AB-CHMINACA, AB-PINACA, and FUBIMINA: Affinity and Potency of Novel Synthetic Cannabinoids in Producing Δ⁹-Tetrahydrocannabinol–Like Effects in Mice

Abstract

Diversion of synthetic cannabinoids for abuse began in the early 2000s. Despite legislation banning compounds currently on the drug market, illicit manufacturers continue to release new compounds for recreational use. This study examined new synthetic cannabinoids, AB-CHMINACA (N-[1-amino-3-methyl-oxobutan-2-yl]-1-[cyclohexylmethyl]-1H-indazole-3-carboxamide), AB-PINACA [N-(1-amino-3-methyl-1-oxobutan-2-yl)-1-pentyl-1H-indazole-3-carboxamide], and FUBIMINA [(1-(5-fluoropentyl)-1H-benzo[d]imadazol-2-yl)(naphthalen-1-yl)methanone], with the hypothesis that these compounds, like those before them, would be highly susceptible to abuse. Cannabinoids were examined in vitro for binding and activation of CB₁ receptors, and in vivo for pharmacological effects in mice and in Δ⁹-tetrahydrocannabinol (Δ⁹-THC) discrimination. AB-CHMINACA, AB-PINACA, and FUBIMINA bound to and activated CB₁ and CB₂ receptors, and produced locomotor suppression, antinociception, hypothermia, and catalepsy. Furthermore, these compounds, along with JWH-018 [1-pentyl-3-(1-naphthoyl)indole], CP47,497 [rel-5-(1,1-dimethylheptyl)-2-[(1R,3S)-3-hydroxycyclohexyl]-phenol], and WIN55,212-2 ([(3R)-2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenyl-methanone, monomethanesulfonate), substituted for Δ⁹-THC in Δ⁹-THC discrimination. Rank order of potency correlated with CB₁ receptor-binding affinity, and all three compounds were full agonists in [³⁵S]GTPγS binding, as compared with the partial agonist Δ⁹-THC. Indeed, AB-CHMINACA and AB-PINACA exhibited higher efficacy than most known full agonists of the CB₁ receptor. Preliminary analysis of urinary metabolites of the compounds revealed the expected hydroxylation. AB-PINACA and AB-CHMINACA are of potential interest as research tools due to their unique chemical structures and high CB₁ receptor efficacies. Further studies on these chemicals are likely to include research on understanding cannabinoid receptors and other components of the endocannabinoid system that underlie the abuse of synthetic cannabinoids.

Introduction

In the 1960s, Raphael Mechoulam’s isolation and elucidation of the chemical structure of Δ⁹-tetrahydrocannabinol (Δ⁹-THC) (Gaoni and Mechoulam, 1964), the primary psychoactive substituent in Cannabis sativa, initiated a concerted effort directed at manipulation of its chemical structure, with one goal being to produce a compound with medicinal effects and no psychoactive properties. Synthesis of phytocannabinoid analogs was followed by development of bicyclic cannabinoids (e.g., CP55,940 [(−)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-trans-4-(3-hydroxypropyl)cyclohexanol]) and aminoalkylindoles [e.g., WIN55,212-2 ([(3R)-2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenyl-methanone, monomethanesulfonate)]. The effort was renewed with the discovery and initial characterization of the endocannabinoid system (Devane et al., 1988), which added the structural templates of arachidonic acid derivative agonists (anandamide and 2-arachidonoylglycerol) (Devane et al., 1992; Hanus et al., 2001) and a pyrazole antagonist, rimonabant (Rinaldi-Carmona et al., 1994). Structure-activity relationship studies focused on delineation of the ways in which these diverse chemical structures could bind to the two identified cannabinoid receptors (CB₁ and CB₂) and differentiation of features that might enhance selectivity for the CB₂ cannabinoid receptor. CB₁ receptor mediation of the marijuana-like psychoactive effects of cannabinoids was confirmed during this time (Wiley et al., 1995b), and the high correlation between binding affinity and potency for producing these psychoactive effects in mice was noted (Compton et al., 1993). The systematic synthesis of cannabinoids for use as research tools to probe the structure and functioning of the cannabinoid receptors or for use as lead candidates in medication development efforts continued to produce a multitude of novel synthetic cannabinoids (e.g., see Manera et al., 2008). After the initial publication of this medicinal chemistry research, much of it lay dormant, with the exception of occasional retrieval by scientists.

Then, in the 2000s, several of the previously reported compounds were identified in confiscated herbal incense labeled as “Spice” (Vardakou et al., 2010). Largely due to the rapid proliferation of information through the internet, the use of JWH-018 [1-pentyl-3-(1-naphthoyl)indole] and other research chemicals for their intoxicating effects was spreading throughout the world. Legal bans of the cannabinoids contained in early products resulted in the emergence of additional compounds not yet illegal, creating a type of “whack-a-mole” situation between drug control agencies and illicit manufacturers. For example, prevalence of JWH-018 faded as it was subsequently replaced by AM-2201 ([1-​(5-​fluoropentyl)-​1H-​indol-​3-​yl]-​1-​naphthalenyl-​methanone) and then by tetramethylcyclopropyl ketone indoles (XLR-11 [(1-​(5-​fluoropentyl)-​1H-​indol-​3-​yl)(2,​2,​3,​3-​tetramethylcyclopropyl)methanone] and UR-144 [(1-​pentyl-​1H-​indol-​3-​yl)(2,​2,​3,​3-​tetramethylcyclopropyl)-​methanone]). Most recently, two indazole carboxamide cannabinoids, AB-CHMINACA (N-[1-amino-3-methyl-oxobutan-2-yl]-1-[cyclohexylmethyl]-1H-indazole-3-carboxamide) and AB-PINACA [N-(1-amino-3-methyl-1-oxobutan-2-yl)-1-pentyl-1H-indazole-3-carboxamide], have achieved prominence, resulting in their temporary placement into Schedule I (i.e., compounds with high abuse potential and no accepted medical use) by the US Drug Enforcement Agency (Drug Enforcement Administration, Department of Justice, 2015). An associated problem with the rapid proliferation of synthetic cannabinoids is detection and identification of their metabolites in biologic fluids. This forensic information is often helpful for detection of use for the purposes of medical treatment, employee screening, or legal prosecution.

The purpose of the present study was to examine the in vitro and in vivo pharmacology of AB-CHMINACA and AB-PINACA (Fig. 1). FUBIMINA [(1-(5-fluoropentyl)-1H-benzo[d]imadazol-2-yl)(naphthalen-1-yl)methanone], an analog of a previously identified synthetic cannabinoid of abuse, AM-2201, was also evaluated (Fig. 1). Assessment centered on assays used to predict the abuse liability of cannabinoids (Wiley and Martin, 2009), including binding and activation of CB₁ receptors, pharmacological equivalence with Δ⁹-THC in a battery of four tests in mice, and Δ⁹-THC discrimination in mice. In the Δ⁹-THC discrimination procedure, results are also presented for synthetic cannabinoids from different chemical classes, including the prototypic indole-derived synthetic cannabinoid JWH-018, a bicyclic cannabinoid CP47,497 [rel-5-(1,1-dimethylheptyl)-2-[(1R,3S)-3-hydroxycyclohexyl]-phenol], and the aminoalkylindole WIN55,212-2. These representative compounds are from different chemical classes of cannabinoids that have been well characterized previously (Compton et al., 1992a,b; Wiley et al., 1998). They were tested in this work to provide a basis for comparison with the structurally innovative compounds shown in Fig. 1. To assist in the development of forensic markers and to examine metabolic transformations, preliminary analysis of urinary metabolites of the three novel synthetic cannabinoids also was undertaken.

Fig. 1.

Chemical structures of JWH-018, AM-2201, FUBIMINA, AB-CHMINACA, and AB-PINACA.

Materials and Methods

Animals

Adult drug naive male ICR mice (31–34 g; Harlan, Frederick, MD) and C57/BL6J mice (20–25 g; The Jackson Laboratory, Bar Harbor, ME) were used in the tetrad battery and drug discrimination experiments, respectively. All mice were housed singly in polycarbonate mouse cages in a temperature-controlled (20–22°C) environment with a 12-hour light/dark cycle (lights on at 6:00 AM). Water was freely available in the home cage. Whereas separate mice, with unlimited access to food, were used for testing each compound dose in the tetrad battery, mice in the drug discrimination experiments were maintained at 85–90% of free-feeding body weights by restricting daily ration of standard rodent chow and were tested repeatedly. At the start of this project, some of these mice had already been trained to discriminate Δ⁹-THC from vehicle; however, others were trained to discriminate Δ⁹-THC de novo. The in vivo studies reported in this manuscript were carried out in accordance with guidelines published in the Guide for the Care and Use of Laboratory Animals (National Research Council, 2011) and were approved by our Institutional Animal Care and Use Committee.

Apparatus

For the tetrad test battery in mice, measurement of spontaneous activity occurred in Plexiglas locomotor activity chambers (47 cm × 25.5 cm × 22 cm). Beam breaks (4 × 8 beam array) were recorded by San Diego Instruments Photobeam Activity System software (San Diego, CA) on a computer located in the experimental room. A standard tail flick device for rodents (Stoelting, Dale, IL) was used to assess antinociception. A digital thermometer (Physitemp Instruments, Clifton, NJ) was used to measure rectal temperature. The ring immobility device consisted of an elevated metal ring (diameter, 5.5 cm; height, 28 cm) attached to a metal stand.

Mice in the drug discrimination experiment were trained and tested in mouse operant chambers (Coulbourn Instruments, Whitehall, PA), housed within light- and sound-attenuating cubicles. Each chamber contained two retractable response levers or nose poke apertures, with stimulus lights located over each lever/aperture, and a separate house light. A food dispenser delivered 20-mg food pellets (Bioserv, Frenchtown, NJ) into a food cup (with a light) centered between the two levers/apertures. Illumination of lights, delivery of food pellets, and recording of lever presses or nose pokes were controlled by a computer-based system (Graphic State Software, version 3.03; Coulbourn Instruments).

Experimental Procedures

All in vitro and in vivo experimental procedures were similar to those described in our previous publication (Wiley et al., 2013), in which we described results of tests with two tetramethylcyclopropyl ketone indoles, XLR-11 and UR-144, that are also classified as synthetic cannabinoids of abuse.

Receptor Binding.

Transfected cell membrane preparations with human CB₁ (hCB₁) and human CB₂ (hCB₂) receptors (PerkinElmer, Waltham, MA) isolated from a HEK-293 expression system were used for cannabinoid-binding assays, as previously described (Zhang et al., 2010). Binding was initiated with the addition of 40 fmol cell membrane proteins to polypropylene assay tubes containing 0.62 nM [³H]CP55,940 (approximately 130 Ci/mmol), a test compound (for displacement studies), and a sufficient quantity of buffer A [50 mM Tris•HCl, 1 mM EDTA, 3 mM MgCl₂, 5 mg/ml bovine serum albumin (BSA), pH 7.4] to bring the total incubation volume to 0.5 ml. Nonspecific binding was determined by the inclusion of 10 μM unlabeled CP55,940. All cannabinoid agonists were prepared from a 10 mM ethanol stock by suspension in buffer A. Following incubation at 30°C for 1 hour, binding was terminated by vacuum filtration through GF/C glass fiber filter plates (PerkinElmer), pretreated in 0.1% (w/v) polyethyleneimine for at least 1 hour, in a 96-well sampling manifold (Brandel, Gaithersburg, MD). Reaction vessels were washed three times with ∼2 ml ice-cold buffer B (50 mM Tris•HCl, 1 mg/ml BSA). The filter plates were air-dried and sealed on the bottom. Liquid scintillate was added to the wells, and the top was sealed. Liquid scintillation spectrometry was used to measure radioactivity after incubating the plates in cocktail for at least 30 minutes. Assays were done in duplicate, and results represent combined data from three independent displacement curves.

Agonist-Stimulated [³⁵S]GTPγS Binding.

G protein–coupled signal transduction ([³⁵S]GTPγ) assays of test compounds were conducted in an incubation mixture consisting of a test compound (0.25 nM–20 µM), GDP (20 μM), [³⁵S]GTPγ (100 pM), and the hCB₁ and hCB₂ membrane preparations described above (40 fmol) in a total volume of 0.45 ml assay buffer [50 mM Tris-HCl, pH 7.4, 1 mM EDTA, 100 mM NaCl, 3 mM MgCl₂, 0.5% (w/v) BSA]. Nonspecific binding was determined in the presence of 100 μM unlabeled GTPγS, and basal binding was determined in the absence of drug. Duplicate samples were incubated for 1 hour at 30°C, and the bound complex was filtered from the reaction mixture, as described above, and counted in a liquid scintillation counter.

Mouse Tetrad.

Each mouse was tested in a tetrad of tests, in which cannabinoid agonists produce a profile of in vivo effects (Martin et al., 1991): suppression of locomotor activity, decreased rectal temperature, antinociception, and catalepsy. Prior to injection, baseline values were obtained for rectal temperature and in the tail flick test in each mouse. In the latter procedure, the mouse’s tail was placed under an intense light (radiant heat), and the latency to remove it (in seconds) was recorded. To minimize tail damage, a 10-second maximal latency was employed. After baseline measurements were taken, mice were injected intraperitoneally with vehicle or drug 30 minutes before being placed into individual activity chambers for a 10-minute session. Immediately upon removal from the chambers, tail-flick latency and rectal temperature were measured again, followed by placement on the elevated ring apparatus at 50 minutes postinjection. The amount of time that the animals remained motionless on the ring during a 5-minute period was recorded. If a mouse fell off the ring during the catalepsy test, it was immediately placed back on and timing was continued for up to nine falls. After the tenth fall, the test was terminated for the mouse.

At least 1 week after completion of all agonist tests, combinations of vehicle or 3 mg/kg rimonabant and active doses of each compound (56 mg/kg Δ⁹-THC, 3 mg/kg AB-CHMINACA, and 30 mg/kg AB-PINACA) were retested in a subset of the same mice. Procedural details were identical to those described above, with the exception that mice received an intraperitoneal injection of vehicle or rimonabant 10 minutes prior to intraperitoneal injection of the agonist test compound.

Because FUBIMINA was not active at doses up to 100 mg/kg i.p., a probe dose of 56 mg/kg (and vehicle) was administered intravenously to separate groups of mice. Evaluation in the tetrad tests proceeded as described above, with the exception that mice were placed into the locomotor chambers 5 minutes after injection and placed on the ring apparatus, 25 minutes postinjection. Subsequently, the effect of 3 mg/kg i.v. rimonabant in combination with the 56 mg/kg i.v. dose of FUBIMINA was assessed. Rimonabant was injected 10 minutes prior to FUBIMINA or vehicle.

Drug Discrimination.

Two groups of adult male mice were trained to discriminate Δ⁹-THC in standard operant chambers, as described previously (Vann et al., 2009). Mice were trained to press one of two levers following intraperitoneal administration of 5.6 mg/kg Δ⁹-THC and to press the other lever after intraperitoneal vehicle injection. Ten consecutive responses on the correct (injection-appropriate) lever resulted in delivery of a food pellet [i.e., fixed ratio 10 (FR10)], whereas responses on the incorrect lever reset the ratio requirement on the correct lever. A double alternation sequence of Δ⁹-THC and vehicle (e.g., drug, drug, vehicle, vehicle) was instituted. Fifteen-minute training sessions were held Monday–Friday until the mice consistently met three criteria, as follows: 1) the first completed FR10 was on the correct lever; 2) ≥80% of the total responding occurred on the correct lever; and 3) response rate was ≥0.17 responses/s.

Substitution tests began after the mice met acquisition criteria. These 15-minute tests usually occurred on Tuesdays and Fridays and were interspersed with training sessions on other weekdays. During test sessions, 10 consecutive responses on either lever delivered reinforcement. To be tested in the experiment, mice must have met the same three criteria as for acquisition on the preceding day and during the previous training session with the alternate training compound (training drug or vehicle). A dose-effect curve was determined with Δ⁹-THC in both groups of mice. The first group of mice was subsequently tested with several cannabinoid tricyclic dibenzopyran analogs (data not shown) and with two indole-derived synthetic cannabinoids (Wiley et al., 2013). In the present study, the first group of mice was tested with JWH-018, WIN55,212-2, and CP47,497, and a second dose-effect curve was determined Δ⁹-THC.

The second group of mice was tested with several noncannabinoid compounds (data not shown). Subsequently, their response requirement was changed from lever presses to nose pokes. This change was necessitated by transition of all of the laboratory’s mouse operant equipment to nose poke apertures and was not specifically related to this study. Additional mice were trained de novo on the nose poke response (using the acquisition procedure described above) and were combined with this second group of mice. With the exception of the actual response (nose poke versus lever press), all other procedural details remained the same (e.g., FR10, food reinforcement, testing criteria). After acquisition of the nose poke response, a dose-effect curve with Δ⁹-THC was determined (i.e., for de novo mice) or redetermined (i.e., for transition mice), followed by dose-effect curve determinations with AB-PINACA, AB-CHMINACA, and FUBIMINA.

Metabolite Analysis.

Twelve mice (n = 4 per drug) were given intraperitoneal injections of 3 mg/kg AB-PINACA, 3 mg/kg AB-CHMINACA, or 100 mg/kg FUBIMINA. Immediately following injections, the mice were placed into metabolism cages and urine was collected over a 24-hour period. Urine from mice dosed with the same compound was pooled for analysis. Samples were extracted using a salting-out liquid-liquid extraction method prior to analysis. Acetonitrile (200 µl) was added to 100 µL urine, and then the samples were vortexed and 50 µl 5 M ammonium acetate was added as a salting out agent. Samples were vortexed and centrifuged at 10,000 rcf for 5 minutes. The top aqueous layer was removed and dried down at 40°C and reconstituted with 50 µl mobile phase A.

Samples were analyzed on a Waters Acquity ultraperformance liquid chromatography system coupled to a Waters Synapt G2 HDMS quadrupole time-of-flight mass spectrometer (Waters, Milford, MA). The mass spectrometer was operated under resolution mode, positive electrospray ionization, source temperature of 150°C, desolvation temperature of 500°C, desolvation gas at 1000 l/h, capillary voltage at 2.99 kV, sampling cone at 35 V, and extraction cone at 4.3 V. The mass spectrometer was externally calibrated from 50 to 1000 m/z using a sodium formate solution. Leucine enkephalin was used as a lockmass to correct for mass shifts during acquisition. Full scan data were collected in both low (4 eV) and high (15–40 eV ramp) collision energies nearly simultaneously for every m/z using MS^E acquisition mode (Bateman et al., 2002).

Samples were separated on an Acquity BEH C18 column (1.7 μm 2.1 × 50 mm) connected to a Vanguard BEH C18 precolumn (1.7 µm × 2.1 × 5 mm) and held at 30°C. Injection volume was 10 µl. A gradient elution with a flow rate of 500 µl/min was used with mobile phase A consisting of water with 0.1% formic acid and mobile phase B consisting of acetonitrile with 0.1% formic acid. The mobile phase composition for AB-PINACA and AB-CHMINACA was held at 90% A for 1.5 minutes, decreased to 55% A over 15 minutes, decreased to 5% A over 3 minutes, and held at 90% A for 2.9 minutes for column re-equilibration. For FUBIMINA the gradient was held at 90% A for .5 minute, decreased to 85% A over 1 minute to 35% A over 15 minutes, then to 5% A over 3 minutes and held at 90% A for 2.9 minutes for column re-equilibration. For EG-18, the gradient was held at 90% A for 0.5 minute, decreased to 65% A over 1 minute and 35% A over 13 minutes, then decreased to 5% A over 5 minutes and held at 90% A for 2.9 minutes for column re-equilibration. All mobile phase composition changes were done linearly.

Drugs and Chemicals

Δ⁹-THC, JWH-018, CP47,497, and rimonabant (the prototypic CB₁ receptor antagonist/inverse agonist) were obtained from the National Institute on Drug Abuse (Bethesda, MD) through the National Institute on Drug Abuse Drug Supply Program. WIN55,212-2 was purchased commercially (Cayman Chemical, Ann Arbor, MI). AB-PINACA, AB-CHMINACA, and FUBIMINA were provided to RTI by the Drug Enforcement Administration. For the in vivo tests, the vehicle for all compounds was 7.8% Polysorbate 80 N.F. (VWR, Marietta, GA) and 92.2% sterile saline USP (Butler Schein, Dublin, OH). All compounds were injected at a volume of 10 ml/kg.

Guanosine-5′-diphosphate, BSA, ammonium acetate, and formic acid were purchased from Sigma-Aldrich (St. Louis, MO). GTPγS was purchased from Roche Diagnostics (Indianapolis, IN). [³⁵S]GTPγS (1150–1300 Ci/mmol) and scintillation fluid (MicroScint 20) were obtained from PerkinElmer Life Sciences (Boston, MA). High-performance liquid chromatography grade acetonitrile and water were purchased from Fisher Scientific (Fairlawn, NJ). Reference standards and metabolite reference standards for all compounds were obtained from Cayman Chemical (Ann Arbor, MI).

Data Analyses

Binding Data Analysis.

Specific binding was calculated by subtracting nonspecific binding from total binding for each concentration of displacing ligand. For displacement studies, curve-fitting and IC₅₀ calculation were done with GraphPad Prism (version 5; GraphPad Software, San Diego, CA), which fits the data to one- and two-site models and compares the two fits statistically. K_i values were estimated from IC₅₀ values using the Cheng-Prusoff equation.

Data for [³⁵S]GTPγS-binding experiments are reported as mean and S.E. of at least three replicates. Specific binding was calculated by subtracting nonspecific binding from total binding and dividing by the total basal binding minus nonspecific binding. Data were plotted and analyzed with GraphPad Prism. Nonspecific binding was subtracted from each sample. Net-stimulated [³⁵S]GTPγS binding was defined as agonist-stimulated minus basal [³⁵S]GTPγS binding, and percent stimulation was defined as (net-stimulated/basal [³⁵S]GTPγS binding) × 100%. Nonlinear iterative regression analyses of agonist concentration-effect curves were performed with GraphPad Prism. Significance was defined as P ≤ 0.05.

Separate factorial analyses of variance (ANOVAs; compound X receptor) were used to determine differences in k_i, EC₅₀, and B_max. Significant differences were further analyzed with Tukey post hoc tests (α = 0.05), as necessary.

Mouse Tetrad.

Spontaneous activity was measured as total number of photocell beam interruptions during the 10-minute session. For the purpose of potency calculation, it was expressed as percentage of inhibition of activity of the vehicle group. Antinociception was expressed as the percentage of maximum possible effect using a 10-second maximum test latency as follows: [(test − control)/(10 − control)] × 100. Rectal temperature values were expressed as the difference between control temperature (before injection) and temperature following drug administration (Δ°C). For catalepsy, the total amount of time (in seconds) that the mouse remained motionless on the ring apparatus (except for breathing and whisker movement) was used as an indication of catalepsy-like behavior. This value was divided by 300 seconds and multiplied by 100 to obtain a percent immobility. For compounds that produced one or more cannabinoid effects, ED₅₀ was calculated separately using least-squares linear regression on the linear part of the dose-effect curve for each measure in the mouse tetrad, plotted against log10 transformation of the dose. ED₅₀ was defined as the dose at which half-maximal effect occurred. Based on data obtained from numerous previous studies with cannabinoids, maximal cannabinoid effect in each procedure was estimated as follows: 100% inhibition of spontaneous activity, 100% maximum possible effect in the tail flick, −6°C change in rectal temperature, and 100% ring immobility. Separate between-subjects ANOVAs were also used to analyze the four measures for each compound. Significant differences from control (vehicle) were further analyzed with Tukey post hoc tests (α = 0.05), as necessary. Factorial ANOVAs (rimonabant dose × compound dose) were used to analyze results of antagonist tests. Significant main effects and interactions were further analyzed with Tukey post hoc tests (α = 0.05), as necessary.

Drug Discrimination.

For each session, percentage of responses on the drug-associated manipulandum and response rate (responses/s) were calculated. Full substitution was defined as ≥80% responding on the drug-associated manipulandum (Vann et al., 2009). ED₅₀ values were calculated on the linear part of the drug manipulandum selection dose-response curve for each drug using least squares linear regression analysis, followed by calculation of 95% confidence intervals (CI). Because mice that responded less than 10 times during a test session did not respond on either manipulandum a sufficient number of times to earn a reinforcer, their data were excluded from analysis of drug manipulandum selection, but their response rate data were included. Response-rate data were analyzed using repeated-measures ANOVA across dose. Significant ANOVAs were further analyzed with Tukey post hoc tests (α = 0.05) to specify differences between means.

Metabolite Identification.

Liquid chromatography/mass spectrometry data were analyzed using Waters MassLynx 4.1 with the aid of the MetaboLynx application manager. Automated data processing with MetaboLynx was supplemented by manual interrogation of the data using mass defect filtering, precursor ion, and fragment ion searching techniques (Grabenauer et al., 2012). Presence of potential metabolites was determined by exact mass match to predicted elemental compositions in the low energy data function. Further refinement of the site of modification was determined by presence of characteristic fragment ions at the same retention time. Metabolites were provisionally identified by their molecular weight, retention time, and fragment ions. Metabolites were compared with reference standards as available.

Results

Cannabinoid Receptor Binding and Agonist-Stimulated [³⁵S]GTPγS Binding.

All three test compounds displaced [³H]CP55,940 at the CB₁ receptor binding site (Fig. 2A), but with varying affinities that ranged from inhibition constants (k_i) 0.59 nM for CP55,940 to 296 nM for FUBIMINA (Table 1). In contrast, only two of the three test compounds (AB-CHMINACA and AB-PINACA) stimulated [³⁵S]GTPγ turnover with reasonable potency (Fig. 2B). Rank order of potency was AB-CHMINACA > CP55,940 > AB-PINACA (Table 1). Furthermore, AB-CHMINACA and AB-PINACA exhibited enhanced efficacy compared with CP55,940 (Table 1) [drug X receptor interaction for B_max: F(3,46) = 10.95, P < 0.05]. Although FUBIMINA also stimulated [³⁵S]GTPγ turnover with efficacy comparable to that obtained with the positive control CP55,940 (Fig. 2B), it did so only at very high concentrations (Table 1), suggesting it serves as a weakly potent agonist at the CB₁ receptor.

Fig. 2.

Effects of CP55,940 (filled squares), AB-PINACA (unfilled squares), AB-CHMINICA (filled circles), and FUBIMINA (unfilled circles) on [³H]CP55,940 displacement (A) and [³⁵S]GTPγS turnover (B) in hCB₁ receptors expressed in HEK-293 cells. Effects of CP55,940 (filled squares), AB-PINACA (unfilled squares), AB-CHMINICA (filled circles), and FUBIMINA (unfilled circles) on [³H]CP55,940 displacement (C) and [³⁵S]GTPγS turnover (D) in hCB₂ receptors expressed in HEK-293 cells. Each concentration-effect curve represents the mean (±S.E.M.) of three to six repetitions.

TABLE 1.

Binding affinity and potency and efficacy for stimulation of [³⁵S]GTPγ turnover at hCB₁ and hCB₂ receptors

For each measure in all columns, n is shown in italics below the S.E.M.

Compound	CB1 Kia	[35S]GTPγ Turnover		CB2 Kia	[35S]GTPγ Turnover
		CB1 EC50b	CB1 Emaxc		CB2 EC50b	CB2 Emaxc
CP55,940	0.59	23.3	124	0.30	2.1	63
	(0.06)	(4.7)	(9)	(0.04)	(0.8)	(4)
	7	12	12	6	10	10
AB-CHMINACA	0.78	7.4	205	0.45	232.4	35
	(0.11)	(1.5)	(14)	(0.03)	(231.2)	(2)
	3	6	6	3	4	4
AB-PINACA	2.87	71	192	0.88	14.9	41
	(0.69)	(20.9)	(25)	(0.00)	(8.4)	(1)
	3	6	6	3	4	4
FUBIMINA	296.1	2466.3	122	23.45	1269.3	56
	(33.5)	(1037.2)	(7)	(3.21)	(557.1)	(9)
	4	6	6	3	6	6

^aValues represent K_i (±S.E.M.) in nM for [³H]CP55,940 displacement at specified (hCB₁ or hCB₂) receptor.

^bValues represent EC₅₀ (±S.E.M.) in nM for [³⁵S]GTPγS binding at specified (hCB₁ or hCB₂) receptor.

^cValues represent percentage of maximal increase (±S.E.M.) for [³⁵S]GTPγS binding over basal at specified (hCB₁ or hCB₂) receptor.

Similar to their effects at the CB₁ receptor, all three test compounds displaced [³H]CP55,940 at the CB₂ receptor binding site (Fig. 2C). Furthermore, affinities of all compounds for the CB₂ receptor exceeded those obtained at the CB₁ receptor by 1.7- to 12.6-fold (Table 1) [main effect of receptor type for k_i: F(1,24) = 60.37, P < 0.05]. In contrast with the greater affinities of the test compounds for the CB₂ receptor, they showed reduced efficacy at this receptor compared with the CB₁ receptor (Fig. 2D) [main effect of receptor type for B_max: F(1,46) = 169.91, P < 0.05]. Whereas CP55,940 and FUBIMINA produced similar efficacies at the CB₂ receptor, efficacies for AB-PINACA and AB-CHMINACA were somewhat lower (Table 1). Potencies for stimulating [³⁵S]GTPγ turnover at the CB₂ receptor varied greatly across the compounds (Table 1). Whereas AB-PINACA was approximately sevenfold less potent than CP55,940, AB-CHMINICA and FUBIMINA were 111- and 604-fold less potent than CP55,940 (Table 1) [main effect of compound for EC₅₀: F(3,46) = 10.58, P < 0.05].

Mouse Tetrad Effects.

For all ANOVAs performed on tetrad test data, significant F values are presented in figure legends of the appropriate figures. Δ⁹-THC, AB-CHMINACA, and AB-PINACA exhibited the complete profile of cannabinoid effects in the tetrad tests in mice, with each compound producing dose-dependent suppression of spontaneous activity, antinociception, hypothermia, and ring immobility (Fig. 3, A–D, respectively). Across the four tests, AB-CHMINACA was 11- to 58-fold more potent than Δ⁹-THC, whereas AB-PINACA was 2- to 14-fold more potent than Δ⁹-THC (Table 2). With the exception of the effect of Δ⁹-THC on spontaneous activity, one or more doses of Δ⁹-THC, AB-CHMINACA, and AB-PINACA significantly affected each measure. Although Δ⁹-THC showed a trend toward decreased spontaneous activity, the effect did not reach statistical significance. In addition to its effects on the tetrad measures, 30 mg/kg AB-PINACA also produced convulsions, flattened body posture (splayed limbs), and labored breathing in most mice within 1 minute after intraperitoneal injection. By the end of tetrad testing, mice had started to recover and were walking around their home cages.

Fig. 3.

Effects of Δ⁹-THC (filled squares), AB-PINACA (unfilled squares), AB-CHMINACA (filled circles), and FUBIMINA (unfilled circles) on locomotor activity (A), antinociception (B), rectal temperature (C), and catalepsy (D). Values represent the mean (±S.E.M.) of six male ICR mice, with the exception that n = 5 for ring immobility for the 100 mg/kg dose of FUBIMINA due to data excluded for one mouse that fell off the ring 10 times. Asterisks (*) indicate significant differences (P < 0.05) compared with respective vehicle (at left side of each panel). Significant F values are as follows: (A) AB-PINACA [F(6,35) = 6.45, P < 0.05] and AB-CHMINACA [F(5,30) = 9.18, P < 0.05]. (B) Δ⁹-THC [F(3,20) = 21.69, P < 0.05], AB-PINACA [F(6,35) = 11.97, P < 0.05], AB-CHMINACA [F(5,30) = 23.43, P < 0.05], and FUBIMINA [F(3,20) = 7.27, P < 0.05]. (C) Δ⁹-THC [F(3,20) = 63.92, P < 0.05], AB-PINACA [F(6,35) = 56.87, P < 0.05], and AB-CHMINACA [F(5,30) = 23.09, P < 0.05]. (D) Δ⁹-THC [F(3,20) = 27.30, P < 0.05], AB-PINACA [F(6,35) = 11.75, P < 0.05], and AB-CHMINACA [F(5,30) = 17.48, P < 0.05].

TABLE 2.

Potencies in the tetrad tests

Values represent ED₅₀ (±95% CIs) in μmol/kg. All compounds were administered intraperitoneally.

Compound	Tetrad Tests ED50
	SA	MPE	RT	RI
mol. wt.	μmol/kg
Δ9-THC	104	34	30	30
(314)	(51–216)	(20–58)	(23–39)	(17–53)
AB-CHMINACA	1.8	2.0	1.1	2.7
(356)	(0.7–4.5)	(1.3–3.0)	(0.7–1.6)	(1.9–3.9)
AB-PINACA	7.6	13.7	5.3	13.9
(330)	(3.3–17.4)	(5.5–34.2)	(3.5–8.0)	(6.8–28.4)
FUBIMINA	Not active	Not active	Not active	Not active
(360)	(>278)	(>278)	(>278)	(>278)

MPE, percentage of maximum possible effect in tail flick test; RI, ring immobility; RT, change in rectal temperature in °C; SA, percentage of inhibition of spontaneous activity.

In contrast, FUBIMINA did not affect spontaneous activity or rectal temperature and did not produce ring immobility at intraperitoneal doses up to 100 mg/kg (Fig. 3). Although i.p. doses of 30 and 100 mg/kg FUBIMINA produced statistically significant increases in antinociception, the magnitude of these increases was small and did not approach the maximal effects observed with Δ⁹-THC, AB-CHMINACA, and AB-PINACA. Despite measurable affinity for the CB₁ receptor, FUBIMINA did not exhibit cannabinoid effects in the tetrad tests following intraperitoneal injection. Consequently, a dose of 56 mg/kg i.v. FUBIMINA was tested (Fig. 4). At this dose, FUBIMINA suppressed locomotor activity and produced antinociception, hypothermia, and catalepsy (Fig. 4, A–D, right, respectively), albeit the magnitudes of its antinociceptive and hypothermic effects were somewhat less than a comparable intraperitoneal dose of Δ⁹-THC.

Fig. 4.

The left side of each panel shows the effects of intraperitoneal injections of vehicle, Δ⁹-THC (56 mg/kg), AB-PINACA (30 mg/kg), and AB-CHMINACA (3 mg/kg) tested in combination with vehicle (unfilled bars) or 3 mg/kg rimonabant (filled bars). The right side of each panel shows the effects of intravenous injections of vehicle and FUBIMINA (56 mg/kg) tested in combination with vehicle (unfilled bars) or 3 mg/kg rimonabant (filled bars). Dependent measures are spontaneous activity (A), antinociception (B), rectal temperature (C), and catalepsy (D). Values represent the mean (±S.E.M.) of six mice per group. Dollar signs ($) indicate significant main effects (P < 0.05) of compound dose, as compared with vehicle. Asterisks (*) and number symbols (#) indicate significant interactions (with post hoc confirmation of difference) between compound (P < 0.05), relative to the vehicle/vehicle condition or the compound plus vehicle condition, respectively. Significant F values are as follows: (A) Δ⁹-THC, AB-PINACA, and AB-CHMINACA [main effect: F(3,40) = 7.81, P < 0.05]; FUBIMINA [main effect: F(1,20) = 31.80, P < 0.05]. (B) Δ⁹-THC, AB-PINACA, and AB-CHMINACA [interaction: F(3,40) = 15.28, P < 0.05]; FUBIMINA [interaction: F(1,20) = 14.44, P < 0.05]. (C) Δ⁹-THC, AB-PINACA, and AB-CHMINACA [interaction: F(3,40) = 5.66, P < 0.05]; FUBIMINA [interaction: F(1,20) = 23.30, P < 0.05]. (D) Δ⁹-THC, AB-PINACA, and AB-CHMINACA [interaction: F(3,40) = 28.55, P < 0.05]; FUBIMINA [interaction: F(1,20) = 19.74, P < 0.05].

Figure 4 also shows the results of antagonism tests. Alone, intraperitoneal doses of 56 mg/kg Δ⁹-THC, 30 mg/kg AB-PINACA, and 3 mg/kg AB-CHMINACA significantly suppressed locomotor activity (Fig. 4A) and produced antinociception (Fig. 4B), hypothermia (Fig. 4C), and catalepsy (Fig. 4D). In each instance, these effects were attenuated by coadministration of 3 mg/kg i.p. rimonabant. Rimonabant (3 mg/kg i.v.) also attenuated the cannabinoid effects of 56 mg/kg FUBIMINA (Fig. 4).

Drug Discrimination in Mice.

Mice trained to lever press for food reward in a Δ⁹-THC discrimination procedure showed full dose-dependent substitution for the 5.6 mg/kg Δ⁹-THC training dose (Fig. 5, top), with a potency similar to that obtained with mice trained in the nose poke procedure (Table 3). The aminoalkylindole WIN55,212-2, a bicyclic cannabinoid CP47,497, and the prototypic indole-derived synthetic cannabinoid JWH-018 also fully and dose-dependently substituted for Δ⁹-THC (Fig. 5, top). Rank order of potency for substitution was JWH-018 > CP47,497 > WIN55,212-2 > Δ⁹-THC (Table 3). Whereas CP47,497 produced response rate decreases at higher doses [F(4,28) = 9.18, P < 0.05], Δ⁹-THC and JWH-018 did not (Fig. 5, bottom), although the highest dose of JWH-018 tested (1 mg/kg) was relatively low compared with those of the other compounds. WIN55,212-2 substantially decreased response rates; however, the small number of mice tested prevented attainment of significance.

Fig. 5.

Effects of Δ⁹-THC (filled squares), WIN55,212-2 (unfilled squares), JWH-018 (filled circles), and CP47,497 (unfilled circles) on percentage of responses that occurred on the Δ⁹-THC–associated lever (Top) and response rate (Bottom). Each point represents the mean (±S.E.M.) of data for male C57/BL6J mice: all doses of Δ⁹-THC (n = 6–7); all doses of JWH-018 (n = 11); at doses of 0.1, 0.3, and 1 mg/kg CP47,497 (n = 8) and WIN55,212-2 (n = 3); at 3 mg/kg CP47,497 (n = 5 for % Δ⁹-THC-lever responding and n = 6 for response rate); at 5.6 mg/kg CP47,497 (n = 1 for % Δ⁹-THC-lever responding and n = 2 for response rate); at 3 mg/kg WIN212-2 (n = 2 for % Δ⁹-THC-lever responding and n = 3 for response rate); at 5.6 mg/kg WIN55,212-2 (n = 1) for response rate. Asterisks (*) indicate significant differences (P < 0.05) compared with vehicle.

TABLE 3.

Potencies for substitution in Δ⁹-THC discrimination

ED₅₀ (±95% CIs) are expressed in μmol/kg. Molecular weights are provided in parentheses. All compounds were administered intraperitoneally.

Compound	CB1 Ki	ED50
	nM	μmol/kg
THC DD (lever)
Δ9-THC (n = 7)	41a	4.5
(314)	(2)	(3.5–5.7)
JWH-018 (n = 11)	9.5a	0.35
(341)	(4.5)	(0.32–0.50)
CP47,947 (n = 8)	9.5b	0.85
(318)	(0.35)	(0.47–1.45)
WIN 55,212-2 (n = 3)	1.9a	0.80
(522)	(0.1)	(0.46–1.38)
THC DD (nose poke)
Δ9-THC (n = 20)	41a	5.7
(314)	(2)	(4.8–6.7)
AB-CHMINACA (n = 8)	0.78	0.34
(356)	(0.11)	(0.22–0.50)
AB-PINACA (n = 9)	2.87	3.78
(330)	(0.69)	(2.42–5.87)
FUBIMINA (n = 6)	296.1	131
(360)	(33.5)	(72–233)

DD, drug discrimination.

^aShowalter et al., 1996.

^bCompton et al., 1993.

Δ⁹-THC also produced full dose-dependent substitution for the 5.6 mg/kg Δ⁹-THC training dose in all mice responding in the nose poke procedure (Fig. 6A), regardless of whether they had received initial training in Δ⁹-THC discrimination with a different response topography (i.e., transition from lever presses to nose pokes; ED₅₀, 5.4 μmol/kg; 95% CI, 4.8–5.7 μmol/kg) or had been trained de novo with the nose poke response (ED₅₀, 5.7 μmol/kg; 95% CI, 4.5–7.6 μmol/kg). Furthermore, Δ⁹-THC did not alter response rates (compared with vehicle) in any of the groups across the dose range tested (Fig. 6B). For the purpose of comparison with the test compounds, data from all mice trained to discriminate Δ⁹-THC using the nose poke response were combined, with a resulting ED₅₀, 5.7 μmol/kg (95% CI, 4.8–6.7 μmol/kg) (Fig. 6C; Table 3). Response rates were significantly increased by 3 mg/kg Δ⁹-THC [F(5,95) = 5.97, P < 0.05], but significant decreases (compared with vehicle) were not observed at any dose (Fig. 6D). During all control tests with vehicle and 5.6 mg/kg Δ⁹-THC across the course of the study, mice responded predominantly on the vehicle- and Δ⁹-THC–associated apertures, respectively (Fig. 6, A and C, left).

Fig. 6.

(Left) Effects of Δ⁹-THC on percentage of responses that occurred on the Δ⁹-THC–associated aperture (A) and response rate (B) in mice trained to discriminate Δ⁹-THC from vehicle in a nose poke procedure following transition from a lever press procedure (filled squares) or de novo (unfilled squares). Each point represents the mean (±S.E.M.) of data for seven male C57/BL6J mice for the transitional group and n = 11–13 for the de novo group. (Right) Effects of Δ⁹-THC (filled squares), AB-PINACA (unfilled squares), AB-CHMINACA (filled circles), and FUBIMINA (unfilled circles) on percentage of responses that occurred on the Δ⁹-THC–associated aperture (C) and response rate (D). Each point represents the mean (±S.E.M.) of data for 18–20 male C57/BL6J mice for Δ⁹-THC and n = 6–9 for the synthetic compounds, except for % Δ⁹-THC–associated aperture responding at 3 mg/kg AB-PINACA (n = 2) and 1 mg/kg AB-CHMINACA (n = 1). Asterisks (*) indicate significant differences compared with respective vehicle.

As shown in Fig. 6C, AB-CHMINACA and AB-PINACA produced full, dose-dependent substitution for Δ⁹-THC. Both compounds were more potent than Δ⁹-THC by 16- and 1.5-fold for AB-CHMINACA and AB-PINACA, respectively (Table 3). Of the two test compounds, the effects of AB-CHMINACA bore the most resemblance to those of Δ⁹-THC, in that the lowest dose producing full substitution (0.3 mg/kg) did not affect response rates (Fig. 6D). Although full substitution also occurred at 1 mg/kg AB-CHMINACA, this effect was accompanied by an overall reduction in response rates [F(3,21) = 36.09, P < 0.05]. In fact, of the seven mice tested at this dose, only one responded on either aperture during the entire session.

The profile of AB-PINACA was distinct from that produced by Δ⁹-THC and AB-CHMINACA, in that full substitution was observed only at a dose (3 mg/kg; Fig. 6C) that also severely reduced response rates [F(4,32) = 16.48, P < 0.05] (Fig. 6D). Only two of nine mice tested with this dose responded on either aperture. Furthermore, the response rate dose-effect function was steep, with no effect at 1.7 mg/kg and nearly complete suppression at a dose (3 mg/kg) only 1/2 log higher (Fig. 6D).

Results of substitution tests with FUBIMINA revealed considerable variability in choice of aperture across the mice. Although FUBIMINA did not fully substitute for Δ⁹-THC (i.e., >80% Δ⁹-THC aperture responding), dose-dependent increases in responding on the Δ⁹-THC aperture (partial substitution) were observed (Fig. 6C; Table 3). FUBIMINA did not significantly affect response rates (Fig. 6D).

Metabolite Identification.

In urine from mice administered AB-PINACA, only monohydroxylations and their corresponding glucuronide conjugates were observed. Three distinct monohydroxylated metabolites were observed, none of which matched the retention times of 4-hydroxy AB-PINACA or 5-hydroxy AB-PINACA reference standards. Fragment ions observed for one of the hydroxylated metabolites were m/z 231, m/z 302, and m/z 330, where m/z 231 is indicative of hydroxylation located on the 1-pentyl-1H-indazole moiety with an attached carbonyl. Other metabolites were identified in in vitro AB-PINACA and confirmed in human urine specimens (Wohlfarth et al., 2015), but not observed in the present study.

Similar to AB-PINACA, a single hydroxylated metabolite and its corresponding glucuronide conjugate were identified in the urine from mice dosed with AB-CHIMINACA. Characteristic fragments observed at both retention times were m/z 328, m/z 356, and m/z 257. The fragment ion at m/z 257 is indicative of hydroxylation on the 1-(cyclohexylmethyl)-1H-indazole moiety with an attached carbonyl. At the time of analysis, no reference standard was available for hydroxylated AB-CHIMINACA; however, Cayman Chemical AB-CHIMINACA metabolites M2, M3A, M4, M5A, and M6 were run as reference standards and were not observed in the in vivo sample.

A summary of the metabolites found in urine from mice dosed FUBIMINA is shown in Table 4, in which I represents the phase I metabolites and II represents the phase II glucuronide conjugates. Unlike for AB-PINACA and AB-CHIMINACA, nonmetabolized, intact parent compound was observed in the pooled in vivo urine sample. Major metabolites identified were multiple monohydroxylations and their corresponding glucuronide conjugates (Fig. 7). Hydroxylation for FUBIMINA was confirmed to occur on the 1-(5-fluoropentyl)-1H-benzimidazole moiety by the presence of a fragment ion at m/z 249 (with the carbonyl attached). Several of the hydroxylated metabolites also had a fragment ion at m/z 161, indicating that hydroxylation was on the benzimidazole moiety. Hydroxylation was also observed on the naphthyl moiety, as determined by the presence of fragment ions at m/z 171, m/z 143, and m/z 233.

TABLE 4.

Metabolic transformations observed in pooled urine from mice dosed with FUBIMINA

Metabolite	FUBIMINA
	I	II
Parent	X
Monohydroxylation	X	X
Dihydrodiol formation	X	X
Saturation	X
Hydration	X	X
Defluorination + hydroxylation	X	X
Defluorination + carboxylation	X
Defluorination + carboxylation + hydroxylation	X

Fig. 7.

Structure of FUBIMINA showing characteristic fragment masses for monohydroxylated metabolites. Hydroxylation was confirmed on the 1-(5-fluoropentyl)-1H-benzimidazole moiety by the presence of a fragment ion at m/z 249. Hydroxylation was also confirmed on the benzimidazole moiety by the presence of a fragment ion at m/z 161 and on the naphthyl moiety by the presence of fragment ions at m/z 171, m/z 143, and m/z 233.

Discussion

Psychoactive cannabinoid agonists produce a characteristic profile of in vitro and in vivo pharmacological effects, including binding to and activating CB₁ receptors, dose-dependent activity in a tetrad battery of tests in mice, and Δ⁹-THC–like discriminative stimulus effects (Wiley and Martin, 2009). In the present study, the in vitro positive control CP55,940 showed low nM K_i for both CB₁ and CB₂ receptors, similar to a number of indole and pyrrole-derived synthetic cannabinoids (Huffman and Padgett, 2005; Wiley et al., 2014a). Furthermore, it stimulated [³⁵S]GTPγS turnover with high potency and efficacy at both receptors, suggesting that CP55,940 would act as a potent CB₁ receptor agonist in vivo. Indeed, this prediction has proved true, as CP55,940 produces cannabimimetic effects in the tetrad battery in mice (Compton et al., 1992b) and substitutes and cross-substitutes for Δ⁹-THC in drug discrimination in rats (Gold et al., 1992; Wiley et al., 1995a). Similarly, the in vivo positive control Δ⁹-THC suppressed locomotor activity and showed hypothermic, antinociceptive, cataleptic, and Δ⁹-THC–like discriminative stimulus effects in mice. Δ⁹-THC–like discriminative stimulus effects were also observed with JWH-018, WIN55,212-2, and CP47,497, as has been shown previously with the former two compounds (Compton et al., 1992a; Wiley et al., 2014b) and with the C-8 homolog of CP47,497 (Gatch and Forster, 2014). Previous research has shown that these three compounds bind to the CB₁ receptor with high affinity and produce cannabimimetic effects in the tetrad battery (Compton et al., 1992a,b; Wiley et al., 1998). Together, these data show that representative compounds from the tetrahydrocannabinol, bicyclic, and aminoalkylindole cannabinoid classes produce similar in vivo and in vitro pharmacological profiles in these assays.

In contrast, the three novel synthetic cannabinoids tested in this study produced distinct profiles in the battery of in vitro and in vivo assays, with quantitative differences compared with typical cannabinoid effects. The profiles of AB-CHMINACA and FUBIMINA most closely matched those obtained previously with other synthetic cannabinoids, differing only quantitatively in their respective affinities, potencies, and efficacies. Similar to CP55,940 (present study) and other full dual CB₁/CB₂ agonists (Huffman and Padgett, 2005), AB-CHMINACA and FUBIMINA displaced [³⁵H]CP55,940 from both cannabinoid receptor types. Although both compounds showed higher CB₂ receptor affinity, FUBIMINA exhibited several-fold greater selectivity than AB-CHMINACA or CP55,940. Both compounds also activated CB₁ receptors with full efficacy comparable to (FUBIMINA) or greater than (AB-CHMINACA) that produced by CP55,940 (present study) or other full agonists such as WIN55,212-2 (Griffin et al., 1998), albeit FUBIMINA showed limited potency that was consistent with its lower binding affinity. Consistent with the magnitudes of their respective CB₁ receptor affinities, AB-CHMINACA produced the full profile of cannabinoid effects in the tetrad battery, whereas FUBIMINA was inactive when administered intraperitoneally and produced a cannabimimetic profile only at a 56 mg/kg i.v. dose. Tetrad effects of AB-CHMINACA and FUBIMINA were attenuated by rimonabant, suggesting CB₁ receptor mediation. In Δ⁹-THC discrimination, 0.3 mg/kg AB-CHMINACA, a dose that did not affect response rates, fully and potently substituted for Δ⁹-THC. This pattern of results resembles that obtained with the control compounds from different chemical classes of cannabinoids (see Fig. 6). In contrast, FUBIMINA only partially substituted for Δ⁹-THC and did so with considerable intrasubject variability. Although tested up to the limits of solubility, FUBIMINA failed to decrease response rates. Together, these results suggest that AB-CHMINACA is a potent and efficacious psychoactive CB₁ receptor agonist that is likely to possess abuse liability in humans, a finding that has been implicitly supported through its recent placement in Schedule I (Drug Enforcement Administration, Department of Justice, 2015). FUBIMINA also appears to share cannabimimetic effects with Δ⁹-THC; however, its low potency may limit its abuse, as illicit manufacturers tend to focus on compounds with greater CB₁ receptor affinity and high potency. Nevertheless, FUBIMINA has been detected in products in Japan (Uchiyama et al., 2014), and, with sufficient concentrations, cannabimimetic effects are likely. Metabolic transformations observed for these compounds were similar to those described for many similarly structured synthetic cannabinoids and primarily consisted of monohydroxylation and glucuronide conjugation, with FUBIMINA also undergoing defluorination, as is typical for synthetic cannabinoids with a 5-fluoropentyl group (e.g., AM-2201).

Although in vitro results showed that AB-PINACA resembled AB-CHMINACA in its high affinities for CB₁ and CB₂ receptors and its high efficacy for stimulation of CB₁ receptors, differences between their profiles emerged in the in vivo experiments. Both compounds produced rimonabant-reversible effects in the complete tetrad battery; however, administration of 30 mg/kg i.p. AB-PINACA was accompanied by short-lived convulsive behavior, an effect that we do not typically observe with Δ⁹-THC or other cannabinoids at doses that produce tetrad effects. In the Δ⁹-THC discrimination procedure, AB-PINACA substituted fully and dose-dependently for Δ⁹-THC, but full substitution was achieved only at a dose that was accompanied by substantial decreases in response rate, with only a small percentage (22%) of mice responding at this dose. Previously, we have observed response rate decreases only with doses of other synthetic cannabinoids that were suprathreshold for full substitution (Fig. 5; Wiley et al., 1995a, 2013). Hence, AB-PINACA’s lack of separation between doses that were Δ⁹-THC-like and those that substantially suppressed responding was unusual compared with the profile seen with other synthetic cannabinoids. These results suggest that AB-PINACA is a potent psychoactive CB₁ receptor agonist, but they also suggest that the doses that induce intoxication may be very close to (or indistinguishable from) doses associated with behavioral toxicity.

In summary, synthetic cannabinoids that were originally developed as research tools or as candidate medications have been diverted to drugs of abuse in the form of products labeled with such terms as “herbal incense,” “fake weed,” “spice,” and “K2.” AB-CHMINACA, AB-PINACA, and FUBIMINA are among the chemicals that have been identified in recent confiscations. The results of the present study demonstrate that the pharmacological effects of AB-CHMINACA, AB-PINACA, and FUBIMINA overlap with those of psychoactive cannabinoids from different chemical classes, including Δ⁹-THC, JWH-018, CP47,497, and WIN55,212-2. Each of these three compounds binds to and activates CB₁ and CB₂ cannabinoid receptors, produces a characteristic tetrad of cannabimimetic effects in mice, and produces dose-dependent increases in responding on the Δ⁹-THC–associated aperture in Δ⁹-THC discrimination. A primary difference among the compounds is their potency, with rank order of potency being correlated with their CB₁ receptor-binding affinities: FUBIMINA < Δ⁹-THC < AB-PINACA < AB-CHMINACA. Notably, all three of these compounds are high efficacy agonists in the [³⁵S]GTPγS-binding assay, as compared with the low partial agonism of Δ⁹-THC. Ironically, AB-PINACA and AB-CHMINACA are of potential interest to the scientific community as research tools due to their unique chemical structures and their high CB₁ receptor efficacies (i.e., >CP55,940). Further use of these chemicals is likely to include greater emphasis on the original purpose for which they were developed: research with a primary goal of increased understanding of cannabinoid receptors and other components of the endocannabinoid system that underlie the abuse of plant-derived and synthetic cannabinoids.

Abbreviations

AB-CHMINACA

N-[1-amino-3-methyl-oxobutan-2-yl]-1-[cyclohexylmethyl]-1H-indazole-3-carboxamide

AB-PINACA

N-(1-amino-3-methyl-1-oxobutan-2-yl)-1-pentyl-1H-indazole-3-carboxamide

AM-2201

[1-​(5-​fluoropentyl)-​1H-​indol-​3-​yl]-​1-​naphthalenyl-​methanone

ANOVA

analysis of variance

BSA

bovine serum albumin

CI

confidence interval

CP47,497

rel-5-(1,1-dimethylheptyl)-2-[(1R,3S)-3-hydroxycyclohexyl]-phenol

CP55,940

(−)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-trans-4-(3-hydroxypropyl)cyclohexanol

FR10

fixed ratio 10

FUBIMINA

(1-(5-fluoropentyl)-1H-benzo[d]imadazol-2-yl)(naphthalen-1-yl)methanone

hCB

human CB

JWH-018

1-pentyl-3-(1-naphthoyl)indole

Δ⁹-THC

Δ⁹-tetrahydrocannabinol

UR-144

(1-​pentyl-​1H-​indol-​3-​yl)(2,​2,​3,​3-​tetramethylcyclopropyl)-​methanone

WIN55,212-2

[(3R)-2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenyl-methanone, monomethanesulfonate

XLR-11

(1-​(5-​fluoropentyl)-​1H-​indol-​3-​yl)(2,​2,​3,​3-​tetramethylcyclopropyl)methanone

Authorship Contributions

Participated in research design: Wiley, Thomas, Lefever.

Conducted experiments: Antonazzo, Cortes, Moore, Patel, Wallgren.

Performed data analysis: Wiley, Thomas, Grabenauer, Moore.

Wrote or contributed to the writing of the manuscript: Wiley, Thomas, Marusich, Lefever, Grabenauer, Moore.