Radioligands for brain 5-HT2 receptor imaging in vivo: why do we need them?

Anatomical and neurochemical organization of the serotonergic system in the mammalian brain and in particular the involvement of the dorsal raphe nucleus in relation to neurological diseases

The brainstem is a neglected brain area in neurodegenerative diseases, including Alzheimer's and Parkinson's disease, frontotemporal lobar degeneration and autonomic dysfunction. In Depression, several observations have been made in relation to changes in one particular the Dorsal Raphe Nucleus (DRN) which also points toward as key area in various age-related and neurodevelopmental diseases. The DRN is further thought to be related to stress regulated processes and cognitive events. It is involved in neurodegeneration, e.g., amyloid plaques, neurofibrillary tangles, and impaired synaptic transmission in Alzheimer's disease as shown in our autopsy findings. The DRN is a phylogenetically old brain area, with projections that reach out to a large number of regions and nuclei of the central nervous system, particularly in the forebrain. These ascending projections contain multiple neurotransmitters. One of the main reasons for the past and current interest in the DRN is its involvement in depression, and its main transmitter serotonin. The DRN also points toward the increased importance and focus of the brainstem as key area in various age-related and neurodevelopmental diseases. This review describes the morphology, ascending projections and the complex neurotransmitter nature of the DRN, stressing its role as a key research target into the neural bases of depression.

Biodistribution and displacement studies of the selective 5-HT2A receptor antagonist 123I-5-I-R91150 in the normal dog

There is increasing interest in mapping receptors in vivo by using functional imaging modalities such as single photon emission tomography (SPET) and positron emission tomography (PET). Since SPET is a more accessible functional imaging modality than PET and, overall, it is more economical, radioligands suitable for this technique are in greater demand. Recently, 123I-5-I-R91150, a radioligand with high selectivity and affinity for 5-HT(2A) receptors in the brain, was introduced for SPET. This study reports on the whole-body distribution and brain uptake of the selective 123I-5-I-R91150 ligand in four normal dogs. The frontal to cerebellar ratio of uptake in time was determined in three dogs. Time-activity curve of venous blood was determined in one dog. Maximal global brain uptake was found at 10-60 min post-injection. Higher brain uptake was noted in the frontal cortical areas compared to the cerebellum. The frontal-cerebellar ratio reached the highest values at 90-180 min. Reversibility and pharmacological selectivity of ligand binding was demonstrated through displacement and blocking studies with the 5-HT(2A) receptor antagonist ketanserin. This study demonstrates that the specific 5-HT(2A) iodinated ligand can be used for imaging and semi-quantification of the 5-HT(2A) receptors in the canine brain in vivo by using SPET.

Delivery of radioligands for positron emission tomography (PET) in the central nervous system

Positron emission tomography (PET) is an imaging technique to monitor the delivery of tracers labeled with positron emitters ((11)C, (13)N, (15)O and (18)F). A wide variety of probes have been labeled to measure biochemical and physiological parameters in the central nervous system (CNS), such as glucose and oxygen metabolism, protein synthesis, blood flow, and neurotransmitter receptor functions. The delivery of labeled compounds to the target tissue, which directly reflect the distribution and kinetics patterns, especially to the neurotransmitter receptors is modulated by several factors, such as regional cerebral blood flow (rCBF), peripheral metabolism, and neurotransmitter concentration in the synaptic cleft. These factors provide misunderstanding of the apparent results, which do not reflect the true state of the CNS. The present paper will summarize several factors that affect the delivery of labeled compounds related to the neurotransmitter receptors in the CNS.

123I-MSP and F[11C]MSP: new selective 5-HT2A receptor radiopharmaceuticals for in vivo studies of neuronal 5-HT2 serotonin receptors. Synthesis, in vitro binding study with unlabelled analogues and preliminary in vivo evaluation in mice

In vitro binding study on bovine brain membranes using [3H]SCH23390, [3H]spiperone, [3H]prazosin and [3H]RP62203 as radioligands (for D1, D2, alpha1 and 5-HT2A receptors respectively) indicate that the new butyrophenones 8-[3-(4-fluorobenzoyl)propyl]-1-methyl-1,3,8-triazaspiro[4,5]de can-4-one (FMSP) and 8-[3-(4-iodobenzoyl)propyl]-1-methyl-1,3,8-triazaspiro[4,5]deca n-4-one (IMSP) exhibit a significantly higher selectivity for the 5-HT2A over D1, D2 and alpha1 receptors. Consequently, the radiolabelled analogues F[11C]MSP and 123I-MSP were prepared in attempt to obtain potential radiopharmaceuticals for in vivo imaging of neuronal 5-HT2A receptors with positron emission tomography (PET) and single photon emission tomography (SPET). F[11C]MSP was synthesized by reaction of [11C]CH3I with 8-[3-(4-fluorobenzoyl)propyl]-1,3,8-triazaspiro[4,5]decan-4- one (DMSP) in 12 +/- 3% radiochemical yield, whereas 123I-MSP was obtained in 82 +/- 8% radiochemical yield by a no-carrier-added Cu(I)-assisted [123I]iododebromination of 8-[3-(4-bromo-benzoyl)propyl]-1-methyl-1,3,8-triazaspiro[4,5]de can-4-ene (BrMSP). In vivo pharmacokinetic and brain binding characterization of 123I-MSP assessed in mice following intravenous injection, showed a fast clearance of 123I-MSP from blood and relatively high initial uptakes in the liver, kidneys and in the lung. Significant uptake and long retention were observed in the brain (up to 1.64% i.d., 60 min p.i.), with a regional accumulation of radioactivity consistent with the reported 5-HT2A receptors distribution in the brain. Frontal cortex to cerebellum ratio of 3.5 was calculated at 60 min p.i. Furthermore, the initial brain uptake was significantly reduced after pretreatment of the animals with ritanserin, a selective 5-HT2 antagonist, and by preinjection of the non-radiolabelled analog IMSP, thus indicating the specificity of the brain uptake. These data suggest that 123I-MSP may be a promising compound for studying the serotoninergic 5-HT2 receptors with SPET. Due to the low specific activity of F[11C]MSP currently obtained by the [11C]methylation reaction, systematic in vivo investigation of F[11C]MSP are as yet not feasable.

Positron emission tomography (PET) methodology for small animals and its application in radiopharmaceutical preclinical investigation

The use and usefulness of positron emission tomography (PET) to quantify the specific and selective in vivo binding of radioligands in small laboratory animals is briefly reviewed up to the end of 1996. Emphasis is placed on practical experience with a dedicated, small diameter, tomograph (built in collaboration with CTI, Knoxville, TN), implementing conventional PET methodology.

Imaging of the serotonergic system: interactions of neuroanatomical and functional abnormalities of depression

For nearly three decades, evidence supporting a role for aberrant serotonergic function in the pathogenesis of depression has accumulated; however, only recently have methodologies and radiotracers suitable for in vivo clinical assessment of depression become available. To date, only a few neurochemical imaging studies have been performed in actively depressed subjects. A preliminary study using single photon emission computed tomography (SPECT) has demonstrated decreased levels of serotonin (5-HT) transporters in the midbrain regions of subjects with major depression. Analysis of the 5-HT2 receptor using positron emission tomography (PET) has suggested that this receptor may not be altered significantly in the depressed brain but may increase in response to antidepressant treatment. These findings are supported by studies in secondary "poststroke" depression that have shown that elevations in 5-HT2 receptor density correlated with the alleviation of symptoms of depressed mood. With the rapid development of novel PET and SPECT radiotracers, future studies of the serotonergic system that evaluate presynaptic (5-HT transporter) and postsynaptic (5-HT1A and 5-HT2A receptors) markers and the interaction of synaptic levels of 5-HT with these sites will make profound contributions to the understanding of the role of the serotonergic synapse in the pathophysiology of depression.

Iodine-123 labelled radiopharmaceuticals and single-photon emission tomography: a natural liaison

Most nuclear medicine departments possess one or more imaging apparatuses for single-photon emission tomography (SPET). Molecules of biological interest to assess metabolism and receptor function are often labelled with 123I, which allows proper SPET imaging. The various methods for radiolabelling are reviewed. As the biological integrity of these agents has been demonstrated for numerous radiopharmaceuticals, the purpose of this review is to summarize the efficacy in various fields of medicine, including the imaging of tumours, infection, myocardium and cerebrum.