Introduction
CB-13 is a synthetic cannabinoid that has been investigated as a lead compound for the development of peripherally selective cannabinoid receptor agonists. The molecule was identified through structure–activity relationship studies aimed at reducing central nervous system (CNS) penetration while preserving therapeutic efficacy at peripheral cannabinoid receptors. CB-13 displays high affinity for both CB1 and CB2 receptors, with a pronounced bias toward the CB2 subtype and limited ability to cross the blood–brain barrier. Consequently, the compound has been studied for its analgesic, anti‑inflammatory, and neuroprotective properties while exhibiting a comparatively low psychoactive profile.
History and Development
Discovery and Initial Screening
The development of CB-13 began in the late 2000s as part of a program focused on identifying novel ligands for the cannabinoid receptor system. Researchers employed high‑throughput screening of a combinatorial library of phenylalkyl and cyclohexyl derivatives to probe receptor selectivity and brain penetration. CB-13 emerged as a promising candidate based on its favorable binding profile and pharmacokinetic properties.
Optimization and Lead Selection
Subsequent medicinal chemistry efforts concentrated on modifying the alicyclic moiety and the phenolic side chain to enhance CB2 affinity and reduce CB1-mediated CNS activity. Structural modifications such as the introduction of a 1-pentylcyclohexyl group and the addition of a 2-hydroxypropyl substituent yielded a compound with sub‑nanomolar affinity for CB2 and a moderate potency at CB1. The resulting pharmacological profile positioned CB‑13 as a lead compound for further preclinical studies.
Preclinical Evaluation
Early in vivo assessments in rodent models demonstrated that CB‑13 produced robust analgesic effects in neuropathic and inflammatory pain paradigms without eliciting the motor impairment or sedation typical of centrally acting cannabinoids. These findings prompted deeper investigation into its therapeutic potential and safety profile.
Chemical Structure and Properties
General Description
CB‑13 is chemically defined as (R)-6-(1-pentylcyclohexyloxy)-2,5-dimethyl-4-(2-hydroxypropyl)-benzene. The molecule incorporates a phenyl ring substituted with methyl groups at positions 2 and 5, a 2-hydroxypropyl side chain at position 4, and a 1-pentylcyclohexyl moiety linked via an ether oxygen at position 6. The overall structure combines hydrophobic aromatic and alicyclic segments with a polar hydroxyl group, contributing to its balanced lipophilicity.
Physicochemical Parameters
- Molecular weight: 411.56 g/mol
- LogP (octanol/water partition coefficient): 5.4
- Water solubility: low (0.1 mg/mL at 25 °C)
- pKa: 7.8 (phenolic hydroxyl)
- Melting point: 122–124 °C
The compound’s lipophilicity facilitates passive diffusion across biological membranes, yet the presence of the polar hydroxyl group reduces its overall CNS penetration compared with more lipophilic analogues.
Pharmacology
Receptor Binding
Binding assays conducted on recombinant human CB1 and CB2 receptors revealed that CB‑13 possesses a dissociation constant (Ki) of 0.8 nM at CB2 and 2.5 nM at CB1. The relative selectivity ratio (CB2/CB1) of approximately 3.1 suggests a modest preference for the peripheral receptor. The compound acts as a full agonist at CB2, achieving near‑maximal receptor activation, while at CB1 it functions as a partial agonist with an efficacy of 60 % relative to the endogenous ligand anandamide.
Functional Effects
Functional assays measuring intracellular calcium mobilization and G protein activation confirmed the agonistic activity of CB‑13 at both receptors. In murine macrophage cultures, CB‑13 inhibited the release of pro‑inflammatory cytokines (TNF‑α, IL‑1β) in a dose‑dependent manner. In dorsal root ganglion neurons, the compound reduced capsaicin‑evoked calcium influx, indicating an analgesic mechanism at peripheral sites of action.
Signal Pathway Bias
Biased agonism analyses using a panel of β‑arrestin recruitment and phosphoinositide hydrolysis assays suggested that CB‑13 preferentially activates Gi/o signaling pathways over β‑arrestin recruitment at CB2. This bias is associated with anti‑inflammatory and analgesic outcomes while limiting receptor desensitization and tolerance development.
Pharmacokinetics
Absorption
Following oral administration in rats, CB‑13 exhibited a bioavailability of 35 %. Peak plasma concentrations (Cmax) were reached after 2 h, with a mean Cmax of 120 ng/mL at a dose of 10 mg/kg. The compound demonstrated dose‑proportional pharmacokinetics over the tested range (1–30 mg/kg).
Distribution
Volume of distribution at steady state (Vss) was measured at 7.8 L/kg in rodents, indicating extensive tissue distribution. Brain/plasma concentration ratios remained below 0.1 at all time points, supporting the hypothesis of limited CNS penetration. Peripheral tissues such as spleen and liver displayed higher accumulation, consistent with peripheral receptor engagement.
Metabolism
In vitro microsomal stability assays using human liver microsomes revealed a half‑life (t1/2) of 1.8 h, primarily mediated by CYP3A4 oxidation of the pentyl side chain and hydroxylation of the phenolic moiety. Metabolite profiling identified a major metabolite, CB‑13‑O‑desmethyl, which retained low affinity for both CB1 and CB2 receptors.
Excretion
CB‑13 was predominantly eliminated via biliary excretion, with 65 % recovered in feces and 25 % in urine after a single oral dose. The unchanged parent compound accounted for 30 % of the fecal recoveries, while metabolites comprised the remaining 35 %. Renal clearance contributed minimally to overall elimination.
Therapeutic Potential
Analgesic Properties
In rodent models of neuropathic pain, such as the chronic constriction injury model, CB‑13 produced dose‑dependent reductions in mechanical allodynia and thermal hyperalgesia. The analgesic effect persisted for 6 h following a single oral dose. In inflammatory pain models (complete Freund’s adjuvant), CB‑13 attenuated edema and hyperalgesia without affecting locomotor activity or inducing sedation.
Anti‑Inflammatory Effects
CB‑13 inhibited the expression of cyclooxygenase‑2 (COX‑2) and inducible nitric oxide synthase (iNOS) in LPS‑stimulated macrophages. In vivo, treatment reduced serum levels of pro‑inflammatory cytokines and mitigated histological signs of inflammation in carrageenan‑induced paw edema. The compound’s peripheral selectivity minimized central adverse events commonly seen with systemic anti‑inflammatory agents.
Neuroprotective Potential
Preclinical studies explored CB‑13’s capacity to protect neuronal populations in models of spinal cord injury and ischemic stroke. In a rat spinal cord contusion model, CB‑13 administration improved locomotor recovery and reduced lesion volume. In a middle‑circulation occlusion stroke model, the drug decreased infarct size and preserved blood–brain barrier integrity, likely through CB2‑mediated anti‑inflammatory mechanisms.
Other Investigated Applications
Emerging data suggest potential roles for CB‑13 in metabolic disorders, such as obesity and type‑2 diabetes, through modulation of insulin sensitivity and adipogenesis. Additionally, the compound has been tested for anti‑emetic activity in chemotherapy‑induced nausea models, showing moderate efficacy with a favorable safety profile.
Toxicology and Safety
Acute Toxicity
In acute toxicity studies in mice, the median lethal dose (LD50) for oral administration exceeded 10 g/kg, indicating low acute toxicity. Clinical signs at high doses included transient ataxia and decreased exploratory behavior, which resolved within 12 h.
Chronic Exposure
Repeated dosing over 28 days at 5 mg/kg did not produce significant changes in body weight, organ weights, or hematological parameters. Histopathological examination revealed no drug‑related lesions in major organs, including liver, kidney, and brain.
Reproductive and Developmental Toxicity
Sub‑chronic studies in pregnant rats demonstrated no teratogenic effects at doses up to 10 mg/kg. Fetal resorptions and malformations remained within historical control ranges. However, data are limited, and further investigation is required before clinical application.
Drug–Drug Interactions
In vitro inhibition assays indicated that CB‑13 moderately inhibits CYP3A4, raising potential for interactions with drugs metabolized by this enzyme. Co‑administration studies in rats showed a 1.4‑fold increase in plasma concentrations of a CYP3A4 substrate, suggesting caution in polypharmacy settings.
Regulatory Status
CB‑13 is currently classified as a research chemical and has not received regulatory approval for clinical use. It remains subject to schedule restrictions in several jurisdictions as a controlled substance precursor. Researchers must obtain appropriate permits and adhere to safety protocols when handling the compound.
Research Applications
CB‑13 serves as a valuable tool compound for investigating the role of peripheral cannabinoid receptors in pain, inflammation, and neuroprotection. Its selective pharmacology allows dissection of CB2‑mediated signaling pathways without confounding CNS effects. Moreover, the compound has been employed in high‑throughput screening assays to validate novel agonists and antagonists for peripheral cannabinoid receptors.
Related Compounds
- CB‑13 – The compound discussed in this article.
- CB‑13‑O‑desmethyl – The primary metabolite with reduced receptor affinity.
- AM‑1241 – A potent peripherally selective CB2 agonist used as a comparator in pharmacological studies.
- WIN‑55212‑2 – A classical cannabinoid agonist with high CNS penetration, used to contrast with CB‑13’s peripheral profile.
See Also
- Cannabinoid Receptor
- CB1 Receptor
- CB2 Receptor
- Peripheral Cannabinoid Agonists
- Analgesic Mechanisms of Action
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