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P-Glycoprotein (P-gp) is an efflux pump located at the blood-brain barrier (BBB) that contributes to the protection of the central nervous system by transporting neurotoxic compounds out of the brain. A decline in P-gp function has been related to the pathogenesis of neurodegenerative diseases. P-gp inducers can increase the P-gp function and are considered as potential candidates for the treatment of such disorders. The P-gp inducer MC111 increased P-gp expression and function in SW480 human colon adenocarcinoma and colo-320 cells, respectively. Our study aims to evaluate the P-gp inducing effect of MC111 in the whole brain in vivo , using the P-gp tracer [ 18 F]MC225 and positron emission tomography (PET). Eighteen Wistar rats were treated with either vehicle solution, 4.5 mg/kg of MC111 (low-dose group), or 6 mg/kg of MC111 (high-dose group). Animals underwent a 60 min dynamic PET scan with arterial-blood sampling, 24 h after treatment with the inducer. Data were analyzed using the 1-tissue-compartment model and metabolite-corrected plasma as the input function. Model parameters such as the influx constant ( K 1 ) and volume of distribution ( V T ) were calculated, which reflect the in vivo P-gp function. P-gp and pregnane xenobiotic receptor (PXR) expression levels of the whole brain were assessed using western blot. The administration of MC111 decreased K 1 and V T of [ 18 F]MC225 in the whole brain and all of the selected brain regions. In the high-dose group, whole-brain K 1 was decreased by 34% ( K 1 -high-dose = 0.20 ± 0.02 vs K 1 -control = 0.30 ± 0.02; p < 0.001) and in the low-dose group by 7% ( K 1 -low-dose = 0.28 ± 0.02 vs K 1 -control = 0.30 ± 0.02; p = 0.42) compared to controls. Whole-brain V T was decreased by 25% in the high-dose group ( V T -high-dose = 5.92 ± 0.41 vs V T -control = 7.82 ± 0.38; p < 0.001) and by 6% in the low-dose group ( V T -low-dose = 7.35 ± 0.38 vs V T -control = 7.82 ± 0.37; p = 0.38) compared to controls. k 2 values did not vary after treatment. The treatment did not affect the metabolism of [ 18 F]MC225. Western blot studies using the whole-brain tissue did not detect changes in the P-gp expression, however, preliminary results using isolated brain capillaries found an increasing trend up to 37% in treated rats. The decrease in K 1 and V T values after treatment with the inducer indicates an increase in the P-gp functionality at the BBB of treated rats. Moreover, preliminary results using brain endothelial cells also sustained the increase in the P-gp expression. In conclusion, the results verify that MC111 induces P-gp expression and function at the BBB in rats. An increasing trend regarding the P-gp expression levels is found using western blot and an increased P-gp function is confirmed with [ 18 F]MC225 and PET.

[ 18 F]MC225 has been developed as a weak substrate of P-glycoprotein (P-gp) aimed to measure changes in the P-gp function at the blood-brain barrier with positron emission tomography. This study evaluates [ 18 F]MC225 kinetics in non-human primates and investigates the effect of both scan duration and P-gp inhibition. Three rhesus monkeys underwent two 91-min dynamic scans with blood sampling at baseline and after P-gp inhibition (8 mg/kg tariquidar). Data were analyzed using the 1-tissue compartment model (1-TCM) and 2-tissue compartment model (2-TCM) fits using metabolite-corrected plasma as the input function and for various scan durations (10, 20, 30, 60, and 91 min). The preferred model was chosen according to the Akaike information criterion and the standard errors (%) of the estimated parameters. For the 91-min scan duration, the influx constant K 1 increased by 40.7% and the volume of distribution ( V T ) by 30.4% after P-gp inhibition, while the efflux constant k 2 did not change significantly. Similar changes were found for all evaluated scan durations. K 1 did not depend on scan duration (10 min- K 1 = 0.2191 vs 91 min- K 1 = 0.2258), while V T and k 2 did. A scan duration of 10 min seems sufficient to properly evaluate the P-gp function using K 1 obtained with 1-TCM. For the 91-min scan, V T and K 1 can be estimated with a 2-TCM, and both parameters can be used to assess P-gp function.

Purpose: [ 18 F]Fluoroethoxybenzovesamicol ([ 18 F]FEOBV) is a radioligand for the vesicular acetylcholine transporter (VAChT), a marker of the cholinergic system. We evaluated the quantification of [ 18 F]FEOBV in rats in control conditions and after partial saturation of VAChT using plasma and reference tissue input models and test-retest reliability., Procedure: Ninety-minute dynamic [ 18 F]FEOBV PET scans with arterial blood sampling were performed in control rats and rats pretreated with 10 μg/kg FEOBV. Kinetic analyses were performed using one- (1TCM) and two-tissue compartmental models (2TCM), Logan and Patlak graphical analyses with metabolite-corrected plasma input, reference tissue Patlak with cerebellum as reference tissue, standard uptake value (SUV) and SUV ratio (SUVR) using 60- or 90-min acquisition. To assess test-retest reliability, two dynamic [ 18 F]FEOBV scans were performed 1 week apart., Results: The 1TCM did not fit the data. Time-activity curves were more reliably estimated by the irreversible than the reversible 2TCM for 60 and 90 min as the influx rate K i showed a lower coefficient of variation (COV, 14-24 %) than the volume of distribution V T (16-108 %). Patlak graphical analysis showed a good fit to the data for both acquisition times with a COV (12-27 %) comparable to the irreversible 2TCM. For 60 min, Logan analysis performed comparably to both irreversible models (COV 14-32 %) but showed lower sensitivity to VAChT saturation. Partial saturation of VAChT did not affect model selection when using plasma input. However, poor correlations were found between irreversible 2TCM and SUV and SUVR in partially saturated VAChT states. Test-retest reliability and intraclass correlation for SUV were good., Conclusion: [ 18 F]FEOBV is best modeled using the irreversible 2TCM or Patlak graphical analysis. SUV should only be used if blood sampling is not possible.

The interaction of dopaminergic and cholinergic neurotransmission in, e.g., Parkinson's disease has been well established. Here, D 2 receptor antagonists were used to assess changes in [ 18 F]-FEOBV binding to the vesicular acetylcholine transporter (VAChT) in rodents using positron emission tomography (PET). After pretreatment with either 10 mg/kg haloperidol, 1 mg/kg raclopride, or vehicle, 90 min dynamic PET scans were performed with arterial blood sampling. The net influx rate ( K i ) was obtained from Patlak graphical analysis, using a metabolite-corrected plasma input function and dynamic PET data. [ 18 F]-FEOBV concentration in whole-blood or plasma and the metabolite-corrected plasma input function were not significantly changed by the pretreatments (adjusted p > 0.07, Cohen's d 0.28-1.89) while the area-under-the-curve (AUC) of the parent fraction of [ 18 F]-FEOBV was significantly higher after haloperidol treatment (adjusted p = 0.022, Cohen's d = 2.51) than in controls. Compared to controls, the AUC of [ 18 F]-FEOBV, normalized for injected dose and body weight, was nonsignificantly increased in the striatum after haloperidol (adjusted p = 0.4, Cohen's d = 1.77) and raclopride (adjusted p = 0.052, Cohen's d = 1.49) treatment, respectively. No changes in the AUC of [ 18 F]-FEOBV were found in the cerebellum (Cohen's d 0.63-0.74). Raclopride treatment nonsignificantly increased K i in the striatum 1.3-fold compared to control rats (adjusted p = 0.1, Cohen's d = 1.1) while it reduced K i in the cerebellum by 28% (adjusted p = 0.0004, Cohen's d = 2.2) compared to control rats. Pretreatment with haloperidol led to a nonsignificant reduction in K i in the striatum (10%, adjusted p = 1, Cohen's d = 0.44) and a 40-50% lower K i than controls in all other brain regions (adjusted p < 0.0005, Cohen's d = 3.3-4.7). The changes in K i induced by the selective D 2 receptor antagonist raclopride can in part be quantified using [ 18 F]-FEOBV PET imaging. Haloperidol, a nonselective D 2 /σ receptor antagonist, either paradoxically decreased cholinergic activity or blocked off-target [ 18 F]-FEOBV binding to σ receptors. Hence, further studies evaluating the binding of [ 18 F]-FEOBV to σ receptors using selective σ receptor ligands are necessary.

In longitudinal PET studies, animals are repeatedly anesthetized which may affect the repeatability of PET measurements. The aim of this study was to assess the effect of anesthesia on the P-gp function as well as the reproducibility of [ 18 F]MC225 PET scans. Thus, dynamic PET scans with blood sampling were conducted in 13 Wistar rats. Seven animals were exposed to isoflurane anesthesia 1 week before the PET scan ("Anesthesia-exposed" PET). A second group of six animals was used to evaluate the reproducibility of measurements of P-gp function at the blood-brain barrier (BBB) with [ 18 F]MC225. In this group, two PET scans were made with a 1 week interval ("Test" and "Retest" PET). Pharmacokinetic parameters were calculated using compartmental models and metabolite-corrected plasma as an input function. "Anesthesia-exposed" animals showed a 28% decrease in whole-brain volume of distribution ( V T ) ( p < 0.001) compared to "Test", where the animals were not previously anesthetized. The V T at "Retest" also decreased (19%) compared to "Test" ( p < 0.001). The k 2 values in whole-brain were significantly increased by 18% in "Anesthesia-exposed" ( p = 0.005) and by 15% in "Retest" ( p = 0.008) compared to "Test". However, no significant differences were found in the influx rate constant K 1 , which is considered as the best parameter to measure the P-gp function. Moreover, Western Blot analysis did not find significant differences in the P-gp expression of animals not pre-exposed to anesthesia ("Test") or pre-exposed animals ("Retest"). To conclude, anesthesia may affect the brain distribution of [ 18 F]MC225 but it does not affect the P-gp expression or function.

The concept of the high-affinity state postulates that a certain subset of G-protein-coupled receptors is primarily responsible for receptor signaling in the living brain. Assessing the abundance of this subset is thus potentially highly relevant for studies concerning the responses of neurotransmission to pharmacological or physiological stimuli and the dysregulation of neurotransmission in neurological or psychiatric disorders. The high-affinity state is preferentially recognized by agonists in vitro. For this reason, agonist tracers have been developed as tools for the noninvasive imaging of the high-affinity state with positron emission tomography (PET). This review provides an overview of agonist tracers that have been developed for PET imaging of the brain, and the experimental paradigms that have been developed for the estimation of the relative abundance of receptors configured in the high-affinity state. Agonist tracers appear to be more sensitive to endogenous neurotransmitter challenge than antagonists, as was originally expected. However, other expectations regarding agonist tracers have not been fulfilled. Potential reasons for difficulties in detecting the high-affinity state in vivo are discussed., (© 2018 The Authors. Medicinal Research Reviews Published by Wiley Periodicals, Inc.)

The estrogen receptor (ER) is a target for endocrine therapy in breast cancer patients. Individual quantification of ERα and ERβ expression, rather than total ER levels, might enable better prediction of the response to treatment. We recently developed the tracer 2- 18 F-fluoro-6-(6-hydroxynaphthalen-2-yl)pyridin-3-ol ( 18 F-FHNP) for assessment of ERβ levels with PET. In the current study, we investigated several pharmacokinetic analysis methods to quantify changes in ERβ availability with 18 F-FHNP PET. Methods: Male nude rats were subcutaneously inoculated in the shoulder with ERα/ERβ-expressing SKOV3 human ovarian cancer cells. Two weeks after tumor inoculation, a dynamic 18 F-FHNP PET scan with arterial blood sampling was acquired from rats treated with vehicle or various concentrations of estradiol (nonspecific ER agonist) or genistein (ERβ-selective agonist). Different pharmacokinetic models were applied to quantify ERβ availability in the tumor. Results: Irreversible-uptake compartmental models fitted the kinetics of 18 F-FHNP uptake better than reversible models. The irreversible 3-tissue-compartment model, which included both the parent and the metabolite input function, gave results comparable to those of the irreversible 2-tissue-compartment model with only a parent input function, indicating that radioactive metabolites contributed little to the tumor uptake. Patlak graphical analysis gave metabolic rates ( K i , the irreversible uptake rate constant) comparable to compartment modeling. The K i values correlated well with ERβ expression but not with ERα, confirming that K i is a suitable parameter to quantify ERβ expression. SUVs at 60 min after tracer injection also correlated ( r 2 = 0.47; P = 0.04) with ERβ expression. A reduction in 18 F-FHNP tumor uptake and K i values was observed in the presence of estradiol or genistein. Conclusion: 18 F-FHNP PET enables assessment of ERβ availability in tumor-bearing rats. The most suitable parameter to quantify ERβ expression is the K i However, a simplified static imaging protocol for determining the SUVs can be applied to assess ERβ levels., (© 2017 by the Society of Nuclear Medicine and Molecular Imaging.)