Structure-activity relationships for 5F-MDMB-PICA and its 5F-pentylindole analogs to induce cannabinoid-like effects in mice

Abstract

Synthetic cannabinoid receptor agonists (SCRAs) are an evolving class of new psychoactive substances found on recreational drug markets worldwide. The indole-containing compound, 5F-MDMB-PICA, is a popular SCRA associated with serious medical consequences, including overdose and hospitalizations. In vitro studies reveal that 5F-MDMB-PICA is a potent agonist at cannabinoid type 1 receptors (CB₁), but little information exists regarding in vivo pharmacology of the drug. To this end, we examined the in vitro and in vivo cannabinoid-like effects produced by 5F-MDMB-PICA and related 5F-pentylindole analogs with differing composition of the head group moiety (i.e., 5F-NNEI, 5F-SDB-006, 5F-CUMYL-PICA, 5F-MMB-PICA). In mouse brain membranes, 5F-MDMB-PICA and its analogs inhibited binding to [³H]rimonabant-labeled CB₁ and displayed agonist actions in [³⁵S]GTPγS functional assays. 5F-MDMB-PICA exhibited the highest CB₁ affinity (K_i = 1.24 nM) and functional potency (EC₅₀ = 1.46 nM), but head group composition markedly influenced activity in both assays. For example, the 3,3-dimethylbutanoate (5F-MDMB-PICA) and cumyl (5F-CUMYL-PICA) head groups engendered high CB₁ affinity and potency, whereas a benzyl (5F-SDB-006) head group did not. In C57BL/6J mice, all 5F-pentylindole SCRAs produced dose- and time-dependent hypothermia, catalepsy, and analgesia that were reversed by rimonabant, indicating CB₁ involvement. In vitro K_i and EC₅₀ values were positively correlated with in vivo ED₅₀ potency estimates. Our findings demonstrate that 5F-MDMB-PICA is a potent SCRA, and subtle alterations to head group composition can have profound influence on pharmacological effects at CB₁. Importantly, measures of CB₁ binding and efficacy in mouse brain tissue seem to accurately predict in vivo drug potency in this species.

Fig. 1

Chemical structures of 5F-pentylindole synthetic cannabinoids showing linker and head group variations, as compared to the parent compound AM-2201.

Fig. 2. Dose-response of 5F-MDMB-PICA and its analogs to compete for CB₁ binding, display CB₁ agonist effects, and produce cannabinoid-like effects in vivo.

Concentration response curves for AM-2201 (red circles), 5F-NNEI (blue squares), 5F-SDB-006 (gray hexagons), 5F-CUMYL-PICA (green diamonds), 5F-MMB-PICA (orange upward triangles), and 5F-MDMB-PICA (purple downward triangles) to inhibit [³H]rimonabant binding (A) and stimulate [³⁵S]GTPγS binding (B) in mouse brain membranes. Dose-response curves for effects of drugs on mean temperature change (Δ °C) (C), catalepsy expressed as percent maximum possible effect (%MPE) (D), and analgesia expressed as %MPE (E) in the triad test. In vitro data are mean ± SD for n = 3 separate experiments performed in triplicate, while in vivo data are mean ± SEM for n = 7–9 mice per dose. Affinity (K_i) and potency values (EC₅₀, ED₅₀) are found in Table 1.

Fig. 3. Time-course and antagonist reversal of cannabinoid-like effects produced by 5F-MDMB-PICA.

Effects of 0.003–0.3 mg/kg s.c. 5F-MDMB-PICA on temperature change (Δ °C) (A), catalepsy expressed as percent maximum possible effect (%MPE) (B), and analgesia expressed as %MPE (C) in the triad test, as assessed every 30 min over the 2 h testing period. Effect of rimonabant (0.1 or 1 mg/kg s.c.) pretreatment on mean temperature Δ (D), mean catalepsy %MPE (E), and mean analgesia %MPE (F) produced by 5F-MDMB-PICA. Data are expressed as mean ± SEM for n = 8–9 mice per dose in time-course plots and mean ± SEM for n = 5–6 mice per condition in antagonist reversal plots. D–F Filled bars & symbols represent statistically significant differences (p < 0.05) compared to the vehicle/vehicle control group.

Fig. 4. Correlations between in vitro and in vivo CB₁ measures in mice.

Relationships between in vitro affinities and potencies in mouse brain tissue and in vivo potencies for cannabinoid-like effects in mice determined in the triad test for AM-2201 (red circles), 5F-NNEI (blue squares), 5F-SDB-006 (gray octagons), 5F-CUMYL-PICA (green diamonds), 5F-MMB-PICA (orange upward triangles), and 5F-MDMB-PICA (purple downward triangles). Spearman rank correlations between log K_i for binding in vitro and in vivo log ED₅₀ parameters of the triad test (A–C). Spearman rank correlations between log EC₅₀ for efficacy in vitro and in vivo log ED₅₀ parameters of the triad test (E–F).