Danforth P. Miller

Thermophysical Properties of Trehalose and Its Concentrated Aqueous Solutions

AbstractPurpose. To address the lack of fundamental thermophysical data for trehalose and its aqueous systems by measuring equilibrium and non-equilibrium properties of such systems. Methods/Results. Differential scanning calorimetry (DSC) and dynamic mechanical analysis were used to measure glass transition temperatures of trehalose and its solutions. X-ray diffractometry was used to verify the structure of amorphous trehalose. Controlled-stress rheometry was used to measure viscosity of several aqueous trehalose systems at ambient and sub-ambient temperatures. Over this temperature range, the density of these solutions was also measured with a vibrating tube densimeter. The equilibrium phase diagram of aqueous trehalose was determined by measuring the solubility and freezing point depression. Conclusions. Our solubility measurements, which have allowed long times for attainment of chemical equilibrium, are substantially different from those reported earlier that used different techniques. Our measurements of the glass transition temperature of trehalose are higher than reported values. A simple model for the glass transition is presented to describe our experimental observations.

Microcalorimetric Measurement of the Interactions Between Water Vapor and Amorphous Pharmaceutical Solids

AbstractPurpose. Use a microcalorimetric technique to measure the interactions between water vapor and amorphous pharmaceutical solids and describe the relationship between long-term physical stability and the storage relative humidity (RH) at constant temperature. Methods. A thermal activity monitor was used to characterize interactions of water vapor with spray-dried amorphous sucrose, lactose, raffinose, and sodium indomethacin. Differential scanning calorimetry was used to measure glass transition temperature, Tg. X-ray powder diffraction was used to confirm that the spray-dried samples were amorphous. Scanning electron microscopy was used to examine particle morphology. Specific surface area was determined by BET analysis of nitrogen and krypton adsorption isotherms. Results. The moisture-induced thermal activity traces (MITATs) of the materials in this study exhibit general behavior that helps explain the effect of moisture content on the physical stability of the glassy phase at a given storage temperature. At some RH threshold, RHm, the MITAT exhibits a dramatic increase in the energy of interaction between water vapor and the glass that cannot be explained by a phase or morphology change. Calorimetric data indicate that water vapor-solid interactions are reversible below RHm; above RHm, energetic hysteresis is observed and water-water interactions predominate. In addition, the MITAT was deconvoluted into sorptive and nonsorptive components, making it possible to assign the observed heat flow to unique thermal events. Samples stored at a RH just below RHm for more than 2 months show no evidence of morphology or phase change. In addition, the MITAT can be deconvoluted into sorptive and nonsorptive components by using a twin-calorimeter arrangement. This analysis provides specificity to the microcalorimetric analysis and helps explain the nature of the physical changes that occur during the hydration glassy phase. Conclusions. The MITAT is a useful tool to determine the onset of moisture-induced physical instability of glassy pharmaceuticals and may find a broad application to determine appropriate storage conditions to ensure long-term physical stability.

Solid-State Stability of Spray-Dried Insulin Powder for Inhalation: Chemical Kinetics and Structural Relaxation Modeling of Exubera Above and Below the Glass Transition Temperature

The effect of temperature on the chemical stability of an amorphous spray-dried insulin powder formulation (Exubera) was evaluated in the solid state at constant moisture content. The chemical stability of the powder was assessed using reversed-phase high-performance liquid chromatography (RP-HPLC) and high-performance-size exclusion chromatography (HP-SEC). The major degradants in spray-dried insulin produced during heat stressing were identified as A21-desamidoinsulin (A21) and high molecular weight protein (HMWP). As expected, the rates of formation of A21 and HMWP were observed to increase with temperature. A stretched-time kinetic model (degradation rate is proportional to the square root of time) was applied to the degradant profiles above and below the glass transition temperature (T(g)) and apparent reaction rate constants were determined. Below T(g), isothermal enthalpy of relaxation measurements were used to assess the effect of temperature on molecular mobility. The formation of A21 and HMWP was found to follow an Arrhenius temperature dependence above and below the T(g). Comparison of reaction rate constants to those estimated from structural relaxation experiments suggests that the reaction pathways to form A21 and HMWP below the T(g) may be coupled with the molecular motions involved in structural relaxation.

Rapid Assessment of the Structural Relaxation Behavior of Amorphous Pharmaceutical Solids: Effect of Residual Water on Molecular Mobility

PurposeUse RH-perfusion microcalorimetry and other analytical techniques to measure the interactions between water vapor and amorphous pharmaceutical solids; use these measurements and a mathematical model to provide a mechanistic understanding of observed calorimetric events.MaterialsIsothermal microcalorimetry was used to characterize interactions of water vapor with a model amorphous system, spray-dried raffinose. Differential scanning calorimetry was used to measure glass transition temperature, Tg. High-sensitivity differential scanning calorimetry was used to measure enthalpy relaxation. X-ray powder diffraction (XRPD) was used to confirm that the spray-dried samples were amorphous. Scanning electron microscopy (SEM) was used to examine particle morphology. Gravimetric vapor sorption was used to measure moisture sorption isotherms. Thermogravimetric analysis (TGA) was used to measure loss on drying.ResultsA moisture-induced thermal activity trace (MITAT) provides a rapid measure of the dependence of molecular mobility on moisture content at a given storage temperature. At some relative humidity threshold, RHm, the MITAT exhibits a dramatic increase in the calorimetric rate of heat flux. Simulations using calorimetric data indicate that this thermal event is a consequence of enthalpy relaxation.ConclusionsRH-perfusion microcalorimetry is a useful tool to determine the onset of moisture-induced physical instability of glassy pharmaceuticals and could find a broad application to determine appropriate storage conditions to ensure long-term physical stability. Remarkably, thermal events measured on practical laboratory timescales (hours to days) are relevant to the stability of amorphous materials on much longer, pharmaceutically relevant timescales (years). The mechanistic understanding of these observations in terms of enthalpy relaxation has added further value to the use of RH-perfusion calorimetry as a rapid means to characterize the molecular mobility of amorphous solids.

Physical Characterization of Tobramycin Inhalation Powder: I. Rational Design of a Stable Engineered-Particle Formulation for Delivery to the Lungs.

A spray-dried engineered particle formulation, Tobramycin Inhalation Powder (TIP), was designed through rational selection of formulation composition and process parameters. This PulmoSphere powder comprises small, porous particles with a high drug load. As a drug/device combination, TOBI Podhaler enables delivery of high doses of drug per inhalation, a feature critical for dry powder delivery of anti-infectives for treatment of cystic fibrosis. The objective of this work was to characterize TIP on both the particle and molecular levels using multiple orthogonal physical characterization techniques. Differential scanning calorimetry (DSC), X-ray powder diffraction (XRPD), electron spectroscopy for chemical analysis (ESCA), and Raman measurements show that a TIP particle consists of two phases: amorphous, glassy tobramycin sulfate with a glass transition temperature of about 100 °C and a gel-phase phospholipid (DSPC) with a gel-to-liquid-crystal transition temperature of about 80 °C. This was by design and constituted a rational formulation approach to provide Tg and Tm values that are well above the temperatures used for long-term storage of TIP. Raman and ESCA data provide support for a core/shell particle architecture of TIP. Particle surfaces are enriched with a porous, hydrophobic coating that reduces cohesive forces, improving powder fluidization and dispersibility. The excellent aerosol dispersibility of TIP enables highly efficient delivery of fine particles to the respiratory tract. Collectively, particle engineering has enabled development of TOBI Podhaler, an approved inhaled drug product that meaningfully reduces the treatment burden to cystic fibrosis patients worldwide.

Advances in Respiratory and Nasal Drug Delivery

T special issue brings together a rich variety of studies that cover key aspects of respiratory and nasal drug delivery: formulation and delivery, deposition, and absorption. These investigations involve local delivery to the lung epithelium or nasal sinuses, or transport to the systemic circulation or the brain. The contributions of this issue reflect the diversity in molecular properties; drugs may be small molecules or macromolecules or vaccine vectors expressing viral antigens, hydrophilic or hydrophobic, soluble or insoluble, crystalline or amorphous. Formulation studies range from mechanistic studies on particle formation during spray drying (Baldelli et al.), to molecular approaches to enhance the action of pyrazinoic acid salts (Durham et al.), to the use of a polymer to improve transgene expression in the development of a nasal Ebola vaccine (Choi et al.). Given the serious concerns about antibiotic-resistant infections, increased attention is being paid to developing treatments for lung infections. Studies in this issue comprise a variety of rational formulation and delivery approaches for anti-infective agents. Das et al. formulate an antifungal agent using PLGA nanoparticles, demonstrating a prolonged lung residence time. Zhou et al. and Costabile et al. formulate antibiotics to treat planktonic and, more importantly, biofilm phenotoypes of Pseudomonas aeruginosa. In accordance with the “hit hard and hit fast” philosophy for treatment of lung infections, Sullivan et al. and Garcia Contreras et al. demonstrate in animal models the delivery of vancomycin and rifampicin, respectively. Here, targeting the lungs enables a significantly higher local concentration and lower systemic levels of drug, reducing off-target effects. Respiratory and nasal drug delivery can be thought of as topical delivery of aerosols to epithelia. While there is a considerable body of literature on formulation and delivery of aerosols, much less is known about what happens after a particle is deposited on an epithelium. The commonly used approach to correlate dissolution rates to oral absorption rates has now been extended to drugs administered via inhalation by studying the dissolution of inhaled corticosteroids of a respirable particle size (Rohrschneider et al.). Beyond dissolution, our understanding of the phenomena that occur on a molecular and cellular level is growing. Motivated by the number of in vivo and ex vivo studies using different animal species to understand nasal drug absorption, Al-Ghabeish et al. provide a comparative study of the expression of several drug transporters in humans and animal species. Transporters in the lungs are also investigated, where Salomon et al. assess an important class of compoundsβ2-adrenergic agonistsfor their influence on organic cation transporters. Given the role of peptides in control of metabolic and other functions, Spetter and Hallschmid review the literature on intranasal delivery of neuropeptides. Excipients, the inactive ingredients in a formulation, are critical components in the formulation of most inhaled drug products, often playing an important role in manufacturability and product performance. Assessment of the safety of small molecule and polymeric excipients is a regulatory expectation for the conduct of clinical trials and the eventual commercialization of new pharmaceutical products. In a concise review, Wolff provides an overview of the toxicological requirements to assess the safety of drugs and excipients intended for pulmonary or nasal delivery. Given that macrophage response is a common observation in preclinical safety studies, Hoffman et al. report on their strategy to differentiate among the druginduced macrophage responses to inhaled medicines. Collectively, these efforts to characterize and understand drug delivery on the particle, cellular, and molecular levels constitute meaningful progress. Most importantly, this work supports the development of medicines that improve patients’ lives. Toward this end, Miller et al. demonstrate how particle engineering has enabled development of an approved inhaled drug product that meaningfully reduces the treatment burden to cystic fibrosis patients. Recognizing the importance of global health, Choi et al. exemplify the frontiers of respiratory and nasal drug delivery through their work on vaccination to offer protection to Ebola infection. In closing, we would like to thank the authors for their contributions to advancing our understanding of the field. Many thanks to our coordinating editor, Ms. Kim Barrett, for her assistance in completing this issue; and to Prof. Gordon Amidon many thanks as well for encouraging us to pursue the creation of the present special issue. We hope that you enjoy these articles and find them to be insightful, thought-provoking, and perhaps a valuable springboard for your own research. David Lechuga-Ballesteros,* Guest Editor Pearl Therapeutics, Redwood City, California 94063, United States Danforth P. Miller, Guest Editor Novartis Pharmaceuticals, San Carlos, California 94070, United States

Comment on ‘‘The effect of charged impurities on a glass transition in a polar medium’’ [J. Chem. Phys. 104, 664 (1996)]

Dakhnovskii and Lubchenko have recently presented a theory to predict the effect of charged impurities on the glass transition temperature (Tg) in a polar medium. We present and cite more extensive sets of experimental results that show that the electrolyte concentration required to induce a 10 K increase in Tg is nearly three orders of magnitude greater than that predicted from their theory.

Physical characterization of Tobramycin Inhalation Powder: II. State Diagram of an Amorphous Engineered Particle Formulation

Tobramycin Inhalation Powder (TIP) is a spray-dried engineered particle formulation used in TOBI Podhaler, a drug-device combination for treatment of cystic fibrosis (CF). A TIP particle consists of two phases: amorphous, glassy tobramycin sulfate and a gel-phase phospholipid (DSPC). The objective of this work was to characterize both the amorphous and gel phases following exposure of TIP to a broad range of RH and temperature. Because, in principle, changes in either particle morphology or the solid-state form of the drug could affect drug delivery or biopharmaceutical properties, understanding physical stability was critical to development and registration of this product. Studies included morphological assessments of particles, thermal analysis to measure the gel-to-liquid crystalline phase transition (Tm) of the phospholipid and the glass transition temperature (Tg) of tobramycin sulfate, enthalpy relaxation measurements to estimate structural relaxation times, and gravimetric vapor sorption to measure moisture sorption isotherms of TIP and its components. Collectively, these data enabled development of a state diagram for TIP-a map of the environmental conditions under which physical stability can be expected. This diagram shows that, at long-term storage conditions, TIP is at least 50 °C below the Tg of the amorphous phase and at least 40 °C below the Tm of the gel phase. Enthalpy relaxation measurements demonstrate that the characteristic structural relaxation times under these storage conditions are many orders of magnitude greater than that at Tg. These data, along with long-term physicochemical stability studies conducted during product development, demonstrate that TIP is physically stable, remaining as a mechanical solid over time scales and conditions relevant to a pharmaceutical product. This met a key design goal in the development of TIP: a room-temperature-stable formulation (3-year shelf life) that obviates the need for refrigeration for long-term storage. This has enabled development of TOBI Podhaler-an approved inhaled drug product that meaningfully reduces the treatment burden of CF patients worldwide.

Ciprofloxacin Dry Powder for Inhalation (ciprofloxacin DPI): Technical design and features of an efficient drug–device combination

Bronchiectasis is a chronic respiratory disease with heterogeneous etiology, characterized by a cycle of bacterial infection and inflammation, resulting in increasing airway damage. Exacerbations are an important cause of morbidity and are strongly associated with disease progression. Many patients with bronchiectasis suffer from two or more exacerbations per year. However, there are no approved therapies to reduce or delay exacerbations in this patient population. Ciprofloxacin DPI is in development as a long-term, intermittent therapy to reduce exacerbations in patients with non-cystic fibrosis (CF) bronchiectasis and evidence of respiratory pathogens. Ciprofloxacin DPI combines drug substance, dry powder manufacturing technology, and an efficient, pocket-sized, dry powder inhaler to deliver an effective antibiotic directly to the site of infection, with minimal systemic exposure and treatment burden. Here we review the drug substance and particle engineering (PulmoSphere™) technology used, and key physical properties of Ciprofloxacin Inhalation Powder, including deposition, delivered dose uniformity, consistency, and stability. Design features of the T-326 Inhaler are described in relation to lung targeting, safety and tolerability of inhalation powders, as well as treatment burden and adherence. If approved, Ciprofloxacin DPI may provide a valuable treatment option for those with frequent exacerbations and respiratory pathogens.