How it works in the brain

Two signaling pathways

The biased agonism hypothesis

A drug that selectively activates one pathway over the other is called a "biased agonist." If β-arrestin2 signaling really is responsible for the most dangerous opioid effects, a strongly G-protein-biased MOR agonist could in principle deliver analgesia and withdrawal suppression with a wider safety margin.

SR-17018 was developed as a test of that hypothesis. The original 2017 paper reported a therapeutic window of 26–105 in mice (compared to 5–21 for morphine and 2–5 for fentanyl)[1].

What follow-up studies showed

Low intrinsic efficacy, not true bias (Gillis et al., 2020)

Gillis and colleagues depleted the available μ-opioid receptors with the irreversible antagonist β-funaltrexamine, then re-measured agonist efficacies. Most "biased" agonists, including SR-17018, showed low intrinsic efficacy — suggesting that what looked like pathway selectivity could largely be explained by partial agonism[2]. A partial agonist will naturally activate downstream pathways unequally depending on the assay's signal amplification.

Bias breaks down at high dose (Fritzwanker et al., 2021)

Under sustained exposure at saturating concentrations, SR-17018 induces an MOR phosphorylation pattern indistinguishable from that of the full agonist DAMGO. The phosphorylation also persists for hours after washout — far longer than for any other tested agonist, including buprenorphine[3]. That implies that at high enough concentrations, SR-17018 doesn't behave like a pathway-selective ligand at all.

Non-competitive, slowly-reversible binding (Stahl et al., 2021)

Stahl and colleagues characterized SR-17018 as a non-competitive MOR agonist that stabilizes the receptor in a wash-resistant G-protein-signaling state. The compound appears to interact with both the orthosteric site (where traditional opioids bind) and a putative allosteric site[4].

Naloxone still works

Because SR-17018's duration of action is long, repeat doses of naloxone may be required. See Safety for the full overdose response.

Current scientific picture

The best provisional summary: SR-17018 is an atypical, non-competitive, partial-agonist-like MOR ligand with apparent G-protein bias under some assay conditions. Its behavior is genuinely unusual, but is no longer considered an unambiguous demonstration of safe biased agonism.