Synthesis of a potential M1 muscarinic agent [76Br]bromocaramiphen

Expanding PET-applications in life sciences with positron-emitters beyond fluorine-18

Positron-emission-tomography (PET) has become an indispensable diagnostic tool in modern nuclear medicine. Its outstanding molecular imaging features allow repetitive studies on one individual and with high sensitivity, though no interference. Rather few positron-emitters with near favourable physical properties, i.e. carbon-11 and fluorine-18, furnished most studies in the beginning, preferably if covalently bound as isotopic label of small molecules. With the advancement of PET-devices the scope of in vivo research in life sciences and especially that of medical applications expanded, and other than "standard" PET-nuclides received increasing significance, like the radiometals copper-64 and gallium-68. Especially during the last decades, positron-emitters of other chemical elements have gotten into the focus of interest, concomitant with the technical advancements in imaging and radionuclide production. With known nuclear imaging properties and main production methods of emerging positron-emitters their usefulness for medical application is promising and even proven for several ones already. Unfortunate decay properties could be corrected for, and β+-emitters, especially with a longer half-life, provided new possibilities for application where slower processes are of importance. Further on, (bio)chemical features of positron-emitters of other elements, among there many metals, not only expanded the field of classical clinical investigations, but also opened up new fields of application. Appropriately labelled peptides, proteins and nanoparticles lend itself as newer probes for PET-imaging, e.g. in theragnostic or PET/MR hybrid imaging. Furthermore, the potential of non-destructive in-vivo imaging with positron-emission-tomography directs the view on further areas of life sciences. Thus, exploiting the excellent methodology for basic research on molecular biochemical functions and processes is increasingly encouraged as well in areas outside of health, such as plant and environmental sciences.

Syntheses of R and S isomers of AF-DX 384, a selective antagonist of muscarinic M2 receptors

Enantiomers of 5,11-dihydro-11-[2-[2-[(N,N-dipropylaminomethyl)piperidin-1- yl]ethylamino]-carbonyl]-6H-pyrido[2,3-b][1,4]benzodiazepin-6-one (AF-DX 384) 1, have been synthesized from (S)-(+) and (R)-(-)-2-[N,N-dipropylaminomethyl]piperidine 4. The enantiomeric excess of 1 has been determined by capillary electrophoresis by using the alpha-highly sulphated cyclodextrin (alpha-HSCD) as chiral selector within the running electrolyte. (S)-(+)-(4) was prepared from (S)-(-)-pipecolic acid in a 4-step procedure (overall yield: 30%, ee: 99%) and (R)-(-)-AF-DX 384 from (R)-(+)-pipecolic acid. The (R)-(-) isomer exhibited in vitro a 23-fold higher affinity than its enantiomer (S)-(+) towards muscarinic receptors of subtype 2.

A selective radiobrominated cocaine analogue for imaging of dopamine uptake sites: pharmacological evaluation and PET experiments

(E)-N-(3-bromoprop-2-enyl)-2beta-carbomethoxy-3beta-4'-tolyl -nortropane or PE2Br, an analogue of cocaine was labelled with the positron emitter 76Br (T1/2=16 h) for pharmacological evaluation in the rat and PET investigation in the monkey. [76Br]PE2Br was obtained by electrophilic substitution from the tributylstannyl precursor with radiochemical yield of 80%. In vivo biodistribution studies of [76Br]PE2Br (20 MBq/nmol) in rats showed a high uptake in the striatum (2.2% ID/g tissue at 15 min p.i.). The striatum to cerebellum radioactivity ratio was 6 at 1 hour p.i. Striatal uptake of [76Br]PE2Br was almost completely prevented by pretreatment with GBR 12909, but citalopram and maprotiline had no effect, confirming the selectivity of the radioligand for the dopamine transporter. PET imaging of the biodistribution of [76Br]PE2Br in the baboon demonstrated rapid and high uptake in the brain (5% ID at 3 min p.i.). The striatal radioactivity concentration reached a plateau at 20 min p.i. (7% ID/100 mL). The uptake in the cortex and cerebellum was very low. A significantly higher uptake in the thalamus was observed. At 1h p.i., the striatum to cerebellum ratio and thalamus to cerebellum ratio were 8 and 1.9 respectively. In competition experiments the radioactivity in the striatum and the thalamus was displaced by 5 mg/kgof cocaine and 5 mg/kg of GBR 12909, but citalopram and maprotiline had no effect. These results showed that [76Br]PE2Br is in vivo a potent and selective radioligand suitable for PET imagingof the dopamine transporter.