Synthesis of carbon-11 labelled SCH 39166, a new selective dopamine D-1 receptor ligand, and preliminary PET investigations

Appl. Radiat. ht.

Vol. 42, No. 5, pp. 451455,

1991

0883-2889/91 $3.00 + 0.00 Copyright 0 1991 Pergamon Press plc

ht. J. Radiar.Appl. Instrum.Part A Printed in Great Britain. All rights reserved

Synthesis of Carbon-l 1 Labelled SCH 39166, a New Selective Dopamine D-l Receptor Ligand, and Preliminary PET Investigations CHRISTER

‘Department Psychiatry

HALLDIN’*, LARS FARDE’,*, ALLEN and GORAN SEDVALL’

BARNETT3

of Psychiatry and Psychology and ‘Ulf Lundahl Research Unit at the Department and Psychology, Karolinska Hospital, 10401 Stockholm, Sweden and ‘Schering-Plough Corporation, Bloomfield, NJ, U.S.A.

of

(Received 24 September 1990) SCH 39166 [(-)-trans-6,7,7a,8,9,13b-hexahydro-3-chloro-2-hydroxy-~-methyl-5~-benzo(d)naphtho(2,1-b)azepine] is a new more selective dopamine D-l receptor antagonist than the widely used SCH 23390. [“CISCH 39166 was prepared by N-methylation of the desmethyl compound SCH 40853 [(-)-fmns6,7,7a,8,9,13b-hexahydro-3-chloro-2-hydroxy-5H-benzo(d)naphtho-(2,l-b)azepine] with [“C]methyl iodide. Reaction in acetone with subsequent straight-phase semi-preparative HPLC resulted in 20-30% radiochemical yield (from EOB and decay-corrected) with a total synthesis time of 35-40 min and a radiochemical purity >99%. The specific activity obtained at EOS was about 1500 Ci/mmol (55 GBq/pmol). [“CISCH 39166 was injected into a Cynomolgus monkey. PET-analysis demonstrated accumulation in the striatum, a region known to have a high density of dopamine D-l receptors. In a displacement experiment, radioactivity in the striatum was markedly reduced after injection of 6 mg unlabelled SCH 23390, thus demonstrating the specificity and reversibility of [“CISCH 39166 binding to dopamine D-l receptors.

Introduction for the endogenous neurotransmitter dopamine exist in at least two distinct subtypes, the dopamine D-l and the D-2 receptors (Kebabian and Calne, 1979). The examination of central dopamine D-l receptor characteristics and function has been hampered because of the previous lack of compounds which selectively binds to dopamine D- 1 receptors. A few years ago the benzazepine SCH 23390 [(R)-( +)8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-lH-3benzazepin-7-01) was described as a potent dopamine D-l receptor antagonist (Iorio, 1981, 1983; Hyttel, 1983). [3H]SCH 23390 was shown to bind selectively to D-l receptors in membranes of the rat and human striatum (Billard et al., 1984; Raisman et al., 1985; Schultz et al., 1985; Pimoule et al., 1985). In animal experiments the dopamine D-l receptors mediate behavioral effects [for a review see Waddington and O’Boyle (1989)]. The pharmacological effect of a selective D-l antagonist on schizophrenic patients has not yet been examined. PET studies with the radiobrominated analog [76Br]SCH 23390 indicated that the analog Receptors

*Author

for correspondence. 451

accumulated in the striatum of the monkey brain (Friedman et al., 1985). [“C]SCH 23390 (Fig. 1) has been prepared from [“C]methyl iodide (Halldin et al., 1986; DeJesus et al., 1987; Ravert et aI., 1987). The usefulness of [“CISCH 23390 as ligand for PETanalysis of central dopamine D- 1 receptor binding in monkey and man is well established (Halldin et al., 1986; Sedvall et al., 1986; Farde et al., 1987). However, [“CISCH 23990 has significant affinity also for 5HT,-receptors and is rapidly metabolized (SO-90%) during the time of a PET-experiment (1 h). The benzonaphthazepine SCH 39166 [(-)-tram6,7,7a,8,9,13b-hexahydro-3-chloro-2-hydroxy-Nmethyl-5H-benzo(d)naphtho-(2,1-b)azepine] has recently been characterized both in vitro and in vivo and has been demonstrated to be a selective dopamine D-l antagonist (Chipkin et al., 1988). In animals SCH 39166 is more slowly metabolized than SCH 23990. Both SCH 23390 and SCH 39166 have nanomolar affinity for dopamine D-l receptors. However, SCH 39166 has an about 25-fold lower affinity for SHT,-receptors and is thus a more selective dopamine D-l receptor ligand than SCH 23390. The compound is undergoing clinical trials as a potential antipsychotic drug.

CHRISTER HALLDIN er al

452

N- “CH,

[“C]SCH

23390

“CH,

0 [“C]SCH Fig. 1. Structural

formulas

39166

of [“C]SCH 39166.

23390 and [“CISCH

In contrast to SCH 23390. SCH 39166 has 2 asymmetric carbon atoms located in a tram conformation (Fig. 1). For the 4 stereo isomers the affinity to dopamine D-l receptors has been investigated in vitro (Berger er al., 1989). The affinity of SCH 39166 for the dopamine D-i receptor is 3OOG500 times higher as compared to the affinity of the other stereo isomers. In the present paper the preparation of [“C]SCH 39166 is described. (“CISCH 39166 was labelled by N-methylation of the free base of the secondary amine in acetone with [“Clmethyl iodide (Scheme I). [“CISCH 39166 was subsequently injected into a Cynomolgus monkey for PET-examination of ligand distribution in brain in zlioo. To examine the specificity and reversibility of [“CISCH 39166 binding to dopamine D-l receptors a displacement experiment was performed.

Experimental General SCH 39166 and SCH 40853 were kindly supplied by Schering-Plough Pharmaceutical Corporation Bloomfield, NJ, U.S.A.. Other chemicals were obtained from commercial sources and were of analytical grade. [“C]Carbon dioxide was produced at the

Scheme

2

4

6 8 10 Time (mid

12

14

Fig. 2. Semipreparative HPLC chromatogram (u.\. ;md radioactivity vs time) using a Waters /I-Poracil column. 1, (“Clmethyl iodide: II. [“C]SCH 39166; II’. SCH 39166: III, acetone: IV, SCH 40853.

Karolinska Hospital with a Scanditronix RNP 16 cyclotron using 16 MeV protons in the IIN(pT x)“C reaction. The gas target was irradiated in a batch production. The [“Clcarbon dioxide produced was trapped in a stainless steel coil cooled with liquid nitrogen before being transferred to the one-pot [“Cjalkyl iodide system. [“C]Methyl iodide was synthesized from [“Clcarbon dioxide utilizing a one-pot reaction set-up similar to that reported elsewhere (Langstrom er (II., 1986. 1987; Halldin c’f trl.. 1990). Semipreparative HPLC was performed using a Shimadzu LC-6A pump, a Rheodyne injector (Type 7126 with a 1 mL loop), a Waters p-Poracil column (300 x 7.8 mm, 10 itm), and an LDC Spectromonitor II (wavelength = 280 nm) in series with a GM tube for radiation detection. [“CISCH 39166 was purified using methylene chloridejmethanol/triethylamine = 94/6/0.06 (methanol/triethylamine preadjusted to pH = 8 with concentrated acetic acid) as the mobile phase with a flow rate of 2.0 mLjmin (Fig. 2). The radiochemical purity of [“CISCH 39166 was analyzed by reverse-phase HPLC using a Kontron 420 pump, a Rheodyne injector (7125 with a IOOpL loop) equipped with a Waters p-Bondapak-Cl8 column (300 x 7.8 mm, 10 pm) and an LDC-Milton Roy 300 u.v.-spectrophotometer (280 nm) in series with a

I

Synthesis of [“CISCH 39166

453

23390 (6 mg, 1 mg/kg iv.) was given as a bolus 18 min after injection of [“C]SCH 39166.

Results and Discussion Chemistry

i 0

2

4 Ii Tinte hnin)

tl

Fig. 3. Analytical HPLC chromatogram (WV.and radioactivity vs time) after HPLC-purification. Top [“CISCH 39166; bottom showing added reference of SCH 39166. Beckman 170 radiodetector. Acetonitrile and 0.01 M phosphoric acid (27/73) was used as the mobile phase with a flow rate of 4.0 mL/min (Fig. 3). [“C]SCH

39166 (Scheme 1)

[“C]Methyl iodide, prepared as describe in detail elsewhere (Halldin et al., 1990) was trapped at room temperature in a reaction vessel (1.0 mL mini-vial, Alltech), containing SCH 40853 (1 .Omg) and acetone (300 pL). The vessel was sealed and heated at 90°C for 5 min. Mobile phase (700 pL) was added before injection on to the semi-preparative HPLC column. [“C]SCH 39166 eluted after 8-9 min with a retention time identical to a standard reference sample (Fig. 2). After evaporation of the mobile phase, the residue was dissolved in 8 mL physiological saline and filtered through a Millipore filter (0.22 pm), yielding a solution which was sterile and free from pyrogens. PET-experiments

[“CISCH 39166 was injected into a Cynomolgus monkey (weight about 4.5 kg and anaesthetized with ketamine) for PET-examination of ligand distribution in the primate brain in Go. The experimental procedure has been described elsewhere (Farde et al., 1985; Sedvall et al., 1991). [“CISCH 39166 (40 MBq, specific activity at time for injection: 800 and 1500 Ci/mmol 30 and 55 GBq/pmol, respectively) was injected iv. into the sural vein. An eight-ring PET system (Scanditronix, PC2048- 15B) was used to follow the regional distribution of radioactivity in the monkey brain for about 60 min (Litton et al., 1990). Regional radioactivity was calculated for each sequential scan, corrected for “C decay and then plotted vs time. In a displacement experiment, a large dose of the dopamine D-l receptor antagonist SCH

SCH 23390 has previously been successfully labelled with “C by alkylation of the desmethyl compound SCH 24518 with [‘VImethyl iodide (Halldin et al., 1986; DeJesus et al., 1987; Ravert et al., 1987). An alternative approach based on reductive carboxylation has been proposed: the alkylation of the secondary amine by direct incorporation of [“Clcarbon dioxide under moderate reaction conditions (Ram et al., 1987). In this approach, the secondary amine is carboxylated with [“Clcarbon dioxide to a carbamate ester followed by the in situ lithium aluminium hydride reduction to give the N-methylated tertiary amine. One obvious advantage is that a [“Clmethyl iodide set-up is not needed. However, the approach has not yet proven useful for routine synthesis. [‘sF]FluoroethylSCH 23390 has also been prepared (Moerlein et al., 1989). The potential of this analog of SCH 23390 as PET-ligand has not yet been clarified. It is, known however, that substitution of the N-methyl group by a N-ethyl group reduces the affinity by about two orders of magnitude (& = 0.4 compared to 41). In the present paper [“CISCH 39166 was labelled by N-methylation of the free base of the secondary amine (SCH 40853) in acetone with [“Clmethyl iodide (9OC for 5min) (Scheme 1). No added base was needed since the free base of the secondary amine was used. Solvent composition, reaction time, temperature and substrate concentration were varied to optimize the incorporation of [“Clmethyl iodide. Optimal radiochemical yields were obtained in both DMF/DMSO and acetone (60-80 and 30-50%, respectively). The yields were somewhat lower as compared to the yields obtained in the synthesis of [“CISCH 23390 [ > 90 and 80%, respectively (Halldin et al., 1986)]. This lower yield is probably due to the use of the more sterically hindered secondary amine (SCH 40853) (Fig. 1). The procedure which has been developed permitted the use of either straight-phase or reverse-phase semi-preparative HPLC for purification. As in the synthesis of [“CISCH 23390 (Halldin et al., 1986) straight-phase was found to be superior to reversephase since the preparation time was shortened and the possibility of contamination by the desmethyl compound (SCH 40853) was eliminated. It was in fact not possible to separate [“C]SCH 39166 from unreacted SCH 40853 during reverse phase semipreparative HPLC conditions. The reaction mixture was dissolved in the mobile phase and injected into the semi-preparative HPLCcolumn without first being evaporated to dryness as was the case in the synthesis of [“CISCH 23390

454

CHRWER HALLDIN et al.

(Halldin et al., 1986). [“CISCH 39166 eluted after 8-9 min, SCH 40853 after 10-I 1 min and unreacted [“Clmethyl iodide after 6-7 min with the same retention times as standard reference samples (Fig. 2). After evaporation of the mobile phase, the residue was dissolved in 8 mL physiological saline and filtered through a Millipore filter (0.22 mm), yielding a solution which was sterile and free from pyrogens. The total radiochemical yield was 20-30% (from EOB and decay-corrected) with a total synthesis time of 35-40 min and a radiochemical purity >99% (Fig. 3). For quantitative PET studies, it is important to obtain radioligands with high specific activity. The number of binding sites is limited and a high dose of radioligand may induce pharmacological effects. The specific activity obtained at EOS was about 1500 Ci/mmol (55 GBq/pmol) which should be satisfactory for studying the dopamine D-l receptor system in the primate brain in ciuo. PET-e.xperiments

[“CISCH 39166 was injected i.v. into a Cynomolgus monkey. There was a rapid accumulation of radioactivity in the brain [Fig. 4(a)]. After 4min

-

1200

S,rI.,“rn cortex

800

Ol Time

(minutes)

fb)

1200

---t

Strl.t”m cun.r

800

1

0

2'0 Time

Fig.

4.

40

6’0

(minutes)

Regional radioactivity (nCi/mL) vs time (min) in the

brain of a Cynomolgus monkey after i.v. administration of 40 MBq of [“CISCH 39166. (a) Control experiment;

(b) displacement experiment.

about 1.5% of radioactivity injected was in the brain. This is comparable to other dopamine receptor ligands such as [“Clraclopride and [“CISCH 23390 (Farde et al., 1987). PET-analysis demonstrated accumulation in the striatum, a region known to have a high density of dopamine D-l receptors [Fig. 4(a)]. In a second experiment, radioactivity in the striatum but not in the receptor poor cerebellum was markedly reduced after injection of 6 mg unlabelled SCH 23390 [Fig. 4(b)]. This displacement experiment indicates the specificity and reversibility of [“CISCH 39166 binding to dopamine D-l receptors, [“CISCH 39166 should be a useful PET ligand for dopamine D-l receptors in man. Acknowledgements-The authors would like to thank Mr Goran Printz for assistance with the radionuclide production. The assistance of the members of the Stockholm PET group involved in the PET experiments is also greatly acknowledged. This work was supported by grants from the Swedish Medical Research Council (03560). the Swedish Natural Science Research Council (K-KU 9973-300). the Bank of Sweden Tercentenary Foundation, the National Institute of Mental Health USA (NIMH, Grant No. 48414). the Ulf Lundahl foundation and the Karolinska Institute.

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