Asymmetric synthesis of (2S)-and (2R)-4-(3-t-butylamino-2-hydroxypropoxy)-benzimidazol-2-[11C]-one ((S)-and (R)-[11C]-CGP 12177) from optically active precursors

[11C]phosgene: Synthesis and application for development of PET radiotracers

Carbon-11-labeled phosgene ([11C]phosgene, [11C]COCl2) is a useful labeling agent that connects two heteroatoms by inserting [11C]carbonyl (11C=O) function in carbamates, ureas, and carbonates, which are components of biologically important heterocyclic compounds and functional groups in drugs as a linker of fragments with in vivo stability. Development of 11C-labeled PET tracers has been performed using [11C]phosgene as a labeling agent. However, [11C]phosgene has not been frequently used for 11C-labeling because preparation of [11C]phosgene required dedicated synthesis apparatus (not commercially available) and had problems in reproducibility and reliability. In our laboratory, an improved method for synthesizing [11C]phosgene using a carbon tetrachloride detection tube kit in environmental air analysis and the automated synthesis system for preparing [11C]phosgene have been developed in 2009. This apparatus has been used for routine synthesis of 11C-labeled tracers 1-4 times/week. Using [11C]phosgene we have developed and produced many PET radiotracers containing [11C]urea and [11C]carbamate moieties. In this review, we report the performance of our method for preparing [11C]phosgene, including automated synthesis apparatus developed in house, and the application of [11C]phosgene for development and production of 11C-labeled PET tracers.

Assessment of the Effects of Age, Gender, and Exercise Training on the Cardiac Sympathetic Nervous System Using Positron Emission Tomography Imaging

BACKGROUND

Using positron emission tomography (PET) imaging, we sought to determine whether normal age or exercise training cause changes in the cardiac sympathetic nervous system function in male or female healthy volunteers.

METHODS

Healthy sedentary participants underwent PET studies before and after 6 months of supervised exercise training. Presynaptic uptake by the norepinephrine transporter-1 function was measured using PET imaging of [(11)C]-meta-hydroxyephedrine, a norepinephrine analog, and expressed as a permeability-surface area product (PSnt in mL/min/mL). Postsynaptic function was measured as β-adrenergic receptor density (β'max in pmol/mL tissue) by imaging the β-receptor antagonist [(11)C]-CGP12177. Myocardial blood flow (MBF in mL/min/mL tissue) was measured by imaging [(15)O]-water.

RESULTS

At baseline, there was no age difference in β'max or MBF but PSnt declined with age (1.12±0.11 young vs 0.87±0.06 old, p = .036). Before training, women had significantly greater MBF (0.87±0.03 vs 0.69±0.03, p < .0001) and PSnt (1.14±0.08 vs 0.75±0.07, p < .001) than men. Training increased VO2 max by 13% (p < .0001), but there were no training effects on β'max, PSnt, or MBF. Greater MBF in females and a trend to increased PSnt post-training persisted.

CONCLUSION

With age, presynaptic uptake as measured by PSnt declines, but there were no differences in β'max. Endurance training significantly increased VO2 max but did not cause any changes in the measures of cardiac sympathetic nervous system function. These findings suggest that significant changes do not occur or that current PET imaging methods may be inadequate to measure small serial differences in a highly reproducible manner.

A Study on the Highly Efficient Synthesis and Pharmaceutical Evaluation of PET Radiopharmaceuticals for the Clinical Application

Positron Emission Tomography (PET) is an advanced non-invasive technology used in the field of nuclear medicine for clinical diagnosis using radiotracers labeled with short-lived positron emitting radionuclides such as (11)C (half-life: 20.4 min), (13)N, (15)O and (18)F. The present study describes an efficient rapid synthesis method for [(11)C]Phosgene ([(11)C]COCl(2)) which is an important potential precursor for preparation of PET radiopharmaceuticals. Catalytic oxidation of [(11)C]CCl(4) using Fe(2)O(3) powder mixed with Fe granules as an oxidizing agent was newly accomplished with a development of fully automated synthetic apparatus. Utilization of produced [(11)C]COCl(2) provided a substantial synthesis of [2-(11)C]thymine as a key intermediate for preparation of [2-(11)C]thymidine, a PET tracer to evaluate cellular proliferation. Direct ring closure reaction of the alkali metal salt of beta-(N-benzoyl-amino)methacrylamide with [(11)C]COCl(2) readily proceeded under mild conditions to afford [2-(11)C]thymine in fair yield reproducibly. By way of further application, a useful PET ligand for beta-adrenoreceptors, S-(-)-[(11)C]CGP-12177 (CGP) was synthesized in markedly high yield with high specific activity and radiochemical purity. CGP for intravenous injection was prepared in 25 min after EOB with a yield of 1.5+/-0.2 GBq. These results of quality control tests demonstrated that CGP preparation is suitable for routine clinical use. Thus, CGP-PET study has been newly added to clinical PET for cardiac functional investigation in Hokkaido University Hospital.

Preparation and pharmaceutical evaluation for clinical application of high specific activity S-(-)[11C]CGP-12177, a radioligand for beta-adrenoreceptors

BACKGROUND

Although S-(-)[C]CGP-12177 is a useful positron emission tomography (PET) ligand for beta-adrenoreceptors, the difficulty in radiolabelling the compound has prevented its extensive clinical application. Recently, we have developed a simple synthesis method for S-(-)[C]CGP-12177. In the present study, we attempted to prepare S-(-)[C]CGP-12177 with a high specific activity for intravenous injection which is feasible for the clinical evaluation of beta-adrenoreceptors.

METHODS

The [C]methane produced during irradiation of a N2--H2 (95/5) mixture with an 18 MeV proton beam (20 microA, 30 min) was chlorinated using Cl2 to yield [C]carbon tetrachloride. S-(-)[C]CGP-12177 was synthesized by reacting the diamino precursor with [C]phosgene produced by oxidizing [C]carbon tetrachloride on a Fe--Fe2O3 column. The product was purified by using reversed phase, high-performance liquid chromatography (RP-HPLC) and the radioactive fraction containing S-(-)[C]CGP-12177 was collected and evaporated to dryness. S-(-)[C]CGP-12177 dissolved in physiological saline was sterilized through a 0.22 microm membrane filter. The radiochemical purity and the mass of the compound were determined with RP-HPLC. The residual organic solvents were determined with GC. Tests for sterility and the presence of bacterial endotoxins were also performed.

RESULTS

S-(-)[C]CGP-12177 for intravenous injection was prepared in 25 min after the end of bombardment with a yield of 1.5+/-0.2 GBq. Specific activity was found to be 385.4+/-133.0 GBq/ micromol at the end of synthesis (EOS) (n=3). Radiochemical purity was found to be more than 99%. Toluene was not detected in the solution. The ethanol concentration was determined to be 60.3+/-52.5 ppm. Tests for sterility and bacterial endotoxins showed negative results.

CONCLUSION

We successfully prepared S-(-)[C]CGP-12177 formulated for intravenous injection with high purity and high specific activity, which is feasible for the clinical evaluation of beta-adrenoreceptors.

PET measures of pre- and post-synaptic cardiac beta adrenergic function

Positron Emission Tomography was used to measure global and regional cardiac beta-adrenergic function in 19 normal subjects and 9 congestive heart failure patients. [(11)C]-meta-hydroxyephedrine was used to image norepinephrine transporter function as an indicator of pre-synaptic function and [(11)C]-CGP12177 was used to measure cell surface beta-receptor density as an indicator of post-synaptic function. Pre-synaptic, but not post-synaptic, function was significantly different between normals and CHF patients. Pre-synaptic function was well matched to post-synaptic function in the normal hearts but significantly different and poorly matched in the CHF patients studied. This imaging technique can help us understand regional sympathetic function in cardiac disease.

Synthesis, radiosynthesis and In vivo evaluation of 5-[3-(4-Benzylpiperidin-1-yl)prop-1-ynyl]-1,3-dihydrobenzoimidazol-2-[11C]one, as a potent NR1A/2B subtype selective NMDA PET radiotracer

Recently, a new series of potent and highly subtype-selective 1-(heteroarylalkynyl)-4-benzylpiperidine antagonists of the NMDA receptors has been described by Pfizer Laboratories. In this series, 5-[3-(4-benzylpiperidin-1-yl)prop-1-ynyl]-1,3-dihydrobenzoimidazol-2-one (1) was identified as a selective antagonist for the NR1(A)/2B subtype, displaying IC(50) values for inhibition of the NMDA responses of 5.3 nM for this subtype (compared to NR1(A)/2A: 35 microM and NR1(A)/2C>100 microM) and was active in rat at a relatively low dosage (10mg/kg po). Derivative 1 has been synthesized in four chemical steps in good overall yield and labelled with carbon-11 at its benzoimidazolone ring using [(11)C]phosgene. The pharmacological profile of [(11)C]-1 was evaluated in vivo in rats with biodistribution studies and brain radioactivity monitored with intracerebral radiosensitive beta-microprobes. The brain uptake of [(11)C]-1 was extremely low (0.07% I.D./mL on average at 30 min) and rather uniform across the different brain structures. This in vivo brain regional distribution of [(11)C]-1 did not match with autoradiographic or binding data obtained with other NR2B subtype-selective NMDA ligands. Competition studies with ifenprodil (20 mg/kg, ip, 30 min before the radiotracer injection) failed to demonstrate specific binding of the radiotracer in the brain. In view of these results, and especially considering the low brain penetration of the radiotracer, [(11)C]-1 does not have the required properties for imaging NMDA receptors using positron emission tomography.

Debenzylation of tertiary amines using phosgene or triphosgene: an efficient and rapid procedure for the preparation of carbamoyl chlorides and unsymmetrical ureas. Application in carbon-11 chemistry

Efficient and rapid preparations of carbamoyl chlorides and unsymmetrical ureas from tertiary amines and phosgene or its safe equivalent triphosgene [bis(trichloromethyl)carbonate, BTC] are described. First, the reaction of stoichiometric amounts of phosgene with secondary amines was revisited, and it was shown that the formation of carbamoyl chlorides in high yields required careful adjustments of experimental conditions and the use of pyridine as an HCl scavenger. A phosgene-mediated dealkylation of triethylamine was observed when this base was used instead of pyridine. Taking advantage of this observation, a strategy of synthesis of carbamoyl chlorides from tertiary amines and phosgene has been developed. N-Alkyl-N-benzyl(substituted)tetrahydroisoquinolines, -piperazines, -piperidines, or -anilines were treated with stoichiometric amounts of phosgene (or BTC) in CH(2)Cl(2). Tertiary amines bearing electron-enriched benzyl group(s) afforded carbamoyl chlorides in excellent yields and without any contamination by symmetrical ureas. Subsequent additions of primary or secondary amines to these carbamoyl chlorides produced unsymmetrical ureas in single-pot and high-yielding operations. This methodology was applied in (11)C-chemistry. From [(11)C]phosgene, a common precursor used in the preparation of radiotracers for positron emission tomography, a rapid and efficient synthesis of (11)C-carbamoyl chlorides and (11)C-unsymmetrical ureas derived from tetrahydroisoquinoline and piperazine is described. The first example of (11)C-amide formation from the reaction of a (11)C-carbamoyl chloride and an organometallic (cyanocuprate or a Grignard reagent in the presence of a nickel catalyst) is also presented.

A simplified and improved synthesis of [11C]phosgene with iron and iron (III) oxide

[11C]Phosgene ([11C]COCl2), a useful precursor for labeling several radiopharmaceuticals, is generally produced by catalytic oxidation of [11C]carbon tetrachloride over Fe granules, although in low yields or with poor reproducibility. In order to develop am improved synthesis of [11C]phosgene, two oxidizing agents, Fe2O3 and CuO, were examined. The yield of [11C]phosgene was significantly increased using Fe2O3 powder mixed with Fe granules, while the use of CuO alone, or CuO powder mixed with Fe granules resulted in an insignificant yield. The yield and specific activity of S- (-) [11C]CGP-12177 synthesized using Fe2O3 powder mixed with Fe granules were markedly higher than those synthesized by the previous methods using Fe granules alone or Fe granules mixed with Fe powder. Thus, in the present study, we developed a simple and practical method for the synthesis of [11C]phosgene, which provided an improved yield of S- (-) [11C]CGP-12177.

Synthesis and in vivo evaluation of [11C]ICI 118551 as a putative subtype selective beta2-adrenergic radioligand

Erytro-(+/-)-1-[2,3-(dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[ iso-propylamino]-2-butanol (ICI 118551) a potent clinically used beta2 adrenergic antagonist, was labelled with carbon-11 (t1/2 = 20.4 min) as a potential radioligand for the non-invasive assessment of beta2 adrenergic receptors in the lung with positron emission tomography (PET). The radiolabelled compound was prepared by reductive N-alkylation of its des-isopropyl precursor with [2-11C]acetone. (+/-)-[11C]ICI 118551 was obtained in greater than 98% radiochemical purity in 30 min with a radiochemical yield of 15 + 5% (non-decay corrected) and a specific radioactivity 2.5 +/- 0.5 Ci/micromol. The biological evaluation of racemic erythro (+/-)-[11C]ICI 118551 in rats and Macaca Nemestrina shows a high radioactivity uptake in lung and heart. However, in both animal models no detectable displacement of lung radioactivity concentration was observed after pre-treatment with propranolol or ICI 118551, which indicates that in this organ, radioligand uptake is mostly due to non-specific binding. The biological data suggest that erythro (+/-)-[11C]ICI 118551 is not adequate to be further developed as a tracer for beta2 adrenergic receptor imaging in vivo.

Selection, design and evaluation of new radioligands for PET studies of cardiac adrenoceptors

Changes in the numbers of human cardiac adrenoceptors (ARs) are associated with various diseases, such as myocardial ischemia, congestive heart failure, cardiomyopathy and hypertension. There is a clear need for capability to assess human cardiac ARs directly in vivo. Positron emission tomography (PET) is an imaging technique that provides this possibility, if effective radioligands can be developed for the targeted ARs. Here, the status of myocardial AR radioligand development for PET is described. Currently, there exist effective radioligands for imaging beta-ARs in human myocardium. One of these, [11C](S)-CGP 12177, is applied extensively to clinical research with PET, sometimes with other tracers of other aspects of the noradrenalin system. Alternative radioligands are in development for beta-ARs, including beta 1-selective radioligands. A promising radioligand for imaging myocardial alpha 1-ARs, [11C]GB67, is now being evaluated in human PET experiments.

Synthesis and in vitro evaluation of [carbonyl-11C]estramustine and [carbonyl-11C]estramustine phosphate

[carbonyl-11C]Estramustine and [carbonyl-11C]estramustine phosphate were synthesized from [11C]phosgene using a one pot procedure. [carbonyl-11C]Estramustine was obtained in 31-43% decay corrected yield based on radioactivity trapped in the reaction vessel. The product was obtained 25 min after the end of radionuclide production with a specific radioactivity of 0.38-1.11 Ci/mumol. A method was developed yielding [carbonyl-11C]estramustine phosphate in 29-45% decay corrected yield based on trapped radioactivity, without purification of the [carbonyl-11C]estramustine intermediate. The product was obtained within 40 min of the end of radionuclide production with a specific radioactivity of 0.59-0.86 Ci/mumol. Results from in vitro experiments suggest that because of their high nonspecific binding, the compounds are unsuitable for positron emission tomography as imaging agents for the estramustine binding protein in cancer.

A novel beta-adrenoceptor ligand for positron emission tomography: evaluation in experimental animals

Myocardial and pulmonary beta-adrenoceptors can be imaged and quantified with the antagonist (S)-4-[3[(1,1-dimethylethyl)amino]-2-hydroxypropoxy]-1,3-dihydro-2H-b enzimidazol-2-[11C]-one (S-[11C]CGP-12177). The synthesis of this ligand (based on the reaction of a precursor with [11C]phosgene) is laborious and in many centers the final product has a low and variable specific activity. This prevents widespread use of S-[11C]CGP-12177 for studies in patients. We prepared S-[11C]CGP-12388, the isopropyl analogue of CGP-12177, by a reliable one-pot procedure and evaluated the radiopharmaceutical for beta-adrenoceptor imaging. Blocking experiments with subtype-selective beta-adrenergic drugs showed that myocardial and pulmonary uptake of S-[11C]CGP-12388 in anesthetized rats reflects ligand binding to beta1- and beta2-adrenoceptors. In this animal model, clearance, metabolism and tissue/plasma ratios of S-[11C]CGP-12388 were similar to those of S-[11C]CGP-12177. A [18F]fluoroisopropyl analogue of CGP-12177 showed less favorable characteristics. S-[11C]CGP-12388 was therefore selected for evaluation in humans and it may become the tracer of choice for clinical studies since it is easily prepared.

Development of radioligands for the imaging of cardiac beta-adrenoceptors using SPECT. Part I: Asymmetric synthesis and structural characterization of five new iodine-containing beta-adrenoceptor antagonist derivatives

The asymmetric synthesis of a series of iodinated beta-adrenoceptor ligands is described. One ligand, (S)-(-)-[1-(2-iodophenoxy)]-3'-(tert-butylamino)-2'-propanol (CYBL3), is based on the beta-adrenoceptor antagonist penbutolol. The other ligands are N-iodovinyl and N-iodoaryl analogues of the beta-adrenoceptor antagonist CGP12177. These have been synthesized from 2-amino-3-nitrophenol. Furthermore, radioiodinated [123I]CYBL3 and [123I](2'S,2"E)-[4-(3'-(1",1"-dimethyl-3"-iodo-2" propenylamino)-2'-hydroxy propoxy)]-benzimidazol-2-one have been prepared by radiolabelling the corresponding trialkyltin precursors using [123I]-NaI in the presence of hydrogen peroxide.

In vivo effect of methyl-quinuclidinyl-benzylate on myocardial beta-adrenoceptor density

The muscarinic receptor antagonist methyl-quinuclidinyl-benzylate decreased myocardial beta-adrenoceptor density Bmax: 20.4 +/- 2.4 pmol/ml tissue versus 33.3 +/- 4 pmol/ml tissue in control dogs (P < 0.001), as assessed by using [11C]CGP-12177 (((2S)-4-(3-t-butyl-amino-2 hydroxypropoxy)-benzimidazol-2-one)) and positron emission tomography. In contrast, atropine did not induce any change in Bmax: 33.7 +/- 3.6 pmol/ml tissue. We hypothetized that methyl-quinuclidinyl-benzylate induced the release of norepinephrine from sympathetic nerve terminals, an effect which could be blocked by guanethidine. Guanethidine alone (10 mg/kg) did not change Bmax: 35.5 +/- 6 pmol/ml tissue. Guanethidine + methyl-quinuclidinyl-benzylate did not induce any significant change in Bmax: 31.5 +/- 5.1 pmol/ml tissue. Therefore, it seems likely that methyl-quinuclidinyl-benzylate acts at the presynaptic level, probably inducing the release of norepinephrine which then causes a down-regulation of beta-adrenoceptors.

Synthesis and preliminary evaluation of (R,S)-1-[2-((carbamoyl-4-hydroxy)phenoxy)-ethylamino]-3-[4-(1-[11C]-met hyl-4-trifluoromethyl-2-imidazolyl)phenoxy]-2-propanol ([11C]CGP 20712A) as a selective beta 1-adrenoceptor ligand for PET

The most selective beta 1-adrenoceptor ligand known at this moment is (S)-1-[2-((carbamoyl-4-hydroxy) phenoxy)ethylamino]-3-[4-(1-methyl-4-trifluoromethyl-2-imidazolyl) phenoxy]-2-propanol (CGP 26505), the S-isomer of CGP 20712A. We prepared the racemic 11C analogue by methylation with [11C]CH3I of the corresponding desmethyl compound using a microwave oven to accelerate the reaction. Several radioactive by-products (about 70% of the non-volatile radioactive products) were formed. After HPLC purification [11C]CGP 20712A with a specific activity of 35 TBq/mmol was dissolved in a propylene glycol-ethanol-saline mixture to prepare it for injection. The total preparation time was 35 min. The radiochemical yield was 5% (calculated from [11C]CH3I, not corrected for decay). The identity of [11C]CGP 20712A was proved by liquid chromatography-mass spectrometry (LC-MS). Tissue distribution studies in male Wistar rats have been performed. At 20 min after injection of the radioligand (0.1 nmol) the DAR [differential absorption ratio = (counts per minute recovered/g tissue)/(counts per min injected/g body weight)] in heart tissue decreased significantly (P < 0.005) from 1.84 +/- 0.11 to 1.21 +/- 0.12 after blocking of beta-adrenoceptors with 500 micrograms (R,S)-propranolol. A preliminary PET study in a Wistar rat showed maximal uptake in the time frame 10-20 min after injection. The ratio of specific/non-specific binding at this interval was 2.6.

Synthesis and evaluation of 1'-[18F]fluorometoprolol as a potential tracer for the visualization of beta-adrenoceptors with PET

(+/-)-1'-[18F]Fluorometoprol 4 was prepared from desisopropylmetoprolol and [18F]fluoroisopropyl tosylate 2 with a radiochemical yield of 2% [corrected for decay to end of bombardment (EOB), synthesis time 90 min]. Synthon 2 was prepared from (S)-1,2-propanediol di(p-toluenesulfonate) in 45% radiochemical yield (EOB, 40 min). Compound 4 shows in two in vitro assays a similar affinity at beta-adrenoceptors (about 0.3 microM) as metoprolol 5, but with a slightly higher beta 1/beta 2-adrenoceptor selectivity ratio (48.6 vs 30.7). In vivo experiments with 4 showed almost no receptor-mediated uptake in the heart, probably because the affinity of (fluoro)metoprolol for the beta 1-adrenoceptors is too low for successful imaging. However, the in vitro experiments suggest that the fluoroisopropyl group is suitable for the synthesis of [18F]fluorinated beta 1-adrenergic receptor binding ligands.

Positron emission tomography with 11C CGP-12177 to assess beta-adrenergic receptor concentration in idiopathic dilated cardiomyopathy

BACKGROUND

Positron emission tomography (PET) with 11C-labeled CGP-12177 (CGP) has been shown to have the potential to noninvasively measure beta-adrenergic receptor concentration in dog heart. The present study was undertaken to evaluate the clinical value of this technique.

METHODS AND RESULTS

Eight normal subjects and 10 patients with heart failure related to an idiopathic cardiomyopathy were studied. Estimation of beta-receptor concentration was based on a graphic method applied on myocardial PET time-concentration curves obtained after an intravenous injection of 11C-CGP followed 30 minutes later by a coinjection of labeled and unlabeled CGP. The clinical tolerance of these injections was good. Left ventricular concentration of beta-receptors was decreased in patients compared with controls (3.12 +/- 0.51 versus 6.60 +/- 1.18 pmol/mL, respectively; p < 0.001). This 53% decrease agrees with previous in vitro data. In eight of the 10 patients, the beta-receptor concentration obtained from PET was compared with the beta-receptor density determined on left ventricular endomyocardial biopsy samples by in vitro binding technique using 3H-CGP-12177. Results obtained with both techniques were correlated (r = 0.79, p = 0.019). Moreover, decreased beta-receptor concentration correlated with the beta-contractile responsiveness to intracoronary dobutamine infusion (r = 0.83, p = 0.003), indicating a direct link between changes in the receptor number and its biological function.

CONCLUSIONS

PET appears to be a safe and reliable method of assessing in vivo changes in the number of left ventricular beta-adrenergic receptor sites of patients with idiopathic cardiomyopathy.