Sean M. Smith

The selective phosphodiesterase 9 (PDE9) inhibitor PF-04447943 (6-[(3S,4S)-4-methyl-1-(pyrimidin-2-ylmethyl)pyrrolidin-3-yl]-1-(tetrahydro-2H-pyran-4-yl)-1,5-dihydro-4H-pyrazolo[3,4-d]pyrimidin-4-one) enhances synaptic plasticity and cognitive function in rodents

Inhibition of phosphodiesterase 9 (PDE9) has been reported to enhance rodent cognitive function and may represent a potential novel approach to improving cognitive dysfunction in Alzheimers disease. PF-04447943, (6-[(3S,4S)-4-methyl-1-(pyrimidin-2-ylmethyl)pyrrolidin-3-yl]-1-(tetrahydro-2H-pyran-4-yl)-1,5-dihydro-4H-pyrazolo[3,4-d]pyrimidin-4-one), a recently described PDE9 inhibitor, was found to have high affinity (Ki of 2.8, 4.5 and 18 nM) for human, rhesus and rat recombinant PDE9 respectively and high selectivity for PDE9 versus PDEs1-8 and 10-11. PF-04447943 significantly increased neurite outgrowth and synapse formation (as indicated by increased synapsin 1 expression) in cultured hippocampal neurons at low (30-100 nM) but not high (300-1000 nM) concentrations. PF-04447943 significantly facilitated hippocampal slice LTP evoked by a weak tetanic stimulus at a concentration of 100 nM but failed to affect response to the weak tetanus at either 30 or 300 nM, or the LTP produced by a theta burst stimulus. Systemic administration of PF-04447943 (1-30 mg/kg p.o.) dose-dependently increased cGMP in the cerebrospinal fluid 30 min after administration indicating target engagement in the CNS of rats. PF-04447943 (1-3 mg/kg p.o.) significantly improved cognitive performance in three rodent cognition assays (mouse Y maze spatial recognition memory model of natural forgetting, mouse social recognition memory model of natural forgetting and rat novel object recognition with a scopolamine deficit). When administered at a dose of 3 mg/kg p.o., which improved performance in novel object recognition, PF-04447943 significantly increased phosphorylated but not total GluR1 expression in rat hippocampal membranes. Collectively these data indicate that PF-04447943 is a potent, selective brain penetrant PDE9 inhibitor that increased indicators of hippocampal synaptic plasticity and improved cognitive function in a variety of cognition models in both rats and mice. Results with PF-04447943 are consistent with previously published findings using a structurally diverse PDE9 inhibitor, BAY73-6199, and further support the suggestion that PDE9 inhibition may represent a novel approach to the palliative remediation of cognitive dysfunction.

The Behavioral and Neurochemical Effects of a Novel d-Amino Acid Oxidase Inhibitor Compound 8 [4H-Thieno [3,2-b]pyrrole-5-carboxylic Acid] and d-Serine

Multiple studies indicate that N-methyl-d-aspartate (NMDA) receptor hypofunction underlies some of the deficits associated with schizophrenia. One approach for improving NMDA receptor function is to enhance occupancy of the glycine modulatory site on the NMDA receptor by increasing the availability of the endogenous coagonists d-serine. Here, we characterized a novel d-amino acid oxidase (DAAO) inhibitor, compound 8 [4H-thieno [3,2-b]pyrrole-5-carboxylic acid] and compared it with d-serine. Compound 8 is a moderately potent inhibitor of human (IC50, 145 nM) and rat (IC50, 114 nM) DAAO in vitro. In rats, compound 8 (200 mg/kg) decreased kidney DAAO activity by ∼96% and brain DAAO activity by ∼80%. This marked decrease in DAAO activity resulted in a significant (p < 0.001) elevation in both plasma (220% of control) and cerebrospinal fluid (CSF; 175% of control) d-serine concentration. However, compound 8 failed to significantly influence amphetamine-induced psychomotor activity, nucleus accumbens dopamine release, or an MK-801 (dizocilpine maleate)-induced deficit in novel object recognition in rats. In contrast, high doses of d-serine attenuated both amphetamine-induced psychomotor activity and dopamine release and also improved performance in novel object recognition. Behaviorally efficacious doses of d-serine (1280 mg/kg) increased CSF levels of d-serine 40-fold above that achieved by the maximal dose of compound 8. These findings demonstrate that pharmacological inhibition of DAAO significantly increases d-serine concentration in the periphery and central nervous system. However, acute inhibition of DAAO appears not to be sufficient to increase d-serine to concentrations required to produce antipsychotic and cognitive enhancing effects similar to those observed after administration of high doses of exogenous d-serine.

The Behavioral and Neurochemical Effects of a Novel D-Amino Acid Oxidase Inhibitor 4H-thieno [3,2-b] pyrrole-5-carboxylic acid (Compound 8) and D-serine

Multiple studies indicate that N-methyl-d-aspartate (NMDA) receptor hypofunction underlies some of the deficits associated with schizophrenia. One approach for improving NMDA receptor function is to enhance occupancy of the glycine modulatory site on the NMDA receptor by increasing the availability of the endogenous coagonists d-serine. Here, we characterized a novel d-amino acid oxidase (DAAO) inhibitor, compound 8 [4H-thieno [3,2-b]pyrrole-5-carboxylic acid] and compared it with d-serine. Compound 8 is a moderately potent inhibitor of human (IC50, 145 nM) and rat (IC50, 114 nM) DAAO in vitro. In rats, compound 8 (200 mg/kg) decreased kidney DAAO activity by ∼96% and brain DAAO activity by ∼80%. This marked decrease in DAAO activity resulted in a significant (p < 0.001) elevation in both plasma (220% of control) and cerebrospinal fluid (CSF; 175% of control) d-serine concentration. However, compound 8 failed to significantly influence amphetamine-induced psychomotor activity, nucleus accumbens dopamine release, or an MK-801 (dizocilpine maleate)-induced deficit in novel object recognition in rats. In contrast, high doses of d-serine attenuated both amphetamine-induced psychomotor activity and dopamine release and also improved performance in novel object recognition. Behaviorally efficacious doses of d-serine (1280 mg/kg) increased CSF levels of d-serine 40-fold above that achieved by the maximal dose of compound 8. These findings demonstrate that pharmacological inhibition of DAAO significantly increases d-serine concentration in the periphery and central nervous system. However, acute inhibition of DAAO appears not to be sufficient to increase d-serine to concentrations required to produce antipsychotic and cognitive enhancing effects similar to those observed after administration of high doses of exogenous d-serine.

The Therapeutic Potential of D-Amino Acid Oxidase (DAAO) Inhibitors

D-amino acid oxidase (DAAO) is a flavoenzyme that degrades D-amino acids through the process of oxidative deamination. DAAO regulation of D-amino acid levels has been associated with several physiological processes ranging from hormone secretion to synaptic transmission and cognition. Recent genetic studies have identified a mutation on chromosome 13 in schizophrenia patients that encodes two gene products (G30 and G72) that are associated with DAAO. Furthermore, DAAO expression and enzyme activity has been reported to be increased in post mortem brain tissue samples from patients with schizophrenia compared to healthy controls. D-serine, a D-amino acid that is regulated by DAAO, is a potent, endogenous co-agonist of the N-methyl-D-aspartic acid (NMDA) receptor. Because NMDA receptor dysfunction is thought to be involved in the positive (psychotic), negative and cognitive symptoms in schizophrenia, there has been much interest in developing potent and selective DAAO inhibitors for the treatment of this disease. Several research reports have been published that describe the synthesis and biological effects of novel, selective, small molecule inhibitors of DAAO. Many of these compounds have been shown, when given systemically, to increase D-serine concentrations in the blood and brain. However, the efficacy of these compounds in behavioral assays that measure antipsychotic potential and pro-cognitive effects in laboratory animals has been inconsistent. This article highlights and reviews research advances for DAAO inhibitors published in peer reviewed journals.

Combined administration of an mGlu2/3 receptor agonist and a 5-HT2A receptor antagonist markedly attenuate the psychomotor-activating and neurochemical effects of psychostimulants

RationaleIt was recently reported that administration of the metabotropic glutamate 2 and 3 (mGlu2/3) receptor agonist prodrug LY2140023 to schizophrenic patients decreased positive symptoms. However, at the single, potentially suboptimal, dose that was tested, LY2140023 trended towards being inferior to olanzapine on several indices of efficacy within the Positive and Negative Syndrome Scale.ObjectivesIn this study, we examined whether the antipsychotic potential of mGlu2/3 receptor agonism can be enhanced with 5-HT2A receptor antagonism.Materials and methodsSpecifically, we characterized the effects of coadministering submaximally effective doses of the 5-HT2A receptor antagonist M100907 (0.2xa0mg/kg) and the mGlu2/3 receptor agonist LY379268 (1xa0mg/kg) on amphetamine-induced and MK-801-induced psychomotor activity in rats, an assay sensitive to antipsychotics. We also determined the effects of coadministering these two compounds on MK-801-induced dopamine and norepinephrine efflux in the nucleus accumbens (NAc).ResultsAt the submaximally effective doses tested, the effects of M100907 and LY379268 on amphetamine-induced and MK-801-induced psychomotor activity were significantly greater when given together than when given separately. Furthermore, coadministration of these doses of M100907 and LY379268 reduced MK-801-induced dopamine efflux in the NAc. This effect on dopamine release was not observed with the administration of either compound alone, even at higher doses that attenuated MK-801-induced psychomotor activity.ConclusionsOur results suggest that a single compound having both mGlu2/3 receptor agonist and 5-HT2A receptor antagonist activity, or coadministration of two compounds selective for these receptors, could be superior in terms of efficacy and/or reduced side-effect liability relative to an mGlu2/3 receptor agonist alone.

T-type calcium channel antagonism produces antipsychotic-like effects and reduces stimulant-induced glutamate release in the nucleus accumbens of rats.

T-type calcium channels are important in burst firing and expressed in brain regions implicated in schizophrenia. Therefore, we examined the effects of novel selective T-type calcium channel antagonists in preclinical assays predictive of antipsychotic-like activity. TTA-A2 blocked the psychostimulant effects of amphetamine and MK-801 and decreased conditioned avoidance responding. These effects appeared mechanism based, rather than compound specific, as two structurally dissimilar T-type antagonists also reduced amphetamine-induced psychomotor activity. Importantly, the ability to reduce amphetamines effects was maintained following 20 days pre-treatment with TTA-A2. To explore the neural substrates mediating the observed behavioral effects, we examined the influence of TTA-A2 on amphetamine-induced c-fos expression as well as basal and stimulant-evoked dopamine and glutamate release in the nucleus accumbens. TTA-A2 decreased amphetamine-induced c-fos expression as well as MK-801-induced, but not basal, glutamate levels in the nucleus accumbens. Basal, amphetamine- and MK-801-induced dopamine efflux was altered. These findings suggest that T-type calcium channel antagonism could represent a novel mechanism for treating schizophrenia.

Discovery and Optimization of a Series of Pyrimidine-Based Phosphodiesterase 10A (PDE10A) Inhibitors through Fragment Screening, Structure-Based Design, and Parallel Synthesis

Screening of a fragment library for PDE10A inhibitors identified a low molecular weight pyrimidine hit with PDE10A Ki of 8700 nM and LE of 0.59. Initial optimization by catalog followed by iterative parallel synthesis guided by X-ray cocrystal structures resulted in rapid potency improvements with minimal loss of ligand efficiency. Compound 15 h, with PDE10A Ki of 8.2 pM, LE of 0.49, and >5000-fold selectivity over other PDEs, fully attenuates MK-801-induced hyperlocomotor activity after ip dosing.

Regulation of dopamine signaling in the striatum by phosphodiesterase inhibitors: novel therapeutics to treat neurological and psychiatric disorders.

Abnormal dopamine neurotransmission has been linked to a wide array of motor, cognitive, and psychiatric disorders. Dopamine binds and regulates intracellular signals through D1-like (D1 and D5) and D2-like (D2, D3, and D4) G-protein coupled receptors. Activation of D1- like receptors stimulates cAMP/PKA signaling via Gs mediated activation of adenylyl cyclase. In contrast, activation of D2-like receptors inhibits cAMP/PKA signaling by Gi inhibition of adenylyl cyclase. In the brain, dopamine signaling is tightly regulated by cyclic nucleotide phosphodiesterases (PDEs). PDEs are a family of enzymes that selectively degrade cAMP and cGMP. There are 11 different families of PDEs that vary in their substrate specificity, kinetic properties, mode of regulation, intracellular localization, and tissue expression patterns. A number of PDE families are highly expressed in the striatum including PDE1, PDE2, PDE4, and PDE10. There is a growing amount of evidence to suggest that these enzymes play a critical role in modulating dopamine signaling and selective inhibitors of these enzymes are currently being explored as novel therapeutics to treat schizophrenia, Huntingtons disease, cognitive disorders and Parkinsons disease. The aim of this review is to summarize the distinct roles of different PDEs in regulating dopamine signaling in the striatum. In addition, we will briefly review the therapeutic potential of selective PDE inhibitors to treat neurological and psychiatric disorders associated with abnormal striatal function.

Characterization of non-nitrocatechol pan and isoform specific catechol-O-methyltransferase inhibitors and substrates.

Reduced dopamine neurotransmission in the prefrontal cortex has been implicated as causal for the negative symptoms and cognitive deficit associated with schizophrenia; thus, a compound which selectively enhances dopamine neurotransmission in the prefrontal cortex may have therapeutic potential. Inhibition of catechol-O-methyltransferase (COMT, EC 2.1.1.6) offers a unique advantage, since this enzyme is the primary mechanism for the elimination of dopamine in cortical areas. Since membrane bound COMT (MB-COMT) is the predominant isoform in human brain, a high throughput screen (HTS) to identify novel MB-COMT specific inhibitors was completed. Subsequent optimization led to the identification of novel, non-nitrocatechol COMT inhibitors, some of which interact specifically with MB-COMT. Compounds were characterized for in vitro efficacy versus human and rat MB and soluble (S)-COMT. Select compounds were administered to male Wistar rats, and ex vivo COMT activity, compound levels in plasma and cerebrospinal fluid (CSF), and CSF dopamine metabolite levels were determined as measures of preclinical efficacy. Finally, novel non-nitrocatechol COMT inhibitors displayed less potent uncoupling of the mitochondrial membrane potential (MMP) compared to tolcapone as well as nonhepatotoxic entacapone, thus mitigating the risk of hepatotoxicity.

Preclinical Characterization of the Phosphodiesterase 10A PET Tracer [(11)C]MK-8193.

PurposeA positron emission tomography (PET) tracer for the enzyme phosphodiesterase 10A (PDE10A) is desirable to guide the discovery and development of PDE10A inhibitors as potential therapeutics. The preclinical characterization of the PDE10A PET tracer [11C]MK-8193 is described.ProceduresIn vitro binding studies with [3H]MK-8193 were conducted in rat, monkey, and human brain tissue. PET studies with [11C]MK-8193 were conducted in rats and rhesus monkeys at baseline and following administration of a PDE10A inhibitor.Results[3H]MK-8193 is a high-affinity, selective PDE10A radioligand in rat, monkey, and human brain tissue. In vivo, [11C]MK-8193 displays rapid kinetics, low test-retest variability, and a large specific signal that is displaced by a structurally diverse PDE10A inhibitor, enabling the determination of pharmacokinetic/enzyme occupancy relationships.Conclusions[11C]MK-8193 is a useful PET tracer for the preclinical characterization of PDE10A therapeutic candidates in rat and monkey. Further evaluation of [11C]MK-8193 in humans is warranted.