Cannabinoids for the treatment of autoimmune and inflammatory skin diseases: A systematic review

In recent years, the medical use of cannabinoids has attracted growing attention worldwide. In particular, anti‐inflammatory properties of cannabinoids led to their emergence as potential therapeutic options for autoimmune and inflammatory disorders. Recent studies have also shown that cannabinoid receptors are widely expressed and have endogenous ligands in the skin, suggesting that the skin has its own endocannabinoid system. The aim of this review is to discuss the potential therapeutic effects of cannabinoids in autoimmune and inflammatory skin diseases. Following an overview of cannabinoids and the endocannabinoid system, we describe the cellular and molecular mechanisms of cannabinoids in skin health and disease. We then review the clinical studies of cannabinoids in autoimmune and inflammatory skin diseases including systemic sclerosis (SSc), dermatomyositis (DM), psoriasis (Pso) and atopic dermatitis (AD). A primary literature search was conducted in July 2023, using PubMed and Web of Science. A total of 15 articles were included after excluding reviews, non‐human studies and in vitro studies from 389 non‐duplicated articles. Available evidence suggests that cannabinoids may be beneficial for SSc, DM, Pso and AD. However, further studies, ideally randomized controlled trials, are needed to further evaluate the use of cannabinoids in autoimmune and inflammatory skin diseases.

[21][22] The aim of this review is to discuss the potential therapeutic effects of cannabinoids for autoimmune and inflammatory skin diseases.Initially, we provide an overview of the endocannabinoid system in the skin.We then describe the roles of cannabinoids in skin homeostasis and its breakdown.Lastly, we review the clinical studies of cannabinoids in autoimmune and inflammatory skin diseases with a special focus on SSc, DM, Pso and AD.

| Cannabinoids
Cannabinoids are a heterogeneous group of compounds that can be divided into the following three classes according to where they are produced: phytocannabinoids (plant), endocannabinoids (body) and synthetic cannabinoids (chemical synthesis). 1 Representative cannabinoids of each class, along with their mechanisms of actions and biological effects, are provided in Table 1.
Phytocannabinoids are produced by the plant C. sativa as secondary metabolites.Over 100 phytocannabinoids have been isolated to date, of which delta-9-tetrahydrocannabinol (Δ 9 -THC) and cannabidiol (CBD) are the most extensively studied.Δ 9 -THC is the primary psychoactive constituent of C. sativa. 23Although Δ 9 -THC has analgesic, antiemetic and anti-inflammatory effects, its medical use is significantly limited due to the psychotropic properties. 24,25[28] Endocannabinoids are natural ligands of cannabinoid receptors that are produced by the enzymatic cleavage of membrane lipid precursors. 1 N-arachidonoylethanolamine (AEA) and 2-arachidonoylglycerol (2-AG) are the most abundant endocannabinoids in the human body. 6,29They are synthesized from arachidonic acid in response to physiological stimuli and are primarily involved in the modulation of inflammation and neurotransmission. 30,31[34] Synthetic cannabinoids refer to the laboratory-synthesized analogs of phytocannabinoids or endocannabinoids. 35Among them, dronabinol, a synthetic Δ 9 -THC and nabilone, a synthetic cannabinoid resembling Δ 9 -THC, are approved by the United States Food and Drug Administration for the treatment of acquired immunodeficiency syndrome-induced anorexia (dronabinol and nabilone) and chemotherapy-induced nausea and vomiting (nabilone). 36,37A number of other synthetic cannabinoids, especially non-psychoactive ones, are under clinical trials for different indications.

| Cannabinoid receptors
There are two types of cannabinoid receptors that have been identified to date: cannabinoid type 1 receptor (CB1R) and cannabinoid type 2 receptor (CB2R). 5,7They are both members of the G proteincoupled receptor superfamily with seven transmembrane spanning domains.CB1R is mainly expressed in the central and peripheral nervous system, where it regulates the release of neurotransmitters. 38,39CB2R, on the other hand, is notably expressed in the

TA B L E 1
The major cannabinoids and their mechanisms of actions.and TRPM8). 42,43Although these putative receptors are activated by endocannabinoids and/or endocannabinoid-like compounds, their roles in the endocannabinoid system are not fully understood. 44

| The endocannabinoid system
The endocannabinoid system consists of endocannabinoids, cannabinoid receptors, endocannabinoid transporters and various enzymes that facilitate the synthesis and degradation of endocannabinoids. 316][47] Regarding the metabolism of endocannabinoids, N-acylphosphatidylethanolamine-specific phospholipase D-like hydrolase (NAPE-PLD) are responsible for the synthesis of AEA and other N-acylethanolamines, 48 while diacylglycerol lipases (DAGLα and DAGLβ) catalyse the synthesis of 2-AG and other monoacylglycerols. 49The degradation of AEA and 2-AG was primarily mediated by fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL), respectively. 50,51N-PEA, a major endocannabinoid-like compound, serves as an alternative substrate to FAAH, thus enhancing the physiological effects of AEA. 51This mechanism is known as the 'entourage effect'.Although AEA and 2-AG are lipophilic and require inter-and intra-cellular carriers, little is known about their transporter system. 52

| The endocannabinoid system in the skin
Skin homeostasis is regulated by a complex network of different cell types and soluble mediators. 53Because the breakdown of this finetuned balance leads to pathological skin conditions, mechanisms for maintaining skin homeostasis have been rigorously studied.[56] It is noteworthy that CB1R and CB2R are widespread in the skin: their expression is observed in various cell types, such as keratinocytes, fibroblasts, sensory neurons and skin immune cells. 13,15 addition, putative cannabinoid receptors including TRP receptors are detected in keratinocytes, sensory neurons and skin immune cells. 57,58Regarding their ligands, AEA and 2-AG, the major endogenous ligands of CB1R and CB2R, are detected in multiple cell types, including those from sweat glands and hair follicles. 59,60PEA, an endocannabinoid-like compound, is also expressed in the skin. 61

| Epidermal proliferation
Multiple members of the endocannabinoid system including CB1R, CB2R, AEA, 2-AG, FAAH, NAPE-PLD and several TRP receptors have been shown to be expressed in human epidermal keratinocytes. 15,54,57,62,63The activation of CB1R by low concentration of AEA (1 μM) prevents the differentiation of 2D-cultured human keratinocytes. 14AEA also inhibits the up-regulation of multiple differentiation markers, such as K1 and K10, in a CB1R-dependent manner. 64Higher concentrations (3-30 μM) of AEA, on the other hand, suppress the proliferation of primary human epidermal keratinocytes while inducing their apoptosis via the sequential activation of CB1R and TRPV1. 65terestingly, Δ 9 -THC and CBD inhibit the proliferation of HPV-16 E6/E7-transformed human keratinocytes independently of CB1R or CB2R, suggesting the role of non-classical cannabinoid receptors in mediating the anti-proliferative actions of these phytocannabinoids. 16llectively, these data suggest that cannabinoids exert antidifferentiative, anti-proliferative or pro-apoptotic effects on epidermal keratinocytes in a concentration-dependent manner.[68] Further studies are warranted to understand the exact mechanisms of actions of cannabinoids on human epidermis.

| Inflammation
][10] Karsak et al demonstrated that 2,4-dinitrofluorobenzene (DNFB)-induced allergic inflammation was exacerbated in CB1R and CB2R double knockout mice, whereas inflammation was attenuated in FAAH knock-out mice with increased levels of AEA. 69Moreover, DNFB-induced skin inflammation is suppressed by the subcutaneous administration of Δ 9 -THC in wild-type mice. 69These results support a protective role of the endocannabinoid system in allergic inflammation in the skin.
Other studies also reported the downregulation of inflammatory cytokines and chemokines by cannabinoids in different models of skin inflammation.For instance, in an in vitro model of allergic contact dermatitis using poly-(I:C)-stimulated human keratinocyte cells, CBD dose-dependently inhibits the release of interleukin (IL)-6, IL-8, tumour necrosis factor (TNF)α and monocyte chemotactic protein (MCP)-2, while up-regulating AEA levels. 17These effects are reversed by the antagonists of CB2R and TRPV1. 17In addition, topical application of CB1R-specific agonist attenuates the skin inflammation in an oxazolone-induced AD animal model. 70Another study reported that topical Δ 9 -THC therapy effectively suppressed DNFB-induced ear swelling and immune cell infiltration not only in wild-type but also in CB1R and CB2R double knock-out mice, suggesting the antiinflammatory activity of Δ 9 -THC that are independent of CB1R or CB2R. 71Overall, these studies support the anti-inflammatory effects of cannabinoids as well as the need to further investigate their CB1R/CB2R-dependent and -independent pathways.

| Fibrosis
Fibrosis is a complex process involving inflammatory responses, fibroblast activation and excessive collagen deposition. 72It is also a hallmark of SSc. 73Akhmetshina et al reported that bleomycininduced dermal fibrosis and leukocyte infiltration were exacerbated in CB2R knock-out mice, while a selective CB2R agonist JWH-133 attenuated these changes in bleomycin-treated wild-type mice. 74o et al also demonstrated that a CBD quinol VCE-004.8 inhibited transforming growth factor (TGF)β-induced collagen synthesis in vitro. 75Importantly, lenabasum, a selective CB2R agonist, has been shown to prevent the progression of bleomycin-induced fibrosis in vivo by stimulating PPARγ signalling. 76These data, along with its favourable safety profile, promote the clinical translation of lenabasum for the treatment of SSc. 77

| Routes of administration of cannabinoids
Various routes of administration of cannabinoids have been developed to date, including oral, transdermal and transmucosal administration. 78Although oral administration is the most popular route of cannabinoids, it suffers from low bioavailability due to gastric instability, hepatic degradation and low water solubility. 79,80In addition, some metabolites can cause psychotropic side effects. 79To avoid these issues, transdermal administration has been exploited as an ideal route of cannabinoid delivery, especially for skin conditions. 81,824][85] In recent years, Reviews, non-human studies and in vitro studies were excluded.
The initial database search yielded 389 articles after removing the duplicates.Of these, 365 articles were excluded during title and abstract screening.Among the 24 articles that underwent full-text review, 9 articles were excluded for the following reasons: reviews (n = 2), non-human studies (n = 2) and in vitro studies (n = 5).In total, 15 articles were included in the systematic review, of which 5 were clinical trials, 7 were cohort studies and 3 were case reports or case series (Table 2).Consensus was reached between the two researchers (A.K and A.Y) on the inclusion and exclusion of all articles.

| Systemic sclerosis
SSc is a connective tissue disease characterized by autoimmunity, vasculopathy and fibrosis of the skin and internal organs. 732][93][94] Based on the encouraging results of preclinical studies using patient fibroblasts and mouse models, 76 a selective CB2R agonist lenabasum has been explored as a potential treatment for SSc.
The first clinical trial of lenabasum in SSc was reported by Spiera et al. 95 In this double-blind, randomized, placebo-controlled phase 2 study (NCT02465437), 42 patients with diffuse cutaneous SSc (dcSSc) on stable medication including immunosuppressants received either oral lenabasum or placebo. 96Lenabasum dosage was randomized to be 5 mg once daily, 20 mg once daily or 20 mg twice daily for the first In an open-label extension, lenabasum continued to show good tolerability, while the ACR CRISS score kept improving. 95th the favourable results of the phase 2 trial, the phase 3 trial was undertaken to investigate the safety, efficacy and tolerability of lenabasum in dcSSc (NCT03398837). 98In this study, 365 patients with dcSSc were randomized and dosed 1:1:1 with lenabasum 20 mg, lenabasum 5 mg or placebo, each twice daily while receiving stable medication including immunosuppressants.The primary endpoint of the ACR CRISS score was not met (0.888 for lenabasum 20 mg vs. 0.887 for placebo at Week 52; p = 0.497 by mixed-effects model repeated-measures analysis).This result was attributed to the remarkable improvement of the ACR CRISS score in both lenabasum-and placebo-treated patients, which reflects the efficacy of the background treatment, especially mycophenolate mofetil.In line with other studies, the safety profile of lenabasum was overall acceptable.
Cannabinoids have also been used for the treatment of vasculopathy-associated symptoms that significantly impair the quality of life of patients with SSc.Cocchiara et al showed that the combination of oral (5 drops twice daily) and local (2 drops at the site of ulcers) CBD oil was associated with a significant reduction of the pain related to skin ulcers, suggesting the use of CBD oil as an analgesic for skin ulcers in SSc. 99Nogueira et al also reported that a male patient with SSc suffering from severe Raynaud's phenomenon and skin ulcers experienced the improvement in and Raynaud's phenomenon and pain after smoking 30 g of C. sativa daily. 100However, no specific metrics were mentioned in this study.In a recent comparative study of SSc patients treated with CBD oil during surgical debridement (n = 25) and those treated with standard local therapy (n = 20), the topical CBD therapy was significantly associated with lower pain scores, higher health assessment scores and an increase in total sleep time. 101

| Dermatomyositis
Dermatomyositis (DM) is an idiopathic inflammatory myopathy with characteristic skin lesions. 102Skin lesions of DM are associated with pruritus, photosensitivity and visible difference, substantially affecting the quality of life of the patients. 103Since active skin lesions of DM are often refractory to conventional treatment, more effective therapeutic options are awaited. 104Recently, antiinflammatory properties of cannabinoids have attracted much attention in DM.For instance, Robinson et al demonstrated in vitro that lenabasum significantly reduced the production of TNFα and type I interferons from peripheral blood mononuclear cells in patients with DM. 105 In the double-blind, randomized, placebo-controlled phase 2 trial (NCT02466243), DM patients with refractory, moderate-toseverely active skin disease were randomized to the lenabasum (n = 11) or placebo (n = 11) group. 106The lenabasum group received 20 mg lenabasum daily for the first 28 days, followed by 20  by mixed-effects model repeated-measures analysis).There were no serious or severe AEs related to lenabasum.In an open-label extension, lenabasum continued to have good tolerability, while the CDASI activity score kept decreasing. 108 the double-blind, randomized, placebo-controlled phase 3 study (NCT03813160), patients were randomized 2:1:2 to lenabasum 20 mg twice daily (n = 69), lenabasum 5 mg twice daily (n = 35) or placebo twice daily (n = 71) for 52 weeks. 109The primary endpoint of Total Improvement Score 110 was not met (28.3 for lenabasum 20 mg twice daily vs. 27.2 for placebo at Week 52; p = 0.331 by mixedeffects model repeated-measures analysis).In the subgroup analysis of the patients with predominant DM with minimal muscle activity, the improvement in CDASI activity score, which was relegated to a secondary endpoint in the phase 3 study, was significantly greater in the lenabasum 20 mg twice daily group than the placebo group at Week 52 (p = 0.006).

| Psoriasis
Pso is a common chronic inflammatory skin disease with complex pathogenesis consisting of genetic, epigenetic and environmental factors. 111Since the skin lesions often develop in highly visible areas such as face, scalp and nails, Pso causes considerable psychosocial disability.The disease burden of Pso further increases with comorbidities such as psoriatic arthritis, cardiovascular diseases and metabolic syndrome. 112,113However, currently used medications are insufficient to control the disease for some patients.Therefore, there is still an unmet need for the development of new treatments for Pso. 114though preliminary, several case reports and case series assessed the efficacy of cannabinoids in Pso.Friedman et al reported that the use of Δ 9 -THC-containing soap and hair oil cleared the scalp lesions after 2 weeks in a male patient with Pso. 66In a retrospective study of CBD-enriched ointment for chronic inflammatory skin diseases, patients with Pso (n = 5) showed an improvement in the Psoriasis Area and Severity Index score 115 at Day 90 (p < 0.001 by the Mann-Whitney test). 83Vincenzi et al also reported that a shampoo containing 0.075% broad-spectrum CBD significantly improved the severity of scalp inflammation within 2 weeks in patients with mild to moderate scalp Pso (n = 22). 67These data suggest that topical CBD is beneficial for Pso skin lesions.However, further studies, especially randomized controlled trials, are needed to further evaluate its safety and efficacy.

| Atopic dermatitis
AD is an allergic inflammatory skin disease characterized by pruritic eruption with a chronic relapsing course. 116With high prevalence of about 3%-20% worldwide, AD poses a major socioeconomic burden in many countries. 117veral studies have evaluated the efficacy of topical cannabinoids for the treatment of AD.Pulvirenti et al showed that twice (n = 16) after 2 weeks of treatment. 84

| Adverse effects of cannabinoids
Due to the widespread distribution of cannabinoid receptors across multiple organs, various AEs have been reported with the medical use of cannabinoids.In a large systematic review, there was an increased risk of short-term AEs with the use of cannabinoids.Among them, the most frequent AEs were dizziness, nausea, fatigue, somnolence, euphoria, disorientation, drowsiness, confusion, hallucination and loss of balance. 123It should be noted that the majority of these events are associated with the use of psychoactive cannabinoids such as dronabinol, nabilone and Δ 9 -THC.Since non-psychoactive cannabinoids have been mainly used for the treatment of autoimmune and inflammatory skin diseases, we summarize the AEs that were reported in the systematic review in this study (Table 2).
Several AEs were reported in the clinical trials of oral lenabasum for SSc and DM.In the phase 2 study (NCT02465437) of lenabasum in SSc, the most common AE attributed to lenabasum was dizziness (22% for lenabasum vs. 13% for placebo), which led to the study discontinuation in one lenabasum-treated patient. 95In the phase 3 trial (NCT03398837) of lenabasum in dcSSc, no serious AEs related to lenabasum were observed and the safety profiles were similar to those reported in the phase 2 trial. 98In the phase 2 trial of lenabasum for DM, no serious or severe AEs occurred in the treatment groups.The most common AEs in the study were mild dizziness (45% for lenabasum vs. 18% for placebo), mild or moderate fatigue (27% vs. 27%), mild dry mouth (36% vs. 18%) and mild diarrhoea (27% vs. 9%). 106In the phase 3 study of lenabasum in DM, the safety profiles were similar to those in the phase 2 trial. 109Collectively, these data suggest that lenabasum is well-tolerated and safely administered.
Topical cannabinoids have a significantly favourable safety profile compared with oral cannabinoids.In the study cohort by Eberlein et al, AEs occurred in nine patients receiving topical N-PEA who reported at least one of the following: stinging, erythema and/ or burning sensation. 121

| CON CLUS ION
This systematic review overviewed the present use of cannabinoids for autoimmune and inflammatory skin diseases (Figure 1, Table 2).
The available data support the safety and efficacy of cannabinoids in SSc, DM, Pso and AD, as well as highlight the need for further studies to confirm their therapeutic use.
Clinical trials for cannabinoids in skin disorders have been extensively conducted in SSc and DM, where randomized controlled trials were performed on oral lenabasum (Table 2).Although promising results were obtained from the phase 2 trials, the phase 3 trials failed to meet the primary endpoint in both diseases, which are attributed to the confounding effect of background immunosuppressants in SSc and the change of primary endpoint from CDASI to Total Improvement Score in DM. 98,109 Nonetheless, subgroup analysis suggested that lenabasum was associated with the improvement of several secondary endpoints in both phase 3 trials.In Pso and AD, the included studies, although limited, support the potential therapeutic benefit of topical cannabinoids.In the future, randomized controlled trials are highly warranted to evaluate the clinical efficacy of cannabinoids more rigorously in these diseases.
In conclusion, available evidence suggests that cannabinoids have the potential therapeutic benefit with good tolerability in SSc, DM, Pso and AD.However, there are no sufficient data to

ACK N OWLED G EM ENTS
None.

Δ 9 -
THC Partial agonist of CB1R and CB2R Pain; nausea/emesis; inflammation; neurologic disorders CBD Inverse agonist and negative allosteric modulator of CB1R and CB2R; agonist of TRPV1 and PPARγ; antagonist of GPR55 Pain; anxiety; epilepsy; inflammation Endocannabinoids AEA Partial agonist of CB1R and weak agonist of CB2R; activator of TRPV1 Reward pathways; appetite; inflammation 2-AG Agonist of CB1R and CB2R; additional affinity for TRPV1, PPARγ and GPR55 Pain; inflammation Synthetic cannabinoids Dronabinol Agonist of CB1R and CB2R Appetite; nausea/emesis Nabilone Agonist of CB1R and CB2R Pain; nausea/emesisimmune system such as spleen, thymus and haematopoietic lineage cells, and modulates immunological responses. 40,41Although CB1R and CB2R have been validated as the main receptors in the endocannabinoid system, the presence of other cannabinoid receptors has been widely investigated.Possible candidates for additional cannabinoid receptors include orphan G protein-coupled receptors (GPR18, GPR55 and GPR119), peroxisome proliferatoractivated nuclear receptors (PPARα and PPARγ) and transient receptor potential (TRP) receptors (TRPV1, TRPV2, TRPV3, TRPV4, TRPA1 researchers have been trying to further improve the efficacy of topical cannabinoids by utilizing new technologies, such as chemical enhancement, physical enhancement, micro-emulsification and nanoparticle carrier systems.[86][87][88][89]4.2 | A systematic reviewA literature search of PubMed and Web of Science was performed according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines90 (Figure1).The search was conducted on 1 July 2023, using the combinations of the following keywords: ('cannabinoid' OR 'cannabis' OR 'cannabidiol') AND ('skin' OR 'cutaneous' OR 'dermatological') AND ('autoimmune' OR 'inflammatory' OR 'systemic sclerosis' OR 'dermatomyositis' OR 'psoriasis' OR 'atopic dermatitis').Additional relevant publications were selected from the reference lists of the retrieved articles.Only original articles in English evaluating the efficacy of cannabinoids in patients with autoimmune or inflammatory skin diseases were included.

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weeks, followed by 20 mg twice daily for the next 8 weeks.Safety and efficacy were assessed at Weeks 4, 8, 12 and 16, with the primary endpoint of the American College of Rheumatology (ACR) Combined Response Index in diffuse cutaneous Systemic Sclerosis (CRISS) score.97At Week 16, the lenabasum group experienced greater improvement in the ACR CRISS score compared with the placebo group (p = 0.07 by two-sided mixed-effects model repeated-measures analysis).Adverse events (AEs) occurred in 63% and 60% of the lenabasum and the placebo group, respectively, with no serious AEs related to lenabasum.
validate their clinical efficacy.Randomized controlled trials with appropriate design are needed to further evaluate the use of cannabinoids for the treatment in autoimmune and inflammatory skin diseases.AUTH O R CO NTR I B UTI O N S Conceptualization: AY, SS; Methodology: AK, AY; Literature search: AK, TY, TF, AY-O, AY; Project administration: AY; Supervision: AY, SS; Writing-original draft: AK; Writing-review and editing: AY, SS.
TF and AY belong to the Social Cooperation Program, Department of Clinical Cannabinoid Research, supported by Japan Cosmetic Published studies of cannabinoids in autoimmune and inflammatory skin diseases.
mg twice daily for the following 56 days.Safety and efficacy were assessed until Day 113, with the primary endpoint of a change in Cutaneous Dermatomyositis Disease Area and Severity Index (CDASI) activity score. 107On Day 113, the lenabasum group showed greater improvement in CDASI activity score than the placebo group (p = 0.038 F I G U R E 1 PRISMA flow chart.The search process is depicted using a flow diagram adapted from the PRISMA guidelines.PRISMA, preferred reporting items for systematic reviews and metaanalyses.TA B L E 2