Andrey Postnov

PET imaging shows loss of striatal PDE10A in patients with Huntington disease

Phosphodiesterase 10A (PDE10A) belongs to a family of enzymes that hydrolyze cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate.1 PDE10A is highly enriched in striatal medium spiny neurons (MSNs), where it regulates intracellular signaling.1 PDE10A has been proposed as a therapeutic target for Huntington disease (HD), a disorder that preferentially affects MSNs, based on the observation that pharmacologic inhibition of PDE10A in transgenic HD mice significantly improved behavioral and neuropathologic abnormalities.2 However, earlier work had shown that striatal PDE10A levels in HD mice already decline to minimal levels before onset of motor symptoms,3 possibly because mutant huntingtin represses PDE10A transcription. Also, postmortem analysis of striatum of 3 patients with HD revealed strong reduction of PDE10A levels.3 Depletion of PDE10A in HD striatum would at first sight seem hard to reconcile with a beneficial effect of PDE10A inhibitors in HD. However, a recent study reported a dramatic increase, rather than decrease, of PDE10A protein in MSNs of HD mice.4 In light of these conflicting results and the strong interest in development of PDE10A inhibitors for clinical use in HD, it is important to determine whether PDE10A levels are affected in the striatum of patients with HD in vivo.

Preclinical Evaluation of a P2X7 Receptor–Selective Radiotracer: PET Studies in a Rat Model with Local Overexpression of the Human P2X7 Receptor and in Nonhuman Primates

The P2X7 receptor (P2X7R) orchestrates neuroinflammation, and this is the basis for an increased interest in the development of antagonists inhibiting P2X7R function in the brain. This study provides the preclinical evaluation of 11C-JNJ-54173717, a PET tracer for P2X7R in both rats and nonhuman primates. Methods: 11C-JNJ-54173717 is a high-affinity radiotracer for the human P2X7R (hP2X7R). Biodistribution and radiometabolite studies were performed. Viral vectors encoding either enhanced green fluorescent protein-hP2X7R or 3flag-hP2X7R were engineered and validated in cell culture. hP2X7R was regionally overexpressed in the rat striatum after stereotactic injection of viral vectors. Dynamic small-animal PET studies were performed in vector-injected rats and in healthy monkeys using 11C-JNJ-54173717. Results: The affinity of JNJ-54173717 was 1.6 ± 0.1 nM in a rat cortex P2X7R membrane binding assay. In a functional assay at the recombinant human and rat P2X7R orthologs, the half maximal inhibitory concentration (IC50) of JNJ-54173717 was 4.2 ± 0.01 nM and 7.6 ± 0.01 nM, respectively. The rat biodistribution study showed that 11C-JNJ-54173717 crossed the blood–brain barrier and was cleared from plasma mainly via the hepatobiliary pathway. A polar radiometabolite was found in rat plasma. No radiometabolites were detected in rat brain. Dynamic small-animal PET showed binding of 11C-JNJ-54173717 in the striatum expressing hP2X7R, with rapid washout from the noninjected control striatum and other brain regions. Likewise, 11C-JNJ-54173717 PET signal was blocked by a chemically distinct P2X7R ligand, indicating specific binding to P2X7R in the monkey brain. Conclusion: JNJ-54173717 is a high-affinity P2X7R antagonist. An animal rat model stably expressing hP2X7R was developed and validated, identifying favorable characteristics for 11C-JNJ-54173717 as a PET radioligand for in vivo visualization of hP2X7R. 11C-JNJ-54173717 selectively visualized P2X7R in the monkey brain, and this radioligand will be further evaluated in a clinical setting to study P2X7R expression levels in neurodegenerative disorders.

PET imaging of TSPO in a rat model of local neuroinflammation induced by intracerebral injection of lipopolysaccharide

OBJECTIVE The goal of this study was to measure functional and structural aspects of local neuroinflammation induced by intracerebral injection of lipopolysaccharide (LPS) in rats using TSPO microPET imaging with [(18)F]DPA-714, magnetic resonance imaging (MRI), in vitro autoradiography and immunohistochemistry (IHC) in order to characterize a small animal model for screening of new PET tracers targeting neuroinflammation. METHODS Rats were injected stereotactically with LPS (50 μg) in the right striatum and with saline in the left striatum. [(18)F]DPA-714 microPET, MRI, in vitro autoradiography and IHC studies were performed at different time points after LPS injection for 1 month. RESULTS Analysis of the microPET data demonstrated high uptake of the tracer in the LPS injected site with an affected-to-non-affected side-binding potential ratio (BPright-to-left) of 3.0 at 3 days after LPS injection. This BP ratio decreased gradually over time to 0.9 at 30 days after LPS injection. In vitro autoradiography ([(18)F]DPA-714) and IHC (CD68, GFAP and TSPO) confirmed local neuroinflammation in this model. Dynamic contrast enhanced (DCE) MRI demonstrated BBB breakdown near the LPS injection site at day 1, which gradually resolved over time and was absent at 1 month after LPS injection. CONCLUSION The LPS model is useful for first screening of newly developed tracers because of the easy design and the robust, unilateral inflammatory reaction allowing the use of the contralateral region as control. Additionally, this model can be used to test and follow up the benefits of anti-inflammatory therapies by non-invasive imaging.

Kinetic modeling and long‐term test‐retest reproducibility of the mGluR5 PET tracer 18F‐FPEB in human brain

18F‐FPEB is a promising PET tracer for studying the metabotropic glutamate subtype 5 receptor (mGluR5) expression in neuropsychiatric disorders. To assess the potential of 18F‐FPEB for longitudinal mGluR5 evaluation in patient studies, we evaluated the long‐term test‐retest reproducibility using various kinetic models in the human brain. Nine healthy volunteers underwent consecutive scans separated by a 6‐month period. Dynamic PET was combined with arterial sampling and radiometabolite analysis. Total distribution volume (VT) and nondisplaceable binding potential (BPND) were derived from a two‐tissue compartment model without constraints (2TCM) and with constraining the K1/k2 ratio to the value of either cerebellum (2TCM‐CBL) or pons (2TCM‐PONS). The effect of fitting different functions to the tracer parent fractions and reducing scan duration were assessed. Regional absolute test‐retest variability (aTRV), coefficient of repeatability (CR) and intraclass correlation coefficient (ICC) were computed. The 2TCM‐CBL showed best fits. The mean 6‐month aTRV of VT ranged from 8 to 13% (CR < 25%) with ICC > 0.6 for all kinetic models. BPND from 2TCM‐CBL with a sigmoid fit for the parent fractions showed the best reproducibility, with aTRV ≤ 7% (CR < 16%) and ICC > 0.9 in most regions. Reducing the scan duration from 90 to 60 min did not affect reproducibility. These results demonstrate for the first time that 18F‐FPEB brain PET has good long‐term reproducibility, therefore validating its use to monitor mGluR5 expression in longitudinal clinical studies. We suggest a 2TCM‐CBL with fitting a sigmoid function to the parent fractions to be optimal for this tracer. Synapse, 2016.

Quantification, Variability, and Reproducibility of Basal Skeletal Muscle Glucose Uptake in Healthy Humans Using 18F-FDG PET/CT

The quantification and variability of skeletal muscle glucose utilization (SMGU) in healthy subjects under basal (low insulin) conditions are poorly known. This information is essential early in clinical drug development to effectively interrogate novel pharmacologic interventions that modulate glucose uptake. The aim of this study was to determine test–retest characteristics and variability of SMGU within and between healthy subjects under basal conditions. Furthermore, different kinetic modeling strategies were evaluated to find the best-fitting model to assess SMGU studied by 18F-FDG. Methods: Six healthy male volunteers underwent 2 dynamic 18F-FDG PET/CT scans with an interval of 24 h. Subjects were admitted to the clinical unit to minimize variability in daily activities and food intake and restrict physical activity. 18F-FDG PET/CT scans of gluteal and quadriceps muscle area were obtained with arterial input. Regions of interest were drawn over the muscle area to obtain time–activity curves and standardized uptake values (SUVs) between 60 and 90 min. Spectral analysis of the data and kinetic modeling was performed using 2-tissue-irreversible (2T3K), 2-tissue-reversible, and 3-tissue-sequential-irreversible (3T5KS) models. Reproducibility was assessed by intraclass correlation coefficients (ICCs) and within-subject coefficient of variation (WSCV). Results: SUVs in gluteal and quadriceps areas were 0.56 ± 0.09 and 0.64 ± 0.07. ICCs (with 90% confidence intervals in parentheses) were 0.88 (0.64–0.96) and 0.96 (0.82–0.99), respectively, for gluteal and quadriceps muscles, and WSCV for gluteal and quadriceps muscles was 2.2% and 3.6%, respectively. The rate of glucose uptake into muscle was 0.0016 ± 0.0004 mL/mL⋅min, with an ICC of 0.94 (0.93–0.95) and WSCV of 6.6% for the 3T5KS model, whereas an ICC of 0.98 (0.92–1.00) and WSCV of 2.8% was obtained for the 2T3K model. 3T5KS demonstrated the best fit to the measured experimental points. Conclusion: Minimal variability in skeletal muscle glucose uptake was observed under basal conditions in healthy subjects. SUV measurements and rate of glucose uptake values were reproducible, with an average WSCV of less than 5%. Compared with SUV, the 3-tissue model adds information about kinetics between blood, intra- and intercellular compartments, and phosphorylation that may highlight the exact mechanisms of metabolic changes after pharmacologic intervention.

Coconut Oil Aggravates Pressure Overload-Induced Cardiomyopathy without Inducing Obesity, Systemic Insulin Resistance, or Cardiac Steatosis

Studies evaluating the effects of high-saturated fat diets on cardiac function are most often confounded by diet-induced obesity and by systemic insulin resistance. We evaluated whether coconut oil, containing C12:0 and C14:0 as main fatty acids, aggravates pressure overload-induced cardiomyopathy induced by transverse aortic constriction (TAC) in C57BL/6 mice. Mortality rate after TAC was higher (p < 0.05) in 0.2% cholesterol 10% coconut oil diet-fed mice than in standard chow-fed mice (hazard ratio 2.32, 95% confidence interval 1.16 to 4.64) during eight weeks of follow-up. The effects of coconut oil on cardiac remodeling occurred in the absence of weight gain and of systemic insulin resistance. Wet lung weight was 1.76-fold (p < 0.01) higher in coconut oil mice than in standard chow mice. Myocardial capillary density (p < 0.001) was decreased, interstitial fibrosis was 1.88-fold (p < 0.001) higher, and systolic and diastolic function was worse in coconut oil mice than in standard chow mice. Myocardial glucose uptake was 1.86-fold (p < 0.001) higher in coconut oil mice and was accompanied by higher myocardial pyruvate dehydrogenase levels and higher acetyl-CoA carboxylase levels. The coconut oil diet increased oxidative stress. Myocardial triglycerides and free fatty acids were lower (p < 0.05) in coconut oil mice. In conclusion, coconut oil aggravates pressure overload-induced cardiomyopathy.

Fatty Acid Amide Hydrolase Inhibition by JNJ-42165279: A Multiple-Ascending Dose and a Positron Emission Tomography Study in Healthy Volunteers

Inhibition of fatty acid amide hydrolase (FAAH) potentiates endocannabinoid activity and is hypothesized to have therapeutic potential for mood and anxiety disorders and pain. The clinical profile of JNJ‐42165279, an oral selective FAAH inhibitor, was assessed by investigating the pharmacokinetics, pharmacodynamics, safety, and binding to FAAH in the brain of healthy human volunteers. Concentrations of JNJ‐42165279 (plasma, cerebrospinal fluid (CSF), urine) and fatty acid amides (FAA; plasma, CSF), and FAAH activity in leukocytes was determined in a phase I multiple ascending dose study. A positron emission tomography study with the FAAH tracer [11C]MK3168 was conducted to determine brain FAAH occupancy after single and multiple doses of JNJ‐42165279. JNJ‐42165279 administration resulted in an increase in plasma and CSF FAA. Significant blocking of brain FAAH binding of [11C]MK3168 was observed after pretreatment with JNJ‐42165279. JNJ‐42165279 produces potent central and peripheral FAAH inhibition. Saturation of brain FAAH occupancy occurred with doses ≥10 mg of JNJ‐42165279. No safety concerns were identified.