Ariadne is a non-hallucinogenic analog in the phenylalkylamine
chemical class of psychedelics that is closely related to an established
synthetic hallucinogen, 2,5-dimethoxy-4-methyl-amphetamine (DOM),
differing only by one methylene group in the α-position to the
amine. Ariadne has been tested in humans including clinical trials
at Bristol-Myers Company that indicate a lack of hallucinogenic effects
and remarkable therapeutic effects, such as rapid remission of psychotic
symptoms in schizophrenics, relaxation in catatonics, complete remission
of symptoms in Parkinson’s disease (PD), and improved cognition
in geriatric subjects. Despite these provocative clinical results,
the compound has been abandoned as a drug candidate and its molecular
pharmacology remained unknown. Here, we report a detailed examination
of the in vitro and in vivo pharmacology of Ariadne and its analogs,
and propose a molecular hypothesis for the lack of hallucinogenic
effects and the therapeutic potential of this compound class. We also
provide a summary of previous clinical and preclinical results to
contextualize the molecular signaling data. Our results show that
Ariadne is a serotonin 5-HT2 receptor agonist, exhibits
modest selectivity over 5-HT1 receptors, has no relevant
activity at 5-HT4,5,7 and other aminergic receptors, and
no substantial affinity at plasma membrane monoamine transporters.
Compared to DOM, Ariadne shows lower signaling potency and efficacy
in multiple signaling pathways examined (Gq, G11, and β-arrestin2) coupled to 5-HT2A receptors.
We confirmed the shift in signaling for an α-propyl analog and
provide a molecular docking rationale for the progressive decrease
in signaling potency with the growing length of the α-substituent.
Ariadne versus DOM exhibits no apparent change in the relative preference
between Gq/11 activation and β-arrestin2 recruitment;
instead, there is a small but consistent drop in efficacy in these
signaling channels. Ariadne acts as a 5-HT2A agonist in
vivo in mice and shows markedly attenuated head twitch response (HTR)
in comparison to its hallucinogenic analogs, consistent with previous
studies in rabbits, cats, and dogs. Hence, we propose the lower 5-HT2A receptor signaling efficacy of this compound class as an
explanatory model for the lack of hallucinogenic effects of Ariadne
in humans and the dramatically attenuated hallucinosis-like effects
in animals (5-HT2A signaling efficacy hypothesis). In terms
of reverse translation of the noted clinical therapeutic effects,
we used an auxilin knockout model of Parkinson’s disease where
Ariadne rescued severe motor deficits in this mouse line, on par with
the effects of l-DOPA, a notable finding considering Ariadne’s
lack of activity at dopamine receptors and transporters. Ariadne emerges
as a prototype of a new drug class, non-hallucinogenic 5-HT2A agonists, with considerable therapeutic potential across psychiatric
and neurological indications.