Glycine transporter type-1 and its inhibitors

The ionotropic glutamate receptor NMDA is allosterically modulated by glycine, a coagonist, its presence is an absolute requirement for receptor activation. The transport of glycine in glutamatergic synapse is carried out by glycine transporter-1 (GlyT1), a Na+/Cl--dependent carrier molecule. The primary role of GlyT1 is to maintain glycine concentrations below saturation level at postsynaptic NMDA receptors. Several isoforms of GlyT1 (a-e) have been identified, which are expressed both in glial and neuronal cell membranes. GlyT1 operates bidirectionally: it decreases synaptic glycine concentration when operates in normal mode and releases glycine from glial cells as operates in a reverse mode. It is expected that non-transportable, non-competitive inhibitors of GlyT1 may have therapeutic value in CNS disorders characterized by hypofunctional NMDA receptor-mediated glutamatergic neurotransmission. Accordingly, GlyT1 inhibitors exhibited antipsychotic profile in a number of animal tests. The first promising in vitro and in vivo experiments with glycine itself, and its N-methyl analogue, sarcosine, had initiated the syntheses of potential GlyT1 inhibitors with more complex structures, in which, however the glycine or sarcosine moiety had always been incorporated. Those attempts led to the development of two compounds, ALX-5407 and Org-24461 with high inhibitory potency; however, none of which is now considered as a drug candidate due, most probably, to safety and/or pharmacokinetic issues. More recently, several structurally new series of highly potent inhibitors with no aminomethylcarboxy group have also been discovered. Some of them might be expected to fulfill all requirements for clinical development. The new generation of GlyT1 inhibitors may represent a novel treatment of patients suffering from schizophrenia and/or other neuropathological conditions.