In vitro and in vivo characterization of [125I]iodomethyllycaconitine in the rat

The in vitro and in vivo binding characteristics of [125I]iodomethyllycaconitine ([125I]iodoMLA) were determined in the rat. [125I]iodoMLA binding to rat cerebral cortex membranes was saturable and reversible and its specific binding represented approximately 70-80% of the total binding. [125I]iodoMLA labeled a single site with Kd = 1.8 +/- 0.4 nM and Bmax = 68 +/- 3 fmol/mg protein. Kinetic analysis revealed a t1/2 for association and dissociation of 10.5 +/- 3.1 and 10.3 +/- 1.6 min, respectively. Pharmacological characterization of [125I]iodoMLA binding indicated that it was specific for the alpha7 nAChR. In vitro brain region binding studies revealed greater binding in regions known to contain high numbers of alpha7 nAChRs. The analysis of the biodistribution of intravenously administered [125I]iodoMLA indicated that it was rapidly cleared and exhibited poor brain penetration; nevertheless, the levels of [125I]iodoMLA in alpha7 nAChR-rich target regions were significantly increased compared to the nontarget region (cerebellum) 60-120 min after administration. No metabolism of MLA by human liver S9 fraction was detected. Our results suggest that [125I]iodoMLA will be a useful radioligand to study the alpha7 nAChR in vitro and in vivo.     

D2 receptor occupancy in conscious rat brain is not significantly distinguished with [3H]‐MNPA, [3H]‐(+)‐PHNO,and [3H]‐raclopride

Positron emission tomography (PET) antagonist ligands such as [¹¹C]‐raclopride are commonly used to study dopamine D2 receptor (D2) binding of antipsychotics. It has been suggested that agonist radioligands bind preferentially to the high‐affinity state of D2 receptor and may provide a more relevant means of assessing D2 occupancy. The main objective of this study was to determine if D2 receptor occupancy (RO) could be differentiated with agonist and antagonist radioligands in vivo. Agonist radioligands [³H]‐MNPA and [³H]‐(+)‐PHNO were synthesized and compared to antagonist [³H]‐raclopride in the in vitro binding and in vivo occupancy studies. In vivo, unanesthetized rats were pretreated with quinpirole (full agonist), aripiprazole (partial agonist), or haloperidol (antagonist) prior to administration of the agonist or antagonist radioligand. All three pretreatment compounds showed equivalent dose‐dependent D2 receptor occupancy in the rat striatum with each radioligand. The in vivo receptor occupancy results suggested that the binding of quinpirole, aripiprazole, and haloperidol to the high or low affinity state of the D2 receptor could not be differentiated using radiolabeled agonists or antagonists, presumably due to a predominance of high affinity states of the D2 receptor in vivo. This hypothesis was supported in part by the in vitro binding results. Our in vitro results show that [³H]‐MNPA binds to D2S transfected CHO cell membranes at a single high affinity site. Displacement of [³H]‐(+)‐PHNO binding by quinpirole and elimination of most [³H]‐(+)‐PHNO binding by the guanine nucleotide GppNHp in striatal membranes suggest that the majority of D2 in striatal tissue is G‐protein coupled. Together, these findings suggest that D2 agonist radioligands produce in vivo receptor occupancy comparable to [³H]‐raclopride. Synapse, 2010. © 2010 Wiley‐Liss, Inc.     

Binding characteristics of [3H]14-methoxymetopon,a high affinity mu-opioid receptor agonist

The highly potent micro -opioid receptor agonist 14-methoxymetopon (4,5alpha-epoxy-3-hydroxy-14beta-methoxy-5beta,17-dimethylmorphinan-6-one) was prepared in tritium labelled form by a catalytic dehalogenation method resulting in a specific radioactivity of 15.9 Ci/mmol. Opioid binding characteristics of [3H]14-methoxymetopon were determined using radioligand binding assay in rat brain membranes. [3H]14-Methoxymetopon specifically labelled a single class of opioid sites with affinity in low subnanomolar range (Ki = 0.43 nm) and maximal number of binding sites of 314 fmol/mg protein. Binding of [3H]14-methoxymetopon was inhibited by ligands selective for the micro -opioid receptor with high potency, while selective kappa-opioids and delta-opioids were weaker inhibitors. 14-Methoxymetopon increased guanosine-5'-O-(3-[35S]thio)-triphosphate ([35S]GTPgammaS) binding with an EC50 of 70.9 nm, thus, providing evidence for the agonist character of this ligand. The increase of [35S]GTPgammaS binding was inhibited by naloxone and selective micro -opioid antagonists, indicating a micro -opioid receptor-mediated action. [3H]14-Methoxymetopon is one of the few nonpeptide mu-opioid receptor agonists available in radiolabelled form up to now. Due to its high affinity and selectivity, high stability and extremely low nonspecific binding (<10%), this radioligand would be an important and useful tool in probing mu-opioid receptor mechanisms, as well as to promote a further understanding of the opioid system at the cellular and molecular level.     

(125)I]3beta-(4-ethyl-3-iodophenyl)nortropane-2beta-carboxylic acid methyl ester ([(125)I]EINT): a potent and selective radioligand for the brain serotonin transporter.

The binding characteristics of [(125)I]3beta-(4-ethyl-3-iodophenyl)nortropane-2beta-carboxylic acid methyl ester ([(125)I]EINT), a high-affinity selective ligand for the serotonin transporter (5-HTT), and its binding characteristics to rat brain membranes were determined. [(125)I]EINT binding to rat cerebral cortex membranes was saturable and reversible, and its specific binding represented approximately 90% of the total binding. [(125)I]EINT labeled a single site with K(d) = 0.22 +/- 0.03 nM and B(max) = 583 +/- 38 fmol/mg protein. Kinetic analysis revealed a t(1/2) for association and dissociation of 20 and 24 min, respectively. Pharmacological characterization of [(125)I]EINT confirmed its high specificity for the 5-HTT. The pattern of brain region distribution in vivo of intravenously administered [(125)I]EINT indicated greater accumulation of the radioligand in 5-HTT-rich brain regions. However, the signal-to-background ratio was low. Thus, [(125)I]EINT appears to be a useful radioligand for studying the 5-HTT in vitro, but it may not be a good in vivo ligand.     

11C]-DASB,a tool for in vivo measurement of SSRI-induced occupancy of the serotonin transporter: PET characterization and evaluation in cats

The in vivo pharmacological profile of [(11)C]-DASB, a new radioligand developed for in vivo imaging of the serotonin transporter (SERT), was evaluated in the cat brain using positron emission tomography (PET). The in vivo distribution of [(11)C]-DASB binding was consistent with the known distribution of SERT sites in the cat brain in vitro with high uptakes of radioactivity in the midbrain and thalamus, intermediate levels in striatum, and modest to low levels of radioactivity in the neocortex and cerebellum, respectively. [(11)C]-DASB binding potential (BP) values ranged from about 0.2 in the neocortex to 2.2 in the midbrain. Radioligand binding in all brain regions except cerebellum was markedly reduced following pretreatment with fluoxetine and citalopram, but was unaffected by pretreatment with GBR12909, maprotiline, and haloperidol, indicating specificity of [(11)C]-DASB binding to the SERT. Two cats were each examined using PET and [(11)C]-DASB in a longitudinal fashion (from 30 min and up to 24 days) following a single i.v. dose of: 1) fluoxetine, and 2) citalopram at different dosages. Both drugs induced similar degrees of SERT occupancy at 30 min postinjection (approximately 90%). A comparison of citalopram and fluoxetine pharmacokinetics in the same animal and at the same dosage (1 mg/kg) showed that citalopram SERT occupancy and plasma half-lives were 9 times and 14 times shorter, respectively, than those of fluoxetine and norfluoxetine. In addition, studies performed after injection of the monoamine oxidase inhibitor tranylcypromine suggested that high levels of synaptic serotonin may compete with [(11)C]-DASB for binding on the SERT. These studies indicate that [(11)C]-DASB is a suitable PET radioligand for measuring drug occupancy of the SERT in vivo and has potential for monitoring in vivo changes in serotonin levels.     

Brain uptake and distribution of the dopamine D3/D2 receptor partial agonist [11C]cariprazine: An in vivo positron emission tomography study in nonhuman primates

Cariprazine is a dopamine D₃/D₂ receptor partial agonist antipsychotic candidate, which binds with high affinity to dopamine D₃ and D₂ receptors (with ～10‐fold higher in vitro affinity to D₃ vs. D₂ receptors) and with moderate affinity to 5‐HT_1A receptors. The main objective of the present molecular imaging investigation was to evaluate the uptake and reversible binding of 11‐C labeled cariprazine in the nonhuman primate brain, in relation to the known distributions of dopamine D₂ and D₃ receptors. We examined the brains of two cynomolgus monkeys at baseline condition as well as during a pharmacological blocking condition, using unlabeled cariprazine or raclopride as blockers before injection of [¹¹C]cariprazine. Of the total injected radioactivity, ～7% entered the brain and ～3–4% remained in the brain after 90 min, indicating good blood brain barrier penetration and slow washout. It was possible to block cariprazine binding with unlabeled cariprazine and raclopride indicating that [¹¹C]cariprazine binds to dopamine D₃/D₂ receptors. Nondisplaceable binding potential (BP_ND) measurements, using a simplified reference tissue model and cerebellum as the reference region, yielded values of ～1.5 and 0.3 in the striatum and thalamus, respectively. Striatum BP_ND values were reduced by 80 and 85% following pretreatment with 0.1 mg/kg IV injection of unlabeled cariprazine and 1 mg/kg IV injection of unlabeled raclopride, respectively. The data confirm that cariprazine, a novel antipsychotic drug candidate, enters the nonhuman primate brain readily and binds to dopamine D₃/D₂ receptors. Furthermore, in PET imaging [¹¹C]cariprazine can effectively visualize dopamine D₃/D₂ receptors in the nonhuman primate brain. Synapse 67:258–264, 2013. © 2013 Wiley Periodicals, Inc.     

Measurement error analysis for the determination of dopamine D2 receptor occupancy using the agonist radioligand [11C]MNPA

The purpose of this study is to investigate errors in quantitative analysis for estimating dopamine D₂ receptor occupancy of antipsychotics with agonist radioligand [¹¹C]MNPA by numerical simulation, with particular attention to the validity of a quantitative approach based on the use of a reference region. Synthetic data were validated using clinical data combined with a bootstrap approach. Time–activity curves (TACs) of [¹¹C]MNPA were simulated, and the reliability of binding potential (BP_ND) and occupancy estimated by nonlinear least square (NLS) fitting and a simplified reference tissue model (SRTM) were investigated for various noise levels and scan durations. In the human positron emission tomography (PET) study with and without antipsychotic, risperidone, the uncertainty of BP_ND and occupancy estimated by SRTM was investigated using resampled TACs based on bootstrap approach with weighted residual errors of fitting. For both NLS and SRTM, it was possible to have <3% of bias in occupancy estimates of [¹¹C]MNPA by 60 mins. However, shortened scan duration degrades the quantification of very small binding potentials, especially in case of SRTM. Observations were replicated on the clinical data. Results showed that dopamine D₂ receptor occupancy by antipsychotics can be estimated precisely in region of interest analysis by SRTM with a longer than 60-min [¹¹C]MNPA PET scan duration.     

Cannabinoid receptor agonist-stimulated [35S]guanosine triphosphate gammaS binding in the brain of C57BL/6 and DBA/2 mice

The two inbred strains of mice C57BL/6 (alcohol-preferring) and DBA/2 (alcohol-avoiding) mice have been shown to differ significantly in their preference for alcohol (EtOH). We have previously demonstrated the differences in the density and the affinity of cannabinoid (CB1) receptors in the brains of the two inbred C57BL/6 and DBA/2 mouse strains. In the present study, we investigated the CB1 receptor agonist-stimulated guanosine-5'-O-(3-[(35)S]thio)-triphosphate ([(35)S]GTPgammaS) binding in plasma membranes (PM) from C57BL/6 and DBA/2 mice. The results indicate that the net CP55,940-stimulated [(35)S]GTPgammaS binding was increased with increasing concentrations of CB1 receptor agonists and GDP. The net CB1 receptor agonist (WIN55,212-2 or HU-210 or CP55,940)-stimulated [(35)S]GTPgammaS binding was reduced significantly (-10% to -12%, P < 0.05) in PM from DBA/2 mice; no significant differences were observed in basal [(35)S]GTPgammaS binding among these strains. Nonlinear regression analysis of net CP55,940-stimulated [(35)S]GTPgammaS binding showed that the B(max) of cannabinoid agonist-stimulated binding was significantly reduced (-24%) in DBA/2 mice (B(max) = 12.43 +/- 0.64 for C57BL/6 and 9.46 +/- 0.98 pmol/mg protein for DBA/2; P < 0.05) without any significant changes in the G protein affinity. The pharmacological specificity of CP55,940-stimulated [(35)S]GTPgammaS binding was examined with CB1 receptor antagonist SR141716A, and these studies indicated that CP55,940-stimulated [(35)S]GTPgammaS binding was blocked by SR141716A, with a decrease in the IC(50) values in the PM from the DBA/2 mouse strain. These results suggest that a signal transduction pathway(s) downstream from the CB1 receptor system may play an important role in controlling the voluntary EtOH consumption by these strains of mice.     

123/125I]RTI-55, an in vivo label for the serotonin transporter.

[123I]RTI-55, an iodinated derivative of the cocaine analog 3 beta-phenyltropane-2 beta-carboxylic acid methyl ester, was evaluated as an agent for in vivo labeling of the serotonin transporter. Labeling of the precursor of RTI-55 with I-123 was efficient and yielded a high specific activity product. After intravenous injection of [123I]RTI-55 into rats, the tracer accumulated in regions with high densities of serotonin and dopamine uptake sites. The distribution of [123I]RTI-55 binding in areas rich in serotonin uptake sites correlated with [3H]serotonin uptake measured in vitro in the same regions. Specific [123I]RTI-55 binding to serotonin uptake sites was inhibited by paroxetine but not by GBR 12,909. Treatment of rats with neurotoxic doses of fenfluramine caused decreases of 66% (in the hypothalamus) to 83% (in the superior colliculi) of specific [125I]RTI-55 binding in all areas except in the striatum and the olfactory tubercles (regions rich in dopamine transporters). These results indicate that [123/125I]RTI-55 binds, although not selectively, to the serotonin transporter in vivo. Furthermore, they suggest that [123I]RTI-55 holds promise as a SPECT imaging agent for the study of the serotonin transporter in humans in health and disease.     

Zolmitriptan, a 5-HT1B/1D receptor agonist for the acute oral treatment of migraine: a multicentre, dose-range finding study

Zolmitriptan is a selective 5-HT_1B/1D receptor agonist for acute oral imgraine theraphy. This randomized, placebo|controlled, parallel-grup study investigated the efficacy and tolerability of oral zolmitriptan (5, 10, 15 and 20 mg) in the tretment of single acute migraine attacks. Of 1181 patients randomized, 840 were evaluable for the primary efficacy analysis. Headache response rates (a reduction in headache intensity from severe or moderate at baseline to mild or no pain at 2 hours post-treatment) were similaracross the zolmitriptan dose groups (66%, 71%, 69% and 77% for 5 mg, 10 mg, 15 mg and 20 mg, respectively) and were significantly higher than that for placebo (19%; all groups P < 0.001). A headache response was reponse was reposted at 1 hour by 40-50% of zolmitriptan recipients (16% placebo). At 2 hours post dose,39-47% of zolmitriptan-treated patients were pain-free, compared with 1% of placebo recipients. Headache recurrence occurred in 21-29% (upper 95% CI 37.1) of zolmitriptan-treated patients and in 65% (95% CI 38.3, 85.8) of placebo recipients. Zolmitriptan was well tolerated at each dose. The most commonly reported adverse events were asthenia, dizziness, paraesthesia and feelings of heaviness. Mostadverse events were of mildor moderate intensity and were efficacy and tolerability profile, the dose response data suggest that lower doses would also offer significant efficacy.     

High occupancy of sigma-1 receptors in the human brain after single oral administration of fluvoxamine: a positron emission tomography study using [11C]SA4503.

BACKGROUND: Sigma-1 receptors might be implicated in the pathophysiology of psychiatric diseases, as well as in the mechanisms of action of some selective serotonin reuptake inhibitors (SSRIs). Among the several SSRIs, fluvoxamine has the highest affinity for sigma-1 receptors (Ki = 36 nM), whereas paroxetine shows low affinity (Ki = 1893 nM). The present study was undertaken to examine whether fluvoxamine binds to sigma-1 receptors in living human brain. METHODS: A dynamic positron emission tomography (PET) data acquisition using the selective sigma-1 receptor ligand [(11)C]SA4503 was performed with arterial blood sampling to evaluate quantitatively the binding of [(11)C]SA4503 to sigma-1 receptors in 15 healthy male volunteers. Each subject had two PET scans before and after randomly receiving a single dose of either fluvoxamine (50, 100, 150, or 200 mg) or paroxetine (20 mg). The binding potential of [(11)C]SA4503 in 9 regions of the brain was calculated by a 2-tissue 3-compartment model. In addition, we examined the effects of functional polymorphisms of the sigma-1 receptor (SIGMAR1) gene on the binding potential of [(11)C]SA4503. RESULTS: Fluvoxamine bound to sigma-1 receptors in all brain regions in a dose-dependent manner, whereas paroxetine did not bind to sigma-1 receptors. However, there was no association between the SIGMAR1 gene polymorphism GC-241-240TT and binding potential. CONCLUSIONS: The study demonstrated that fluvoxamine bound to sigma-1 receptors in living human brain at therapeutic doses. These findings suggest that sigma-1 receptors may play an important role in the mechanism of action of fluvoxamine.     

In vitro characterization of SLV308 (7-[4-methyl-1-piperazinyl]-2(3H)-benzoxazolone, monohydrochloride): a novel partial dopamine D2 and D3 receptor agonist and serotonin 5-HT1A receptor agonist

Present Parkinson's disease treatment strategies are far from ideal for a variety of reasons; it has therefore been suggested that partial dopamine receptor agonism might be a potential therapeutic approach with potentially fewer side effects. In the present study, we describe the in vitro characterization of the nonergot ligand SLV308 (7-[4-methyl-1-piperazinyl]-2(3H)-benzoxazolonemonohydrochloride). SLV308 binds to dopamine D(2), D(3), and D(4) receptors and 5-HT(1) (A) receptors and is a partial agonist at dopamine D(2) and D(3) receptors and a full agonist at serotonin 5-HT(1) (A) receptors. At cloned human dopamine D(2,L) receptors, SLV308 acted as a potent but partial D(2) receptor agonist (pEC(50) = 8.0 and pA(2) = 8.4) with an efficacy of 50% on forskolin stimulated cAMP accumulation. At human recombinant dopamine D(3) receptors, SLV308 acted as a partial agonist in the induction of [(35)S]GTPgammaS binding (intrinsic activity of 67%; pEC(50) = 9.2) and antagonized the dopamine induction of [(35)S]GTPgammaS binding (pA(2) = 9.0). SLV308 acted as a full 5-HT(1) (A) receptor agonist on forskolin induced cAMP accumulation at cloned human 5-HT(1) (A) receptors but with low potency (pEC(50) = 6.3). In rat striatal slices SLV308 concentration-dependently attenuated forskolin stimulated accumulation of cAMP, as expected for a dopamine D(2) and D(3) receptor agonist. SLV308 antagonized the inhibitory effect of quinpirole on K(+)-stimulated [(3)H]-dopamine release from rat striatal slices (pA(2) = 8.5). In the same paradigm, SLV308 had antagonist properties in the presence of quinpirole (pA(2) = 8.5), but the partial D(2) agonist terguride had much stronger antagonistic properties. In conclusion, SLV308 combines high potency partial agonism at dopamine D(2) and D(3) receptors with full efficacy low potency serotonin 5-HT(1) (A) receptor agonism and is worthy of profiling in in vivo models of Parkinson's disease.     

Increment of in vivo binding of [H]SCH 23390, a dopamine D1 receptor ligand, induced by cyclic AMP-dependent protein kinase in rat brain

The effects of cyclic AMP (cAMP)-related compounds on in vivo [(3)H]SCH 23390 binding to striatal dopamine D(1) receptors were investigated using autoradiography in order to clarify the possible regulation of the cAMP-dependent mechanisms in the in vivo ligand-receptor bindings in the living brain. Intrastriatal infusion of the cAMP analogue, N6,2'-O-dibutyryl-cyclic AMP (db-cAMP; 5, 25 and 100 nmol/side) produced a dose-dependent increase of in vivo [(3)H]SCH 23390 binding in conscious rats. This increasing effect of [(3)H]SCH 23390 binding completely disappeared by 6 h after the infusion of db-cAMP. A similar increase of in vivo [(3)H]SCH 23390 binding to striatal D(1) receptors was also observed by intrastriatal injection of 8-bromo-cyclic AMP (8Br-cAMP, 100 nmol/side). Pretreatment with Rp-cyclic AMP triethylamine (Rp-cAMPS, 100 nmol/side), an inhibitor of the cAMP-dependent protein kinase (PKA), completely blocked the increasing effect of [(3)H]SCH 23390 binding induced by db-cAMP. In contrast, in vitro [(3)H]SCH 23390 binding was not significantly altered by intrastriatal infusion of db-cAMP, which indicated that the maximum number of binding sites (B(max)) for D(1) receptors was not changed. The kinetic analysis employed the graphical method indicated that a db-cAMP-induced increase of in vivo [(3)H]SCH 23390 binding was mainly due to an increase in the bimolecular association rate constant (k(on)). These results strongly indicate that the PKA-mediated phosphorylation may play a pivotal role in the regulating the in vivo [(3)H]SCH 23390 dopamine D(1) receptor binding in intact rat brain.     

Efficacy and safety of olorinab,a full agonist of the cannabinoid receptor 2, for the treatment of abdominal pain in patients with irritable bowel syndrome: Results from a phase 2b randomized placebo-controlled trial (CAPTIVATE)

Background

Olorinab is a highly selective, peripherally acting, full agonist of cannabinoid receptor 2. This study assessed the efficacy and safety of olorinab to treat abdominal pain in patients with irritable bowel syndrome with diarrhea (IBS-D) and constipation (IBS-C).

Methods

CAPTIVATE was a phase 2b, randomized, double-blind, placebo-controlled, parallel-group trial. Eligible participants aged 18–70 years with IBS-C and IBS-D diagnosed per Rome IV received olorinab 10 mg, 25 mg, or 50 mg three times daily (TID) or placebo TID for 12 weeks. The primary endpoint was the change in patient-reported average abdominal pain score (AAPS) from baseline to Week 12.

Key Results

A total of 273 participants were randomized to receive olorinab 10 mg (n = 67), olorinab 25 mg (n = 67), olorinab 50 mg (n = 69), or placebo (n = 70). Although a treatment response was observed across all groups, the weekly change in average AAPS from baseline to Week 12 was not significantly different between placebo and any olorinab dose. In a prespecified subgroup analysis of participants with a baseline AAPS ≥6.5, olorinab 50 mg (n = 35) significantly improved AAPS compared with placebo (n = 30) (p = 0.014). Adverse event rates were comparable between olorinab and placebo and there were no reported serious adverse events or deaths.

Conclusion and Inferences

Although olorinab was well-tolerated and improved weekly AAPS, the primary endpoint was not met. However, in participants with moderate-to-severe pain at baseline (AAPS ≥6.5), olorinab 50 mg significantly improved weekly AAPS compared with placebo. ClinicalTrials.gov : NCT04043455.     

Characterization of in vivo pharmacokinetic properties of the dopamine D1 receptor agonist DAR-0100A in nonhuman primates using PET with [11C] NNC112 and [11C] raclopride

DAR-0100A, the active enantiomer of dihydrexidine, is a potent dopamine D1 agonist under investigation for treatment of cognitive impairment and negative symptoms of schizophrenia. We measured the dose–occupancy relationship for DAR-0100A at D1 receptors using positron emission tomography (PET) imaging in baboons with [¹¹C] NNC112 and its binding to D2 with [¹¹C] raclopride. Two baboons were scanned with [¹¹C] NNC112 at baseline and after three different doses of DAR-0100A. Two baboons were scanned with [¹¹C] raclopride at baseline and after one dose of DAR-0100A. Occupancy (ΔBP_ND) was computed in the striatum and cortex. A clear relationship was observed between plasma concentration of DAR-0100A and ΔBP_ND. ΔBP_ND was larger in the striatum than in the cortex, consistent with reports showing that 25% of [¹¹C] NNC112 BP_ND in the cortex is attributed to 5-HT_2A. Plasma EC₅₀ estimates ranged from 150 to 550 ng/mL according to the constraints on the model. There was no detectable effect of DAR-0100A on [¹¹C] raclopride BP_ND. These data suggest that at doses likely to be administered to patients, occupancy will not be detectable with [¹¹C] NNC112 PET and binding of DAR-0100A to D2 will be negligible. This is the first demonstration with PET of a significant occupancy by a full D1 agonist in vivo.     

In vivo labelling of the adenosine A2A receptor in mouse brain using the selective antagonist [3H]SCH 58261

The selective A2A receptor antagonist [3H]SCH 58261 was injected intravenously in mice and the radioactivity accumulating in various brain regions was determined by tissue sampling. Radioactivity levels in regions of interest such as the striatum were highest 15 min after injection and quickly declined thereafter (30 min and 1 h postinjection) in a time-dependent manner. The amount of labelling was ranked as follows: striatum (4.6 +/- 0.3 fmol/mg protein) > cortex > hippocampus > pons = hypothalamus > cerebellum (0.5 +/- 0.05 fmol/mg protein). Specific labelling of the A2A receptor occurred in striatum and cortex because significantly less radioactivity accumulated in these areas from adenosine A2A receptor knockout mice as compared to wild-type littermates. In control outbred CD1 mice, a striatum-to-cerebellum ratio of 7.6 +/- 0.6 was found. At 30 min postinjection, the nonselective adenosine receptor antagonist caffeine reduced the radioactivity due to [3H]SCH 58261 in the striatum by 32% at 1 mg/kg i.p. and by 66% at the stimulant dose of 6.25 mg/kg i.p. Radioactivity in the striatum was lowered, respectively, by 66 and 86% 30 min after injection of 3 or 10 mg/kg i.p. doses of unlabelled SCH 58261. The present results indicate that [3H]SCH 58261 directly labels striatal A2A receptors in vivo. Thus [3H]SCH 58261 is an excellent tool for studying brain distribution and A2A receptor occupancy of various compounds ranging from xanthines, such as caffeine, to other A2A antagonists.     

ABT-089, a neuronal nicotinic receptor partial agonist, for the treatment of attention-deficit/hyperactivity disorder in adults: results of a pilot study.

BACKGROUND: This pilot study was designed to evaluate ABT-089, a neuronal nicotinic receptor partial agonist, as treatment for adult attention-deficit/hyperactivity disorder (ADHD). METHODS: Adults with ADHD received placebo, 2 mg, 4 mg, or 20 mg of ABT-089 for 2 weeks each in a randomized, double-blind, placebo-controlled, 4 x 4 Latin square design for a total of 8 weeks. In addition to the primary outcome, the Conner's Adult ADHD Rating Scale (CAARS), secondary rating scales, and neuropsychological and safety assessments were completed. RESULTS: A total of 11 adults with well-characterized ADHD completed this crossover study. ABT-089 b.i.d. was superior to placebo for the CAARS Total Symptom Score, which was the primary endpoint (placebo: 38.0 +/- 1.9; 2 mg b.i.d.: 32.2 +/- 1.9, one-tail p = .021; 4 mg b.i.d.: 33.2 +/- 1.9, p = .047; 20 mg b.i.d.: 33.5 +/- 1.9, p = .056). ABT-089 was also superior to placebo for the CAARS ADHD Index and Hyperactive/Impulsive scores and the Clinical Global Impression-ADHD Severity score. On the clinical efficacy endpoints, CAARS Total Symptom Score and CAARS Hyperactive/Impulsive score, a shallow inverted U-shaped dose-response curve was observed; however, the dose-response curve for attention and memory effects as measured by computerized cognitive testing seemed dose-linear. No clinically meaningful findings in safety assessments or side effect profile were observed. CONCLUSIONS: Data from this pilot study suggest that ABT-089 might be effective in treating adult ADHD and that it is well tolerated. On the basis of these promising results, larger, parallel-group ABT-089 studies of longer duration are warranted.     

Distribution of corticotropin‐releasing factor (CRF) receptor binding in the mouse brain using a new,high‐affinity radioligand, [125I]‐PD‐Sauvagine

The corticotropin‐releasing factor (CRF) family of peptides includes CRF and three urocortins, which signal through two distinct G‐protein coupled receptors, CRF₁ and CRF₂. Although the cellular distribution of CRF receptor expression has been well characterized at the mRNA level, the localization of receptor protein, and, by inference, of functional receptors, has been limited by a lack of reliable immunohistochemical evidence. Recently, a CRF‐related peptide, termed PD‐sauvagine, was isolated from the skin of the frog, Pachymedusa dacnicolor, and validated as a high‐affinity ligand for CRF receptor studies. A radiolabeled analog, [¹²⁵I]‐PD‐sauvagine, with high signal‐to‐noise ratio, was used in autoradiographic studies to map the distribution of CRF receptor binding sites in the mouse brain. Through the use of receptor‐deficient mice and subtype‐specific antagonists, CRF₁ and CRF₂ binding sites were isolated, and found to be readily reconcilable with regional patterns of mRNA expression. Binding site distributions within a given structure sometimes differed from mRNA patterns, however, particularly in laminated structures of the isocortex, hippocampus, and cerebellum, presumably reflecting the trafficking of receptors to their operational homes on neuronal (mostly dendritic) processes. Binding patterns of [¹²⁵I]‐PD‐sauvagine provided independent assessments of controversial receptor localizations, failing to provide support for CRF₁ expression in central autonomic components of the limbic forebrain, the locus coeruleus and cerebellar Purkinje cells, or for CRF₂ in any aspect of the cerebellar cortex. Though lacking in ideal resolution, in vitro binding of the PD‐sauvagine radioligand currently provides the most sensitive and accurate available tool for localizing CRF receptors in rodent brain.     

Modification of the binding of [H]MK-801 to brain regions and spinal cord of rats treated chronically with U-50,488H, a κ-opioid receptor agonist

Male Sprague-Dawley rats were rendered tolerant to U-50,488H by twice-daily injections of the drug (25 mg/kg, i.p.) for 4 days. In tolerant rats, the binding of [³H]MK-801 was increased in pons and medulla and corpus striatum but decreased in midbrain and hippocampus and was due to changes in B_max values. In U-50,488H-abstinent rats, the binding of [³H]MK-801 was increased in pons and medulla and hippocampus, and decreased in midbrain and amygdala. In hippocampus, the B_max of [³H]MK-801 was increased but the K_d was decreased whereas in amygdala and pons and medulla, the changes were due to alterations in the B_max values. Previous studies have shown that NMDA receptor antagonists block the tolerance to the analgesic action of U-50,488H in rodents. The present studies demonstrate differential changes in the NMDA receptors of brain regions of U-50,488H-tolerant and -abstinent rats.