Pharmacological evaluation of synthetic cannabinoids identified as constituents of spice

Abstract

In recent years, many synthetic cannabinoid (CB) receptor agonists have appeared on the market as constituents of herbal incense mixtures known as "spice". Contrary to the declared use, they are perorally consumed as a replacement for marijuana to get "high". In many cases, detailed information on the physicochemical and pharmacological properties of the synthetic compounds found in spice preparations is lacking. We have now evaluated a large series of heterocyclic compounds, 1,3-disubstituted indole and 2-azaindole derivatives known or assumed to be CB₁ receptor agonists, many of which have previously been identified in forensic samples. The mainly observed structural variations to circumvent restriction by law were bioisosteric exchanges of functional groups in known CB₁ agonists. We analyzed the structure-activity relationships of compounds at human CB₁ and CB₂ receptors based on affinities obtained in radioligand binding studies, and determined their efficacy in cAMP accumulation assays. Moreover, we investigated the activities of the compounds at the orphan G protein-coupled receptors GPR18 and GPR55 both of which are known to interact with cannabinoids. Most of the investigated compounds behaved as potent full agonists of CB₁ and CB₂ receptors with affinities in the low nanomolar to subnanomolar concentration range. Some compounds were moderately potent GPR55 antagonists, while none interacted with GPR18. Most derivatives were predicted to cross the blood-brain barrier as determined by bioinformatics tools. These data are useful for assessing synthetic cannabinoids and will be helpful for predicting pharmacological properties of novel compounds that appear on the illicit drug market.

Keywords: GPR18; GPR55; Indazoles; Indoles; Structure-activity relationship; Synthetic cannabinoid.

Fig. 1

Structures and affinities of standard CB receptor agonists

Fig. 2

Functional properties of investigated compounds determined in cAMP accumulation assays, in the presence of forskolin (10 µM). Test concentration was 1 µM or 10 µM, depending on the determined K _i value. The selected concentration corresponds to the concentration at which a maximal effect was observed. All experiments were carried out three to five times, each in duplicate. a Compounds 4–18; b compounds 19–27; c compounds 28–24. All results were normalized to maximal receptor activation by the full agonist CP55,940 (2)

Fig. 3

Concentration-dependent inhibition of cAMP accumulation by XLR-12 (41). All experiments were carried out three to five times, each in duplicate

Fig. 4

a Affinities of investigated compounds at the CB₁ receptor plotted against logP values. b Affinities of investigated compounds at the CB₂ receptor plotted against logP values