Development Of New Pet And Spect Radiotracers

National Institute On Drug Abuse

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Abstract

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). Based on radiation dosimetry and toxicological studies in animals that demonstrated the safety of 5-[I-125]iodo-A-85380 (5IA) and 2-fluoro-A-85380 (2FA) for use in humans, INDs allowing the administration of these radioligands to humans were submitted. In collaboration with Yale University, the first human studies with 5IA and SPECT were performed and confirmed the safety of doses sufficient to visualize alpha4beta2 nAChRs in the human brain. 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. Methodology was developed and data were collected in kinetic studies of the thalamus in non-human primates (NHPs) for estimating the densities of nAChRs and, in collaboration with researchers from the University of Michigan, visualizing the loss of these receptors in the striatum of unilaterally MPTP-lesioned NHPs with 2FA and PET. 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. Therefore, to collect sufficient data to quantify receptor binding with PET after the bolus administration of 2FA, up to 4 - 6 hours of scanning are required. Such prolonged scanning could limit the practical use of 2FA in human studies, especially due to the 1.8 h half-life of [F-18]. Efforts were directed at developing radioligands with faster kinetics for imaging nAChRs in vivo. To that end, six novel analogs of A-85380 and A-84543 with calculated log P (Clog P) values in the range of 1.2 to 3 were synthesized, radiolabeled and evaluated in vitro and in vivo. Molecular modeling of the structures of these compounds revealed differences in their conformational profiles and the electronic properties, providing insight into structure-activity relationships with nAChRs. The molecular modeling was performed in collaboration with staff in the Medicinal Chemistry Section of the Medications Discovery Research Branch. Increasing the lipophilicity from Clog P = 0.6 (2FA) yielded ligands with faster blood brain barrier penetration and increased nonspecific binding. Although [11C]methyl-PVC, the best radioligand from the series, exhibits specific in vivo receptor binding that is half that of 2FA, [11C]methyl-PVC may be useful for studying the in vivo occupancy of nAChRs by endogenous or exogenous ligands. From in vitro and in vivo studies with these radioligands (Kd values at 37 degrees C: 9 to 600 pM, Clog P values: 0.6 to 3), an algorithm was developed for predicting binding potential (BP). BP is the one of the most important characteristic of a radioligand as it characterizes the specificity of radioligand receptor binding and reflects its suitability for qualitive and quantitative in vivo imaging. BP values for six radioligands calculated from their Kds at 37 oC, their Clog Ps, and the density of alpha4beta2 nAChRs in each brain region of interest and this algorithm were in good agreement with those observed in in vivo in Rhesus monkeys, suggesting that this algorithm might be useful for screening potential compounds for a new radioligand for alpha4beta2 nAChRs with improved characteristics. As part of a new effort to develop radioligands for in vivo imaging of cannabinoid receptors, we synthesized and evaluated in vitro twelve new analogs of SR141716, a selective ligand for CB1 subtype of cannabinoid receptors. Two of these compounds are potentially suitable for use as in vivo imaging agents. Three additional compounds in this series reversed the inhibition of excitatory transmission produced by WIN 55,212-2, a cannabinoid receptor agonist, suggesting that they may be of interest for the treatment of marijuana and cocaine addictions. This work was performed in collaboration with staff in the Cellular Neurophysiology Section of the Cellular Neurobiology Research Branch.