Collybolide was first isolated from R. maculata in 1911,[4][5][6] but its structure remained unsolved until the 1970s, when
X-ray crystallography yielded the structure of a collybolide
epimer, isocollybolide,[7] and 1H and 13C
NMR elucidated the structure and relative stereochemistry of collybolide.[8] Importantly, early reports were unable to confidently assign the
absolute configuration of collybolide; a 1986 isolation of a collybolide congener noted that the absolute configuration of the series "remains to be determined",[9] and a 2001
circular dichroism study was only able to tentatively infer which enantiomer naturally occurred based on
density functional theory calculations.[10] A 2016 report claimed to have conclusively assigned the absolute configuration of collybolide by
X-ray crystallography,[1] but a following 2022 report noted that the
Flack parameter accompanying the 2016 crystal structure was inconclusive,[2] and could not be used to confidently assign its absolute stereochemistry.
Purported kappa-opioid receptor agonism
Collybolide is a sesquiterpene that contains a furyl-δ-lactone, a structural feature shared with the diterpene natural product
salvinorin A. Salvinorin A is a hallucinogen that acts via high-potency agonism of the human
kappa-opioid receptor (KOR), and collybolide's structural similarity to salvinorin A prompted a 2016 team to investigate collybolide's activity at the KOR, in the hopes of discovering a new, non-nitrogenous opioid.[1]Radioligand displacement and functional assays showed collybolide binding to (Ki = 0.9 nM) and activating the human KOR, and an in vivo assay described collybolide inhibiting
chloroquine-induced itch in mice at an extremely low dose (IC50 = 0.08 mg/kg). These results attracted widespread attention in the biomedical community, as collybolide appeared to be a potent and selective KOR agonist that might be developed into a new treatment for pain or
pruritus,[11][12] lacking the adverse effects of typical
mu-opioid receptor agonist pain treatments. These claims of KOR agonism also attracted the attention of the recreational
psychedelic community.[13]
Independent chemical synthesis and biological assay of collybolide in 2022 found that it was devoid of opioid activity.[2] Radioligand displacement assays showed only weak (Ki = 794 nM) binding of collybolide to the human KOR, and functional assays showed that collybolide does not activate KOR signaling at concentrations up to 10 μM (measured by
35S]GTPγS binding,
cAMP accumulation, and
beta-arrestin recruitment assays). Shevick et al. note the presence of surface-modifying agents in the 2016 assay procedures, in addition to low percent stimulation in the 2016 [35S]GTPγS assay, that may have caused noise in the data to be mistaken as signal.[2] The source of the false positive result for KOR agonism in the 2016 study has yet to be rigorously identified. However, the findings and conclusions of the 2022 study – that collybolide was incorrectly assigned as a KOR agonist – explain why no credible reports of collybolide's psychoactivity have surfaced.[14][15]
Chemical synthesis
The 2022 reevaluation of collybolide's KOR activity leveraged access to both natural and unnatural enantiomers of collybolide via total synthesis.[2][16] Key features of the synthesis included an enantioselective
Diels-Alder reaction using the Hayashi-Jørgensen
proline organocatalyst, and an enamine
[3,3]-sigmatropic rearrangement to stereoselectively install a late-stage benzoyloxy (BzO) group.
^Hansske, Friedrich; Paululat, Thomas; Gerlitz, Martin; Gruen-Wollny, Iris (June 9, 2005).
"WO2005051376 - Arzneimittel enthaltend Collybolide". WIPO. Archived from the original on May 11, 2023. Retrieved May 11, 2023.{{
cite web}}: CS1 maint: bot: original URL status unknown (
link)