Estimating Cardiac 5-HT2B Safety Margins for Repeated Low-Dose Psilocybin Using an Exposure-Response Model
Repeated low-dose psilocybin is being developed as a scalable outpatient treatment for mood and anxiety disorders, but chronic exposure raises concern because psilocin binds the cardiac serotonin 5-HT2B receptor, whose sustained agonism causes drug-induced valvular heart disease (VHD). We evaluated this risk using an exposure-response model that incorporates functional efficacy and exposure duration rather than binding affinity alone. Plasma psilocin concentrations were converted into the time-integrated increment in 5-HT2B Gq signaling above endogenous serotonergic tone ({Delta}TIA) and calibrated against drugs and conditions with known valvular outcomes. All modeled exposures known to cause human VHD scored {Delta}TIA [≥] +172 %{middle dot}h/day, whereas exposures not associated with VHD scored [≤] +28. A candidate 3 mg daily psilocybin regimen scored {Delta}TIA +3, roughly two orders of magnitude below the weakest valvulopathic exposure. This safety margin arises from psilocins low-efficacy partial agonism at 5-HT2B (Emax {approx}51.8% of serotonin, compared with 96% for norfenfluramine) and its short half-life ({approx} 2.5 h), which prevents accumulation and produces brief daily receptor engagement. In support of the model, rats receiving continuous psilocin for 12 days at plasma concentrations {approx}2.4-fold above the projected human peak for 3 mg daily psilocybin showed no valvular lesions by blinded histopathology. This exposure duration however cannot exclude slowly developing fibrosis. Emerging human data, including serial echocardiography in repeated LSD microdosing and a large observational cohort, are also agreement with the model. Collectively, these findings suggest a favorable safety margin for daily, sub-hallucinogenic psilocybin use in clinical indications. Nevertheless, continued pharmacological and clinical investigations should include prospective echocardiographic monitoring to advance the clinical safety profile of sub-hallucinogenic psilocybin and support its evaluation across a broad array of therapeutic programs. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=54 SRC="FIGDIR/small/739440v1_ufig1.gif" ALT="Figure 1"> View larger version (10K): org.highwire.dtl.DTLVardef@a2d696org.highwire.dtl.DTLVardef@b64fbaorg.highwire.dtl.DTLVardef@12c41d4org.highwire.dtl.DTLVardef@95e9cd_HPS_FORMAT_FIGEXP M_FIG C_FIG Three key determinants of cardiac safety margins for repeated low-dose psilocybin are shown. Psilocin is a low-efficacy partial agonist at 5-HT2B (ceiling {approx}52% vs 96% for norfenfluramine; left). Its short half-life yields a brief daily pulse of receptor engagement rather than a sustained plateau (center). The resulting integrated 5-HT2B signal ({Delta}TIA) at 3 mg daily lies roughly two orders of magnitude below valvulopathic exposures, and continuous in vivo exposure produced no valvulopathy (right).