Serotonin transporter occupancy by escitalopram and citalopram in the non-human primate brain: a [(11)C]MADAM PET study

Improving SUVR quantification by correcting for radiotracer clearance in tissue

Standardized Uptake Value Ratio (SUVR) is a widely reported semi-quantitative positron emission tomography (PET) outcome measure, partly because of its ease of measurement from short scan durations. However, in brain, SUVR is often a biased estimator of the gold-standard distribution volume ratio (DVR) due to non-equilibrium conditions, i.e., clearance of the radiotracer in relevant tissues. Factors that affect radiotracer metabolism and clearance such as medication or subject groups could lead to artificial differences in SUVR. This work developed a correction that reduces the bias in SUVR (estimated from a short 15-30 min PET imaging session) by accounting for the effects of tracer clearance observed during the late SUVR time window. The proposed correction takes the form of a one-step non-linear algebraic transform of SUVR that is a function of radiotracer dependent parameters such as clearance rates from the reference and target tissues, and population averaged reference region clearance rate (k 2 , ref ). An important observation was the need for accurate estimation of radiotracer clearance rate in target tissue, which was addressed with a regression based model. Simulations and human data from two different radiotracers (healthy controls for [11C]LSN3172176, healthy controls and Parkinson's disease subjects for [18F]FE-PE2I) were used to validate the correction and evaluate its benefits and limitations. SUVR correction in human data significantly reduced mean SUVR bias across brain regions and subjects (from ∼25% for SUVR to <10% for corrected SUVR). This correction also significantly reduced the variability of this bias across brain regions for both tracers (approximately 50% for [11C]LSN3172176, 20% for [18F]FE-PE2I). Future work should investigate the benefits of using corrected SUVR in other populations and with different tracers.

Concise Review: Recent advances in neuroimaging techniques to assist clinical trials on cell-based therapies in neurodegenerative diseases

Neurodegenerative diseases, such as Alzheimer's disease (AD) and Parkinson's disease (PD), are progressive disorders for which a curative therapy is still lacking. Cell-based therapy aims at replacing dysfunctional cellular populations by repairing damaged tissue and by enriching the microenvironment of selective brain areas, and thus constitutes a promising disease-modifying treatment of neurodegenerative diseases. Scientific research has engineered a wide range of human-derived cellular populations to help overcome some of the logistical, safety, and ethical issues associated with this approach. Open-label studies and clinical trials in human participants have employed neuroimaging techniques, such as positron emission tomography (PET) and magnetic resonance imaging (MRI), to assess the success of the transplantation, to evaluate the functional integration of the implanted tissue into the host environment and to understand the pathophysiological changes associated with the therapy. Neuroimaging has constituted an outcome measure of large, randomized clinical trials, and has given answers to clarify the pathophysiology underlying some of the complications linked with this therapy. Novel PET radiotracers and MRI sequences for the staging of neurodegenerative diseases and to study alterations at molecular level significantly expands the translational potential of neuroimaging to assist pre-clinical and clinical research on cell-based therapy in these disorders. This concise review summarizes the current use of neuroimaging in human studies of cell-based replacement therapy and focuses on future application of PET and MRI techniques to evaluate the pathophysiology and treatment efficacy, as well as to aid patient selection and as an outcome measure to improve treatment success.

Quantification of PET infusion studies without true equilibrium: A tissue clearance correction

In some positron emission tomography (PET) studies, a reversibly binding radioligand is administered as a constant infusion to establish true equilibrium for quantification. This approach reduces scanning time and simplifies data analysis, but assumes similar behavior of the radioligand in plasma across the study population to establish true equilibrium in all subjects. Bias in outcome measurements can result if this assumption is not met. This work developed and validated a correction that reduces bias in total distribution volume (VT) estimates when true equilibrium is not present. This correction, termed tissue clearance correction (TCC), took the form V T = V T ( A ) / ( 1 + β γ V T ( A ) ) , where β is the radioligand clearance rate in tissue, γ is a radiotracer-specific constant, and VT(A) is the apparent VT. Simulations characterized the robustness of TCC across imperfect values of γ and β and demonstrated reduction to false positive rates. This approach was validated with human infusion data for three radiotracers: [18F]FPEB, (-)-[18F]flubatine, and [11C]UCB-J. TCC reduced bias in VT estimates for all radiotracers and significantly reduced intersubject variance in VT for [18F]FPEB data in some brain regions. Thus, TCC improves quantification of data acquired from PET infusion studies.

Tobacco Smoking in People Is Not Associated with Altered 18-kDa Translocator Protein Levels: A PET Study

The effects of tobacco smoking on the immune system of the brain are not well elucidated. Although nicotine is immunosuppressive, other constituents in tobacco smoke have inflammatory effects. PET imaging of the 18-kDa translocator protein (TSPO) provides a biomarker for microglia, the primary immunocompetent cells of the brain. This work compared brain TSPO levels in 20 tobacco smokers (abstinent for at least 2 h) and 20 nonsmokers using a fully quantitative modeling approach for the first time, to our knowledge. Methods: ¹¹C-PBR28 (N-((2-(methoxy-¹¹C)-phenyl)methyl)-N-(6-phenoxy-3-pyridinyl)acetamide) PET scans were acquired with arterial blood sampling to estimate the metabolite-corrected input function. ¹¹C-PBR28 volumes of distribution were estimated throughout the brain with multilinear analysis. Results: Statistical analyses revealed no evidence of significant differences in regional ¹¹C-PBR28 volumes of distribution between smokers and nonsmokers (whole-brain Cohen d = 0.09) despite adequate power to detect medium effect sizes. Conclusion: These findings inform previous PET studies reporting lower TSPO radiotracer concentrations in the brain (measured as SUV) for tobacco smokers than for nonsmokers by demonstrating the importance of accounting for radiotracer concentrations in plasma. These findings suggest that nonsmokers and smokers have comparable TSPO levels in the brain. Additional work with other biomarkers is needed to fully characterize the effects of tobacco smoking on the brain immune system.

Effect of clinically relevant doses of vortioxetine and citalopram on serotonergic PET markers in the nonhuman primate brain

Vortioxetine is a multimodal antidepressant approved for treatment of major depressive disorder. Preclinical studies have demonstrated that the mechanism of action of vortioxetine might be different from selective serotonin reuptake inhibitors (SSRIs), including larger serotonin (5-HT) release and direct modulation of several 5-HT receptors. In the current positron emission tomography (PET) study, we evaluated the mechanism of action of vortioxetine by comparing its effect to the SSRI citalopram on the binding of [¹¹C]AZ10419369 to the 5-HT_1B receptor in the nonhuman primate brain. Initially, the 5-HT transporter (5-HTT) binding of vortioxetine was determined by [¹¹C]MADAM PET measurements before and after administration of vortioxetine (0.1-3.0 mg/kg) and data were used to confirm clinically relevant dosing in subsequent PET measurements with [¹¹C]AZ10419369. The 5-HT_1B receptor binding was significantly decreased after 0.3 mg/kg of citalopram in the dorsal raphe nucleus (5%), as well as after 0.3 mg/kg of vortioxetine in six brain regions (~25%) or 1.0 mg/kg of vortioxetine in all 12 examined regions (~48%). Moreover, there was no effect of 1.0 mg/kg of vortioxetine on the binding of [¹¹C]Cimbi-36 to the 5-HT_2A receptor, which has comparable sensitivity to 5-HT release as [¹¹C]AZ10419369 binding. In conclusion, at clinically relevant doses, vortioxetine induced larger reductions in [¹¹C]AZ10419369 binding than citalopram. These observations suggest that vortioxetine binds to the 5-HT_1B receptor at clinically relevant doses. Future studies are warranted to evaluate the role of the 5-HT_1B receptor in the therapeutic effects of vortioxetine and as a potential target for the development of novel antidepressant drugs.

Target Engagement of a Phosphodiesterase 2A Inhibitor Affecting Long-Term Memory in the Rat

Inhibition of phosphodiesterase 2A (PDE2A) has been proposed as a potential approach to enhance cognitive functioning and memory through boosting intracellular cGMP/cAMP and enhancing neuroplasticity in memory-related neural circuitry. Previous preclinical studies demonstrated that PDE2A inhibitors could reverse N-methyl-D-aspartate receptor antagonist (5S,10R)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine or ketamine-induced memory deficit. Here, we report that the potent and selective PDE2A inhibitor 4-(1-azetidinyl)-7-methyl-5-[1-methyl-5-[5-(trifluoromethyl)-2-pyridinyl]-1H-pyrazol-4-yl]-imidazo[5,1-f][1,2,4]triazine (PF-05180999) enhances long-term memory in a contextual fear conditioning model in the rat at the oral dose of 0.3 mg/kg. Target engagement at this efficacious dose was explored using in vivo autoradiography. Converse to the results of a decrease of PDE2A binding (target occupancy) by the PDE2A inhibitor, a paradoxical increase (up to 40%) in PDE2A binding was detected. However, a typical target occupancy curve could be generated by PF-05180999 at much higher doses. In vitro experiments using recombinant PDE2A protein or rat brain homogenate that contains native PDE2A protein demonstrated that increased cGMP after initial PDE2A inhibition could be responsible for the activation of PDE2A enzyme via allosteric binding to the GAF-B domain, leading to positive cooperativity of the dormant PDE2A enzymes. Our results suggest that when evaluating target engagement of PDE2A inhibitors for memory disorder in clinical setting with occupancy assays, the efficacious dose may not fall on the typical receptor/target curve. On the contrary, an increase in PDE2A tracer binding is likely seen. Our results also suggest that when evaluating target occupancy of enzymes, potential regulation of enzyme activities should be considered.

Potential Effect of Prolonged Sevoflurane Anesthesia on the Kinetics of [11C]Raclopride in Non-human Primates

Purpose

Positron emission tomography (PET) in non-human primates (NHP) is commonly performed under anesthesia, with sevoflurane being a widely used inhaled anesthetic. PET measurement in NHP can be repeated, and a difference in radioligand kinetics has previously been observed between the first and second PET measurement on the same day using sevoflurane anesthesia. In this study, we evaluated the effect of prolonged sevoflurane anesthesia on kinetics and binding potential (BP_ND) of [¹¹C]raclopride in NHP.

Procedures

Three cynomolgus monkeys underwent two to three PET measurements with [¹¹C]raclopride under continuous sevoflurane anesthesia on the same day. The concentration of sevoflurane was adjusted according to the general conditions and safety parameters of the NHP. Time to peak (TTP) radioactivity in the striatum was estimated from time-activity curves (TACs). The BP_ND in the striatum was calculated by the simplified reference tissue model using the cerebellum as reference region.

Results

In each NHP, the TTP became shorter in the later PET measurements than in the first one. Across all measurements (n = 8), concentration of sevoflurane correlated with TTP (Spearman’s ρ = − 0.79, p = 0.03), but not with BP_ND (ρ = − 0.25, p = 0.55).

Conclusions

These data suggest that sevoflurane affects the shape of TACs but has no evident effect on BP_ND in consecutive PET measurements.

Characterization of [11C]Lu AE92686 as a PET radioligand for phosphodiesterase 10A in the nonhuman primate brain

Purpose

[¹¹C]Lu AE92686 is a positron emission tomography (PET) radioligand that has recently been validated for examining phosphodiesterase 10A (PDE10A) in the human striatum. [¹¹C]Lu AE92686 has high affinity for PDE10A (IC₅₀ = 0.39 nM) and may also be suitable for examination of the substantia nigra, a region with low density of PDE10A. Here, we report characterization of regional [¹¹C]Lu AE92686 binding to PDE10A in the nonhuman primate (NHP) brain.

Methods

A total of 11 PET measurements, seven baseline and four following pretreatment with unlabeled Lu AE92686 or the structurally unrelated PDE10A inhibitor MP-10, were performed in five NHPs using a high resolution research tomograph (HRRT). [¹¹C]Lu AE92686 binding was quantified using a radiometabolite-corrected arterial input function and compartmental and graphical modeling approaches.

Results

Regional time-activity curves were best described with the two-tissue compartment model (2TCM). However, the distribution volume (V_T) values for all regions were obtained by the Logan plot analysis, as reliable cerebellar V_T values could not be derived by the 2TCM. For cerebellum, a proposed reference region, V_T values increased by ∼30 % with increasing PET measurement duration from 63 to 123 min, while V_T values in target regions remained stable. Both pretreatment drugs significantly decreased [¹¹C]Lu AE92686 binding in target regions, while no significant effect on cerebellum was observed. Binding potential (BP_ND) values, derived with the simplified reference tissue model (SRTM), were 13–17 in putamen and 3–5 in substantia nigra and correlated well to values from the Logan plot analysis.

Conclusions

The method proposed for quantification of [¹¹C]Lu AE92686 binding in applied studies in NHP is based on 63 min PET data and SRTM with cerebellum as a reference region. The study supports that [¹¹C]Lu AE92686 can be used for PET examinations of PDE10A binding also in substantia nigra.

Electronic supplementary material

The online version of this article (doi:10.1007/s00259-016-3544-9) contains supplementary material, which is available to authorized users.

PET Neurochemical Imaging Modes

PET has deep roots in neuroscience stemming from its first application in brain tumor and brain metabolism imaging. PET emerged over the past few decades and continues to play a prominent role in the study of neurochemistry in the living human brain. Over time, neurochemical imaging with PET has been expanded to address a host of research questions related to, among many others, protein density, drug occupancy, and endogenous neurochemical release. Each of these imaging modes has distinct design and analysis considerations that are critical for enabling quantitative measurements. The number of considerations required for a neurochemical PET study can make it unapproachable. This article aims to orient those interested in neurochemical PET imaging to three of the common imaging modes and to provide some perspective on needs that exist for expansion of neurochemical PET imaging.