Reduced Tolerance and Asymmetrical Crosstolerance to Effects of the Indole Quinuclidinone Analog PNR-4-20, a G Protein-Biased Cannabinoid 1 Receptor Agonist in Mice: Comparisons with Δ9-Tetrahydrocannabinol and JWH-018.

Most cannabinoid 1 receptor (CB₁R) agonists will signal through both G protein-dependent and -independent pathways in an unbiased manner. Recruitment of β-arrestin 2 desensitizes and internalizes receptors, producing tolerance that limits therapeutic utility of cannabinoids for chronic conditions. We developed the indole quinuclidinone (IQD) analog (Z)-2-((1-(4-fluorobenzyl)-1H-indol-3-yl)methylene)quinuclidin-3-one (PNR-4-20) as a novel G protein-biased agonist at CB₁Rs, and the present studies determine if repeated administration of PNR-4-20 produces lesser tolerance to in vivo effects compared with unbiased CB₁R agonists Δ⁹-tetrahydrocannabinol (Δ⁹-THC) and 1-pentyl-3-(1-naphthoyl)indole (JWH-018). Adult male National Institutes of Health Swiss mice were administered comparable doses of PNR-4-20 (100 mg/kg), Δ⁹-THC (30 mg/kg), or JWH-018 (3 mg/kg) once per day for five consecutive days to determine tolerance development to hypothermic, antinociceptive, and cataleptic effects. Persistence of tolerance was then determined after a drug abstinence period. We found that unbiased CB₁R agonists Δ⁹-THC and JWH-018 produced similar tolerance to these effects, but lesser tolerance was observed with PNR-4-20 for hypothermic and cataleptic effects. Tolerance to the effects of PNR-4-20 completely recovered after drug abstinence, while residual tolerance was always observed with unbiased CB₁R agonists. Repeated treatment with PNR-4-20 and Δ⁹-THC produced asymmetric crosstolerance to hypothermic effects. Importantly, binding studies suggest PNR-4-20 produced significantly less downregulation of CB₁Rs relative to Δ⁹-THC in hypothalamus and thalamus of chronically treated mice. These studies suggest that the G protein-biased CB₁R agonist PNR-4-20 produces significantly less tolerance than unbiased cannabinoid agonists, and that the IQD analogs should be investigated further as a novel molecular scaffold for development of new therapeutics.