Corrections in Dose Assessment of99mTc Radiolabeled Aerosol Particles Targeted to Central Human Airways Using Planar Gamma Camera Imaging

Imaging Aerosol Deposition with Two-Dimensional Gamma Scintigraphy

Several imaging modalities have been employed to quantify lung dose and the distribution of the dose of orally inhaled aerosols in vivo. Two-dimensional (2D, or planar) imaging using gamma scintigraphy is the most widely used of these modalities. Two-dimensional gamma scintigraphy studies are accomplished using a single- or dual-headed gamma camera. The formulation to be tested is admixed with the gamma emitting radioisotope ^99mtechnetium, which serves as a surrogate for the drug. This article provides details as to how 2D gamma scintigraphy images should be acquired and analyzed using recently standardized methods. Based on the new guidelines, the investigator should confirm that the drug formulation is unchanged with the addition of the radioisotope, determine the amount of radioactivity needed for inhalation to obtain appropriate radioactivity counts in the lungs, perform quality control procedures for the gamma camera, identify the lung borders of the study subject using a reference image such as an X-ray computed tomography scan, a ventilation scan, or a transmission scan, acquire a lung transmission image to correct for attenuation of radioactivity by lung tissue, instruct the subject how to inhale the radiolabel-drug mixture and record associated breathing parameters, acquire anterior and/or posterior views of the lungs and any other regions of interest (i.e., oropharynx, stomach) and assess the acquired images for total and regional dose to the lungs. Total dose should be assessed after identification of the right lung border and appropriate correction for tissue attenuation. Regional dose should be quantified as a normalized outer/inner deposition ratio (O/I) and expressed as the penetration index (PI). Mass balance should be performed as needed. By following the standardized methods, 2D gamma scintigraphy data from studies in different laboratories may be compared and combined, leading to multi-center studies and more rapid development of new medications and devices for inhaled therapies.

Three-Dimensional Quantitative Co-Mapping of Pulmonary Morphology and Nanoparticle Distribution with Cellular Resolution in Nondissected Murine Lungs

Deciphering biodistribution, biokinetics, and biological effects of nanoparticles (NPs) in entire organs with cellular resolution remains largely elusive due to the lack of effective imaging tools. Here, light sheet fluorescence microscopy in combination with optical tissue clearing was validated for concomitant three-dimensional mapping of lung morphology and NP biodistribution with cellular resolution in nondissected ex vivo murine lungs. Tissue autofluorescence allowed for label-free, quantitative morphometry of the entire bronchial tree, acinar structure, and blood vessels. Co-registration of fluorescent NPs with lung morphology revealed significant differences in pulmonary NP distribution depending on the means of application (intratracheal instillation and ventilator-assisted aerosol inhalation under anesthetized conditions). Inhalation exhibited a more homogeneous NP distribution in conducting airways and acini indicated by a central-to-peripheral (C/P) NP deposition ratio of unity (0.98 ± 0.13) as compared to a 2-fold enhanced central deposition (C/P = 1.98 ± 0.37) for instillation. After inhalation most NPs were observed in the proximal part of the acini as predicted by computational fluid dynamics simulations. At cellular resolution patchy NP deposition was visualized in bronchioles and acini, but more pronounced for instillation. Excellent linearity of the fluorescence intensity-dose response curve allowed for accurate NP dosimetry and revealed ca. 5% of the inhaled aerosol was deposited in the lungs. This single-modality imaging technique allows for quantitative co-registration of tissue architecture and NP biodistribution, which could accelerate elucidation of NP biokinetics and bioactivity within intact tissues, facilitating both nanotoxicology studies and the development of nanomedicines.

Measurement techniques for respiratory tract deposition of airborne nanoparticles: a critical review

Determination of the respiratory tract deposition of airborne particles is critical for risk assessment of air pollution, inhaled drug delivery, and understanding of respiratory disease. With the advent of nanotechnology, there has been an increasing interest in the measurement of pulmonary deposition of nanoparticles because of their unique properties in inhalation toxicology and medicine. Over the last century, around 50 studies have presented experimental data on lung deposition of nanoparticles (typical diameter≤100 nm, but here≤300 nm). These data show a considerable variability, partly due to differences in the applied methodologies. In this study, we review the experimental techniques for measuring respiratory tract deposition of nano-sized particles, analyze critical experimental design aspects causing measurement uncertainties, and suggest methodologies for future studies. It is shown that, although particle detection techniques have developed with time, the overall methodology in respiratory tract deposition experiments has not seen similar progress. Available experience from previous research has often not been incorporated, and some methodological design aspects that were overlooked in 30-70% of all studies may have biased the experimental data. This has contributed to a significant uncertainty on the absolute value of the lung deposition fraction of nanoparticles. We estimate the impact of the design aspects on obtained data, discuss solutions to minimize errors, and highlight gaps in the available experimental set of data.

Effect of nanoparticles exposure on fractional exhaled nitric oxide (FENO) in workers exposed to nanomaterials

Fractional exhaled nitric oxide (FENO) measurement is a useful diagnostic test of airway inflammation. However, there have been few studies of FENO in workers exposed to nanomaterials. The purpose of this study was to examine the effect of nanoparticle (NP) exposure on FENO and to assess whether the FENO is increased in workers exposed to nanomaterials (NM). In this study, both exposed workers and non-exposed controls were recruited from NM handling plants in Taiwan. A total of 437 subjects (exposed group = 241, non-exposed group = 196) completed the FENO and spirometric measurements from 2009–2011. The authors used a control-banding (CB) matrix to categorize the risk level of each participant. In a multivariate linear regression analysis, this study found a significant association between risk level 2 of NP exposure and FENO. Furthermore, asthma, allergic rhinitis, peak expiratory flow rate (PEFR), and NF-κB were also significantly associated with FENO. When the multivariate logistic regression model was adjusted for confounders, nano-TiO₂ in all of the NM exposed categories had a significantly increased risk in FENO > 35 ppb. This study found associations between the risk level of NP exposure and FENO (particularly noteworthy for Nano-TiO₂). Monitoring FENO in the lung could open up a window into the role nitric oxide (NO) may play in pathogenesis.

Computational and bioengineered lungs as alternatives to whole animal, isolated organ, and cell-based lung models

Development of lung models for testing a drug substance or delivery system has been an intensive area of research. However, a model that mimics physiological and anatomical features of human lungs is yet to be established. Although in vitro lung models, developed and fine-tuned over the past few decades, were instrumental for the development of many commercially available drugs, they are suboptimal in reproducing the physiological microenvironment and complex anatomy of human lungs. Similarly, intersubject variability and high costs have been major limitations of using animals in the development and discovery of drugs used in the treatment of respiratory disorders. To address the complexity and limitations associated with in vivo and in vitro models, attempts have been made to develop in silico and tissue-engineered lung models that allow incorporation of various mechanical and biological factors that are otherwise difficult to reproduce in conventional cell or organ-based systems. The in silico models utilize the information obtained from in vitro and in vivo models and apply computational algorithms to incorporate multiple physiological parameters that can affect drug deposition, distribution, and disposition upon administration via the lungs. Bioengineered lungs, on the other hand, exhibit significant promise due to recent advances in stem or progenitor cell technologies. However, bioengineered approaches have met with limited success in terms of development of various components of the human respiratory system. In this review, we summarize the approaches used and advancements made toward the development of in silico and tissue-engineered lung models and discuss potential challenges associated with the development and efficacy of these models.

Challenges in assessing regional distribution of inhaled drug in the human lungs

INTRODUCTION

Both the total amount of drug deposited in the lungs (whole lung deposition) and the amount deposited in different lung regions (regional lung deposition) are potentially important factors that determine the safety and efficacy of inhaled drugs. Radionuclide imaging is well established for quantifying the whole lung deposition of inhaled drugs, but the assessment of regional lung deposition is less straightforward, because of the complex nature of the lung anatomy.

AREAS COVERED

This review describes the challenges and problems associated with quantifying regional lung deposition by the two-dimensional (2D) radionuclide imaging method of gamma scintigraphy, and by the three-dimensional (3D) radionuclide imaging methods of single-photon-emission computed tomography (SPECT) and positron-emission tomography (PET). The advantages and disadvantages of each method for assessing regional lung deposition are discussed.

EXPERT OPINION

Owing to its 2D nature, gamma scintigraphy provides limited information about regional lung deposition. SPECT provides regional lung deposition data in three dimensions, but usually involves a (99m)Tc radiolabel. PET enables the regional lung deposition of radiolabeled drug molecules to be quantified in three dimensions, but poses the greatest logistical and technical difficulties. Despite their more challenging nature, 3D imaging methods should be considered as an alternative to gamma scintigraphy whenever the determination of regional lung deposition of pharmaceutical aerosols is a major study objective.

Comparing lung regions of interest in gamma scintigraphy for assessing inhaled therapeutic aerosol deposition

BACKGROUND

Two-dimensional gamma scintigraphy is an important technique used to evaluate the lung deposition from inhaled therapeutic aerosols. Images are divided into regions of interest and deposition indices are derived to quantify aerosol distribution within the intrapulmonary airways. In this article, we compared the different approaches that have been historically used between different laboratories for geometrically defining lung regions of interest. We evaluated the effect of these different approaches on the derived indices classically used to assess inhaled aerosol deposition in the lungs. Our primary intention was to assess the ability of different regional lung templates to discriminate between central and peripheral airway deposition patterns generated by inhaling aerosols of different particle sizes.

METHODS

We investigated six methods most commonly reported in the scientific literature to define lung regions of interest and assessed how different each of the derived regional lung indices were between the methods to quantify regional lung deposition. We used monodisperse albuterol aerosols of differing particle size (1.5, 3, and 6 μm) in five mild asthmatic subjects [forced expiratory volume in 1 sec (FEV(1)) 90% predicted] to test the different approaches of each laboratory.

RESULTS

We observed the areas of geometry used to delineate central (C) and peripheral (P) lung regions of interest varied markedly between different laboratories. There was greater similarity between methods in values of penetration index (PI), defined as P/C aerosol counts normalized by P/C krypton ventilation counts, compared to nonnormalized C/P or P/C aerosol count-ratios. Normalizing the aerosol deposition P/C count-ratios by the ventilation P/C count-ratios, reduced the variability of the data. There was dependence of the regional lung deposition indices on the size of the P region of interest in that, as P increased, C/P count-ratios decreased and P/C count-ratios increased, whereas PI was less affected by variations in the P area. We found particle size, itself, strongly influenced the indices of regional aerosol deposition such that C/P count-ratios increased with increasing particle size for each method and conversely, P/C count-ratios and PI decreased.

CONCLUSIONS

Different approaches used to determine pulmonary regions of interest and quantify aerosol deposition produce different results. Our research highlights a genuine need for a consensus to standardize the methodology to facilitate data comparison between laboratories on aerosol deposition.

Deposition, imaging, and clearance: what remains to be done?

Deposition and clearance studies are used during product development and in fundamental research. These studies mostly involve radionuclide imaging, but pharmacokinetic methods are also used to assess the amount of drug absorbed through the lungs, which is closely related to lung deposition. Radionuclide imaging may be two-dimensional (gamma scintigraphy or planar imaging), or three-dimensional (single photon emission computed tomography and positron emission tomography). In October 2009, a group of scientists met at the "Thousand Years of Pharmaceutical Aerosols" conference in Reykjavik, Iceland, to discuss future research in key areas of pulmonary drug delivery. This article reports the session on "Deposition, imaging and clearance." The objective was partly to review our current understanding, but more importantly to assess "what remains to be done?" A need to standardize methodology and provide a regulatory framework by which data from radionuclide imaging methods could be compared between centers and used in the drug approval process was recognized. There is also a requirement for novel radiolabeling methods that are more representative of production processes for dry powder inhalers and pressurized metered dose inhalers. A need was identified for studies to aid our understanding of the relationship between clinical effects and regional deposition patterns of inhaled drugs. A robust methodology to assess clearance from small conducting airways should be developed, as a potential biomarker for therapies in cystic fibrosis and other diseases. The mechanisms by which inhaled nanoparticles are removed from the lungs, and the factors on which their removal depends, require further investigation. Last, and by no means least, we need a better understanding of patient-related factors, including how to reduce the variability in pulmonary drug delivery, in order to improve the precision of deposition and clearance measurements.

Risks from accidental exposures to engineered nanoparticles and neurological health effects: a critical review

There are certain concerns regarding the safety for the environment and human health from the use of engineered nanoparticles (ENPs) which leads to unintended exposures, as opposed to the use of ENPs for medical purposes. This review focuses on the unintended human exposure of ENPs. In particular, possible effects in the brain are discussed and an attempt to assess risks is performed.

Animal experiments have shown that investigated ENPs (metallic nanoparticles, quantum dots, carbon nanotubes) can translocate to the brain from different entry points (skin, blood, respiratory pathways). After inhalation or instillation into parts of the respiratory tract a very small fraction of the inhaled or instilled ENPs reaches the blood and subsequently secondary organs, including the CNS, at a low translocation rate. Experimental in vivo and in vitro studies have shown that several types of ENPs can have various biological effects in the nervous system. Some of these effects could also imply that ENPs can cause hazards, both acutely and in the long term. The relevance of these data for risk assessment is far from clear. There are at present very few data on exposure of the general public to either acute high dose exposure or on chronic exposure to low levels of air-borne ENPs. It is furthermore unlikely that acute high dose exposures would occur. The risk from such exposures for damaging CNS effects is thus probably very low, irrespective of any biological hazard associated with ENPs.

The situation is more complicated regarding chronic exposures, at low doses. The long term accumulation of ENPs can not be excluded. However, we do not have exposure data for the general public regarding ENPs. Although translocation to the brain via respiratory organs and the circulation appears to be very low, there remains a possibility that chronic exposures, and/or biopersistent ENPs, can influence processes within the brain that are triggering or aggravating pathological processes.

In general, the present state of knowledge is unsatisfactory for a proper risk assessment in this area. Crucial deficits include lack of exposure data, the absence of a proper dose concept, and that studies often fail in adequate description of the investigated ENPs.

The use of combined single photon emission computed tomography and X-ray computed tomography to assess the fate of inhaled aerosol

BACKGROUND

Gamma camera imaging is widely used to assess pulmonary aerosol deposition. Conventional planar imaging provides limited information on its regional distribution. In this study, single photon emission computed tomography (SPECT) was used to describe deposition in three dimensions (3D) and combined with X-ray computed tomography (CT) to relate this to lung anatomy. Its performance was compared to planar imaging.

METHODS

Ten SPECT/CT studies were performed on five healthy subjects following carefully controlled inhalation of radioaerosol from a nebulizer, using a variety of inhalation regimes. The 3D spatial distribution was assessed using a central-to-peripheral ratio (C/P) normalized to lung volume and for the right lung was compared to planar C/P analysis. The deposition by airway generation was calculated for each lung and the conducting airways deposition fraction compared to 24-h clearance.

RESULTS

The 3D normalized C/P ratio correlated more closely with 24-h clearance than the 2D ratio for the right lung [coefficient of variation (COV), 9% compared to 15% p < 0.05]. Analysis of regional distribution was possible for both lungs in 3D but not in 2D due to overlap of the stomach on the left lung. The mean conducting airways deposition fraction from SPECT for both lungs was not significantly different from 24-h clearance (COV 18%). Both spatial and generational measures of central deposition were significantly higher for the left than for the right lung.

CONCLUSIONS

Combined SPECT/CT enabled improved analysis of aerosol deposition from gamma camera imaging compared to planar imaging. 3D radionuclide imaging combined with anatomical information from CT and computer analysis is a useful approach for applications requiring regional information on deposition.

Origin and fate of dietary nanoparticles and microparticles in the gastrointestinal tract

Humans have evolved with oral exposure to dietary microparticles and nanoparticles as a normal occurrence but the ever-growing exploitation of nanotechnology is likely to increase exposure further, both qualitatively and quantitatively. Moreover, unlike the situation with respirable particles, relatively little is known about gastrointestinal intake and handling of nanoparticles. With a long term interest in gut exposure and responses to dietary microparticles, our group is now applying its expertise to nanoparticles in the gastrointestinal tract. Here we aim to address (i) the current challenges associated with the characterisation of particle-host or particle-cell interactions, (ii) the origin and mechanisms of uptake of particles in the gastrointestinal tract, especially via the Peyer's patch and (iii) potential cellular effects of nanoparticles in the generation of reactive oxygen species and inflammasome activation, or microparticles in their adjuvant activity in pro-inflammatory signalling and immune responsiveness.