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Pharmaceutical Co. (Osaka, Japan). Nuclear magnetic resonance (NMR)
spectra were recorded on a JEOL JNM-ECX400P spectrometer (Tokyo,
Japan) at ambient temperature. Signal patterns are indicated as follows: s,
singlet; d, doublet; t, triplet; q, quartet; m, multiplet; and br, broad signal.
Coupling constants (J values) are given in hertz (Hz). Carbon-11 was pro-
duced by an 14N(p, a)11C nuclear reaction using a CYPRIS HM-12S Cy-
clotron (Sumitomo Heavy Industry, Tokyo, Japan). An automated radiolabel-
ing system was used for heating the reaction mixture, dilution, HPLC injec-
tion, fraction collection, evaporation, and sterile filtration. Purification with
semipreparative HPLC was performed on a JASCO system (Tokyo, Japan).
Analytical HPLC was performed on a Shimadzu system (Kyoto, Japan)
equipped with pumps and a UV detector, and effluent radioactivity was
measured with an RLC700 radio analyzer (Aloka, Tokyo, Japan). The
columns used for analytical and semipreparative HPLC were COSMOSIL
Fig. 1. Representative Chromatogram of [11C-Carbonyl] 1
UV absorption: 288 nm. Left: HPLC for purification of reaction mixture. Right: Ana-
lytical HPLC after preparative HPLC purification.
added at the same temperature. After 1 min at 30 °C, the re-
action gave a complex mixture in which the desired [11C-
methyl]ramelteon accompanied by many kinds of side prod-
C
18 AR-II (Nacalai Tesque).
(S)-N-[2-(1,6,7,8-Tetrahydro-2H-indeno[5,4-b]furan-8-yl)ethyl]-
acetamide (3) (S)-2-1,6,7,8-Tetrahydro-2H-indeno[5,4-b]furan-8-yl)-
ucts was observed. Consequently, the chemical purity of iso- ethylamine hydrochloride (2)17) (1.0 g, 4.2 mmol) was dissolved in 2 M
lated [11C-methyl]ramelteon was extremely low (ꢂ28% at
UV absorbance, lꢁ288 nm). The introduction of a protect-
ing group to improve this synthetic route requires additional
NaOH (10 ml) and THF (10 ml). Acetyl chloride (330 mg, 4.2 mmol) was
added to the solution. It was stirred at room temperature for 30 min and then
extracted with ethyl acetate (10 ml, 3 times). Combined organic layers were
dried over sodium sulfate and evaporated to dryness under vacuum. The
work for the deprotection to elongate the time which could
be disadvantageous for the synthesis of a short-lived PET
probe. Therefore, such trial was thus abandoned.
crude product was purified by flash chromatography eluting with a ratio of
dichloromethane : ethyl acetate of 2 : 1 to give 3 (932 mg, 90%) as a white
solid. 1H-NMR (400 MHz, CDCl3) d: 6.94 (d, Jꢁ8.0 Hz, 1H), 6.60 (d,
Jꢁ8.0 Hz, 1H), 5.72 (br s, 1H), 4.67—4.47 (m, 2H), 3.35—3.07 (m, 5H),
2.90—2.74 (m, 5H), 2.31—2.22 (m, 1H), 2.05—1.98 (m, 1H), 1.96 (s, 3H),
1.83—1.88 (m, 1H), and 1.66—1.59 (m, 1H). [a]D24 ꢀ85.1° (cꢁ0.52,
ethanol). MS (electrospray ionization (ESI)) m/z: 246 [MꢄH]ꢄ, 244
[MꢀH]ꢀ.
On the contrary, the synthesis of [11C-methyl]ramelteon by
the route B was quite successful. Thus, according to the
method reported by Luthra and co-workers,16) [11C]propionyl
chloride was prepared by reacting [11C]CO2 with ethyl mag-
nesium bromide and then with phthaloyl chloride in the exis-
tence of di-tert-butylpyridine. The resulting [11C]propionyl
chloride was distilled into an acetonitrile solution of 2 and
triethylamine at ꢀ10 °C. The acylation reaction was com-
pleted as soon as the [11C]propionyl chloride was introduced
in the reaction mixture. The total synthesis time was 22—
26 min from the end of bombardment (EOB) to the pharma-
ceutical formulation. A decay-corrected radiochemical yield
[11C-Carbonyl]ramelteon ([11C-Carbonyl] 1) [11C]CO2 was trapped
from the gas target in a column containing molecular sieves (300 mg) at
30—50 °C. After trapping, the column was warmed to 180 °C, the released
[11C]CO2 was bubbled into a diethyl ether solution of 0.1 M ethyl magnesium
bromide (1.0 ml) at ꢀ10 °C with a flow of He of 20 ml/min. When transfer
of radioactivity was complete, the He flow was stopped and a solution of
phthaloyl dichloride (200 ml) and 2,5-di-tert-butylpyridine (100 ml) in dry di-
ethyl ether (100 ml) was added. The mixture was heated to 130 °C, and the
formed [11C]propionyl chloride was distilled into a solution of amine hy-
drochloride 2 (0.5—0.7 mg) and triethylamine (50 ml) in acetonitrile (500 ml)
based on [11C]CO2 was 22—43% (nꢁ5) and the isolated ra- at ꢀ10 °C with a flow of He of 50 ml/min. The mixture was placed at 0 °C
dioactivity was 4.8—9.0 GBq (nꢁ5, at end of synthesis) with
ꢃ99% radiochemical and chemical purities. Specific ra-
for 1 min and then diluted with HPLC eluent (1 ml). The reaction mixture
was purified by reverse-phase HPLC (COSMOSIL C18 AR-II 10 mm
(i.d.)ꢅ250 mm, 5 ml/min, with a 1 : 1 ratio of CH3CN : 10 mM HCOONH4).
The desired fraction (retention timeꢁ5—6 min) was collected into a flask
dioactivity at the end of synthesis was 43—162 GBq/mmol
(nꢁ5) (Fig. 1). The chemical identity of [11C]ramelteon was
containing ascorbic acid (200 ml), evaporated to dryness under vacuum, and
the residue was taken up in polysorbate 80 : propylene glycol : saline with a
0.1 : 1 : 10 volume ratio (4 ml). The formulated solution was used to establish
the chemical and radiochemical purity and specific radioactivity by analyti-
cal HPLC (COSMOSIL C18 AR-II, 4.6 mm (i.d.)ꢅ150 mm, 1 ml/min, and
CH3CN : 100 mM HCOONH4 at a 50 : 50 ratio). The total synthesis time was
22—26 min and 5.0—9.0 GBq of [11C]ramelteon was constantly isolated
with 43—162 GBq/mmol of the specific radioactivity (nꢁ5) with a good
radiochemical yield (22—43%, decay corrected based on [11C]CO2). Chemi-
cal purity (UV; lꢁ288 nm) and radiochemical purity were always greater
than 99%.
confirmed by co-injection with ramelteon on an analytical
HPLC. Thus, efficient synthesis of [11C-carbonyl]ramelteon
from the corresponding primary amine precursor 2 by
[11C]propionylation was achieved.
Conclusion
In summary, ramelteon, a selective melatonin receptor ag-
onist ramelteon was efficiently labeled with 11C. Comprising
two labeling methods, [11C]methylation and [11C]acylation,
the latter was much superior, giving [11C]ramelteon in 22—
43% decay-corrected radiochemical yield from [11C]CO2
with ꢃ99% chemical and radiochemical purities. Isolated ra-
dioactivity was 4.8—9.0 GBq sufficient for both animal and
human PET studies. The total synthesis time was 22—26 min
[11C-Methyl]ramelteon ([11C-Methyl] 2) [11C]CO2 was converted to
[11C]CH3I via a reaction with 0.1 M lithium aluminum hydride in THF and
n
subsequent reaction with hydroiodic acid.18) 1.6 M BuLi (0.1 ml) was added
to a solution of 3 (5 mg) in anhydrous THF (0.4 ml) at ꢀ15 °C and it was left
for 10 min. Then, [11C]CH3I was introduced to the mixture at the same tem-
perature. The mixture was left at 30 °C for 1 min and then it was quenched
with 10% HCOOH in acetonitrile (1.0 ml). The mixture was purified by the
and the specific radioactivity at the end of synthesis was HPLC condition described above. The total synthesis time was 24—27 min
to give [11C]ramelteon with ꢂ28% chemical purity (UV; lꢁ288 nm) and
ꢃ98% radiochemical purity (nꢁ2).
43—162 GBq/mmol. The resulting [11C]ramelteon could be
used as a sharper PET probe for in vivo studies on melatonin
receptors involved in circadian rhythms.
Acknowledgements This work was supported in part by a consignment
expense for the Molecular Imaging Program on “Research Base for Explor-
ing New Drugs” from the Ministry of Education, Culture, Sports, Science
and Technology (MEXT) of Japan. We would like to thank Mr. Masahiro
Kurahashi (Sumitomo Heavy Industry Accelerator Service Ltd.) for operat-
ing the cyclotron.
Experimental
All chemicals and solvents were purchased from Wako Pure Chemical In-
dustries (Osaka, Japan), Tokyo Kasei Kogyo (Tokyo, Japan), Nacalai Tesque
(Kyoto, Japan), and ABX (Radeberg, Germany), and were used without fur-
ther purification. Ramelteon for cold standards was purchased from Takeda
Takashima-Hirano, Misato
