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| Formula | C28H30O8 | 
| Molar mass | 494.540 g·mol−1 | 
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Herkinorin is an opioid analgesic that is an analogue of the natural product salvinorin A. It was discovered in 2005 during structure-activity relationship studies into neoclerodane diterpenes, the family of chemical compounds of which salvinorin A is a member. [1]
Unlike salvinorin A, which is a selective κ-opioid receptor agonist with no significant μ-opioid receptor affinity, herkinorin is predominantly a μ-opioid receptor agonist. Compared to salvinorin A, herkinorin has 47× lower affinity for κ-opioid receptors (Ki = 90 nM vs Ki = 1.9 nM), and at least 25× higher affinity for μ-opioid receptors (Ki = 12 nM vs Ki >1000 nM), where it acts as a full agonist (IC50 = 0.5 μM, Emax = 130% vs DAMGO). [2] [3] Herkinorin is a semi-synthetic compound, made from salvinorin B, which is most conveniently made from salvinorin A by deacetylation, since, while both salvinorin A and salvinorin B are found in the plant Salvia divinorum , salvinorin A is present in larger quantities. [4]
A study in primates showed it to act peripherally as both a μ- and κ-opioid receptor agonist, with a fast onset of action. The study did not find any evidence of central activity in primates and questions whether herkinorin's effects are due entirely to peripheral binding. [5] Unlike most μ-opioid receptor agonists, herkinorin does not promote the recruitment of β-arrestin 2 to the intracellular domain of the μ-opioid receptor, or induce receptor internalization. [6] This means that herkinorin may not produce tolerance and dependence in the same way as other opioids, although some development of tolerance through other mechanisms has been observed, [7] and some other analogues related to herkinorin can recruit β-arrestins. [8]