Elsevier

Neurochemistry International

Volume 98, September 2016, Pages 89-93
Neurochemistry International

Glycine transporter2 inhibitors: Getting the balance right

https://doi.org/10.1016/j.neuint.2015.12.007Get rights and content

Abstract

Neurotransmitter transporters are targets for a wide range of therapeutically useful drugs. This is because they have the capacity to selectively manipulate the dynamics of neurotransmitter concentrations and thereby enhance or diminish signalling through particular brain pathways. High affinity glycine transporters (GlyTs) regulate extracellular concentrations of glycine and provide novel therapeutic targets for neurological disorders.

Introduction

Neurotransmitter transporters are targets for a wide range of therapeutically useful drugs. This is because they have the capacity to selectively manipulate the dynamics of neurotransmitter concentrations and thereby enhance or diminish signalling through particular brain pathways. High affinity glycine transporters (GlyTs) regulate extracellular concentrations of glycine and provide novel therapeutic targets for neurological disorders (Eulenburg et al., 2005, Vandenberg et al., 2014).

Glycine is both an inhibitory and an excitatory neurotransmitter. It activates strychnine-sensitive inhibitory glycine receptors and is a co-agonist with glutamate on the excitatory N-methyl-d-aspartate (NMDA) subtype of glutamate receptors. Thus, GlyTs have the potential to influence both inhibitory glycinergic and excitatory glutamatergic neurotransmission (Berger et al., 1998, Eulenburg et al., 2005). Two human subtypes of GlyTs have been identified, GlyT1 and GlyT2. GlyT1 is predominantly expressed in glial cells surrounding both excitatory and inhibitory synapses, whereas GlyT2 is predominantly expressed in the brainstem and spinal cord where it is associated with presynaptic inhibitory glycinergic neurons (Liu et al., 1992, Liu et al., 1993, Kim et al., 1994, Zafra et al., 1995a, Zafra et al., 1995b, Kinney et al., 2003).

The GlyTs are members of the Na+/Cl dependent family of neurotransmitter transporters, that also includes transporters for γ-aminobutyric acid, noradrenaline, dopamine and serotonin (Liu et al., 1992, Liu et al., 1993, Amara and Kuhar, 1993, Kim et al., 1994, Broer and Gether, 2012). Characterisation of the different physiological roles of the two GlyT subtypes has opened the possibility of pharmacologically manipulating glycine concentrations as a potential means to treat specific disorders. GlyT1 inhibitors are thought to provide potential treatments for schizophrenia, alcohol addiction and pain, while inhibition of GlyT2 has the potential to alleviate pain (Sur and Kinney, 2004, Eulenburg et al., 2005, Harvey and Yee, 2013, Vandenberg et al., 2014).

Inhibitory glycinergic neurons are highly abundant in the dorsal horn of the spinal cord, particularly in lamina III (Pfeiffer et al., 1984, Mitchell et al., 1993, Zeilhofer et al., 2012), and contribute to inhibition of nociceptive signalling. These neurons have important roles in segregating nociceptive and non-noxious information pathways (von Hehn et al., 2012, Zeilhofer et al., 2012) and dysfunction of glycinergic systems in the spinal cord, together with GABAergic systems (Yaksh, 1989, Sivilotti and Woolf, 1994, Sorkin and Puig, 1996, Lynch and Callister, 2006, Torsney and MacDermott, 2006, Lu et al., 2013), contribute to allodynia associated with neuropathic and inflammatory pain. In this review we will discuss the pharmacological properties of inhibitors of the GlyT2 subtype of glycine transporters and their potential use for the treatment of neuropathic and inflammatory pain. As part of this discussion we will address the potential for unwanted effects of GlyT2 inhibitors. Transient inhibition of neuronal GlyT2 can initially elevate extracellular glycine concentrations and stimulate glycinergic neurotransmission, but paradoxically, long term inhibition may lead to a reduced capacity to load glycine into synaptic vesicles and reduce glycinergic neurotransmission (Fig. 1). These considerations will need to be addressed if the therapeutic potential of GlyT2 inhibitors is to be realised.

Section snippets

The roles of GlyT2 in controlling inhibitory glycinerigic neurotransmission

Vesicular glycine release from presynaptic terminals leads to a rapid increase in synaptic glycine concentrations to approximately 3 mM (Beato, 2008) and activation of postsynaptic inhibitory glycine receptors. The termination of glycine neurotransmission is achieved by a combination of diffusion of glycine from the synapse and active uptake of glycine by GlyT1 into glial cells and GlyT2 into the presynaptic neurons (Eulenburg et al., 2005, Beato, 2008). Glycine transport by GlyT1 is coupled to

ALX1393

ALX1393 is a potent GlyT2 inhibitor with an IC50 of 10–25 nM and an approximate 200-fold selectivity for GlyT2 over GlyT1 (Caulfield et al., 2001, Mingorance-Le Meur et al., 2013) (Fig. 2). ALX1393 has been studied in animal models of pain using either intravenous or intrathecal injections (Xu et al., 2005, Morita et al., 2008, Haranishi et al., 2010, Nishikawa et al., 2010, Mingorance-Le Meur et al., 2013). A single intravenous injection of 0.01 mg/kg ALX1393 reduces mechanical allodynia over

ORG25543 and related compounds

The analgesic actions of ORG25543 (Fig. 2) are similar to ALX1393 (Morita et al., 2008). However, unlike ALX1393, ORG25543 is an irreversible GlyT2 inhibitor (Mingorance-Le Meur et al., 2013). Intrathecal and intravenous injections of ORG25543 reduce allodynia associated with nerve ligation injury, streptozotocin-induced diabetic pain model, and Complete Freunds Adjuvent-induced inflammatory pain. The acute actions of high doses of ORG25543 in lamina X neurons of mouse spinal cord slices show

Phenoxymethylbenzamides

A series of phenoxymethylbenzamides that inhibit GlyT2 with mid nM potency have recently been developed by Takahashi et al. (2014). One of these compounds, GT-0198, has an IC50 of 105 nM (Fig. 2), shows good in vivo pharmacokinetic parameters and has been tested in a mouse model of neuropathic pain (Omori et al., 2015). Intravenous, oral and intrathecal injections of GT-0198 reduce allodynia in sciatic nerve injured mice. In this model the analgesic effects of GT-0198 were comparable to similar

Multiple target inhibitors

Whilst there is growing evidence that inhibition of GlyT2 has potential for the treatment of neuropathic pain, drugs that modulate many other targets have also shown significant efficacy in chronic pain trials. These include serotonin transporter inhibitors, Ca2+ channel (α2 subunits) inhibitors, opioid receptor agonists, cycloxygenase 2 inhibitors, voltage-gated Na+ channel inhibitors, TrpV1 channel inhibitors, α2-adrenceptor agonists and NMDA receptor inhibitors. In addition, a variety of

NAGly and related lipids

NAGly is one of a number of endogenous acyl-amino acids that have biological effects (Fig. 2) (Connor et al., 2010, Carland et al., 2014). NAGly is produced in a variety of organs, but its highest concentrations are found in the spinal cord (Huang et al., 2001). NAGly is a partial, non-competitive, inhibitor of GlyT2 with minimal activity at other closely related transporters, such as GlyT1 and GAT1 (Wiles et al., 2006). The IC50 for NAGly inhibition of GlyT2 is approximately 3 μM and it

VVZ-149

VVZ-149 is a GlyT2 inhibitor that also has inhibitory effects on 5HT2A receptors and P2X3 receptors. 5HT2A and P2X3 receptors have also been suggested to be potential targets for the treatment of neuropathic pain and so this approach targets three distinct systems. VVZ-149 has modest potency at all three targets, with IC50s of 0.86 μM at GlyT2, 1.3 μM at 5HT2A and 0.87 μM at P2X3 receptors (Pang et al., 2012). The main criteria in the search that lead to the identification of VVZ-198 as an

Conclusions

There is a growing body of evidence to suggest that GlyT2 inhibitors have the potential to be developed into analgesics for the treatment of neuropathic or chronic pain. However, at this stage there are many questions as to what is the optimal design of a GlyT2 inhibitor. Are competitive or non-competitive inhibitors likely to most useful? Will partial inhibitors be better able to control long-term inhibition of GlyT2 whilst also maintaining the capacity to load synaptic vesicles? Should drugs

Acknowledgements

Work in the lab of RJV, JCE and RMR are supported by a grant from the Australian NHMRC APP1082570. SNM is supported by an Australian Postgraduate Award.

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