The therapeutic potential of glycine transporter-1 inhibitors

While current antipsychotic medications are often efficacious for the positive symptoms of schizophrenia, there remains a critical need for compounds with improved tolerability and efficacy for the negative symptoms and cognitive dysfunction associated with this disease. There is a growing body of evidence suggesting that the potentiation of N-methyl-D-aspartate (NMDA) receptor function may be a useful approach for the treatment of schizophrenia. One proposed strategy for this potentiation is to increase synaptic levels of the neurotransmitter glycine by blocking the glycine transporter-1. Since glycine acts as a required co-agonist for the NMDA receptor complex; this approach allows an increase in the effectiveness of normal glutamatergic signalling at the NMDA receptor complex. Recent preclinical research, focused on the development and testing of novel glycine transporter-1 inhibitors, suggests that this approach may be feasible. Converging clinical evidence suggesting therapeutic efficacy following the potentiation of glycinergic activity further supports this approach. Clinical studies with novel glycine re-uptake inhibitors will provide critical information regarding the therapeutic utility and tolerability of this treatment for schizophrenia and other disorders associated with NMDA receptor hypofunction.     

The glycine transporter-1 inhibitors NFPS and Org 24461: a pharmacological study

The in vitro and in vivo pharmacology of two glycine transporter-1 (GlyT1) inhibitors, N[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)-propyl]sarcosine (NFPS) and R,S-(+/-)N-methyl-N-[(4-trifluoromethyl)phenoxy]-3-phenyl-propylglycine (Org 24461), was studied. NFPS and Org 24461 inhibited the uptake of [3H]glycine in hippocampal synaptosomal preparation with IC(50) values of 0.022 and 2.5 microM. Neither NFPS nor Org 24461 (0.1 microM) showed significant binding to alpha-1, alpha-2, and beta-adrenoceptors, D(1) and D(2) dopamine receptors, and 5-HT(1A) and 5-HT(2A) serotonin receptors in membranes prepared from rat brain or to cloned 5-HT(6) and 5-HT(7) receptors. At 10 microM concentrations, binding affinity was measured for NFPS to 5-HT(2A) and 5-HT(2C) serotonin receptors and alpha-2 adrenoceptors and for NFPS and Org 24461 to 5-HT(7) serotonin receptors. Glycine (0.1 mM) and sarcosine (5 mM) increased [3H]glycine efflux from superfused rat hippocampal slices preloaded with [3H]glycine. NFPS and Org 24461 (0.1 mM) did not influence [3H]glycine efflux, however, they inhibited glycine-induced [3H]glycine release. These findings indicate that NFPS and Org 24461 selectively inhibit glycine uptake without being substrates of the transporter protein. Several antipsychotic tests were used to characterize antipsychotic effects of NFPS and Org 24461 in vivo. These compounds did not alter apomorphine-induced climbing and stereotypy in a dose of 10 mg/kg p.o. in mice and did not induce catalepsy in a dose of 10 mg/kg i.p. in rats. The ID(50) values of NFPS were 21.4 mg/kg and higher than 30 mg/kg i.p. for inhibition of phencyclidine (PCP)- and D-amphetamine-induced hypermotility in mice and these values were 2.5 and 8.6 mg/kg i.p. for Org 24461. NFPS and Org 24461 did not exhibit anxiolytic effects in light-dark test in mice, in the meta-chlorophenylpiperazine (mCPP)-induced anxiety test (minimal effective dose or MED was higher than 3 mg/kg i.p.) and in the Vogel conflict drinking test in rats (MED was higher than 10 mg/kg i.p.). Both NFPS and Org 24461 (1-10 mg/kg i.p.) reversed PCP-induced changes in EEG power spectra in conscious rats. These data support the view that GlyT1 inhibitors may have potential importance in treatment of negative symptoms of schizophrenia.     

Glycine transporter-1: a new potential therapeutic target for schizophrenia

The hypofunction hypothesis of glutamatergic neurotransmission via N-methyl-D-aspartate (NMDA) receptors in the pathophysiology of schizophrenia suggests that increasing NMDA receptor function via pharmacological manipulation could provide a new therapeutic strategy for schizophrenia. The glycine modulatory site on NMDA receptor complex is the one of the most attractive therapeutic targets for schizophrenia. One means of enhancing NMDA receptor neurotransmission is to increase the availability of the obligatory co-agonist glycine at modulatory site on the NMDA receptors through the inhibition of glycine transporter-1 (GlyT-1) on glial cells. Some clinical studies have demonstrated that the GlyT-1 inhibitor sarcosine (N-methylglycine) shows antipsychotic activity in patients with schizophrenia. Currently, a number of pharmaceutical companies have been developing novel and selective GlyT-1 inhibitors for the treatment of schizophrenia. A recent double blind phase II study demonstrated that the novel GlyT-1 inhibitor RG1678 has a robust and clinically meaningful effect in patients with schizophrenia. In this article, the author reviews the recent findings on the GlyT-1 as a potential therapeutic target of schizophrenia.     

The therapeutic potential of aromatase inhibitors

The third generation aromatase inhibitors are both remarkably potent and specific endocrine agents inhibiting aromatase activity and reducing circulating oestrogen levels in postmenopausal women to levels never previously seen. Their therapeutic potential is consequently much greater than the earlier prototype drugs. Their excellent side-effect profile also allows for potential wider indications in the treatment of oestrogen-related diseases, including breast cancer. It still remains to determine whether their potent endocrine effects translate into increased therapeutic benefit. In advanced breast cancer, aromatase inhibitors have been shown to have improved efficacy and toxicity profiles when compared with progestins, aminoglutethimide and tamoxifen. Aromatase inhibitors have also been used in the neoadjuvant setting, where they have been shown to achieve higher response rates than tamoxifen and to be more successful at downstaging tumours. Early results comparing an aromatase inhibitor with tamoxifen in the adjuvant setting in early breast cancer show anastrozole to be superior to tamoxifen in terms of both disease-free survival and a lower incidence of new contralateral tumours. There was also a more favourable side-effect profile, which has implications for potential future prophylactic treatment. Additionally, since aromatase inhibitors have different mechanisms of action, unlike antioestrogens, they may be particularly useful as chemopreventive agents if oestrogens are themselves genotoxic. Aromatase inhibitors have been used to date almost exclusively in postmenopausal women. The potential of combining them with luteinising hormone-releasing hormone analogues allows the possibility of treating premenopausal women with either oestrogen receptor-positive breast cancer or benign conditions such as cyclical breast pain, fibroadenomata, recurrent cystic disease or endometriosis. There is also the potential for their use in men with conditions such as gynaecomastia or prostate cancer. These new generation aromatase inhibitors may well have an increasing role in the future management of a number of conditions in addition to breast cancer.     

The therapeutic potential of aromatase inhibitors

The third generation aromatase inhibitors are both remarkably potent and specific endocrine agents inhibiting aromatase activity and reducing circulating oestrogen levels in postmenopausal women to levels never previously seen. Their therapeutic potential is consequently much greater than the earlier prototype drugs. Their excellent side-effect profile also allows for potential wider indications in the treatment of oestrogen-related diseases, including breast cancer. It still remains to determine whether their potent endocrine effects translate into increased therapeutic benefit. In advanced breast cancer, aromatase inhibitors have been shown to have improved efficacy and toxicity profiles when compared with progestins, aminoglutethimide and tamoxifen. Aromatase inhibitors have also been used in the neoadjuvant setting, where they have been shown to achieve higher response rates than tamoxifen and to be more successful at downstaging tumours. Early results comparing an aromatase inhibitor with tamoxifen in the adjuvant setting in early breast cancer show anastrozole to be superior to tamoxifen in terms of both disease-free survival and a lower incidence of new contralateral tumours. There was also a more favourable side-effect profile, which has implications for potential future prophylactic treatment. Additionally, since aromatase inhibitors have different mechanisms of action, unlike antioestrogens, they may be particularly useful as chemopreventive agents if oestrogens are themselves genotoxic. Aromatase inhibitors have been used to date almost exclusively in postmenopausal women. The potential of combining them with luteinising hormone-releasing hormone analogues allows the possibility of treating premenopausal women with either oestrogen receptor-positive breast cancer or benign conditions such as cyclical breast pain, fibroadenomata, recurrent cystic disease or endometriosis. There is also the potential for their use in men with conditions such as gynaecomastia or prostate cancer. These new generation aromatase inhibitors may well have an increasing role in the future management of a number of conditions in addition to breast cancer.     

The therapeutic potential of microbial proteasome inhibitors

The proteasome influences cellular homeostasis through the degradation of regulatory proteins, many of which are also involved in disease pathogenesis. In particular, numerous regulatory proteins associated with tumor growth, such as cyclins, cyclin-dependent kinase inhibitors, tumor suppressors, and NF-κB inhibitors are degraded by the proteasome. Proteasome inhibitors can stabilize these regulatory proteins, resulting in the suppression of tumor development and the regulation of immune responses. Thus, proteasome inhibitors are promising candidate antitumor agents and immune-regulatory agents. Bortezomib is the first-in-class proteasome inhibitor approved for the treatment of multiple myeloma. Despite its high efficiency, however, a large proportion of patients do not attain sufficient clinical response due to toxicity and drug resistance. Therefore, the development of new proteasome inhibitors with improved pharmacological properties is needed. Natural products produced by microorganisms are a promising source of such compounds. This review provides an overview of proteasome inhibitors produced by microorganisms, with special focus on inhibitors isolated from actinomycetes.     

The therapeutic potential of PDE4 inhibitors

Phosphodiesterase enzymes are responsible for the inactivation of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). Phosphodiesterase 4 (PDE4) is a cAMP specific phosphodiesterase expressed in inflammatory cells such as eosinophils. Inhibition of PDE4 results in an elevation of cAMP in these cells, which in turn downregulates the inflammatory response. The anti-inflammatory effects of PDE4 inhibitors have been well documented both in vitro and in vivo in a variety of animal models. The potential use of PDE4 inhibitors as anti-inflammatory agents for the treatment of asthma and other inflammatory disorders has received considerable attention from the pharmaceutical industry, but to date, there are no selective PDE4 inhibitors on the market. Early PDE4 inhibitors, typified by rolipram, suffered from dose-limiting side effects, including nausea and emesis, which severely restricted their therapeutic utility. Second generation compounds, including CDP840 and SB207499 (Ariflo), have been identified with reduced side effect liability. Recent evidence suggests a correlation between side effects and the ability of compounds to bind at the so-called high affinity rolipram binding site (HPDE), whilst beneficial effects appear to correlate with binding at the catalytic site. A number of companies are actively pursuing compounds which exhibit improved affinity for the catalytic site and reduced affinity for the HPDE, in the expectation that this will provide compounds with an improved therapeutic index.     

Glutamate-based therapeutic approaches: inhibitors of glycine transport

A growing body of evidence suggests that activation of the glutamatergic system, particularly N-methyl-D-aspartate (NMDA) receptor function, may be a viable approach to the treatment of schizophrenia, and potentially other cognitive disorders. The excitotoxicity associated with direct NMDA receptor agonists limits their therapeutic potential, and the glycine modulatory site of the NMDA receptor has received growing interest as a therapeutic target. One approach to enhance NMDA receptor function is to increase the availability of the necessary co-agonist glycine at this modulatory site through inhibition of glycine reuptake from the synapse via glycine transporter-1 (GlyT1). Both preclinical and clinical evidence provide support for this approach, as do recent findings demonstrating the regulation of dopaminergic neurotransmission by GlyT1 inhibition. As a result, several groups have focused on the development of novel GlyT1 inhibitors. In addition, recent electrophysiological findings and data from transgenic mouse models suggest that GlyT1 might also play a role in terminating the actions of glycine at strychnine-sensitive glycine receptors, and therefore GlyT1 antagonists also have potential for the treatment of conditions where activation of inhibitory pathways in the central nervous system might be beneficial.     

The therapeutic potential of small molecule TACE inhibitors

No abstract provided     

The therapeutic potential for phospholipase A2 inhibitors

Modulation of pro-inflammatory lipid mediator production by inhibition of phospholipase A2 (PLA2) activity remains a potential target for development of new drugs for the treatment of rheumatoid arthritis or other inflammatory diseases. Evidence now exists that more than one isoform of PLA2, including types IIa and V 14 and 85 kDa PLA2, is required for production of lipid mediators. Specific isoforms may be associated with production of either the prostaglandin or leukotriene class of lipid mediators in different cell types such as monocytes and neutrophils. Characterisation of isoform-selective inhibitors in models of inflammatory disease, such as rat adjuvant arthritis or phorbol myristate acetate (PMA)-induced mouse ear oedema, suggests that both 14 kDa and 85 kDa PLA2 may contribute to the development of the disease state and that both forms of PLA2 may be targets for modulation of inflammatory disease. Much work remains to clarify fully the relative roles of different PLA2 in the aetiology of inflammatory disease and to identify potent, selective and efficacious inhibitors of each PLA2.     

The therapeutic potential of inhibitors of the trypanothione cycle

There is an urgent need for new drugs in the treatment of human African trypanosomiasis, Chagas' disease and leishmaniasis. This article provides an overview of current drugs, formulations and their deficiencies. Targets for the design of new drugs and the rational provided for targeting enzymes of the trypanothione cycle are described. Biochemical aspects of the cycle and the currently investigated target trypanothione reductase are discussed as are the several classes of inhibitors and their in vitro potencies. Evidence is provided for considering the tryparedoxins as a new target for antiprotozoal chemotherapy and a summary of glutathione-based inhibitors with significant in vitro activity is reported.     

The therapeutic potential of inhibitors of the trypanothione cycle

There is an urgent need for new drugs in the treatment of human African trypanosomiasis, Chagas’ disease and leishmaniasis. This article provides an overview of current drugs, formulations and their deficiencies. Targets for the design of new drugs and the rational provided for targeting enzymes of the trypanothione cycle are described. Biochemical aspects of the cycle and the currently investigated target trypanothione reductase are discussed as are the several classes of inhibitors and their in vitro potencies. Evidence is provided for considering the tryparedoxins as a new target for antiprotozoal chemotherapy and a summary of glutathione-based inhibitors with significant in vitro activity is reported.     

d-Serine and a glycine transporter-1 inhibitor enhance social memory in rats

Rationale  

Glutamatergic abnormalities are involved in the etiology of schizophrenia. Clinical evidence demonstrates that positive modulation of “glycine modulatory sites” on N-methyl-d-aspartic acid (NMDA) receptors improve cognitive deficits as well as positive and negative symptoms in schizophrenic patients.     

Characterization of SSR103800, a selective inhibitor of the glycine transporter-1 in models predictive of therapeutic activity in schizophrenia

On native human, rat and mouse glycine transporter-1(GlyT1), SSR130800 behaves as a selective inhibitor with IC50 values of 1.9, 5.3 and 6.8 nM, respectively. It reversibly blocked glycine uptake in mouse brain cortical homogenates, increased extracellular levels of glycine in the rat prefrontal cortex, and potentiated NMDA-mediated excitatory postsynaptic currents in rat hippocampal slices. SSR103800 (30 mg/kg, p.o.) decreased MK-801- and PCP-induced locomotor hyperactivity in rodents. SSR103800 (1 and 10 mg/kg, p.o.) attenuated social recognition deficit in adult rats induced by neonatal injections of PCP (10 mg/kg, s.c., on post-natal day 7, 9 and 11). SSR103800 (3 mg/kg, p.o.) counteracted the deficit in short-term visual episodic-like memory induced by a low challenge dose of PCP (1 mg/kg, i.p.), in PCP-sensitized rats (10 mg/kg, i.p.). SSR103800 (30 mg/kg, i.p.) increased the prepulse inhibition of the startle reflex in DBA/1J mice. SSR103800 decreased defensive- and despair-related behaviors in the tonic immobility test in gerbils (10 and 30 mg/kg, p.o.) and in the forced-swimming procedure in rats (1 and 3 mg/kg, p.o.), respectively. These findings suggest that SSR103800 may have a therapeutic potential in the management of the core symptoms of schizophrenia and comorbid depression states.     

The therapeutic potential of novel aromatase inhibitors in breast cancer

For over 20 years, aromatase inhibitors have played a role in the treatment of breast cancer. However, until relatively recently, their utility has been limited by a lack of selectivity and significant toxicity. Several second and third generation aromatase inhibitors which are highly selective for the aromatase enzyme have reached clinical trials this decade. As a result of these initial studies, aromatase inhibitors are now established as second line agents in the treatment of postmenopausal women with advanced breast cancer. They are now under evaluation in several other settings, including adjuvant and neoadjuvant therapy, and as part of combination treatment in premenopausal women. This review focuses on recent and ongoing clinical trials and the effect these have had on the clinical use of aromatase inhibitors.     

The therapeutic potential of O6-alkylguanine DNA alkyltransferase inhibitors

Resistance to O(6-)alkylating agents can be overcome by depletion of the DNA repair protein, O(6)-alkylguanine DNA alkyltransferase. Inhibitors of this protein act as pseudosubstrates and, so far, O(6)-benzylguanine and lomeguatrib have been tested in clinical trials. Inherently non-toxic, optimum doses for protein depletion have been established for both agents. Myelosuppression of alkylating agents is significantly enhanced when used in combination with these agents, necessitating significant reductions in standard doses. Consequently, no improvement in efficacy is seen. Strategies to limit myelotoxicity are complex and will be very difficult to apply clinically. O(6)-alkylguanine DNA alkyltransferase inhibition may also potentiate the toxicity of other agents such as cyclophosphamide and irinotecan. Other mechanisms of DNA repair are also important and drugs targeting some of these systems are in early phase clinical trials.     

The therapeutic potential of CETP inhibitors: a patent review

Introduction: Epidemiological studies have identified that high levels of low-density lipoprotein-cholesterol (LDL-C) and low levels of high-density lipoprotein-cholesterol (HDL-C) are two independent causes of cardiovascular disease (CVD). Statins, niacin and fibrate are used for the treatment of CVD. However, some defects are shown in the treatment process. Thus, there is a demand for better treatment strategies that confer preferable efficacy with fewer side effects. Cholesteryl ester transfer protein (CETP) promotes the movement of CEs from HDL to LDL and VLDL in exchange for triglycerides (TGs).

Areas covered: In this review, we reviewed the development and therapeutic applications of CETP inhibitors. A comprehensive review of the patents and pharmaceutical applications between 2009 and 2017 has been highlighted.

Expert opinion: Recently, CETP inhibitors have attracted considerable interest in atherosclerosis-related disease. There are four drugs (torcetrapib, anacetrapib, evacetrapib and dalcetrapib) that have been clinically evaluated in phase III clinical trials and showed promising results in raising HDL-C levels, but there were suboptimal performances in reducing the risk of cardiovascular events with all the compounds. The correlation between plasma HDL-C levels and CVD incidence needs further verification. The timeline is still long for CETP inhibitors to emerge from the treatment of CVD.     

Norepinephrine transporter inhibitors and their therapeutic potential

The norepinephrine transporter (NET) is located in the plasma membrane of noradrenergic neurons, where it functions to take up synaptically released norepinephrine (NE). The NET thus serves as the primary mechanism for the inactivation of noradrenergic signaling. Some potent and selective or mixed NET inhibitors (e.g., reboxetine and atomoxetine) have been successfully developed to treat a variety of mental disorders such as depression and attention deficit hyperactivity disorder (ADHD). However, to date, only a very limited number of NET-selective inhibitors are available. New potent and selective NET inhibitors may find application in the treatment of mental disorders or in PET imaging, and may enhance our basic understanding of these illnesses. In the present review, both previously reported and newly designed NET inhibitors, as well as their therapeutic and imaging potential, will be discussed. Two types of molecules, the conformationally constrained tropanes and the piperidine-based nocaine/modafinil hybrid ligands, represent new leads and provide good opportunities for discovering novel potent and selective NET inhibitors that are useful as therapies and imaging agents for the NET.