While pharmacological blockade of
dopamine D2 receptor can effectively suppress the psychotic or positive symptoms of
schizophrenia, there is no satisfactory medication for the negative and
cognitive symptoms of
schizophrenia in spite of the proliferation of second generation
antipsychotic drugs. Excitements over a new class of third generation
antipsychotics that might possibly fill this urgent medical need have been prompted by the recent development of
glycine transporter 1 (GlyT1) inhibitors. The impetus of this novel pharmacological strategy stems directly from the prevailing hypothesis that negative and
cognitive symptoms are attributable to the hypofunction of glutamatergic signalling via the
N-methyl-D-aspartate (
NMDA) receptor in the brain. Inhibition of GlyT1 reduces clearance of extra-cellular
glycine near
NMDA receptor-containing synapses, and thereby increases baseline occupancy of the
glycine-B site at the NR1 subunit of the
NMDA receptor, which is a prerequisite of channel activation upon stimulation by the excitatory
neurotransmitter glutamate. Pharmacological inhibition of GlyT1 is expected to boost
NMDA receptor function and therefore alleviate persistent negative and
cognitive symptoms without excessive risk of excitotoxicity associated with direct
NMDA receptor agonists. The recently completed phase III clinical trials of the Roche compound,
bitopertin (a.k.a.
RG1678 or RO-4917838) had initially raised hope that this new class of drugs might represent the first successful translation of the
glutamate hypothesis of
schizophrenia to the clinic. However, the outcomes of the multi-centre
bitopertin clinical trials have been disappointing. The present review seeks to examine this promise through a critical survey of the latest clinical and preclinical findings on the therapeutic potential of GlyT1 inhibition or down-regulation.