Synthesis and pharmacological evaluation of newly detected synthetic cannabinoid receptor agonists AB-4CN-BUTICA, MMB-4CN-BUTINACA, MDMB-4F-BUTICA, MDMB-4F-BUTINACA and their analogs

Abstract

Synthetic cannabinoid receptor agonists (SCRAs) continue to make up a significant portion new psychoactive substances (NPS) detected and seized worldwide. Due to their often potent activation of central cannabinoid receptors in vivo, use of SCRAs can result in severe intoxication, in addition to other adverse health effects. Recent detections of AB-4CN-BUTICA, MMB-4CN-BUTINACA, MDMB-4F-BUTICA and MDMB-4F-BUTINACA mark a continuation in the appearance of SCRAs bearing novel tail substituents. The proactive characterization campaign described here has facilitated the detection of several new SCRAs in toxicological case work. Here we detail the synthesis, characterization, and pharmacological evaluation of recently detected SCRAs, as well as a systematic library of 32 compounds bearing head, tail, and core group combinations likely to appear in future. In vitro radioligand binding assays revealed most compounds showed moderate to high affinity at both CB₁ (pK _i = < 5 to 8.89 ± 0.09 M) and CB₂ (pK _i = 5.49 ± 0.03 to 9.92 ± 0.09 M) receptors. In vitro functional evaluation using a fluorescence-based membrane potential assay showed that most compounds were sub-micromolar to sub-nanomolar agonists at CB₁ (pEC₅₀ = < 5 to 9.48 ± 0.14 M) and CB₂ (pEC₅₀ = 5.92 ± 0.16 to 8.64 ± 0.15 M) receptors. An in silico receptor-ligand docking approach was utilized to rationalize binding trends for CB₂ with respect to the tail substituent, and indicated that rigidity in this region (i.e., 4-cyanobutyl) was detrimental to affinity.

Keywords: SCRAs; cannabinoid receptor 1 agonists; cannabinoids; docking; in vitro evaluation; pharmacology; synthesis; synthetic cannabinoid.

Figure 1

Δ⁹-THC (1), JWH-018 (2), and selected SCRAs MDMB-FUBINACA (3) and ADB-BUTINACA (4).

Figure 2

Detected SCRAs AB-4CN-BUTICA (5), MMB-4CN-BUTINACA (6), MDMB-4F-BUTICA (7), and MDMB-4F-BUTINACA (8).

Figure 3

Structures of amino acid-derived 4-cyanobutyl and 4-fluorobutyl-substitued indole, indazole and 7-azaindole SCRAs explored in this study. AB, [(S)-2-amino-3-methylbutanamide]; ADB, [(S)-2-amino-3,3-dimethylbutanamide]; APP, [(S)-2-amino-3-phenylpropanamide]; MMB, [methyl (S)-2-amino-3-methylbutanoate]; MDMB, [methyl (S)-2-amino-3,3-dimethylbutanoate]; MPP, [methyl ((S)-2-amino-3-phenylpropanoate]; 4-CN-BUT, 4-cyanobutyl; 4F-BUT, 4-fluorobutyl; ICA, indole-3-carboxamide; INACA, indazole-3-carboxamide; 7AICA, 7-aza-indole-3-carboxamide.

Scheme 1

Reagents and conditions: (a) (i) NaH, alkylbromide, DMF, 0 °C–rt, 1 h; (ii) (CF₃CO)₂O, 0 °C–rt, 2 h; (b) KOH, MeOH, PhMe, reflux, 2 h; (c) EDC•HCl, HOBt•H₂O, amino acid•HCl, Et₃N, DMF, rt, 18 h, 29–99%. (d) NaH, alkylbromide, DMF, 0°C–rt, 24 h, 81%; (e) 1 M aq. NaOH, MeOH, rt, 48 h, 98%.

Figure 4

Functional activities of compounds 5–40 at hCB₁ and hCB₂, relative to CP55,940. Efficacies are normalised to maximal response of 1 μM CP55,940. Vertical sets indicate the same heterocyclic core (left = indole; centre = indazole; right = 7-azaindole). Graphs are grouped by tail, with rows indicating alkyl tails, either 4-cyanobutyl (rows 1 and 3), or 4-fluorobutyl (rows 2 and 4). Dashed lines indicate where curve could not be fit. Data is expressed as means ± SEM from 3 to 6 independent experiments.

Figure 5

Computationally predicted binding modes of 9 (orange) and 26 (green) overlayed at CB₁ (A), PDB:6N4B and CB₂ (B), PDB:6PT0 (57, 58). The hydrophobic regions of the binding site (grey) were evaluated with SiteMap (67). Glide docking was performed using the Schrodinger computational chemistry suite (66).