Carbon-11-labelling of a novel, trishomocubane-derived, high affinity and selectivity DAT ligand

Complete text of publication follows: Objectives: Parkinson's disease, schizophrenia, attention deficit disorder and drug abuse are related to abnormalities within the brain's dopaminergic system. The neuronal dopamine transporter (DAT) plays a key role in regulating the synaptic concentration of dopamine and thus dopamine neurotransmission in the brain. Since the DAT can be considered as a marker of the integrity and number of the presynaptic striatal dopamine-producing neurons, considerable efforts have been spent in recent years on the design and development of DAT-selective radioligands for use in Positron Emission Tomography (PET) studies. Notably, the tropane PE2I and its fluorinated analogue LBT-999 were identified as having high affinity and selectivity for the DAT over the norepinephrine transporter (NET) and the serotonin transporter (SERT). Besides tropanes, only a few bicyclic frameworks, e.g. bicyclo[2.2.2]octanes, have delivered compounds with high affinity for the DAT. Recently, novel poly-carbocyclic DAT ligands with selectivity over the NET and the SERT were reported. The lead compound of this series (1, N-methyl-N-(3-fluoro) benzyl-pentacyclo[5.4.0.02,6.03,10.05,9] undec-8-ylamine, Ki = 1.2 nM, ≥ 8300-fold selectivity over NET and SERT) was selected as a potential candidate for imaging the DAT with PET and isotopically labelled with carbon-11 using [11C]methyl triflate. Methods: The trishomocubane derivatives 1 (reference) and 2 (precursor for labelling with carbon-11) were prepared from commercially available Cookson's diketone in 6 and 7 steps, respectively. Carbon-11 labelling of 1 was performed using a TRACERLab FX-C Pro synthesizer (GEMS) and comprises (1) trapping at -10 C of [11C]MeOTf in acetone (0.4 mL) containing the nor-derivative 2 (0.6-0.9 mg, free base) and aq. 3N NaOH (8 μL); (2) heating at 110 C for 2 min; (3) concentration to dryness and taking up the residue in 1.0 mL of the HPLC mobile phase; (4) purification using semi-preparative reversed-phase HPLC (Waters X-Terra PrepR MS C-18 - eluent: CH3CN / H2O / TEA: 85 / 15 / 0.1 (v:v:v) - flow rate: 8 mL/min - detection at 254 nm) and (5) SepPakR Plus C-18-based formulation for i.v. injection. Results: Compound 2 was obtained in about 70% overall yield from Cookson's diketone. Methylation was performed at room temperature for 14 hrs using 37% aq. CH2O and NaBH(OAc)3 in 1, 2-dichloroethane to afford compound 1 in 92% yield. Carbon-11-labelled 1 ([11C]-1) was obtained in 14-20% decay-corrected yields, based on starting [11C]carbon dioxide. Typically, starting from a 74 GBq cyclotron-produced [11C]carbon dioxide batch, 2.8 to 4.0 GBq of [11C]-1, ≥ 99% radiochemically pure and ready to inject, were obtained in 39 min (including HPLC purification, Rt: 9.5-10.0 min). Specific radioactivities ranged from 75 to 150 GBq/μmol. Conclusions: The trishomocubane DAT ligand 1 was successfully labelled with carbon-11. Biodistribution studies and PET-imaging (Focus 220 Concorde) are currently underway in rodents to evaluate the potential of this novel radioligand to image the DAT in vivo