Evaluation of CLINDE as potent translocator protein (18 kDa) SPECT radiotracer reflecting the degree of neuroinflammation in a rat model of microglial activation

BACKGROUND

The translocator protein (TSPO; 18 kDa), the new name of the peripheral-type benzodiazepine receptor, is localised in mitochondria of glial cells and expressed in very low concentrations in normal brain. Their expression rises after microglial activation following brain injury. Accordingly, TSPO are potential targets to evaluate neuroinflammatory changes in a variety of CNS disorders.

PURPOSE

To date, only a few effective tools are available to explore TSPO by SPECT. We characterised here 6-chloro-2-(4'iodophenyl)-3-(N,N-diethyl)-imidazo[1,2-a]pyridine-3-acetamide or CLINDE in a rat model with different stages of excitotoxic lesion.

METHODS

Excitotoxicity was induced in male Wistar rats by unilateral intrastriatal injection of different amounts of quinolinic acid (75, 150 or 300 nmol). Six days later, two groups of rats (n = 5-6/group) were i.v. injected with [(125)I]-CLINDE (0.4 MBq); one group being pre-injected with PK11195 (5 mg/kg). Brains were removed 30 min after tracer injection and the radioactivity of cerebral areas measured. Complementary ex vivo autoradiography, in vitro autoradiography ([(3)H]-PK11195) and immunohistochemical studies (OX-42) were performed on brain sections.

RESULTS

In the control group, [(125)I]-CLINDE binding was significantly higher (p < 0.001) in lesioned than that in intact side. This binding disappeared in rats pre-treated with PK11195 (p < 0.001), showing specific binding of CLINDE to TSPO. Ex vivo and in vitro autoradiographic studies and immunohistochemistry were consistent with this, revealing a spatial correspondence between radioactivity signal and activated microglia. Regression analysis yielded a positive relation between the ligand binding and the degree of neuroinflammation.

CONCLUSION

These results demonstrate that CLINDE is suitable for TSPO in vivo SPECT imaging to explore their involvement in neurodegenerative disorders associated with microglial activation.

DPA-714, a new translocator protein-specific ligand: synthesis, radiofluorination, and pharmacologic characterization

The translocator protein (18 kDa) (TSPO), formerly known as the peripheral benzodiazepine receptor, is dramatically upregulated under pathologic conditions. Activated microglia are the main cell type expressing the TSPO at sites of central nervous system pathology. Radioligands for the TSPO can therefore measure active disease in the brain. This article details the synthesis, radiofluorination, and pharmacologic evaluation of a new TSPO-specific pyrazolopyrimidine, DPA-714.

METHODS

The affinity of DPA-714 for the TSPO was measured in rat kidney membranes with (3)H-PK11195. The in vitro functional activity of DPA-714 was measured in a steroidogenic assay in which the ability of DPA-714 to increase pregnenolone synthesis was measured with rat C6 glioma cells. The radiofluorination of DPA-714 was achieved by nucleophilic (18)F-fluoride displacement of the tosylate precursor. (18)F-DPA-714 was assessed in rats harboring unilateral quinolinic acid (QA) lesions. In addition, pretreatment experiments were performed with PK11195 (5 mg/kg), DPA-714 (1 mg/kg), and DPA-713 (1 mg/kg). The in vivo binding and biodistribution of (18)F-DPA-714 were determined in a baboon with PET. Experiments involving presaturation with PK11195 (1.5 mg/kg) and displacement with DPA-714 (1 mg/kg) were conducted to evaluate the specificity of radioligand binding.

RESULTS

In vitro binding studies revealed that DPA-714 displayed a high affinity for the TSPO (dissociation constant, 7.0 nM). DPA-714 stimulated pregnenolone synthesis at levels 80% above the baseline. (18)F-DPA-714 was prepared at a 16% radiochemical yield and a specific activity of 270 GBq/mumol. In rats harboring unilateral QA lesions, an 8-fold-higher level of uptake of (18)F-DPA-714 was observed in the ipsilateral striatum than in the contralateral striatum. Uptake in the ipsilateral striatum was shown to be selective because it was inhibited to the level in the contralateral striatum in the presence of PK11195, nonlabeled DPA-714, or DPA-713. PET studies demonstrated rapid penetration and good retention of (18)F-DPA-714 in the baboon brain. Pretreatment with PK11195 effectively inhibited the uptake of (18)F-DPA-714 in the whole brain, indicating its selective binding to the TSPO. The injection of nonlabeled DPA-714 20 min after the injection of (18)F-DPA-714 resulted in radioligand washout, demonstrating the reversibility of (18)F-DPA-714 binding.

CONCLUSION

(18)F-DPA-714 is a specific radioligand for the TSPO, displaying promising in vivo properties and thus warranting further investigation.

Partial recovery of dopaminergic pathway after graft of adult mesenchymal stem cells in a rat model of Parkinson's disease

Cellular therapy with adult stem cells appears as an opportunity for treatment of Parkinson's disease. To validate this approach, we studied the effects of transplantation of rat adult bone-marrow mesenchymal stem cells in a rat model of Parkinson's disease. Animals were unilaterally lesioned in the striatum with 6-hydroxydopamine. Two weeks later, group I did not undergo grafting, group II underwent sham grafting, group III was intra-striatal grafted with cells cultured in an enriched medium and group IV was intra-striatal grafted with cells cultured in a standard medium. Rotational amphetamine-induced behavior was measured weekly until animals were killed 6 weeks later. One week after graft, the number of rotations/min was stably decreased by 50% in groups III and IV as compared with groups I and II. At 8 weeks post-lesion, the density of dopaminergic markers in the nerve terminals and cell bodies, i.e. immunoreactive tyrosine hydroxylase, membrane dopamine transporter and vesicular monoamine transporter-2 was significantly higher in group III as compared with group I. Moreover, using microdialysis studies, we observed that while the rate of pharmacologically induced release of dopamine was significantly reduced in lesioned versus intact striatum in no grafted rats, it was similar in both sides in animals transplanted with mesemchymal stem cells. These data demonstrate that graft of adult mesemchymal stem cells reduces behavioral effects induced by 6-hydroxydopamine lesion and partially restores the dopaminergic markers and vesicular striatal pool of dopamine. This cellular approach might be a restorative therapy in Parkinson's disease.

Ex vivo and in vivo evaluation of (2R,3R)-5-[(18)F]-fluoroethoxy- and fluoropropoxy-benzovesamicol, as PET radioligands for the vesicular acetylcholine transporter

Molecular imaging of the vesicular acetylcholine transporter (VAChT) using positron emission tomography (PET) may provide insights into early diagnosis and better understanding of Alzheimer's disease. We further characterized the VAChT ligand (2R,3R)-5-FEOBV (1) and developed new fluoropropoxy analogues. Ex vivo studies of the new nonradiolabeled analogues (2R,3R)-5-FPOBV (2) (k(D) = 0.7 nM) and (2S,3S)-5-FPOBV (3) (k(D) = 8.8 nM) were performed in rat brain and showed an enantioselective inhibition of (-)-5-[(125)I]-IBVM uptake in striatum, cortex, and hippocampus (e.g., 74% for 2 and only 54% for 3 in the cortex). Radiochemical procedures were developed to produce [(18)F]1 and [(18)F]2 as potential imaging agent for the VAChT. The radiochemistry was carried out in a one step procedure, with radiolabeling yields of 17 and 2.6% (range: 1-5.4), respectively, nondecay corrected with good specific activity: 124-338 GBq/micromol. The radiochemical purity was greater than 98%. The biological (ex vivo and in vivo) properties of these radioligands were evaluated in rats and showed a low (less then 0.1% of the injected dose) and homogeneous brain uptake. The in vivo PET study of [(18)F]2 performed in baboon also revealed rapid defluorination as the main problem. Therefore [(18)F]1 and [(18)F]2 appear to be unsuitable for in vivo imaging of the VAChT using PET.

New fluoro-diphenylchalcogen derivatives to explore the serotonin transporter by PET

A series of fluorinated diphenylchalcogen derivatives, possessing a sulfur or an oxygen bridge, has been prepared with the aim to get a suitable radiotracer to image the SERT in vivo using positron emission tomography (PET). The compounds were synthesized and assayed toward the serotonin (SERT), dopamine (DAT), and norepinephrine (NET) transporters. Among the developed series, five compounds display a high SERT affinity (K(i): 0.27-2.91 nM range) and can be labeled either with carbon-11 or fluorine-18.

Evaluation of a Solid Bolus Suitable for Esophageal Scintigraphy

This study was designed to select a suitable solid bolus for esophageal scintigraphy. Optimally, a bolus should leave minimal residual buccal and pharyngeal activity after being swallowed. We compared the oropharyngeal behavior of three boluses, i.e., omelette, egg white, and paté of 1- and 3-ml volume. Thirty patients without dysfunction of the upper esophageal sphincter were recruited for the study. Scintigraphy interpretation was based on the results of condensed images and time activity curves. A total of 108 oropharyngeal transits were analyzed. First we determined the most appropriate volume (1 or 3 ml) of paté, omelette, and egg white (i.e., the volume with the least residual oropharyngeal activity). Buccal or pharyngeal bolus retention occurred significantly less frequently with 1 ml paté than 3 ml (p = 0.03) and also less frequently with 3 ml egg white than with 1 ml egg white (p = 0.03), and the mean buccal bolus retention index was lower using 3 ml omelette than 1 ml omelette (p = 0.03). Then we identified the most suitable of the three selected boluses. Both oral and pharyngeal residues were higher for paté (1 ml) than for omelette (p = 0.02 and 0.05), and pharyngeal residue was significantly lower for omelette (3 ml) than for egg white (3 ml) (p = 0.02). In conclusion, a 3-ml bolus of radiolabeled omelette seems to be the most suitable bolus for exploration of esophageal transit, and its use could enhance the potential of scintigraphy in the assessment of esophageal disorders.

Decreased vesicular acetylcholine transporter and α4β2 nicotinic receptor density in the rat brain following 192 IgG-saporin immunolesioning

Degeneration of cholinergic neurons is a well known characteristic of Alzheimer's disease (AD). Two radioligands were studied in a rat model of cholinergic degeneration to evaluate their potential efficacy for molecular imaging of AD. Following specific cholinergic-cell immunolesioning with 192 IgG-saporin (SAP), ex vivo autoradiography was performed with (123)IBVM, a radioligand which targets the vesicular acetylcholine transporter (VAChT). Following the decay of (123)I, the same animals had in vitro autoradiography performed with (125)I-A-85380, a marker for nicotinic acetylcholine receptors (nAChRs). As expected significant, widespread decreases in (123)IBVM uptake were observed in SAP treated animals. Moderate but significant reductions in (125)I-A-85380 binding in the hippocampus (Hip) and cerebellum (Cbm) were also observed following SAP immunolesioning. The results with (123)IBVM confirm and extend previous work investigating the uptake of radioiodinated IBVM in this animal model. The results with (125)I-A-85380 are unique and are in contrast with work performed in this animal model with other nAChR radioligands, indicating the favourable properties of this radioligand for molecular imaging.

Ageing and reproduction: is polycystic ovary syndrome an exception?

BACKGROUND

Polycystic ovary syndrome (PCOS) is a frequent cause of infertility. Despite an impressive number of reports, few have evaluated the influence of age upon fertility. We present the outcomes of three infertile women with PCOS who achieved spontaneous pregnancies when ageing.

CASE REPORTS

Three patients with PCOS were monitored for more than 20 years. PCOS was confirmed by clinical data (oligo/amenorrhoea, infertility, hirsutism), hormonal measures and ovarian ultrasonography. All three infertile patients were treated for several years using numerous ovulation induction protocols with varying responses. When ageing, they gained more regular cycles and spontaneously became pregnant at 39, 40 1/2 and 36 years of age, more than 5 years after induction treatment was stopped, and in spite of increasing weight in each of them.

CONCLUSIONS

These clinical observations suggest improved fertility in some PCOS ageing women. The positive impact of ageing on cycle regularisation in PCOS has recently been claimed but the fertility outcome was not evaluated. Ovary ageing results in diminution of the follicular cohort in both normal and PCOS women, associated with decreased inhibin B and anti-müllerian hormone (AMH) levels. Lower inhibin B levels induce FSH enhancement, with a rise in FSH rate per follicle which may determine better follicle maturation, regular and ovulatory cycles in PCOS ageing women. The best proof of this improved fertility was the occurrence of spontaneous pregnancies which never occurred previously.

Comparative analysis of different scintigraphic approaches to assess pulmonary ventilation

A study was carried out to investigate the predictive value of 81-metastable-krypton (81mKr) distribution, high-size 99-metastable-technetium (99mTc) aerosol deposition and low-size 99mTc aerosol (Technegas) deposition on the pulmonary ventilation evaluated by 133-xenon (133Xe) lung scintigraphy, and to assess the correlation between the 81mKr distribution, the 99mTc aerosols deposition, and the respiratory parameters of patients with chronic obstructive pulmonary disease (COPD). Twenty COPD patients were included. The 81mKr, 133Xe, and 99mTc aerosol lung scintigraphies were successively carried out. The 81mKr distribution and 99mTc deposition were compared to the 133Xe distribution at equilibrium and to the 133Xe clearance. Sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of 81mKr and Technegas lung scintigraphies to detect alterations in ventilation revealed by 133Xe were defined. The 81mKr distribution and 99mTc deposition according to respiratory parameters were described using a principal component analysis. Compared to 133Xe distribution, a significantly higher distribution of 81mKr in the upper parts of the lungs in the more severe patients (p = 0.05), a significantly higher deposition of Technegas in the lower parts of the lungs (p = 0.0008), and a significantly higher deposition in the central parts of the high-size 99mTc aerosol were observed (p = 0.0001). The PPV and the NPV were, respectively, 0.54 and 0.58 for 81mKr and 0.54 and 0.55 for Technegas. There was a significant negative correlation between 81mKr distribution and 133Xe clearance (p = 0.0001) between Technegas deposition and 133Xe clearance (p = 0.0007), and between 99mTc diethylene-triamino-penta-acetate (DTPA) deposition and 133Xe clearance (p = 0.001). Both the 81mKr peripheral distribution and Technegas peripheral deposition correlated negatively with increased obstruction, as measured by forced expiratory volume in 1 sec (FEV1). Peripheral deposition of the high-size 99mTc aerosol deposition correlated with the inspiration/expiration time ratio. In conclusion, 81mKr and 99mTc aerosols' lung scintigraphies do not reflect exactly the pulmonary ventilation as measured by 133Xe scintigraphy.

Chronic dietary alpha-linolenic acid deficiency alters dopaminergic and serotoninergic neurotransmission in rats

This study examined the effects of dietary alpha-linolenic acid [18:3(n-3)] deficiency on dopaminergic serotoninergic neurotransmission systems in 60-d-old male rats. Rats were fed semipurified diets containing either peanut oil [the (n-3)-deficient group] or peanut plus rapeseed oil (control group). We measured the densities of the serotonin-2 (5-HT2) receptors and the dopamine-2 (D2) receptors by autoradiography and membrane-binding assays in relation to the fatty acid composition and levels of endogenous monoamines in three cerebral regions: the frontal cortex, the striatum and the cerebellum. Long-term feeding of the (n-3)-deficient diet induced a significantly higher 5-HT2 receptor density in the frontal cortex compared with the control rats without any difference in the endogenous serotonin concentrations. The results also showed some modification of dopaminergic neurotransmission specifically in the frontal cortex in the rats deficient in alpha-linolenic acid, with a significantly lower density of D2 receptors and a significantly lower concentration of endogenous dopamine than in control animals. Moreover, there were lower levels of (n-3) fatty acids in all the regions studied in the deficient rats, balanced by greater levels of (n-6) fatty acids. These results suggest that chronic consumption of an alpha-linolenic acid-deficient diet could induce modifications of the neurotransmission pathways; this might induce the behavioral disturbances previously described in this fatty acid-deficient animal model.

Synthesis and in vitro evaluation of N-substituted aza-trozamicol analogs as vesicular acetylcholine transporter ligands

As dysfunction of cerebral cholinergic neurotransmission is one of the main features in patients with Alzheimer's disease, in vivo imaging of the vesicular acetylcholine transporter (VAChT) can be of great value for the early diagnosis of this disease. Two series of positional isomers of m-iodobenzyltrozamicol (MIBT): 3-hydroxy-4-(N-phenylpiperazinyl)piperidine and 4-hydroxy-3-(N-phenylpiperazinyl)piperidine substituted by benzyl, aryl, alkyl or vinyl groups at the nitrogen have been synthesized. These compounds have been evaluated in vitro by competition studies and five compounds (N-benzyl derivatives) showed high affinity for the VAChT (11nM<IC(50)<66nM). These compounds will be soon radiolabeled with [(125)I] for further biological evaluation using single photon emission computed tomography (SPECT).

Synthesis and in vitro evaluation of novel derivatives of diphenylsulfide as serotonin transporter ligands

As the serotonin transporter (SERT) is involved in several neurodegenerative and psychiatric disorders, radiopharmaceuticals to image the SERT by PET or SPECT would be very valuable in studying these diseases. For the development of imaging agents, we have synthesized novel derivatives of recently reported diphenylsulfide SERT ligands, in which the sulfur atom linking the two phenyl rings was replaced by an oxygen, sulfinyl, sulfonyl, amino or carbon group. Three of these exhibited good to high in vitro affinity (0.5 nM < K(i) < 11 nM) and selectivity for the SERT over the other monoamine transporters.

Synthesis and biological evaluation of a series of flavone derivatives as potential radioligands for imaging the multidrug resistance-associated protein 1 (ABCC1/MRP1)

Multidrug resistance (MDR) is one of the major problems affecting the treatment of cancer. In vivo visualization and quantification of MDR proteins would be of great value to better select the therapeutic strategy. Six flavone-based compounds were synthesized and evaluated for their cytotoxic activity and MDR-reversing capacity using hMRP1 or hMDR1 overexpressing cell lines for in vitro assays. All the flavone derivatives were highly selective for hMRP1-expressing cell lines. These derivatives each used at 4muM (a non-cytotoxic concentration) enhance significantly the sensitivity of hMRP1-mediated MDR cell line toward doxorubicin toxicity. Their MDR-reversing capacity suggests that, in particular, the 4'-fluoroalkyloxy and 4'-iodo apigenin derivatives are potential new radiopharmaceuticals to visualize in vivo MRP1-mediated MDR phenomenon by PET or SPECT.

[(11)C]MADAM, a new serotonin transporter radioligand characterized in the monkey brain by PET

The aim of this study was to explore the potential of a new selective serotonin transporter (5-HTT) inhibitor, N,N-dimethyl-2-(2-amino-4-methylphenylthio)benzylamine (MADAM, K(i)=1.65 nM), as a PET radioligand for examination of 5-HTT in the nonhuman primate brain. MADAM was radiolabeled by an N-methylation reaction using [(11)C]methyl triflate and the binding was characterized by PET in four cynomolgus monkeys. Metabolite levels in plasma were measured by gradient high-performance liquid chromatography (HPLC). The radiochemical incorporation yield of [(11)C]MADAM was 75-80% and the specific radioactivity at the time of administration was 34-652 GBq/micromol (n=8). The highest uptake of radioactivity was observed in striatum, thalamus, mesencephalon, and the lower brainstem. Lower binding was detected in neocortex and the lowest radioactive uptake was found in the cerebellum. This distribution is in accordance with the known expression of 5-HTT in vitro. The fraction of the total radioactivity in monkey plasma representing unchanged [(11)C]MADAM was 20% at 45 min after injection, as measured by gradient HPLC. Pretreatment measurements, using unlabeled citalopram, GBR 12909, and maprotiline, as well as a displacement measurement, using unlabeled MADAM, confirmed that [(11)C]MADAM binds selectively and reversibly to 5-HTT, and support the use of the cerebellum as reference region. The present characterization of binding in the monkey brain suggests that [(11)C]MADAM is a potential PET radioligand for quantitative studies of 5-HTT binding in the human brain.