The value of indium-labelled leucocytes in clinical practice

Measurement of eosinophil kinetics in healthy volunteers

Radiolabelled leukocyte scans are widely used in nuclear medicine to locate sites of infection and inflammation. Radiolabelling of leukocyte subpopulations can also yield valuable information on cell trafficking and kinetics in vivo, but care must be taken to minimize inadvertent cell activation ex vivo. Here, we describe the use of autologous indium(111)-labelled eosinophils to measure eosinophil intravascular life-span and monitor their distribution and fate using gamma camera imaging in healthy non-atopic individuals.

Imaging stem cell therapy for the treatment of peripheral arterial disease

Arteriosclerotic cardiovascular diseases are among the leading causes of morbidity and mortality worldwide. Therapeutic angiogenesis aims to treat ischemic myocardial and peripheral tissues by delivery of recombinant proteins, genes, or cells to promote neoangiogenesis. Concerns regarding the safety, side effects, and efficacy of protein and gene transfer studies have led to the development of cell-based therapies as alternative approaches to induce vascular regeneration and to improve function of damaged tissue. Cell-based therapies may be improved by the application of imaging technologies that allow investigators to track the location, engraftment, and survival of the administered cell population. The past decade of investigations has produced promising clinical data regarding cell therapy, but design of trials and evaluation of treatments stand to be improved by emerging insight from imaging studies. Here, we provide an overview of pre-clinical and clinical experience using cell-based therapies to promote vascular regeneration in the treatment of peripheral arterial disease. We also review four major imaging modalities and underscore the importance of in vivo analysis of cell fate for a full understanding of functional outcomes.

A (99m)Tc-labeled dual-domain cytokine ligand for imaging of inflammation

INTRODUCTION

Interleukin (IL)-1 and IL-18 are potent proinflammatory cytokines in inflammation-related diseases. Their actions are regulated by IL-1 receptor antagonist (IL-1ra) and IL-18 binding protein (IL-18bp). This study was designed to (99m)Tc-radiolabel an IL-1ra and IL-18bp dual-domain cytokine ligand, IL-18bp-Fc-IL-1ra, for specific inflammation targeting.

METHODS

The (99m)Tc-IL-18bp-Fc-IL-1ra was obtained by direct labeling via 2-iminothiolane reduction. Competitive binding of (99m)Tc-labeled and unlabeled IL-18bp-Fc-IL-1ra to rat polymorphonuclear leukocytes was assessed in vitro. A mouse ear edema model was used to evaluate specific targeting properties of (99m)Tc-IL-18bp-Fc-IL1ra in vivo. The correlation between (99m)Tc-IL-18bp-Fc-IL-1ra uptake and (111)In-labeled polymorphonuclear neutrophil infiltration was studied using ischemic-reperfused rat hearts.

RESULTS

Direct (99m)Tc-labeling yielded a stable dual-domain cytokine radioligand with radiochemical purity greater than 95% after gel filtration. Competitive binding studies showed specific targeting of (99m)Tc-IL-18bp-Fc-IL-1ra to inflammatory cells. The (99m)Tc-IL-18bp-Fc-IL-1ra uptake was 1.80±0.17 % injected dose per gram (%ID/g) in the inflamed ear without blocking, whereas uptake in the presence of IL-18bp-Fc-IL-1ra was 1.09±0.08 %ID/g (P<.05). The amounts of IL-1β and IL-18 were significantly increased in the inflamed ears compared to the vehicle controls. A significant correlation of (99m)Tc-IL-18bp-Fc-IL-1ra with (111)In-labeled neutrophil distribution was observed in the ischemic-reperfused hearts (P<.001).

CONCLUSION

Targeting proinflammatory cytokines with (99m)Tc-IL-18bp-Fc-IL-1ra may provide a suitable approach for specific detection of inflammatory sites.

Advances in cardiovascular molecular imaging for tracking stem cell therapy

The high mortality rate associated with cardiovascular disease is partially due to the lack of proliferative cells in the heart. Without adequate repair following myocardial infarction, progressive dilation can lead to heart failure. Stem cell therapies present one promising option for treating cardiovascular disease, though the specific mechanisms by which they benefit the heart remain unclear. Before stem cell therapies can be used safely in human populations, their biology must be investigated using innovative technologies such as multi-modality molecular imaging. The present review will discuss the basic principles, labelling techniques, clinical applications, and drawbacks associated with four major modalities: radionuclide imaging, magnetic resonance imaging, bioluminescence imaging, and fluorescence imaging.

Advances in radionuclide molecular imaging in myocardial biology

Molecular imaging is a new and evolving field that employs a targeted approach to noninvasively assess biologic processes in vivo. By assessing key elements in specific cellular processes prior to irreversible end-organ damage, molecular tools will allow for earlier detection and intervention, improving management and outcomes associated with cardiovascular diseases. The goal of those working to expand this field is not just to provide diagnostic and prognostic information, but rather to guide an individual's pharmacological, cell-based, or genetic therapeutic regimen. This article will review molecular imaging tools in the context of our current understanding of biological processes of the myocardium, including angiogenesis, ventricular remodeling, inflammation, and apoptosis. The focus will be on radiotracer-based molecular imaging modalities with an emphasis on clinical application. Though this field is still in its infancy and may not be fully ready for widespread use, molecular imaging of myocardial biology has begun to show promise of clinical utility in acute and chronic ischemia, acute myocardial infarction, congestive heart failure, as well as in more global inflammatory and immune-mediated responses in the heart-like myocarditis and allogeneic cardiac transplant rejection. With continued research and development, molecular imaging promises to be an important tool for the optimization of cardiovascular care.

New molecular imaging targets to characterize myocardial biology

Molecular imaging represents a targeted approach to noninvasively assess biologic (both physiologic and pathologic) processes in vivo. Ideally the goal of molecular imaging is not just to provide diagnostic and prognostic information based on identification of the molecular events associated with a pathologic process but rather to guide individually tailored pharmacologic, cell-based, or genetic therapeutic regimens. This article reviews the recent advances in myocardial molecular imaging in the context of the cardiovascular processes of angiogenesis, apoptosis, inflammation, and ventricular remodeling. The focus is on radiotracer-based single photon emission computed tomography and positron emission tomography molecular imaging approaches.

Radionuclides delivery systems for nuclear imaging and radiotherapy of cancer

The recent developments of nuclear medicine in oncology have involved numerous investigations of novel specific tumor-targeting radiopharmaceuticals as a major area of interest for both cancer imaging and therapy. The current progress in pharmaceutical nanotechnology field has been exploited in the design of tumor-targeting nanoscale and microscale carriers being able to deliver radionuclides in a selective manner to improve the outcome of cancer diagnosis and treatment. These carriers include chiefly, among others, liposomes, microparticles, nanoparticles, micelles, dendrimers and hydrogels. Furthermore, combining the more recent nuclear imaging multimodalities which provide high sensitivity and anatomical resolution such as PET/CT (positron emission tomography/computed tomography) and SPECT/CT (combined single photon emission computed tomography/computed tomography system) with the use of these specific tumor-targeting carriers constitutes a promising rally which will, hopefully in the near future, allow for earlier tumor detection, better treatment planning and more powerful therapy. In this review, we highlight the use, limitations, advantages and possible improvements of different nano- and microcarriers as potential vehicles for radionuclides delivery in cancer nuclear imaging and radiotherapy.

Imaging stem cells implanted in infarcted myocardium

Stem cell-based cellular cardiomyoplasty represents a promising therapy for myocardial infarction. Noninvasive imaging techniques would allow the evaluation of survival, migration, and differentiation status of implanted stem cells in the same subject over time. This review describes methods for cell visualization using several corresponding noninvasive imaging modalities, including magnetic resonance imaging, positron emission tomography, single-photon emission computed tomography, and bioluminescent imaging. Reporter-based cell visualization is compared with direct cell labeling for short- and long-term cell tracking.

Assessment of myocardial inflammation produced by experimental coronary occlusion and reperfusion with 99mTc-RP517, a new leukotriene B4 receptor antagonist that preferentially labels neutrophils in vivo

BACKGROUND

99mTc-RP517 is a new leukotriene B4 (LTB4) receptor antagonist developed for imaging acute inflammation or infection. A unique property of 99mTc-RP517 is its ability to label white blood cells in vivo after intravenous injection. The goals of this study were to determine relative 99mTc-RP517 binding to human leukocyte subtypes and the 99mTc-RP517 uptake pattern in canine myocardium where inflammation was induced by either coronary occlusion and reperfusion or tumor necrosis factor alpha (TNFalpha) injection.

METHODS AND RESULTS

Fluorescence-activated cell sorter analysis was performed on whole human blood (n=2) and isolated neutrophils (n= 4) with a fluorescent analog of 99mTc-RP517, [F]-RP517. In whole blood, [F]-RP517 (500 nmol/L) preferentially labeled neutrophils. On isolated neutrophils, [F]-RP517 (10 nmol/L) binding was inhibited by 44% when LTB4 (400 nmol/L) was added. 99mTc-RP517 was injected intravenously in anesthetized, open-chest dogs before coronary occlusion (90 minutes) and reperfusion (120 minutes) (n=9) or before intramyocardial TNFalpha injection (n=3). Ex vivo images of heart slices were acquired. The left ventricle was divided into 72 segments for flow and 99mTc-RP517 uptake analysis. There was an inverse exponential relationship between 99mTc-RP517 uptake and occlusion flow (r=0.73). In the same 15 segments, 99mTc-RP517 uptake was highly correlated with the neutrophil enzyme myeloperoxidase (r=0.91). Ex vivo images revealed tracer uptake in the reperfused area (ischemic to normal count ratio=2.7+/-0.2).

CONCLUSIONS

RP517 binds to the neutrophil LTB4 receptor after intravenous injection. After reperfusion, 99mTc-RP517 uptake correlated with myeloperoxidase and was observed on ex vivo images, indicating that this tracer may have potential as an inflammation-imaging agent.

Heme is a potent inducer of inflammation in mice and is counteracted by heme oxygenase

Various pathologic conditions, such as hemorrhage, hemolysis and cell injury, are characterized by the release of large amounts of heme. Recently, it was demonstrated that heme oxygenase (HO), the heme-degrading enzyme, and heme are able to modulate adhesion molecule expression in vitro. In the present study, the effects of heme and HO on inflammation in mice were analyzed by monitoring the biodistribution of radiolabeled liposomes and leukocytes in conjunction with immunohistochemistry. Small liposomes accumulate in inflamed tissues by diffusion because of locally enhanced vascular permeability, whereas leukocytes actively migrate into inflammatory areas through specific adhesive interactions with the endothelium and chemotaxis. Exposure to heme resulted in a dramatic increase in liposome accumulation in the pancreas, but also intestines, liver, and spleen exhibited significantly increased vascular permeability. Similarly, intravenously administered heme caused an enhanced influx of radiolabeled leukocytes into these organs. Immunohistochemical analysis showed differential up-regulation of the adhesion molecules ICAM-1, P-selectin, and fibronectin in liver and pancreas in heme-treated animals. Heme-induced adhesive properties were accompanied by a massive influx of granulocytes into these inflamed tissues, suggesting an important contribution to the pathogenesis of inflammatory processes. Moreover, inhibition of HO activity exacerbated heme-induced granulocyte infiltration. Here it is demonstrated for the first time that heme induces increased vascular permeability, adhesion molecule expression, and leukocyte recruitment in vivo, whereas HO antagonizes heme-induced inflammation possibly through the down-modulation of adhesion molecules.

18F-FDG labelling of human leukocytes

Radiolabelled leukocytes are useful for the imaging of inflammation and infection, and 18F-fluorodeoxyglucose (18F-FDG) is known to concentrate in metabolically active cells. We evaluated the feasibility of leukocyte labelling with 18F-FDG using ACD and heparin anticoagulants at 20 degrees C and 37 degrees C, with and without gentle mixing during incubation. With leukocytes (WBC) harvested from 20 ml blood, studies were performed using 18F-FDG in concentrations of 3.7-74 MBq (0.1-2.0 mCi). 18F-FDG WBC stability in platelet-poor plasma was assessed at 1-4 h. Satisfactory labelling efficiency was achieved with incubation in heparin-saline at 37 degrees C for 30 min (62.7+/-1.6%), and was further enhanced by mixing during incubation (78.1+/-3.9%). Cell labelling was predominantly of granulocytes (78.5+/-1.4%). 18F-FDG WBC was relatively stable in platelet-poor plasma for up to 4 h, and no cell staining was observed in viability studies using trypan blue. These results indicate the feasibility of leukocyte labelling with 18F-FDG, providing an approach that may be useful in PET imaging of inflammation and infection.

The usefulness of Tc-99m HMPAO labeled WBC SPECT in eosinophilic gastroenteritis

Tc-99m HMPAO labeled WBC SPECT was performed in 5 patients with eosinophilic gastroenteritis before and after successful medical therapy. The imaging findings were graded according to the following imaging scheme; grade 0, no uptake; grade 1, uptake < bone marrow; grade 2, uptake < liver; grade 3, uptake > liver. In no patient was the diagnosis made radiologically or with colonoscopy. The sites of involvement were identified with Tc-99m HMPAO WBC imaging in all patients before treatment and the radionuclide imaging studies were all negative after therapy.

Synthesis, physicochemical properties, and biological evaluation of hydroxypyranones and hydroxypyridinones: novel bidentate ligands for cell-labeling

The synthesis of a range of hydroxypyranones and hydroxypyridinones with potential for the chelation of indium(III) is described. The crystal structures of two of the indium complexes are presented. The distribution coefficients of the ligands and the corresponding iron(III), gallium(III), and indium(III) complexes are reported. Good linear relationships between the distribution coefficients of the iron and gallium complexes and iron and indium complexes were obtained. In contrast a nonlinear relationship was obtained between the distribution coefficient of the free ligand and the distribution coefficient of the three groups of complexes. This latter relationship was used to identify compounds with optimal cell labeling properties. Two such compounds both 6-(alkoxymethyl)-3-hydroxy-4H-pyran-4-ones have been compared with tropolone for their ability to label human leucocytes with 111In. The leucocyte labeling efficiencies of the selected ligands were greater and the in-vitro plasma stabilities were similar to that of 111In-tropolonate. These results suggest that the new bidentate ligands may offer advantages over those currently used for cell-labeling.

Technetium-99m scintigraphy: more accurate assessment of ulcerative colitis with exametazime-labelled leucocytes than with antigranulocyte antibodies

To compare two technetium-99m scintigraphic techniques - 99mTc-labelled antibodies against granulocyte non-specific cross-reacting antigen-95 and 99mTc-exametazime labelled leucocytes in ulcerative colitis - 23 consecutive patients undergoing colonoscopy were investigated in a prospective and randomized study. In each patient the two scans and colonoscopy and biopsy were performed within 10 days. Scans, endoscopy and histology were independently graded for degree of inflammation in eight different colorectal segments for each patient. Active inflammation in one or more segments was present on endoscopy in 22 patients and on histology in 17 patients. Twenty-two patients had increased uptake on the antibody scan and 21 patients on the exametazime scan. Twelve patients showed the same disease extent with both scan methods (total colitis, n=10; distal colitis, n=2). Compared with endoscopy, sensitivity for inflammation in individual segments was 0.51 for antibody scan and 0.87 for exametazime scan; specificity was 0.67 and 0.55, respectively. The predictive value for presence of inflammation was 0.66 for antibody scan and 0.72 for exametazime; the predictive value for absence of inflammation was 0.52 and 0.77, respectively. Segmental scan uptake of endoscopically or histologically visualized inflammation was consistently lower for antigranulocyte antibodies than for exametazime. It is concluded that in patients with active ulcerative colitis, inflammation is better visualized with 99mTc-exametazime labelled leucocytes than with 99mTc-labelled antigranulocyte antibodies. The antibody technique offers the advantage of in vivo labelling, but is less reliable than the exametazime method for imaging of colonic inflammation.

Superparamagnetically labelled neutrophils as potential abscess-specific contrast agent for MRI

In order to evaluate the potential of superparamagnetically labelled neutrophils as inflammation-specific contrast agents for magnetic resonance imaging (MRI), various types of magnetite particles were associated with isolated human neutrophils in vitro and subsequent effects on characteristic cellular functions investigated. Exposure to uncoated magnetite caused strong irreversible cell aggregation, whereas uptake of polystyrene-embedded magnetite microcrystals (Estapor M1-0.70/60) occurred with only minor changes in neutrophil adhesion properties, cellular metabolism, and chemotactic behaviour in vitro. As revealed by MR phantom imaging, Estapor-labelled neutrophils generated visible contrast above a threshold concentration of 1-2 micrograms Fe/g in a tissue-equivalent environment. Intravenous administration of labelled neutrophils to rabbits resulted in local magnetite accumulation up to 2.4 micrograms Fe/g tissue in artificial inflammation sites. Subsequent T2-weighted imaging of an intramuscular abscess clearly demonstrated the expected hypointense area surrounding the typically bright core of inflammation.

The utility of [99mTc]HMPAO-leukocytes for imaging infection

Technetium-99m hexamethylpropylene amine oxime (HMPAO)-labeled leukocytes are well established for the investigation of inflammatory disease. Their kinetics and normal distribution are similar to those of indium-111-labeled leukocytes except for nonspecific activity in urine, kidneys, gall bladder, and bowel, which results from the elution of secondary 99mTc-labeled HMPAO complexes. The principal clinical indications for [99mTc]HMPAO-leukocytes include inflammatory bowel disease (IBD), osteomyelitis, soft tissue sepsis, and, to a lesser extent, occult fever. The superior resolution and count density of 99mTc places [99mTc]HMPAO-leukocytes at an advantage over 111In-leukocytes in IBD, especially for the identification of small bowel involvement in patients with Crohn's disease. However, quantification of disease activity is more difficult than with 111In. Technetium-99m HMPAO-leukocytes are indicated for most forms of acute soft tissue and abdominal sepsis, although when compared with 111In, it may be more difficult to demonstrate communication between an abdominal abscess and bowel lumen. Chronic osteomyelitis, including infected joint prostheses, are better approached with 111In-labeled leukocytes. Occult fever and fever of unknown origin (FUO) are more controversial. There is still a place for gallium-67 in FUO, of which there is a wide spectrum of causes. Occult fever implies a pyogenic cause for an undiagnosed fever and should probably be imaged with 111In-leukocytes. With the advances being made in other imaging modalities and in interventional radiology, there is a clear need for radionuclide agents that can be used for whole-body screening in patients with undiagnosed fever. Such agents may include fluorine-18-fluorodeoxy-glucose and radiolabeled monoclonal antibodies to endothelial adhesion molecules activated at the foci of inflammation.

Do technetium-99m hexamethylpropylene amine oxime-labeled leukocytes truly reflect the mucosal inflammation in patients with ulcerative colitis?

Twenty-five patients with ulcerative colitis and nine controls with macroscopically non-inflamed colon were investigated with technetium-99m hexamethylpropylene amine oxime-labeled leukocyte scintigraphy and colonoscopy with biopsies. The interval between leukocyte scintigraphy and colonoscopy was < or = 14 days in all patients with ulcerative colitis and < or = 30 days in eight of nine controls. Scintigrams were obtained at approximately 45 min and 4 h after injection of labeled leukocytes. One nuclear physician, one internist, and one pathologist graded blindly and independently of each other the degree of active inflammation in seven different colonic segments for each patient, using 4-grade scales for scans and macroscopically and histologically viewed inflammation, respectively. A positive correlation between endoscopic and histologic grading of all colonic segments and scan gradings for all subjects and for ulcerative colitis patients separately was found (all, p < 0.001). By means of kappa statistics, the inter-observer agreement between scintigraphic grading at 45 min and endoscopy was, for all subjects, 0.32 (95% confidence interval (CI), 0.20-0.44; p < 0.001) and, for patients with ulcerative colitis, 0.19 (CI, 0.07-0.31; p < 0.001). When 17 patients who had complete colonoscopies were divided into those with total, extensive, or distal colitis, leukocyte scintigraphy underestimated the extension of active inflammation. A simple scintigraphic scoring system reflects the colonic inflammation viewed endoscopically and histologically in patients with ulcerative colitis but underestimates the presence of active inflammation in individual colonic segments.

Indium labelled leucocyte scanning in pyrexia of unknown origin

Pyrexia of unknown origin (PUO), the commonest cause of which is occult infection, represents a difficult diagnostic problem. Radiolabelled white cell scanning provides a non-invasive and potentially useful diagnostic tool in this condition, but its efficacy remains controversial. In a retrospective study, we analysed 30 111In white cell scans, performed between 1983 and 1988 in 25 patients with PUO, strictly defined as a fever of at least 3 weeks duration reaching 38.3 degrees C on more than three occasions and investigated in hospital for a minimum of 7 days. Eleven studies were on post-operative patients who, although developing fever within 1 week of surgery, satisfied the above criteria. Overall, the diagnostic sensitivity and specificity were 55% and 74% respectively, with an overall accuracy of 67%. This improved to 91% in the post-operative patients compared to an accuracy of 52% in spontaneous PUO. Accuracy did not correlate with duration of symptoms, leucocytosis or index of clinical suspicion. In conclusion, although 111In white cell scanning is not particularly useful for the investigation of spontaneous PUO, it does have a role in post-operative PUO.

Imaging of bone infection with labelled white blood cells: role of contemporaneous bone marrow imaging

The uptake of white blood cells (WBC) into normal bone marrow may lead to difficulty in detecting bone infection. Twenty-one patients in whom the WBC scan was equivocal or positive underwent a technetium 99m colloid scan to show the distribution of bone marrow. Six patients had a positive WBC scan, and in five of them a discordant colloid scan confirmed infection. However, in one the colloid scan was concordant, indicating that the WBC activity was not due to infection but the result of normal bone marrow uptake. Fifteen patients had an equivocal WBC scan. In 14, infection was excluded by a concordant scan, and 1 patient with a discordant scan was lost to follow-up. We conclude that the combination of a WBC scan and a colloid scan is an effective method to distinguish infection from normal bone marrow activity and, in particular, reduces the number of incorrect diagnoses of infection.