Synthesis of several MPP derivatives for ^(99m)Tc-labelling and evaluated as potential 5-HT_(1A) receptor imaging agents

The(2-methoxyphenyl) piperazine(MPP) was selected as the functional group and conjugated to dithiocarbamate through different spacers.A series of new MPP derivatives(MPPnDTC,n = 2-6) were synthesized and radiolabelled with 99mTc-nitrido core or 99mTc-tricarboxyl core as potential 5-HT1A receptor imaging agents.All the six 99mTc-labelled complexes were lipophilic and neutral.Biodistribution results showed that those radiotracers had moderate initial brain and hippocampus uptake.There have no significant relation was observed between the biological properties of these tracers with the length of its carbon chain.The radioactivity concentrations of hippocampus of 99mTcN-MPP2DTC,99mTcN-MPP3DTC,99mTcN-MPP4DTC,99mTcN-MPP5DTC,99mTcN-MPP6DTC and 99mTc(CO)3-MPP3DTC at 2 min post-injection time(p.i.) were 0.43,1.15,0.99,1.04,1.13 and 0.83 %ID/g,respectively.

Discovery and development of brain-penetrant ^(18)F-labeled radioligands for neuroimaging of the sigma-2 receptors

We have discovered and synthesized a series of indole-based derivatives as novel sigma-2(σ_(2))receptor ligands.Two ligands with high σ_(2) receptor affinity and subtype selectivity were then radiolabeled with F-18 in good radiochemical yields and purities,and evaluated in rodents.In biodistribution studies in male ICR mice,radioligand[18F]9,or 1-(4-(5,6-dimethoxyisoindolin-2-yl)butyl)-4-(2-[18F]fluoroethoxy)-1H-indole,was found to display high brain uptake and high brain-to-blood ratio.Pretreatment of animals with the selective σ_(2) receptor ligand CM398 led to significant reductions in both brain uptake(29%-54%)and brain-to-blood ratio(60%-88%)of the radioligand in a dose-dependent manner,indicating high and saturable specific binding of[18F]9 to σ_(2) receptors in the brain.Further,ex vivo autoradiography in male ICR mice demonstrated regionally heterogeneous specific binding of[18F]9 in the brain that is consistent with the distribution pattern of σ_(2) receptors.Dynamic positron emission tomography imaging confirmed regionally distinct distribution and high levels of specific binding for[18F]9 in the rat brain,along with appropriate tissue kinetics.Taken together,results from our current study indicated the novel radioligand[18F]9 as the first highly specific and promising imaging agent for σ_(2) receptors in the brain.

Synthesis and characterization of the two enantiomers of a chiral sigma-1 receptor radioligand:(S)-(+)-and(R)-(-)-[^(18)F]FBFP

Racemic[^(18)F]FBFP([^(18)F]1)proved to be a potentσ_(1) receptor radiotracer with superior imaging properties.The pure enantiomers of unlabeled compounds(S)-and(R)-1 and the corresponding iodonium ylide precursors were synthesized and characterized.The two enantiomers(S)-1 and(R)-1 exhibited comparable high affinity forσ_(1) receptors and selectivity overσ_(2) receptors.The Ca^(2+) fluorescence assay indicated that(R)-1 behaved as an antagonist and(S)-1 as an agonist forσ_(1) receptors.The ^(18)F-labeled enantiomers(S)-and(R)-[^(18)F]1 were obtained in>99%enantiomeric purity from the corresponding enantiopure iodonium ylide precursors with radiochemical yield of 24.4%±2.6%and molar activity of 86–214 GBq/μmol.In ICR mice both(S)-and(R)-[^(18)F]1displayed comparable high brain uptake,brain-to-blood ratio,in vivo stability and binding specificity in the brain and peripheral organs.In micro-positron emission tomography(PET)imaging studies in rats,(S)-[^(18)F]1 exhibited faster clearance from the brain than(R)-[^(18)F]1,indicating different brain kinetics of the two enantiomers.Both(S)-and(R)-[^(18)F]1 warrant further evaluation in primates to translate a single enantiomer with more suitable kinetics for imaging theσ_(1) receptors in humans.