Krista Fowles

In Vivo Imaging of Endogenous Pancreatic β-Cell Mass in Healthy and Type 1 Diabetic Subjects Using 18F-Fluoropropyl-Dihydrotetrabenazine and PET

The ability to noninvasively measure endogenous pancreatic β-cell mass (BCM) would accelerate research on the pathophysiology of diabetes and revolutionize the preclinical development of new treatments, the clinical assessment of therapeutic efficacy, and the early diagnosis and subsequent monitoring of disease progression. The vesicular monoamine transporter type 2 (VMAT2) is coexpressed with insulin in β-cells and represents a promising target for BCM imaging. Methods: We evaluated the VMAT2 radiotracer 18F-fluoropropyl-dihydrotetrabenazine (18F-FP-(+)-DTBZ, also known as 18F-AV-133) for quantitative PET of BCM in healthy control subjects and patients with type 1 diabetes mellitus. Standardized uptake value was calculated as the net tracer uptake in the pancreas normalized by injected dose and body weight. Total volume of distribution, the equilibrium ratio of tracer concentration in tissue relative to plasma, was estimated by kinetic modeling with arterial input functions. Binding potential, the steady-state ratio of specific binding to nondisplaceable uptake, was calculated using the renal cortex as a reference tissue devoid of specific VMAT2 binding. Results: Mean pancreatic standardized uptake value, total volume of distribution, and binding potential were reduced by 38%, 20%, and 40%, respectively, in type 1 diabetes mellitus. The radiotracer binding parameters correlated with insulin secretion capacity as determined by arginine-stimulus tests. Group differences and correlations with β-cell function were enhanced for total pancreas binding parameters that accounted for tracer binding density and organ volume. Conclusion: These findings demonstrate that quantitative evaluation of islet β-cell density and aggregate BCM can be performed clinically with 18F-FP-(+)-DTBZ PET.

Affinity and selectivity of [11C]-(+)-PHNO for the D3 and D2 receptors in the rhesus monkey brain in vivo

Although [11C]‐(+)‐PHNO has enabled quantification of the dopamine‐D3 receptor (D3R) in the human brain in vivo, its selectivity for the D3R is not sufficiently high to allow us to disregard its binding to the dopamine‐D2 receptor (D2R). We quantified the affinity of [11C]‐(+)‐PHNO for the D2R and D3R in the living primate brain. Two rhesus monkeys were examined on four occasions each, with [11C]‐(+)‐PHNO administered in a bolus + infusion paradigm. Varying doses of unlabeled (+)‐PHNO were coadministered on each occasion (total doses ranging from 0.09 to 5.61 μg kg−1). The regional binding potential (BPND) and the corresponding doses of injected (+)‐PHNO were used as inputs in a model that quantified the affinity of (+)‐PHNO for the D2R and D3R, as well as the regional fractions of the [11C]‐(+)‐PHNO signal attributable to D3R binding. (+)‐PHNO in vivo affinity for the D3R (Kd/fND ∼ 0.23–0.56 nM) was 25‐ to 48‐fold higher than that for the D2R (Kd/fND ∼ 11–14 nM). The tracer limits for (+)‐PHNO (dose associated with D3R occupancy ∼ 10%) were estimated at ∼0.02–0.04 μg kg−1 injected mass for anesthetized primate and at 0.01–0.02 μg kg−1 for awake human positron emission tomography (PET) studies. Our data enabled a rational design and interpretation of future PET studies with [11C]‐(+)‐PHNO. Synapse, 2012.

Evaluation of [(11)C]MRB for assessment of occupancy of norepinephrine transporters: Studies with atomoxetine in non-human primates.

[(11)C]MRB is one of the most promising radioligands used to measure brain norepinephrine transporters (NET) with positron emission tomography (PET). The objective of this study was to evaluate the suitability of [(11)C]MRB for drug occupancy studies of NET using atomoxetine (ATX), a NET uptake inhibitor used in the treatment of depression and attention-deficit hyperactivity disorder (ADHD). A second goal of the study was identification of a suitable reference region. Ten PET studies were performed in three anesthetized rhesus monkeys following an infusion of ATX or placebo. [(11)C]MRB arterial input functions and ATX plasma levels were also measured. A dose-dependent reduction of [(11)C]MRB volume of distribution was observed after correction for [(11)C]MRB plasma free fraction. ATX IC(50) was estimated to be 31 ± 10ng/mL plasma. This corresponds to an effective dose (ED(50)) of 0.13mg/kg, which is much lower than the therapeutic dose of ATX in ADHD (1.0-1.5mg/kg). [(11)C]MRB binding potential BP(ND) in the thalamus was estimated to be 1.8 ± 0.3. Defining a reference region for a NET radiotracer is challenging due to the widespread and relatively uniform distribution of NET in the brain. Three regions were evaluated for use as reference region: caudate, putamen and occipital cortex. Caudate was found to be the most suitable for preclinical drug occupancy studies in rhesus monkeys. The IC(50) estimate obtained using MRTM2 BP(ND) without arterial blood sampling was 21 ± 3ng/mL (using caudate as the reference region). This study demonstrated that [(11)C]MRB is suitable for drug occupancy studies of NET.

Awake Nonhuman Primate Brain PET Imaging with Minimal Head Restraint: Evaluation of GABAA-Benzodiazepine Binding with 11C-Flumazenil in Awake and Anesthetized Animals

Neuroreceptor imaging in the nonhuman primate (NHP) is valuable for translational research approaches in humans. However, most NHP studies are conducted under anesthesia, which affects the interpretability of receptor binding measures. The aims of this study were to develop awake NHP imaging with minimal head restraint and to compare in vivo binding of the γ-aminobutyric acid type A (GABAA)-benzodiazepine radiotracer 11C-flumazenil under anesthetized and awake conditions. We hypothesized that 11C-flumazenil binding potential (BPND) would be higher in isoflurane-anesthetized monkeys. Methods: The small animal PET scanner was fitted to a mechanical device that raised and tilted the scanner 45° while the awake NHP was tilted back 35° in a custom chair for optimal brain positioning, which required acclimation of the animals to the chair, touch-screen tasks, intravenous catheter insertion, and tilting. For PET studies, the bolus–plus–constant infusion method was used for 11C-flumazenil administration. Two rhesus monkeys were scanned under the awake (n = 6 scans) and isoflurane-anesthetized (n = 4 scans) conditions. An infrared camera was used to track head motion during PET scans. Under the awake condition, emission and head motion-tracking data were acquired for 40–75 min after injection. Anesthetized monkeys were scanned for 90 min. Cortisol measurements were acquired during awake and anesthetized scans. Equilibrium analysis was used for both the anesthetized (n = 4) and the awake (n = 5) datasets to compute mean BPND images in NHP template space, using the pons as a reference region. The percentage change per minute in radioactivity concentration was calculated in high- and low-binding regions to assess the quality of equilibrium. Results: The monkeys acclimated to procedures in the NHP chair necessary to perform awake PET imaging. Image quality was comparable between awake and anesthetized conditions. The relationship between awake and anesthetized values was BPND (awake) = 0.94 BPND (anesthetized) + 0.36 (r2 = 0.95). Cortisol levels were significantly higher under the awake condition (P < 0.05). Conclusion: We successfully performed awake NHP imaging with minimal head restraint. There was close agreement in 11C-flumazenil BPND values between awake and anesthetized conditions.

Adenosine 2A Receptor Occupancy by Tozadenant and Preladenant in Rhesus Monkeys

Motor symptoms in Parkinson disease (PD) are caused by a loss of dopamine input from the substantia nigra to the striatum. Blockade of adenosine 2A (A2A) receptors facilitates dopamine D2 receptor function. In phase 2 clinical trials, A2A antagonists (istradefylline, preladenant, and tozadenant) improved motor function in PD. We developed a new A2A PET radiotracer, 18F-MNI-444, and used it to investigate the relationship between plasma levels and A2A occupancy by preladenant and tozadenant in nonhuman primates (NHP). Methods: A series of 20 PET experiments was conducted in 5 adult rhesus macaques. PET data were analyzed with both plasma-input (Logan graphical analysis) and reference-region–based (simplified reference tissue model and noninvasive Logan graphical analysis) methods. Whole-body PET images were acquired for radiation dosimetry estimates. Human pharmacokinetic parameters for tozadenant and preladenant were used to predict A2A occupancy in humans, based on median effective concentration (EC50) values estimated from the NHP PET measurements. Results: 18F-MNI-444 regional uptake was consistent with A2A receptor distribution in the brain. Selectivity was demonstrated by dose-dependent blocking by tozadenant and preladenant. The specific-to-nonspecific ratio was superior to that of other A2A PET radiotracers. Pharmacokinetic modeling predicted that tozadenant and preladenant may have different profiles of A2A receptor occupancy in humans. Conclusion: 18F-MNI-444 appears to be a better PET radiotracer for A2A imaging than currently available radiotracers. Assuming that EC50 in humans is similar to that in NHP, it appears that tozadenant will provide a more sustained A2A receptor occupancy than preladenant in humans at clinically tested doses.

In Vivo Evaluation of 18F-MNI698: An 18F-Labeled Radiotracer for Imaging of Serotonin 4 Receptors in Brain

Serotonin 4 receptors (5-hydroxytryptamine receptor 4 [5HT4R]) hold promise as a novel therapeutic approach to multiple brain disorders, including Alzheimer and Huntington disease. In vivo imaging of these receptors with selective 5HT4R radiotracers and PET would be valuable to investigate alterations in 5HT4R in different brain disorders and to assist drug discovery. In this study, 18F-MNI698 was evaluated as a potential PET radiotracer for imaging of 5HT4R in the brain. Methods: Eighteen PET studies were performed in 3 adult rhesus monkeys. The radiotracer was administered as a bolus intravenous injection or bolus plus constant infusion (time that would be required to inject the bolus at the infusion rate = 60 min), and arterial blood was collected for data quantification. Kinetic models were used to estimate distribution volumes and binding potentials, for which the cerebellum was used as a reference region. 18F-MNI698 test–retest variability and upper mass dose limits were determined. Preblocking studies using several doses of SB204070, a selective 5HT4R antagonist, were performed. Results: 18F-MNI698 avidly entered the monkey brain (peak percentage injected dose of ∼6.6%), and its brain distribution was consistent with known 5HT4R densities. At 120 min after bolus injection and after the start of radiotracer infusion, only less than 5% and approximately 10% parent compound was present in blood, respectively. Measured binding potentials were underestimated by 22%–36% when noninvasive methods were used for data quantification in comparison with invasive methods. A good agreement was found between test–retest measurements. The radiotracer upper mass dose limit (<5% occupancy) was determined to be 13.1 μg per 70 kg of body weight. SB204070 blocked the radiotracer binding in a dose-dependent manner. Conclusion: Data indicate that 18F-MNI698 is a promising PET radiotracer for imaging of 5HT4R in the brain, and human studies are warranted based on these study results.

Imaging drug-induced dopamine release in rhesus monkeys with [11C]PHNO versus [11C]raclopride PET

Objective: The radiotracer [C]PHNO may have advantages over other dopamine D2/D3 receptor ligands because as an agonist it measures only the high affinity—functionally active—D2/D3 receptors and not the low affinity receptors. Ginovart et al. (2006) demonstrated improved sensitivity over [C]raclopride for measuring amphetamine-induced changes in synaptic dopamine levels. Our aimwas to take advantage of the strength of [C]PHNO for measuring the small dopamine signal induced by nicotine. Previously, Marenco et al. (2004) reported that a nicotine challenge (0.01–0.06 mg/kg, IV) yielded a 5% reduction and an amphetamine challenge (0.4 mg/kg, IV) in a 28% reduction in [C]raclopride binding potential in the caudate and putamen in monkeys. The goal of this study was to compare the sensitivity of [C]PHNO PET to that of [C] raclopride PET with nicotineand amphetamine-induced dopamine release in nonhuman primates.

Awake nonhuman primate brain PET imaging without head restraint

Introduction: Brain studies in awake nonhuman primates (NHP) have particular relevance to systems neuroscience and the study of disease. For many years, PET has used NHP studies for development of new radiopharmaceuticals and for the evaluation of the mechanisms of action of pharmacological agents. However, the vast majority of PET NHP studies are conducted under anesthesia, which has substantial effects on PET metabolic and receptor bindingmeasures. In a fewcenters, awakeNHPPET imaging has been employed using head fixation. The goal of thiswork is to develop awake NHP PET imaging without head restraint by translating head motion correction technology used in human studies.

Kinetic modeling of [11C]PHNO in rhesus monkeys using the Focus 220 PET scanner

Objective: [C]PHNO is a radioligand suitable to image D2/D3 receptors which has twomain advantages over other D2 PET radioligands. First, it is an agonist and thus may be more sensitive to synaptic levels of dopamine (Willeit et al., 2006). Secondly, it is the first PET tracer permitting measurement of the D3 subtype of dopamine receptors. These receptors are mainly concentrated in the pallidum and substantia nigra. The goal of this study was to evaluate different kinetic modeling strategies to quantify [C]PHNO binding in non-human primates using the high resolution scanner Focus 220.

PET evaluation of the TSPO ligands [F-18]FEPPA, [F-18]PRB06, and [F-18]PBR111 in nonhuman primate

Methods: Thirteen scans were performed in 3 isoflurane-anesthetized baboons on the HR+ scanner with the 3 ligands. Studies were performed under baseline conditions, and following pre-block or displacement with PK11195 at doses up to 7 mg/kg. Metabolite-corrected arterial input functions were acquired. Early summed images were registered to each animals MR scan that were subsequently registered to a baboon brain template, and time–activity curves were generated for modeling analysis. Total volumes of distribution (VT) were determined from oneand two-tissue (1T, 2T) modeling, and baseline and pre-block values were compared to assess the magnitude of specific binding.