Endocannabinoids acting via CB₁ receptors (CB₁R) play a critical role in regulating energy homeostasis, which was the rationale for the pharmaceutical development of CB₁R antagonists for the treatment of obesity. Although the first-in-class CB₁R antagonist rimonabant proved to be effective in mitigating obesity and its multiple cardiometabolic complications, it was withdrawn from clinical use due to CNS-mediated neuropsychiatric side effects, which halted the further therapeutic development of the whole class of these compounds. Compared to the brain, CB₁Rs are expressed at low yet functional levels in peripheral organs involved in regulating energy homeostasis, including liver, skeletal muscle, adipose tissue and endocrine pancreas. In recent preclinical studies, selective targeting of these receptors by 'second generation' peripherally restricted CB₁R antagonists replicated the metabolic benefits of rimonabant in rodent models of obesity and diabetes without causing CNS-mediated side effects. Increased CB₁R activity also contributes to complex, multifactorial disorders such as various forms of tissue fibrosis, treatment of which may benefit from simultaneous engagement of more than one therapeutic target. Accordingly, novel 'third generation' hybrid inhibitors of peripheral CB₁R and inducible NO synthase were tested in mouse models of liver and pulmonary fibrosis where their antifibrotic efficacy was found to exceed the efficacy of drugs that inhibit only one of these targets. In this review, we will discuss the challenges and opportunities offered by second and third generation CB₁R antagonists and their potential therapeutic uses.

Idiopathic pulmonary fibrosis (IPF) is a life-threatening disease without effective treatment, highlighting the need for identifying new targets and treatment modalities. The pathogenesis of IPF is complex, and engaging multiple targets simultaneously might improve therapeutic efficacy. To assess the role of the endocannabinoid/cannabinoid receptor 1 (endocannabinoid/CB ₁ R) system in IPF and its interaction with inducible nitric oxide synthase (iNOS) as dual therapeutic targets, we analyzed lung fibrosis and the status of the endocannabinoid/CB ₁ R system and iNOS in mice with bleomycin-induced pulmonary fibrosis (PF) and in lung tissue and bronchoalveolar lavage fluid (BALF) from patients with IPF, as well as controls. In addition, we investigated the antifibrotic efficacy in the mouse PF model of an orally bioavailable and peripherally restricted CB ₁ R/iNOS hybrid inhibitor. We report that increased activity of the endocannabinoid/CB ₁ R system parallels disease progression in the lungs of patients with idiopathic PF and in mice with bleomycin-induced PF and is associated with increased tissue levels of interferon regulatory factor-5. Furthermore, we demonstrate that simultaneous engagement of the secondary target iNOS by the hybrid CB ₁ R/iNOS inhibitor has greater antifibrotic efficacy than inhibition of CB ₁ R alone. This hybrid antagonist also arrests the progression of established fibrosis in mice, thus making it a viable candidate for future translational studies in IPF.

Liver fibrosis, a consequence of chronic liver injury and a way station to cirrhosis and hepatocellular carcinoma, lacks effective treatment. Endocannabinoids acting via cannabinoid-1 receptors (CB ₁ R) induce profibrotic gene expression and promote pathologies that predispose to liver fibrosis. CB ₁ R antagonists produce opposite effects, but their therapeutic development was halted due to neuropsychiatric side effects. Inducible nitric oxide synthase (iNOS) also promotes liver fibrosis and its underlying pathologies, but iNOS inhibitors tested to date showed limited therapeutic efficacy in inflammatory diseases. Here, we introduce a peripherally restricted, orally bioavailable CB ₁ R antagonist, which accumulates in liver to release an iNOS inhibitory leaving group. In mouse models of fibrosis induced by CCl ₄ or bile duct ligation, the hybrid CB ₁ R/iNOS antagonist surpassed the antifibrotic efficacy of the CB ₁ R antagonist rimonabant or the iNOS inhibitor 1400W, without inducing anxiety-like behaviors or CB ₁ R occupancy in the CNS. The hybrid inhibitor also targeted CB ₁ R-independent, iNOS-mediated profibrotic pathways, including increased PDGF, Nlrp3/Asc3, and integrin αvβ6 signaling, as judged by its ability to inhibit these pathways in cnr1 ^–/– but not in nos2 ^–/– mice. Additionally, it was able to slow fibrosis progression and to attenuate established fibrosis. Thus, dual-target peripheral CB ₁ R/iNOS antagonists have therapeutic potential in liver fibrosis.

Seven-transmembrane receptors signal via G-protein- and β-arrestin-dependent pathways. We describe a peripheral CB₁R antagonist (MRI-1891) highly biased toward inhibiting CB₁R-induced β-arrestin-2 (βArr2) recruitment over G-protein activation. In obese wild-type and βArr2-knockout (KO) mice, MRI-1891 treatment reduces food intake and body weight without eliciting anxiety even at a high dose causing partial brain CB₁R occupancy. By contrast, the unbiased global CB₁R antagonist rimonabant elicits anxiety in both strains, indicating no βArr2 involvement. Interestingly, obesity-induced muscle insulin resistance is improved by MRI-1891 in wild-type but not in βArr2-KO mice. In C2C12 myoblasts, CB₁R activation suppresses insulin-induced akt-2 phosphorylation, preventable by MRI-1891, βArr2 knockdown or overexpression of CB₁R-interacting protein. MRI-1891, but not rimonabant, interacts with nonpolar residues on the N-terminal loop, including F108, and on transmembrane helix-1, including S123, a combination that facilitates βArr2 bias. Thus, CB₁R promotes muscle insulin resistance via βArr2 signaling, selectively mitigated by a biased CB₁R antagonist at reduced risk of central nervous system (CNS) side effects.

Introduction: Neurotensin is a gut-brain peptide hormone, a 13 amino acid neuropeptide found in the central nervous system and in the GI tract. The neurotensinergic system is implicated in various physiological and pathological processes related to neuropsychiatric and metabolic machineries, cancer growth, food, and drug intake. NT mediates its functions through its two G protein-coupled receptors: neurotensin receptor 1 (NTS1/NTSR1) and neurotensin receptor 2 (NTS2/NTSR2). Over the past decade, the role of NTS3/NTSR3/sortilin has also gained importance in human pathologies. Several approaches have appeared dealing with the discovery of compounds able to modulate the functions of this neuropeptide through its receptors for therapeutic gain. Areas covered: The article provides an overview of over four decades of research and details the drug discovery approaches and patented strategies targeting NTSR in the past decade. Expert opinion: Neurotensin is an important neurotransmitter that enables crosstalk with various neurotransmitter and neuroendocrine systems. While significant efforts have been made that have led to selective agonists and antagonists with promising in vitro and in vivo activities, the therapeutic potential of compounds targeting the neurotensinergic system is still to be fully harnessed for successful clinical translation of compounds for the treatment of several pathologies.