Emerging cannabinoids differ in behavioral safety margins in mice: a comparative hazard proximity analysis of locomotor suppression and catalepsy-like immobility

Purpose Novel cannabinoids, including tetrahydrocannabihexol (THCH), tetrahydrocannabivarin (THCV), hexahydrocannabinol (HHC), hexahydrocannabihexol (HHCH), and hexahydrocannabiphorol (HHCP), have raised toxicological concerns, but their acute neurobehavioral effects remain poorly understood. This study compared the in vivo potencies of six cannabinoids using locomotor suppression and catalepsy-like immobility as indices of psychomotor suppression and toxic motor impairment, respectively. Methods Male C57BL/6J mice received intraperitoneal administration of Δ9-tetrahydrocannabinol (THC), HHC, THCH, THCV, HHCH, or HHCP. Catalepsy-like immobility was assessed using the bar test 20 min after dosing (maximum latency, 300 s), and spontaneous locomotor activity was recorded for 10 min beginning 35 min after dosing. ED50 values were determined for locomotor suppression and catalepsy-like immobility, and the Hazard Proximity Index (HPI, %) was calculated as (ED50_locomotor / ED50_catalepsy) × 100, with higher values indicating a narrower behavioral safety margin. Results All cannabinoids reduced spontaneous locomotor activity. THC, THCH, HHCH, and HHCP produced robust catalepsy-like immobility, whereas HHC induced only weak, non-significant immobility. THCV increased immobility only at higher doses and did not reach 50% catalepsy within the tested dose range. HPI values were 25.6% for THC, 45.1% for THCH, 5.1% for HHC, 33.2% for HHCH, and 3.7% for HHCP; HPI was not calculated for THCV. Conclusions Psychomotor suppression occurred at lower doses than catalepsy-like toxicity. Some side-chain-extended cannabinoids, particularly THCH and HHCH, showed a narrower separation between these endpoints. These findings provide comparative ED50 benchmarks and introduce the HPI as a novel ED50-based screening metric for the relative prioritization of emerging cannabinoids in forensic toxicology.