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Potential receptor imaging agents based on Tc-99m for the in vivo visualization of the peripheral benzodiazepine receptor (PBR) have been designed on the basis of the information provided by the previously published structure-affinity relationship studies, which suggested the existence of tolerance to voluminous substituents in the receptor area interacting with 3-position of the quinoline nucleus of 2-quinolinecarboxamides 5. In the first step of the investigation, the stereoelectronic features of the above-indicated receptor area were also probed by means of 4-phenyl-3-[(1-piperazinyl)methyl]-2-quinolinecarboxamide derivatives bearing different substituents on the terminal piperazine nitrogen atom (compounds 6a-f). The structure-affinity relationship data confirmed the existence of a tolerance to bulky lipophilic substituents and stimulated the design of bifunctional ligands based on the 4-phenyl-3-[(1-piperazinyl)methyl]-2-quinolinecarboxamide moiety (compounds 6h,j,k,m). The submicromolar PBR affinity of rhenium complexes 6j,m suggests that the presence of their metal-ligand moieties with encaged rhenium is fairly compatible with the interaction with the PBR binding site. Thus, in order to obtain information on the in vivo behavior of these bifunctional ligands, (99m)Tc-labeled compounds 6h,k were synthesized and evaluated in preliminary biodistribution and single photon emission tomography (SPET) studies. The results suggest that both tracers do not present a clear preferential distribution in tissues rich in PBR, probably because of their molecular dimensions, which may hamper both the intracellular diffusion toward PBR and the interaction with the binding site.

The previous exploration of the structure-affinity relationships concerning 4-phenyl-2-quinolinecarboxamide peripheral benzodiazepine receptor (PBR) ligands 6 showed as an interesting result the importance of the presence of a chlorine atom in the methylene carbon at position 3 of the quinoline nucleus. The subnanomolar PBR affinity shown by N-benzyl-3-chloromethyl-N-methyl-4-phenyl-2-quinolinecarboxamide (6b) suggested its chlorine atom to be replaced with other halogens in order to optimize the interaction of the quinolinecarboxamide derivatives with PBR and to develop suitable candidates for positron emission tomography (PET) or single photon emission computed tomography (SPECT) studies. The binding studies led to the discovery of fluoromethyl derivative 6a, which showed an IC(50) value of 0.11nM and is, therefore, one of the most potent PBR ligands so far described. Fluoromethyl derivative 6a has been labeled with (11)C (t(1/2)=20.4min, beta(+)=99.8%) starting from the corresponding des-methyl precursor (14) using [(11)C]CH(3)I in the presence of tetrabutylammonium hydroxide in DMF with a 35-40% radiochemical yield (corrected for decay) and 1.5Ci/mumol of specific radioactivity. Ex vivo rat biodistribution and inhibition (following intravenous pre-administration of PK11195) studies showed that [(11)C]6a rapidly and specifically accumulated in PBR-rich tissues such as heart, lung, kidney, spleen, and adrenal, and at a lower level in other peripheral organs and in the brain. The images obtained in mouse with small animal YAP-(S)PET essentially confirmed the result of the ex vivo biodistribution experiments. The biological data suggest that [(11)C]6a is a promising radioligand for peripheral benzodiazepine receptor PET imaging in vivo.

3-Quinolinecarboxamides have been synthesized and evaluated for their binding to the human NK(1) receptor. Several secondary amide derivatives show NK(1) receptor affinity in the picomolar range. The most active compound, hydroxymethylcarboxamide 3h showing an IC(50) value in the subpicomolar range, behaved as an agonist of NK(1) receptor in endothelial cell proliferation, inositol phosphate turnover, and NO-mediated cyclic GMP accumulation, thus proving it to be the first non-peptide NK(1) receptor agonist showing very high potency.

The peripheral-type benzodiazepine receptors (PBRs) are only minimally expressed in normal brain parenchyma, where they are primarily localized in glial cells. Their basal expression rises in different neurodegenerative disorders, due to the presence of infiltrating inflammatory cells and activated microglia. [11C]PK11195, a selective PBR antagonist, has been used for the in vivo PET monitoring of neurodegeneration in clinical observations. We recently developed and labeled with carbon-11 three new carboxamide derivatives: [11C]VC193M, [11C]VC195 and [11C]VC198M. Aim of this study was to evaluate these ligands for the in vivo measuring of PBRs expression in neurodegenerations and compare their kinetic behavior with that of the reference tracer [11C]PK11195. Radioligands were evaluated in a preclinical model of Huntington's disease consisting in the monolateral striatal injection of quinolinic acid (QA). Activated microglia and astrocytic gliosis was present only within the affected striatum. A concomitant increase in radioactivity accumulation was observed for all the tracers examined (P<0.01). Among the new compounds, [11C]VC195 showed higher levels of lesioned/unlesioned striatum ratios (3.28+/-0.44), in comparison with [11C]VC193M and [11C]VC198M (2.69+/-0.53 and 1.52+/-0.36, respectively), but slightly inferior to that observed for [11C]PK11195 (3.76+/-1.41).In conclusion, the results of the study indicate that [11C]VC195 is a promising candidate for in vivo PET monitoring of neurodegenerative processes but its in vivo behavior overlap that of [11C]PK11195.

The novel quinoline-2-carboxamide derivatives N-[methyl-C-11]-3-methyl-4-phenyl-N-(phenyl-methyl)quinoline-2-carboxamide ([C-11]4), (+/-)-N-[methyl-C-11]-3-methyl-N-(1-methylpropyl)-4- phenylquinoline-2-carboxamide ([C-11]5), and (+/-)-N-[methyl-C-11]-3-methyl-4-(2-fluorophenyl)-N-(1-methylpropyl)quinoline-2-carboxamide ([C-11]6) were labeled with carbon-11 (t(1/2) = 20.4 min, beta (+) = 99.8%) as potential radioligands for the noninvasive assessment of peripheral benzodiazepine type receptors (PBR) in vivo with positron emission tomography (PET). The radiosynthesis consisted of N-methylation of the desmethyl precursors 3-methyl-4-phenyl-N-(phenylmethyl)quinoline-2-carboxamide (4a), (+/-)-3-methyl-N-(1-methylpropyl)-4-phenylquinoline-2-carboxamide (5a), and (+/-)-4-(2-fluorophenyl)-3-methyl-N-(1-methylpropyl)quinoline-2-carboxamide (6a) with either [C-11]methyl iodide or [C-11]methyl triflate in the presence of tetrabutylammonium hydroxide or potassium hydroxide in dimethylformamide. The radioligands [C-11]4, [C-11]5, and [C-11]6 were synthesized with over 99% radiochemical purity in 30 min, 30 +/- 5% radiochemical yield, calculated at the end of synthesis (EOS) non-decay-corrected, and 2.5 +/- 1.2 Ci/mu mol of specific radioactivity. Inhibition studies in rats following intravenous pre-administration of 1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxamide (PK 11195, 1) showed high specific binding to PER of [C-11]4, [C-11]5, and [C-11]6 in heart, lung, kidney, adrenal gland, spleen, and brain. The biological data suggest that [C-11]5, [C-11]6, and particularly [C-11]4 are promising radioligands for PER imaging in vivo with PET.