Rapastinel
Rapastinel is a novel antidepressant that is under development by Allergan as an adjunctive therapy for the treatment of treatment-resistant major depressive disorder. It is a centrally active, intravenously administered amidated tetrapeptide that acts as a selective, weak partial agonist of an allosteric site of the glycine site of the NMDA receptor complex. The drug is a rapid-acting and long-lasting antidepressant as well as robust cognitive enhancer by virtue of its ability to both inhibit and enhance NMDA receptor-mediated signal transduction.
On March 3, 2014, the U.S. FDA granted Fast Track designation to the development of rapastinel as an adjunctive therapy in treatment-resistant major depressive disorder. As of 2015, the drug had completed phase II clinical development for this indication. On January 29, 2016, Allergan announced that rapastinel had received Breakthrough Therapy designation from the U.S. FDA for adjunctive treatment of major depressive disorder.
Rapastinel belongs to a group of compounds, referred to as glyxins, that were derived via structural modification of B6B21, a monoclonal antibody that similarly binds to and modulates the NMDA receptor. The glyxins were invented by Joseph Moskal, the co-founder of Naurex. Glyxins and B6B21 do not bind to the glycine site of the NMDA receptor but rather to a different regulatory site on the NMDA receptor complex that serves to allosterically modulate the glycine site. As such, rapastinel is technically an allosteric modulator of the glycine site of the NMDA receptor, and hence is more accurately described as a functional glycine site weak partial agonist.
In addition to its antidepressant effects, rapastinel has been shown to enhance memory and learning in both young adult and learning-impaired, aging rat models. It has been shown to increase Schaffer collateral-CA1 long-term potentiation in vitro. In concert with a learning task, rapastinel has also been shown to elevate gene expression of hippocampal NR1, a subunit of the NMDA receptor, in three-month-old rats. Neuroprotective effects have also been demonstrated in Mongolian Gerbils by delaying the death of CA1, CA3, and dentate gyrus pyramidal neurons under glucose and oxygen-deprived conditions. Additionally, rapastinel has demonstrated antinociceptive activity, which is of particular interest, as both competitive and noncompetitive NMDA receptor antagonists are ataxic at analgesic doses, while rapastinel and other glycine subunit ligands are able to elicit analgesia at non-ataxic doses.
Apimostinel, an analogue of rapastinel with the same mechanism of action but greatly improved potency, is being developed by the same company as a follow-on compound to rapastinel.
On March 6, 2019, Allergan announced rapastinel failed to differentiate from placebo during phase III trials. Its development has since been discontinued.
