Albert D. Windhorst

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.

Δ9-Tetrahydrocannabinol Induces Dopamine Release in the Human Striatum

The influence of cannabis on mental health receives growing scientific and political attention. An increasing demand for treatment of cannabis dependence has refueled the discussion about the addictive potential of cannabis. A key feature of all addictive drugs is the ability to increase synaptic dopamine levels in the striatum, a mechanism involved in their rewarding and motivating effects. However, it is currently unknown if cannabis can stimulate striatal dopamine neurotransmission in humans. Here we show that Δ9-tetrahydrocannabinol (THC), the main psychoactive component in cannabis, induces dopamine release in the human striatum. Using the dopamine D2/D3 receptor tracer [11C]raclopride and positron emission tomography in seven healthy subjects, we demonstrate that THC inhalation reduces [11C]raclopride binding in the ventral striatum and the precommissural dorsal putamen but not in other striatal subregions. This is consistent with an increase in dopamine levels in these regions. These results suggest that THC shares a potentially addictive property with other drugs of abuse. Further, it implies that the endogenous cannabinoid system is involved in regulating striatal dopamine release. This allows new directions in research on the effects of THC in neuropsychiatric disorders, such as schizophrenia.

Rapid Decrease in Delivery of Chemotherapy to Tumors after Anti-VEGF Therapy: Implications for Scheduling of Anti-Angiogenic Drugs

Current strategies combining anti-angiogenic drugs with chemotherapy provide clinical benefit in cancer patients. It is assumed that anti-angiogenic drugs, such as bevacizumab, transiently normalize abnormal tumor vasculature and contribute to improved delivery of subsequent chemotherapy. To investigate this concept, a study was performed in non-small cell lung cancer (NSCLC) patients using positron emission tomography (PET) and radiolabeled docetaxel ([(11)C]docetaxel). In NSCLC, bevacizumab reduced both perfusion and net influx rate of [(11)C]docetaxel within 5 hr. These effects persisted after 4 days. The clinical relevance of these findings is notable, as there was no evidence for a substantial improvement in drug delivery to tumors. These findings highlight the importance of drug scheduling and advocate further studies to optimize scheduling of anti-angiogenic drugs.

Blood–brain barrier P-glycoprotein function in Alzheimer's disease

A major pathological hallmark of Alzheimers disease is accumulation of amyloid-β in senile plaques in the brain. Evidence is accumulating that decreased clearance of amyloid-β from the brain may lead to these elevated amyloid-β levels. One of the clearance pathways of amyloid-β is transport across the blood-brain barrier via efflux transporters. P-glycoprotein, an efflux pump highly expressed at the endothelial cells of the blood-brain barrier, has been shown to transport amyloid-β. P-glycoprotein function can be assessed in vivo using (R)-[(11)C]verapamil and positron emission tomography. The aim of this study was to assess blood-brain barrier P-glycoprotein function in patients with Alzheimers disease compared with age-matched healthy controls using (R)-[(11)C]verapamil and positron emission tomography. In 13 patients with Alzheimers disease (age 65 ± 7 years, Mini-Mental State Examination 23 ± 3), global (R)-[(11)C]verapamil binding potential values were increased significantly (P = 0.001) compared with 14 healthy controls (aged 62 ± 4 years, Mini-Mental State Examination 30 ± 1). Global (R)-[(11)C]verapamil binding potential values were 2.18 ± 0.25 for patients with Alzheimers disease and 1.77 ± 0.41 for healthy controls. In patients with Alzheimers disease, higher (R)-[(11)C]verapamil binding potential values were found for frontal, parietal, temporal and occipital cortices, and posterior and anterior cingulate. No significant differences between groups were found for medial temporal lobe and cerebellum. These data show altered kinetics of (R)-[(11)C]verapamil in Alzheimers disease, similar to alterations seen in studies where P-glycoprotein is blocked by a pharmacological agent. As such, these data indicate that P-glycoprotein function is decreased in patients with Alzheimers disease. This is the first direct evidence that the P-glycoprotein transporter at the blood-brain barrier is compromised in sporadic Alzheimers disease and suggests that decreased P-glycoprotein function may be involved in the pathogenesis of Alzheimers disease.

Relationship of Cerebrospinal Fluid Markers to 11C-PiB and 18F-FDDNP Binding

The purpose of this study was to investigate the potential relationships between cerebrospinal fluid (CSF) measurements of β-amyloid-1–42 (Aβ1-42) and total tau to 11C-Pittsburgh compound B (11C-PiB) and 2-(1-{6-[(2-18F-fluoroethyl)(methyl)amino]-2-naphthyl}ethylidene) malononitrile (18F-FDDNP) binding as measured using PET. Methods: A total of 37 subjects were included, consisting of 15 patients with Alzheimer disease (AD), 12 patients with mild cognitive impairment, and 10 healthy controls. All subjects underwent a lumbar puncture and PET using both 11C-PiB and 18F-FDDNP. For both PET tracers, parametric images of binding potential were generated. Potential associations of CSF levels of Aβ1-42 and tau with 11C-PiB and 18F-FDDNP binding were assessed using Pearson correlation coefficients and linear regression analyses. Results: For both global 11C-PiB and 18F-FDDNP binding, significant correlations with CSF levels of Aβ1-42 (r = −0.72 and −0.37, respectively) and tau (r = 0.58 and 0.56, respectively) were found across groups (all P < 0.001, except P < 0.05 for correlation between 18F-FDDNP and Aβ1-42). Linear regression analyses showed that, adjusted for regional volume, age, sex, and diagnosis, global 11C-PiB uptake had an inverse association with Aβ1-42 CSF levels (standardized β = −0.50, P < 0.001), whereas there was a positive association between global 18F-FDDNP binding and tau CSF levels (standardized β = 0.62, P < 0.01). Conclusion: The good agreement between these 2 different types of biomarkers (i.e., CSF and PET) provides converging evidence for their validity. The inverse association between 11C-PiB and CSF tau Aβ1-42 confirms that 11C-PiB measures amyloid load in the brain. The positive association between 18F-FDDNP and CSF tau suggests that at least part of the specific signal of 18F-FDDNP in AD patients is due to tangle formation.

(R)- and (S)-[11C]verapamil as PET-tracers for measuring P-glycoprotein function: in vitro and in vivo evaluation

The mdr1 gene product P-glycoprotein (P-gp) is involved in the bioavailability and pharmacokinetics of various drugs. Racemic [(11)C]verapamil has been used to image P-gp expression in vivo. A racemic tracer, however, is not suitable for quantification. The purpose of the present study was to identify the most appropriate enantiomer of [(11)C]verapamil as a potential PET-tracer for quantifying P-gp function. The two enantiomers, (R)- and (S)-[(11)C]verapamil, were synthesized and studied in vivo. For the in vivo model mdr1a/1b double gene knock-out and wild type mice were used. The in vitro study made use of the LLC-PK1 MDR cell line to examine the P-gp mediated transport of both enantiomers. The biodistribution of (R)- and (S)-[(11)C]verapamil in dKO and WT mice demonstrated no stereoselectivity of verapamil for P-gp in the blood-brain barrier and in the testes. In addition, no significant differences in P-gp transport for both enantiomers were observed in the in vitro experiments. Previous studies have shown that (R)-verapamil is metabolized less in man and that it has lower affinity for calcium channels. Since (R)- and (S)-verapamil have equal transport for P-gp, the (R)-enantiomer seems to be the best and safest candidate as PET-tracer for measuring P-gp function in vivo.

Detection of Alzheimer Pathology In Vivo Using Both 11C-PIB and 18F-FDDNP PET

11C-Pittsburgh Compound-B (11C-PIB) and 18F-(2-(1-{6-[(2-[18F]fluoroethyl)(methyl)amino]-2-naphthyl}ethylidene) (18F-FDDNP) have been developed as PET tracers for in vivo imaging of pathology in Alzheimers disease (AD). The purpose of this study was to directly compare these tracers in patients with AD, patients with mild cognitive impairment (MCI), and healthy controls. Methods: Paired 11C-PIB and 18F-FDDNP scans were acquired in 14 patients with AD, 11 patients with amnestic MCI, and 13 controls. For both tracers, parametric images of binding potential (BPND) were generated. Global cortical BPND was assessed using ANOVA. In addition, regional patterns of BPND were compared between diagnostic groups using ANOVA for repeated measures. Results: Global cortical BPND of 11C-PIB showed higher binding in patients with AD than in controls and patients with MCI. 18F-FDDNP uptake was higher in patients with AD than in controls, but MCI could not be distinguished from AD or from controls. Global BPND values of both tracers were moderately correlated (r = 0.45; P = 0.005). In MCI, BPND of 11C-PIB showed a bimodal distribution, whereas values for 18F-FDDNP were more widespread, with more MCI patients demonstrating increased uptake. Regional 11C-PIB binding showed different patterns across diagnostic groups, as AD patients showed an overall increase in binding, with the lowest binding in the medial temporal lobe. With 18F-FDDNP, patterns were similar across diagnostic groups. For all groups, highest values were observed in the medial temporal lobe. Conclusion: Differences in BPND between patients with AD, patients with MCI, and controls were more pronounced for 11C-PIB. The difference in regional binding, the moderate correlation, and the discrepant findings in MCI suggest that they measure related, but different, characteristics of the disease.

Impact of molecular imaging on the diagnostic process in a memory clinic

[11C]Pittsburgh compound B ([11C]PIB) and [18F]‐2‐fluoro‐2‐deoxy‐D‐glucose ([18F]FDG) PET measure fibrillar amyloid‐β load and glucose metabolism, respectively. We evaluated the impact of these tracers on the diagnostic process in a memory clinic population.

Preclinical evaluation and validation of [18F]HX4, a promising hypoxia marker for PET imaging

Hypoxia has been shown to be an important microenvironmental parameter influencing tumor progression and treatment efficacy. Patient guidance for hypoxia-targeted therapy requires evaluation of tumor oxygenation, preferably in a noninvasive manner. The aim of this study was to evaluate and validate the uptake of [18F]HX4, a novel developed hypoxia marker for PET imaging. A heterogeneous accumulation of [18F]HX4 was found within rat rhabdomyosarcoma tumors that was significantly (P < 0.0001) higher compared with the surrounding tissues, with temporal increasing tumor-to-blood ratios reaching a plateau of 7.638 ± 0.926 and optimal imaging properties 4 h after injection. [18F]HX4 retention in normal tissues was found to be short-lived, homogeneous and characterized by a fast progressive temporal clearance. Heterogeneity in [18F]HX4 tumor uptake was analyzed based on 16 regions within the tumor according to the different orthogonal planes at the largest diameter. Validation of heterogeneous [18F]HX4 tumor uptake was shown by a strong and significant relationship (r = 0.722; P < 0.0001) with the hypoxic fraction as calculated by the percentage pimonidazole-positive pixels. Furthermore, a causal relationship with tumor oxygenation was established, because combination treatment of nicotinamide and carbogen resulted in a 40% reduction (P < 0.001) in [18F]HX4 tumor accumulation whereas treatment with 7% oxygen breathing resulted in a 30% increased uptake (P < 0.05). [18F]HX4 is therefore a promising candidate for noninvasive detection and evaluation of tumor hypoxia at a macroscopic level.

Evaluation of Tracer Kinetic Models for Quantification of P-Glycoprotein Function using (R)-[11C]Verapamil and PET

Diminished P-glycoprotein (P-gp)-mediated transport across the Blood–brain barrier may play an important role in several neurodegenerative disorders. In previous studies, a racemic mixture of (R)-[11C]verapamil and (S)-[11C]verapamil has been used as tracer for assessing P-gp function using positron emission tomography (PET). Quantification, however, is compromised by potential differences in kinetics between these two isomers. The aim of the present study was to evaluate the kinetics of pure (R)-[11C]verapamil in humans and to develop a tracer kinetic model for the analysis of P-gp-mediated transport of (R)-[11C]verapamil, including the putative contribution of its radioactive metabolites. Dynamic (R)-[11C]verapamil PET scans of 10 male volunteers were analysed with various single- or two-tissue compartment models, with separate compartments for N-dealkylated and N-demethylated metabolites, assuming that either (R)-[11C]verapamil alone or (R)-[11C]verapamil and any combination of metabolites cross the BBB. In addition, six of the subjects underwent two (R)-[11C]verapamil scans to evaluate test–retest reliability. One hour after injection, 50% of total plasma radioactivity consisted of labelled metabolites. Most models fitted the data well and the analysis did not point to a definite ‘best’ model, with differences in optimal model between subjects. The lowest mean test–retest variability (2.9%) was found for a single-tissue model without any metabolite correction. Models with separate metabolite compartments lead to high test–retest variability. Assuming that differences in kinetics of (R)-[11C]verapamil and N-dealkylated metabolites are small, a one input, one-tissue model with correction for N-demethylated metabolites only leads to a good compromise between fit quality and test–retest variability.