The ability to image and quantify nicotinic receptors in human brain may be crucial to better understanding the neurobiology of smoking addiction. Analogs of A-85380 and A-84543 have been evaluated as imaging agents for nicotinic acetylcholine receptors (nAChRs) with positron emission tomography (PET) and single photon emission computed tomography (SPECT). In collaboration with Yale University, the SPECT studies with 5IA revealed that cerebral nAChRs can be quantified in human non-smoking volunteers. Toxicological studies in animals demonstrated the safety of 2-[F-18]fluoro-A-85380 (2FA). The first human PET studies in non-smokers with 2-[F-18]fluoro-A-85380 (2FA) demonstrated the feasibility and safety quantitatively imaging nAChRs in the thalamus and visualizing these receptors in brain regions containing low to moderate receptor densities and that multiple studies on a single volunteer are possible within dosimetry limits. Preliminary analysis of PET studies in smokers with 2FA demonstrate increased densities of nicotinic receptors in many brain regions compared to the brains of non-smokers. Mice studies showed that the administration of the non-selective inhibitor of cytochrome P450, cimetidine, substantially slowed the rapid in vivo metabolism of 5IA, suggesting that the use of this or similar compounds could reduce the dose of radioactivity needed to successfully image nAChRs in human volunteers. In collaboration with researchers from the University of Michigan, the loss of nAChRs in the striatum of unilaterally MPTP-lesioned NHPs with 2FA and PET was demonstrated. Kinetic studies with 2FA and PET in NHPs revealed that 2FA accumulates relatively slowly in brain, partially because its low lipophilicity slows its blood-brain-barrier penetration. A series of 5-heteryl-6-halogeno-A-85380 derivatives with binding affinity (Ki) at the nicotinic acetylcholine receptor (nAChR) ranging from 3 to 150 pM and a lipophilicity (logD) range of -1.6 to +1.5 has been synthesized as potential PET ligands. Most ligands of the series exhibited a higher binding affinity at the alpha4beta2* subtype of nAChRs than epibatidine. Molecular modeling studies revealed an important role of the orientation of the external heterocyclic ring on the binding affinity of the ligands with nAChRs. Two compounds of the series were radiolabeled with 18F. Recent PET studies with one of these compounds ([18F]NIDA52189) demonstrated that its binding potential values in Rhesus monkey brain was ca. 2.5 times that of 2-[F-18]F-A-85380, the only available PET radiotracer for imaging cerebral nAChR in humans. Preliminary toxicology and dosimetry studies suggest that [F-18]NIDA52189 is suitable for quantitative imaging of extrathalamic nAChRs. Imaging of the central CB1 cannabinoid receptors by PET will improve our understanding of the roles of these receptors in the brain. Existing PET radiotracers for imaging CB1 are very lipophilic compounds with high non-specific to specific binding ratios and as a result are not adequate for quantitative studies. A 14 member series of analogs of 1-[(N-methyl-piperidin-2-yl)methyl]-3-naphthoylindole, a CB1 agonist with high binding affinity, with reduced lipophilicities has been synthesized. The lead compound shows a Ki of 0.9 nM, which represents one of the highest affinities observed for CB1 receptor ligands; further, it has a cLogD value, which should reduce non specific binding, a problem observed with all previously tested ligands. Our study showed that substitutions to the lead compound at the 4 position on the naphthyl ring gave similar Ki values that allowed us to vary the lipophilicity of the ligand, giving cLogD values in the range of 2.7 to 4.5. Substituting a fluorine atom for hydrogen gave a derivative with the lowest Ki value, 0.7 nM, and an unexpectedly low experimental LogD of 2.6. A 4-nitro precursor for nucleophic radio-fluorination of this compound has been used giving good radiochemiccal yields and specific activity. When the entomerically purified tracer was used in an ex-vivo study on mice the target to non-target ratios were about 1.7-1.8. for measurements from the hippocampus to brain stem regions. Other tracers are planned including the 4-(2-hydroxyethyl)naphthoyl derivative which has a better combination of lipophilicity and binding. Corticotropin-Releasing Hormone (CRH) acts as a major regulator of the hypothalamic-pituitary-adrenal (HPA) axis coordinating neuroendocrine, autonomic, immune, and behavioral responses to stress. It is prevalent in the central nervous system (CNS) where it acts as a neurotransmitter. A high-affinity, nonpeptide radioligand for the CRHR1 was synthesized and showed distribution in rat brain consistent with CRHR1 receptor subtype using in vitro autoradiography. This is the first nonpeptide radiotracer combining high affinity and appropriate lipophilicity that penetrates the blood-brain barrier and hence has the potential to be used for PET imaging studies. In vivo visualization of changes in the CRH1 receptor or its occupancy would further the understanding of the role of stress in drug abuse.
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