Guido Slegers

Apoptosis-detecting radioligands: current state of the art and future perspectives

This review provides a critical and thorough overview of the radiopharmaceutical development and in vivo evaluation of all apoptosis-detecting radioligands that have emerged so far, along with their possible applications in nuclear medicine. The following SPECT and PET radioligands are discussed: all forms of halogenated Annexin V (i.e. 123I-labelled, 124I-labelled, 125I-labelled, 18F-labelled), 99mTc/94mTc-labelled Annexin V derivatives using different chelators and co-ligands (i.e. BTAP, Hynic, iminothiolane, MAG3, EDDA, EC, tricarbonyl, SDH) or direct 99mTc-labelling, 99mTc-labelled Annexin V mutants and 99mTc/18F-radiopeptide constructs (i.e. AFIM molecules), 111In-DTPA-PEG-Annexin V, 11C-Annexin V and 64Cu-, 67Ga- and 68Ga-DOTA-Annexin V. In addition, the potential role and clinical relevance of anti-PS monoclonal antibodies and other alternative apoptosis markers are reviewed, including: anti-Annexin V monoclonal antibodies, radiolabelled caspase inhibitors and substrates and mitochondrial membrane permeability targeting radioligands. Nevertheless, major emphasis is placed on the group of Annexin V-based radioligands, in particular 99mTc-Hynic-Annexin V, since this molecule is by far the most extensively investigated and best-characterised apoptosis marker at present. Furthermore, the newly emerging imaging modalities for in vivo detection of programmed cell death, such as MRI, MRS, optical, bioluminescent and ultrasound imaging, are briefly described. Finally, some future perspectives are presented with the aim of promoting the development of potential new strategies in pursuit of the ideal cell death-detecting radioligand.

PET visualization of microglia in multiple sclerosis patients using [11C] PK11195

Activated microglia are involved in the immune response of multiple sclerosis (MS). The peripheral benzodiazepine receptor (PBR) is expressed on microglia and up‐regulated after neuronal injury. [11C]PK11195 is a positron emission tomography (PET) radioligand for the PBR. The objective of the present study was to investigate [11C]PK11195 imaging in MS patients and its additional value over magnetic resonance imaging (MRI) concerning the immuno‐pathophysiological process. Seven healthy and 22 MS subjects were included. Semiquantitative [11C]PK11195 uptake values were assessed with normalization on cortical grey matter. Uptake in Gadolinium‐lesions was significantly increased compared with normal white matter. Uptake in T2‐lesions was generally decreased, suggesting a PBR down‐regulation. However, uptake values increased whenever a clinical or MR‐relapse was present, suggestive for a dynamic process with a transient PBR up‐regulation. During disease progression, an increase of normal‐appearing white matter (NAWM) uptake was found, propagating NAWM as the possible real burden of disease. In conclusion, [11C]PK11195 and PET are able to demonstrate inflammatory processes with microglial involvement in MS.

Prefrontal 5-HT2a receptor binding index, hopelessness and personality characteristics in attempted suicide

BACKGROUND Depression, hopelessness, impaired problem solving capacities and deficient serotonergic functions have been identified as major causes of suicidal behaviour. In general, the relation between biological markers of attempted suicide and psychological functions has been investigated using indirect peripheral markers of, e.g. the serotonergic system. Recently, functional neuroimaging techniques with radioligands allow direct in vivo assessment of the neurobiological status of the central nervous system. METHODS We studied the binding index of serotonin-(2a) (5-HT(2a)) receptors in the frontal cortex of attempted suicide patients (n=9) and normal controls (n=13) using [123I]5-I-R91150, a highly selective 5-HT(2a) receptor ligand. Moreover, we measured personality characteristics (using Cloningers Temperament and Character Inventory) and levels of hopelessness (using Becks Hopelessness Scale), and studied the association between 5-HT(2a) receptor binding index, hopelessness and these personality dimensions. RESULTS When compared to normal controls, attempted suicide patients had a significantly lower binding potential of frontal 5-HT(2a) receptors, a higher level of hopelessness, a higher score on the temperament dimension harm avoidance and lower scores on the character dimensions self-directedness and cooperativeness. A significant correlation was found between harm avoidance, hopelessness and binding index in the population of patients that attempted suicide. LIMITATIONS The limited number of patients and potential ingestion of psychotropic drugs may influence the results of the study. CONCLUSIONS Lower central serotonergic function, hopelessness and harm avoidance are interrelated phenomena, which may increase the probability of the occurrence of attempted suicide.

Assessment of neuroinflammation and microglial activation in Alzheimer's disease with radiolabelled PK11195 and single photon emission computed tomography - A Pilot Study

Objectives: Inflammation contributes to degeneration in Alzheimer’s disease (AD), not simply as a secondary phenomenon, but primarily as a significant source of pathology. [123I]iodo-PK11195 is a single photon emission computed tomography (SPECT) ligand for the peripheral benzodiazepine receptor, the latter being expressed on microglia (brain resident macrophages) and upregulated under inflammatory circumstances. The objectives were to assess AD inflammation by detecting [123I]iodo-PK11195 uptake changes and investigate how uptake values relate with perfusion SPECT and neuropsychological findings. Methods: Ten AD and 9 control subjects were included. [123I]iodo-PK11195 SPECT images were realigned into stereotactic space where binding indices, normalized on cerebellar uptake, were calculated. Results: The mean [123I]iodo-PK11195 uptake was increased in AD patients compared with controls in nearly all neocortical regions; however, statistical significance was only reached in the frontal and right mesotemporal regions. Significant correlations were found between regional increased [123I]iodo-PK11195 uptake and cognitive deficits. Conclusions: [123I]iodo-PK11195 is a cellular disease activity marker and allows in vivo assessment of microglial inflammation in AD.

Quantitative Tumor Apoptosis Imaging Using Technetium-99m–HYNIC Annexin V Single Photon Emission Computed Tomography

PURPOSE Radiolabeled annexin V may allow for repetitive and selective in vivo identification of apoptotic cell death without the need for invasive biopsy. This study reports on the relationship between quantitative technetium-99m- (99mTc-) 6-hydrazinonicotinic (HYNIC) radiolabeled annexin V tumor uptake, and the number of tumor apoptotic cells derived from histologic analysis. PATIENTS AND METHODS Twenty patients (18 men, two women) suspected of primary (n = 19) or recurrent (n = 1) head and neck carcinoma were included. All patients underwent a spiral computed tomography (CT) scan, 99mTc-HYNIC annexin V tomography, and subsequent surgical resection of the suspected primary or recurrent tumor. Quantitative 99mTc-HYNIC annexin V uptake in tumor lesions divided by the tumor volume, derived from CT, was related to the number of apoptotic cells per tumor high-power field derived from terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick end-labeling (TUNEL) assays performed on sectioned tumor slices. RESULTS Diagnosis was primary head and neck tumor in 18 patients, lymph node involvement of a cancer of unknown primary origin in one patient, and the absence of recurrence in one patient. Mean percentage absolute tumor uptake of the injected dose per cubic centimeter tumor volume derived from tomographic images was 0.0003% (standard deviation [SD], 0.0004%) at 1 hour postinjection (PI) and 0.0001% (SD, 0.0000%) at 5 to 6 hours PI (P =.012). Quantitative 99mTc-HYNIC annexin V tumor uptake correlated well with the number of apoptotic cells if only tumor samples with no or minimal amounts of necrosis were considered. CONCLUSION In the absence of necrosis, absolute 99mTc-HYNIC annexin V tumor uptake values correlate well with the number of apoptotic cells derived from TUNEL assays.

Microglial imaging with positron emission tomography and atrophy measurements with magnetic resonance imaging in multiple sclerosis : a correlative study

Objective: The objectives of the present study were to assess brain atrophy in multiple sclerosis (MS) patients during different disease stages and to investigate by PET and [11C]PK11195, a marker of microglial activation, the relationship between inflammation, atrophy and clinically relevant measures. Methods: Eight healthy subjects and 22 MS patients were included. Semiquantitative [11C]PK11195 uptake values, with normalization on cortical grey matter, were measured for magnetic resonance imaging T2- and T1-lesions and normal appearing white matter (NAWM). As atrophy index we used the ratio of the amount of white and grey matter divided by the ventricular size, using an optimized a priori based segmentation algorithm (SPM99). Results: Atrophy was significantly greater in MS patients compared to age-matched controls. A significant correlation was found between brain atrophy and both disease duration and disability, as measured with the Expanded Disability Status Scale. For NAWM, [11C]PK11195 uptake increased with the amount of atrophy, while T2-lesional [11C]PK11195 uptake values decreased according to increasing brain atrophy. Conclusions: The present study suggests that brain atrophy, correlating with disease duration and disability, is directly related to NAWM and T2-lesional inflammation as measured by microglial activation.

Nuclear medicine imaging to predict response to radiotherapy: a review

PURPOSE To review available literature on positron emission tomography (PET) and single photon emission computerized tomography (SPECT) for the measurement of tumor metabolism, hypoxia, growth factor receptor expression, and apoptosis as predictors of response to radiotherapy. METHODS AND MATERIALS Medical literature databases (Pubmed, Medline) were screened for available literature and critically analyzed as to their scientific relevance. RESULTS Studies on 18F-fluorodeoxyglucose PET as a predictor of response to radiotherapy in head-and-neck carcinoma are promising but need confirmation in larger series. 18F-fluorothymine is stable in human plasma, and preliminary clinical data obtained with this marker of tumor cell proliferation are promising. For imaging tumor hypoxia, novel, more widely available radiopharmaceuticals with faster pharmacokinetics are mandatory. Imaging of ongoing apoptosis and growth factor expression is at a very early stage, but results obtained in other domains with radiolabeled peptides appear promising. Finally, for most of the tracers discussed, validation against a gold standard is needed. CONCLUSION Optimization of the pharmacokinetics of relevant radiopharmaceuticals as well as validation against gold-standard tests in large patient series are mandatory if PET and SPECT are to be implemented in routine clinical practice for the purpose of predicting response to radiotherapy.

Estimates of regional cerebral blood flow and 5-HT2A receptor density in impulsive, aggressive dogs with 99mTc-ECD and 123I-5-I-R91150

Impulsive aggression in dogs has an important impact on human public health. Better insight into the pathophysiology of this phenomenon could lead to more adequate diagnosis and treatment. Indirect in vivo research on peripheral body fluids and post-mortem studies in impulsive animals and humans indicate a deficient serotonergic system in general and disturbances in the serotonin-2A (5-HT2A) receptor in particular. In this study, brain perfusion and the 5-HT2A receptors were examined in impulsive, aggressive dogs, in comparison with a group of normally behaving animals. In order to decide which dogs to include in this study, owners were asked to describe the general behaviour of the dogs, the circumstances in which aggression occurred and their conduct during aggressive acts. Finally, 19 dogs were retained for this study, showing, according to different behavioural specialists, disinhibited dominance aggression. Functional imaging studies were performed on all these dogs. Single-photon emission tomography (SPET) was used to measure regional brain perfusion using technetium-99m labelled ethyl cysteinate dimer (ECD). The 5-HT2A receptor binding properties were investigated using the selective radioligand iodine-123 labelled 5-I-R91150. A significant increase in uptake of the 5-HT2A radioligand was noted in all cortical areas. No significant alterations were found in regional cortical perfusion, indicating that the increased binding index was not a consequence of increased tracer delivery. This study supports a role for the serotonergic system in canine impulsive aggression.

Imaging of the 5-HT2A system: age-, gender-, and Alzheimer’s disease-related findings

Serotonin (5-HT) and more specifically the 5-HT(2A) receptor is involved in cognitive and non-cognitive behavior and plays an important role in Alzheimers disease (AD). The objective was to assess the 5-HT(2A) binding potential (BP) in healthy volunteers and AD with SPECT and 123I-5-I-R91150, a selective radio-iodinated 5-HT(2A) receptor antagonist. Twenty-six controls and nine AD patients were included. A semiquantitive analysis with normalization on cerebellar uptake provided estimates of BP for 26 cortical regions of interest. An age-related decline of neocortical BP was found (11.6% per decade). Compared to age-matched controls, a generally decreased neocortical BP in AD was found with a significant regional reduction in the orbitofrontal, prefrontal, lateral frontal, cingulate, sensorimotor, parietal inferior, and occipital region. These results are in line with previous postmortem, in vitro, and PET findings. The age-related decline highlights the necessity for matched advanced age study samples. The fact that the 5-HT(2A) receptor is differentially affected in AD patients has implications for both the etiological basis and therapeutic management of AD.

Is there a role for agonist gastrin-releasing peptide receptor radioligands in tumour imaging?

Gastrin-releasing peptide (GRP) has been shown to be a tumour growth stimulating agent for a number of normal and human cancer cell lines. The tumour growth effect is a direct result of GRP binding to membrane G-protein coupled GRP receptors (GRP-R) on the cell surface. Available data on the role of GRP and GRP-R in human lung, prostate, breast, colorectal and gastric carcinoma are reviewed and it is suggested that radiolabelled agonists are preferable to antagonists for imaging and therapy as they appear to be internalised, yielding a higher target/background ratio. The use of rhenium or indium radiolabels for therapy may provide a new approach to GRP/bombesin expressing tumours.