SA4503: A Novel Sigma1 Receptor Agonist

The Sigma Receptors in Alzheimer's Disease: New Potential Targets for Diagnosis and Therapy

Sigma (σ) receptors are a class of unique proteins with two subtypes: the sigma-1 (σ1) receptor which is situated at the mitochondria-associated endoplasmic reticulum (ER) membrane (MAM), and the sigma-2 (σ2) receptor, located in the ER-resident membrane. Increasing evidence indicates the involvement of both σ1 and σ2 receptors in the pathogenesis of Alzheimer's disease (AD), and thus these receptors represent two potentially effective biomarkers for emerging AD therapies. The availability of optimal radioligands for positron emission tomography (PET) neuroimaging of the σ1 and σ2 receptors in humans will provide tools to monitor AD progression and treatment outcomes. In this review, we first summarize the significance of both receptors in the pathophysiology of AD and highlight AD therapeutic strategies related to the σ1 and σ2 receptors. We then survey the potential PET radioligands, with an emphasis on the requirements of optimal radioligands for imaging the σ1 or σ2 receptors in humans. Finally, we discuss current challenges in the development of PET radioligands for the σ1 or σ2 receptors, and the opportunities for neuroimaging to elucidate the σ1 and σ2 receptors as novel biomarkers for early AD diagnosis, and for monitoring of disease progression and AD drug efficacy.

Widespread cell stress and mitochondrial dysfunction occur in patients with early Alzheimer's disease

Cell stress and impaired oxidative phosphorylation are central to mechanisms of synaptic loss and neurodegeneration in the cellular pathology of Alzheimer's disease (AD). In this study, we quantified the in vivo expression of the endoplasmic reticulum stress marker, sigma 1 receptor (S1R), using [¹¹C]SA4503 positron emission tomography (PET), the mitochondrial complex I (MC1) with [¹⁸F]BCPP-EF, and the presynaptic vesicular protein SV2A with [¹¹C]UCB-J in 12 patients with early AD and in 16 cognitively normal controls. We integrated these molecular measures with assessments of regional brain volumes and cerebral blood flow (CBF) measured with magnetic resonance imaging arterial spin labeling. Eight patients with AD were followed longitudinally to estimate the rate of change of the physiological and structural pathology markers with disease progression. The patients showed widespread increases in S1R (≤ 27%) and regional reduction in MC1 (≥ -28%) and SV2A (≥ -25%) radioligand binding, brain volume (≥ -23%), and CBF (≥ -26%). [¹⁸F]BCPP-EF PET MC1 binding (≥ -12%) and brain volumes (≥ -5%) showed progressive reductions over 12 to 18 months, suggesting that they both could be used as pharmacodynamic indicators in early-stage therapeutics trials. Associations of reduced MC1 and SV2A and increased S1R radioligand binding with reduced cognitive performance in AD, although exploratory, suggested a loss of metabolic functional reserve with disease. Our study thus provides in vivo evidence for widespread, clinically relevant cellular stress and bioenergetic abnormalities in early AD.

Cutamesine Overcomes REM Sleep Deprivation-Induced Memory Loss: Relationship to Sigma-1 Receptor Occupancy

PURPOSE

Rapid eye movement (REM) sleep deprivation (SD) decreases cerebral sigma-1 receptor expression and causes cognitive deficits. Sigma-1 agonists are cognitive enhancers. Here, we investigate the effect of cutamesine treatment in the REM SD model.

PROCEDURES

Sigma-1 receptor occupancy (RO) in the rat brain by cutamesine was determined using 1-[2-(3,4-dimethoxyphenethyl)]-4-(3-phenylpropyl)piperazine ([(11)C]SA4503) and positron emission tomography (PET), and tissue cutamesine levels were measured by ultra performance liquid chromatography (UPLC)-MS. RO was calculated from a Cunningham-Lassen plot, based on the total distribution volume of [(11)C]SA4503 determined by Logan graphical analysis. Cognitive performance was assessed using the passive avoidance (PA) test.

RESULTS

Cutamesine at a dose of 1.0 mg/kg reversed REM SD-induced cognitive deficit and occupied 92 % of the sigma-1 receptor population. A lower dose (0.3 mg/kg) occupied 88 % of the receptors but did not significantly improve cognition.

CONCLUSION

The anti-amnesic effect of cutamesine in this animal model may be related to longer exposure at a higher dose and/or drug binding to secondary targets.

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.

Potentiation of nerve growth factor-induced neurite outgrowth by fluvoxamine: role of sigma-1 receptors, IP3 receptors and cellular signaling pathways

BACKGROUND

Selective serotonin reuptake inhibitors (SSRIs) have been widely used and are a major therapeutic advance in psychopharmacology. However, their pharmacology is quite heterogeneous. The SSRI fluvoxamine, with sigma-1 receptor agonism, is shown to potentiate nerve-growth factor (NGF)-induced neurite outgrowth in PC 12 cells. However, the precise cellular and molecular mechanisms underlying potentiation by fluvoxamine are not fully understood. In this study, we examined the roles of cellular signaling pathways in the potentiation of NGF-induced neurite outgrowth by fluvoxamine and sigma-1 receptor agonists.

METHODS AND FINDINGS

The effects of three SSRIs (fluvoxamine, sertraline, paroxetine) and three sigma-1 receptor agonists (SA4503, 4-phenyl-1-(4-phenylbutyl) piperidine (PPBP), and dehydroepiandrosterone (DHEA)-sulfate) on NGF-induced neurite outgrowth in PC12 cells were examined. Also examined were the effects of the sigma-1 receptor antagonist NE-100, inositol 1,4,5-triphosphate (IP(3)) receptor antagonist, and specific inhibitors of signaling pathways in the potentiation of NGF-induced neurite outgrowth by selective sigma-1 receptor agonist SA4503. Fluvoxamine (but not sertraline or paroxetine) and the sigma-1 receptor agonists SA4503, PPBP, and DHEA-sulfate significantly potentiated NGF-induced neurite outgrowth in PC12 cells in a concentration-dependent manner. The potentiation by fluvoxamine and the three sigma-1 receptor agonists was blocked by co-administration of the selective sigma-1 receptor antagonist NE-100, suggesting that sigma-1 receptors play a role in blocking the enhancement of NGF-induced neurite outgrowth. Moreover, the potentiation by SA4503 was blocked by co-administration of the IP(3) receptor antagonist xestospongin C. In addition, the specific inhibitors of phospholipase C (PLC-gamma), phosphatidylinositol 3-kinase (PI3K), p38MAPK, c-Jun N-terminal kinase (JNK), and the Ras/Raf/mitogen-activated protein kinase (MAPK) signaling pathways blocked the potentiation of NGF-induced neurite outgrowth by SA4503.

CONCLUSION

These findings suggest that stimulation of sigma-1 receptors and subsequent interaction with IP(3) receptors, PLC-gamma, PI3K, p38MAPK, JNK, and the Ras/Raf/MAPK signaling pathways are involved in the mechanisms of action of sigma-1 receptor agonists such as fluvoxamine and SA4503.

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.

Mapping of human cerebral sigma1 receptors using positron emission tomography and [11C]SA4503

The objective of this study was to establish the kinetic analysis for mapping sigma(1) receptors (sigma1Rs) in the human brain by positron emission tomography (PET) with [(11)C]SA4503. The sigma1Rs are considered to be involved in various neurological and psychiatric diseases. [(11)C]SA4503 is a recently developed radioligand with high and selective affinity for sigma1Rs, and we have first applied it to clinical studies. Nine healthy male subjects each underwent a dynamic 90-min PET scan after injection of [(11)C]SA4503. In addition to the baseline measurement, three of the nine subjects underwent a second [(11)C]SA4503-PET after partial blockade of sigma1Rs by oral administration of haloperidol, a sigma receptor antagonist. Full kinetic analysis using two times nonlinear estimations was applied for fitting a two-tissue three-compartment model to determine the binding potential (BP) and total distribution volume (tDV) of [(11)C]SA4503. Graphical analysis with a Logan plot was also applied for estimations of tDV. The regional distribution patterns of BP and tDV in 11 regions were compatible with those of previously reported sigma1Rs in vitro. The reduced binding sites of sigma1Rs by haloperidol were appropriately evaluated. The tDVs derived from the two methods matched each other well. The Logan plot offered images of the tDV, which reflected sigma1R densities, and the tDV in the images decreased after haloperidol loading. Moreover, comparison of BPs calculated with and without metabolite correction for plasma input function indicated that the metabolite correction could be omitted. We concluded that this method enables the quantitative analysis of sigma1Rs in the human brain.

Phencyclidine-induced cognitive deficits in mice are improved by subsequent subchronic administration of fluvoxamine: role of sigma-1 receptors

This study was undertaken to examine the effects of the selective serotonin reuptake inhibitors fluvoxamine and paroxetine on cognitive deficits in mice after repeated administration of the N-methyl-D-aspartate receptor antagonist phencyclidine (PCP). In the novel object recognition test, repeated administration of PCP (10 mg/kg/day, 10 days) significantly decreased the exploratory preference in the retention test session, but not in the training test session. PCP-induced cognitive deficits were significantly improved by subsequent subchronic (2-week) administration of fluvoxamine (20 mg/kg/day), but not paroxetine (10 mg/kg/day). Furthermore, the effect of fluvoxamine on PCP-induced cognitive deficits was antagonized by co-administration of the selective sigma-1 receptor antagonist NE-100 (1 mg/kg/day). Moreover, PCP-induced cognitive deficits were also significantly improved by subsequent subchronic (2-week) administration of the selective sigma-1 receptor agonist SA4503 (1 mg/kg/day) or neurosteroid dehydroepiandrosterone 3-sulfate (DHEA-S; 25 mg/kg/day). The effects of SA4503 or DHEA-S were also antagonized by co-administration of NE-100 (1 mg/kg/day), suggesting the role of sigma-1 receptors in the active mechanisms of these drugs. In contrast, acute single administration of these drugs (fluvoxamine, paroxetine, SA4503) alone or combination with NE-100 did not alter PCP-induced cognitive deficits. The present study suggests that agonistic activity of fluvoxamine at sigma-1 receptors plays a role in the active mechanisms of fluvoxamine on PCP-induced cognitive deficits in mice. Therefore, sigma-1 receptor agonists such as fluvoxamine would be potential therapeutic drugs for the treatment of the cognitive deficits of schizophrenia.

Sigma1 and sigma2 receptor binding affinity and selectivity of SA4503 and fluoroethyl SA4503

SA4503, a potent sigma(1) receptor agonist, is reported as having 103-fold higher affinity for sigma(1) (IC(50) = 17.4 nM) than sigma(2) (IC(50) = 1,784 nM) sites in guinea pig brain membranes. Modest structural changes appear to have major effects on sigma(1)/sigma(2) selectivity. The fluoroethyl analog, FE-SA4503, is described as having high primary affinity for sigma(2) sites (IC(50) = 2.11 nM) and a weaker interaction with sigma(1) sites (IC(50) = 6.48 nM). SA4503 and FE-SA4503 have been radiolabeled for PET studies, and both bind selectively to sigma(1) receptors in animal and human brain in vivo. We prepared SA4503 and FE-SA4503 as reference compounds for radioligand development efforts. In our hands, SA4503 is 14-fold selective for sigma(1) (K(i) = 4.6 nM) over sigma(2) (K(i) = 63.1 nM) sites in guinea pig brain homogenates. Further, FE-SA4503 exhibits the same 14-fold selectivity for sigma(1) (K(i) = 8.0 nM) over sigma(2) (K(i) = 113.2 nM) receptors. The main differences from previously reported values stem from sigma(2) affinity determinations. This protocol, displacement of [(3)H]DTG binding to sigma(2) sites using (+)-pentazocine (200 nM) to mask sigma(1) sites, was validated by the proper rank order of sigma(2) inhibitory potencies shown by a panel of additional ligands: ifenprodil > haloperidol > DTG >> (+)-pentazocine. Robust Pearson correlation (r = 1.0, P = 0.002; slope = 0.97) was observed for our pK(i) values against those from a prior study by others. The findings have bearing on structure-activity relationships for this active series, and on conclusions that might be drawn from experiments relying upon defined sigma(1)/sigma(2) binding selectivity.

Evaluation of (+)-p-[11C]methylvesamicol for mapping sigma1 receptors: a comparison with [11C]SA4503

Vesamicol is a leading compound for positron emission tomography (PET) and single photon emission computed tomography (SPECT) tracers for mapping the vesicular acetylcholine transporter (VAChT). Recently, we found that (+)-p-methylvesamicol ((+)-PMV) has low affinity for VAChT (K(i)=199 nM), but has moderate to high affinity for sigma receptors: K(i)=3.0 nM for sigma1 and K(i)=40.7 nM for sigma2, and that sigma1-selective SA4503 (K(i)=4.4 nM for sigma1 and K(i)=242 nM for sigma2) has moderate affinity for VAChT (K(i)=50.2 nM). In the present study, we examined the potential of (+)-[11C]PMV as a PET radioligand for mapping sigma1 receptors as compared with [11C]SA4503. In rat brain, similar regional distribution patterns of (+)-[11C]PMV and [11C]SA4503 were shown by tissue dissection and by ex vivo autoradiography. Blocking experiments using (+/-)-PMV, (-)-vesamicol, SA4503, haloperidol and (+/-)-pentazocine showed that the two tracers specifically bound to sigma1 receptors, and that [11C]SA4503 exhibited greater specific binding than (+)-[11C]PMV. No sign of VAChT-specific binding by [11C]SA4503 was observed in the striatum, which is rich in VAChT sites. In conclusion, (+)-[11C]PMV specifically bound to sigma1 receptors in the brain, but to a lesser extent than [11C]SA4503, suggesting that (+)-[11C]PMV is a less preferable PET ligand than [11C]SA4503. On the other hand, the moderate affinity of [11C]SA4503 for VAChT is negligible in vivo.

Evaluation of [18F]fluorinated sigma receptor ligands in the conscious monkey brain

PET-imaging of the sigma receptors is very helpful to understand processes, e.g., several central nervous system (CNS)-diseases in which the sigma receptors are involved. The [(18)F]fluoroethylated analogs of SA4503 and SA5845 ([(18)F]FE-SA4503 and [(18)F]FE-SA5845) were evaluated in conscious monkeys to estimate its suitability for human application for PET. Conscious monkeys (Macaca Mulatta) were either scanned with [(18)F]FE-SA4503 or [(18)F]FE-SA5845 (n = 3 for both groups, 220-802 MBq). After a dynamic study of 120 min, radioactivity was displaced by intravenous (i.v.) injection of haloperidol (1 mg/kg). One month later the same set of three monkeys were scanned with [(18)F]FE-SA4503 for 120 min and "cold" SA4503 (1 mg/kg) was infused to displace the radioactivity, and the other three monkeys were pretreated with haloperidol (1 mg/kg) before the 120-min PET-scan with [(18)F]FE-SA5845. Cortical areas (cingulate, frontal, occipital, parietal, temporal), striatum, and thalamus showed high radioactivity uptake. Infusion of haloperidol displaced the radioactivity levels of the two radioligands. The same effect was found for [(18)F]FE-SA4503 after SA4503 displacement. Pretreatment with haloperidol blocked the [(18)F]FE-SA5845 binding to give PET-images with low and uniform uptake in the brain. The findings demonstrated the reversible binding of the two radioligands. Metabolite analysis showed that 14% and 23% parent compound of [(18)F]FE-SA5845 and [(18)F]FE-SA4503, respectively, at 120 min postinjection was present in plasma. Kinetic analysis showed that the binding potential of [(18)F]FE-SA5845 was higher in all brain regions than that of [(18)F]FE-SA4503 (4.75-8.79 vs. 1.65-4.04). The highest binding potential was found in the hippocampus, followed by the cortical regions, thalamus, cerebellar hemisphere, striatum and vermis. Both [(18)F]FE-SA compounds bound specifically to cerebral sigma receptors of the monkey and have potential for mapping sigma receptors in the human brain.

An increase of sigma receptors in the aged monkey brain

We evaluated in vivo the effect of aging on the sigma(1) receptors in the monkey brain by the quantitative analysis of the binding of [11C]SA4503 to sigma(1) receptors with positron emission tomography. Based on a three-compartment model, the influx rate constant K(1) of [11C]SA4503 from plasma to brain across the blood-brain barrier in all 10 regions investigated became smaller in the aged monkeys (20-28 years old, n=5) than in the young adult monkeys (4-8 years old, n=5), but the reduction was not significant due to the individual differences. On the other hand, the binding potential, which was calculated as the ratio of the association rate constant k(3) to the dissociation rate constant k(4) for the binding of [11C]SA4503 to sigma(1) receptors in the brain, significantly increased in nine of the brain regions of the aged monkeys to the 160-210% levels of the young monkeys. We concluded that the sigma(1) receptor binding sites increased in the aging process of the monkey brain.

Evidence for sigma-1-like receptors in isolated rat liver mitochondrial membranes

1. Sigma (sigma) receptors have generated a great deal of interest on the basis of their possible roles in various pathologies, including cytoprotection. Although the exact function of sigma-1 (sigma(1)) receptors is not yet known, their role in the regulation of intracellular Ca(2+) levels and sterol biosynthesis, functions that could be assigned to mitochondria, are the only mechanisms described. 2. Using preparations of purified rat liver and brain mitochondria we demonstrate herein the presence of sigma-like binding sites. [(3)H](+)-pentazocine, a sigma(1) radioligand was used to label these sites. 3. In the liver, [(3)H](+)-pentazocine labelled one class of binding sites with high affinity (K(d)=3 nM), similar to that observed in liver microsomes and synaptic membranes. These sites were located on the outer mitochondrial membranes and displayed high affinity for other sigma(1) ligands namely, haloperidol, ifenprodil, carbetapentane or 1,3-di(2-tolyl)guanidine (DTG). 4. The presence of sigma(1) receptors on liver mitochondria was confirmed using double fluorescence immunostaining. 5. [(3)H](+)-pentazocine binding sites were also found on brain mitochondria but they appeared pharmacologically distinct to the liver ones as [(3)H](+)-pentazocine and typical sigma(1) ligands displayed lower affinities for these sites. Nevertheless, [(3)H](+)-pentazocine binding on both liver and brain mitochondria was modulated by progesterone, a putative endogenous ligand for sigma receptors. 6. Our data demonstrates the presence of [(3)H](+)-pentazocine binding sites with pharmacological characteristics identical to sigma(1) receptors on rat liver mitochondrial membranes. The pharmacological significance of these sites and their role on mitochondrial function remain unknown.

Preclinical evaluation of [11C]SA4503: radiation dosimetry, in vivo selectivity and PET imaging of sigma1 receptors in the cat brain

Our previous in vivo study with rats has demonstrated that 11C-labeled 1-(3,4-dimethoxyphenethyl)-4-(3-phenylpropyl)piperazine ([11C]SA4503) is a potential radioligand for mapping CNS sigmal receptors by positron emission tomography (PET). In the present study, we further characterized this ligand. The radiation absorbed-dose of [11C]SA4503 in humans estimated with the tissue distribution in mice, was higher in the liver, kidney and pancreas than in other organs studied, but was low enough for clinical use. The brain uptake of [11C]SA4503 in mice was reduced to approximately 60-70% by co-injection of carrier SA4503 and haloperidol, but not by co-injection of any of six ligands for sigma2 or other receptors, for which SA4503 showed in vitro >100 times weaker affinity than for signal receptor. In the cat brain, the uptake in the cortex was higher than that in the cerebellum. The radioactivity in the cortex and cerebellum accumulated for the first 10 min and then gradually decreased until 81.5 min in the baseline measurement, but rapidly decreased in the carrier-loading condition. The receptor-mediated uptake was estimated to be approximately 60-65% of the total radioactivity in the cortex and cerebellum at 76 min after tracer injection. We have concluded that [11C]SA4503 has the potential for mapping sigma1 receptor by PET.

[(3)H]-trimetazidine mitochondrial binding sites: regulation by cations, effect of trimetazidine derivatives and other agents and interaction with an endogenous substance

Trimetazidine, an antiischaemic drug, has been shown to restore impaired mitochondrial functions. Specific binding sites for [(3)H]-trimetazidine have been previously detected in liver mitochondria. In the present study we confirm this observation and provide additional evidence for the involvement of these sites in the pharmacological effects of the drug. Inhibition experiments using a series of trimetazidine derivatives revealed the presence of three classes of binding sites. An N-benzyl substituted analogue of trimetazidine exhibited a very high affinity (K(i)=7 nM) for one of these classes of sites. Compounds from different pharmacological classes were evaluated for their ability to inhibit [(3)H]-trimetazidine binding. Among the drugs tested pentazocine, ifenprodil, opipramol, perphenazine, haloperidol, and to a lower extent prenylamine, carbetapentane and dextromethorphan competed with high affinity, suggesting a similarity of high affinity [(3)H]-trimetazidine sites with sigma receptors. [(3)H]-Trimetazidine binding was modulated by pH. Neutral trimetazidine had about 10 fold higher affinity than protonated trimetazidine for its mitochondrial binding sites. Various cations also affected [(3)H]-trimetazidine binding. Ca(2+) was the most potent inhibitor and totally suppressed the binding of [(3)H]-trimetazidine to the sites of medium affinity. An endogenous cytosolic ligand was able to displace [(3)H]-trimetazidine from its binding sites. Its activity was not affected by boiling for 15 min, suggesting a non-protein compound. These data suggest that mitochondrial [(3)H]-trimetazidine binding sites could have a physiological relevance and be involved in the antiischaemic effects of the drug.

Effects of Hypericum perforatum extract on ethanol intake, and on behavioral despair: a search for the neurochemical systems involved

The present study investigated the possible involvement of sigma receptors and of serotonergic mechanisms in the effects of Hypericum perforatum extract (HPE) on immobility time in the forced swimming test (FST) and on ethanol intake in Marchigian Sardinian alcohol-preferring rats. The HPE employed was a dry extract containing 0.3% hypericin and 3.8% hyperforin. Intraperitoneal pretreatment with 20 mg/kg of the sigma receptor antagonist rimcazole (RIM), 30 min prior to HPE, completely suppressed the antiimmobility effect of HPE (3 intragastric injections of 250 mg/kg). Intracerebroventricular pretreatment with 5, 7-dihydroxytryptamine (5,7-DHT), which produced a marked depletion of brain serotonin, reduced the antiimmobility effect, although this reduction was not as pronounced as that of RIM. On the other hand, the inhibitory effect of HPE on 10% ethanol intake was modified neither by 5,7-DHT nor by RIM pretreatment. These results suggest that the antidepressant-like effect of HPE in the FST may be mediated by interaction with sigma receptors and to some extent by increased serotonergic neurotransmission. On the other hand, these mechanisms appear to be unimportant for the effect of HPE on ethanol intake.

In vivo evaluation of [(11)C]SA4503 as a PET ligand for mapping CNS sigma(1) receptors

The potential of the (11)C-labeled selective sigma(1) receptor ligand 1-(3,4-dimethoxyphenethyl)-4-(3-phenylpropyl)piperazine ([(11)C]SA4503) was evaluated in vivo as a positron emission tomography (PET) ligand for mapping sigma(1) receptors in rats. SA4503 is known to have a high affinity (IC(50) = 17.4 nM) and a higher selectivity (sigma(1)/sigma(2) = 103) for the sigma(1) receptor. A high and increasing brain uptake of [(11)C]SA4503 was found. Pre-, co- and postinjection of cold SA4503 significantly decreased uptake of [(11)C]SA4503 in the brain, spleen, heart, lung, and kidney in which sigma receptors are present as well as in the skeletal muscle. In the blocking study with one of four sigma receptor ligands including haloperidol, (+)-pentazocine, SA4503, and (-)-pentazocine (in the order of their affinity for sigma(1) receptor subtype), SA4503 and haloperidol significantly reduced the brain uptake of [(11)C]SA4503 to approximately 30% of the control, but the other two benzomorphans did not. A high specific uptake of [(11)C]SA4503 by the brain was also confirmed by ex vivo autoradiography (ARG) and PET. Ex vivo ARG showed a higher uptake in the vestibular nucleus, temporal cortex, cingulate cortex, inferior colliculus, thalamus, and frontal cortex, and a moderate uptake in the parietal cortex and caudate putamen. Peripherally, the blocking effects of the four ligands depended on their affinity for sigma(1) receptors. No (11)C-labeled metabolite was detected in the brain 30 min postinjection, whereas approximately 20% of the radioactivity was found as (11)C-labeled metabolites in plasma. These results have demonstrated that the (11)C-labeled sigma(1) receptor ligand [(11)C]SA4503 has a potential for mapping sigma(1) receptors in the central nervous system and peripheral organs.

Synthesis and in vivo evaluation of [11C]SA6298 as a PET sigma1 receptor ligand

The potential of a 11C-labeled selective sigma1 receptor ligand, 1-(3,4-dimethoxyphenethyl)-4-[3-(3,4-dichlorophenyl)propyl]piperazine ([11C]SA6298), was evaluated as a positron emission tomography (PET) ligand for mapping sigma, receptors in the central nervous system and peripheral organs. [11C]SA6298 was synthesized by methylation of the desmethyl SA6298 with [11C]CH3I, with the decay-corrected radiochemical yield of 39 +/- 5% based on [11C]CH3I and with the specific activity of 53 +/- 17 TBq/mmol within 20 min from end of bombardment (EOB). In mice, the uptake of [11C]SA6298 was significantly decreased by carrier loading in the brain, liver, spleen, heart, lung, small intestine, and kidney in which sigma receptors are present as well as in the skeletal muscle. Pretreatment with SA6298 also blocked the uptake of [11C]SA6298 by these organs except for the small intestine, but significant displacement of [11C]SA6298 by posttreatment with SA6298 was observed only in the heart, lung, and muscle. In the blocking study with one of the eight sigma receptor ligands, including haloperidol, SA6298, NE-100, (+)-pentazocine, SA4503, (-)-pentazocine, (+)-3-PPP, and (+)-SKF 10,047 (in the order of the affinity for sigma1 receptor subtype), only SA6298 and an analog SA4503 significantly reduced the brain uptake of [11C]SA6298 to approximately 80% of the control, but the other six ligands did not. Peripherally, the uptake of [11C]SA6298 by the organs described above was decreased predominantly by SA6298 or SA4503, but the blocking effects of the other five ligands except for NE-100 depended on their affinity for sigma1 receptors. The saturable brain uptake of [11C]SA6298, approximately 20%, was also observed by tissue dissection method in rats and by PET in a cat. Ex vivo autoradiography of the rat brain showed a high uptake in the cortex and thalamus. In the cat brain a relatively high uptake was found in the cortex, thalamus, striatum, and cerebellum. These results have indicated a receptor-mediated uptake of the tracer to some extent in the brain and peripheral organs. However, the tracer has a limited potential for the PET study of the brain receptors because of a relatively high nonspecific binding.

Acute and chronic administration of the selective sigma1 receptor agonist SA4503 significantly alters the activity of midbrain dopamine neurons in rats: An in vivo electrophysiological study

In this study, we examined the effect of the acute and repeated administration of the selective sigma (sigma)1 receptor agonist 1-(3, 4-dimethoxyphenethyl)-4-(3-phenylpropyl)piperazine dihydrochloride (SA4503) on the number and firing pattern of spontaneously active dopamine (DA) neurons in substantia nigra pars compacta (SNC) and ventral tegmental area (VTA) in anesthetized, male Sprague-Dawley rats. This was accomplished using the technique of in vivo extracellular single unit recording. The intravenous administration of SA4503 (0.01-1.28 mg/kg) did not significantly alter the firing rate or pattern of spontaneously active DA neurons in either the SNC or VTA. A single injection of either 0.1 or 0.3 mg/kg i.p. of SA4503 did not alter the number of spontaneously active SNC and VTA DA neurons. In contrast, a single injection of 1 mg/kg i.p. of SA4503 produced a significant decrease and increase in the number of spontaneously active SNC and VTA DA neurons, respectively. Overall, the firing pattern parameters of spontaneously active SNC DA neurons were altered more significantly than those of spontaneously active VTA DA neurons following the acute administration of SA4503. The repeated administration (one injection per day for 21 days) of 0.3 and 1 mg/kg i.p. of SA4503 produced a significant increase in the number of spontaneously active VTA DA neurons. In addition, the repeated administration of SA4503 produced a greater alteration of the firing pattern of spontaneously active VTA compared to SNC DA neurons. Our results suggest that the administration of SA4503 significantly alters the activity of spontaneously active midbrain DA neurons, particularly those in the VTA following repeated administration.