Gert Luurtsema

Microglia Activation in Recent-Onset Schizophrenia: A Quantitative (R)-[11C]PK11195 Positron Emission Tomography Study

BACKGROUND Schizophrenia is a brain disease involving progressive loss of gray matter of unknown cause. Most likely, this loss reflects neuronal damage, which should, in turn, be accompanied by microglia activation. Microglia activation can be quantified in vivo using (R)-[(11)C]PK11195 and positron emission tomography (PET). The purpose of this study was to investigate whether microglia activation occurs in patients with recent-onset schizophrenia. METHODS Ten patients with recent-onset schizophrenia and 10 age-matched healthy control subjects were included. A fully quantitative (R)-[(11)C]PK11195 PET scan was performed on all subjects, including arterial sampling to generate a metabolite-corrected input curve. RESULTS Compared with control subjects, binding potential of (R)-[(11)C]PK11195 in total gray matter was increased in patients with schizophrenia. There were no differences in other PET parameters. CONCLUSIONS Activated microglia are present in schizophrenia patients within the first 5 years of disease onset. This suggests that, in this period, neuronal injury is present and that neuronal damage may be involved in the loss of gray matter associated with this disease. Microglia may form a novel target for neuroprotective therapies in schizophrenia.

Pharmacoresistance in epilepsy: A pilot PET study with the p-glycoprotein substrate R-[(11)C]verapamil

Summary:  Purpose and Methods: Regional overexpression of the multidrug transporter P‐glycoprotein (P‐gp) in epileptic brain tissue may lower target site concentrations of antiepileptic drugs and thus contribute to pharmacoresistance in epilepsy. We used the P‐gp substrate R‐[11C]verapamil and positron emission tomography (PET) to test for differences in P‐gp activity between epileptogenic and nonepileptogenic brain regions of patients with drug‐resistant unilateral temporal lobe epilepsy (n = 7). We compared R‐[11C]verapamil kinetics in homologous brain volumes of interest (VOIs) located ipsilateral and contralateral to the seizure focus. Results: Among different VOIs, radioactivity was highest in the choroid plexus. The hippocampal VOI could not be used for data analysis because it was contaminated by spill‐in of radioactivity from the adjacent choroid plexus. In several other temporal lobe regions that are known to be involved in seizure generation and propagation ipsilateral influx rate constants K1 and efflux rate constants k2 of R‐[11C]verapamil were descriptively increased as compared to the contralateral side. Parameter asymmetries were most prominent in parahippocampal and ambient gyrus (K1, range: −3.8% to +22.3%; k2, range: −2.3% to +43.9%), amygdala (K1, range: −20.6% to +31.3%; k2, range: −18.0% to +38.9%), medial anterior temporal lobe (K1, range: −8.3% to +14.5%; k2, range: −14.5% to +31.0%) and lateral anterior temporal lobe (K1, range: −20.7% to +16.8%; k2, range: −24.4% to +22.6%). In contrast to temporal lobe VOIs, asymmetries were minimal in a region presumably not involved in epileptogenesis located outside the temporal lobe (superior parietal gyrus, K1, range: −3.7% to +4.5%; k2, range: −4.2% to +5.8%). In 5 of 7 patients, ipsilateral efflux (k2) increases were more pronounced than ipsilateral influx (K1) increases, which resulted in ipsilateral reductions (10%–26%) of R‐[11C]verapamil distribution volumes (DV). However, for none of the examined brain regions, any of the differences in K1, k2 and DV between the epileptogenic and the nonepileptogenic hemisphere reached statistical significance (p > 0.05, Wilcoxon matched pairs test). Conclusions: Even though we failed to detect statistically significant differences in R‐[11C]verapamil model parameters between epileptogenic and nonepileptogenic brain regions, it cannot be excluded from our pilot data in a small sample size of patients that regionally enhanced P‐gp activity might contribute to drug resistance in some patients with temporal lobe epilepsy.

Fully automated synthesis module for the high yield one-pot preparation of 6-[F-18]fluoro-L-DOPA

Abstract A fully automated one-pot synthesis of 6-[ 18 F ]fluoro- l -DOPA, an important radiopharmaceutical for studies on the presynaptic dopamine metabolism with positron emission tomography, is described. 6-[ 18 F ]Fluoro- l -DOPA was prepared in high radiochemical yield (33±4%, c.f.d.) and radiochemical purity (>99%) in 45 min synthesis time by a fluorodestannylation reaction, followed by acidic removal of the protecting groups. CFCl3 was found to be a better solvent for the fluorodestannylation reaction than CHCl3 or acetonitrile. In CFCl3, [ 18 F ]F2 was a superior fluorinating agent over [ 18 F ]acetyl hypofluorite.

Development of a Tracer Kinetic Plasma Input Model for (R)-[11C]PK11195 Brain Studies

(R)-[11C]PK11195 ([1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl]-3-isoquinoline carboxamide) is a ligand for the peripheral benzodiazepine receptor, which, in the brain, is mainly expressed on activated microglia. Using both clinical studies and Monte Carlo simulations, the aim of this study was to determine which tracer kinetic plasma input model best describes (R)-[11C]PK11195 kinetics. Dynamic positron emission tomography (PET) scans were performed on 13 subjects while radioactivity in arterial blood was monitored online. Discrete blood samples were taken to generate a metabolite corrected plasma input function. One-tissue, two-tissue irreversible, and two-tissue reversible compartment models, with and without fixing K1/k2 ratio, k4 or blood volume to whole cortex values, were fitted to the data. The effects of fixing parameters to incorrect values were investigated by varying them over a physiologic range and determining accuracy and reproducibility of binding potential and volume of distribution using Monte Carlo simulations. Clinical data showed that a two-tissue reversible compartment model was optimal for analyzing (R)-[11C]PK11195 PET brain studies. Simulations showed that fixing the K1/k2 ratio of this model provided the optimal trade-off between accuracy and reproducibility. It was concluded that a two-tissue reversible compartment model with K1/k2 fixed to whole cortex value is optimal for analyzing (R)-[11C]PK11195 PET brain studies.

PET Imaging of Tumor Hypoxia Using 18F-Fluoroazomycin Arabinoside in Stage III–IV Non–Small Cell Lung Cancer Patients

Tumor hypoxia hampers the efficacy of radiotherapy because of its increased resistance to ionizing radiation. The aim of the present study was to estimate the potential added clinical value of the specific hypoxia tracer 18F-fluoroazomycin arabinoside (18F-FAZA) over commonly used 18F-FDG in the treatment of advanced-stage non–small cell lung cancer (NSCLC). Methods: Eleven patients with stage III or stage IV NSCLC underwent 18F-FDG and 18F-FAZA PET before chemoradiotherapy. The maximum standardized uptake value (SUVmax) was used to depict 18F-FDG uptake, and the tumor-to-background (T/B) ratio and tumor fractional hypoxic volume (FHV) were used to quantify hypoxia. The spatial correlation between 18F-FDG and 18F-FAZA uptake values was investigated using voxel-based analysis. Partial-volume correction was applied. Results: All 11 patients showed clear uptake of 18F-FAZA in the primary tumor. However, different patterns of 18F-FDG and 18F-FAZA uptake distributions were observed and varied widely among different tumors. No significant correlation was observed between 18F-FDG SUVmax and 18F-FAZA T/B ratio (P = 0.055). The median FHV of 1.4 was 48.4% (range, 5.0–91.5). A significant positive correlation was found between the 18F-FAZA T/B ratio and FHV of 1.4 (P < 0.001). There was no correlation between the lesion size and FHV or between the 18F-FDG SUVmax and FHV. The pattern of tumoral 18F-FDG uptake was rather homogeneous, whereas 18F-FAZA uptake was more heterogeneous, suggesting that 18F-FAZA identifies hypoxic areas within metabolically active areas of tumor. A significant correlation between 18F-FDG SUVmax and lesion size (P = 0.002) was observed. Conclusion: 18F-FAZA PET imaging is able to detect heterogeneous distributions of hypoxic subvolumes out of homogeneous 18F-FDG background in a clinical setting. Therefore, 18F-FAZA might be considered a tool for guiding dose escalation to the hypoxic fraction of the tumor.

Day-to-Day Test–Retest Variability of CBF, CMRO2, and OEF Measurements Using Dynamic 15O PET Studies

PurposeWe assessed test–retest variability of cerebral blood flow (CBF), cerebral blood volume (CBV), cerebral metabolic rate of oxygen (CMRO2), and oxygen extraction fraction (OEF) measurements derived from dynamic 15O positron emission tomography (PET) scans.ProceduresIn seven healthy volunteers, complete test–retest 15O PET studies were obtained; test–retest variability and left-to-right ratios of CBF, CBV, OEF, and CMRO2 in arterial flow territories were calculated.ResultsWhole-brain test–retest coefficients of variation for CBF, CBV, CMRO2, and OEF were 8.8%, 13.8%, 5.3%, and 9.3%, respectively. Test–retest variability of CBV left-to-right ratios was <7.4% across all territories. Corresponding values for CBF, CMRO2, and OEF were better, i.e., <4.5%, <4.0%, and <1.4%, respectively.ConclusionsThe test–retest variability of CMRO2 measurements derived from dynamic 15O PET scans is comparable to within-session test–retest variability derived from steady-state 15O PET scans. Excellent regional test–retest variability was observed for CBF, CMRO2, and OEF. Variability of absolute CBF and OEF measurements is probably affected by physiological day-to-day variability of CBF.

(R)-[11C]Verapamil PET studies to assess changes in P-glycoprotein expression and functionality in rat blood-brain barrier after exposure to kainate-induced status epilepticus

BackgroundIncreased functionality of efflux transporters at the blood-brain barrier may contribute to decreased drug concentrations at the target site in CNS diseases like epilepsy. In the rat, pharmacoresistant epilepsy can be mimicked by inducing status epilepticus by intraperitoneal injection of kainate, which leads to development of spontaneous seizures after 3 weeks to 3 months. The aim of this study was to investigate potential changes in P-glycoprotein (P-gp) expression and functionality at an early stage after induction of status epilepticus by kainate.Methods(R)-[11C]verapamil, which is currently the most frequently used positron emission tomography (PET) ligand for determining P-gp functionality at the blood-brain barrier, was used in kainate and saline (control) treated rats, at 7 days after treatment. To investigate the effect of P-gp on (R)-[11C]verapamil brain distribution, both groups were studied without or with co-administration of the P-gp inhibitor tariquidar. P-gp expression was determined using immunohistochemistry in post mortem brains. (R)-[11C]verapamil kinetics were analyzed with approaches common in PET research (Logan analysis, and compartmental modelling of individual profiles) as well as by population mixed effects modelling (NONMEM).ResultsAll data analysis approaches indicated only modest differences in brain distribution of (R)-[11C]verapamil between saline and kainate treated rats, while tariquidar treatment in both groups resulted in a more than 10-fold increase. NONMEM provided most precise parameter estimates. P-gp expression was found to be similar for kainate and saline treated rats.ConclusionsP-gp expression and functionality does not seem to change at early stage after induction of anticipated pharmacoresistant epilepsy by kainate.

Radiopharmaceuticals for Assessing ABC Transporters at the Blood-Brain Barrier

ABC transporters protect the brain by transporting neurotoxic compounds from the brain back into the blood. P‐glycoprotein (P‐gp) is the most investigated ABC (efflux) transporter, as it is implicated in neurodegenerative diseases such as Alzheimers disease. Altered function of P‐gp can be studied in vivo, using Positron Emission Tomography (PET). To date, several radiopharmaceuticals have been developed to image P‐gp function in vivo. So far, attempts to image expression levels of P‐gp using radiolabeled P‐gp inhibitors have not been successful. Improved knowledge of compound behavior toward P‐gp from in vitro studies should increase predictability of in vivo outcome.

SAR Studies on Tetrahydroisoquinoline Derivatives: The Role of Flexibility and Bioisosterism To Raise Potency and Selectivity toward P-glycoprotein

The development of P-glycoprotein (P-gp) ligands remains of considerable interest, mostly for investigating the proteins structure and transport mechanism. In recent years, many different generations of ligands have been tested for their ability to modulate P-gp activity. The aim of the present work is to perform SAR studies on tetrahydroisoquinoline derivatives in order to design potent and selective P-gp ligands. For this purpose, the effect of bioisosteric replacement and the role of flexibility have been investigated, and four series of tetrahydroisoquinoline ligands have been developed: (a) 2-aryloxazole bioisosteres, (b) elongated analogues, (c) 2H-chromene, and (d) 2-biphenyl derivatives. The results showed that both 2-biphenyl derivative 20b and elongated derivative 6g behaved as strong P-gp substrates. In conclusion, important aspects for developing potent and selective P-gp ligands have been highlighted, providing a solid starting point for further optimization.

Myocardial Oxygen Extraction Fraction Measured Using Bolus Inhalation of 15O-Oxygen Gas and Dynamic PET

The aim of this study was to determine the accuracy of oxygen extraction fraction (OEF) measurements using a dynamic scan protocol after bolus inhalation of 15O2. The method of analysis was optimized by investigating potential reuse of myocardial blood flow (MBF), perfusable tissue fraction, and blood and lung spillover factors derived from separate 15O-water and C15O scans. Methods: Simulations were performed to assess the accuracy and precision of OEF for a variety of models in which different parameters from 15O-water and C15O scans were reused. Reproducibility was assessed in 8 patients who underwent one 10-min dynamic scan after bolus injection of 1.1 GBq of 15O-water, two 10-min dynamic scans after bolus inhalation of 1.4 GBq of 15O2, and a 6-min static scan after bolus inhalation of 0.8 GBq of C15O for region-of-interest definition. Results: Simulations showed that accuracy and precision were lowest when all parameters were determined from the 15O2 scan. The optimal accuracy and precision of OEF were obtained when fixing MBF, perfusable tissue fraction, and blood spillover to values derived from a 15O-water scan and estimating spillover from the pulmonary gas volume using an attenuation map. Optimal accuracy and precision were confirmed in the patient study, showing an OEF test–retest variability of 13% for the whole myocardium. Correction of spillover from pulmonary gas volume requires correction of the lung time–activity curve for pulmonary blood volume, which could equally well be obtained from a 15O-water rather than C15O scan. Conclusion: Measurement of OEF is possible using bolus inhalation of 15O2 and a dynamic scan protocol, with optimal accuracy and precision when other relevant parameters, such as MBF, are derived from an additional 15O-water scan.