Synthesis of fluorine substituted pyrazolopyrimidines as potential leads for the development of PET-imaging agents for the GABAA receptors

Neuroimaging of GABA(A) receptors offers the potential for a better diagnosis of diseases related to a dysfunction of the GABAergic neurotransmission. A series of potent fluorinated analogues of the pyrazolopyrimidine Indiplon has been synthesized and evaluated in vitro as potential agents for imaging the GABA(A) receptor by means of positron emission tomography (PET). The most promising compound N-(3-fluoropropyl)-N-[3-[3-(thiophene-2-carbonyl)-pyrazolo[1,5-a]pyrimidin-7-yl]-phenyl]-acetamide (5b) showed an IC(50) value of 2.78+/-0.63 nM comparable to the lead compound Indiplon (IC(50) 3.29+/-0.37 nM), thus making it an interesting candidate for further investigations. In addition to the fluorinated reference compounds, suitable precursors for (18)F-radiolabelling studies have been synthesized.

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.

Postnatal development and kinetics of [3H]gaboxadol binding in rat brain: in vitro homogenate binding and quantitative autoradiography

The postnatal development of the binding of the GABA(A) receptor agonist [(3)H]gaboxadol in rat brain was investigated. Using brain tissue from rats obtained at postnatal days 1, 10, 25, and >25 (adult), the binding of [(3)H]gaboxadol and the benzodiazepine [(3)H]flunitrazepam to GABA(A) receptors was compared in homogenate binding assays and quantitative receptor autoradiography. Kinetic and equilibrium data obtained in homogenate binding studies revealed two different [(3)H]gaboxadol affinities. A kinetically derived K(D) of 3.7 nM in adult cerebellum, calculated from the association and dissociation rate constants k(on) (1.45 x 10(8) M(-1) min(-1)) and k(off) (0.54 min(-1)) was contrasted by an equilibrium K(D) of 38.6 nM, obtained by homologous competition experiments. Quantitative analysis of autoradiographic data revealed an increase in specific [(3)H]gaboxadol binding sites during brain development, which resembles the anatomical and temporal pattern of the postnatal expression of the extrasynaptic delta subunit of GABA(A) receptors. In conclusion, by the radioligand binding data obtained on native tissue, binding of gaboxadol to GABA(A) receptors located outside the synaptic junctions could be postulated.

Radiosynthesis and biological evaluation of an 18F-labeled derivative of the novel pyrazolopyrimidine sedative-hypnotic agent indiplon

INTRODUCTION

Gamma amino butyric acid type A (GABA(A)) receptors are involved in a variety of neurological and psychiatric diseases, which have promoted the development and use of radiotracers for positron emission tomography imaging. Radiolabeled benzodiazepine antagonists such as flumazenil have most extensively been used for this purpose so far. Recently, the non-benzodiazepine pyrazolopyrimidine derivative indiplon with higher specificity for the alpha(1) subtype of the GABA(A) receptor has been introduced for treatment of insomnia. The aim of this study was the development and biological evaluation of an (18)F-labeled derivative of indiplon.

METHODS

Both [(18)F]fluoro-indiplon and its labeling precursor were synthesized by two-step procedures starting from indiplon. The radiosynthesis of [(18)F]fluoro-indiplon was performed using the bromoacetyl precursor followed by multiple-stage purification using semipreparative HPLC and solid phase extraction. Stability, partition coefficients, binding affinities and regional brain binding were determined in vitro. Biodistribution and radiotracer metabolism were studied in vivo.

RESULTS

[(18)F]Fluoro-indiplon was readily accessible in good yields (38-43%), with high purity and high specific radioactivity (>150 GBq/micromol). It displays high in vitro stability and moderate lipophilicity. [(18)F]Fluoro-indiplon has an affinity to GABA(A) receptors comparable to indiplon (K(i)=8.0 nM vs. 3.4 nM). In vitro autoradiography indicates high [(18)F]fluoro-indiplon binding in regions with high densities of GABA(A) receptors. However, ex vivo autoradiography and organ distribution studies show no evidence of specific binding of [(18)F]fluoro-indiplon. Furthermore, the radiotracer is rapidly metabolized with high accumulation of labeled metabolites in the brain.

CONCLUSIONS

Although [(18)F]fluoro-indiplon shows good in vitro features, it is not suitable for in vivo imaging studies because of its metabolism. Structural modifications are needed to develop derivatives with higher in vivo stability.

[11C]SMe-ADAM, an imaging agent for the brain serotonin transporter: synthesis, pharmacological characterization and microPET studies in rats

N,N-Dimethyl-2-(2-amino-4-methylthiophenylthio)benzylamine (SMe-ADAM, 1) is a highly potent and selective inhibitor of the serotonin transporter (SERT). This compound was labeled with carbon-11 by methylation of the S-desmethyl precursor 10 with [(11)C]methyl iodide to obtain the potential positron emission tomography (PET) radioligand [(11)C]SMe-ADAM. The radiochemical yield was 27 +/- 5%, and the specific radioactivity was 26-40 GBq/micromol at the end of synthesis. Ex vivo and in vivo biodistribution experiments in rats demonstrated a rapid accumulation of the radiotracer in brain regions known to be rich in SERT, such as the thalamus/hypothalamus region (3.59 +/- 0.41%ID/g at 5 min after injection). The specific uptake reached a thalamus to cerebellum ratio of 6.74 +/- 0.95 at 60 min postinjection. The [(11)C]SMe-ADAM uptake in the thalamus was significantly decreased by pretreatment with fluoxetine to 38 +/- 11% of the control value. Furthermore, no metabolites of [(11)C]SMe-ADAM could be detected in the SERT-rich regions of the rat brain. It is concluded that [(11)C]SMe-ADAM may be a suitable PET ligand for SERT imaging in the living brain.

Autoradiography of 2-[18F]F-A-85380 on nicotinic acetylcholine receptors in the porcine brain in vitro

Noninvasive molecular imaging of subtypes of neuronal nicotinic acetylcholine receptors (nAChRs) will provide information on the role of these receptors in neurodegenerative diseases. The binding of the positron emission tomography ligand 2-[18F]F-A-85380 to nAChRs was investigated in the porcine brain by quantitative autoradiography in vitro. The high-affinity binding of 2-[18F]F-A-85380 to each of the investigated 12 brain areas was saturable and apparently monophasic (e.g., apparent KD value of 1.72 nM in the thalamus). The highest density of specific binding sites was observed in the thalamus (1,158 fmol/mg protein), and the lowest density was measured in the cerebellar gray matter (11 fmol/mg protein). An attempt to assess nAChR subtype specificity of 2-[18F]F-A-85380 was made by competitive autoradiography. Binding of 2-[18F]F-A-85380 coincubated with 2-F-A-85380, epibatidine, cytisine, or methyllycaconitine, respectively, indicated a specificity of 2-[18F]F-A-85380 to beta2-containing nAChRs in the porcine brain. The autoradiographic data confirmed the suitability of swine as a model for the evaluation of radioligands designed for imaging of nAChR subtypes in the living brain.

Norchloro-fluoro-homoepibatidine: specificity to neuronal nicotinic acetylcholine receptor subtypes in vitro

The subtype-specificity of newly synthesised epibatidine-related compounds, norchloro-fluoro-homoepibatidine (NCFHEB) and derivatives, to neuronal nicotinic acetylcholine receptors (nAChR) has been investigated. NCFHEBs were assayed in competitive binding assays to (+/-)-[(3)H]epibatidine-labelled rat thalamic nAChRs and human alpha4beta2, alpha3beta4, and alpha7 nAChRs, expressed in stably transfected HEK-293 and SH-SY5Y cells. The binding affinity of (+)-NCFHEB (K(i): 0.064 nM) and (-)-NCFHEB (K(i): 0.112 nM) to human alpha4beta2 nAChR is in the same order of magnitude as that of epibatidine (K(i): 0.014 nM). However, because the affinity of both NCFHEB-enantiomers to human alpha3beta4 nAChR is up to 65 times lower than that of epibatidine, the alpha4beta2 subtype-specificity of NCFHEB is increased up to 1,400% compared to epibatidine.

Signal transduction pathways in mouse microglia N-11 cells activated by advanced glycation endproducts (AGEs)

Deposition of cross-linked insoluble protein aggregates such as amyloid plaques is characteristic for Alzheimer's disease. Microglial activation by these extracullar deposits has been proposed to play a crucial role in functional degeneration as well as cell death of neurones. A sugar-derived post-translational modification of long-lived proteins, advanced glycation endproducts (AGEs), activate specific signal transduction pathways, resulting in the up-regulation of various pro-inflammatory signals such as cytokines [interleukin-6 (IL-6), tumour necrosis factor-alpha (TNF-alpha)] and inducible nitric oxide synthase (iNOS). Our goal was to study AGE-activated signal transduction pathways involved in the induction of pro-inflammatory effectors in the murine microglial cell line N-11. Chicken egg albumin-AGE (CEA-AGE), used as model AGE, induces nitric oxide (NO), TNF-alpha and IL-6 production. The AGE receptor, RAGE, and the transcription factor, nuclear factor kappa B (NF-kappaB), appear to be involved in all pathways, since a neutralizing RAGE antibody and a peptide inhibiting NF-kappaB translocation down-regulated NO, TNF-alpha and IL-6 production. NO and TNF-alpha, but not IL-6 production appear to be regulated independently, since NOS inhibitors did not decrease TNF-alpha secretion and a neutralizing TNF-alpha antibody did not reduce NO production, while employment of NOS inhibitors reduced significantly the secretion of IL-6. Inhibition of the MAP-kinase-kinase (MEK) and phosphatidylinositol 3-kinase (PI3K) pathway, but not that of mitogen-activated protein kinase-p38 (MAPK-p38), reduced NO, TNF-alpha and IL-6 significantly, suggesting that simultaneous activation of the first two pathways is necessary for the AGE-induced induction of these pro-inflammatory stimuli.

Beta-amyloid peptide potentiates inflammatory responses induced by lipopolysaccharide, interferon -gamma and 'advanced glycation endproducts' in a murine microglia cell line

beta-Amyloid (Abeta) plaques are characteristic hallmarks of Alzheimer's disease (AD). In AD, it has been suggested that activation of microglial cells might be the link between Abeta deposition and neuronal degeneration. Activated microglia are associated with senile plaques and produce free radicals and inflammatory cytokines. However, it is still not clear whether Abeta needs a prestimulated environment to exert its proinflammatory potential. Advanced glycation endproducts (AGEs), protein-bound oxidation products of sugars, have been shown to accumulate in senile plaques and could induce a silent but chronic inflammation in the AD brain. We tested whether Abeta acts as an amplifier of a submaximal proinflammatory response initiated by exposure to chicken egg albumin-AGE, lipopolysaccharide or interferon-gamma. Synthetic Abeta was used to produce three different samples (Abeta-fibrilar; Abeta-aggregated; Abeta-AGE), which were characterized for beta-sheeted fibrils by the thioflavin-T test and electron microscopy. As markers of microglial activation, nitric oxide, interleukin-6, macrophage-colony stimulation factor and tumour necrosis factor-alpha production was measured. All three Abeta samples alone could not induce a detectable microglial response. The combination of Abeta preparations, however, with the coinducers provoked a strong microglial response, whereby Abeta-AGE and fibrilar Abeta were more potent inflammatory signals than aggregated Abeta. Thus, Abeta in senile plaques can amplify microglial activation by a coexisting submaximal inflammatory stimulus. Hence, anti-inflammatory therapeutics could either target the primary proinflammatory signal (e.g. by limiting AGE-formation by AGE inhibitors or cross-link breakers) or the amplifyer Abeta (e.g. by limiting Abeta production by beta- or gamma-secretase inhibitors).

Glycoxidative stress creates a vicious cycle of neurodegeneration in Alzheimer's disease--a target for neuroprotective treatment strategies?

Accumulation of Advanced Glycation Endproducts (AGEs) in the brain is a feature of ageing and degeneration, especially in Alzheimer's disease (AD). Increased AGE levels explain many of the neuropathological and biochemical features of AD such as extensive protein crosslinking (beta-amyloid and MAP-tau), glial activation, oxidative stress and neuronal cell death. Oxidative stress and AGEs initiate a positive feedback loop, where normal age-related changes develop into a pathophysiological cascade. Combined intervention using antioxidants, anti-inflammatory drugs and AGE-inhibitors may be a promising neuroprotective strategy.

In vitro-prepared advanced glycation end-products and the modulating potential of their low-molecular weight degradation products in IRPTC-A rat proximal-tubular derived kidney epithelial cell line

Low-molecular advanced glycation end-products (AGEs)-degradation products resulting from a proteolysis of tissue or circulating AGEs represent up to 80% of AGE plasma immunoreactivity. These AGE peptides contribute to the dramatic increase in AGE levels in end-stage renal disease even in the absence of diabetes. Because glomerular filtered AGE-degradation products may accumulate within intracellular compartments of proximal tubular epithelial cells, we investigated whether there is a pathway potentially mediating damaging effects of AGE-degradation products by perturbation of the function of the tubuloepithelium. Proximal tubular-derived rat kidney cells (IRPTC) were incubated with high-molecular AGEs highly modified by incubation of bovine serum albumin (BSA) with glucose for 50 days in vitro, and with low-molecular AGE-degradation products derived from proteolytic cleavage and isolated in the molecular range between 1 and 30 kDa. The proliferation of IRPTC (3H-thymidine incorporation) was reduced to 89+/-1% and 69+/-2% after 24 hr of incubation with BSA-AGE and BSA-AGE-degradation products, respectively. The cell viability of IRPTC was reduced significantly to 59+/-15% and 31+/-13% after 144 hr of incubation with BSA-AGE and BSA-AGE-degradation products, respectively. Conditioned media obtained from IRPTC incubated for 72 hr with BSA-AGE and its degradation products increased the proliferation rate of renal fibroblasts (RFb) to 222+/-24% and 449+/-40%, respectively. Incubation of IRPTC with BSA-AGE-degradation products increased the expression of endothelin-1 (ET-1) mRNA to 210% after 1 hr; the expression of platelet-derived growth factor-B (PDGF-B) mRNA reached 184% after 2 hr. Regarding the toxicity of AGEs to the kidney, low-molecular weight AGE-degradation products possibly form an individual fraction with a comparatively higher toxic potential.

Protein "AGEing"--cytotoxicity of a glycated protein increases with its degree of AGE-modification

Non-enzymatic glycation of proteins with reducing sugars and subsequent transition metal-catalyzed oxidations leads to the formation of protein-bound "advanced glycation endproducts" (AGEs). They accumulate on long-lived proteins including on and in the vicinity of the beta-amyloid plaques in Alzheimer's disease (AD). Since the AGE modification of a protein increases with time, and such a "long-term incubation" might also occur in the AD brain, we investigated whether an increase in the cytotoxic effects of an AGE-modified model protein occurs over time. Bovine serum albumin (BSA) was modified by glucose for defined time periods, and the viability of SH-SY5Y neuroblastoma cells, incubated with the differentially AGE-modified BSA samples, was measured with the MTT assay. Cytotoxicity of the AGE-modified BSAs increased in correlation to the incubation time with glucose. Among the AGE-specific markers, browning (OD 400) correlated best with cytotoxicity, followed by AGE-specific fluorescence and the defined AGE, carboxymethyllysine. Since AGEs accumulate in AD over time, they may be one of the "age-related" factors contributing to neuronal cell death in Alzheimer's disease.

Advanced glycation endproducts co-localize with inducible nitric oxide synthase in Alzheimer's disease

Advanced glycation endproducts (AGEs), protein-bound oxidation products of sugars, have been shown to be involved in the pathophysiological processes of Alzheimer's disease (AD). AGEs induce the expression of various pro-inflammatory cytokines and the inducible nitric oxide synthase (iNOS) leading to a state of oxidative stress. AGE modification and resulting crosslinking of protein deposits such as amyloid plaques may contribute to the oxidative stress occurring in AD. The aim of this study was to immunohistochemically compare the localization of AGEs and beta-amyloid (Abeta) with iNOS in the temporal cortex (Area 22) of normal and AD brains. In aged normal individuals as well as early stage AD brains (i.e. no pathological findings in isocortical areas), a few astrocytes showed co-localization of AGE and iNOS in the upper neuronal layers, compared with no astrocytes detected in young controls. In late AD brains, there was a much denser accumulation of astrocytes co-localized with AGE and iNOS in the deeper and particularly upper neuronal layers. Also, numerous neurons with diffuse AGE but not iNOS reactivity and some AGE and iNOS-positive microglia were demonstrated, compared with only a few AGE-reactive neurons and no microglia in controls. Finally, astrocytes co-localized with AGE and iNOS as well as AGE and were found surrounding mature but not diffuse amyloid plaques in the AD brain. Our results show that AGE-positive astrocytes and microglia in the AD brain express iNOS and support the evidence of an AGE-induced oxidative stress occurring in the vicinity of the characteristic lesions of AD. Hence activation of microglia and astrocytes by AGEs with subsequent oxidative stress and cytokine release may be an important progression factor in AD.

Advanced glycation endproducts change glutathione redox status in SH-SY5Y human neuroblastoma cells by a hydrogen peroxide dependent mechanism

The reaction of proteins with reducing sugars leads to the formation of 'advanced glycation endproducts' (AGEs). They accumulate in Alzheimer's disease brain in the vicinity of beta-amyloid plaques. AGEs are cytotoxic by a mechanism involving reactive oxygen species, which implies that they could compromise glutathione redox status. In this study, we show that AGEs (BSA-AGE and beta-amyloid-AGE) persistently increase the ratio of oxidized to reduced glutathione in a dose- and time-dependent manner in SH-SY5Y neuroblastoma cells. The level of oxidized glutathione accounted to 10-14% and persisted for up to 24 h in the presence of added AGEs. In contrast, the unmodified beta-amyloid peptides A beta (1-40) and A beta (25-35) had no significant effect on glutathione redox status. The AGE-induced increase in oxidized glutathione could be prevented by the radical scavengers N-acetylcysteine, alpha-lipoic acid and 17beta-estradiol or by application of catalase, indicating that superoxide and hydrogen peroxide production precedes the AGE-mediated depletion of reduced glutathione.

Differences in the modulating potential of advanced glycation end product (AGE) peptides versus AGE proteins

Differences in the modulating potential of advanced glycation end product (AGE) peptides versus AGE proteins. Advanced glycation end products (AGEs), identified as irreversible products of a complex reaction of carbonyl groups of reducing sugars with free protein amino groups, are characterized by resistance to proteolytic degradation. The incomplete digestion of AGEs results in low molecular weight AGEs accumulating in the blood of diabetic and uremic patients. We hypothesized that the accumulation of these compounds may contribute to the dysfunction and/or degeneration of tubular epithelial cells. Our study examined whether low-molecular-weight AGE peptides and high-molecular-weight AGE proteins affect the functional cellular properties of two tubular epithelial cell lines: immortalized human kidney tubular epithelial (IHKE) and immortalized rat renal proximal tubular cells (IRPTCs). Parameters of cellular damage and growth behavior were cell counting, analysis of the cellular metabolic activity (MTT assay), as well as cellular proliferation (3[H]-thymidine-incorporation). IHKE treated with bovine serum albumin-AGE (BSA-AGE 50) or BSA-AGE-Pep 50 revealed a decrease in cellular metabolic activity as compared with controls after 48 hours of incubation (73 +/- 9% for BSA-AGE 50 and 62 +/- 11% for BSA-AGE-Pep 50 vs. 89 +/- 8% for BSA Co 50). Low molecular weight BSA-AGE-Pep 50 induced a significantly greater cellular damage in IRPTCs as compared with high molecular weight BSA-AGE 50 after 144 hours of incubation (59 +/- 15% for BSA-AGE 50 vs. 31 +/- 13% for BSA-AGE-Pep 50). The decrease in metabolic activity correlated well with a decrease in cellular proliferation. The results suggest a higher toxic potential of low molecular weight AGE peptides compared with high molecular weight AGE proteins in IRPTC and IHKE. This may provide evidence that low molecular weight degradation products of AGE-modified proteins have an important risk potential.

Immunochemical detection of imidazolone in uremia and rheumatoid arthritis

The advanced glycation end-product imidazolone is formed by reaction of arginine with 3-deoxyglucosone (3-DG), a reactive intermediate of the Maillard reaction, whose formation is non-oxidative. Using an antibody specific to this 3-DG-derived AGE, we demonstrated the presence of imidazolone-modified proteins in vivo in the urine and dialysate of patients with chronic renal failure, in the synovial fluid of patients with rheumatoid arthritis, as well as in vitro in human serum and human serum albumin incubated with glucose. Furthermore, we could show that in uremic patients the dimeric form of beta(2)-microglobulin is more susceptible to imidazolone modification than the monomeric one. Thus, the immunochemical detection of imidazolone may be a good marker for 3-DG-derived AGE modification in vivo and in vitro permitting a differentiation between the oxidative and the non-oxidative pathway of AGE generation.

Time course of cytokine mRNA expression in kidneys of rats with unilateral ureteral obstruction

The development of renal interstitial fibrosis (RIF) is related to the expression and excretion of cytokines and growth factors. Thus, we investigated the time course of mRNA expression of cytokines known as causative factors in a model of RIF in rats before and on day 10 after unilateral ureteral obstruction (UUO), when first signs of fibrosis were visible, as well as during progressive RIF. UUO causes a fivefold increase in mRNA expression of monocyte chemoattractant protein 1 15 days after surgery as compared with contralateral kidneys. The level remains elevated about three-fold up to day 25. The mRNA of the fibrogenic cytokine transforming growth factor beta 1 (TGF-beta1) is increased two- to threefold during the time course, whereas the mRNAs of platelet-derived growth factor B chain (PDGF-B) and its receptor beta (PDGF-Rbeta) increase after UUO, reaching their maxima on days 10-15. PDGF-B mRNA increase up to day 15, marking the onset of fibrosis, and decreases thereafter, whereas the expression of the PDGF-Rbeta mRNA remains elevated more than threefold over the entire study period. Incubation of cultured renal fibroblasts with TGF-beta1 and/or PDGF-B suggests that their specific action on cell growth and proliferation is maintained even when they are used in combination. The sustained elevation of TGF-beta1 and PDGF-B/PDGF-Rbeta mRNA levels confirms the assumption of a particular involvement of these cytokines in the pathogenesis of RIF. The mRNA expression of the gap junctional protein connexin 43 in ureteral ligated kidneys is increased sixfold already 5 days after UUO. In this way, the increased connexin 43 mRNA levels indicate a possible function in the remodeling of the kidney tissue after tubular damage and fibrosis.