Samidorphan, also known as Lybalvi, is a novel opioid-system modulator, it is similar to naltrexone. It functions primarily as a μ-opioid receptor antagonist and is used primarily in combination with antipsychotics to reduce their metabolic dysfunction-associated adverse effects. Samidorphan was first approved by the FDA in May 2021. It is used in combination with olanzapine, an antipsychotic, for the treatment of bipolar disorder and for the treatment of schizophrenia in adults. It is demonstrated that the addition of samidorphan to olanzapine helps mitigate the metabolic-related adverse effects of olanzapine; presumably, this is due to opioid receptor signaling, though the exact mechanism remains to be determined. Samidorphan functions primarily as a μ-opioid receptor antagonist and as a κ/δ-opioid receptor partial agonist. Since it is an opioid antagonist, this drug can potentiate opioid withdrawal. Samidorphan acts as an antagonist at the μ-opioid, a partial agonist at both the κ- and δ-opioid receptors in vitro. Samidorphan inhibits the exchange of GTP for GDP which is required to activate the G-protein complex. This prevents the Gi subunit of the mu opioid receptor from inhibiting adenylate cyclase, which can therefore continue to catalyze ATP into cAMP. cAMP increases the excitability in spinal cord pain transmission neurons which allows the patient to feel pain rather than the analgesic effects of opioids. The inhibition of Mu-type opioid receptors also prevents the Gi subunit of the mu opioid receptor from activating the inwardly rectifying potassium channel increasing K+ conductance which would cause hyperpolarization. Samidorphan also prevents the gamma subunit of the mu opioid receptor from inhibiting the N-type calcium channels on the neuron. This allows calcium to enter the neuron and depolarize. The inhibition of mu-opioid receptors prevents hyperpolarization in the neuron, allowing it to fire at a normal rate. The neuron is able to depolarize and the high concentration of calcium releases GABA into the synapse which binds to GABA receptors. GABA receptors inhibits dopamine cell firing in the pain transmission neurons. This prevents the analgesic and depressive effects of opioids, preventing opioid overdose. GABA also inhibits dopamine cell firing in the reward pathway which is the main cause of addiction to opioids and other drugs.

Samidorphan Pathway References