FDG-PET imaging of pulmonary inflammation in healthy volunteers after airway instillation of endotoxin

Multimodality molecular imaging of the lung

The continued progression of chronic lung disease despite current treatment options has led to the increasing evaluation of molecular imaging tools for diagnosis, treatment planning, drug discovery, and therapy monitoring. Concurrently the development of multimodality positron emission tomography (PET) / computed tomography (CT), single-photon emission computed tomography (SPECT)/CT, and magnetic resonance imaging (MRI)/PET scanners has opened the potential for more sophisticated imaging biomarker probes. Here we review the potential uses of multimodality imaging tools, the established uses of molecular imaging in nononcologic lung pathophysiology and drug discovery, and some of the technical challenges in multimodality molecular imaging of the lung.

Molecular imaging of influenza and other emerging respiratory viral infections

Research on the pathogenesis and therapy of influenza and other emerging respiratory viral infections would be aided by methods that directly visualize pathophysiologic processes in patients and laboratory animals. At present, imaging of diseases, such as swine-origin H1N1 influenza, is largely restricted to chest radiograph and computed tomography (CT), which can detect pulmonary structural changes in severely ill patients but are more limited in characterizing the early stages of illness, differentiating inflammation from infection or tracking immune responses. In contrast, imaging modalities, such as positron emission tomography, single photon emission CT, magnetic resonance imaging, and bioluminescence imaging, which have become useful tools for investigating the pathogenesis of a range of disease processes, could be used to advance in vivo studies of respiratory viral infections in patients and animals. Molecular techniques might also be used to identify novel biomarkers of disease progression and to evaluate new therapies.

Lung regional metabolic activity and gas volume changes induced by tidal ventilation in patients with acute lung injury

RATIONALE

During acute lung injury (ALI), mechanical ventilation can aggravate inflammation by promoting alveolar distension and cyclic recruitment-derecruitment. As an estimate of the intensity of inflammation, metabolic activity can be measured by positron emission tomography imaging of [(18)F]fluoro-2-deoxy-D-glucose.

OBJECTIVES

To assess the relationship between gas volume changes induced by tidal ventilation and pulmonary metabolic activity in patients with ALI.

METHODS

In 13 mechanically ventilated patients with ALI and relatively high positive end-expiratory pressure, we performed a positron emission tomography scan of the chest and three computed tomography scans: at mean airway pressure, end-expiration, and end-inspiration. Metabolic activity was measured from the [(18)F]fluoro-2-deoxy-D-glucose uptake rate. The computed tomography scans were used to classify lung regions as derecruited throughout the respiratory cycle, undergoing recruitment-derecruitment, and normally aerated.

MEASUREMENTS AND MAIN RESULTS

Metabolic activity of normally aerated lung was positively correlated both with plateau pressure, showing a pronounced increase above 26 to 27 cm H(2)O, and with regional Vt normalized by end-expiratory lung gas volume. This relationship did not appear to be caused by a higher underlying parenchymal metabolic activity in patients with higher plateau pressure. Regions undergoing cyclic recruitment-derecruitment did not have higher metabolic activity than those collapsed throughout the respiratory cycle.

CONCLUSIONS

In patients with ALI managed with relatively high end-expiratory pressure, metabolic activity of aerated regions was associated with both plateau pressure and regional Vt normalized by end-expiratory lung gas volume, whereas no association was found between cyclic recruitment-derecruitment and increased metabolic activity.

Radiolabeled antiviral drugs and antibodies as virus-specific imaging probes

A number of small-molecule drugs inhibit viral replication by binding directly to virion structural proteins or to the active site of a viral enzyme, or are chemically modified by a viral enzyme before inhibiting a downstream process. Similarly, antibodies used to prevent or treat viral infections attach to epitopes on virions or on viral proteins expressed on the surface of infected cells. Such drugs and antibodies can therefore be thought of as probes for the detection of viral infections, suggesting that they might be used as radiolabeled tracers to visualize sites of viral replication by single-photon emission computed tomography (SPECT) or positron emission tomography (PET) imaging. A current example of this approach is the PET imaging of herpes simplex virus infections, in which the viral thymidine kinase phosphorylates radiolabeled thymidine analogues, trapping them within infected cells. One of many possible future applications might be the use of a radiolabeled hepatitis C protease inhibitor to image infection in animals or humans and provide a quantitative measure of viral burden. This article reviews the basic features of radionuclide imaging and the characteristics of ideal tracer molecules, and discusses how antiviral drugs and antibodies could be evaluated for their suitability as virus-specific imaging probes. The use of labeled drugs as low-dose tracers would provide an alternative application for compounds that have failed to advance to clinical use because of insufficient in vivo potency, an unsuitable pharmacokinetic profile or hepato- or nephrotoxicity.

Assessment of lung inflammation with 18F-FDG PET during acute lung injury

OBJECTIVE

The purpose of this review is to describe the current experimental and clinical data regarding the fundamentals and applications of (18)F-FDG PET during acute lung injury (ALI) and acute respiratory distress syndrome (ARDS).

CONCLUSION

Lung inflammation is a key feature of ALI. During ALI, FDG PET can be used to monitor lung neutrophils, which are essential cells in the pathophysiologic mechanisms of ALI. Pulmonary FDG kinetics are altered during experimental and human ALI and are associated with regional lung dysfunction, histologic abnormalities, and prognosis. FDG PET may be a valuable noninvasive method for gaining comprehensive understanding of the mechanisms of ALI/ARDS and for evaluating therapeutic interventions.

Lungs of patients with acute respiratory distress syndrome show diffuse inflammation in normally aerated regions: a [18F]-fluoro-2-deoxy-D-glucose PET/CT study

OBJECTIVE

Neutrophilic inflammation plays a key role in the pathogenesis of acute respiratory distress syndrome (ARDS) and acute lung injury (ALI). Positron emission tomography (PET) with [F]-fluoro-2-deoxy-D-glucose (FDG) can be used to image cellular metabolism that, during lung inflammatory processes, likely reflects neutrophils activity. The aim of this study was to assess the magnitude and regional distribution of inflammatory metabolic activity in the lungs of patients with ALI/ARDS by PET with FDG.

DESIGN

Prospective clinical investigation.

PATIENTS

Ten patients with ALI/ARDS; four spontaneously breathing and two mechanically ventilated subjects, without known lung disease, served as controls.

INTERVENTIONS

In each individual we performed an FDG PET/computed tomography of the thorax.

MEASUREMENTS AND MAIN RESULTS

FDG cellular influx rate constant (Ki) was computed for the imaged lung field and for regions of interest, grouping voxels with similar density. In all patients with ALI/ARDS, Ki was higher than in controls, also after accounting for the increased lung density. Ki values differed greatly among patients, but in all patients Ki of the normally aerated regions was much higher (2- to 24-fold) than in controls. Whereas in some patients the highest Ki values corresponded to regions with the lowest aeration, in others these regions had lower Ki than normally and mildly hypoaerated regions.

CONCLUSION

In patients with ALI/ARDS, undergoing mechanical ventilation since days, the metabolic activity of the lungs is markedly increased across the entire lung density spectrum. The intensity of this activation and its regional distribution, however, vary widely within and between patients.

[18F]fluorodeoxyglucose positron emission tomography for lung antiinflammatory response evaluation

RATIONALE

Few noninvasive biomarkers for pulmonary inflammation are currently available that can assess the lung-specific response to antiinflammatory treatments. Positron emission tomography with [(18)F]fluorodeoxyglucose (FDG-PET) is a promising new method that can be used to quantify pulmonary neutrophilic inflammation.

OBJECTIVES

To evaluate the ability of FDG-PET to measure the pulmonary antiinflammatory effects of hydroxymethylglutaryl-coenzyme A reductase inhibitors (statins) and recombinant human activated protein C (rhAPC) in a human model of experimentally-induced lung inflammation.

METHODS

Eighteen healthy volunteers were randomized to receive placebo, lovastatin, or rhAPC before intrabronchial segmental endotoxin challenge. FDG-PET imaging was performed before and after endotoxin instillation. The rate of [(18)F]FDG uptake was calculated as the influx constant K(i) by Patlak graphical analysis. Bronchoalveolar lavage (BAL) was performed to determine leukocyte concentrations for correlation with the PET imaging results.

MEASUREMENTS AND MAIN RESULTS

There was a statistically significant decrease in K(i) in the lovastatin-treated group that was not seen in the placebo-treated group, suggesting attenuation of inflammation by lovastatin treatment despite a small decrease in BAL total leukocyte and neutrophil counts that was not statistically significant. No significant decrease in K(i) was observed in the rhAPC-treated group, correlating with a lack of change in BAL parameters and indicating no significant antiinflammatory effect with rhAPC.

CONCLUSIONS

FDG-PET imaging is a sensitive method for quantifying the lung-specific response to antiinflammatory therapies and may serve as an attractive platform for assessing the efficacy of novel antiinflammatory therapies at early phases in the drug development process. Clinical trial registered with www.clinicaltrials.gov (NCT00741013).

Induction and concurrent taxanes enhance both the pulmonary metabolic radiation response and the radiation pneumonitis response in patients with esophagus cancer

PURPOSE

The primary aim of this study was to assess pulmonary radiation toxicity quantitatively in patients who received thoracic radiotherapy combined with induction and/or concurrent chemotherapy with or without taxanes for esophageal cancer.

METHODS AND MATERIALS

The study subjects were 139 patients treated at the University of Texas M.D. Anderson Cancer Center for esophageal cancer and who had undergone [(18)F]-fluorodeoxyglucose positron emission tomography/computed tomography between November 1, 2003 and December 15, 2007 for disease restaging after chemoradiotherapy. The patients were grouped into those who had not received taxanes (Group 1), those who had received induction or concurrent taxanes (Group 2), and those who had received both induction and concurrent taxanes (Group 3). Clinical pulmonary toxicity was scored using the National Cancer Institute Common Terminology Criteria for Adverse Events, version 3. Linear regression was applied to the fluorodeoxyglucose uptake vs. radiation dose to determine the pulmonary metabolic radiation response (PMRR) for each case. The clinical toxicity scores and PMRR among the groups were evaluated for significance differences.

RESULTS

The crude rate of pneumonitis symptoms was 46%, 62%, and 74% for Group 1, 2, and 3, respectively. The analysis of variance test of log(PMRR) by treatment was significant (p = .0046). Group 3 had a 61% greater PMRR compared with Group 1 (p = .002). Group 2 had a 38% greater PMRR compared with Group 1 (p = .015). Finally, Group 3 had a 17% greater PMRR compared with Group 2 (p = .31). A PMRR enhancement ratio of 1.60 (95% confidence interval, 1.19-2.14) was observed for Group 3 vs. Group 1.

CONCLUSION

Patients given induction and concurrent taxane chemotherapy had a significantly greater PMRR and clinical pneumonitis symptoms compared with the patients whose chemotherapy regimen did not include taxanes.

Recent trends in soft-tissue infection imaging

This article discusses the current techniques and future directions of infection imaging with particular attention to respiratory, central nervous system, abdominal, and postoperative infections. The agents currently in use localize to areas of infection and inflammation. An infection-specific imaging agent would greatly improve the utility of scintigraphy in imaging occult infections. The superior spatial resolution of (18)F-fluorodeoxyglucose positron emission tomography ((18)F-FDG-PET) and its lack of reliance on a functional immune system, gives this agent certain advantages over the other radiopharmaceuticals. In respiratory tract infection imaging, an important advancement would be the ability to quantitatively delineate lung inflammation, allowing one to monitor the therapeutic response in a variety of conditions. Current studies suggest PET should be considered the most accurate quantitative method. Scintigraphy has much to offer in localizing abdominal infection as well as inflammation. We may begin to see a gradual increase in the usage of (18)F-FDG-PET in detecting occult abdominal infections. Commonly used modalities for imaging inflammatory bowel disease are scintigraphy with (111)In-oxine/(99m)Tc-HMPAO labeled autologous white blood cells. The literature on central nervous system infection imaging is relatively scarce. Few clinical studies have been performed and numerous new agents have been developed for this use with varying results. Further studies are needed to more clearly delineate the future direction of this field. In evaluating the postoperative spine, (99m)Tc-ciprofloxacin single-photon emission computed tomography (SPECT) was reported to be >80% sensitive in patients more than 6 months after surgery. FDG-PET has also been suggested for this purpose and may play a larger role than originally thought. It appears PET/computed tomography (CT) is gaining support, especially in imaging those with fever of unknown origin or nonfunctional immune systems. Although an infection-specific agent is lacking, the development of one would greatly advance our ability to detect, localize, and quantify infections. Overall, imaging such an agent via SPECT/CT or PET/CT will pave the way for greater clinical reliability in the localization of infection.

Modeling pulmonary kinetics of 2-deoxy-2-[18F]fluoro-D-glucose during acute lung injury

RATIONALE AND OBJECTIVES

Dynamic positron emission tomographic imaging of the radiotracer 2-deoxy-2-[(18)F]fluoro-D-glucose ((18)F-FDG) is increasingly used to assess metabolic activity of lung inflammatory cells. To analyze the kinetics of (18)F-FDG in brain and tumor tissues, the Sokoloff model has been typically used. In the lungs, however, a high blood-to-parenchymal volume ratio and (18)F-FDG distribution in edematous injured tissue could require a modified model to properly describe (18)F-FDG kinetics.

MATERIALS AND METHODS

We developed and validated a new model of lung (18)F-FDG kinetics that includes an extravascular/noncellular compartment in addition to blood and (18)F-FDG precursor pools for phosphorylation. Parameters obtained from this model were compared with those obtained using the Sokoloff model. We analyzed dynamic PET data from 15 sheep with smoke or ventilator-induced lung injury.

RESULTS

In the majority of injured lungs, the new model provided better fit to the data than the Sokoloff model. Rate of pulmonary (18)F-FDG net uptake and distribution volume in the precursor pool for phosphorylation correlated between the two models (R(2)=0.98, 0.78), but were overestimated with the Sokoloff model by 17% (P< .05) and 16% (P< .0005) compared to the new one. The range of the extravascular/noncellular (18)F-FDG distribution volumes was up to 13% and 49% of lung tissue volume in smoke- and ventilator-induced lung injury, respectively.

CONCLUSION

The lung-specific model predicted (18)F-FDG kinetics during acute lung injury more accurately than the Sokoloff model and may provide new insights in the pathophysiology of lung injury.

Radiation pneumonitis: correlation of toxicity with pulmonary metabolic radiation response

PURPOSE

To characterize the relationship between radiation pneumonitis (RP) clinical symptoms and pulmonary metabolic activity on post-treatment [(18)F]-fluorodeoxyglucose positron emission tomography (FDG-PET).

PATIENTS AND METHODS

We retrospectively studied 101 esophageal cancer patients who underwent restaging FDG-PET/computed tomography imaging 3-12 weeks after completing thoracic radiotherapy. The National Institutes of Health Common Toxicity Criteria, version 3, was used to score the RP clinical symptoms. Linear regression was applied to the FDG-PET/computed tomography images to determine the normalized FDG uptake vs. radiation dose. The pulmonary metabolic radiation response (PMRR) was quantified as this slope. Modeling was performed to determine the interaction of PMRR, mean lung dose (MLD), and the percentage of lung receiving >20 Gy with RP outcomes.

RESULTS

Of the 101 patients, 25 had Grade 0, 10 had Grade 1, 60 had Grade 2, 5 had Grade 3, and 1 had Grade 5 RP symptoms. Logistic regression analysis demonstrated that increased values of both MLD and PMRR were associated with a greater probability of RP clinical symptoms (p = 0.032 and p = 0.033, respectively). Spearman's rank correlation found no association between the PMRR and the dosimetric parameters (planning target volume, MLD, percentage of lung receiving >5-30 Gy). Twofold cross-validation demonstrated that the combination of MLD and PMRR was superior to either alone for assessing the development of clinical RP symptoms. The combined MLD (or percentage of lung receiving >20 Gy) and PMRR had a greater sensitivity and accuracy (53.3% and 62.5%, respectively) than either alone.

CONCLUSION

The results of this study have demonstrated a significant correlation between RP clinical symptoms and the PMRR measured by FDG-PET/computed tomography after thoracic radiotherapy.

MRI: the next frontier for cystic fibrosis clinicians?

Evaluation of: Puderbach M, Eichinger M Haeselbarth J et al. Assessment of morphological MRI for pulmonary changes in cystic fibrosis (CF) patients. Invest. Radiol. 42, 715-725 (2007). There is growing interest in assessing pulmonary disease in patients with cystic fibrosis (CF) with sensitive imaging techniques, such as thin-section computed tomography (CT) scans of the chest, to detect early abnormalities and to evaluate the response of therapeutic interventions. However, the higher radiation dose of CT compared with a chest radiograph remains a concern as the cumulative dose could potentially be harmful, if used routinely over decades. The current prospective study evaluates the ability of morphological MRI, a radiation-free imaging modality, to detect pulmonary pathology in CF patients. MRI examinations were found to be comparable to CT scans and may, therefore, potentially be a useful imaging technology to evaluate lung disease in CF patients.

The opportunities and challenges of developing imaging biomarkers to study lung function and disease

Recent advances in imaging offer exciting opportunities to develop and validate lung-specific biomarkers as valuable adjuncts to diagnosis, tests of treatment efficacy, and/or treatment monitoring. State-of-the-art structural, functional, and molecular imaging methods allow the lungs to be visualized noninvasively in vivo at submillimeter and subsecond spatial and temporal scales. However, the development and validation of imaging biomarkers present some special challenges, including the following: equipment evaluation, procedure standardization, data regarding reproducibility and replication, interrater variability, the production and measurement of reference standards, sensitivity to interventions or disease progression, intersubject variance, choice of image reconstruction and segmentation algorithms, automated versus observer-dependent image analysis, data acquisition during conditions of standardized lung volume, whether a reliable association can be demonstrated between the imaging biomarker and a clinical endpoint, and whether its use will have a favorable cost-effective impact on drug development or disease management. Establishing such performance characteristics, especially for single investigators at single institutions, can be daunting if not impossible for costly biomarkers such as imaging. Therefore, to take full advantage of the opportunities presented by state-of-the-art imaging methods, new approaches to analytic and clinical validation must be developed in collaboration with industry, foundation, and federal funding agencies.

Radiation pneumonitis: local dose versus [18F]-fluorodeoxyglucose uptake response in irradiated lung

PURPOSE

To quantify the relationship between the local radiation dose received and the posttreatment positron emission tomography/computed tomography (PET/CT) [(18)F]2-fluoro-2-deoxyglucose (FDG) uptake in the lung.

METHODS AND MATERIALS

The data from 36 patients treated for esophageal cancer with thoracic radiotherapy who underwent restaging PET/CT imaging between 4 and 12 weeks after radiotherapy completion were evaluated. Their treatment planning CT was registered with the restaging PET/CT. Using histogram analysis, the voxel average FDG-PET uptake vs. radiation dose was obtained for each case. Hierarchical linear regression models for each patient were applied to study the variation in the linear trends between cases. Deviation of the dose-response curve from a linear model was tested.

RESULTS

The median time between radiotherapy completion and FDG-PET imaging was 40 days (range, 26-70 days). The median of the mean standard uptake value in the lung that received 0-5 Gy was 0.63 (range, 0.36-1.27), 5-10 Gy was 0.77 (range, 0.40-1.35), 10-20 Gy was 0.80 (range, 0.40-1.72), and >20 Gy was 1.08 (range, 0.44-2.63). A hierarchical linear regression model of the radiation dose and normalized FDG uptake per case found an adequate fit with the linear model, and the addition of quadratic and logarithmic functions did not improve the fit. The 36 cases had a posterior mean of slopes range of 0.0048-0.069.

CONCLUSION

The regional dose vs. radiation pneumonitis response was evaluated with FDG-PET/CT imaging. Statistical modeling found a linear relationship. The slope of this relationship varied over an order of magnitude, reflecting the range of the underlying biological response to radiation among the study population.

Regulation of lipopolysaccharide-induced increases in neutrophil glucose uptake

The pathogenesis of many lung diseases involves neutrophilic inflammation. Neutrophil functions, in turn, are critically dependent on glucose uptake and glycolysis to supply the necessary energy to meet these functions. In this study, we determined the effects of p38 mitogen-activated protein kinase and hypoxia-inducible factor (HIF)-1, as well as their potential interaction, on the expression of membrane glucose transporters and on glucose uptake in murine neutrophils. Neutrophils were harvested and purified from C57BL/6 mice and stimulated with lipopolysaccharide (LPS) in the presence or absence of specific p38 and HIF-1 inhibitors. Glucose uptake was measured as the rate of [3H]deoxyglucose (DG) uptake. We identified GLUT-1 in mouse neutrophils, but neither GLUT-3 nor GLUT-4 were detected using Western blot analysis, even after LPS stimulation. LPS stimulation did not increase GLUT-1 protein levels but did cause translocation of GLUT-1 from the cell interior to the cell surface, together with a dose-dependent increase in [3H]DG uptake, indicating that glucose uptake is regulated in these cells. LPS also activated both p38 and the HIF-1 pathway. Inhibitors of p38 and HIF-1 blocked GLUT-1 translocation and [3H]DG uptake. These data suggest that LPS-induced increases in neutrophil glucose uptake are mediated by GLUT-1 translocation to the cell surface in response to sequential activation of neutrophil p38 and HIF-1alpha in neutrophils. Given that neutrophil function and glucose metabolism are closely linked, control of the latter may represent a new target to ameliorate the deleterious effects of neutrophils on the lungs.

Visualizing lung function with positron emission tomography

Positron emission tomography (PET) provides three-dimensional images of the distributions of radionuclides that have been inhaled or injected into the lungs. By using radionuclides with short half-lives, the radiation exposure of the subject can be kept small. By following the evolution of the distributions of radionuclides in gases or compounds that participate in lung function, information about such diverse lung functions as regional ventilation, perfusion, shunt, gas fraction, capillary permeability, inflammation, and gene expression can be inferred. Thus PET has the potential to provide information about the links between cellular function and whole lung function in vivo. In this paper, recent advancements in PET methodology and techniques and information about lung function that have been obtained with these techniques are reviewed.