Determination of plasma [18F]-6-fluorodopa during positron emission tomography: elimination and metabolism in carbidopa treated subjects

An investigation of the metabolism of [18F]-6-fluorodopa (FDOPA) given to carbidopa treated subjects for scanning by positron emission tomography (PET) has been carried out by analysis of plasma. Reverse phase ion pair HPLC and alumina extraction were employed to fractionate and identify the [18F]-labelled compounds of plasma over a two hour period. During this time, the plasma levels of both total 18F and FDOPA decreased as a bi-exponential function of time. The rates of 18F, but not FDOPA, elimination were observed to decrease with age. In addition to FDOPA, only one other major peak of radioactivity was resolved by HPLC. Identification of this compound as the O-methylated derivative of FDOPA (MeFDOPA) is based on its shared HPLC elution time with in vitro synthesized O-[methyl-14C]-FDOPA. The ratio of the concentration of MeFDOPA to FDOPA (MeFDOPA/FDOPA) in plasma increased linearly with time, and the slope of this linear relationship decreased with the age of the individual.     

A Method for the In Vivo Investigation of the Serotonergic 5-HT2 Receptors in the Human Cerebral Cortex Using Positron Emission Tomography and 18F-Labeled Setoperone

Following previous validation in baboons, we have studied the characteristics of [18F]setoperone as a radioligand for investigating serotonergic 5-hydroxytryptamine2 (5-HT2) receptors in the normal, unmedicated human brain with positron emission tomography (PET); subjects orally pretreated with therapeutic amounts of ketanserin, sulpiride, or prazosin were also studied to evaluate the specificity and sensitivity of [18F]setoperone brain specific binding. In controls (n = 10), the tracer showed a clear-cut retention in both frontal cortex and striatum (known to contain a high density of 5-HT2 receptors) relative to cerebellum (known to be devoid of 5-HT2 receptors). In the seven young controls (20-39 years old), the frontal cortex/cerebellum and striatum/cerebellum ratios increased during the first hour to reach similar values of 2.53 +/- 0.12 and 2.38 +/- 0.11 (mean +/- SEM), respectively, and were essentially stable during the second hour. Pretreatment with ketanserin (a 5-HT2 blocker) significantly reduced the frontal cortex/cerebellum ratio to 0.7-1.0 at 65 min, whereas the striatum/cerebellum ratio was significantly, but only partially, reduced. During sulpiride treatment (a D2 blocker), the frontal cortex/cerebellum ratio was not altered, whereas the striatum/cerebellum ratio was significantly, but only partially, reduced. With prazosin pretreatment (an alpha 1-adrenergic blocker), neither the frontal cortex/cerebellum nor the striatum/cerebellum ratio was modified. These data in humans with PET demonstrate that [18F]setoperone labels with high sensitivity and selectivity 5-HT2 receptors in the frontal cortex; in striata, however, binding is to both 5-HT2 and D2 receptors. The deproteinated-to-whole plasma radio-activity concentration ratio increased with time following injection. The mean percentage of intact [18F]setoperone, in deproteinated plasma, was 82, 74, 53, 45, 30, and 22% at 5, 10, 20, 30, 60, and 110 min following injection, respectively. These data indicate that [18F]setoperone (a) is significantly bound to plasma proteins and (b) is significantly metabolized into several labeled metabolites that are much more hydrophilic than setoperone and, hence, presumably do not cross the blood-brain barrier. These results suggest the suitability of [18F]setoperone data for modeling of 5-HT2 receptor binding in brain.     

18F]fluoroacetate positron emission tomography for hepatocellular carcinoma and metastases: an alternative tracer for [11C]acetate?

[11C]Acetate (ACT) positron emission tomography/computed tomography (PET/CT) is useful in the detection of hepatocellular carcinoma (HCC). This study aimed to evaluate whether [18F]fluoroacetate (FAC) could be an alternative analogue of [11C]ACT for the diagnosis of HCC. [18F]FAC was synthesized using the precursor t-butyl 2-(methanesulfonyloxy)ethanoate. Five volunteer patients with known HCC were recruited after consent. Whole-body [18F]FAC PET/CT was performed at 20 minutes and 1 hour postinjection and compared to [11C]ACT PET/CT at 20 minutes postinjection to assess biodistribution and tumor uptake characteristics. Qualitative and semiquantitative analyses were performed with statistical correlations on the physiologic organs of accumulation and HCC lesions for both tracers. [18F]FAC was obtained with 99% radiochemical purity, and the reaction yield was 16.0% with 1-hour synthesis time. The biodistribution of [18F]FAC on PET/CT was significantly different from that of [11C]ACT (p < .05) by the lack of preferential uptake in any specific organ, particularly the pancreas, resembling the pattern of blood-pool retention although partly metabolized via the bowel. There was no significant defluorination, and none of the [11C]ACT-avid HCC lesions showed increased [18F]FAC activity. These were different from the results reported on other species. [18F]FAC may not be a potential alternative tracer for [11C]ACT in PET/CT evaluation of HCC in human subjects.     

Determination of unchanged [18F]dopamine in human and non-human primate plasma during positron emission tomography studies: a new solid-phase extraction method comparable to radio-thin-layer chromatography analysis

Routine determination of [¹⁸F]DOPA and its metabolites in plasma is essential for assessment and quantification of presynaptic dopamine function in vivo using a modeling approach with positron emission tomography (PET). The determination of unchanged [¹⁸F]DOPA from human and non-human primate plasma using solid-phase extraction (SPE) with Sep-Pak cartridges during PET dopaminergic studies is described here. The results from the studies showed that this new approach in comparsion to a method such as thin-layer chromatography (TLC) possessed a simplicity, rapidity and accuracy as well as good correlation between the two techniques (p<0.0001). A proposed procedure involving radio-analysis on alumina plates (Al₂O₃) was also developed with an excellent correlation compared to the conventional C₁₈ plates (r=0.96). Thus it could be concluded that the SPE on either C₁₈ or alumina cartridges (Waters) compared to radio-TLC analysis on C₁₈ and alumina systems, appears to be a useful analytical method suitable for correcting the input arterial function in routine clinical PET neurotransmission studies.     

Determination of unchanged [F]dopamine in human and non-human primate plasma during positron emission tomography studies: a new solid-phase extraction method comparable to radio-thin-layer chromatography analysis

Routine determination of [¹⁸F]DOPA and its metabolites in plasma is essential for assessment and quantification of presynaptic dopamine function in vivo using a modeling approach with positron emission tomography (PET). The determination of unchanged [¹⁸F]DOPA from human and non-human primate plasma using solid-phase extraction (SPE) with Sep-Pak cartridges during PET dopaminergic studies is described here. The results from the studies showed that this new approach in comparsion to a method such as thin-layer chromatography (TLC) possessed a simplicity, rapidity and accuracy as well as good correlation between the two techniques (p<0.0001). A proposed procedure involving radio-analysis on alumina plates (Al₂O₃) was also developed with an excellent correlation compared to the conventional C₁₈ plates (r=0.96). Thus it could be concluded that the SPE on either C₁₈ or alumina cartridges (Waters) compared to radio-TLC analysis on C₁₈ and alumina systems, appears to be a useful analytical method suitable for correcting the input arterial function in routine clinical PET neurotransmission studies.     

Radiosynthesis of a ¹⁸F-labeled 2,3-diarylsubstituted indole via McMurry coupling for functional characterization of cyclooxygenase-2 (COX-2) in vitro and in vivo

The radiosynthesis of 3-(4-[(18)F]fluorophenyl)-2-(4-methylsulfonylphenyl)-1H-indole [(18)F]-3 as potential PET radiotracer for functional characterization of cyclooxygenase-2 (COX-2) in vitro and in vivo is described. [(18)F]-3 was prepared by McMurry cyclization of a (18)F-labeled intermediate with low valent titanium and zinc via a two-step procedure in a remote controlled synthesizer unit including HPLC purification and solid phase extraction. In this way [(18)F]-3 was synthesized in 80 min synthesis time in 10% total decay corrected yield from [(18)F]fluoride in radiochemical purity >98% and a specific activity of 74-91 GBq/μmol (EOS). [(18)F]-3 was evaluated in vitro using pro-inflammatory stimulated THP-1 and COX-2 expressing tumor cell lines (FaDu, A2058, HT-29), where the radiotracer uptake was shown to be consistent with up regulated COX-2 expression. The stability of [(18)F]-3 was determined by incubation in rat whole blood and plasma in vitro and by metabolite analysis of arterial blood samples in vivo, showing with 75% of original compound after 60 min an acceptable high metabolic stability. However, no substantial tumor accumulation of [(18)F]-3 could be observed by dynamic small animal PET studies on HT-29 tumor-bearing mice in vivo. This may be due to the only moderate COX-1/COX-2 selectivity of 3 as demonstrated by both cellular and enzymatic cyclooxygenase inhibition assay in vitro. Nevertheless, the new approach first using McMurry cyclization in (18)F-chemistry gives access to (18)F-labeled diarylsubstituted heterocycles that hold promise as radiolabeled COX-2 inhibitors.     

N-(3-[18F]fluoropropyl)-spiperone: The preferred 18F labeled spiperone analog for positron emission tomographic studies of the dopamine receptor

The ligands currently used for PET studies of the dopamine receptor are fluorine-18-labeled spiperone (FSp) and carbon-11 or fluorine-18-labeled N-methyl-spiperone. All three of these ligands have drawbacks in either their chemical preparation or their biological behavior. We have previously prepared a series of N-fluoroalkyl-spiperone derivatives which are simple to prepare in high radiochemical yield. N-[¹⁸F]fluoropropyl-spiperone (3-F-Pr-Sp) and N-[¹⁸F]fluoroethyl-spiperone (2-F-Et-Sp) were the most promising ligands. In vitro competitive binding studies showed affinities for the dopamine receptor of 3-F-Pr-Sp > FSp > 2-F-Et-Sp. Brain extraction studies in a primate model showed that FSp, 2-F-Et-Sp, and 3-F-Pr-Sp were not completely extracted by the brain. High bone uptake and kidney clearance was observed with 3-F-Pr-Sp, while 2-F-Et-Sp cleared through the intestine in rats. This is in contrast to FSp where clearance is through the kidney. Studies to evaluate the extraction of metabolites in the brain were carried out by administering large doses (10 mCi) of FSp, 2-F-Et-Sp and 3-F-Pr-Sp to rats and reinjecting the metabolites in blood into other rats. These experiments showed that < 0.02% of the metabolites from FSp and 3-F-Pr-Sp entered the brain, while 0.5% of the metabolites from 2-F-Et-Sp entered the brain. The majority of the activity present in the cerebellum after the administration of 2-F-Et-Sp is metabolites; therefore 2-F-Et-Sp is unsuitable for PET imaging studies. PET imaging studies in baboons and in one normal human volunteer with 3-F-Pr-Sp showed a high striatum-to-cerbellum ratio, showing that 3-F-Pr-Sp can replace ligands currently in use to study dopamine receptors.     

Evaluation of 18F-labeled acetylcholinesterase substrates as PET radiotracers

Four 18F-labeled acetylcholinesterase (AChE) substrates, (S)-N-[18F]fluoroethyl-2-piperidinemethyl acetate (1), (R)-N-[18F]fluoroethyl-3-pyrrolidinyl acetate (2), N-[18F]fluoroethyl-4-piperidinyl acetate (3), and (R)-N-[18F]fluoroethyl-3-piperidinyl acetate (4), were evaluated for in vivo blood and brain metabolism in mice, brain pharmacokinetics in rats monkeys (M. nemistrina) using PET imaging. All 18F-labeled compounds were compared to N-[11C]methyl-4-piperidinyl propionate (PMP). Compound 1 was completely metabolized within 1 min in mouse blood and brain. This compound had relatively fast regional brain pharmacokinetics and poor discrimination between brain regions with different AChE concentration. Compound 4 showed relatively slower blood metabolism and slower pharmacokinetics than compound 1 but again poor discrimination between brain regions. Both compounds 1 and 4 showed different kinetic profiles than PMP in PET studies. Compound 3 had the slowest blood metabolism and slower pharmacokinetics than PMP. Compound 2 showed highly encouraging characteristics with an in vivo metabolism rate, primate brain uptake, and regional brain pharmacokinetics similar to [11C]PMP. The apparent hydrolysis rate constant k3 in primate cortex was very close to that of [11C]PMP. This compound has potential to be a good PET radiotracer for measuring brain AChE activity. The longer lifetime of 18F would permit longer imaging times and allows preparation of radiotracer batches for multiple patients and delivery of the tracer to other facilities, making the technique more widely available to clinical investigators.     

Lumped constant for [(18)F]fluorodeoxyglucose in skeletal muscles of obese and nonobese humans

Quantitative 2-[(18)F]fluoro-2-deoxy-D-glucose ([(18)F]FDG) positron emission tomography (PET) has been widely used to calculate glucose utilization in skeletal muscle. FDG-PET results depend partly on the lumped constant (LC), which accounts for the differences in the transport and phosphorylation between [(18)F]FDG and glucose. In this study, we estimated the LC for [(18)F]FDG directly in normal and in insulin-resistant obese subjects by combining FDG PET with the microdialysis technique. Eight obese [age 29.4 +/- 1.0 yr, body mass index (BMI) 33.6 +/- 1.0 kg/m(2)] and eight nonobese (age 25.0 +/- 1.0 yr, BMI 23.1 +/- 1.0 kg/m(2)) males were studied during euglycemic hyperinsulinemia (1 mU. kg(-1).min(-1) for 150 min). Muscle blood flow was measured using (15)O-labeled water and PET. Muscle [(18)F]FDG uptake (rGU(FDG)) was calculated with Patlak graphic analysis. Interstitial glucose concentration of the quadriceps femoris muscle was measured simultaneously with [(18)F]FDG scanning using microdialysis. Muscle glucose uptake (by microdialysis, rGU(MD)) was calculated by multiplying glucose extraction by regional muscle blood flow. A significant correlation was found between rGU(MD) and rGU(FDG) (r = 0.78, P < 0.01). The LC was determined as the ratio of the rGU(FDG) to the rGU(MD). The LC averaged 1.16 +/- 0.16 and was similar in the obese and nonobese subjects (1.15 +/- 0.11 vs. 1.16 +/- 0.07, respectively, not significant). In conclusion, the microdialysis technique can be reliably combined with FDG PET to measure glucose uptake in skeletal muscle. Direct measurements with these two independent techniques suggest an LC value of 1.2 for [(18)F]FDG in human skeletal muscle during insulin stimulation, and the LC appears not to be sensitive to insulin resistance.     

Synthesis and radiopharmacological investigation of 3-[4′-[18F]fluorobenzylidene]indolin-2-one as possible tyrosine kinase inhibitor

The radiosynthesis and radiopharmacological evaluation of 3-[4′-[¹⁸F]fluorobenzylidene]indolin-2-one, a derivative of tyrosine kinase inhibitor SU5416, is described. The radiosynthesis was accomplished by Knoevenagel condensation of 4-[¹⁸F]fluorobenzaldehyde with oxindole in a remotely controlled synthesis module. The reaction conditions were optimized through screening the influence of different bases on the radiochemical yield. The radiotracer was obtained after a two-step labelling procedure in 4% decay-corrected radiochemical yield at a specific activity of 48–61 GBq/μmol within 90 min. The radiochemical purity after semi-preparative HPLC purification exceeded 98%.The biodistribution was studied in Wistar rats. After distribution the radiotracer was rapidly accumulated in the adrenals, liver and kidneys, however, it was cleared from these and the most other organs. Only the adipose tissue remained the activity over 60 min. Unexpected high transient uptake was observed in the brain, pancreas, heart and lung. The fast clearance of 3-[4′-[¹⁸F]fluorobenzylidene]indolin-2-one was caused by excretion, approximately one half each was renal and biliary excreted and the other part cleared by metabolic processes. In arterial blood plasma two more polar metabolites were found by radio-HPLC. After 20 min post-injection, only 12% of intact radiotracer has been detected. Consequently, in small animal PET studies with FaDu tumour bearing mice no specific uptake in the tumours could be observed.     

N-[18F]fluoroethyl-4-piperidyl acetate ([18F]FEtP4A): A PET tracer for imaging brain acetylcholinesterase in vivo

N-[(18)F]Fluoroethyl-4-piperidyl acetate ([(18)F]FEtP4A) was synthesized and evaluated as a PET tracer for imaging brain acetylcholinesterase (AchE) in vivo. [(18)F]FEtP4A was previously prepared by reacting 4-piperidyl acetate (P4A) with 2-[(18)F]fluoroethyl bromide ([(18)F]FEtBr) at 130 degrees C for 30 min in 37% radiochemical yield using an automated synthetic system. In this work, [(18)F]FEtP4A was synthesized by reacting P4A with 2-[(18)F]fluoroethyl iodide ([(18)F]FEtI) or 2-[(18)F]fluoroethyl triflate ([(18)F]FEtOTf in improved radiochemical yields, compared with [(18)F]FEtBr under the corresponding condition. Ex vivo autoradiogram of rat brain and PET summation image of monkey brain after iv injection of [(18)F]FEtP4A displayed a high radioactivity in the striatum, a region with the highest AchE activity in the brain. Moreover, the distribution pattern of (18)F radioactivity was consistent with that of AchE in the brain: striatum>frontal cortex>cerebellum. In the rat and monkey plasma, two radioactive metabolites were detected. However, their presence might not preclude the imaging studies for AchE in the brain, because they were too hydrophilic to pass the blood-brain barrier and to enter the brain. In the rat brain, only [(18)F]fluoroethyl-4-piperidinol ([(18)F]FEtP4OH) was detected at 30 min postinjection. The hydrolytic [(18)F]FEtP4OH displayed a slow washout and a long retention in the monkey brain until the PET experiment (120 min). Although [(18)F]FEtP4A is a potential PET tracer for imaging AchE in vivo, its lower hydrolytic rate and lower specificity for AchE than those of [(11)C]MP4A may limit its usefulness for the quantitative measurement for AchE in the primate brain.     

Synthesis and in vivo evaluation of [18F]-4-[5-(4-methylphenyl)-3-(trifluoromethyl)-1H-pyrazol-1-yl]benzenesulfonamide as a PET imaging probe for COX-2 expression

Synthesis of [18F]4-[5-(4-methylphenyl)-3-(trifluoromethyl)-1H-pyrazol-1-yl]benzenesulfonamide ([18F]celecoxib), a selective COX-2 inhibitor, is achieved via a bromide to [18F]F- exchange reaction. Synthesis of the precursor for radiolabeling was achieved from 4'-methylacetophenone in four steps with 22% overall yield. Under non-radioactive conditions, fluorination was achieved using TBAF in DMSO at 135 degrees C in 80% yield. Synthesis of [18F]celecoxib was achieved using [18F]TBAF in DMSO at 135 degrees C in 10+/-2% yield (EOS) with >99% chemical and radiochemical purities. The specific activity was 120+/-40 mCi/micromol (EOB). [18F]celecoxib was found to be stable in ethanol, however, de[18F]fluorination (6.5%) was observed after 4 h in 10% ethanol-saline solution. Rodent PET studies show bone labeling indicating in vivo de[18F]fluorination of [18F]celecoxib. PET studies in baboon indicated a lower rate of de[18F]fluorination than rat and retention of radioactivity in brain regions consistent with the known distribution of COX-2. A radiolabeling method that can generate consistent high specific activity is needed for routine human use.     

Metabolism of 2-[18F]fluoro-2-deoxyglucose in tumor-bearing rats: Chromatographic and enzymatic studies

The activities of hexokinase and glucose-6-phosphatase, as well as the in vivo metabolic products of 2-[¹⁸F]fluoro-2-deoxyglucose ([¹⁸F]FDG) (45 min after an i.v. injection), were determined from several tissues of Rous sarcoma implanted rats. The HK/G-6-Pase ratio was found to be high in brain and tumor, and low in liver with intermediate values for kidney and muscle. In accordance with the measured enzyme activities about 90% of the ¹⁸F was found as [¹⁸F]FDG-6-P in brain, heart and tumor, whereas most of its was as [¹⁸F]FDG in liver and kidney. In addition three minor metabolites, tentatively identified as nucleotide-derivatives of [¹⁸F]FDG, were formed. Our results suggest that at least Rous sarcoma tumor effectively converts [¹⁸F]FDG to [¹⁸F]FDG-6-P and thus PET studies with [¹⁸F]FDG can be applied to tumor diagnosis and to quantitative measurement of glucose utilization in tumor tissue according to the model of Sokoloff.(⁹)     

Metabolic fates of 2′-deoxy-5-[18F]fluorouridine in tumor-bearing mice and human plasma

The metabolic fate of 2′-deoxy-5-[¹⁸F]fluorouridine ([¹⁸F]FdUrd), a useful positron emission tomography (PET) tracer of nucleic acid metabolism in tumors, was investigated in mice and humans. A rapid increase in labeled catabolites was found in mouse and human plasma. In mouse FM3A mammary carcinoma, the corresponding catabolites were also detected in addition to metabolites which were activated by the nucleic acid metabolism. From a biodistribution study of β-[³H]alanine, α-[¹⁸F]fluoro-β-alanine, a major catabolite, was assumed to be taken up twice as much by tumor than by the brain. Nucleic acid metabolism in brain tumors by [¹⁸F]FdUrd-PET may be assessed using normal brain regions as a reference.     

2-Deoxy-2-[18F]fluoro-d-galactose as an In Vivo tracer for imaging galactose metabolism in tumors with positron emission tomography

The feasibility of 2-deoxy-2-[¹⁸F]fluoro-D-galactose ([¹⁸F]FdGal) for imaging galactose metabolism in tumors with positron emission tomography (PET), was investigated using two hepatomas, Yoshida sarcoma, or glioma in rats, and mouse mammary carcinoma. In hepatoma-bearing rats the highest uptake of [¹⁸F]FdGal was observed in the liver followed by the kidney and tumor. The tumor uptake increased with time, and the high uptake ratios of tumor to organ were observed except for the liver and kidney. Tumor uptake was also measured in all tumors. As main metabolites in all tumors, [¹⁸F]FdGal 1-phosphate and UDP-[¹⁸F]FdGal were found by HPLC. Two hepatomas showed a slightly higher uptake and a larger percentage of UDP derivative than the other three tumors. By autoradiography the brain tumor was visualized clearly. These results indicate that [¹⁸F]FdGal has potential as a tracer for imaging galactose metabolism in tumors with PET.     

18F Labeled benzimidazole derivatives as potential radiotracer for positron emission tomography (PET) tumor imaging

This article reported the synthesis and bioevaluation of two [¹⁸F] labeled benzimidazole derivatives, 4-(5-(2-[¹⁸F] fluoro-4-nitrobenzamido)-1-methyl-1H-benzimidazol-2-yl) butanoic acid ([¹⁸F] FNBMBBA, [¹⁸F]a1) and 3-(2-fluoroethyl)-7-methyl-2-propyl-3H-benzimidazole-5-carboxylic acid ([¹⁸F] FEMPBBA, [¹⁸F]b1) for PET tumor imaging. The preparation [¹⁸F] FEMPBBA was completed in 1 h with overall radiochemical yield of 50–60% (without decay corrected). Biodistribution assay in S180 tumor bearing mice of both compounds were carried out, and the results are both meaningful. [¹⁸F] FEMPBBA which can be taken as a revision of [¹⁸F] FNBMBBA got an excellent result, and has significant advantages in some aspects compared with L-[¹⁸F] FET and [¹⁸F]-FDG in the same animal model, especially in tumor/brain uptake ratio. The tumor/brain uptake ratio of [¹⁸F] FEMPBBA gets to 4.81, 7.15, and 9.8 at 30 min, 60 min and 120 min, and is much higher than that of L-[¹⁸F] FET (2.54, 2.92 and 2.95) and [¹⁸F]-FDG (0.61, 1.02, 1.33) at the same time point. The tumor/muscle and tumor/blood uptake ratio of [¹⁸F] FEMPBBA is also higher than that of L-[¹⁸F] FET at 30 min and 60 min. This result indicates compound [¹⁸F] FEMPBBA is a promising radiotracer for PET tumor imaging.     

Synthesis and preliminary biological evaluation of O6-[4-(2-[18F]fluoroethoxymethyl)benzyl]guanine as a novel potential PET probe for the DNA repair protein O6-alkylguanine-DNA alkyltransferase in cancer chemotherapy

A novel fluorine-18-labeled O6-benzylguanine (O6-BG) derivative, O6-[4-(2-[18F]fluoroethoxymethyl)benzyl]guanine (O6-[18F]FEMBG, [18F]1), has been synthesized for evaluation as a potential positron emission tomography (PET) probe for the DNA repair protein O6-alkylguanine-DNA alkyltransferase (AGT) in cancer chemotherapy. The appropriate radiolabeling precursor N(2,9)-bis(p-anisyldiphenylmethyl)-O6-[4-(hydroxymethyl)benzyl]guanine (6) and reference standard O6-[4-(2-fluoroethoxymethyl)benzyl]guanine (O6-FEMBG, 1) were synthesized from 1,4-benzenedimethanol and 2-amino-6-chloropurine in four or six steps, respectively, with moderate to excellent chemical yields. The target tracer O6-[18F]FEMBG was prepared in 20-35% radiochemical yields by reaction of MTr-protected precursor 6 with [18F]fluoroethyl bromide followed by quick deprotection reaction and purification with a simplified Silica Sep-Pak method. Total synthesis time was 60-70 min from the end of bombardment. Radiochemical purity of the formulated product was >95%, with a specific radioactivity of >1.0 Ci/micromol at the end of synthesis. The activity of unlabeled O6-FEMBG was evaluated via an in vitro AGT oligonucleotide assay. Preliminary findings from biological assay indicate that the synthesized analogue has similarly strong inhibiting effect on AGT in comparison with O6-BG and O6-4-fluorobenzylguanine (O6-FBG). The results warrant further in vivo evaluation of O6-[18F]FEMBG as a new potential PET probe for AGT.     

Preparation and evaluation of 17-ethynyl-substituted 16α-[18F]fluoroestradiols: Selective receptor-based PET imaging agents

We have prepared and studied six new analogs of 16α-fluoroestradiol (FES): 17α- and 17β-ethynyl-FES (7 [FEES]and 7a), and the 11β-ethyl (8 and 8a) and 11β-methoxy (9 and 9a) derivatives, novel estrogen receptor-based PET imaging agents. The relative binding affinity (RBA) for the estrogen receptor (ER) versus FES is increased for 7, 9 and 9a but decreased for 7a, 8 and 8a. All six analogs have been labeled in the 16α position with ¹⁸F by the nucleophilic displacement of the corresponding 16β-trifluoromethanesulfonate with nBu₄N¹⁸F. Subsequent ethynylation with lithium trimethylsilylacetylide yielded the FEES analogs (total synthesis time: 120 min; effective specific activity: 200–2400 Ci/mmol). Selective uptake in the uterus was high for [¹⁸F)7, [¹⁸F]8, [¹⁸F]9 and [¹⁸F]9a (% ID/g values at 1 h: 11.2, 12.9, 9.9 and 8.3, respectively), while uptake was effectively blocked by coinjection of an excess of unlabeled estradiol. The FEES analogs, [¹⁸F]7, [¹⁸F]8 and [¹⁸F]9, exhibited the highest selectivity, in terms of target (uterus)-to-blood ratios, ever seen amongst estrogen radiopharmaceuticals, 154, 145 and 169, respectively. The analogs [¹⁸F]7a and [¹⁸F]8a displayed no uptake in the uterus, consistent with their low RBAs. Metabolism studies revealed that most of the uterine activity is unmetabolized while the blood exhibits a rapid and subsequently sustained mixture of metabolites. The muscle shows a metabolic profile intermediate to the uterus and blood. These analogs provide an array of desirable characteristics for the optimal PET imaging of ER-rich target tissues.     

Positron emission tomography: quantitative measurement of brain acetylcholinesterase activity using radiolabeled substrates

A new method for quantitative measurement of brain acetylcholinesterase (AChE) activity in living human brain using positron emission tomography (PET) is described. We tested several radiolabeled lipophilic acetylcholine analogs, e.g., N-methylpiperidyl esters, which readily entered the brain via the blood-brain barrier, were hydrolyzed selectively by AChE, and were then trapped in the brain. Among them, and tested and N-[11C]methylpiperidin-4-yl acetate ([11C]MP4A) was chosen as the tracer for PET. Quantitative measurement of cortical AChE was accomplished by fitting the time course of cerebral radioactivity concentration measured by PET and the metabolite-corrected arterial plasma input function using a nonlinear least-squares fitting method. Normal control studies of subjects with a wide range in age (24-89 years) showed no decrease in AChE activity in the cerebral cortex with age. Studies on patients with Alzheimer's disease demonstrated a widespread reduction of AChE activity in the cerebral cortex (more profound in early-onset than in late-onset Alzheimer's disease). Parkinson's disease and progressive supranuclear palsy, clinically similar disorders, could be differentiated with [11C]MP4A/PET studies. Simple methods without using an arterial input function are also proposed. The method provides a quantitative measure of the cholinergic aspect of brain function and proved to be useful in diagnosis of neurodegenerative disorders including Alzheimer's disease.     

6-[18F]fluoro-A-85380, a novel radioligand for in vivo imaging of central nicotinic acetylcholine receptors

A novel positron emission tomography (PET) radiotracer, 6-[18F]fluoro-3-(2(S)-azetidinylmethoxy)pyridine (6-[18F]fluoro-A-85380, 6-[18F]FA) was synthesized by a no-carrier-added fluorination. In vitro 6-[18F]FA bound to nicotinic acetylcholine receptors (nAChRs), with very high affinity (Kd 28 pM). In PET studies, 6-[18F]FA specifically labeled central nAChRs in the brain of the Rhesus monkey and demonstrated highest levels of accumulation of radioactivity in brain regions enriched with the alpha4beta2 subtype of nAChR. 6-[18F]FA exhibited a target-to-non-target ratio (estimated as radioactivity in the thalamus to that in the cerebellum) of binding in primate brain similar to that previously determined for a labeled analog of epibatidine, [18F]FPH. In contrast to [18F]FPH, the novel tracer is expected to exhibit substantially less toxicity. Thus, the novel radioligand, 6-[18F]FA, appears to be a suitable candidate for imaging nAChRs in human brain.