Iclepertin

Iclepertin (BI 425809) is a potent and selective inhibitor of glycine transporter type 1 (GlyT1) with an IC50 of 5.0 nanomolar in human SK-N-MC cells, developed by Boehringer Ingelheim for the treatment of cognitive impairment associated with schizophrenia (CIAS). The compound is an orally bioavailable azabicyclo[3.1.0]hexane methanone bearing two trifluoromethyl groups and a methylsulfonyl substituent, with three defined stereocenters and a molecular weight of 512.42 g/mol. By blocking the reuptake of glycine from the synaptic cleft into astrocytes and presynaptic terminals, iclepertin increases synaptic glycine concentration at the glycine-B co-agonist site of the N-methyl-D-aspartate (NMDA) receptor, thereby enhancing glutamatergic neurotransmission. This mechanism addresses the NMDA receptor hypofunction hypothesis of schizophrenia, which posits that reduced glutamatergic signaling contributes to the cognitive deficits observed in the disorder.

Pharmacokinetically, iclepertin exhibits a terminal half-life of 34 to 59 hours, supporting once-daily oral dosing with steady-state conditions achieved by day 6. Oral bioavailability is approximately 72 percent for a 25 mg tablet under fasted conditions. The compound is metabolized predominantly (90 percent or greater) by cytochrome P450 3A4, rendering it sensitive to strong CYP3A4 inhibitors (approximately 6-fold AUC increase with itraconazole) and inducers (approximately 90 percent AUC reduction with rifampicin). In Phase I studies, oral administration of 10 mg produced a dose-dependent approximately 50 percent increase in cerebrospinal fluid glycine levels, confirming central target engagement.

The Phase II clinical programme produced divergent results across indications. In a 509-patient randomized, double-blind, placebo-controlled trial in schizophrenia (NCT02832037), iclepertin at 10 mg in the published literature for 12 weeks produced a statistically significant improvement on the MATRICS Consensus Cognitive Battery overall composite T-score, with an adjusted mean difference of 1.98 points and a standardized effect size of 0.34 versus placebo. However, a parallel 610-patient Phase II trial in mild-to-moderate Alzheimer's disease dementia (NCT02788513) failed to demonstrate any dose-response relationship on the ADAS-Cog11 primary endpoint. A subsequent Phase II trial combining iclepertin with computerized cognitive training (NCT03859973, n=200) also failed to meet its primary endpoint. The compound received United States Food and Drug Administration Breakthrough Therapy Designation for CIAS in May 2021. In January 2025, Boehringer Ingelheim reported that the Phase III CONNEX programme, comprising three replicate randomized controlled trials enrolling 1835 patients across 338 sites in 41 countries with 26 weeks of treatment at 10 mg in the published literature, failed to meet its primary or key secondary endpoints (pooled MCCB adjusted mean difference 0.127, 95 percent confidence interval negative 0.396 to 0.650, p=0.63). Iclepertin was well tolerated across all trials, with adverse event rates numerically similar to or lower than placebo. The compound is not approved in any jurisdiction.

This monograph reviews the chemistry and synthesis of iclepertin; the molecular pharmacology of GlyT1 inhibition and the NMDA receptor enhancement mechanism; the comprehensive pharmacokinetic characterization including CYP3A4 drug-drug interactions; preclinical efficacy in MK-801 deficit and social recognition models; the complete clinical evidence base across schizophrenia, Alzheimer's disease, and cognitive training paradigms; reconstitution, sourcing, and handling considerations; adverse event and safety signals; and a comparative assessment of five alternative NMDA-enhancing or GlyT1-targeting compounds (bitopertin, sarcosine, D-serine, PF-03463275, luvadaxistat) against iclepertin on five competency standards.