The Synthetic Cathinone α‐PPP Acts as a Competitive Antagonist at Human 5‐HT2A Receptors
Authors: Chen Yiming , Blough Bruce , Murnane Kevin , Canal Clinton 
Published: June 2021

Synthetic cathinones (SC) are β‐ketone analogs of amphetamines, and produce similar psychoactive effects. Some can elicit serious psychiatric events such as psychosis and delirium. Lethal overdoses have also been reported. Despite these toxicities, the receptor pharmacology of cathinones remains unclear. All SC, like amphetamines, primarily target monoamine transporters, however, off‐target effects may also contribute to adverse events. We hypothesized that at high doses, SC can target G protein‐coupled receptors—including activation of serotonin 5‐HT2A and blockade of muscarinic M1—that can contribute to psychosis and delirium. Radioligand competition binding assays were performed to determine the binding affinities of nine novel SC at human 5‐HT2A and M1 receptors transiently expressed in HEK293 cells. [3H]Ketanserin (1.6 nM) and [3H]N‐methyl‐scopolamine (1.0 nM) were used to label 5‐HT2A and M1 receptors, respectively. None of the drugs exhibited affinity at the M1 receptor (no displacement of [3H]N‐methyl‐scopolamine up to 10 μM, N=2 replicates), however, two SC—alpha‐pyrrolidinopropiophenone (α‐PPP) and 4‐methyl‐α‐PPP—had appreciable affinity at 5‐HT2A receptors (pKi=5.7 ± 0.1 (2.0 μM), N=3 and pKi=5.4 ± 0.1 (4.3 μM), N=2), respectively). Considering α‐PPP's affinity at the dopamine and norepinephrine transporters (~1.3 and 2.0 μM, respectively, Eshleman et al., 2017), our data suggested that α‐PPP may also have physiologically relevant activity at 5‐HT2A receptors. We therefore tested its functional activity at 5‐HT2A receptors transiently expressed in HEK293 cells using a FRET‐based inositol phosphate 1 (IP1) assay (Cisbio, IP‐One HTRF). Contrary to our hypothesis, α‐PPP did not activate 5‐HT2A receptors (up to 100 μM). In subsequent tests, it caused a dose‐dependent, rightward shift of the 5‐HT2A agonist (±)DOI dose‐response curve (pA2=5.7± 0.2, N=4), without altering (±)DOI's maximal response. These data showed that α‐PPP has competitive interactions at the 5‐HT2A receptors. To assess in vivo 5‐HT2A activity, we assessed in FVB mice α‐PPP's ability to block the (±)DOI‐elicited head‐twitch response, a 5‐HT2A‐dependent behavior. α‐PPP dose‐dependently blocked the (±)DOI‐elicited HTR (28% and 88% decrease at 3 and 10 mg/kg, respectively, N=6–7/group). To corroborate a 5‐HT2A mechanism, we tested 3′,4′‐Methylenedioxy‐α‐pyrrolidinopropiophenone (MDPPP), a structurally similar SC that we found had no affinity at 5‐HT2A (>50 μM)—MDPPP did not significantly block the HTR up to 10 mg/kg ((±)DOI alone 13 ± 1 HTRs vs. MDPPP 10 mg/kg, 10 ± 2 HTRs, P=0.2, N=5–6/group). Considering reported doses of α‐PPP by users of 50 mg or higher with repeated P.O. administration, that SC readily cross the blood‐brain barrier, and that several structurally‐similar drugs reach micromolar levels in the brain at comparable doses, we conclude that α‐PPP likely engages and blocks 5‐HT2A receptors in the brains of human drug users.Support or Funding InformationR21DA040907This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.