Synthesis andin vivo evaluation in mice of [123I]-(4-fluorophenyl)[1-(3-iodophenethyl)piperidin-4-yl]methanone as a potential SPECT-tracer for the serotonin 5-HT2A receptor

The Benzoylpiperidine Fragment as a Privileged Structure in Medicinal Chemistry: A Comprehensive Review

The phenyl(piperidin-4-yl)methanone fragment (here referred to as the benzoylpiperidine fragment) is a privileged structure in the development of new drugs considering its presence in many bioactive small molecules with both therapeutic (such as anti-cancer, anti-psychotic, anti-thrombotic, anti-arrhythmic, anti-tubercular, anti-parasitic, anti-diabetic, and neuroprotective agents) and diagnostic properties. The benzoylpiperidine fragment is metabolically stable, and it is also considered a potential bioisostere of the piperazine ring, thus making it a feasible and reliable chemical frame to be exploited in drug design. Herein, we discuss the main therapeutic and diagnostic agents presenting the benzoylpiperidine motif in their structure, covering articles reported in the literature since 2000. A specific section is focused on the synthetic strategies adopted to obtain this versatile chemical portion.

5-HT radioligands for human brain imaging with PET and SPECT

The serotonergic system plays a key modulatory role in the brain and is the target for many drug treatments for brain disorders either through reuptake blockade or via interactions at the 14 subtypes of 5-HT receptors. This review provides the history and current status of radioligands used for positron emission tomography (PET) and single photon emission computerized tomography (SPECT) imaging of human brain serotonin (5-HT) receptors, the 5-HT transporter (SERT), and 5-HT synthesis rate. Currently available radioligands for in vivo brain imaging of the 5-HT system in humans include antagonists for the 5-HT(1A), 5-HT(1B), 5-HT(2A), and 5-HT(4) receptors, and for SERT. Here we describe the evolution of these radioligands, along with the attempts made to develop radioligands for additional serotonergic targets. We describe the properties needed for a radioligand to become successful and the main caveats. The success of a PET or SPECT radioligand can ultimately be assessed by its frequency of use, its utility in humans, and the number of research sites using it relative to its invention date, and so these aspects are also covered. In conclusion, the development of PET and SPECT radioligands to image serotonergic targets is of high interest, and successful evaluation in humans is leading to invaluable insight into normal and abnormal brain function, emphasizing the need for continued development of both SPECT and PET radioligands for human brain imaging.

In vivo evaluation in rodents of [(123)I]-3-I-CO as a potential SPECT tracer for the serotonin 5-HT(2A) receptor

INTRODUCTION

[(123)I]-(4-fluorophenyl)[1-(3-iodophenethyl)piperidin-4-yl]methanone ([(123)I]-3-I-CO) is a potential single photon emission computed tomography tracer with high affinity for the serotonin 5-HT(2A) receptor (K(i)=0.51 nM) and good selectivity over other receptor (sub)types. To determine the potential of the radioligand as a 5-HT(2A) tracer, regional brain biodistribution and displacement studies will be performed. The influence of P-glycoprotein blocking on the brain uptake of the radioligand will also be investigated.

METHODS

A regional brain biodistribution study and a displacement study with ketanserin were performed with [(123)I]-3-I-CO. Also, the influence of cyclosporin A (50 mg/kg) on the brain distribution of the radioligand was investigated. For the displacement study, ketanserin (1 mg/kg) was administered 30 min after injection of [(123)I]-3-I-CO.

RESULTS

The initial brain uptake of [(123)I]-3-I-CO was quite high, but a rapid wash-out of radioactivity was observed. Cortex-to-cerebellum binding index ratios were low (1.1 - 1.7), indicating considerable aspecific binding and a low specific 'signal' of the radioligand. Tracer uptake was reduced to the levels in cerebellum (a 60% reduction) after ketanserin displacement. Administration of cyclosporin A resulted in a doubling of the brain radioactivity concentration.

CONCLUSIONS

Although [(123)I]-3-I-CO showed adequate brain uptake and could be displaced by ketanserin, high aspecific binding to brain tissue was responsible for very low cortex-to-cerebellum binding index ratios, possibly limiting the potential of the radioligand as a serotonin 5-HT(2A) receptor tracer. We also demonstrated that [(123)I]-3-I-CO is probably a weak substrate for the P-glycoprotein efflux transporter.