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

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.

In vitro and in vivo sigma 1 receptor imaging studies in different disease states

The sigma receptor system has been classified into two distinct subtypes, sigma 1 (σ1R) and sigma 2 (σ2R). Sigma 1 receptors (σ1Rs) are involved in many neurodegenerative diseases and different central nervous system disorders such as Alzheimer's disease, Parkinson's disease, schizophrenia, and drug addiction, and pain. This makes them attractive targets for developing radioligands as tools to gain a better understanding of disease pathophysiology and clinical diagnosis. Over the years, several σ1R radioligands have been developed to image the changes in σ1R distribution and density providing insights into their role in disease development. Moreover, the involvement of both σ1Rs and σ2Rs with cancer make these ligands, especially those that are σ2R selective, great tools for imaging different types of tumors. This review will discuss the principles of molecular imaging using PET and SPECT, known σ1R radioligands and their applications for labelling σ1Rs under different disease conditions. Furthermore, this review will highlight σ1R radioligands that have demonstrated considerable potential as biomarkers, and an opportunity to fulfill the ultimate goal of better healthcare outcomes and improving human health.

18 F-Labeled benzylpiperazine derivatives as highly selective ligands for imaging σ1 receptor with positron emission tomography

We report the design, synthesis, and evaluation of a new series of benzylpiperazine derivatives as selective σ1 receptor ligands. All seven ligands possessed low nanomolar affinity for σ1 receptors (Ki (σ1 ) = 0.31-4.19 nM) and high subtype selectivity (Ki (σ2 )/Ki (σ1 ) = 50-2448). The fluoroethoxy analogues also exhibited high selectivity toward the vesicular acetylcholine transporter (Ki (VAChT)/Ki (σ1 ) = 99-18252). The corresponding radiotracers [18 F]13, [18 F]14, and [18 F]16 with high selectivity (Ki (σ2 )/Ki (σ1 ) > 100, Ki (VAChT)/Ki (σ1 ) > 1000) were prepared in 42% to 55% radiochemical yields (corrected for decay), greater than 99% radiochemical purity (RCP), and molar activity of about 120 GBq/μmol at the end of synthesis (EOS). All three radiotracers showed high initial brain uptake in mouse (8.37-11.48% ID/g at 2 min), which was not affected by pretreatment with cyclosporine A, suggesting that they are not substrates for permeability-glycoprotein (P-gp). Pretreatment with SA4503 or haloperidol resulted in significantly reduced brain uptake (35%-62% decrease at 30 min). In particular, [18 F]16 displayed high brain-to-blood ratios and high in vivo metabolic stability. Although it may not be an optimal neuroimaging agent because of its slow kinetics in the mouse brain, [18 F]16 can serve as a lead compound for further structural modifications to explore new potential radiotracers for σ1 receptors.

Imaging sigma receptors in the brain: New opportunities for diagnosis of Alzheimer's disease and therapeutic development

The sigma-1 (σ₁) receptor is a chaperone protein located on the mitochondria-associated membrane of the endoplasmic reticulum, while the sigma-2 receptor (σ₂) is an endoplasmic reticulum-resident membrane protein. Recent evidence indicates that both of these receptors figure prominently in the pathophysiology of Alzheimer's disease (AD) and thus are targets for the development of novel, disease-modifying therapeutic strategies. Radioligand-based molecular imaging technique such as positron emission tomography (PET) imaging is a powerful tool for the investigation of protein target expression and function in living subjects. In this review, we survey the development of PET radioligands for the σ₁ or σ₂ receptors and assess their potential for human imaging applications. The availability of PET imaging with σ₁ or σ₂ receptor-specific radioligands in humans will allow the investigation of these receptors in vivo and lead to further understanding of their respective roles in AD pathogenesis and progression. Moreover, PET imaging can be used in target occupancy studies to assess target engagement and correlate receptor occupancy and therapeutic response of σ₁ receptor agonists and σ₂ receptor antagonists currently in clinical trials. It is expected that neuroimaging of σ₁ and σ₂ receptors in the brain will shed new light on AD pathophysiology and may provide us with new biomarkers for diagnosis of AD and efficacy monitoring of emerging AD therapeutic strategies.

1-(4-[18F]Fluorobenzyl)-4-[(tetrahydrofuran-2-yl)methyl]piperazine: A Novel Suitable Radioligand with Low Lipophilicity for Imaging σ1 Receptors in the Brain

We have designed and synthesized novel piperazine compounds with low lipophilicity as σ1 receptor ligands. 1-(4-Fluorobenzyl)-4-[(tetrahydrofuran-2-yl)methyl]piperazine (10) possessed a low nanomolar σ1 receptor affinity and a high selectivity toward the vesicular acetylcholine transporter (>2000-fold), σ2 receptors (52-fold), and adenosine A2A, adrenergic α2, cannabinoid CB1, dopamine D1, D2L, γ-aminobutyric acid A (GABAA), NMDA, melatonin MT1, MT2, and serotonin 5-HT1 receptors. The corresponding radiotracer [18F]10 demonstrated high brain uptake and extremely high brain-to-blood ratios in biodistribution studies in mice. Pretreatment with the selective σ1 receptor agonist SA4503 significantly reduced the level of accumulation of the radiotracer in the brain. No radiometabolite of [18F]10 was observed to enter the brain. Positron emission tomography and magnetic resonance imaging confirmed suitable kinetics and a high specific binding of [18F]10 to σ1 receptors in rat brain. Ex vivo autoradiography showed a reduced level of binding of [18F]10 in the cortex and hippocampus of the senescence-accelerated prone (SAMP8) compared to that of the senescence-accelerated resistant (SAMR1) mice, indicating the potential dysfunction of σ1 receptors in Alzheimer's disease.

Evaluation of the Enantiomer Specific Biokinetics and Radiation Doses of [(18)F]Fluspidine-A New Tracer in Clinical Translation for Imaging of σ₁ Receptors

The enantiomers of [(18)F]fluspidine, recently developed for imaging of σ₁ receptors, possess distinct pharmacokinetics facilitating their use in different clinical settings. To support their translational potential, we estimated the human radiation dose of (S)-(-)-[(18)F]fluspidine and (R)-(+)-[(18)F]fluspidine from ex vivo biodistribution and PET/MRI data in mice after extrapolation to the human scale. In addition, we validated the preclinical results by performing a first-in-human PET/CT study using (S)-(-)-[(18)F]fluspidine. Based on the respective time-activity curves, we calculated using OLINDA the particular organ doses (ODs) and effective doses (EDs). The ED values of (S)-(-)-[(18)F]fluspidine and (R)-(+)-[(18)F]fluspidine differed significantly with image-derived values obtained in mice with 12.9 μSv/MBq and 14.0 μSv/MBq (p < 0.025), respectively. A comparable ratio was estimated from the biodistribution data. In the human study, the ED of (S)-(-)-[(18)F]fluspidine was calculated as 21.0 μSv/MBq. Altogether, the ED values for both [(18)F]fluspidine enantiomers determined from the preclinical studies are comparable with other (18)F-labeled PET imaging agents. In addition, the first-in-human study confirmed that the radiation risk of (S)-(-)-[(18)F]fluspidine imaging is within acceptable limits. However, as already shown for other PET tracers, the actual ED of (S)-(-)-[(18)F]fluspidine in humans was underestimated by preclinical imaging which needs to be considered in other first-in-human studies.

(18)F-Labeled 1,4-Dioxa-8-azaspiro[4.5]decane Derivative: Synthesis and Biological Evaluation of a σ1 Receptor Radioligand with Low Lipophilicity as Potent Tumor Imaging Agent

We report the syntheses and evaluation of series of novel piperidine compounds with low lipophilicity as σ1 receptor ligands. 8-(4-(2-Fluoroethoxy)benzyl)-1,4-dioxa-8-azaspiro[4.5]decane (5a) possessed high affinity (K(i) = 5.4 ± 0.4 nM) for σ1 receptors and selectivity for σ2 receptors (30-fold) and the vesicular acetylcholine transporter (1404-fold). [(18)F]5a was prepared using a one-pot, two-step labeling procedure in an automated synthesis module, with a radiochemical purity of >95%, and a specific activity of 25-45 GBq/μmol. Cellular association, biodistribution, and autoradiography with blocking experiments indicated specific binding of [(18)F]5a to σ1 receptors in vitro and in vivo. Small animal positron emission tomography (PET) imaging using mouse tumor xenograft models demonstrated a high accumulation in human carcinoma and melanoma. Treatment with haloperidol significantly reduced the accumulation of the radiotracer in tumors. These findings suggest that radiotracer with suitable lipophilicity and appropriate affinity for σ1 receptors could be used for tumor imaging.

Distinctive in vivo kinetics of the new σ1 receptor ligands (R)-(+)- and (S)-(-)-18F-fluspidine in porcine brain

Because of their involvement in growth and survival signaling cascades, the σ(1) receptors (σ(1)Rs) represent a novel target for the treatment of cancer and several brain diseases such as depression and neurodegeneration. From a series of σ1R-specific (18)F-fluoroalkylated spirocyclic piperidines, we have chosen (18)F-fluspidine for detailed investigation of the in vivo kinetics of the (R)-(+)- and (S)-(-)-enantiomers to identify their potential for imaging in humans.

METHODS

Enantiopure tosylate precursors for radiolabeling were obtained using chiral preparative high-performance liquid chromatography and used for radiosynthesis of both (18)F-fluspidine enantiomers by nucleophilic substitution with K-(18)F-F-Kryptofix 222-carbonate complex in a synthesis module. Brain pharmacokinetics were investigated by dynamic PET studies in piglets under baseline and blocking conditions using the highly selective σ1R agonist SA4503. Standardized uptake values (SUVs) were calculated for 24 MR-defined brain regions. Total distribution volume (V(T)) and binding potentials (k3'/k4) of (S)-(-)- and (R)-(+)-(18)F-fluspidine were estimated. Furthermore, V(T) values were estimated by graphical analysis using Logan plots.

RESULTS

The (S)- and (R)-tosylates were obtained in excellent enantiomeric purities (>98% and >96% enantiomeric excess, respectively). (S)-(-)- and (R)-(+)-(18)F-fluspidine were synthesized within approximately 70 min (radiochemical yield, 35%-45%; specific activity, 650-870 GBq/μmol; radiochemical purity, >99%). Both radiotracers displayed different brain uptake kinetics. Although the initial brain uptake was similar, the SUV at the end of the study differed significantly (P < 0.05), with (R)-(+)-(18)F-fluspidine showing about 60%-150% higher values. Administration of SA4503 reduced SUV almost equally for both radiotracers by approximately 65%. Furthermore, k(3)' was significantly decreased under blocking conditions in almost all regions ((S)-(-)-(18)F-fluspidine, -90%-95%; (R)-(+)-(18)F-fluspidine, -70%-90%) whereas effects on k(4) differed according to the particular brain region. V(T) estimated by both graphical analysis using Logan plots and full nonlinear kinetic analysis revealed significant inhibition for both radiotracers under blocking conditions.

CONCLUSION

Both (S)-(-)- and (R)-(+)-(18)F-fluspidine appear to be suitable for σ1R imaging in humans. The different pharmacokinetics of (S)-(-)-(18)F-fluspidine and (R)-(+)-(18)F-fluspidine may have the potential for application in the diagnostics of different pathologic conditions.

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.

Synthesis and biological evaluation of ¹⁸F labeled fluoro-oligo-ethoxylated 4-benzylpiperazine derivatives for sigma-1 receptor imaging

We report the synthesis and evaluation of a series of fluoro-oligo-ethoxylated 4-benzylpiperazine derivatives as potential σ(1) receptor ligands. In vitro competition binding assays showed that 1-(1,3-benzodioxol-5-ylmethyl)-4-(4-(2-fluoroethoxy)benzyl)piperazine (6) exhibits low nanomolar affinity for σ(1) receptors (K(i)=1.85 ± 1.59 nM) and high subtype selectivity (σ(2) receptor: K(i)=291 ± 111 nM; K(i)σ(2)/K(i)σ(1)=157). [(18)F]6 was prepared in 30-50% isolated radiochemical yield, with radiochemical purity of >99% by HPLC analysis after purification, via nucleophilic (18)F(-) substitution of the corresponding tosylate precursor. The logD(pH 7.4) value of [(18)F]6 was found to be 2.57 ± 0.10, which is within the range expected to give high brain uptake. Biodistribution studies in mice demonstrated relatively high concentration of radiotracers in organs known to contain σ(1) receptors, including the brain, lungs, kidneys, heart, and spleen. Administration of haloperidol 5 min prior to injection of [(18)F]6 significantly reduced the concentration of radiotracers in the above-mentioned organs. The accumulation of radiotracers in the bone was quite low suggesting that [(18)F]6 is relatively stable to in vivo defluorination. The ex vivo autoradiography in rat brain showed high accumulation of radiotracers in the brain areas known to possess high expression of σ(1) receptors. These findings suggest that [(18)F]6 is a suitable radiotracer for imaging σ(1) receptors with PET in vivo.

Synthesis and biological evaluation of novel 4-benzylpiperazine ligands for sigma-1 receptor imaging

We report the synthesis and evaluation of 4-benzylpiperazine ligands (BP-CH(3), BP-F, BP-Br, BP-I, and BP-NO(2)) as potential σ(1) receptor ligands. The X-ray crystal structure of BP-Br, which crystallized with monoclinic space group P2(1)/c, has been determined. In vitro competition binding assays showed that all the five ligands exhibit low nanomolar affinity for σ(1) receptors (K(i)=0.43-0.91nM) and high subtype selectivity (σ(2) receptor: K(i)=40-61nM; K(i)σ(2)/K(i)σ(1)=52-94). [(125)I]BP-I (1-(1,3-benzodioxol-5-ylmethyl)-4-(4-iodobenzyl)piperazine) was prepared in 53±10% isolated radiochemical yield, with radiochemical purity of >99% by HPLC analysis after purification, via iododestannylation of the corresponding tributyltin precursor. The logD value of [(125)I]BP-I was found to be 2.98±0.17, which is within the range expected to give high brain uptake. Biodistribution studies in mice demonstrated relatively high concentration of radiolabeled substances in organs known to contain σ(1) receptors, including the brain, lung, kidney, heart, and spleen. Administration of haloperidol 5min prior to injection of [(125)I]BP-I significantly reduced the concentration of radioactivity in the above-mentioned organs. The accumulation of radiolabeled substance in the thyroid was quite low suggesting that [(125)I]BP-I is relatively stable to in vivo deiodination. These findings suggest that the binding of [(125)I]BP-I to σ(1) receptors in vivo is specific.

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.

Development of PET radiopharmaceuticals and their clinical applications at the Positron Medical Center

The Positron Medical Center has developed a large number of radiopharmaceuticals and 36 radiopharmaceuticals have been approved for clinical use for studying aging and geriatric diseases, especially brain functions. Positron emission tomography (PET) has been used to provide a highly advanced PET-based diagnosis. The current status of the development of radiopharmaceuticals, and representative clinical and methodological results are reviewed.

Successive positron emission tomography measurement of cerebral blood flow and neuroreceptors in the human brain: an 11C-SA4503 study

For evaluating a newly developed radioligand for positron emission tomography (PET), successive measurements of regional cerebral blood flow (rCBF) and the kinetics of the radioligand in the same subject are preferable in the first clinical trial. In this study, we demonstrate an example for the study of (11)C-labeled 1-(3,4-dimethoxyphenethyl)-4-(3-phenylprophyl) piperazine ((11)C-SA4503) for mapping sigma(1) receptors in the human brain. Five healthy male subjects underwent two successive PET scans with (15)O-H(2)O to measure the rCBF and with (11)C-SA4503 (dynamic 60-min scan). The brain kinetics of (11)C-SA4503 was evaluated using the time-activity curve (TAC) of tissue in each of the 11 regions of the brain and the metabolite-corrected TAC of plasma on the basis of a two-tissue compartment fourparameter model. The estimated parameters were four rate constants: K (1), influx from plasma to brain tissue; k (2), efflux from tissue to plasma; k (3), association between tracer and receptors; and k (4), dissociation of tracer-receptor complex, and the binding potential (BP), k (3)/k (4). (11)CSA4503 was distributed all over the brain, and the TACs exhibited an accumulation pattern in all the 11 regions. K (1) of (11)C-SA4503 correlated with rCBF, but the other three rate constant parameters and BP did not. The regional difference in the BP of (11)C-SA4503 is compatible with those of sigma(1) receptors. In conclusion, successive PET measurements of rCBF and the brain kinetics of radioligand-neuroreceptor binding are useful for the first clinical trial of a newly developed radioligand for mapping neuroreceptors, and the protocol is applicable to pathophysiological studies of brain disorders.

A change of in vivo characteristics depending on specific activity of radioiodinated (+)-2-[4-(4-iodophenyl)piperidino]cyclohexanol [(+)-pIV] as a ligand for sigma receptor imaging

The radioiodinated (+)-p-iodovesamicol [(+)-pIV], which shows a high binding affinity for sigma-1 (sigma-1) receptors, is prepared by an exchange reaction. The specific activity (SA) is fairly low and therefore is insufficient for clinical use. In this study, we prepared (+)-[(125)I]pIV with a high SA from tributylstannyl precursor and compared the in vivo characteristics between high and low SA by imaging sigma-1 receptors in the central nervous system. In the biodistribution study, a difference in brain accumulation was observed between the two methods. At 30 min postinjection, the brain accumulation (1.58%ID/g) of low SA [0.6-1.1 TBq/mmol (16-30 Ci/mmol)] (+)-[(125)I]pIV was higher than that (1.34%ID/g) of high SA [>88.8 TBq/mmol (>2400 Ci/mmol)] (+)-[(125)I]pIV. In the blocking study, the brain uptake of high SA (+)-[(125)I]pIV was reduced more significantly by the coadministration of sigma ligands such as pentazocine, haloperidol or SA4503 than that of low SA (+)-[(125)I]pIV. These results showed that nonspecific binding of high SA (+)-[(125)I]pIV in the brain was lower than that of low SA (+)-[(125)I]pIV, and high SA (+)-[(125)I]pIV bound more specifically to sigma-1 receptors in the brain than low SA (+)-[(125)I]pIV. In contrast, in the blood-binding study, high SA (+)-[(125)I]pIV (58.4%) bound to blood cells with higher affinity than low SA (+)-[(125)I]pIV (46.0%). In metabolite studies, blood metabolites of high SA (+)-[(125)I]pIV (57.3+/-3.5%) were higher than those of low SA (+)-[(125)I]pIV (45.5+/-4.1%) at 30 min postinjection. Higher SA may be apt to bind to blood cells with higher affinity and to be metabolized faster.

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.

Synthesis and evaluation of fluorine-18-labeled SA4503 as a selective sigma1 receptor ligand for positron emission tomography

The [(18)F]fluoromethyl analog of the sigma(1) selective ligand 1-(3,4-dimethoxyphenethyl)-4-(3-phenylpropyl)piperazine dihydrochloride (SA4503) ([(18)F]FM-SA4503) was prepared and its potential evaluated for the in vivo measurement of sigma(1) receptors with positron emission tomography (PET). FM-SA4503 had selective affinity for the sigma(1) receptor (K(i) for sigma(1) receptor, 6.4 nM; K(i) for sigma(2) receptor, 250 nM) that was compatible with the affinity of SA4503 (K(i) for sigma(1) receptor, 4.4 nM; K(i) for sigma(2) receptor, 242 nM). [(18)F]FM-SA4503 was synthesized by (18)F-fluoromethylation of O-demethyl SA4503 in the radiochemical yield of 2.9-16.6% at the end of bombardment with a specific activity of 37.8-283 TBq/mmol at the end of synthesis. In mice, the uptake of [(18)F]FM-SA4503 in the brain was gradually increased for 30 min after injection, and then decreased. In the blocking study, brain uptake was significantly decreased by co-injection of haloperidol to 32% of control, and FM-SA4503 to 52% of control. In PET study of the monkey brain, high uptake was found in the cerebral cortex, thalamus and striatum. The radioactivity level of [(18)F]FM-SA4503 in the brain regions gradually increased over a period of 120 min after injection, followed by a stable plateau phase until 180 min after injection. In pretreatment with haloperidol measurement of the monkey brain, the radioactivity level was 22-32% and 11-25% of the baseline at 60 and 180 min, respectively, after injection, suggesting high receptor-specific binding. [(18)F]FM-SA4503 showed specific binding to sigma(1) receptors in mice and monkeys; therefore, [(18)F]FM-SA4503 has the potential for mapping sigma(1) receptors in the brain.

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.

A feasibility study of [11C]SA4503-PET for evaluating sigma1 receptor occupancy by neuroleptics: The binding of haloperidol to sigma1 and dopamine D2-like receptors

We investigated feasibility of positron emission tomography (PET) with [11C]SA4503 for evaluating the sigma1 receptor occupancy rate by neuroleptics. Haloperidol, which is well known to bind dopamine D2-like receptor (D2R) as well as to be a representative non-selective antagonist for sigma1 receptor (sigma1R), was selected as a model drug. Three healthy male subjects underwent 60-min [11C]raclopride-PET and 90-min [11C]SA4503-PET scans successively at a 120-min interval twice in a day for baseline measurement and on another day for haloperidol-loading measurement 16 hours after peroral administration of 3 mg of haloperidol. Binding potential (BP) of [11C]raclopride and [11C]SA4503 was quantitatively evaluated and the sigma1R and D2R occupancy rates were determined. D2R occupancy rates by haloperidol were 64% and 62% in the caudate and putamen, respectively, 16 h after the administration, while sigma1R occupancy rates were approximately 80% in all seven regions investigated including the caudate, putamen and cerebellum 18 h after the administration, suggesting that the sigma1R receptor occupancy rate by haloperidol was slightly larger than the D2R receptor occupancy rate. We concluded that [11C]SA4503-PET can be used for evaluating the sigma1R occupancy rates by neuroleptics or other drugs.

Synthesis and in Vitro Evaluation of Iodinated Derivatives of Piperazine as a New Ligand for Sigma Receptor Imaging by Single Photon Emission Computed Tomography

A new series of radioiodinated analogues of 1-[2-(3,4-dimethoxyphenyl)ethyl]-4-(3-phenylpropyl)piperazine (SA4503) was synthesized and evaluated as a potential brain sigma-1 receptor imaging ligands by single photon emission computed tomography (SPECT). Iodinated analogues of SA4503 (4a-c) were prepared from piperazine in a high yield. The in vitro competition binding studies using [3H] DTG (sigma-1, 2), [3H] (+)-pentazocine (sigma-1), and [3H] DTG in the presence of carbetapentane (sigma-2) as sigma receptor selective radioligands were revealed that iodinated analogues 4a-c possess high affinities to sigma receptors (IC50: 4a=7.1, 4b=31.0, and 4c=77.3 nM). In particular, the affinity of 4a, bearing iodine at ortho position on the phenyl ring, was 4.4 times greater than SA4503, and 3 times greater than that of haloperidol. The meta-iodo analogue 4b was the same to SA4503, the lead compound. The radioiodinated derivatives, [125I] 4a, 4b were synthesized no-carrier-added from the corresponding tributyltin precursors by the iododestannylation reaction with high yields. The binding of [125I] 4a, 4b have been characterized in the rat brain membranes. These compounds were indicated single population binding to sigma receptor with high affinity (4a: Kd=1.86+/-0.34 nM, Bmax=205+/-28.9 fmol/mg protein, 4b: Kd=3.30+/-0.51 nM, Bmax=231.5+/-13.8 fmol/mg protein). In vitro blocking studies were confirmed that the high specificity of 4a, 4b. These results suggest that radioiodinated 4a and 4b are promising sigma receptors imaging ligand for pursuing further in vivo studies.

In Vivo Evaluation of Radioiodinated 1-[2-(3,4-Dimethoxyphenyl)ethyl]-4-(3-phenylpropyl)-piperazine Derivatives as New Ligands for Sigma Receptor Imaging Using Single Photon Emission Computed Tomography

New series of radioiodinated analogues of 1-[2-(3,4-dimethoxyphenyl)ethyl]-4-[3-(2-iodophenyl)propyl]piperazine (o-BON) and 1-[2-(3,4-dimethoxyphenyl)ethyl]-4-[3-(3-iodophenyl)propyl]piperazine (m-BON) were evaluated as single photon emission computed tomography (SPECT) radiopharmaceuticals for mapping sigma receptors in the central nervous system (CNS) and peripheral organs. In vivo biodistribution studies of [125I] o- and m-BON in mice demonstrated high initial uptakes and prolonged retention in the brain. In contrast to high brain uptake and retention, the blood accumulations were low, resulting in good brain-blood ratios (7.9-9.2). In the other tissues, high uptake of [125I] o- and m-BON were observed in the liver, kidney, heart, lung, and pancreas. Moreover, selective interactions of [125I] o- and m-BON with sigma receptors were confirmed by pretreatment experiments with various sigma and other receptor ligands. Haloperidol posttreatment induced decreases in the accumulation of [125I] o- and m-BON. These data suggest that [125I] o- and m-BON binding to sigma receptors is reversible and competitive. Furthermore, ex vivo autoradiograms of [125I] o- and m-BON in rats showed high uptake in the parietal cortex, vestibular nucleus, and pons nucleus and moderate uptake in the thalamus, inferior colliculus, hippocampus, hypothalamus, and temporal cortex. These ex vivo autoradiograms were comparable with the histochemical distribution of sigma receptors. Furthermore, the uptake of [125I] o- and m-BON reflected quantitative amounts of sigma receptor in the brain. These results demonstrated that radiolabeled o- and m-BON have good characteristics for mapping sigma receptors in the CNS and the peripheral organs with SPECT.

Evaluation of [11C]SA5845 and [11C]SA4503 for imaging of sigma receptors in tumors by animal PET

Sigma receptors are expressed in a wide variety of tumor cell lines, and are expressed in proliferating cells. A radioligand for the visualization of sigma receptors could be useful for selective detection of primary tumors and their metastases, and for non-invasive assessment of tumor proliferative status. To this end we evaluated two sigma receptor ligands, [11C]SA5845 and [11C]SA4503. In an in vitro study, AH109A hepatoma showed moderate densities of sigma1 and sigma2 receptors, and VX-2 carcinoma showed a high density of sigma2 receptors: Bmax (fmol/mg protein) for sigma1 vs. sigma2, 1,700 vs. 1,200 for AH109A hepatoma and 800 vs. 10,000 for VX-2 carcinoma. In a cell growth assay in vitro, neither SA5845 nor SA4503 (<10 microM) showed any inhibitory effect on proliferation of the AH109A hepatoma cells. In rats, the uptake of [11C]SA5845 and [11C]SA4503 in AH109A tissues was accumulated over the first 60 minutes; however, the uptake of both tracers increased by co-injection with haloperidol as a sigma receptor ligand. On the other hand, in the PET studies of rabbits, the uptake of [11C]SA5845 in the VX-2 carcinoma was relatively higher than that of [11C]SA4503, because of a much higher density of sigma2 receptors compared to sigma1 receptors in the VX-2 tissue, and the uptake of both tracers in the VX-2 tissue was decreased by carrier-loading and pre-treatment with haloperidol ([11C]SA5845, 53% and 26%, respectively; [11C]SA4503, 41% and 22%, respectively at 30 minutes after injection). Therefore, [11C]SA5845 and [11C]SA4503 may be potential ligands for PET imaging of sigma receptor-rich tumors.

Synthesis and evaluation of 2-[18F]fluoro-CP-118,954 for the in vivo mapping of acetylcholinesterase

5,7-Dihydro-3-[2-[1-(2-fluorobenzyl)-4-piperidinyl]ethyl]-6H-pyrrolo[3,2,f]-1,2-benzisoxazol-6-one (2-flouro-CP-118,954; 1), a potent acetylcholinesterase (AChE) inhibitor, was prepared as a radioligand by reductive alkylation of CP-144,885 the debenzylated form of CP 118,954, with 2-[18F]fluorobenzaldehyde. The decay-corrected radiochemical yield was 25-30% and the effective specific activity was 41-53 GBq/micromol. Tissue distribution studies of 2-[18F]fluoro-CP-118,954 ([18F]1) in mice showed that the regional brain distribution correlated well with the known density of AChE in the mouse brain. A high level of uptake in the striatum was also shown at all time points in the olfactory tubercle, which is known to have dopaminergic neurons. Blocking studies showed that radioligand uptake in all brain regions was not altered by either the dopamine receptor antagonists or the sigma receptor agonist. On the other hand, radioligand uptake in both the striatum and the olfactory tubercle was significantly blocked (80%) by ligand 1. The low level of bone uptake over time suggested that [18F]1 underwent little in vivo metabolic defluorination. The lack of metabolite formation in the mouse brain indicated that the regional distribution was attributed to [18F]1. These results demonstrated that [18F]1 binds specifically and selectively to AChE in mice and appears to be a suitable radioligand for the in vivo mapping of AChE.

Improved synthesis of [11C]SA4503, [11C]MPDX and [11C]TMSX by use of [11C]methyl triflate

Recently we have clinically used three new radioligands, [11C]SA4503, [11C]MPDX, and [11C]TMSX, for mapping sigma1, adenosine A1, and adenosine A2A receptors, respectively, in the human brain by positron emission tomography. These radioligands are synthesized by methylation of the respective demethyl precursor with [11C]methyl iodide. Here we demonstrate the improved syntheses of these compounds by use of [11C]methyl triflate, a highly reactive alternative to [11C]methyl iodide.

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.

Different brain kinetics of two sigma 1 receptor ligands, [3H](+)-pentazocine and [11C]SA4503, by P-glycoprotein modulation

We compared the brain kinetics of radiolabeled (+)-pentazocine and SA4503, which have a high and selective affinity for sigma(1) receptors. Brain uptake of [(11)C]SA4503 was high after intravenous injection followed by a gradual decrease in mice, whereas that of [(3)H](+)-pentazocine rapidly decreased. The brain uptake of the two radioligands was dose-dependently reduced, but the reduction of [(3)H](+)-pentazocine was found at higher doses. Percentages of the saturable binding of [(3)H](+)-pentazocine was much lower than that of [(11)C]SA4503. The brain uptake of [(3)H](+)-pentazocine was greatly blocked by SA4503 at a dose of 2 micromol/kg, while that of [(11)C]SA4503 was blocked by (+)-pentazocine at a dose of 20 micromol/kg and over. When mice were treated with cyclosporin A, a P-glycoprotein modulator, the uptake of [(3)H](+)-pentazocine was enhanced, but that of [(11)C]SA4503 was not. Under control and P-glycoprotein-modulated conditions, the brain uptake of both radioligands was reduced by haloperidol, another representative sigma receptor ligand, to a different extent. We concluded that the P-glycoprotein modulation resulted in the different brain kinetics of the two radioligands. The radiolabeled SA4503 is suitable as an in vivo probe, but radiolabeled (+)-pentazocine is not.

Sigma1 and dopamine D2 receptor occupancy in the mouse brain after a single administration of haloperidol and two dopamine D2-like receptor ligands

We investigated sigma(1) and dopamine D(2) receptor occupancy in mouse brain after a single injection of haloperidol, nemonapride, or spiperone using [(11)C]SA4503 and [(11)C]raclopride, respectively. Co-injection of the three compounds significantly blocked the uptake of each radioligand. Six hours later, only haloperidol blocked [(11)C]SA4503 uptake, while all three reduced [(11)C]raclopride uptake. Sigma(1) receptor occupancy by haloperidol was reduced to 19% at day 2 when D(2) receptor occupancy disappeared. [(11)C]SA4503 would be applicable to the investigation of sigma(1) receptor occupancy of antispychotic drugs using PET.

Synthesis and evaluation of 11C- and 18F-labeled 1-[2-(4-alkoxy-3-methoxyphenyl)ethyl]-4-(3-phenylpropyl)piperazines as sigma receptor ligands for positron emission tomography studies

We prepared sigma(1)-receptor selective 1-([4-methoxy-(11)C]-3,4-dimethoxyphenethyl)-4-(3-phenylpropyl)piperazine ([(11)C]SA4503) and its fluorinated analog 1-([4-methoxy-(11)C]3,4-dimethoxyphenethyl)-4-[3-(4-fluorophenyl)propyl]piperazine ([(11)C]SA5845), and their [(11)C]ethoxy and [(18)F]fluoroethoxy analogs, and evaluated their potential for positron emission tomography studies. [(11)C]SA4503 is most selective for sigma(1) receptors, and the other five showed affinities for sigma(1) and sigma(2) receptors with a different extent. All radioligands showed the receptor-specific binding in the brain, and visualized similar regional brain distributions by ex vivo autoradiography. The [(11)C]ethoxy analogs were relatively labile for metabolism.

Age-related changes of the binding of [3h]SA4503 to sigma1 receptors in the rat brain

We have recently developed 1-([3-O-methyl-11C]3,4-dimethoxyphenethyl)-4-(3-phenylpropyl) piperazine ([11C]SA4503) as a selective radioligand for mapping sigma1 receptors in the brain by positron emission tomography (PET). In the present short communication we evaluated the age-related changes of the binding of this ligand to sigma1 receptors in Fisher-344 rats (1.5-, 6-, 12-, and 24-month-old) by the in vitro binding assay. We also measured the binding of [3H](+)-pentazocine to sigma1 receptors and the binding of [3H]1,3-di-O-tolylguanidine to sigma2 receptors, which are current standard methods. The specific binding of the three radioligands increased age-dependently. Both Kd and Bmax values of the 24-month-old rats for each radioligand were significantly higher than those of the young rats (1.5- and 6-month-old). The increased numbers of both sigma1 and sigma2 receptor subtypes in the aged rats compensate for the lowered affinity, and rather enhanced the radioligand-receptor binding. The results contrast strikingly with the age-dependent decrease in the dopaminergic, cholinergic and glutamatergic receptors that are reported to be correlated with the sigma receptors, and indicate that a PET study with [11C]SA4503 to evaluate the aging process in humans would be of great interest.

Visualization of sigma1 receptors in eyes by ex vivo autoradiography and in vivo positron emission tomography

Sigma receptors are present on the neurons of the central nervous system and in peripheral organs. They have also been demonstrated in ocular tissues by in vitro membrane binding assays. We have investigated whether sigma(1) receptors can be demonstrated in rat eyes by ex vivo autoradiography using [(11)C]SA4503, a selective radioligand. We also tested whether in vivo positron emission tomography (PET) can be used to show sigma(1) receptors in rabbit eyes. In rats, a high accumulation of [(11)C]SA4503 was found in the iris-ciliary body and retina. A carrier-loading experiment showed that the receptor-specific binding of [(11)C]SA4503 was approximately 75% of the total binding in the brain. Sigma(1) receptors were also detected in the projecting terminals of the retina to the superior colliculus. PET showed radioactivity in the anterior segment including the iris-ciliary body and retina, and pretreatment or displacement by a sigma receptor ligand (haloperidol), suggested that the PET signal reflects radioligand-receptor binding. The high density of sigma(1) receptors in the iris-ciliary body and retina was confirmed by ex vivo autoradiography. In conclusion, the iris-ciliary body and retina are rich in sigma(1) receptors, and PET may be used to investigate the in vivo distribution of these neuroreceptors in the eye.

Synthesis and evaluation of [18F]fluoroethyl SA4503 as a PET ligand for the sigma receptor

The sigma receptor might be involved in several diseases in the central nervous system. It occurs in the endocrine, immune, and other peripheral organ systems and is expressed in a variety of human tumors. The [18F]fluoroethyl analog of the sigma1-selective ligand SA4503 ([18F]FE-SA4503) was prepared and evaluated in animals to investigate its suitability for in vivo measurement of sigma receptors with positron emission tomography (PET). [18F]FE-SA4503 was synthesized by [18F]fluoroethylation of the corresponding O-demethyl precursor in an overall radiochemical yield of 4-7% (EOB) with a specific activity of >100 TBq/mmol. The radioligand had higher in vitro affinity for the sigma receptor than SA4503 (IC(50) sigma1 6.48 nM, IC50 sigma2 2.11 nM). [18F]FE-SA4503 was injected into mice. Uptake could be blocked by co-injection of the sigma receptor ligands haloperidol, pentazocine, and cold SA4503, but not with other receptor ligands. Ex vivo autoradiography studies in rats showed regional distribution in the brain similar to [11C]SA4503. Hippocampus, thalamus, and cortical areas were clearly delineated by [18F]FE-SA4503. The uptake was blocked by SA4503 treatment. In the rat brain, only a small portion of metabolites (6.6% of brain radioactivity) was detected at 30 min postinjection, whereas in plasma the fraction of metabolites amounted to 51.3% of plasma radioactivity. The kinetics of [18F]FE-SA4503 was measured with PET in the conscious monkey brain. High uptake values were found in the cortex, thalamus, cerebellum, and striatum, reaching a plateau value at 30 min postinjection. It is concluded that [18F]FE-SA4503 showed specific binding to sigma receptors in three animal species.

[11C]Raclopride binding was reduced in vivo by sigma(1) receptor ligand SA4503 in the mouse brain, while [11C]SA4503 binding was not by raclopride

[11C]Raclopride is widely used as a representative dopamine D(2)-like receptor ligand in positron emission tomography (PET) studies, and [11C]1-(3,4-dimethoxyphenethyl)-4-(3-phenylpropyl)piperazine dihydrochloride ([11C]SA4503) is a recently developed selective ligand for mapping sigma(1) receptors in the brain. The striatal uptake of [11C]raclopride in mice was reduced by co-injection of an excess amount of SA4503, in spite of the fact that raclopride had no effect on the brain uptake of [11C]SA4503 as shown in a previous study. The blocking effect of SA4503 on the striatal uptake of [11C]raclopride was dose-dependent, but disappeared by 1 h or 6 h after intraperitoneal injection of SA4503. The brain uptake of [11C]SA4503 was not affected by a dopamine transporter inhibitor GBR 12909, nor was [11C]beta-CIT-FP inhibited by SA4503. The IC(50) values of raclopride for sigma(1) and sigma(2) receptor subtypes measured in vitro were 11800 nM and 4950 nM, respectively, suggesting that the affinity was too low for [11C]raclopride to bind in vivo to sigma receptors. On the other hand, the IC(50) value of SA4503 for dopamine D(2) receptors was 470 nM, that is approximate 1/25 of the affinity of raclopride for the dopamine D(2) receptors. Therefore, possible explanations for the partial blocking effects of SA4503 on the striatal uptake of [11C]raclopride are: (1) an excess amount of SA4503 may reduce the [11C]raclopride uptake due to its low affinity for dopamine D(2) receptors, or (2) SA4503 may enhance endogenous dopamine release, which results in the competitive inhibition of the [11C]raclopride uptake. These findings support that both [11C]raclopride and [11C]SA4503 are selective in vivo ligands for dopamine D(2)-like receptors and sigma(1) receptors, respectively, in spite of the partial blocking effect of SA4503 on the striatal uptake of [11C]raclopride.

Development of Theme in Radiochemistry

The disintegration rates of 222Rn and its daughters in natural water were determined successfully by the use of the integral counting method with a liquid scintillation spectrometer. A significant advantage of this method is its freedom from the quenching effect. Moreover, when plural alpha-, and beta-emitters are present, their total amounts can be determined. The simple extrapolation of integral counting curve to zero pulse-height, however, do not give the true disintegration rate for the soft beta-emitters (Emax < 200 keV), because the liquid scintillator (LS) has a relatively high detection threshold. Therefore, the zero detection threshold of the liquid scintillation spectrometer was determined by measuring standard 3H samples, and a modified integral counting method which extrapolates the integral counting curve to the zero detection threshold was proposed. The method has been successfully applied to various beta-emitters, 222Rn samples, and coloured samples of beta-emitters, giving more accurate absolute disintegration rate than the conventional integral counting method and the efficiency tracing method. In the course of the study determining 222Rn by liquid scintillation counting, we observed unexpected phenomena; the air luminescence from gaseous space above LS, and the temperature dependence of pulse-height spectra. As for the former phenomenon, we proposed a method for correcting errors due to air luminescence, a method for determining alpha-emitters by the air luminescence, and a rapid calibration method for 222Rn detectors. As for the latter phenomenon, we observed that the pulse-height spectra for alpha, and beta-emitters in LS are shifted toward higher pulse-height with decreasing temperature. We found that the fluorescence intensities of the solvent of LS (toluene) is promoted at lower temperatures, and that not only toluene, but also the fluorescence intensity of a number of aromatic hydrocarbons and aliphatic hydrocarbons show the same effect as toluene. Other unexpected results are existence of metals in number of enzymes, and discrepancies between the experimental value for Kurie plot of allowed beta-emitters and the value which would be expected according to Fermi's theory, the results of which would affect the transmission probability of potential barrier for alpha-particles.