Synthesis of 2-deoxy-2[82Br]bromo-d-mannose and related compounds and their biodistributions in mice

The synthesis of 2-deoxy-2-[⁸²Br]bromo-3,4,6-tri-O-acetyl--d-mannopyranosyl chloride (compound 3b) and 2-deoxy-2-[⁸²Br]bromo-d-mannose (compound 4d), and their biodistributions in mice are described. The reaction of 3,4,6-tri-O-acetyl-d-glucal (compound 2) with unlabeled and labeled bromine chloride (compounds 1a and 1b) generated in situ from the oxidation of bromide with N-chlorosuccinimide gave unlabeled and labeled 2-deoxy-2-bromo-3,4,6-tri-O-acetyl--d-mannopyranosyl chloride (compounds 3a and 3b) with a radiochemical yield of 58% (chemical yield, 63%). The hydrolysis of compounds 3a and 3b with 2N HCl gave 2-deoxy-2-bromo-d-mannose (compounds 4a and 4b) with a radiochemical yield of 72%. The biodistribution of compounds 3b and 4b after injection in mice indicated that 2% of the total injected radioactivity rapidly accumulated in the brain, while 6% of the total injected radioactivity accumulated in the heart; however, the radioactivity started to decline in these two organs after 15 min.Visiting scholar from the School of Pharmacy, Shanghai First Medical College, People's Republic of China     

Imaging of I2-imidazoline receptors by small-animal PET using 2-(3-fluoro-[4-11C]tolyl)-4,5-dihydro-1H-imidazole ([11C]FTIMD)

IntroductionImidazoline receptors (IRs) have been established as distinct receptors, and have been categorized into at least two subtypes (I₁R and I₂R). I₂Rs are associated with depression, Alzheimer's disease, Huntington's disease and Parkinson's disease. A few positron emission tomography (PET) probes for I₂Rs have been synthesized, but a selective PET probe has not been evaluated for the imaging of I₂Rs by PET. We labeled a selective I₂R ligand 2-(3-fluoro-4-tolyl)-4,5-dihydro-1H-imidazole (FTIMD) with ¹¹C and performed the first imaging of I₂Rs by PET using 2-(3-fluoro-[4-¹¹C]tolyl)-4,5-dihydro-1H-imidazole ([¹¹C]FTIMD).Methods[¹¹C]FTIMD was prepared by a palladium-promoted cross-coupling reaction of the tributylstannyl precursor and [¹¹C]methyl iodide in the presence of tris(dibenzylideneacetone)dipalladium(0) and tri(o-tol)phosphine. Biodistribution was investigated in rats by tissue dissection. [¹¹C]FTIMD metabolites were measured in brain tissues and plasma. Dynamic PET scans were acquired in rats, and the kinetic parameters estimated.Results[¹¹C]FTIMD was successfully synthesized with a suitable radioactivity for the injection. Co-injection with 0.1 mg/kg of cold FTIMD and BU224 induced a significant reduction in the brain-to-blood ratio 15 and 30 min after the injection. In metabolite analysis, unchanged [¹¹C]FTIMD in the brain was high (98%) 30 min after the injection. In PET studies, high radioactivity levels were observed in regions with a high density of I₂R. The radioactivity levels and V_T values in the brain regions were prominently reduced by 1.0 mg/kg of BU224 pretreatment as compared with control.Conclusion[¹¹C]FTIMD showed specific binding to I₂Rs in rat brains with a high density of I₂R.     

PET studies with l-[1-11C]tyrosine,l-[methyl-11C]methionine and 18F-fluorodeoxyglucose in prolactinomas in relation to bromocryptine treatment

Aspects of metabolism in prolactinomas were investigated by positron emission tomography using l-[1-¹¹C]tyrosine, l-[methyl-¹¹C]methionine and ¹⁸F-fluorodeoxyglucose (¹⁸FDG). Using l-[1-¹¹C]tyrosine, four patients were monitored prior to and 18 h after an injection of 50 mg bromocryptine. At 18 h after bromocryptine intervention, l-[1-¹¹C]tyrosine uptake into tumour was reduced with 28% (P<0.07). A correlation analysis of the bromocryptine-induced decrease in l-[1-¹¹C]tyrosine uptake and the reduction of serum prolactin levels indicated that the action of bromocryptine on prolactin synthesis and prolactin release is not coupled. In the untreated situation, the four patients were investigated with ¹⁸FDG as well, but the prolactinomas could not be visualized. Three untreated patients were studied with l-[methyl-¹¹C]methionine. The tumour-imaging potential of l-[methyl-¹¹C]methionine and l-[1-¹¹C]tyrosine appeared to be nearly equivalent for prolactinomas. Unlike prolactinoma tissue, the salivary glands showed a pronounced preference for l-[1-¹¹C]tyrosine as compared to l-[methyl-¹¹C]methionine. l-[1-¹¹C]tyrosine is a valuable tool to obtain information on the metabolism and treatment of prolactinomas.     

Identification and regional distribution in rat brain of radiometabolites of the dopamine transporter PET radioligand [<Superscript>11</Superscript>C]PE2I

Purpose We aimed to determine the composition of radioactivity in rat brain after intravenous administration of the dopamine transporter radioligand, [¹¹C]PE2I. Methods PET time-activity curves (TACs) and regional brain distribution ex vivo were measured using no-carrier-added [¹¹C]PE2I. Carrier-added [¹¹C]PE2I was administered to identify metabolites with high-performance liquid radiochromatography (RC) or RC with mass spectrometry (LC-MS and MS-MS). The stability of [¹¹C]PE2I was assessed in rat brain homogenates. Results After peak brain uptake of no-carrier-added [¹¹C]PE2I, there was differential washout rate from striata and cerebellum. Thirty minutes after injection, [¹¹C]PE2I represented 10.9 ± 2.9% of the radioactivity in plasma, 67.1 ± 11.0% in cerebellum, and 92.5 ± 3.2% in striata, and was accompanied by two less lipophilic radiometabolites. [¹¹C]PE2I was stable in rat brain homogenate for at least 1 h at 37°C. LC-MS identified hydroxylated PE2I (1) (m/z 442) and carboxyl-desmethyl-PE2I (2) (m/z 456) in brain. MS-MS of 1 gave an m/z 442→424 transition due to H₂O elimination, so verifying the presence of a benzyl alcohol group. Metabolite 2 was the benzoic acid derivative. Ratios of ex vivo measurements of [¹¹C]PE2I, [¹¹C]1, and [¹¹C]2 in striata to their cognates in cerebellum were 6.1 ± 3.4, 3.7 ± 2.2 and 1.33 ± 0.38, respectively, showing binding selectivity of metabolite [¹¹C]1 to striata. Conclusion Radiometabolites [¹¹C]1 and [¹¹C]2 were characterized as the 4-hydroxymethyl and 4-carboxyl analogs of [¹¹C]PE2I, respectively. The presence of the pharmacologically active [¹¹C]1 and the inactive [¹¹C]2 is a serious impediment to successful biomathematical analysis.     

In vivo imaging of adenosine A2A receptors in rat and primate brain using [11C]SCH442416

Purpose The aim of this study was to evaluate the suitability of [¹¹C]SCH442416 for the in vivo imaging of adenosine A_2A receptors.Methods In rats and Macaca nemestrina, we evaluated the time course of the cerebral distribution of [¹¹C]SCH442416. Furthermore, in rats we investigated the rate of metabolic degradation, the inhibitory effects of different drugs acting on adenosine or dopamine receptors and the modification induced by the intrastriatal administration of quinolinic acid (QA).Results The rate of metabolic degradation of [¹¹C]SCH442416 in rats was slow; 60 min after tracer injection, more than 40% of total plasma activity was due to unmetabolised [¹¹C]SCH442416. At the time of maximum uptake, radioactive metabolites represented only 6% of total extractable activity in the cerebellum and less than 1% in the striatum. In the striatum, the region with the highest expression of A_2A receptors, the in vivo uptake of [¹¹C]SCH442416 was significantly reduced only by drugs acting on A_2A receptors or by QA, a neurotoxin that selectively reduces the number of intrastriatal GABAergic neurons. Position emission tomography (PET) studies in monkeys indicated that the tracer rapidly accumulates in brain, reaching maximum uptake between 5 and 10 min. Twenty minutes after the injection, radioactivity concentration in the striatum was two times that in the cerebellum.Conclusion The specificity of binding, the rank order of regional distribution in the brain of rats and M. nemestrina, the good signal to noise ratios and the low amount of radioactive metabolites in brain and periphery indicate that [¹¹C]SCH442416 is a promising tracer for the in vivo imaging of A_2A adenosine receptors using PET.     

[carbonyl-11C]Desmethyl-WAY-100635 (DWAY) is a potent and selective radioligand for central 5-HT1A receptors in vitro and in vivo

 [carbonyl-¹¹C]Desmethyl-WAY-100635 (DWAY) is possibly a low-level metabolite appearing in plasma after intravenous administration of [carbonyl-¹¹C]WAY-100635 to human subjects for positron emission tomographic (PET) imaging of brain 5-HT_1A receptors. In this study we set out to assess the ability of DWAY to enter brain in vivo and to elucidate its possible interaction with 5-HT_1A receptors. Desmethyl-WAY-100635 was labelled efficiently with carbon-11 (t _1/2 = 20.4 min) in high specific radioactivity by reaction of its descyclohexanecarbonyl analogue with [carbonyl-¹¹C]cyclohexanecarbonyl chloride. The product was separated in high radiochemical purity by high-performance liquid chromatography (HPLC) and formulated for intravenous injection. Rats were injected intravenously with DWAY, sacrificed at known times and dissected to establish radioactivity content in brain tissues. At 60 min after injection, the ratios of radioactivity concentration in each brain region to that in cerebellum correlated with previous in vitro and in vivo measures of 5-HT_1A receptor density. The highest ratio was about 22 in hippocampus. Radioactivity cleared rapidly from plasma; HPLC analysis revealed that DWAY represented 55% of the radioactivity in plasma at 5 min and 33% at 30 min. Only polar radioactive metabolites were detected. Subsequently, a cynomolgus monkey was injected intravenously with DWAY and examined by PET. Maximal whole brain uptake of radioactivity was 5.7% of the administered dose at 5 min after injection. The image acquired between 9 and 90 min showed high radioactivity uptake in brain regions rich in 5-HT_1A receptors (e.g. frontal cortex and neocortex), moderate uptake in raphe nuclei and low uptake in cerebellum. A transient equilibrium was achieved in cortical regions at about 60 min, when the ratio of radioactivity concentration in frontal cortex to that in cerebellum reached 6. The corresponding ratio for raphe nuclei was about 3. Radioactive metabolites appeared rapidly in plasma, but these were all more polar than DWAY, which represented 52% of the radioactivity in plasma at 4 min and 20% at 55 min. In a second PET experiment, in which a cynomolgus monkey was pretreated with the selective 5-HT_1A receptor antagonist, WAY-100635, at 25 min before DWAY injection, radioactivity in all brain regions was reduced to that in cerebellum. Autoradiography of post mortem human brain cryosections after incubation with DWAY successfully delineated 5-HT_1A receptor distribution. Receptor-specific binding was eliminated in the presence of the selective 5-HT_1A receptor agonist, 8-OH-DPAT [(±)-8-hydroxy-2-dipropylaminotetralin]. These findings show that: (a) intravenously administered DWAY is well able to penetrate brain in rat and monkey, (b) DWAY is a highly effective radioligand for brain 5-HT_1A receptors in rat and monkey in vivo and for human brain in vitro, and (c) the metabolism and kinetics of DWAY appear favourable to successful biomathematical modelling of acquired PET data. Thus, DWAY warrants further evaluation as a radioligand for PET studies of 5-HT_1A receptors in human brain. Received 1 October and in revised form 12 December 1997     

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.     

Peripheral metabolism of (R)-[11C]verapamil in epilepsy patients

Purpose (R)-[¹¹C]verapamil is a new PET tracer for P-glycoprotein-mediated transport at the blood-brain barrier. For kinetic analysis of (R)-[¹¹C]verapamil PET data the measurement of a metabolite-corrected arterial input function is required. The aim of this study was to assess peripheral (R)-[¹¹C]verapamil metabolism in patients with temporal lobe epilepsy and compare these data with previously reported data from healthy volunteers. Methods Arterial blood samples were collected from eight patients undergoing (R)-[¹¹C]verapamil PET and selected samples were analysed for radiolabelled metabolites of (R)-[¹¹C]verapamil by using an assay that measures polar N-demethylation metabolites by solid-phase extraction and lipophilic N-dealkylation metabolites by HPLC. Results Peripheral metabolism of (R)-[¹¹C]verapamil was significantly faster in patients compared to healthy volunteers (AUC of (R)-[¹¹C]verapamil fraction in plasma: 29.4 ± 3.9 min for patients versus 40.8 ± 5.0 min for healthy volunteers; p < 0.0005, Student’s t-test), which resulted in lower (R)-[¹¹C]verapamil plasma concentrations (AUC of (R)-[¹¹C]verapamil concentration, normalised to injected dose per body weight: 25.5 ± 2.1 min for patients and 30.5 ± 5.9 min for healthy volunteers; p = 0.038). Faster metabolism appeared to be mainly due to increased N-demethylation as the polar [¹¹C]metabolite fraction was up to two-fold greater in patients. Conclusions Faster metabolism of (R)-[¹¹C]verapamil in epilepsy patients may be caused by hepatic cytochrome P450 enzyme induction by antiepileptic drugs. Based on these data caution is warranted when using an averaged arterial input function derived from healthy volunteers for the analysis of patient data. Moreover, our data illustrate how antiepileptic drugs may decrease serum levels of concomitant medication, which may eventually lead to a loss of therapeutic efficacy.     

The Bücherer-Strecker synthesis of d- and l-(1-11C)tyrosine and the in vivo study of l-(1-11C)tyrosine in human brain using positron emission tomography

The synthesis of d-and l-(1-¹¹C)tyrosine, starting with ¹¹C-cyanide, is reported. dl-(1-¹¹C)Tyrosine was prepared by the Bücherer-Strecker reaction, from carrier added ¹¹C-cyanide with an incorporation of 80% in 20 min. The isolation of the pure d- and l-amino acid isomers from the enantiomeric mixture was accomplished within 15 min by preparative HPLC using a chiral stationary phase and a phosphate buffer as the mobile phase. Typically, the total synthesis time was 50 min (including purification) from end of trapping of ¹¹C-cyanide, with a radiochemical yield of d- and l-amino acid of 40%–60%. The d- and l-(1-¹¹C)tyrosine were both obtained optically pure, with a carrier added specific activity of 0.3–0.5 Ci/mmol and a radiochemical purity better than 99%. The ¹¹C labelled l-tyrosine was used in an in vivo study in the human brain using positron emission tomography (PET).     

Effect of reserpine on the brain uptake of carbon 11 methamphetamine and itsN-propagyl derivative,deprenyl

The enantiomers of methamphetamine (MAMP) and itsN-propagyl derivative, deprenyl, were labelled with carbon 11, and the tissue distribution of these labelled compounds in mice was studied. Both enantiomers of¹¹C-MAMP rapidly entered into the brain and then disappeared according to a single exponential curve. The enantiomers of¹¹C-deprenyl were also rapidly distributed to various organs in the same manner. With regard to elimination, however, a stereoselective, long-term retention of radioactivity in the brain, heart and lung, due to its irreversible binding with monoamine oxidase B, was observed forl-¹¹C-deprenyl. In reserpinized mice, the initial brain uptake of both thel andd forms of¹¹C-MAMP was significantly decreased. On the other hand, the brain uptake of both enantiomers of¹¹C-deprenyl was slightly increased by pretreatment with reserpine. A significant and non-stereoselective elevation of the lung uptake of¹¹C-deprenyl was also seen in reserpinized mice. In addition, both the relative tissue distribution and ratios of radioactivity in the brain compared with blood or heart at 1 and 5 min after the injection of¹¹C-labelled methanol in mice were not changed by reserpine. These results indicate that the transport or binding processes of these amines rather than the blood flow might be altered by reserpine. There would be an important role of the pK_a values of amines in both processes. The reduction of brain uptake as well as the change in ratio between brain and heart ofl-11C-MAMP in reserpinized mice 1 min after injection were reversed by treatment with amphetamine in a dose-related manner. D-Amphetamine was found to be several times more potent than the correspondingl-form in this regard. The present results reveal some possibility that the transport or binding processes of MAMP in the brain may be regulated by cathecholaminergic neurotransmission.     

Synthesis and in vivo evaluation of the putative breast cancer resistance protein inhibitor [11C]methyl 4-((4-(2-(6,7-dimethoxy-1,2,3,4-tetrahydroisoquinolin-2-yl)ethyl)phenyl)amino-carbonyl)-2-(quinoline-2-carbonylamino)benzoate

IntroductionThe multidrug efflux transporter breast cancer resistance protein (BCRP) is highly expressed in the blood-brain barrier (BBB), where it limits brain entry of a broad range of endogenous and exogenous substrates. Methyl 4-((4-(2-(6,7-dimethoxy-1,2,3,4-tetrahydroisoquinolin-2-yl)ethyl)phenyl)amino-carbonyl)-2-(quinoline-2-carbonylamino)benzoate (1) is a recently discovered BCRP-selective inhibitor, which is structurally derived from the potent P-glycoprotein (P-gp) inhibitor tariquidar. The aim of this study was to develop a new PET tracer based on 1 to map BCRP expression levels in vivo.MethodsCompound 1 was labelled with ¹¹C in its methyl ester function by reaction of the corresponding carboxylic acid 2 with [¹¹C]methyl triflate. Positron emission tomography (PET) imaging of [¹¹C]-1 was performed in wild-type, Mdr1a/b^(?/?), Bcrp1^(?/?) and Mdr1a/b^(?/?)Bcrp1^(?/?) mice (n=3 per mouse type) and radiotracer metabolism was assessed in plasma and brain.ResultsBrain-to-plasma ratios of unchanged [¹¹C]-1 were 4.8- and 10.3-fold higher in Mdr1a/b^(?/?) and in Mdr1a/b^(?/?)Bcrp1^(?/?) mice, respectively, as compared to wild-type animals, but only modestly increased in Bcrp1^(?/?) mice. [¹¹C]-1 was rapidly metabolized in vivo giving rise to a polar radiometabolite which was taken up into brain tissue.ConclusionOur data suggest that [¹¹C]-1 preferably interacts with P-gp rather than BCRP at the murine BBB which questions its reported in vitro BCRP selectivity. Consequently, [¹¹C]-1 appears to be unsuitable as a PET tracer to map cerebral BCRP expression.     

Role of NMDA receptor upon [14C]acetate uptake into intact rat brain

Objective  To clarify the role of N-methyl-d-aspartate (NMDA) receptors upon [¹⁴C]acetate uptake in the rodent central nervous system (CNS), ibotenic acid (IBO) was infused into the right striatum of the rat brain. Methods  Autoradiograms of [¹⁴C]acetate uptake in the brain for 2 h following the infusion of IBO (10 μg/μl) were obtained in both non-treated and MK-801 (1 mg/kg, i.v.) pretreated rats. The effect of MK-801 on [¹⁴C]acetate uptake in the normal rat brain was also studied. Results  Infusion of IBO significantly decreased [¹⁴C]acetate uptake in the infused side of the striatum. The expression of monocarboxylate transporter-1 was not altered, suggesting that the activity of tricarboxylic acid (TCA) cycle in glial cells might be depressed. Pretreatment with MK-801 completely blocked the decreasing effect of IBO on [¹⁴C]acetate uptake. MK-801 also increased [¹⁴C]acetate uptake in the whole brain of normal rats. Conclusions  These results indicate the important roles of NMDA receptors on [¹⁴C]acetate uptake in the intact rat brain.     

The time course of metabolites in human plasma after 6-[18F]fluoro-l-m-tyrosine administration

For the investigation of intracerebral dopamine metabolism, 6-[¹⁸F]fluoro-l-m-tyrosine (FmT) has been proposed as an alternative tracer that, unlike 6-[¹⁸F]fluoro-l-dopa, is not subjected to O-methylation. We have studied the time course of FmT and its labelled metabolites in plasma after intravenous injection into humans, employing a method that we have developed and validated to analyze labelled metabolites of FmT in plasma. The study population comprised 38 subjects, 14 of whom were taking an aromatic amino acid decarboxylase inhibitor (carbidopa), and obtained arterialized venous blood samples at various times after an intravenous injection of between 185 and 370 MBq of FmT. The major metabolite of FmT present in plasma was 6-[¹⁸F]fluoro-3-hydroxyphenylacetic acid. The time course of the fraction of radioactivity in plasma attributable to FmT was fitted well by the decay of two exponential functions. The fast component of the decay accounted for 40%–50% of the radioactivity and had a half-life of about 5 min. The slow component of the decay had a half-life of about 6 h in the subjects not taking carbidopa and 20 h in the patients taking carbidopa. When the total available FmT was calculated for each individual subject and expressed as a proportion of total radioactivity, this quantity did not differ significantly from that determined from the corresponding population mean. There was significantly more (15% on average) FmT available over the course of the 2-h experiment in the group pretreated with carbidopa. Our results demonstrate that the major metabolite in plasma after an intravenous injection of FmT is 6-[¹⁸F]fluoro-3-hydroxyphenylacetic acid. The clearance of FmT from plasma compares well with the clearance of intravenously administered levodopa. Carbidopa increases significantly the availability of FmT in plasma. These results verify, in a large number of human subjects, earlier analyses of FmT metabolism in other species. We validate a population-derived approach that can adequately describe the time course of FmT in plasma, alleviating the need for metabolite analysis. Received 2 March and in revised form 25 June 1999     

O-(2-[18F]Fluoroethyl)-l-tyrosine and l-[methyl-11C]methionine uptake in brain tumours: initial results of a comparative study

O-(2-[¹⁸F]Fluoroethyl)-l-tyrosine (FET) is a recently described amino acid analogue that has shown high accumulation in animal tumours. The aim of this study was to compare the uptake of FET with that of l-[methyl-¹¹C]methionine (MET) in patients with suspected primary or recurrent intracerebral tumours. Sixteen consecutive patients with intracerebral lesions were studied on the same day by positron emission tomography (PET) using MET and FET. Uptake of FET and MET was quantified by standardized uptake values. Tracer kinetics for normal brain and intracerebral lesions were compared. On the basis of the MET-PET studies, viable tumour tissue was found in 13 patients. All tumours showed rapid uptake of FET and were visualized with high contrast. Mean uptake of FET for normal grey matter, white matter and tumour tissue was 1.1±0.2, 0.8±0.2 and 2.7±0.8 SUV, respectively. In all three tissues, uptake of MET was slightly higher (1.4±0.2, 0.9±0.1 and 3.3±1.0 SUV; P<0.01). However, contrast between tumour and normal tissues was not significantly different between MET and FET. Uptake of FET in non-neoplastic lesions (1.0±0.1 SUV) was significantly lower than in tumour tissue (P=0.007). For all lesions there was a close correlation (r=0.98) between MET and FET uptake. In conclusion, in PET studies of human brain tumours, the uptake and image contrast of FET appear to be very similar to those of MET. The specificity of FET for tumour tissue is promising but has to be addressed in a larger series of patients with non-neoplastic lesions. Received 24 October 1999 and in revised form 19 January 2000     

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.     

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.     

Synthesis and biological evaluation of 1-(4-[f]fluorobenzyl)-4-[(5,6-dimethoxy-1-oxoindan-2-yl)methyl]piperidine for in vivo studies of acetylcholinesterase

We synthesized and evaluated 1-(4-fluorobenzyl)-4-[(5,6-dimethoxy-1-oxoindan-2-yl)methyl]piperidine (4-FDP), which is an analog of donepezil. The 4-[¹⁸F]FDP was prepared by reductive alkylation of debenzylated donepezil with 4-[¹⁸F]fluorobenzaldehyde in high radiochemical yield (decay-corrected, 40–52%) and with high effective specific activity (30–38 GBq/μmol). Tissue distribution studies in mice demonstrated nonspecific distribution of the 4-[¹⁸F]FDP in brain regions, suggesting that this radioligand may not be a suitable agent for in vivo studies of acetylcholinesterase (AChE), despite its potent in vitro biological activity.     

Improved Stability and Practicality for Synthesis of 4-Borono-2-[18F]fluoro-l-phenylalanine by Combination of [18O]O2 Single-Use and [18F]CH3COOF Labeling Agents

Purpose4-Borono-2-[¹⁸F]fluoro-l-phenylalanine ([¹⁸F]FBPA) synthesized with [¹⁸F]F₂, produced using the ¹⁸O(p, n)¹⁸F reaction, has been reported for increasing radioactivity. However, a dedicated system and complex procedure is required to reuse the costly [¹⁸O]O₂ gas; also, the use of [¹⁸F]F₂ as a labeling agent reduces the labeling rate and radiochemical purity. We developed a stable and practical method for [¹⁸F]FBPA synthesis by combining [¹⁸F]F₂, produced using a [¹⁸O]O₂ single-use system, and a [¹⁸F]CH₃COOF labeling agent.MethodsThe produced [¹⁸F]F₂ was optimized, and then [¹⁸F]FBPA was synthesized. For passivation of the target box, 0.5% F₂ was pre-irradiated in argon. Gaseous products were discarded; the target box was filled with [¹⁸O]O₂ gas, and then irradiated (first irradiation). Then, the [¹⁸O]O₂ gas was discarded, 0.05–0.08% F₂ in argon was fed into the target box, and it was again irradiated (second irradiation). The [¹⁸F]F₂ obtained after this was passed through a CH₃COONa column, converting it into the [¹⁸F]CH₃COOF labeling agent, which was then used for [¹⁸F]FBPA synthesis.ResultsThe mean amount of as-obtained [¹⁸F]F₂ was 55.0 ± 3.3 GBq and that of as-obtained [¹⁸F]CH₃COOF was 21.6 ± 1.4 GBq after the bombardment. The radioactivity and the radiochemical yield based on [¹⁸F]F₂ of [¹⁸F]FBPA were 4.72 ± 0.34 GBq and 12.2 ± 0.1%, respectively. The radiochemical purity and molar activity were 99.3 ± 0.1% and 231 ± 22 GBq/mmol, respectively.ConclusionWe developed a method for [¹⁸F]FBPA production, which is more stable and practical compared with the method using [¹⁸O]O₂ gas-recycling and [¹⁸F]F₂ labeling agent.     

Synthesis and PET studies of [11C-cyano]letrozole (Femara), an aromatase inhibitor drug

IntroductionAromatase, a member of the cytochrome P450 family, converts androgens such as androstenedione and testosterone into estrone and estradiol, respectively. Letrozole (1-[bis-(4-cyanophenyl)methyl]-1H-1,2,4-triazole; Femara) is a high-affinity aromatase inhibitor (K_i=11.5 nM) that has Food and Drug Administration approval for breast cancer treatment. Here we report the synthesis of carbon-11-labeled letrozole and its assessment as a radiotracer for brain aromatase in the baboon.MethodsLetrozole and its precursor (4-[(4-bromophenyl)-1H-1,2,4-triazol-1-ylmethyl]benzonitrile) were prepared in a two-step synthesis from 4-cyanobenzyl bromide and 4-bromobenzyl bromide, respectively. The [¹¹C]cyano group was introduced via tetrakis(triphenylphosphine)palladium(0)-catalyzed coupling of [¹¹C]cyanide with the bromo precursor. Positron emission tomography (PET) studies in the baboon brain were carried out to assess regional distribution and kinetics, reproducibility of repeated measures and saturability. Log D, the free fraction of letrozole in plasma and the [¹¹C-cyano]letrozole fraction in arterial plasma were also measured.Results[¹¹C-cyano]Letrozole was synthesized in 60 min with a radiochemical yield of 79–80%, with a radiochemical purity greater than 98% and a specific activity of 4.16±2.21 Ci/μmol at the end of bombardment (n=4). PET studies in the baboon revealed initial rapid and high uptake and initial rapid clearance, followed by slow clearance of carbon-11 from the brain, with no difference between brain regions. Brain kinetics was not affected by coinjection of unlabeled letrozole (0.1 mg/kg). The free fraction of letrozole in plasma was 48.9%, and log D was 1.84.Conclusion[¹¹C-cyano]Letrozole is readily synthesized via a palladium-catalyzed coupling reaction with [¹¹C]cyanide. Although it is unsuitable as a PET radiotracer for brain aromatase, as revealed by the absence of regional specificity and saturability in brain regions such as amygdala, which are known to contain aromatase, it may be useful in measuring letrozole distribution and pharmacokinetics in the brain and peripheral organs.     

Kinetics of radioiodinated species in subcellular fractions from rat hearts following administration of iodine-123-labelled 15-(p-iodophenyl)-3-(R,S)-methylpentadecanoic acid (123I-BMIPP)

It is recognized that iodine-123-labelled 15-(p-iodophenyl)-3-(R,S)-methylpentadecanoic acid (¹²³IBMIPP) slowly washes out of the myocardium. The mechanism for the washout was investigated in normal rat hearts by analyses of the subcellular distribution and lipid classes based on the BMIPP metabolism. Rat hearts were excised at 1–120 min after intravenous injection of¹²³I-BMIPP. After counting the radioactivity, the hearts were digested with Nagarse and homogenized, and then fractionated into the cytosolic, mitochondrial, microsomal and crude nuclear fractions by centrifugations. The radioactivity of each fraction was counted, and the lipid classes were analysed by radio-thin-layer chromatographic and high-performance liquid chromatographic methods. The heart uptake of 1231-BMIPP was maximal at 5 min (6.81%±0.36% ID/g), and 41% of the radioactivity disappeared within 120 min. The myocardial radioactivity was immediately distributed into the cytosolic, mitochondrial, microsomal and crude nuclear fractions. The distribution (%) of each fraction was almost identical from 5 min through 120 min. The cytosolic fraction was always the major site of radioactivity deposition (60%), and the time-activity curve of the cytosolic fraction paralleled that of the whole heart throughout the 120-min study period. In the cytosolic fraction, most of the radioactivity was incorporated into the triglyceride class, and the rest was present in the free fatty acid, phospholipid (phosphatidylcholine) and diglyceride classes. In the mitochondrial fraction, the radioactivity was mostly incorporated into the phospholipid class (phosphatidylethanolamine), followed by free fatty acids. The final metabolite of¹²³I-BMIPP,¹²³I-p-iodophenylacetic acid (¹²³I-PIPA), initially appeared in the mitochondrial fraction as early as 1 min, and subsequently in the cytosolic fraction at 5 min. Another intermediary metabolite,¹²³I-p-iodophenyldodecanoic acid (¹²³I-PIPC₁₂), was found only in the mitochondrial fraction after 5 min. In conclusion, the slow washout kinetics of¹²³I-BMIPP from the myocardium mainly reflects the turnover rate of the triglyceride pool in the cytosol. The BMIPP metabolism, i.e. initial -oxidation followed by subsequent cycles of -oxidation, was confirmed in vivo. The participation of the mitochondria in the metabolism was also proven.