were prepared using spray-drying with different LL proportions. The particle size, density, morphology, crystallinity, water content, hygroscopicity, antibacterial activity, in vitro aerosolization performance, and stability of each formulation were characterized. NTM DPIs were suitable for inhalation and amorphous with a corrugated surface. The analysis indicated that the water content and hygroscopicity were decreased with the addition of LL, whilst the antibacterial activity of NTM was maintained. The optimal formulation ND₂ (NTM:LL = 30:1) showed high fine particle fraction values (85.14 ± 8.97%), which was 2.78-fold those of ND₀ (100% NTM). It was stable after storage at 40 ± 2 °C, 75 ± 5% relative humidity (RH). The additional LL in NTM DPI successfully reduced the hygroscopicity and improved the aerosolization performance. NTM DPIs were proved to be a feasible and desirable approach for the treatment of LRTI.

High dose dry powder inhalers to overcome the challenges of tuberculosis treatment

Tuberculosis (TB) is a major global health burden. The emergence of the human immunodeficiency virus (HIV) epidemic and drug resistance has complicated global TB control. Pulmonary delivery of drugs using dry powder inhalers (DPI) is an emerging approach to treat TB. In comparison with the conventional pulmonary delivery for asthma and chronic obstructive pulmonary disease (COPD), TB requires high dose delivery to the lung. However, high dose delivery depends on the successful design of the inhaler device and the formulation of highly aerosolizable powders. Particle engineering techniques play an important role in the development of high dose dry powder formulations. This review focuses on the development of high dose dry powder formulations for TB treatment with background information on the challenges of the current treatment of TB and the potential for pulmonary delivery. Particle engineering techniques with a particular focus on the spray drying and a summary of the developed dry powder formulations using different techniques are also discussed.

Formulation techniques for high dose dry powders

Delivery of drugs to the lungs via dry powder inhaler (DPI) is a promising approach for the treatment of both local pulmonary conditions and systemic diseases. Though DPIs are widely used for the pulmonary deposition of potent bronchodilators, anticholinergics, and corticosteroids, there is growing interest in the utilization of this delivery system for the administration of high drug doses to the lungs, as made evident by recent regulatory approvals for anti-microbial, anti-viral and osmotic agents. However, the formulation of high dose DPIs carries several challenges from both a physiological and physicochemical standpoint. This review describes the various formulation techniques utilized to overcome the barriers associated with the pulmonary delivery of high dose powders.

Azithromycin-loaded respirable microparticles for targeted pulmonary delivery for the treatment of pneumonia

Pneumonia is a major contributor to infection-based hospitalizations and deaths in the United States. Antibiotics such as azithromycin (AZM), although effective at managing pneumonia, often suffer from off-target diffusion and poor bioavailability when administered orally or via intravenous injection. The formation of biofilms at the disease sites makes the treatment more complicated by protecting bacteria from antimicrobial agents and thus necessitating a much higher dosage of antibiotics to eradicate the biofilms. As such, targeted pulmonary delivery of antibiotics has emerged as a promising alternative by providing direct access to the lung while also allowing higher local therapeutic concentrations but minimal systemic exposure. In this study, AZM was encapsulated in N-fumaroylated diketopiperazine (FDKP) microparticles for efficient pulmonary delivery. Both in vitro and in vivo results demonstrated that AZM@FDKP-MPs administered via intratracheal insufflation achieved at least a 3.4 times higher local concentration and prolonged retention times compared to intravenous injection and oral administration, suggesting their potential to better manage bacterial pneumonia.

A review of co-milling techniques for the production of high dose dry powder inhaler formulation

Drug delivery by inhalation offers several advantages compared to other dosage forms, including rapid clinical onset, high bioavailability, and minimal systemic side effects. Drug delivery to the lung can be achieved as liquid suspensions or solutions in nebulizers and pressurized metered-dose inhalers (pMDI), or as dry powders in dry powder inhalers (DPIs). Compared to other delivery systems, DPIs are, in many cases, considered the most convenient as they are breath actuated and do not require the use of propellants. Currently, the delivery of low drug doses for the treatment of lung conditions such as asthma and chronic obstructive pulmonary disease are well established, with numerous commercial products available on the market. The delivery of low doses can be achieved from either standard carrier- or aggregate-based formulations, which are unsuitable in the delivery of high doses due to particle segregation associated with carrier active site saturation and the cohesiveness of micronized aggregates which have poor flow and de-agglomeration properties. High-dose delivery is required for the treatment of lung infection (i.e. antibiotics) and in the emerging application of drug delivery for the management of systemic conditions (i.e. diabetes). Therefore, there is a demand for new methods for production of high-dose dry powder formulations. This paper presents a review of co-mill processing, for the production of high-efficiency inhalation therapies, including the jet mill, mechanofusion, or ball mill methodologies. We investigate the different techniques, additives, and drugs studied, and impact on performance in DPI systems.

On the effects of blending, physicochemical properties, and their interactions on the performance of carrier-based dry powders for inhalation - A review

Blending drug and carrier powders to produce homogeneous drug-carrier adhesive mixtures is a key step in the production of dry powder inhaler (DPI) formulations. Although the blending conditions can result in different conclusions or probably change the outcome of a study entirely if being selected differently, there is a scarcity of data on the influence of blending processes on the physicochemical properties of bulk powder formulations and the follow-on effects on DPI performance. This paper provides an overview of the interactions between variables related to blending conditions (e.g. blending equipment, time, speed and sequence as well as environmental humidity) and powder physicochemical properties (e.g. size distribution, shape distribution, density, anomeric composition, electrostatic charge, surface, and bulk properties), and their effects on the performance of adhesive mixtures for inhalation in terms of drug content homogeneity, drug-carrier adhesion, and drug aerosolisation behaviour. The relevance of carrier payload, batch size and segregation was also discussed. Challenges and future directions were identified. This review therefore contributes towards a better understanding of the blending process, powder physicochemical properties, and their interlinked effects on the fundamental understanding of adhesive mixtures for inhalation. The knowledge gained is essential to ensure optimum blending and thereby controlled functionality of DPIs.

Effect of compression pressure on inhalation grade lactose as carrier for dry powder inhalations

Introduction:

This study focused on the potential effects of compression forces experienced during lactose (InhaLac 70, 120, and 230) storage and transport on the flowability and aerosol performance in dry powder inhaler formulation.

Materials and Methods:

Lactose was subjected to typical compression forces 4, 10, and 20 N/cm². Powder flowability and particle size distribution analysis of un-compressed and compressed lactose was evaluated by Carr's index, Hausner's ratio, the angle of repose and by laser diffraction method. Aerosol performance of un-compressed and compressed lactose was assessed in dispersion studies using glass twin-stage-liquid-impenger at flow rate 40-80 L/min.

Results:

At compression forces, the flowability of compressed lactose was observed same or slightly improved. Furthermore, compression of lactose caused a decrease in in vitro aerosol dispersion performance.

Conclusion:

The present study illustrates that, as carrier size increases, a concurrent decrease in drug aerosolization performance was observed. Thus, the compression of the lactose fines onto the surfaces of the larger lactose particles due to compression pressures was hypothesized to be the cause of these observed performance variations. The simulations of storage and transport in an industrial scale can induce significant variations in formulation performance, and it could be a source of batch-to-batch variations.

Condensational growth of combination drug-excipient submicrometer particles for targeted high-efficiency pulmonary delivery: evaluation of formulation and delivery device

OBJECTIVES

The objective of this study was to investigate the in-vitro particle-size growth of combination drug and excipient submicrometer aerosols generated from a series of formulations and two aerosol delivery devices.

METHODS

Submicrometer combination drug and excipient particles were generated experimentally using both the capillary aerosol generator and the Respimat inhaler. Budesonide and albuterol sulfate were used as model drugs and were formulated with sodium chloride, citric acid and mannitol as excipients in various ratios. Aerosol growth was evaluated in-vitro in a coiled-tube geometry designed to provide residence times and thermodynamic conditions consistent with the airways.

KEY FINDINGS

Submicrometer combination drug : excipient aerosols when exposed to simulated respiratory conditions increased to micrometer size suitable for pulmonary deposition. It was possible to control the aerosol growth ratio by altering: (1) the hygroscopic excipient, (2) the drug : excipient ratio and (3) the drug. The applicability of this approach was demonstrated using the capillary aerosol generator and the Respimat inhaler.

CONCLUSIONS

The enhanced excipient growth approach may enable the delivery of submicrometer aerosol particles that increase in size within the airways and result in high percentages of pulmonary deposition.

Formulation and Evaluation of Insulin Dry Powder for Inhalation

PURPOSE

Dry powder formulation of insulin for pulmonary administration was prepared to obtain increased drug deposition in the alveolar absorptive region. The deposition was studied by investigating the dispersion and deaggregation of insulin from the carrier lactose using an Andersen cascade impactor and twin stage impinger. The subsequent absorption following the deposition was studied by in vivo method.

METHODS

Insulin in solution with absorption promoters was lyophilized. The powder was incorporated with lactose of different grades and their combinations as carriers to deliver using an inhaler device. Solid-state characteristics of the carrier as well as the drug powder were assessed by particle size and distribution measurement. The flow properties such as moisture content, powder density, angle of repose, and carr's compressibility index of the powder mixture were determined. The aerosol behavior of the powder was studied by dispersion using rotahaler(c) connected to a twin-stage impinger (TSI) and an eight-stage Andersen cascade impactor (ACI) operating at different flow rates of 30-90 l/min. The in vivo performance was studied by deliverance to the respiratory tract of guinea pigs. The intratracheal bioavailability with respective to intravenous route was calculated by measuring the blood glucose reduction.

RESULTS

The coarser particles of lactose in fractions of carrier containing a wide particle size distribution impacted in the preseperator of cascade impactor, and only the particle less than 10 microm size entered stage 0-stage 7. Formulation containing 1:1 mixture of Respitose ML006 (62%<50 microm) and Respitose ML003 (37.8%<50 microm) as carrier imparts well deaggregation of insulin, and higher deposition leads to 52.3% of fine particle fraction at 60 Lit/min and in vivo bioavailability of 82%.

CONCLUSIONS

Insulin formulations containing 1:1 mixture of Respitose ML006 and Respitose ML003 as carrier can impart deeper deposition of drug particles and cause higher bioavailability. This suggests that carrier used in the formulation influenced the amount of insulin deposition in the alveolar region of the lung. Hence, it was concluded that the availability of insulin for systemic absorption depends on the particle size of the drug as well as the carrier lactose.

Carrier-based strategies for targeting protein and peptide drugs to the lungs

With greater interest in delivery of protein and peptide-based drugs to the lungs for topical and systemic activity, a range of new devices and formulations are being investigated. While a great deal of recent research has focused on the development of novel devices, attention must now be paid to the formulation of these macromolecular drugs. The emphasis in this review will be on targeting of protein/peptide drugs by inhalation using carriers and ligands.

Stability and aerodynamic behaviour of glucocorticoid particles prepared by a supercritical fluids process

Particle processing techniques using supercritical fluids (SF) are potential alternative technologies to design particles for inhalation. Powders of budesonide and flunisolide were prepared using solution enhanced dispersion by supercritical fluids (SEDS) process. The aim was to determine thermodynamic stability of different polymorphs of flunisolide including new forms from SEDS technology and to characterise micronised and SEDS produced powders of budesonide and flunisolide for their suitability as inhalation powders. Acetone and methanol solutions of budesonide and flunisolide, with a concentration of 2.5 mg/ml, were used for the particle preparation. The pressure was 100 bar and temperatures were 60 degrees C or 80 degrees C. The flow rates of CO(2) and drug solution were 9 ml/min and 0.3 ml/min, respectively. Chemical purity of different polymorphs of flunisolide was estimated using high performance liquid chromatography (HPLC) and thermal behaviour was determined using differential scanning calorimetry (DSC). Particle morphology and surface examination were performed using scanning electron microscopy (SEM) and atomic force microscopy (AFM), respectively. The particle size distribution and density of the powders were determined with the help of Coulter Counter and helium pycnometer respectively. The in vitro deposition of the powders was studied using multistage liquid impinger (MLI). From the stability study, it was found that the two forms of flunisolide, polymorphs II and hemihydrate, were the most stable. Flunisolide form III was transformed to hemihydrate during the stability study. The chemical purity of the material was increased after SEDS processing. SEDS produced powders of budesonide and flunisolide form III from acetone showed narrow volumetric particle size distributions with 90% of the particles below 4 microm and geometric mean size around 3 microm. However, in the MLI study, budesonide powder obtained from SEDS with acetone showed favorable deposition in the lower stages with a mass median aerodynamic diameter (MMAD) of around 3 microm whilst the flunisolide form III was preferentially deposited in the higher stages of the MLI with MMAD of over 5 microm, due to aggregation of the particles. Particles of budesonide and flunisolide, in the size range, suitable for inhalation, were reproducibly produced using SEDS.

Preparation of Disodium Fluorescein Powders in Association with Lauric and Capric Acids

In previous studies micronized disodium fluorescein (DF) powders were coated with lauric or capric acid. This was a precursor to evaluating the effects of the fatty acids on the hygroscopic growth of the DF aerosols. Studies of the association of fatty acids with DF reflected a limited interpretation of the data obtained. The present study examines more closely the phenomena involved in coating the DF. It is concluded that lateral interactions due to the hydrophobic aliphatic tails of the fatty acids give rise to adsorption, aggregation, and flocculation, all of which cause inflections in the association isotherm rendering it nonlinear.

Fundamental effects of particle morphology on lung delivery: predictions of Stokes' law and the particular relevance to dry powder inhaler formulation and development

Key factors that contribute to the aerodynamic properties of aerosol particles are found in Stokes' law. These factors may be monitored or controlled to optimize drug delivery to the lungs. Predictions of the aerodynamic behavior of therapeutic aerosols can be derived in terms of the physical implications of particle slip, shape and density. The manner in which each of these properties have been used or studied by pharmaceutical scientists to improve lung delivery of drugs is readily understood in the context of aerosol physics. Additional improvement upon current aerosol delivery of particulates may be predicted by further theoretical scrutiny.

Study of the hygroscopic properties of selected pharmaceutical aerosols using single particle levitation

PURPOSE

To use a single particle levitation technique to investigate the equilibrium water sorption characteristics in both the evaporation and growth of four respiratory drugs at 37 degrees C: atropine sulfate (AS), isoproterenol hydrochloride (IPHC) and isoproterenol hemisulfate (IPHS) and disodium cromoglycate (DSCG).

METHODS

The equilibrium water content was measured as a function of relative humidity (RH) by a single particle levitation technique using an electrodynamic balance (EDB). The change of water content was determined by the voltage required to balance the weight of the levitated particle electrostatically. The water activities of bulk samples were also measured. Growth ratios were determined and compared with values in the literature.

RESULTS

Crystallization or deliquescence was not observed for AS, IPHC and IPHS. The hysteresis in the water cycle was not observed for any of the drugs. At RH approximately 0%, AS particles still contain about 5% water but IPHC and IPHS particles do not contain any residual water. The aerodynamic growth ratio from RH 0% to 99.5% is 2.60, 2.86, 2.42 and 1.26 for AS, IPHC, IPHS and DSCG, respectively. Supersaturated droplets of IPHC and IPHS are expected to exist in the ambient conditions. DSCG is in a solid state in the RH range of 10-90%.

CONCLUSIONS

It is expected that some aerosolized drugs of low solubility may experience supersaturation before they enter the human body and this could exert a significant influence both on particle loss before inhalation and on the deposition of the drugs in the lungs. The EDB is a convenient and reliable tool for studying the hygroscopic properties of pharmaceutical aerosols, especially for supersaturated solutions.

Study of hygroscopic properties of aqueous mixtures of disodium fluorescein and sodium chloride using an electrodynamic balance

PURPOSE

The purposes of this study are: a) to demonstrate the use of an Electrodynamic Balance (EDB) to investigate the hygroscopic properties of pharmaceutical aerosols; and b) to evaluate the applicability of the Zdanovskii-Stokes-Robinson model (ZSR) in the associated data analysis with multicomponent pharmaceutical aerosols.

METHODS

The compositional dependence of the water activity of two model materials commonly employed in the study of pharmaceutical aerosols, namely, NaCl and Disodium Fluorescein (DF), was investigated using an EDB. The water contents of single levitated droplets of NaCl and DF and their mixtures at mass ratios of 1:3, 1:1, 3:1, and 6:1 from dilute concentration to high supersaturation were determined as a function of relative humidity (RH).

RESULTS

At decreasing ambient RH, supersaturated aqueous NaCl droplets lose water and crystallize to form dry solid particles at an RH of approximately 50%. Aqueous DF droplet continues to lose water until it reaches a final state containing about 20% by mass of residual water. Mixed solutions of DF and NaCl crystallize at an RH of approximately 50% and then continue to lose water at lower RHs. The resulting "dried" particle still contains water whose amount depends on the mass ratios of DF and NaCl in the mixture. Good prediction of water activity of the DF-NaCl mixture can be achieved with the ZSR model. Collection of a full set of water activity-composition data at each mass ratio of DF-NaCl requires only a few hours.

CONCLUSIONS

The EDB, together with the application of the ZSR model in data treatment, appears to be a valuable tool for studying the hygroscopic properties of pharmaceutical aerosols.

Behavior of hygroscopic pharmaceutical aerosols and the influence of hydrophobic additives

The high temperature and relative humidity in the lung can result in the hygroscopic growth of susceptible aerosol particles or droplets. The term hygroscopic growth describes the increase in particle diameter which occurs as the result of association with water vapor. The influence of hygroscopicity upon lung deposition of aerosols has been a productive area of research in industrial hygiene, environmental sciences, and inhalation toxicology. Many pharmaceutical inhalation aerosols display hygroscopic behavior in their passage through the airways; however, the effect has been neglected. Controlling the phenomenon of hygroscopic growth and, thus, the related lung deposition of aerosols might result in the therapeutic advantage of targeting the site of action. Such an approach might also allow identification of the location of pharmacologic receptor sites in the lung. This Review discusses an approach to achieving control of hygroscopic growth of aerosol particles. Theoretical and experimental studies have indicated that inhaled particle diameters increased significantly for drugs commonly administered to the lung. The presence of certain additives, notably glycerol, cetyl alcohol, and lauric and capric acids, has been demonstrated to reduce the growth of particles under conditions approaching those in the lung. Very few quantitative studies of the nature discussed herein have appeared in the literature. It is conceivable that an aerosol particle could be fabricated of known initial size and density, and by implication, deposition characteristic, and this might be induced to follow specific growth kinetics to enhance deposition in a particular region of the lung. Thus, physical targeting of regions within the lung might be achieved.