A brief history of gamma scintigraphy

Illuminating Insights: Clinical Study Harnessing Pharmacoscintigraphy for Permeation Study of Radiolabeled Nimesulide Topical Formulation in Healthy Human Volunteers

An alternative to oral administration for the delivery of therapeutic substances is the topical route, which frequently has comparable efficacy but may have a better tolerability profile. Gamma scintigraphy is a noninvasive technique that involves the application of radioactive substances to conduct biodistribution studies of therapeutic substances delivered through various routes. Nimesulide (NSD) was radiolabeled with technetium pertechnetate (Technetium99m [99mTc]) and this radiolabeled drug complex (99mTc-NSD) was used to prepare a topical gel formulation. The permeation of the radiolabeled drug from the topical gel was determined by gamma scintigraphy on human volunteers. The region of interest was calculated for the quantification of permeated radiolabeled drugs. This was observed that the mean percentage permeation of 99mTc-NSD was found to be 0.32 ± 0.22 to 36.37 ± 2.86 at 5 and 240 min. It was demonstrated that gamma scintigraphy may be a noninvasive and reliable technique for the determination of drug permeation through topical routes.

Radiolabeling Method for Lyophilizate for Dry Powder Inhalation Formulations

Human lung deposition data is non-mandatory for drug approval but very useful for the development of orally inhaled drug products. Lung deposition of inhaled drugs can be quantified by radionuclide imaging, for which one of the first considerations is the method used to radiolabel formulations. In this study, we report the development of a radiolabeling method for lyophilizate for dry powder inhalation (LDPI) formulations. TechneCoatTM is one method that can radiolabel drug particles without using solvents. In this method, particles are radiolabeled with a dispersion of 99mTc-labeled nanoparticles called TechnegasTM. Because a LDPI formulation is not comprised of particles but is a lyophilized cake aerosolized by air impact, the TechneCoat method cannot be used for the radiolabeling of LDPI formulations. We therefore modified the TechneCoat apparatus so that LDPI formulations were not aerosolized by the Technegas flow. Radiolabeling using a modified TechneCoat apparatus was validated with model LDPI formulations of interferon alpha (IFN). IFN of 99mTc-unlabeled, IFN of 99mTc-labeled, and 99mTc of 99mTc-labeled LDPI formulations showed similar behavior, and differences from IFN of 99mTc-unlabeled LDPI formulations were within ±15% in aerodynamic particle size distribution measurement. Our radiolabeling method for LDPI formulations may be useful for the quantification of drug deposition in human lungs.

Recent Advances in Inflammation and Treatment of Small Airways in Asthma

Small airways were historically considered to be almost irrelevant in the development and control of pulmonary chronic diseases but, as a matter of fact, in the past few years we have learned that they are not so "silent". Asthma is still a worldwide health issue due to the great share of patients being far from optimal management. Several studies have shown that the deeper lung inflammation plays a critical role in asthma pathogenesis, mostly in these not well-controlled subjects. Therefore, assessing the degree of small airways inflammation and impairment appears to be a pivotal step in the asthmatic patient's management. It is now possible to evaluate them through direct and indirect measurements, even if some obstacles still affect their clinical application. The success of any treatment obviously depends on several factors but reaching the deeper lung has become a priority and, for inhaled drugs, this is strictly connected to the molecule's size. The aim of the present review is to summarize the recent evidence concerning the small airway involvement in asthma, its physiopathological characteristics and how it can be evaluated in order to undertake a personalized pharmacological treatment and achieve a better disease control.

Dry powder inhalation: past, present and future

INTRODUCTION

Early dry powder inhalers (DPIs) were designed for low drug doses in asthma and COPD therapy. Nearly all concepts contained carrier-based formulations and lacked efficient dispersion principles. Therefore, particle engineering and powder processing are increasingly applied to achieve acceptable lung deposition with these poorly designed inhalers. Areas covered: The consequences of the choices made for early DPI development with respect of efficacy, production costs and safety and the tremendous amount of energy put into understanding and controlling the dispersion performance of adhesive mixtures are discussed. Also newly developed particle manufacturing and powder formulation processes are presented as well as the challenges, objectives, and new tools available for future DPI design. Expert opinion: Improved inhaler design is desired to make DPIs for future applications cost-effective and safe. With an increasing interest in high dose drug delivery, vaccination and systemic delivery via the lungs, innovative formulation technologies alone may not be sufficient. Safety is served by increasing patient adherence to the therapy, minimizing the use of unnecessary excipients and designing simple and self-intuitive inhalers, which give good feedback to the patient about the inhalation maneuver. For some applications, like vaccination and delivery of hygroscopic formulations, disposable inhalers may be preferred.

Aerosol Delivery of siRNA to the Lungs. Part 1: Rationale for Gene Delivery Systems

This article reviews the pulmonary route of administration, aerosol delivery devices, characterization of pulmonary drug delivery systems, and discusses the rationale for inhaled delivery of siRNA. Diseases with known protein malfunctions may be mitigated through the use of siRNA therapeutics. The inhalation route of administration provides local delivery of siRNA therapeutics for the treatment of various pulmonary diseases, however barriers to pulmonary delivery and intracellular delivery of siRNA exists. siRNA loaded nanocarriers can be used to overcome the barriers associated with the pulmonary route, such as anatomical barriers, mucociliary clearance, and alveolar macrophage clearance. Apart from naked siRNA aerosol delivery, previously studied siRNA carrier systems comprise of lipidic, polymeric, peptide, or inorganic origin. Such siRNA delivery systems formulated as aerosols can be successfully delivered via an inhaler or nebulizer to the pulmonary region. Preclinical animal investigations of inhaled siRNA therapeutics rely on intratracheal and intranasal siRNA and siRNA nanocarrier delivery. Aerosolized siRNA delivery systems may be characterized using in vitro techniques, such as dissolution test, inertial cascade impaction, delivered dose uniformity assay, laser diffraction, and laser Doppler velocimetry. The ex vivo techniques used to characterize pulmonary administered formulations include the isolated perfused lung model. In vivo techniques like gamma scintigraphy, 3D SPECT, PET, MRI, fluorescence imaging and pharmacokinetic/pharmacodynamics analysis may be used for evaluation of aerosolized siRNA delivery systems. The use of inhalable siRNA delivery systems encounters barriers to their delivery, however overcoming the barriers while formulating a safe and effective delivery system will offer unique advances to the field of inhaled medicine.

Techniques of assessing small airways dysfunction

The small airways are defined as those less than 2 mm in diameter. They are a major site of pathology in many lung diseases, not least chronic obstructive pulmonary disease (COPD) and asthma. The small airways are frequently involved early in the course of these diseases, with significant pathology demonstrable often before the onset of symptoms or changes in spirometry and imaging. Despite their importance, they have proven relatively difficult to study. This is in part due to their relative inaccessibility to biopsy and their small size which makes their imaging difficult. Traditional lung function tests may only become abnormal once there is a significant burden of disease within them. This has led to the term 'the quiet zone' of the lung. In recent years, more specialised tests have been developed which may detect these changes earlier, perhaps offering the possibility of earlier diagnosis and intervention. These tests are now moving from the realms of clinical research laboratories into routine clinical practice and are increasingly useful in the diagnosis and monitoring of respiratory diseases. This article gives an overview of small airways physiology and some of the routine and more advanced tests of airway function.

In vitro, in vivo and ex vivo models for studying particle deposition and drug absorption of inhaled pharmaceuticals

Delivery of therapeutic agents via the pulmonary route has gained significant attention over the past few decades because this route of administration offers multiple advantages over traditional routes that include localized action, non-invasive nature and favorable lung-to-plasma ratio. However, assessment of post administration behavior of inhaled pharmaceuticals-such as deposition of particles over the respiratory airways, interaction with the respiratory fluid and movement across the air-blood barrier-is challenging because the lung is a very complex organs that is composed of airways with thousands of bifurcations with variable diameters. Thus, much effort has been put forward to develop models that mimic human lungs and allow evaluation of various pharmaceutical and physiological factors that influence the deposition and absorption profiles of inhaled formulations. In this review, we sought to discuss in vitro, in vivo and ex vivo models that have been extensively used to study the behaviors of airborne particles in the lungs and determine the absorption of drugs after pulmonary administration. We have provided a summary of lung cast models, cascade impactors, noninvasive imaging, intact animals, cell culture and isolated perfused lung models as tools to evaluate the distribution and absorption of inhaled particles. We have also outlined the limitations of currently used models and proposed future studies to enhance the reproducibility of these models.

The importance of imaging and physiology measurements in assessing the delivery of peripherally targeted aerosolized drugs

Considerable recent effort has been directed towards developing new aerosol formulations and delivery devices that can target drugs to the lung periphery. In order to determine the efficacy of targeted drug therapy, it is essential that the peripheral lung region be adequately assessed. Imaging of the airways structure and pathology has greatly advanced in the last decade and this rate of growth is accelerating as new technologies become available. Lung imaging continues to play an important role in the study of the peripheral airways and, when combined with state-of-the-art lung function measurements and computational modeling, can be a powerful tool for investigating the effects of inhaled medication. This article focuses on recent strategies in imaging and physiological measurements of the lungs that allow the assessment of inhaled medication delivered to the periphery and discusses how these methods may help to further optimize and refine future aerosol delivery technology.

Comparative pharmacoscintigraphic and pharmacokinetic evaluation of two new formulations of inhaled insulin in type 1 diabetic patients

In this open, single-dose study, we compared the lung deposition and bioavailability of two newly developed insulin formulations for pulmonary delivery. Twelve type 1 diabetic patients were administered the two insulin products (2 U/kg b.w.), which had been radiolabelled with (99m)Tc. The formulations were either microparticles of insulin without excipients (F1) or lipid-coated insulin microparticles (F2). Lung deposition was assessed by γ-scintigraphy imaging performed immediately after administration. Bioavailability was evaluated by quantifying serum insulin levels over a period of 6 h. Lung deposition was found to be 50 ± 9% and 24 ± 8% for the F1 and F2 formulations, respectively. The insulin AUC₀₋₃₆₀ ratio of F1/F2 was 188%, which was consistent with scintigraphic imaging. The concordance between imaging and biological results suggests that the lower bioavailability of F2 is due to its lower lung deposition and not to a reduced absorption into the blood stream. Additional in vitro experiments indicated that the lower performance of F2 was most probably related to a lower disaggregation efficiency of the powder when administered at a sub-optimal flow rate. The two formulations showed interesting pharmacokinetic profiles (T(max) of 26 and 16 min for F1 and F2, respectively) that mimic the physiological insulin secretion pattern. The bioavailability of the developed formulations was within the range of other DPI insulin formulations that have reached the final stages of clinical development.

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.

Validation of computational fluid dynamics in CT-based airway models with SPECT/CT

PURPOSE

To compare the results obtained by using numerical flow simulations with the results of combined single photon emission computed tomography (SPECT) and computed tomography (CT) and to demonstrate the importance of correct boundary conditions for the numerical methods to account for the large amount of interpatient variability in airway geometry.

MATERIALS AND METHODS

This study was approved by all relevant institutional review boards. All patients gave their signed informed consent. In this study, six patients with mild asthma (three men; three women; overall mean age, 46 years ± 17 [standard deviation]) underwent CT at functional residual capacity and total lung capacity, as well as SPECT/CT. CT data were used for segmentation and computational fluid dynamics (CFD) simulations. A comparison was made between airflow distribution, as derived with (a) SPECT/CT through tracer concentration analysis, (b) CT through lobar expansion measurement, and (c) CFD through flow computer simulation. Also, the heterogeneity of the ventilation was examined.

RESULTS

Good agreement was found between SPECT/CT, CT, and CFD in terms of airflow distribution and hot spot detection. The average difference for the internal airflow distribution was less than 3% for CFD and CT versus SPECT/CT. Heterogeneity in ventilation patterns could be detected with SPECT/CT and CFD.

CONCLUSION

This results of this study show that patient-specific computer simulations with appropriate boundary conditions yield information that is similar to that obtained with functional imaging tools, such as SPECT/CT.

SUPPLEMENTAL MATERIAL

http://radiology.rsna.org/lookup/suppl/doi:10.1148/radiol.10100322/-/DC1.

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.

Flow analyses in the lower airways: patient-specific model and boundary conditions

Computational fluid dynamics (CFD) is increasingly applied in the respiratory domain. The ability to simulate the flow through a bifurcating tubular system has increased the insight into the internal flow dynamics and the particular characteristics of respiratory flows such as secondary motions and inertial effects. The next step in the evolution is to apply the technique to patient-specific cases, in order to provide more information about pathological airways. This study presents a patient-specific approach where both the geometry and the boundary conditions (BC) are based on individual imaging methods using computed tomography (CT). The internal flow distribution of a 73-year-old female suffering from chronic obstructive pulmonary disease (COPD) is assessed. The validation is performed through the comparison of lung ventilation with gamma scintigraphy. The results show that in order to obtain agreement within the accuracy limits of the gamma scintigraphy scan, both the patient-specific geometry and the BC (driving pressure) play a crucial role. A minimal invasive test (CT scan) supplied enough information to perform an accurate CFD analysis. In the end it was possible to capture the pathological features of the respiratory system using the imaging and computational fluid dynamics techniques. This brings the introduction of this new technique in the clinical practice one step closer.

Pharmacoscintigraphic and pharmacokinetic evaluation of tobramycin DPI formulations in cystic fibrosis patients

Tobramycin dry powder formulations were evaluated by gamma scintigraphy and pharmacokinetic methods. In an open single-dose, three-treatment, three-period, cross-over study, nine cystic fibrosis patients received both the two test products and the reference product Tobi (nebulizer solution) in order to assess lung deposition and systemic comparative bioavailability of the two investigational inhaled products versus the marketed inhaled comparator product. The percentage of dose (mean+/-SD) in the whole lung was 53.0+/-10.0% for the tobramycin Form 1, 34.1+/-12.4% for the tobramycin Form 2 and 7.6+/-2.7% for the comparator product Tobi. Lung deposition expressed as a percentage of the nominal dose was thus estimated to be 7.0 and 4.5 times higher for the Tobra Form 1 and Tobra Form 2 than for the Tobi, respectively. Furthermore, the systemic bioavailability (adjusted to correspond to the same drug dose as that of the comparator product deposited in the lung) was found to be 1.6 times higher for the comparator product Tobi than for the two DPI formulations. The principal advantages of the DPI formulations include reduced systemic availability and thus, side effects, and higher dose levels of the drug at the site of drug action.

Analytical technique to recover the third dimension in planar imaging of inhaled aerosols: (1) impact on spatial quantification

An analytical algorithm is described for converting planar scintigraphic images of aerosol distributions in the lungs to an equivalent three-dimensional (3D) representation. The recovery of volumetric information should benefit regional quantification. The technique has been validated using simulated planar images of eleven known aerosol distributions in ten realistic lungs. Global and regional 3D parameters, such as the total activity deposition (A), the penetration index (PI) and the relative penetration index (rPI), were quantified on the planar images and on their 3D representation. Random and systematic errors of the estimation were measured. Finally, the performance of planar imaging was compared with that of single-photon emission computed tomography (SPECT). SPECT images were simulated for the same aerosol distributions in the same subjects and quantified for A, PI, and rPI. The systematic errors in A, PI and rPI obtained from planar imaging were 8.9%, 64.8%, and 54.1%, respectively, using the two-dimensional (2D) analysis; they improved significantly to 4.4%, 19.0%, and 25.5% with the 3D analysis (p < 0.01). The corresponding values for SPECT were 5.2%, 9.8%, and 15.7%, significantly better for PI and rPI (p < 0.01). The random errors of A were similar for all techniques being about 5%; those of PI and rPI measurements were significantly higher for planar imaging (<or=14%) than SPECT (<or=8%). In conclusion, 3D spatial parameters can be derived from planar imaging that are significantly more accurate in characterizing different aerosol depositions than their 2D counterpart. However, the errors remain significantly higher than with SPECT.

Lung Deposition of Mannitol Powder Aerosol in Healthy Subjects

Mannitol as a dry powder aerosol is used for bronchoprovocation testing and to enhance mucus clearance in people with excessive airway secretions. The dose and distribution of the deposited aerosol in the lung was investigated using fast single photon emission tomography (SPECT) imaging. Mannitol powder (3 microm particle size) was produced by spray drying and radiolabeled with (99m)Tc-DTPA. Approximately 60 mg of radiolabeled mannitol (containing 52-68 MBq of (99m)Tc-DTPA) was administered to 10 healthy subjects using the Inhalator dry powder inhaler (DPI), and SPECT images (1 min each) were collected. Thirteen percent to 31% of the dose of mannitol loaded in the inhaler deposited in the lungs and the deposited dose correlated positively with the peak inhalation air flow. The regional aerosol lung distribution, as expressed by the penetration index (i.e., ratio of peripheral to central deposition in the lung) varied from 0.31 to 0.88, which however showed no dependency on any flow parameters. The variation in response to the same dose of mannitol within the asthmatic population may in part be explained by these findings.

SPECT Imaging for Radioaerosol Deposition and Clearance Studies

Planar gamma camera scintigraphy is well established for measuring the deposition and clearance of radioaerosols. Single photon emission computed tomography (SPECT) provides threedimensional (3D) reconstructions of the radioactivity distribution, thus avoiding the compression of 3D data into two-dimensional (2D) images and potentially offering superior assessment of aerosol deposition patterns. However, SPECT has traditionally been associated with long imaging times, making it unsuitable for measuring deposition and clearance of radioaerosols with fast clearance. Multi-detector SPECT systems can collect complete SPECT studies in <1 min, allowing both initial deposition and clearance over time to be assessed by dynamic SPECT. Simultaneous transmission measurement with an external source provides attenuation correction for absolute activity quantification as well as aiding in the definition of the lung volume of interest. A dynamic SPECT imaging protocol has been developed to allow fast imaging from the oropharynx to the abdomen using gamma cameras with limited axial field of views. This allows activity quantification not only in the lungs, but also in areas outside the thorax. However, fast dynamic SPECT imaging is technically and computationally more demanding and provides less scope for reducing the radioactivity administered to the subjects. It has been shown that dynamic SPECT, compared to planar imaging, is more sensitive in detecting changes in deposition as measured by the Penetration Index (PI). Thus, SPECT can better differentiate between large and small airways, which is important for lung regional analysis.

Radionuclide imaging technologies and their use in evaluating asthma drug deposition in the lungs

Whole lung and regional lung deposition of inhaled asthma drugs in the lungs can be quantified using either two-dimensional or three-dimensional radionuclide imaging methods. The two-dimensional method of gamma scintigraphy has been the most widely used, and is currently considered the industry standard, but the three-dimensional methods (SPECT, single photon emission computed tomography; and PET, positron emission tomography) give superior regional lung deposition data and will undoubtedly be used more frequently in future. Recent developments in radionuclide imaging are described, including an improved algorithm for assessing regional lung deposition in gamma scintigraphy, and a patent-protected radiolabelling method (TechneCoat), applicable to both gamma scintigraphy and SPECT. Radionuclide imaging data on new inhaled asthma products provide a milestone assessment, and the data form a bridge between in vitro testing and a full clinical trials program, allowing the latter to be entered with increased confidence.