Preclinical acute toxicity studies and rodent-based dosimetry estimates of the novel sigma-1 receptor radiotracer [(18)F]FPS

Dosimetry and Toxicity Studies of the Novel Sulfonamide Derivative of Sulforhodamine 101([18F]SRF101) at a Preclinical Level

BACKGROUND

The SR101 N-(3-[18F]Fluoropropyl) sulfonamide ([18F]SRF101) is a Sulforhodamine 101 derivative that was previously synthesised by our group. The fluorescent dye SR101 has been reported as a marker of astroglia in the neocortex of rodents in vivo.

OBJECTIVE

The aim of this study was to perform a toxicological evaluation of [18F]SRF101 and to estimate human radiation dosimetry based on preclinical studies.

METHODS

Radiation dosimetry studies were conducted based on biokinetic data obtained from a mouse model. A single-dose toxicity study was carried out. The toxicological limit chosen was <100 μg, and allometric scaling with a safety factor of 100 for unlabelled SRF101 was selected.

RESULTS

The absorbed and effective dose estimated using OLINDA/EXM V2.0 for male and female dosimetric models presented the same tendency. The highest total absorbed dose values were for different sections of the intestines. The mean effective dose was 4.03 x10-3 mSv/MBq and 5.08 x10-3 mSv/MBq for the male and female dosimetric models, respectively, using tissue-weighting factors from ICRP-89. The toxicity study detected no changes in the organ or whole-body weight, food consumption, haematologic or clinical chemistry parameters. Moreover, lesions or abnormalities were not found during the histopathological examination.

CONCLUSION

The toxicological evaluation of SRF101 verified the biosafety of the radiotracer for human administration. The dosimetry calculations revealed that the radiation-associated risk of [18F]SRF101 would be of the same order as other 18F radiopharmaceuticals used in clinical applications. These study findings confirm that the novel radiotracer would be safe for use in human PET imaging.

Biodistribution and Radiation Dosimetry of 18F-FTC-146 in Humans

The purpose of this study was to assess safety, biodistribution, and radiation dosimetry in humans for the highly selective σ-1 receptor PET agent 18F-6-(3-fluoropropyl)-3-(2-(azepan-1-yl)ethyl)benzo[d]thiazol-2(3H)-one (18F-FTC-146). Methods: Ten healthy volunteers (5 women, 5 men; age ± SD, 34.3 ± 6.5 y) were recruited, and written informed consent was obtained from all participants. Series of whole-body PET/MRI examinations were acquired for up to 3 h after injection (357.2 ± 48.8 MBq). Blood samples were collected, and standard vital signs (heart rate, pulse oximetry, and body temperature) were monitored at regular intervals. Regions of interest were delineated, time-activity curves were calculated, and organ uptake and dosimetry were estimated. Results: All subjects tolerated the PET/MRI examination well, and no adverse reactions to 18F-FTC-146 were reported. High accumulation of 18F-FTC-146 was observed in σ-1 receptor-dense organs such as the pancreas and spleen, moderate uptake in the brain and myocardium, and low uptake in bone and muscle. High uptake was also observed in the kidneys and bladder, indicating renal tracer clearance. The effective dose of 18F-FTC-146 was 0.0259 ± 0.0034 mSv/MBq (range, 0.0215-0.0301 mSv/MBq). Conclusion: First-in-human studies with clinical-grade 18F-FTC-146 were successful. Injection of 18F-FTC-146 is safe, and absorbed doses are acceptable. The potential of 18F-FTC-146 as an imaging agent for a variety of neuroinflammatory diseases is currently under investigation.

The development of radiotracers for imaging sigma (σ) receptors in the central nervous system (CNS) using positron emission tomography (PET)

Sigma (σ) receptors are unique mammalian proteins, distributed in the central nervous system and elsewhere, which are increasingly implicated in the pathophysiology of virtually all major central nervous system disorders. The heterogeneous but wide distribution of σ1 in the brain has prompted the development of selective radiotracers for imaging these sites using positron emission tomography (PET). To date, some 50 carbon-11-labelled and fluorine-18-labelled candidate PET radioligands targeting σ receptors have been reported. The historical development of selective σ1 receptor ligands as potential PET imaging agents, as well as the radiochemistry and application of the most recently developed examples, is described herein.

Human radiation dosimetry of 6-[18F]FDG predicted from preclinical studies

PURPOSE

The authors are developing 6-[(18)F]fluoro-6-deoxy-D-glucose (6-[(18)F]FDG) as an in vivo tracer of glucose transport. While 6-[(18)F]FDG has the same radionuclide half-life as 2-[(18)F]fluoro-2-deoxy-D-glucose (2-[(18)F]FDG) which is ubiquitously used for PET imaging, 6-[(18)F]FDG has special biologic properties and different biodistributions that make it preferable to 2-[(18)F]FDG for assessing glucose transport. In preparation for 6-[(18)F]FDG use in human PET scanning, the authors would like to determine the amount of 6-[(18)F]FDG to inject while maintaining radiation doses in a safe range.

METHODS

Rats were injected with 6-[(18)F]FDG, euthanized at specified times, and tissues were collected and assayed for activity content. For each tissue sample, the percent of injected dose per gram was calculated and extrapolated to that for humans in order to construct predicted time-courses. Residence times were calculated as areas under the curves and were used as inputs to OLINDA/EXM in order to calculate the radiation doses.

RESULTS

Unlike with 2-[(18)F]FDG for which the urinary bladder wall receives the highest absorbed dose due to urinary excretion, with 6-[(18)F]FDG there is little urinary excretion and osteogenic cells and the liver are predicted to receive the highest absorbed doses: 0.027 mGy/MBq (0.100 rad/mCi) and 0.018 mGy/MBq (0.066 rad/mCi), respectively. Also, the effective dose from 6-[(18)F]FDG, i.e., 0.013 mSv/MBq (0.046 rem/mCi), is predicted to be approximately 30% lower than that from 2-[(18)F]FDG.

CONCLUSIONS

6-[(18)F]FDG will be safe for use in the PET scanning of humans.

Extended acute toxicity study of (188) Re-liposome in rats

Liposomes can selectively target cancer sites and carry payloads, thereby improving diagnostic and therapeutic effectiveness as well as reducing toxicity. To evaluate therapeutic strategies, it is essential to use animal models reflecting important safety aspects before clinical application. As our previous study found that a high dosage (185 of MBq) of (188) Re-N,N-bis (2-mercaptoethyl)-N',N'-diethylethylenediamine-labeled pegylated liposomes ((188) Re-liposome) induced a decrease in white blood cell (WBC) count in Sprague-Dawley rats 7 days postinjection, the objective of the present study was to investigate extended acute radiotoxicity of (188) Re-liposome. Rats were administered via intravenous (i.v.) injection with (188) Re-liposome (185, 55.5 and 18.5 MBq), normal saline as a blank control or non-radioactive liposome as a vehicle control. Mortality, clinical signs, food consumption, body weights, urinary, biochemical and hematological analyzes were examined. In addition, gross necropsy and histopathological examinations were also performed at the end of the follow-up period. None of the rats died and no clinical sign was observed during the 28-day study period. Only male rats receiving (188) Re-liposome at a high dosage (185 MBq) displayed a slight weight loss compared with the control rats. In both male and female rats, the WBC counts of both high-dose and medium-dose (55.5 MBq) groups reduced significantly 7 days postinjection, but recovered to the normal range on Study Day 29. There was no significant difference in urinary analyzes, biochemical parameters and histopathological assessments between the (188) Re-liposome-treated and control groups. The information generated from the present study on extended acute toxicity of (188) Re-liposome will serve as a safety reference for radiopharmaceuticals in early-phase clinical trials.

Synthesis and in vivo evaluation of [18F]N-(2-benzofuranylmethyl)-N'-[4-(2-fluoroethoxy)benzyl]piperazine, a novel σ1 receptor PET imaging agent

N-(2-Benzofuranylmethyl)-N'-[4-(2-fluoroethoxy)benzyl]piperazine (6, σ(1)K(i)=2.6 nM) was radiolabeled with fluorine-18 to provide a potential σ(1) receptor radioligand for use in positron emission tomography (PET). Radiofluorination of the appropriate tosylate precursor furnished [(18)F]6 with a specific activity of 45 GBq/μmol, in an average radiochemical yield of 18% and greater than 98% radiochemical purity. MicroPET imaging in Papio hamadryas baboon brain revealed [(18)F]6 uptake consistent with σ receptor distribution, and specificity for σ receptors was demonstrated in a haloperidol pre-treated animal. [(18)F]6 possesses suitable properties for PET imaging of σ(1) receptors, and further investigation of this σ(1) receptor tracer is warranted.

Synthesis and in vitro biological evaluation of carbonyl group-containing analogues for σ1 receptors

To identify the ligands for σ(1) receptors that are potent and selective, analogues of prezamicol and trozamicol scaffolds of carbonyl-containing vesicular acetylcholine transporter (VAChT) inhibitors were explored. Of the 23 analogues synthesized and tested, 5 displayed very high affinity for σ(1) (K(i) = 0.48-4.05 nM) and high selectivity for σ(1) relative to σ(2) receptors (σ(1)/σ(2) selectivity of >749-fold). Four of the five compounds (14a, 14b, 14c, and 14e) showed very low affinity for VAChT (K(i) > 290 nM), and the fifth compound (14g) showed moderate affinity for VAChT (K(i) = 44.2 nM). The compound [1'-(4-fluorobenzyl)-3'-hydroxy[1,4']bipiperidinyl-4-yl]-(4-fluorophenyl)methanone (14a) displayed very high affinity and selectivity for σ(1) receptor (K(i) = 0.48 nM, σ(1)/σ(2) > 3600). All four of these most promising compounds (14a, 14b, 14c, and 14e) can be radiosynthesized with fluorine-18 or carbon-11, which will allow further evaluation of their properties as PET probes for imaging σ(1) receptor in vivo.

Molecular imaging of σ receptors: synthesis and evaluation of the potent σ1 selective radioligand [18F]fluspidine

PURPOSE

Neuroimaging of σ(1) receptors in the human brain has been proposed for the investigation of the pathophysiology of neurodegenerative and psychiatric diseases. However, there is a lack of suitable (18)F-labelled PET radioligands for that purpose.

METHODS

The selective σ(1) receptor ligand [(18)F]fluspidine (1'-benzyl-3-(2-[(18)F]fluoroethyl)-3H-spiro[[2]benzofuran-1,4'-piperidine]) was synthesized by nucleophilic (18)F(-) substitution of the tosyl precursor. In vitro receptor binding affinity and selectivity were assessed by radioligand competition in tissue homogenate and autoradiographic approaches. In female CD-1 mice, in vivo properties of [(18)F]fluspidine were evaluated by ex vivo brain section imaging and organ distribution of intravenously administered radiotracer. Target specificity was validated by organ distribution of [(18)F]fluspidine after treatment with 1 mg/kg i.p. of the σ receptor antagonist haloperidol or the emopamil binding protein (EBP) inhibitor tamoxifen. In vitro metabolic stability and in vivo metabolism were investigated by LC-MS(n) and radio-HPLC analysis.

RESULTS

[(18)F]Fluspidine was obtained with a radiochemical yield of 35-45%, a radiochemical purity of ≥ 99.6% and a specific activity of 150-350 GBq/μmol (n = 6) within a total synthesis time of 90-120 min. In vitro, fluspidine bound specifically and with high affinity to σ(1) receptors (K (i) = 0.59 nM). In mice, [(18)F]fluspidine rapidly accumulated in brain with uptake values of 3.9 and 4.7%ID/g and brain to blood ratios of 7 and 13 at 5 and 30 min after intravenous application of the radiotracer, respectively. By ex vivo autoradiography of brain slices, resemblance between binding site occupancy of [(18)F]fluspidine and the expression of σ(1) receptors was shown. The radiotracer uptake in the brain as well as in peripheral σ(1) receptor expressing organs was significantly inhibited by haloperidol but not by tamoxifen. Incubation with rat liver microsomes led to a fast biotransformation of fluspidine. After an incubation period of 30 min only 13% of the parent compound was left. Seven metabolites were identified by HPLC-UV and LC-MS(n) techniques. However, [(18)F]fluspidine showed a higher metabolic stability in vivo. In plasma samples ∼ 94% of parent compound remained at 30 min and ∼ 67% at 60 min post-injection. Only one major radiometabolite was detected. None of the radiometabolites crossed the blood-brain barrier.

CONCLUSION

[(18)F]Fluspidine demonstrated favourable target affinity and specificity as well as metabolic stability both in vitro and in animal experiments. The in vivo properties of [(18)F]fluspidine offer a high potential of this radiotracer for neuroimaging and quantitation of σ(1) receptors in vivo.

Preliminary evaluation of acute toxicity of (188) Re-BMEDA-liposome in rats

Liposomes can selectively target cancer sites and carry payloads, thereby improving diagnostic and therapeutic effectiveness and reducing toxicity. To evaluate therapeutic strategies, it is essential to use animal models reflecting important safety aspects before clinical application. The objective of this study was to investigate acute radiotoxicity of ¹⁸⁸Re-N,N-bis (2-mercaptoethyl)-N',N'-diethylethylenediamine (BMEDA)-labeled pegylated liposomes (¹⁸⁸Re-BMEDA-liposome) in Sprague-Dawley rats. Rats were administered with ¹⁸⁸Re-BMEDA-liposome, normal saline as blank or non-radioactive liposome as vehicle control via intravenous injection and observed for 14 days. Examinations were conducted with respect to mortality, clinical signs, food consumption, body weight and hematological and biochemical analyses. In addition, gross necropsy, histopathological examinations and cytogenetic analyses were also performed. None of the rats died and no clinical sign was observed during the 14-day study period. Rats administered with ¹⁸⁸Re-BMEDA-liposome at dosage of 185 MBq displayed a significant weight loss compared with the control from study day (SD) 1 to SD 4, and the white blood cell count reduced to 5-10% of initial value (female: 18.55 ± 6.58 to 0.73 ± 0.26 x 10³  µl⁻¹; male: 14.52 ± 5.12 to 1.43 ± 0.54 x 10³  µl⁻¹) 7 days-post injection, but were found to have recovered on SD 15. There were no significant differences in biochemical parameters and histopathological assessments between the ¹⁸⁸Re-BMEDA-liposome-treated and control groups. The frequencies of dicentric chromosomes were associated with dosage of ¹⁸⁸Re-BMEDA-liposome. The information generated from this study on acute toxicity will serve as a safety reference for further subacute toxicity study in rats and human clinical trials.

Early development of sigma-receptor ligands

Sigma receptors (σ-1 and σ-2) are non-opioid proteins implicated in the pathophysiology of various neurological disorders and cancer. The σ-1 subtype is a chaperon protein widely distributed in the CNS and peripheral tissues. These receptors are involved in the modulation of K+- and Ca2+-dependent signaling cascades at the endoplasmic reticulum and modulation of neurotransmitter release. σ-1 receptors are emerging targets for the treatment of neurophychiatric diseases (schizophrenia and depression) and cocaine addiction. σ-2 receptors are lipid raft proteins. They are highly expressed on many tumor cells and hence considered potential targets for anticancer drugs. σ receptors bind to a diverse class of pharmacological compounds like cocaine, methamphetamine, benzomorphans like (±)-pentazocine, (±)-SKF-10,047 and endogenous neurosteroids and sphingolipids. In this review we focus on the early development of σ receptor-specific ligands and radiolabeling agents.

The sigma receptor: evolution of the concept in neuropsychopharmacology

Although originally proposed as a subtype of opioid receptors, the sigma receptor is now confirmed to be a non-opioid receptor that binds diverse classes of psychotropic drugs. Sigma receptors are subdivided into two subtypes, sigma-1 and sigma-2. The sigma-1 receptor is a 25-kDa protein possessing one putative transmembrane domain and an endoplasmic reticulum retention signal. Sigma-1 receptors are highly expressed in deeper laminae of the cortex, olfactory bulb, nuclei of mesencephalon, hypothalamus, and Purkinje cells in the brain. Sigma-1 receptors are predominantly localized at the endoplasmic reticulum of both neurons and oligodendrocytes. From behavioral studies, sigma-1 receptors were shown to be involved in higher-ordered brain functions including memory and drug dependence. The actions mediated by sigma-1 receptors at the cellular level can be considered either as acute or chronic. The acute actions include the modulation of ion channels (i.e., K+ channel, NMDA receptors, IP3 receptors) and the sigma-1 receptor translocation. Chronic actions of sigma-1 receptors are basically considered to be the result of an up- or down regulation of the sigma-1 receptor itself. For example, the upregulation of sigma-1 receptors per se, even without exogenous ligands, promotes cellular differentiation and reconstitution of lipid microdomains (lipid rafts) in cultured cells. These findings together suggest that sigma-1 receptors might possess a constitutive biological activity, and that sigma-1 receptor ligands might merely work as modulators of the innate activity of this protein. Recent in vitro and in vitro studies strongly point to the possibility that sigma-1 receptors participate in membrane remodeling and cellular differentiation in the nervous system.

Small molecule receptors as imaging targets

The aberrant expression and function of certain receptors in tumours and other diseased tissues make them preferable targets for molecular imaging. PET and SPECT radionuclides can be used to label specific ligands with high affinity for the target receptors. The functional information obtained from imaging these receptors can be used to better understand the systems under investigation and for diagnostic and therapeutic applications. This review discusses some of the aspects of receptor imaging with small molecule tracers by PET and SPECT and reviews some of the tracers for the receptor imaging of tumours and brain, heart and lung disorders.

In vivo characterization of radioiodinated (+)-2-[4-(4-iodophenyl) piperidino] cyclohexanol as a potential sigma-1 receptor imaging agent

In this study, the (+)-enantiomer of radioiodinated 2-[4-(4-iodophenyl)piperidino]cyclohexanol [(+)-[(125)I]-p-iodovesamicol] [(+)-[(125)I]pIV], which is reported to bind with high affinity to sigma-1 receptors in vitro, was tested for its usefulness in imaging sigma-1 receptors in the central nervous system (CNS) in vivo. In biodistribution studies, significant amounts (approximately 3% of the injected dose) of (+)-[(125)I]pIV accumulated in rat brain, and its retention was prolonged. In blocking studies, the accumulation of (+)-[(125)I]pIV in the rat brain was significantly reduced by the coadministration of sigma-ligands such as pentazocine (5.0 micromol), haloperidol (0.5 micromol) or SA4503 (0.5 micromol). The blocking effect of pentazocine (selective sigma-1 ligand) was similar to the blocking effects of SA4503 and haloperidol [nonselective sigma (sigma-1 and sigma-2) ligands]. Ex vivo autoradiography of the rat brain at 45 min following intravenous injection of (+)-[(125)I]pIV showed high localization in brain areas rich in sigma-1 receptors. Thus, the distribution of (+)-[(125)I]pIV was thought to bind to sigma-1 receptors in the CNS in vivo. These results indicate that radioiodinated (+)-pIV may have the potential to image sigma-1 receptors in vivo.

Preclinical acute toxicity studies and dosimetry estimates of the novel sigma-1 receptor radiotracer, [18F]SFE

[(18)F]1-(2-Fluoroethyl)-4-[(4-cyanophenoxy)methyl]piperidine ([(18)F]SFE) is a novel, selective, high-affinity sigma-1 receptor radioligand that has been preclinically well characterized in rodents. To support an investigational new drug (IND) application for the first evaluation of [(18)F]SFE in humans, single-organ and whole-body radiation adsorbed doses associated with [(18)F]SFE injection were estimated from rat distribution data. In addition, single- and multiple-dose toxicity studies were conducted in rabbits and in dogs. Multiple-dose toxicity studies in rabbits and single-dose toxicity studies in beagles suggest at least a 100-fold safety margin for humans studies at a mass dose limit of 4.0 mug per intravenous injection, based on the combined no observable adverse effect levels (NOAEL, mg/m(2)) measured in these species. Radiation dosimetry estimates obtained from rat biodistribution analyses of [(18)F]SFE suggest that most tissues would receive about 0.010-0.020 mGy/MBq, while the adrenal glands, brain, bone, liver, lungs, and spleen would receive slightly higher doses (0.024-0.044 mGy/MBq). The adrenal glands were identified as the critical organ, because they received the highest adsorbed radiation dose. The total exposure resulting from a 5 mCi administration of [(18)F]SFE is well below the FDA-defined limits for yearly cumulative and per-study exposures to research participants. These combined results support the expectation that [(18)F]SFE will be safe for use in human positron emission tomography (PET) imaging studies with the administration of 5 mCi and a mass dose equal to or less than 4.0 mug SFE per injection.

[123I]TPCNE--a novel SPET tracer for the sigma-1 receptor: first human studies and in vivo haloperidol challenge

[123I]TPCNE (1(trans-[123I]iodopropen-2-yl)-4-[(4-cyanophenoxy)methyl] piperidine; Ki = 0.67 nM; log P = 3.36) is a novel sigma-1 receptor SPET ligand. In this study, we developed an optimized labeling method for [123I]TPCNE and investigated the kinetics, binding characteristics, and whole-body distribution of this tracer for the first time in humans. We also performed a challenge with the sigma-1 receptor antagonist haloperidol against [123I]TPCNE. Seven healthy volunteers were recruited. Dynamic brain SPET scans were performed following i.v. administration of 185 MBq [123I]TPCNE in all seven subjects. Three of the subjects were given oral haloperidol (2.5 mg) approximately 1 h before the scan. The dynamic data were analyzed with both reversible and irreversible compartmental models.[123I]TPCNE showed high uptake in brain and liver. All non-haloperidol-treated subjects showed a high whole-brain uptake (average: 8.7% of injected activity). No significant clearance of the tracer was seen up to 30 h post injection. In the haloperidol-treated subjects, the time-activity curves clearly demonstrated clearance of the tracer from the brain. Regional radioactivity concentrations were reduced by haloperidol from 42% in the cerebellum to 73% in the thalamus.[(123)I]TPCNE demonstrated high brain uptake, with highest binding found in the posterior cingulate. A region in which binding was unaffected by haloperidol pretreatment could not be identified, and the time-activity data were best described by an irreversible model.

In vivo evaluation in rats of [(18)F]1-(2-fluoroethyl)-4-[(4-cyanophenoxy)methyl]piperidine as a potential radiotracer for PET assessment of CNS sigma-1 receptors

INTRODUCTION

Sigma-1 receptors are expressed throughout the mammalian central nervous system (CNS) and are implicated in several psychiatric disorders, including schizophrenia and depression. We have recently evaluated the high-affinity (K(D)=0.5+/-0.2 nM, log P=2.9) sigma-1 receptor radiotracer [(18)F]1-(3-fluoropropyl)-4-(4-cyanophenoxymethyl)piperidine, [(18)F]FPS, in humans. In contrast to appropriate kinetics exhibited in baboon brain, in the human CNS, [(18)F]FPS does not reach pseudoequilibrium by 4 h, supporting the development of a lower-affinity tracer [Waterhouse RN, Nobler MS, Chang RC, Zhou Y, Morales O, Kuwabara H, et al. First evaluation of the sigma-1 receptor radioligand [(18)F]1-3-fluoropropyl-4-((4-cyanophenoxy)-methyl)piperidine ([(18)F]FPS) in healthy humans. Neuroreceptor Mapping 2004, July 15-18th, Vancouver, BC Canada 2004]. We describe herein the in vivo evaluation in rats of [(18)F]1-(2-fluoroethyl)-4-[(4-cyanophenoxy)methyl]piperidine ([(18)F]SFE) (K(D)=5 nM, log P=2.4), a structurally similar, lower-affinity sigma-1 receptor radioligand.

METHODS

[(18)F]SFE was synthesized (n=4) as previously described in good yield (54+/-6% EOB), high specific activity (2.1+/-0.6 Ci/micromol EOS) and radiochemical purity (98+/-1%) and evaluated in awake adult male rats.

RESULTS

Similar to [(18)F]FPS, regional brain radioactivity concentrations [percentage of injected dose per gram of tissue (%ID/g), 15 min] for [(18)F]SFE were highest in occipital cortex (1.86+/-0.06 %ID/g) and frontal cortex (1.76+/-0.38 %ID/g), and lowest in the hippocampus (1.01+/-0.02%ID/g). Unlike [(18)F]FPS, [(18)F]SFE cleared from the brain with approximately 40% reduction in peak activity over a 90-min period. Metabolite analysis (1 h) revealed that [(18)F]SFE was largely intact in the brain. Blocking studies showed a large degree (>80%) of saturable binding for [(18)F]SFE in discrete brain regions.

CONCLUSIONS

We conclude that [(18)F]SFE exhibits excellent characteristics in vivo and may provide a superior PET radiotracer for human studies due to its faster CNS clearance compared to [(18)F]FPS.

N-[18F]4'-fluorobenzylpiperidin-4yl-(2-fluorophenyl) acetamide ([18F]FBFPA): a potential fluorine-18 labeled PET radiotracer for imaging sigma-1 receptors in the CNS

A series of brain uptake studies and PET imaging studies were conducted with the sigma(1) selective imaging agent, [(18)F]FBFPA. The results of the study indicate that this radiotracer readily crosses the blood-brain barrier and labels sigma(1) receptors in vivo. In vivo blocking studies with a sigma(1) selective ligand and a nonselective sigma(1)/sigma(2) receptor ligand indicates that [(18)F]FBFPA labels sigma(1) and not sigma(2) receptors in rodent brain. PET imaging studies demonstrated a high uptake in regions of rhesus monkey brain having a high density of sigma(1) receptors. The uptake of [(18)F]FBFPA was displaced by the sigma ligand, haloperidol (1 mg/kg, i.v.). In vivo blocking studies indicate that the progesterone blocked the brain uptake of [(18)F]FBFPA in rat brain. These data indicate that [(18)F]FBFPA is a potential radiotracer for imaging sigma(1) receptors in the CNS in vivo with PET.