Glycine transport inhibitors for the treatment of schizophrenia: symptom and disease modification.

Schizophrenia is a severe neuropsychiatric disorder for which there is no adequate current treatment. Recent theories about the molecular basis of schizophrenia focus on disturbances of glutamatergic neurotransmission, particularly at NMDA-type glutamate receptors (NMDARs). NMDARs are regulated in vivo by the amino acids glycine and D-serine. Glycine levels, in turn, are regulated by glycine transporter type 1 (GlyT1), which serves to maintain low subsaturating glycine levels in the vicinity of the NMDAR. Therefore, one proposed approach to the treatment of schizophrenia is via the inhibition of GlyT1-mediated transport. During the past decade, several well-tolerated, high-affinity glycine transport inhibitors (GTIs) have been developed that demonstrate the ability to potentiate NMDAR-mediated neurotransmission in animal models relevant to schizophrenia. In addition, clinical trials have been conducted with sarcosine (N-methylglycine), a naturally occurring GTI. Issues related to clinical proof-of-concept studies with high-affinity GTIs in schizophrenia are discussed in this review.