Equilibrium modeling of 5-HT(2A) receptors with [18F]deuteroaltanserin and PET: feasibility of a constant infusion paradigm

ABSTRACT. [¹⁸F]Altanserin has emerged as a promising positron emission tomography (PET) ligand for serotonin-2A (5-HT_2A) receptors. The deuterium substitution of both of the 2′-hydrogens of altanserin ([¹⁸F]deuteroaltanserin) yields a metabolically more stable radiotracer with higher ratios of parent tracer to radiometabolites and increased specific brain uptake than [¹⁸F]altanserin. The slower metabolism of the deuterated analog might preclude the possibility of achieving stable plasma and brain activities with a bolus plus constant infusion within a reasonable time frame for an ¹⁸F-labeled tracer (T_1/2 110 min). Thus, the purpose of this study was to test the feasibility in human subjects of a constant infusion paradigm for equilibrium modeling of [¹⁸F]deuteroaltanserin with PET. Seven healthy male subjects were injected with [¹⁸F]deuteroaltanserin as a bolus plus constant infusion lasting 10 h postinjection. PET acquisitions and venous blood sampling were performed throughout the infusion period. Linear regression analysis revealed that time-activity curves for both specific brain uptake and plasma [¹⁸F]deuteroaltanserin concentration stabilized after about 5 h. This permitted equilibrium modeling and estimation of V^′₃ (ratio of specific uptake to total plasma parent concentration) and the binding potential V₃ (ratio of specific uptake to free plasma parent concentration). Cortical/cerebellar ratios were increased by 26% relative to those we previously observed with [¹⁸F]altanserin using similar methodology in a somewhat older subject sample. These results demonstrate feasibility of equilibrium imaging with [¹⁸F]deuteroaltanserin and suggest that it may be superior to [¹⁸F]altanserin as a PET radioligand.     

Comparison of [(18)F]altanserin and [(18)F]deuteroaltanserin for PET imaging of serotonin(2A) receptors in baboon brain: pharmacological studies

The regional distribution in brain, distribution volumes, and pharmacological specificity of the PET 5-HT(2A) receptor radiotracer [(18)F]deuteroaltanserin were evaluated and compared to those of its non-deuterated derivative [(18)F]altanserin. Both radiotracers were administered to baboons by bolus plus constant infusion and PET images were acquired up to 8 h. The time-activity curves for both tracers stabilized between 4 and 6 h. The ratio of total and free parent to metabolites was not significantly different between radiotracers; nevertheless, total cortical R(T) (equilibrium ratio of specific to nondisplaceable brain uptake) was significantly higher (34-78%) for [(18)F]deuteroaltanserin than for [(18)F]altanserin. In contrast, the binding potential (Bmax/K(D)) was similar between radiotracers. [(18)F]Deuteroaltanserin cortical activity was displaced by the 5-HT(2A) receptor antagonist SR 46349B but was not altered by changes in endogenous 5-HT induced by fenfluramine. These findings suggest that [(18)F]deuteroaltanserin is essentially equivalent to [(18)F]altanserin for 5-HT(2A) receptor imaging in the baboon.     

PET quantification of 5-HT2A receptors in the human brain: a constant infusion paradigm with [18F]altanserin.

[18F]altanserin has been used to label serotonin 5-HT2A receptors, which are believed to be important in the pathophysiology of schizophrenia and depression. The purpose of this study was to test the feasibility of a constant infusion paradigm for equilibrium modeling of [18F]altanserin with PET. Kinetic modeling with [18F]altanserin may be hampered by the presence of lipophilic radiometabolites observed in plasma after intravenous administration. METHODS: Eight healthy volunteers were injected with [18F]altanserin as a bolus (208+/-9 MBq [5.62+/-0.25 mCi]) plus constant infusion (65+/-3 MBq/h [1.76+/-0.08 mCi/h]) ranging from 555 to 626 min (615+/-24 min) after injection. PET acquisitions (10-20 min) and venous blood sampling were performed every 30-60 min throughout the infusion period. RESULTS: Linear regression analysis revealed that time-activity curves for both brain activity and plasma [18F]altanserin and metabolite concentrations stabilized after about 6 h. This permitted equilibrium modeling and estimation of V3' (ratio of specific uptake [cortical-cerebellar] to total plasma parent concentration after 6 h). Values of V3' ranged from 1.57+/-0.38 for anterior cingulate cortex to 1.02+/-0.39 for frontal cortex. The binding potential V3 (ratio of specific uptake to free plasma parent concentration after 6 h, using group mean f1) was also calculated and ranged from 169+/-41 for anterior cingulate cortex to 110+/-42 for frontal cortex. From 6 h onward, the rate of change for V3' and V3 was only 1.11+/-1.69 %/h. CONCLUSION: These results demonstrate the feasibility of equilibrium imaging with [18F]altanserin over more than 5 radioactive half-lives and suggest a method to overcome difficulties associated with lipophilic radiolabeled metabolites. The stability in V3 and V3' once equilibrium is achieved suggests that a single PET acquisition obtained at 6 h may provide a reasonable measure of 5-HT2A receptor density.     

Imaging the norepinephrine transporter with positron emission tomography: initial human studies with (<Emphasis Type="Italic">S,S</Emphasis>)-[<Superscript>18</Superscript>F]FMeNER-D<Subscript>2</Subscript>

Introduction (S,S)-[¹⁸F]FMeNER-D₂ is a recently developed positron emission tomography (PET) ligand for in vivo quantification of norepinephrine transporter. A monkey occupancy study with the radioligand indicated that (S,S)-[¹⁸F]FMeNER-D₂ can be useful for quantitative PET analysis. In this preliminary study, regional distributions in the living human brain were evaluated. Materials and methods Brain PET measurements were performed for a total of 255 min after the injection of 188.3 ± 5.7 MBq of (S,S)-[¹⁸F]FMeNER-D₂ in four healthy male subjects. Regions of interests were drawn on the thalamus and the caudate in the coregistered MRI/PET images. Results (S,S)-[¹⁸F]FMeNER-D₂ displayed good brain penetration and selective retention in regions rich in norepinephrine reuptake sites. The transient peak equilibrium was reached during the PET measurements. The ratios of radioactivity uptake in the thalamus to that in the caudate were 1.50 ± 0.06 for the time period of 90–255 min. Conclusion The present preliminary investigation indicates that (S,S)-[¹⁸F]FMeNER-D₂ has suitable characteristics for probing the norepinephrine reuptake system with PET in the human brain.     

Distribution of the 5-HT<Subscript>1A</Subscript> receptor antagonist [<Superscript>18</Superscript>F]FPWAY in blood and brain of the rat with and without isoflurane anesthesia

Purpose To determine whether brain and plasma equilibrium of a proposed PET tracer for 5-HT_1A, [¹⁸F]FPWAY, can be achieved in a sufficiently short time for practical use of the brain to plasma equilibrium distribution ratio (DR) to monitor receptor availability with and without isoflurane anesthesia. Methods Awake (n=4) and isoflurane-anesthetized (n=4) rats were administered a continuous 60 min intravenous infusion of [¹⁸F]FPWAY with timed arterial blood sampling. Brains of the isoflurane-anesthetized rats were scanned with the ATLAS small animal PET scanner; awake rats were not. All rats were killed at 60 min and scanned postmortem for 15 min, followed by brain slicing for autoradiography. Several regions of interest (ROIs) were defined in the PET images as well as in the autoradiographic images. Regional DRs were calculated as total activity in the brain ROI divided by plasma [¹⁸F]FPWAY activity. Results DRs in the anesthetized animals were constant between 30 and 60 min, indicating that near equilibrium between brain and plasma had been achieved by ∼30 min. DRs determined from postmortem PET data were higher in the isoflurane-anesthetized rats by 24% (not significant) and 33% (p=0.065) in whole brain and hippocampus, respectively. DRs determined from autoradiographic data were greater in isoflurane-anesthetized rats in medial hippocampus, lateral hippocampus, and cerebellum by 33% (p=0.054), 63% (p<0.01), and 32% (p<0.05), respectively. Conclusion [¹⁸F]FPWAY could be an appropriate ligand for monitoring changes in receptor availability in the serotonergic system using a bolus/infusion paradigm. One possible explanation for higher DRs in anesthetized rats may be a reduction in endogenous 5-HT secretion under isoflurane anesthesia.     

Uptake of 4-borono-2-[<Superscript>18</Superscript>F]fluoro-<Emphasis Type="SmallCaps">L</Emphasis>-phenylalanine in sporadic and neurofibromatosis 2-related schwannoma and meningioma studied with PET

Purpose Meningiomas and schwannomas associated with neurofibromatosis 2 (NF2) are difficult to control by microsurgery and stereotactic radiotherapy alone. Boron neutron capture therapy (BNCT) is a chemically targeted form of radiotherapy requiring increased concentration of boron-10 in tumour tissue. PET with the boron carrier 4-borono-2-[¹⁸F]fluoro-L-phenylalanine ([¹⁸F]FBPA) allows investigation of whether 4-borono-L-phenylalanine (BPA) concentrates in NF2 tumours, which would make BNCT feasible. Methods We studied dynamic uptake of [¹⁸F]FBPA in intracranial meningiomas (n=4) and schwannomas (n=6) of five sporadic and five NF2 patients. Tracer input function and cerebral blood volume were measured. [¹⁸F]FBPA uptake in tumour and brain was assessed with a three-compartmental model and graphical analysis. These, together with standardised uptake values (SUVs), were used to define tumour-to-brain [¹⁸F]FBPA tissue activity gradients. Results Model fits with three parameters K ₁ (transport), k ₂ (reverse transport) and k ₃ (intracellular metabolism) were found to best illustrate [¹⁸F]FBPA uptake kinetics. Maximum SUV was two- to fourfold higher in tumour as compared with normal brain and independent of NF2 status. The increased uptake was due to higher transport of [¹⁸F]FBPA in tumour. In multiple-time graphical analysis (MTGA, Gjedde-Patlak plot) the tumour-to-brain [¹⁸F]FBPA influx constant (K _i -MTGA) ratios varied between 1.8 and 5.4 in NF2-associated tumours while in sporadic tumours the ratio was 1–1.4. Conclusion [¹⁸F]FBPA PET offers a viable means to evaluate BPA uptake in meningiomas and schwannomas in NF2. Based on our results on tumour uptake of [¹⁸F]FBPA, some of these benign neoplasms may be amenable to BNCT. Financial support: This work was sponsored in part by the Department of Army, Grant No. DAMD17-00-1-0545. The US Army Medical Research Acquisition Activity, 820 Chandler Street, Fort Detrick, MD 21702-5014, USA, is the awarding and administering acquisition office. The content of the information of this paper does not necessarily reflect the position or the policy of the US Government.     

Peripheral metabolism of [F]FDDNP and cerebral uptake of its labelled metabolites

[¹⁸F]FDDNP is a positron emission tomography (PET) tracer for determining amyloid plaques and neurofibrillary tangles in the brain in vivo. In order to quantify binding of this tracer properly, a metabolite-corrected plasma input function is required. The purpose of the present study was to develop a sensitive method for measuring [¹⁸F]FDDNP and its radiolabelled metabolites in plasma. The second aim was to assess whether these radiolabelled metabolites enter the brain.In humans, there was extensive metabolism of [¹⁸F]FDDNP. After 10 min, more than 80% of plasma radioactivity was identified as polar ¹⁸F-labelled fragments, probably formed from N-dealkylation of [¹⁸F]FDDNP. These labelled metabolites were reproduced in vitro using human hepatocytes. PET studies in rats showed that these polar metabolites can penetrate the blood–brain barrier and result in uniform brain uptake.     

Biodistribution and radiation dosimetry of the norepinephrine transporter radioligand (<Emphasis Type="Italic">S</Emphasis>,<Emphasis Type="Italic">S</Emphasis>)-[<Superscript>18</Superscript>F]FMeNER-D<Subscript>2</Subscript>: a human whole-body PET study

Purpose (S,S)-[¹⁸F]FMeNER-D₂ is a recently developed positron-emission tomography (PET) radioligand for in vivo quantification of the norepinephrine transporter system. The aim of this study was to provide dosimetry estimates for (S,S)-[¹⁸F]FMeNER-D₂ based on human whole-body PET measurements. Methods PET scans were performed for a total of 6.4 h after the injection of 168.9 ± 31.5 MBq of (S,S)-[¹⁸F]FMeNER-D₂ in four healthy male subjects. Volumes of interest were drawn on the coronal images. Estimates of the absorbed dose of radiation were calculated using the OLINDA software. Results Uptake was largest in lungs, followed by liver, bladder, brain and other organs. Peak values of the percent injected dose (%ID) at a time after radioligand injection were calculated for the lung (21.6%ID at 0.3 h), liver (5.1%ID at 0.3 h), bladder (12.2%ID at 6 h) and brain (2.3%ID at 0.3 h). The largest absorbed dose was found in the urinary bladder wall (0.039 mGy/MBq). The calculated effective dose was 0.017 mSv/MBq. Conclusion Based on the distribution and dose estimates, the estimated radiation burden of (S,S)-[¹⁸F]FMeNER-D₂ is lower than that of [¹⁸F]FDG. The radioligand would allow multiple PET examinations in the same research subject per year.     

Reproducibility of 5-HT<Subscript>2A</Subscript> receptor measurements and sample size estimations with [<Superscript>18</Superscript>F]altanserin PET using a bolus/infusion approach

Purpose To determine the reproducibility of measurements of brain 5-HT_2A receptors with an [¹⁸F]altanserin PET bolus/infusion approach. Further, to estimate the sample size needed to detect regional differences between two groups and, finally, to evaluate how partial volume correction affects reproducibility and the required sample size. Methods For assessment of the variability, six subjects were investigated with [¹⁸F]altanserin PET twice, at an interval of less than 2 weeks. The sample size required to detect a 20% difference was estimated from [¹⁸F]altanserin PET studies in 84 healthy subjects. Regions of interest were automatically delineated on co-registered MR and PET images. Results In cortical brain regions with a high density of 5-HT_2A receptors, the outcome parameter (binding potential, BP₁) showed high reproducibility, with a median difference between the two group measurements of 6% (range 5–12%), whereas in regions with a low receptor density, BP₁ reproducibility was lower, with a median difference of 17% (range 11–39%). Partial volume correction reduced the variability in the sample considerably. The sample size required to detect a 20% difference in brain regions with high receptor density is approximately 27, whereas for low receptor binding regions the required sample size is substantially higher. Conclusion This study demonstrates that [¹⁸F]altanserin PET with a bolus/infusion design has very low variability, particularly in larger brain regions with high 5-HT_2A receptor density. Moreover, partial volume correction considerably reduces the sample size required to detect regional changes between groups.     

NanoPET imaging of [18F]fluoromisonidazole uptake in experimental mouse tumours

Purpose The purpose of this study was to assess the potential and utility of ultra-high-resolution hypoxia imaging in various murine tumour models using the established hypoxia PET tracer [¹⁸F]fluoromisonidazole ([¹⁸F]FMISO).Methods [¹⁸F]FMISO PET imaging was performed with the dedicated small-animal PET scanner NanoPET (Oxford Positron Systems) and ten different human tumour xenografts in nude mice as well as B16 melanoma tumours in syngeneic Balb/c mice. For comparison, [¹⁸F]fluorodeoxyglucose ([¹⁸F]FDG) PET scans were also performed in the mice bearing human tumour xenografts.Results In 10 out of 11 experimental tumour models, [¹⁸F]FMISO PET imaging allowed clear-cut visualisation of the tumours. Inter- and intratumoural heterogeneity of tracer uptake was evident. In addition to average TMRR (tumour-to-muscle retention ratio including all voxels in a volume of interest (VOI)), the parameters TMRR_75% and TMRR₅ (tumour-to-muscle retention ratio including voxels of 75% or more of the maximum radioactivity in a VOI and the five hottest pixels, respectively) also served as measures for quantifying the heterogeneous [¹⁸F]FMISO uptake in the tumours. The variability observed in [¹⁸F]FMISO uptake was related neither to tumour size nor to the injected mass of the radiotracer. The pattern of normoxic and hypoxic regions within the human tumour xenografts, however, correlated with glucose metabolism as revealed by comparison of [¹⁸F]FDG and [¹⁸F]FMISO images.Conclusion This study demonstrates the feasibility and utility of [¹⁸F]FMISO for imaging murine tumour models using NanoPET.     

Comparison of the pharmacokinetics of 68Ga-DOTATOC and [18F]FDG in patients with metastatic neuroendocrine tumours scheduled for 90Y-DOTATOC therapy

Purpose The purpose of this study was to evaluate and compare, by means of dynamic PET, the pharmacokinetics of ⁶⁸Ga-DOTATOC, a tracer which reflects the expression of somatostatin receptors (SSTRs), and of [¹⁸F]FDG, a marker of tumour viability, in patients with metastatic neuroendocrine tumours (NETs) in whom ⁹⁰Y-DOTATOC therapy was planned.Materials and methods Fifteen patients (63 lesions) with confirmed metastatic NETs were enrolled in this study. Dynamic [¹⁸F]FDG and ⁶⁸Ga-DOTATOC PET scans were performed on two different days in the same week. The data analysis was based on qualitative and quantitative analysis using a two-tissue compartment model with a blood compartment and a non-compartment model based on the fractal dimension (FD). Multivariate analysis was used for evaluation of the kinetic data.Results Enhanced [¹⁸F]FDG uptake was observed in 43/63 lesions. ⁶⁸Ga-DOTATOC showed pathologically enhanced uptake in all evaluated patients and in 57/63 lesions. Discordant scintigraphic results for [¹⁸F]FDG and ⁶⁸Ga-DOTATOC were observed in 6/15 patients. Global SUV was defined as the SUV measured in the last frame (55–60 min p.i.) of the dynamic series, for each tracer. The median global SUV uptake was 7.9 for ⁶⁸Ga-DOTATOC and 4.6 for [¹⁸F]FDG. The selection of patients for ⁹⁰Y-DOTATOC therapy was based on the uptake of ⁶⁸Ga-DOTATOC. Multiple linear regression analysis was applied to determine the effect of each kinetic parameter (K ₁–k ₄, V _B) on the global SUV of both tracers. The highest positive t-ratio was found for K ₁ (receptor binding), followed by k ₃ (cellular internalisation) and V _B (fractional blood volume), when using the global ⁶⁸Ga-DOTATOC uptake (SUV) as a target variable. Analysis of the [¹⁸F]FDG data revealed the highest positive t-ratio for V _B, followed by k ₃ (phosphorylation) and K ₁ (influx). The comparison of global SUV, K ₁–k ₄ and the FD for [¹⁸F]FDG and ⁶⁸Ga-DOTATOC did not show any statistically significant correlation. The only parameter that demonstrated a significant linear correlation between the tracers was V _B.Conclusion ⁶⁸Ga-DOTATOC is a promising tool for evaluation of the expression of SSTR2 in NETs. The combination of [¹⁸F]FDG and ⁶⁸Ga-DOTATOC dynamic PET studies provides different information regarding the biological properties of lesions in patients with metastatic NETs in whom ⁹⁰Y-DOTATOC therapy is planned. While the global ⁶⁸Ga-DOTATOC uptake is influenced mostly by K ₁, the global [¹⁸F]FDG uptake is mostly influenced by V _B. Only patients with enhanced ⁶⁸Ga-DOTATOC uptake (SUV >5.0) were referred to ⁹⁰Y-DOTATOC therapy.     

In-vivo imaging characteristics of two fluorinated flumazenil radiotracers in the rat

Purpose  [¹¹C]Flumazenil shows promise as a clinical and research PET radiotracer to image changes in GABA_A central benzodiazepine receptor (cBZR), but its widespread use has been limited by practical limitations of [¹¹C]. This study evaluated the imaging characteristics of two fluorinated PET radiotracers in rats in vivo: [¹⁸F]fluoroflumazenil ([¹⁸F]FFMZ) and [¹⁸F]flumazenil ([¹⁸F]FMZ). Methods  PET acquisitions were performed on a small-animal scanner following injection of [¹⁸F]FFMZ in nine rats and [¹⁸F]FMZ in eight rats. The following treatments were investigated: (1) injection of the tracer dose, (2) presaturation then injection of the tracer dose, and (3) injection of the tracer dose followed by a displacement injection. Unchanged tracer was measured in plasma and brain structures in four animals 10 and 30 min after injection, and ex-vivo autoradiography was also performed. Results  For both [¹⁸F]FFMZ and [¹⁸F]FMZ maximal brain activity peaked rapidly, and was highest in the hippocampus (1.12±0.06 SUV, 1.24±0.10 SUV, respectively), and lowest in the pons (1.00±0.07 SUV, 1.03±0.09 SUV, respectively). By 50 min after injection, maximal uptake for [¹⁸F]FFMZ and [¹⁸F]FMZ had decreased in the hippocampus to 18±3% and 80±1% (p<0.01), respectively. The presaturation and displacement studies showed a higher nonspecific component for [¹⁸F]FFMZ than for [¹⁸F]FMZ. Metabolite studies showed that at 30 min only 10% of the signal was from [¹⁸F]FFMZ in the brain. This nonspecific binding was apparent on autoradiography. In contrast, [¹⁸F]FMZ accounted for >70% of the signal in the brain, which resulted in well-defined regional binding on autoradiography. Conclusion  These results demonstrate that [¹⁸F]FMZ is a superior radiotracer to [¹⁸F]FFMZ for in-vivo PET imaging of the GABA_A/cBZR, having slower metabolism and leading to lower concentrations of metabolites in the brain that results in a substantially better signal-to-noise ratio.     

[18F]Altanserin and small animal PET: Impact of multidrug efflux transporters on ligand brain uptake and subsequent quantification of 5-HT2A receptor densities in the rat brain

IntroductionThe selective 5-hydroxytryptamine type 2a receptor (5-HT_2AR) radiotracer [¹⁸F]altanserin is a promising ligand for in vivo brain imaging in rodents. However, [¹⁸F]altanserin is a substrate of P-glycoprotein (P-gp) in rats. Its applicability might therefore be constrained by both a differential expression of P-gp under pathological conditions, e.g. epilepsy, and its relatively low cerebral uptake. The aim of the present study was therefore twofold: (i) to investigate whether inhibition of multidrug transporters (MDT) is suitable to enhance the cerebral uptake of [¹⁸F]altanserin in vivo and (ii) to test different pharmacokinetic, particularly reference tissue-based models for exact quantification of 5-HT_2AR densities in the rat brain.MethodsEighteen Sprague-Dawley rats, either treated with the MDT inhibitor cyclosporine A (CsA, 50 mg/kg, n = 8) or vehicle (n = 10) underwent 180-min PET scans with arterial blood sampling. Kinetic analyses of tissue time–activity curves (TACs) were performed to validate invasive and non-invasive pharmacokinetic models.ResultsCsA application lead to a two- to threefold increase of [¹⁸F]altanserin uptake in different brain regions and showed a trend toward higher binding potentials (BP_ND) of the radioligand.ConclusionsMDT inhibition led to an increased cerebral uptake of [¹⁸F]altanserin but did not improve the reliability of BP_ND as a non-invasive estimate of 5-HT_2AR. This finding is most probable caused by the heterogeneous distribution of P-gp in the rat brain and its incomplete blockade in the reference region (cerebellum). Differential MDT expressions in experimental animal models or pathological conditions are therefore likely to influence the applicability of imaging protocols and have to be carefully evaluated.     

Carbon-11 pb-12: an attempt to visualize the dopamine d4 receptor in the primate brain with positron emission tomography

The dopamine D₄ receptor (D₄R) is expressed in low density in various extrastriatal brain regions. This receptor subtype is discussed in relation to the pathophysiology and treatment of schizophrenia but no selective positron emission tomography (PET) ligand is available to date to study the distribution in vivo. The arylpiperazine derivative N-[2-[4-(4-chlorophenyl)piperazin-1-yl]ethyl]-3-methoxybenzamide (PB-12) is a novel, high-affinity ( K_i=0.040 nM) and selective D₄R ligand. We radiolabeled PB-12 with carbon-11 (t_1/2 20.4 min) by O-methylation of the corresponding desmethyl analogue N-[2-[4-(4-chlorophenyl)piperazin-1-yl]ethyl]-3-hydroxybenzamide (LM-190) with [¹¹C]methyl triflate. Derivative LM-190 was prepared by condensing 3-hydroxybenzoic acid with the appropriate amine. For the radiolabeling, the incorporation yield was >90% and the total synthesis time including high performance liquid chromatography (HPLC) purification was about 35 min. The specific radioactivity of [¹¹C]PB-12 at time of injection was 67–118 GBq·μmol⁻¹. PET studies in a cynomolgus monkey showed a high uptake and widespread distribution of radioactivity in the brain, including the neocortex and thalamus. About 40% of total radioactivity in plasma represented unchanged radioligand at 60 min after injection as determined by HPLC. Pretreatment with the D₄R ligand 3-{[4-(4-chlorophenyl)piperazin-1-yl]methyl}-1H-pyrollo[2,3-b]pyridine (L-745,870) prior to radioligand injection failed to demonstrate receptor-specific binding in the monkey brain. Furthermore, the brain radioactivity distribution was left unaffected by pretreating with unlabeled PB-12. This failure to detect a D₄R-specific signal may be related to a very low density of the D₄R in primate brain, insufficient binding affinity of the radioligand, and a high background of nonspecific binding. It can be concluded from these findings that [¹¹C]PB-12 is not suitable to visualize the D₄R in the primate brain with PET.     

GMP-compliant radiosynthesis of [18F]altanserin and human plasma metabolite studies

[¹⁸F]altanserin is the preferred radiotracer for in-vivo labeling of serotonin 2A receptors by positron emission tomography (PET). We report a modified synthesis procedure suited for reliable production of multi-GBq amounts of [¹⁸F]altanserin useful for application in humans. We introduced thermal heating for drying of [¹⁸F]fluoride as well as for the reaction instead of microwave heating. We furthermore describe solid phase extraction and HPLC procedures for quantitative determination of [¹⁸F]altanserin and metabolites in plasma. The time course of arterial plasma activity with and without metabolite correction was determined. 90 min after bolus injection, 38.4% of total plasma activity derived from unchanged [¹⁸F]altanserin. Statistical comparison of kinetic profiles of [¹⁸F]altanserin metabolism in plasma samples collected in the course of two ongoing studies employing placebo, the serotonin releaser dexfenfluramine and the hallucinogen psilocybin, revealed the same tracer metabolism. We conclude that metabolite analysis for correction of individual plasma input functions used in tracer modeling is not necessary for [¹⁸F]altanserin studies involving psilocybin or dexfenfluramine treatment.     

Multicompartmental study of fluorine-18 altanserin binding to brain 5HT2 receptors in humans using positron emission tomography

Serotoninergic type 2 (5HT₂) receptors have been implicated in the regulation of many brain functions in humans and may play a role in several neurological and psychiatric diseases. Fluorine-18 altanserin has been proposed as a new radiotracer for the study of 5HT₂ receptors by PET because of its high affinity for 5HT₂ receptors (Ki: 0.13 nM) and its good specificity in in vitro studies. Dynamic PET studies were carried out in 12 healthy volunteers after intravenous injection of 0.1 mCi/kg [¹⁸F]altanserin. Ninety minutes after injection, we observed mainly cortical binding. Basal ganglia and cerebellum showed very low uptake and the frontal cortex to cerebellum ratio was about 3. To evaluate the quantitative distribution of this ligand in the brain, we used two different methods of data analysis: a four-compartment model was used to achieve quantitative evaluation of rate constants (K ₁ and k ₂ through k ₆) by non-linear regression, and a multiple-time graphical analysis technique for reversible binding was employed for the measurement of k ₁/k ₂ and k ₃/k ₄ ratios. Using both methods, we found significant differences in binding capacity (estimated by k ₃/k ₄ = B _max/K _d) between regions, the values increasing as follows: occipital, limbic, parietal, frontal and temporal cortex. After correction of values obtained by the graphical method for the existence of non-specific binding, results generated by the two methods were consistent.     

Comparisons of [18F]-1-deoxy-1-fluoro-scyllo-inositol with [18F]-FDG for PET imaging of inflammation, breast and brain cancer xenografts in athymic mice

Introduction

The aim of the study was to evaluate the uptake of [¹⁸F]-1-deoxy-1-fluoro-scyllo-inositol ([¹⁸F]-scyllo-inositol) in human breast cancer (BC) and glioma xenografts, as well as in inflammatory tissue, in immunocompromised mice. Studies of [¹⁸F]-2-fluoro-2-deoxy-d-glucose ([¹⁸F]-FDG) under the same conditions were also performed.

Methods

Radiosynthesis of [¹⁸F]-scyllo-inositol was automated using a commercial synthesis module. Tumour, inflammation and normal tissue uptakes were evaluated by biodistribution studies and positron emission tomography (PET) imaging using [¹⁸F]-scyllo-inositol and [¹⁸F]-FDG in mice bearing subcutaneous MDA-MB-231, MCF-7 and MDA-MB-361 human BC xenografts, intracranial U-87 MG glioma xenografts and turpentine-induced inflammation.

Results

The radiosynthesis of [¹⁸F]-scyllo-inositol was automated with good radiochemical yields (24.6%±3.3%, uncorrected for decay, 65±2 min, n=5) and high specific activities (≥195 GBq/μmol at end of synthesis). Uptake of [¹⁸F]-scyllo-inositol was greatest in MDA-MB-231 BC tumours and was comparable to that of [¹⁸F]-FDG (4.6±0.5 vs. 5.5±2.1 %ID/g, respectively; P=.40), but was marginally lower in MDA-MB-361 and MCF-7 xenografts. Uptake of [¹⁸F]-scyllo-inositol in inflammation was lower than [¹⁸F]-FDG. While uptake of [¹⁸F]-scyllo-inositol in intracranial U-87 MG xenografts was significantly lower than [¹⁸F]-FDG, the tumour-to-brain ratio was significantly higher (10.6±2.5 vs. 2.1±0.6; P=.001).

Conclusions

Consistent with biodistribution studies, uptake of [¹⁸F]-scyllo-inositol was successfully visualized by PET imaging in human BC and glioma xenografts, with lower accumulation in inflammatory tissue than [¹⁸F]-FDG. The tumour-to-brain ratio of [¹⁸F]-scyllo-inositol was also significantly higher than that of [¹⁸F]-FDG for visualizing intracranial glioma xenografts in NOD SCID mice, giving a better contrast.     

[<Superscript>18</Superscript>F]EF3 is not superior to [<Superscript>18</Superscript>F]FMISO for PET-based hypoxia evaluation as measured in a rat rhabdomyosarcoma tumour model

Purpose  The aim of this investigation was to quantitatively compare the novel positron emission tomography (PET) hypoxia marker 2-(2-nitroimidazol-1-yl)-N-(3[¹⁸F],3,3-trifluoropropyl)acetamide ([¹⁸F]EF3) with the reference hypoxia tracer [¹⁸F]fluoromisonidazole ([¹⁸F]FMISO). Methods  [¹⁸F]EF3 or [¹⁸F]FMISO was injected every 2 days into two separate groups of rats bearing syngeneic rhabdomyosarcoma tumours. In vivo PET analysis was done by drawing regions of interest on the images of selected tissues. The resulting activity data were quantified by the percentage of injected radioactivity per gram tissue (%ID/g) and tumour to blood (T/B) ratio. The spatial distribution of radioactivity was defined by autoradiography on frozen tumour sections. Results  The blood clearance of [¹⁸F]EF3 was faster than that of [¹⁸F]FMISO. The clearance of both tracers was slower in tumour tissue compared with other tissues. This results in increasing T/B ratios as a function of time post tracer injection (p.i.). The maximal [¹⁸F]EF3 tumour uptake, compared to the maximum [¹⁸F]FMISO uptake, was significantly lower at 2 h p.i. but reached similar levels at 4 h p.i. The tumour uptake for both tracers was independent of the tumour volume for all investigated time points. Both tracers showed heterogeneous intra-tumoural distribution. Conclusions  [¹⁸F]EF3 tumour uptake reached similar levels at 4 h p.i. compared with tumour retention observed after injection of [¹⁸F]FMISO at 2 h p.i. Although [¹⁸F]EF3 is a promising non-invasive tracer, it is not superior over [¹⁸F]FMISO for the visualisation of tumour hypoxia. No significant differences between [¹⁸F]EF3 and [¹⁸F]FMISO were observed with regard to the intra-tumoural distribution and the extra-tumoural tissue retention.     

An in vivo comparison of cis- and trans-[18F]mefway in the nonhuman primate

Introduction

[¹⁸F]Mefway is a serotonin 5-HT_1A PET radiotracer with high specificity and favorable in vivo imaging properties. The chemical structure of [¹⁸F]mefway permits ¹⁸F labeling in either the cis or trans positions at the 4-cyclohexyl site. We have previously reported on the in vivo kinetics of trans-[¹⁸F]mefway in the nonhuman primate. In this work, we compare the in vivo binding of cis-[¹⁸F]mefway and trans-[¹⁸F]mefway to evaluate the properties of cis-[¹⁸F]mefway for 5-HT_1A PET imaging.

Methods

The cis- and trans-[¹⁸F]mefway tracers were synthesized via nucleophilic substitution with their respective tosyl precursors. Two monkeys (one male, one female) were given bolus injections of both cis- and trans-labeled [¹⁸F]mefway in separate experiments. Dynamic scans were acquired for 90 min with a microPET P4 scanner. Time–activity curves were extracted in the areas of the mesial temporal cortex (MTC), anterior cingulate gyrus (aCG), insular cortex (IC), raphe nuclei (RN) and cerebellum (CB). The in vivo behavior of the radiotracers was compared based upon the nondisplaceable binding potential (BP_ND) using the CB as a reference region.

Results

Averaged over the two subjects, BP_ND values were as follows: MTC: 7.7, 0.58; aCG: 4.95, 0.32; IC: 3.27, 0.2; and RN: 3.05, 0.13, for trans-[¹⁸F]mefway and cis-[¹⁸F]mefway, respectively.

Conclusion

The cis-labeled [¹⁸F]mefway tracer has low specific binding throughout the 5-HT_1A regions of the brain compared to trans-[¹⁸F]mefway, suggesting that the target-to-background binding of cis-[¹⁸F]mefway may limit its use for in vivo assessment of 5-HT_1A binding.     

Biodistribution and human dosimetry of enantiomer-1 of the synthetic leucine analog anti-1-amino-2-fluorocyclopentyl-1-carboxylic acid

Introduction

The enantiomerically enriched (ee=90%, enantiomer 1) synthetic amino acid (R,S)-anti-1-amino-2-fluorocyclopentyl-1-carboxylic acid (anti-2-[¹⁸F]FACPC-1) accumulates in malignant cells by elevated transport through the sodium-independent system-l (leucine preferring) amino acid transporter. The purpose of this study was to evaluate in vivo uptake and single-dose toxicity of anti-2-[¹⁸F]FACPC-1 in animals as well as the individual organ and whole-body dose in humans.

Methods

A DU145 xenograft rodent model was used to measure anti-2-[¹⁸F]FACPC-1 uptake at 15, 30 and 60 min post-injection. Animals were sacrificed and organs harvested to measure the percent injected activity per organ and to calculate residence time. Anti-2-[¹⁸F]FACPC-1 toxicity was assessed using a single microdose (37–74 MBq/kg) in nonhuman primates. Their vital signs were monitored for 2 h post-injection for drug-related effects. Human biodistribution studies were collected by sequential whole-body PET/CT scans on six healthy volunteers (three male and three female) for 120 min following a single 247±61 MBq bolus injection of anti-2-[¹⁸F]FACPC-1. Estimates of radiation dose from anti-2-[¹⁸F]FACPC-1 to the human body were calculated using recommendations of the MIRD committee and MIRDOSE 3.0 software.

Results

High anti-2-[¹⁸F]FACPC-1 residence time was observed in the pancreas of the rodent model compared to the human data. No abnormal treatment-related observations were made in the nonhuman primate toxicity studies. Human venous blood showed no metabolites of anti-2-[¹⁸F]FACPC-1 in the first 60 min post-injection. All volunteers showed initially high uptake in the kidneys followed by a rapid washout phase. The estimated effective dose equivalent was 0.0196 mSv/MBq.

Conclusion

Anti-2-[¹⁸F]FACPC-1 showed low background uptake in the brain, thoracic and abdominal cavities of humans, suggesting a possible use for detecting malignant tissues in these regions.