Positron emission tomography analysis of [11C]KW-6002 binding to human and rat adenosine A2A receptors in the brain.

Adenosine A(2A) receptors are found on striatal neurones projecting to the external pallidum. KW-6002 (istradefylline) is a potent and selective antagonist for the adenosine A(2A) receptors in the CNS and acts to inhibit the excessive activity of this pathway in the MPTP marmoset model of PD, thus relieving parkinsonism. The objectives of this study were to investigate the regional binding of the novel positron emission tomography tracer [(11)C]KW-6002 in the healthy human brain and the rat brain, along with receptor occupancy by cold KW-6002 at varying doses in human. The highest [(11)C]KW-6002 uptake in the rat brain was seen in striatum and lower levels in cortex and cerebellum. Brain [(11)C]KW-6002 uptake was well characterized in humans by a two-tissue compartmental model with a blood volume term, and the ED(50) of cold KW-6002 was 0.5 mg in the striatum. Over 90% receptor occupancy was achieved with daily oral doses of greater than 5 mg. In humans, blockable binding was present in all gray matter structures including the cerebellum, which has not been reported to express A(2A) receptors. MRS 1745, an A(2B) receptor selective antagonist, had no effect on the cerebellar binding of [(11)C]KW-6002 in rats, suggesting that this blockable signal is unlikely to result from an affinity for adenosine A(2B) receptors.

Glucose metabolism in brain tumours can be estimated using [18F]2-fluorodeoxyglucose positron emission tomography and a population-derived input function scaled using a single arterialised venous blood sample.

The aim of this study was to assess simplified methods for deriving input functions for estimating glucose metabolism using 18F-FDG-PET. Nine glioma patients underwent paired 18F-FDG-PET scans as part of a phase II study and the data used to estimate the metabolic rate of glucose (MRGlu) using a population-derived input function (arterial data from 14 scans) scaled using a single arterial blood sample taken at 20 min. Paired studies were performed in four further glioma patients with stable disease at least four months following radiotherapy to determine whether scaling the population-derived input function using a 20-min arterialised venous or venous sample further simplified the method. The heated hand method was used to obtain arterialised venous blood that approximated arterial blood. In the 9 phase II glioma patients, there was a good, statistically significant correlation between the MRGlu values estimated using the individual arterial input functions and the single arterial sample scaled population-derived input functions (r(2)=0.88, p<0.001, n=36). Blood samples collected during three scans on two of the stable disease patients showed no significant difference between the arterialised venous and arterial plasma concentrations of 18F (p>0.1, n=15) when the degree of arterialisation of the blood was monitored and maintained using a thermocouple. A significant difference was found between the plasma arterial and venous levels of 18F. There was an excellent correlation between MRGlu estimated using an arterial input function and a population-derived input function scaled using a single arterialised venous blood sample (r(2)=0.98, n=12). The method was reproducible with less than 4.4% variation between repeat tumour scans. Therefore, a population-derived input function scaled using a single arterialised venous blood sample at 20 min can be used for estimating MRGlu using 18F-FDG PET in glioma patients.

In vivo evaluation of [18F]fluoroetanidazole as a new marker for imaging tumour hypoxia with positron emission tomography.

Development of hypoxia-targeted therapies has stimulated the search for clinically applicable noninvasive markers of tumour hypoxia. Here, we describe the validation of [(18)F]fluoroetanidazole ([(18)F]FETA) as a tumour hypoxia marker by positron emission tomography (PET). Cellular transport and retention of [(18)F]FETA were determined in vitro under air vs nitrogen. Biodistribution and metabolism of the radiotracer were determined in mice bearing MCF-7, RIF-1, EMT6, HT1080/26.6, and HT1080/1-3C xenografts. Dynamic PET imaging was performed on a dedicated small animal scanner. [(18)F]FETA, with an octanol-water partition coefficient of 0.16+/-0.01, was selectively retained by RIF-1 cells under hypoxia compared to air (3.4- to 4.3-fold at 60-120 min). The radiotracer was stable in the plasma and distributed well to all the tissues studied. The 60-min tumour/muscle ratios positively correlated with the percentage of pO(2) values <5 mmHg (r=0.805, P=0.027) and carbogen breathing decreased [(18)F]FETA-derived radioactivity levels (P=0.028). In contrast, nitroreductase activity did not influence accumulation. Tumours were sufficiently visualised by PET imaging within 30-60 min. Higher fractional retention of [(18)F]FETA in HT1080/1-3C vs HT1080/26.6 tumours determined by dynamic PET imaging (P=0.05) reflected higher percentage of pO(2) values <1 mmHg (P=0.023), lower vessel density (P=0.026), and higher radiobiological hypoxic fraction (P=0.008) of the HT1080/1-3C tumours. In conclusion, [(18)F]FETA shows hypoxia-dependent tumour retention and is, thus, a promising PET marker that warrants clinical evaluation.

Comparison of methodologies for the in vivo assessment of 18FLT utilisation in colorectal cancer.

Fluorine-18 3'-deoxy-3'-fluorothymidine (18FLT) is a tissue proliferation marker which has been suggested as a new tumour-specific imaging tracer in positron emission tomography (PET). The objectives of this study were to investigate the pharmacokinetics of 18FLT in patients with colorectal cancer, defining methodologies for the quantitative analysis of the in vivo 18FLT uptake and subsequently assessing the accuracy of semi-quantitative measures. Dynamic acquisitions over a single field of view of interest identified by computed tomography were carried out for up to 60 min following injection of 18FLT (360 +/- 25 MBq). Dynamic arterial blood sampling was carried out in order to provide a blood input function. Simultaneous venous samples were also taken in order to investigate their potential utilisation in deriving a hybrid input function. Arterial and venous blood samples at 5, 15, 30, 60 and 90 min p.i. were used for metabolite analysis. Eleven patients with primary and/or metastatic colorectal cancer were studied on a lesion by lesion basis (n = 21). All acquired images were reconstructed using ordered subsets expectation maximisation and segmented attenuation correction. Time-activity curves were derived by image region of interest (ROI) analysis and image-based input functions were obtained using abdominal or thoracic aorta ROIs. Standardised uptake values (SUVs) were calculated to provide semi-quantitative indices of uptake, while non-linear regression (NLR) methodology in association with a three-compartment model and Patlak analysis were carried out to derive the net influx constant Ki. The metabolite analysis revealed two radioactive metabolites, with the parent compound representing approximately 80% of the total radioactivity in the 30-min plasma sample. In the case of NLR, better fits were obtained with a 3k model (i.e. k4 = 0) for both lesion and bone marrow time-activity curves. For the same lesions, a high correlation was observed between the Ki derived from either Patlak analysis or NLR(3k) and the corresponding SUVs. Our results also suggest that the quantitative behaviour of 18FLT in vivo (up to 60 min p.i.) may be characterised using a 3k model or Patlak analysis in combination with image-derived input functions. The good correlation found between the SUVs (at 60 min) and Ki values supports the use of semi-quantitative indices to assess the proliferation rate of colorectal cancer lesions in vivo with 18FLT.

Radiosynthesis of 4-[(2-chloroethyl)(2-[(11)C]ethyl)amino]-phenoxycarbonyl-l-glutamic acid a half mustard prodrug as a potential probe for imaging antibody- and gene-directed enzyme prodrug therapy with positron emission tomography.

The potential antibody directed prodrug therapy half-mustard prodrug 4-[(2-chloroethyl)(2-ethyl)amino]-phenoxycarbonyl-L-glutamic acid was synthesised by reductive alkylation of 4-[(2-chloroethyl)amino]-phenoxycarbonyl-L-glutamic acid using acetaldehyde. 4-[(2-chloroethyl)[(11)C](2-ethyl)amino]phenoxycarbonyl-L-glutamic acid was synthesized with 18-22% decay corrected radiochemical yield in 45 min from EOB by reductive alkylation of 4-[(2-chloroethyl)amino]-phenoxycarbonyl-L-glutamic acid using [(11)C]acetaldehyde. [(11)C]Acetaldehyde was prepared in 60% decay corrected radiochemical yield by oxidation of [(11)C]ethanol over heated copper oxide. The radiosynthesis of [(11)C]ethanol was re-examined and optimized. 4-[(2-chloroethyl)(2-ethyl)amino]-phenoxycarbonyl-L-glutamic acid was found to have affinity for carboxypeptidase G2; the K(m) and V(max) were 99.4-115.9 microM (n=3) and 3.6-5.0 microM/min, respectively, at a carboxypeptidase G2 concentration of 0.0247 U/ml.

In vitro selectivity, in vivo biodistribution and tumour uptake of annexin V radiolabelled with a positron emitting radioisotope.

The availability of a noninvasive method to detect and quantify apoptosis in tumours will enable tumour response to several cancer therapies to be assessed. We have synthesised two radiotracers, annexin V and the N-succinimidyl-3-iodobenzoic acid (SIB) derivative of annexin V, labelled with radio-iodine ((124)I and (125)I) and provided proof of the concept by assessing specific binding and biodistribution of these probes to apoptotic cells and tumours. We have also assessed the tumour uptake of [(124)I]annexin V in a mouse model of apoptosis. RIF-1 cells induced to undergo apoptosis in vitro showed a drug concentration-dependent increased binding of [(125)I]annexin V and [(125)I]SIB-annexin V. In the same model system, there was an increase in terminal deoxynucleotidyl transferase-mediated nick end labelling (TUNEL)-positive cells and a decrease in clonogenic survival. Radiotracer binding was completely inhibited by preincubation with unlabelled annexin V. In RIF-1 tumour-bearing mice, rapid distribution of [(125)I]SIB-annexin V-derived radioactivity to kidneys was observed and the radiotracer accumulated in urine. The binding of [(125)I]SIB-annexin V to RIF-1 tumours increased by 2.3-fold at 48 h after a single intraperitoneal injection of 5-fluorouracil (165 mg kg(-1) body weight), compared to a 4.4-fold increase in TUNEL-positive cells measured by immunostaining. Positron emission tomography images with both radiotracers demonstrated intense localisation in the kidneys and bladder. Unlike [(124)I]SIB-annexin V, [(124)I]annexin V also showed localisation in the thyroid region presumably due to deiodination of the radiolabel. [(124)I]SIB-annexin V is an attractive candidate for in vivo imaging of apoptosis by PET.

In vivo imaging of cellular proliferation in colorectal cancer using positron emission tomography.

BACKGROUND: and aims: Positron emission tomography (PET) using (18)F labelled 2-fluoro-2-deoxy-D-glucose ((18)FDG) is an established imaging tool, although the recent development of a biologically stable thymidine analogue [18F] 3'-deoxy-3-fluorothymidine ((18)FLT) has allowed PET to image cellular proliferation by utilising the salvage pathway of DNA synthesis. In this study, we have compared uptake of (18)FLT and (18)FDG with MIB-1 immunohistochemistry to evaluate the role of PET in quantifying in vivo cellular proliferation in colorectal cancer (CRC). PATIENTS AND METHODS: Patients with resectable, primary, or recurrent CRC were prospectively studied. Thirteen lesions from 10 patients (five males, five females), median age 68 years (range 54-87), were evaluated. Patients underwent (18)FDG and (18)FLT PET scanning. Tracer uptake within lesions was quantified using standardised uptake values (SUVs). Histopathological examination and MIB-1 immunohistochemistry were performed on all lesions, and proliferation quantified by calculating a labelling index (% of MIB-1 positively stained nuclei within 1500 tumour cells). RESULTS: Histology confirmed adenocarcinoma in 12 of 13 lesions; the remaining lesion was reactive. All eight extrahepatic lesions were visualised using both (18)FLT and (18)FDG. Three of the five resected liver metastases were also avid for (18)FLT and showed high proliferation, while the remaining two lesions which demonstrated no uptake of (18)FLT had correspondingly very low proliferation. There was a statistically significant positive correlation (r =0.8, p<0.01) between SUVs of the tumours visualised with (18)FLT and the corresponding MIB-1 labelling indices. No such correlation was demonstrated with (18)FDG avid lesions (r =0.4). CONCLUSIONS: (18)FLT PET correlates with cellular proliferation markers in both primary and metastatic CRC. This technique could provide a mechanism for in vivo grading of malignancy and early prediction of response to adjuvant chemotherapy.

Potential impact of [18F]3'-deoxy-3'-fluorothymidine versus [18F]fluoro-2-deoxy-D-glucose in positron emission tomography for colorectal cancer.

Fluorine-18 labelled fluoro-2-deoxy- d-glucose ((18)FDG) positron emission tomography (PET) imaging demonstrates the increased glucose consumption of malignant cells, but problems with specificity have led to the development of new PET tracers. [(18)F]3'-deoxy-3'-fluorothymidine ((18)FLT) is a new tracer which images cellular proliferation by entering the salvage pathway of DNA synthesis. In this study we compared the cellular uptake of (18)FLT and (18)FDG in patients with colorectal cancer (CRC). Seventeen patients with 50 primary or metastatic CRC lesions were prospectively recruited. Lesions were initially identified using computed tomography. Patients underwent both (18)FDG and (18)FLT scanning. Semi-quantitative analysis of tracer uptake was carried out using standardised uptake values. All the primary tumours ( n=6) were visualised by both tracers, with (18)FDG showing on average twice the uptake of (18)FLT. Similar uptake of both tracers was seen in lung and peritoneal lesions, with (18)FLT imaging five of the six lung lesions and all of the peritoneal lesions. Of the 32 colorectal liver metastases, 11 (34%) were seen as avid for (18)FLT, compared with 31 (97%) for (18)FDG. No correlation was seen between the uptake of the two tracers ( R(2)=0.03). (18)FLT shows a high sensitivity in the detection of extrahepatic disease but poor sensitivity for the imaging of colorectal liver metastases, making it unlikely to have a role as a diagnostic tracer in CRC. We have demonstrated that (18)FDG and (18)FLT image two distinct processes. The prognostic implications of the uptake of (18)FLT need to be assessed in terms of response to chemoradiotherapy and survival.

Cancer Research UK procedures in manufacture and toxicology of radiotracers intended for pre-phase I positron emission tomography studies in cancer patients.

Radiolabelled compounds formulated for injection (radiopharmaceuticals), are increasingly being employed in drug development studies. These can be used in tracer amounts for either pharmacokinetic or pharmacodynamic studies. Such radiotracer studies can also be carried out early in man, even prior to conventional Phase I clinical testing. The aim of this document is to describe procedures for production and safety testing of oncology radiotracers developed for imaging by positron emission tomography in cancer patients. We propose strategies for overcoming the inability to produce compounds in sufficient quantities via the radiosynthetic routes for full chemical characterisation and toxicology testing including (i) independent confirmation as far as possible that the stable compound associated with the radiopharmaceutical is identical to the non-labelled compound, (ii) animal toxicity studies with > or = 10 times (typically 100 times) the intended tracer dose in humans scaled by body surface area, and (iii) patient monitoring during the radiotracer positron emission tomography clinical trial.

Evaluation of [4-O-methyl-(11)C]KW-6002 as a potential PET ligand for mapping central adenosine A(2A) receptors in rats.

KW-6002, a xanthine-based adenosine A(2A) antagonist, was labelled with the positron emitter carbon-11 by O-methylation of its precursor, KF23325, using [(11)C]iodomethane and was evaluated in rats as a putative in vivo radioligand for positron emission tomography (PET). Following intravenous injection of [(11)C]KW-6002, radioactivity was measured in blood, plasma, peripheral tissues, and in discrete brain tissues over a 2-h time period commensurate with PET scanning. In brain, [(11)C]KW-6002 showed highest retention in striata, with evidence of saturable binding, and lowest retention in frontal cortex (a tissue low in adenosine A(2A) receptors). PET scanning with [(11)C]KW-6002 demonstrated a specific signal in the striata which could be described using compartmental modelling. Specific binding was, however, also detected in extrastriatal regions, including brain areas reported to have low adenosine A(2A) receptor density. Blocking studies with the A(1) selective antagonist KF15372 and the non xanthine-type A(2A) antagonist ZM 241385 failed to elucidate the nature of this binding. Thus, although [(11)C]KW-6002 shows some potential for development as a PET ligand for quantifying striatal adenosine A(2A) receptor function, its in vivo selectivity requires further investigation.

Comparative biodistribution and metabolism of carbon-11-labeled N-[2-(dimethylamino)ethyl]acridine-4-carboxamide and DNA-intercalating analogues.

The tricyclic carboxamide N-[2-(dimethylamino)ethyl]acridine-4-carboxamide (DACA) is a DNA-intercalating agent capable of inhibiting both topoisomerases I and II and is currently in Phase II clinical trial. Many related analogues have been developed, but despite their potent in vitro cytotoxicities, they exhibit poor extravascular distribution. As part of an ongoing drug development program to obtain related "minimal intercalators" with lower DNA association constants, we have compared the biodistribution and metabolite profiles of the prototype compound, DACA, with three analogues to aid rational drug selection. All of these compounds share a common structural feature, N-dimethyl side chain, which was radiolabeled with the positron-emitting radioisotope, carbon-11. This strategy was selected because it allows promising candidates emerging from preclinical studies in animals to be evaluated rapidly in humans using positron emission tomography (PET). The acridine DACA, the phenazine SN 23490, the pyridoquinoline SN 23719, and the dibenzodioxin SN 23935 were found to be cytotoxic in in vitro assays with an IC50 of 1.4-1.8 microM, 0.4-0.6 microM, 1.3-1.6 microM, and 24-36 microM, respectively, in HT29, U87MG, and A375M cell lines. Ex vivo biodistribution studies with carbon-11 radiolabeled compounds in mice bearing human tumor xenografts showed rapid clearance of 11C-radioactivity (parent drug and metabolites) from blood and the major organs. Rapid hepatobiliary clearance and renal excretion were also observed. There was low [<5% of injected dose/gram (%ID/g)] and variable uptake of 11C-radioactivity in three tumor types for all of the compounds. Tumor (U87MG) to blood 11C-radioactivity for [11C]DACA, [11C](9-methoxyphenazine-1-carboxamide (SN 23490), [11C]2-(4-pyridyl)quinoline-8-carboxamide (SN 23719), and [11C]dibenzo[1,4]dioxin-1-carboxamide (SN 23935) at 30 min were 2.9 +/- 1.1, 2.3 +/- 0.6, 2.6 +/- 0.6, and 0.7 +/- 0.2, respectively. For SN 23719, the distribution of 11C-radioactivity in normal tissues and tumors determined ex vivo was in broad agreement with that determined in vivo by whole body PET scanning. [11C]DACA was rapidly and extensively metabolized to several plasma metabolites and a major tumor metabolite. In contrast, [11C]SN 23935, [11C]SN 23490, and [11C]SN 23719 showed less extensive metabolism. In the tumor samples, the parent [11C]DACA and [11C]SN 23935 represented between 0.3 and 1.5%ID/g, whereas [11C]SN 23490 and [11C]SN 23719 represented between 1.5 and 2.8%ID/g. In conclusion, by using a strategy with 11C-labeling, we have determined the tissue distribution and metabolic stability of novel tricyclic carboxamides with the view of selecting analogues with potentially better in vivo activity against solid tumors. SN 23490 and SN 23719 had more favorable distribution and metabolic stability compared with DACA and SN 23935 and may warrant further development. The radiolabeling strategy used allows ex vivo and in vivo evaluation of promising anticancer agents in animals and offers the potential of rapid translation to studies in humans using PET.

Pharmacokinetic evaluation of N-[2-(dimethylamino)ethyl]acridine-4-carboxamide in patients by positron emission tomography.

PURPOSE: To evaluate tumor, normal tissue, and plasma pharmacokinetics of N-[2-(dimethylamino)ethyl]acridine-4-carboxamide (DACA). The study aimed to determine the pharmacokinetics of carbon-11-labeled DACA ([11C]DACA) and evaluate the effect of pharmacologic doses of DACA on radiotracer kinetics. PATIENTS AND METHODS: [11C]DACA (at 1/1,000 phase I starting dose) was administered to 24 patients with advanced cancer (pre-phase I) or during a phase I trial of DACA in five patients. Positron emission tomography (PET) was performed to assess pharmacokinetics and tumor blood flow. Plasma samples were analyzed for metabolite profile of [11C]DACA. RESULTS: There was rapid systemic clearance of [11C]DACA over 60 minutes (1.57 and 1.46 L x min(-1) x m(-2) in pre-phase I and phase I studies, respectively) with the production of several radiolabeled plasma metabolites. Tumor, brain, myocardium, vertebra, spleen, liver, lung, and kidneys showed appreciable uptake of 11C radioactivity. The area under the time-versus-radioactivity curves (AUC) showed the highest variability in tumors. Of interest to potential toxicity, maximum radiotracer concentrations (Cmax) in brain and vertebra were low (0.67 and 0.54 m(2) x mL(-1), respectively) compared with other tissues. A moderate but significant correlation was observed for tumor blood flow with AUC (r = 0.76; P =.02) and standardized uptake value (SUV) at 55 minutes (r = 0.79; P =.01). A decrease in myocardial AUC ( P =.03) and splenic and myocardial SUV ( P =.01 and.004, respectively) was seen in phase I studies. Significantly higher AUC, SUV, and Cmax were observed in tumors in phase I studies. CONCLUSION: The distribution of [11C]DACA and its radiolabeled metabolites was observed in a variety of tumors and normal tissues. In the presence of unlabeled DACA, pharmacokinetics were altered in myocardium, spleen, and tumors. These data have implications for predicting activity and toxicity of DACA and support the use of PET early in drug development.

Radiolabelled tracers and anticancer drugs for assessment of therapeutic efficacy using PET.

Positron Emission Tomography (PET) has the potential to improve efficacy of established and novel cancer therapies and to assist more rapid and rational progression of promising novel therapies into the clinic. This is due to PET's unrivalled sensitivity and ability to monitor the pharmacokinetics and pharmacodynamics of drugs and biochemicals radiolabelled with short -lived positron emitting radioisotopes. PET is a multidisciplinary science which employs chemists, biologists, mathematical modellers, pharmacologists as well as clinicians. Clinical research questions in oncology determine the methodological challenges faced by these other disciplines. Within this context we focus on the developments of the radiolabelled compounds that have underpinned the clinical work in oncology for monitoring tumour and normal tissue pharmacokinetics, assessment of tumour response, cell proliferation, gene expression, hypoxia, multidrug resistance and status of receptors on tumours.

Transjugular intrahepatic portosystemic shunt versus distal splenorenal shunt--a comparative study.

BACKGROUND/AIMS: No general consensus exists regarding the proper surgical management of recurrent variceal bleeding due to hepatic cirrhosis. Transjugular intrahepatic portosystemic shunt and distal splenorenal shunt are increasingly being performed in the management of these patients. The present study was undertaken to compare the efficacy, complications and survival rate of these two procedures. METHODOLOGY: Sixty-seven patients with alcoholic liver cirrhosis of Child-Pugh's class A (n = 22) and class B (n = 45) with recurrent variceal bleeding not controlled by conservative means underwent either transjugular intrahepatic portosystemic shunt placement (n = 35) or a distal splenorenal shunt operation (n = 32). These patients were followed for a mean of 887 +/- 189 days. Both groups were compared with respect to the rates of survival, recurrence of gastrointestinal bleeding, encephalopathy, ascitis, shunt blockade and other relevant biochemical parameters. RESULTS: Patients who underwent a distal splenorenal shunt operation had lower rates of recurrence of gastrointestinal bleeding (6.25% vs. 25.71%), encephalopathy (18.75% vs. 42.86%) shunt blockade (6.25% vs. 68.57%) and lower mean fasting blood ammonia levels (56.70 +/- 7.10 mumol/L vs. 61.70 +/- 5.70 mumol/L). However the rate of ascitis was higher amongst these patients (40.63% vs. 11.43%). There was no significant difference in the midterm survival rates between these groups (81.25% vs. 80.00%). Both procedures were effective in controlling functional renal failure, splenomegaly and features of hypersplenism. CONCLUSIONS: Distal splenorenal shunt operation is a better therapeutic option than transjugular intrahepatic portosystemic shunt placement for control of recurrent variceal bleeding due to hepatic cirrhosis.

An automated radiosynthesis of 2-[11C]thymidine using anhydrous [11C]urea derived from [11C]phosgene.

2-[11C]Thymidine has been produced from [11C]methane via [11C]phosgene and [11C]urea. Anhydrous [11C]urea was prepared from [11C]phosgene by reaction with liquid ammonia. This novel approach avoids the problems associated with the synthesis of anhydrous [11C]urea from [11C]cyanide. A fully automated system based on a modular approach and under PLC control has been developed. The system provides 2-[11C]thymidine reliably and reproducibly for clinical PET studies. The radiosynthesis takes 45-50 min from [11C]methane and the average yield was 1.5-3.3 GBq (40-90 mCi). The specific radioactivity was typically in the range 29.6-51.8 GBq mumol-1 (0.8-1.4 Ci mumol-1) at EOS corresponding to 6-12 micrograms of stable thymidine. The radiochemical yield of 2-[11C]thymidine was ca. 14% from [11C]methane.

Measurement of changes in opioid receptor binding in vivo during trigeminal neuralgic pain using [11C] diprenorphine and positron emission tomography.

The binding of [11C]diprenorphine to mu, kappa, and delta subsites in cortical and subcortical structures was measured by positron emission tomography in vivo in six patients before and after surgical relief of trigeminal neuralgia pain. The volume of distribution of [11C]diprenorphine binding was significantly increased after thermocoagulation of the relevant trigeminal division in the following areas: prefrontal, insular, perigenual, mid-cingulate and inferior parietal cortices, basal ganglia, and thalamus bilaterally. In addition to the pain relief associated with the surgical procedure, there also was an improvement in anxiety and depression scores. In the context of other studies, these changes in binding most likely resulted from the change in the pain state. The results suggest an increased occupancy by endogenous opioid peptides during trigeminal pain but cannot exclude coexistent down-regulation of binding sites.

Evaluation of [methyl-3H]L655,708 and [ethyl-3H]RY80 as putative PET ligands for central GABA(A) receptors containing alpha5 subunit.

Two selective radioligands of gamma aminobutyric acid (GABA)A receptors containing the alpha5 subunit, [3H]L655,708 and [3H]RY80, were evaluated in rats as potential in vivo tracers for positron emission tomography (PET). Brain uptake index (BUI), a measure of first pass extraction, was moderate for [3H]L655,708 (BUI of 59%) and good for [3H]RY80 (BUI of 96%). This finding was consistent with their in vitro binding to plasma proteins of approximately 76% and 50%, respectively. Following intravenous injection of either radioligand, radioactivity in plasma was measured and uptake characteristics were assessed in brain within a time period relevant to PET scanning (up to 90 min). Discrete brain regions, such as frontal cortex, striatum, hypothalamus, thalamus, hippocampus, colliculi, medulla, and cerebellum, were sampled and the temporal distribution of radioactivity analysed. Despite the reasonable delivery to the brain, neither of the radioligands had sufficient retention in the tissues rich in alpha5-containing GABA(A) receptors to achieve a good selective signal. For both radioligands, a maximal tissue:cerebellum ratio of 1.5 was seen in hippocampus at 10 min after injection. Thus, neither of the compounds studied shows potential for further development as an in vivo PET ligand.

Characterisation of the appearance of radioactive metabolites in monkey and human plasma from the 5-HT1A receptor radioligand, [carbonyl-11C]WAY-100635--explanation of high signal contrast in PET and an aid to biomathematical modelling.

N-(2-(4-(2-Methoxy-phenyl)-1-piperazin-1-yl)ethyl)-N-(2-pyridyl)++ +cyclohexanecarboxamide (WAY-100635), labelled in its amido carbonyl group with 11C (t1/2 = 20.4 min), is a promising radioligand for the study of brain 5-HT1A receptors with positron emission tomography (PET). Thus, in PET experiments in six cynomolgus monkeys and seven healthy male volunteers, [carbonyl-11C]WAY-100635 was taken up avidly by brain. Radioactivity was retained in regions rich in 5-HT1A receptors, such as occipital cortex, temporal cortex and raphe nuclei, but cleared rapidly from cerebellum, a region almost devoid of 5-HT1A receptors. [Carbonyl-11C]WAY-100635 provides about 3- and 10-fold higher signal contrast (receptor-specific to nonspecific binding) than [O-methyl-11C]WAY-100635 in receptor-rich areas of monkey and human brain, respectively. To elucidate the effect of label position on radioligand behaviour and to aid in the future biomathematical interpretation of the kinetics of regional cerebral radioactivity uptake in terms of receptor-binding parameters, HPLC was used to measure [carbonyl-11C]WAY-100635 and its radioactive metabolites in plasma at various times after intravenous injection. Radioactivity cleared rapidly from monkey and human plasma. Parent radioligand represented 19% of the radioactivity in monkey plasma at 47 min and 8% of the radioactivity in human plasma at 40 min. [Carbonyl-11C]desmethyl-WAY-100635 was below detectable limits in monkey plasma and at most a very minor radioactive metabolite in human plasma. [11C]Cyclohexanecarboxylic acid was identified as a significant radioactive metabolite. In human plasma this maximally represented 21% of the radioactivity at 10 min after radioligand injection. All other major radioactive metabolites in monkey and human plasma were even more polar. No-carrier-added [carbonyl-11C]cyclohexanecarboxylic acid was prepared in the laboratory and after intravenous administration into cynomolgus monkey was shown with PET to give only a low uptake of radioactivity into brain tissue. The acid rapidly gave rise to several radioactive metabolites of higher polarity in plasma. The observed lack of any significant metabolism of [carbonyl-11C]WAY-100635 to highly lipophilic or pharmacologically potent radioactive compounds is consistent with its high signal contrast in primate brain.

In vivo saturation kinetics of two dopamine transporter probes measured using a small animal positron emission tomography scanner.

When estimated in vitro, the parameters which describe the binding of radiolabelled analogues of cocaine to sites on the dopamine transporter are very much influenced by the methodology used. In the present study, a small animal positron emission tomography (PET) scanner was used to estimate in vivo saturation kinetics for two carbon-11 labelled compounds presently used to monitor dopamine terminal function. The binding of [11C]CFT (WIN 35,428) in rat striatum was adequately described by a single-site model, giving an apparent dissociation constant corresponding to an intravenous dose of 242 nmol/kg. In contrast, the binding of [11C]RTI-121 was better described by a two-site model with the 'high-affinity' site or state (dissociation constant = 1 nmol/kg) being significantly occupied at doses routinely used in PET scanning. Such findings cannot readily be predicted from in vitro work, but could aid in both the choice of ligand and the model used in quantification of scan data. While multi-dose in vivo PET studies are difficult in man, rat PET can easily be employed either pre-clinically for putative radioligands, or experimentally, to study drug interactions and receptor occupancy related to functional efficacy.