Cannabidiol (CBD; C₂₁H₃₀O₂) is a phytocannabinoid without psychoactive activities produced by Cannabis Sativa L and has a structural similarity to Δ⁹- tetrahydrocannabinol (THC; C₂₁H₃₀O₂), the primary psychotropic congener of this cannabis plant. Both cannabinoids are lipophilic compounds characterized by long half-life, bioaccumulation, and shared common metabolic pathways within the cytochrome family, drug carriers, and plasma protein binding substrates (127). Both of them also have anti-inflammatory activity and possible antiviral potential. Nonetheless, the majority of studies indicate the immunosuppressive and anti-inflammatory effects of cannabidiol in various immunological reactions and inflammations (70). CBD, unlike THC, is a non-toxic compound with a high safety margin and drug tolerance, even at doses up to 1500 mg/day. Recently, it was demonstrated that cannabidiol has anti-inflammatory effects in chronic inflammatory diseases preclinical models, apoptotic effects on the mammalian cells (70), or effects that contribute to the host of the viral infection response (128, 129).

Interleukin-6 is effectively suppressed by cannabidiol in numerous models of inflammation, including diabetes, asthma, pancreatitis, and hepatitis. In vivo, cannabidiol use resulted in an IL-6 decrease in ex vivo lipopolysaccharide-stimulated peritoneal macrophages in acute pancreatitis and bronchial-alveolar lavage fluid in lipopolysaccharide-induced pneumonia. Also, in mice with the induced asthma-like disease, IL-4, IL-5, and IL-13 cytokines and chemokines caused in the lungs of mice were shown to be suppressed by CBD (70). Cannabidiol decreased lung inflammation in asthma and acute pneumonia mouse models by inhibiting the production of pro-inflammatory cytokines by immune cells and suppressing the exuberant immune response (130, 131). Nichols and Kaplan (2020) (70) have shown that CBD inhibits the production of pro-inflammatory cytokines such as interleukin IL-1α and β, IL -2, IL-6, Il-17A interferon-γ inducible protein 10, monocyte chemoattractant protein-1, tumor necrosis factor -α and macrophage inflammatory protein-1α, which are associated with the occurrence of polyorgan inflammation and high mortality caused by SARS-CoV-2 (70). The current research results point out that CBD’s immunotherapeutic and anti-inflammatory properties may limit the cytokine storm and reduce the effects of exaggerated inflammation in patients with severe respiratory tract viral infections and ARDS often associated with COVID-19 (124).

To date, no studies about interactions between drugs used to treat SARS-CoV-2 infection and CBD have been published, but considering the inhibition of interleukin 6 receptor by Tocilizumab and the anti-inflammatory role of cannabidiol in the treatment of severe respiratory viral infections, a synergic, significant effect of these compounds on the reduction of inflammation in the acute course of COVID-19 can be expected ( Figure 5 ).

The constant mutations of SARS-CoV-2 make it imperative to identify an effective medication for patients suffering from COVID -19. The possibility of using cannabinoids to treat severe cases of this disease is increasingly being discussed.

The novel coronavirus binds to cellular receptors via angiotensin-converting enzyme 2 (ACE2), characteristic of pulmonary tissue, oral and nasal mucosa, kidneys, IG, and testicles. Furthermore, smokers and patients with chronic obstructive pulmonary disease have been reported to be more susceptible to COVID-19 and develop a severe form of the illness, as they present a high level of ACE2 expression. Therefore, it is believed that ACE2 expression in the oral, respiratory, and intestinal epithelium may provide SARS-CoV-2 with a vital entry point into the host cells, and ACE2 modulation in these tissues may limit SARS-CoV-2 binding to ACE2 receptors and thus reduce susceptibility to COVID-19 (57, 132–134). A study by Wang et al. on artificial human 3D models of the tissues mentioned above allowed these authors to identify 13 extracts from Cannabis sativa with a significant amount of CBD, which affects the expression of the ACE2 gene and the level of the ACE2 protein. Preliminary experiments also demonstrated that the extracts studied may decrease the concentration of the transmembrane protease serine 2 (TMPRSS2), a protein essential in the process of the viruses entering into cells (57).

The cannabinoid system consists of two cannabinoid receptors: CB1, present in the central nervous system (CNS), and CB2, in the immune system. Various ligands activate cannabinoid receptors: endogenous (anandamide, AEA; 2-arachidonoylglycerol, 2-AG), exogenous (e.g., phytocannabinoids from Cannabis sativa L.), or synthetic (135). The most critical phytocannabinoids include psychoactive delta-9-tetrahydrocannabinol (THC) and non-psychoactive cannabidiol (CBD). In contrast to THC, a partial antagonist of both CB1 and CB2 receptors, CBD is a partial antagonist of CB2 and only a weak antagonist of CB1 (136–138). Other cannabinoids are found in dried cannabis in much lower amounts. Cannabigerol and cannabichromene inhibit AEA re-uptake (139). The MOA of numerous other phytocannabinoids, such as cannabidivarin, cannabidiol, or cannabielsoin, has not been fully explored yet (138). When developing an adequate preparation based on cannabis flower extracts, it must be remembered that cannabis contains over 100 identified cannabinoids, Δ⁹-THC and CBD being the best-known ones, and other compounds like terpenes (140, 141). Terpenes and cannabinoids may interact with each other, affecting a given extract’s overall therapeutic effect. It is assumed that a quote is more potent than a single compound; therefore, it is essential to study the impact of a whole extract obtained from a plant rather than a single compound (142, 143). It has been known that anandamide (AEA) endocannabinoid, an endogenous antagonist with a high affinity to CB1, decreases IL-6 production. In contrast, THC, a partial antagonist of CB1 and CB2, inhibits IL-12 and IFN-γ release (144–148). Another phytocannabinoid (E)-β-caryophyllene [(E)-BCP], as a functional antagonist of CB2, inhibits both the production of pro-inflammatory cytokines in the peripheral blood induced by lipopolysaccharides (LPS) and the LPS-incited phosphorylation of Erk1/2 and JNK1/2 in monocytes (149–151). CBD impedes the expression of Il-6, IL-8, and TNF-α in in vitro models of allergic contact dermatitis and bone and joint inflammation. On the other hand, delta-9-tetrahydrocannabinol reduces the release of TNF-α, IL-1β, IL-6, and IL-8 in MG63 cells incited with LPS, which points to an essential role of the CB2 receptor in the anti-inflammatory response (152, 153). COVID-19 patients show macrophages, monocytes, and low levels of lymphocytes accountable for acute lung injury and leading to acute respiratory distress syndrome or even death. There are two groups of macrophages involved in the inflammatory response, cytokine production, phagocytosis, cell proliferation, and tissue repair: classically activated macrophages (M1) and alternatively activated macrophages (M2) (154–157). CB2 receptors are known as macrophage polarization regulators in inflammation. The use of an antagonist reduces the proliferation of inflammation-stimulating macrophages (M1) and increases the commonness of the second type of macrophages, which have an opposite effect (M2) (148, 158).

In SARS-CoV2 infection, there is a change in cytokine production, very similar to a cytokine storm, accompanied by excessive release of immune cells. Mesenchymal stromal cells (MSCs) have an anti-inflammatory effect. Their use may decrease the production of inflammatory-inducing compounds, which could improve the condition of the lungs previously damaged by, e.g., the flu virus (64, 159–161). MSCs raise the level of peripheral lymphocytes, simultaneously lowering the number of immune cells producing cytokines. Furthermore, MSCs produce leukemia inhibitory factor (LIF), which helps counteract the cytokine storm in viral pneumonia and stimulates CB2 receptors (162–164). The proposed MSC treatment for COVID-19 patients and proper stimulation of CB2 receptors will allow for the repair of damaged stem cells and immune response stimulation. It has been observed that MSCs do not stimulate the synthesis of ACE2 and TMPRSS2 proteins, which are associated with SARS-CoV-2 infection (165–167).

It was demonstrated that, in comparison to women, men are more prone to SARS-CoV-2 infection, which lower estrogen concentrations could explain. This beneficial property of estrogens and the immunosuppressive effect, which reduces excessive inflammation, may be associated with the CB2 cannabinoid receptor, a well-known immune response modulator (168–172).

Selective stimulation of CB2 may limit inflammation in COVID-19 patients through inflammatory cascade control in several checkpoints by reducing cytokine production, limiting immune cell proliferation, or producing antibodies, thus eliminating acute immune response (148, 173, 174). Currently, there are no CB2 antagonists approved for human use. Therefore, while searching for a commercially available CB2 receptor inhibitor is ongoing, treatment with CBD may be an alternative solution for COVID-19 patients (148).

Numerous experimental studies on rodents have shown that CB1 activation is essential for an effective immune response in bacterial infections, whereas CB2 activation prevents further damage caused by inflammation in sepsis due to an immunosuppressive effect (175). El Biali et al. (2020) (175) report a few human studies identifying potential relationships between the endocannabinoid system and the immune response. For instance, it was found that a genetic polymorphism in CB2 (CBQ63R), which reduces CB2 responses, can be linked with a higher probability of hospitalization in small children infected with RSV (n = 83), with the risk of developing severe ARTI being two times higher in allele Q carriers (OR = 2.148; 95% CI:1.09–4.22), and three times higher in the carriers of the QQ genotype (OR = 3.28; 95% CI: 1.22–8.71) (175, 176).

The FDA approved two forms of delta-9-tetrahydrocannabinol (dronabinol and nabilone) for treating some of the adverse effects of chemotherapy (i.e., nausea and vomiting) and for recovery of the appetite in wasting diseases like AIDS. In 2018, CBD was permitted for treating two types of pediatric epilepsy: Dravet syndrome and Lennox-Gastaut syndrome. Apart from these four indications, the most solid evidence of using cannabinoids with desired therapeutic outcomes is observed in chronic pain (including neuropathic pain) and MS-related muscle spasticity (125). Numerous in vitro and in vivo studies on animal and human cells suggest that CBD has an immunosuppressive and anti-inflammatory effect through direct inhibition of microglial cells and T cells, induction of apoptosis in regulatory T cells, or through myeloid-derived regulatory T cell induction (70). In vitro studies in animals suggest that Cannabis sativa extracts have an anti-bacterial and limited anti-fungal properties (177). A study conducted among healthy volunteers (n = 10) demonstrated that a dose of 30 mg of water-soluble or fat-soluble CBD significantly decreased the TNF level in peripheral blood mononuclear cells stimulated with bacterial lipopolysaccharide (178).

To date, there have been no studies investigating the effect of cannabinoids on SARS-CoV-2 infection. Furthermore, there is no epidemiological data on COVID-19 incidence in people using cannabinoids for medical or recreational purposes. A paper by Esposito lists four features of CBD that warrant its use, namely: 1) Cannabis sativa extracts have been proven to regulate the expression of two receptors that are of crucial importance for SARS-CoV-2 in a cellular model, 2) it has been demonstrated that CBD has an extensive range of immunomodulatory and anti-inflammatory effects, which may reduce the over-production of that leads to acute lung injury, 3) as a PPARγ antagonist, CBD may present a direct anti-viral effect, 4) as a PPARγ antagonist, CBD may inhibit the process of pulmonary fibrosis (179).

The characteristic feature of severe SARS-CoV-2 infection is the uncontrolled release of cytokines IL-1β, IL-6, and CCL2, and pro-inflammatory molecules, together with a decrease in the number of NK (natural killer) cells that may cause the cytokine storm. There is much to suggest that the severe course of infection does not stem from the viremia per se but depends on the degree of immune dysregulation. To limit mortality in severe cases of COVID-19, it is necessary to develop new therapeutic options to mitigate the cytokine storm (180). Esposito et al. (2020) indicate that due to the rapid spread of the pandemic, the ideal drug candidate should already be used in treating other diseases, have a good safety profile, and act to mitigate the cytokine storm through immunomodulation rather than immunosuppression (179). Recently, it was shown that Cannabis sativa extracts with a high concentration of CBD down-regulate the activity of ACE2 and TMPRSS2 enzymes, which are vital for SARS-CoV-2 to enter the human body (181). In recent years, CBD has been the subject of much research due to its broad spectrum of therapeutic effects, including anti-seizure, calming, sleep-inducing, anti-psychotic, anti-cancer, anti-inflammatory, and neuroprotective effects (182). What has been emphasized is the lack of adverse psychotropic effects of cannabidiol and its favorable safety profile in humans (182). The pharmacological activity of CBD was tested in patients suffering from various conditions, including respiratory diseases characterized by acute lung injury (179). CBD effect on adenosine A2A receptors limited leukocyte migration to the lungs, which was accompanied by a significant decrease in pro-inflammatory cytokine (TNF-α and IL-6) and chemokine (MCP-1/MIP-2/CXCL2) release, thus clearly improving the compromised lung function (131, 183).

CBD has also been studied as a molecule with a potential anti-viral effect. CBD acts by interacting with nuclear hormone receptors PPAR. It was shown that down-regulation of PPARγ expression by alveolar macrophages significantly reduces lung inflammation and enhances regeneration after viral respiratory tract infections (184). Preventive and therapeutic administration of PPARγ antagonists decreased morbidity and mortality related to influenza A virus infection (185). However, the use of full PPARγ agonists has several adverse effects, including the risk of cardiovascular complications, cardiac insufficiency, and stroke. It needs to be verified whether CBD, as a weak antagonist of PPARγ, could be used without causing such adverse effects (179). There is no direct proof of the anti-viral activity of cannabinoids in viral infections. Such products have been described in vitro studies. Lowe et al. demonstrated an anti-viral effect of CBD against the hepatitis C virus (HCV), but not against HBV, in cell lines for producing these viruses (128). Anti-viral activity of CBD was also confirmed against Kaposi’s sarcoma-associated herpesvirus (KSHV) in a model of KSHV-infected human dermal microvascular endothelial cells (HMVECs) (129). In yet another study, CBD mitigated the effects of neuroinflammation induced by Theiler’s murine encephalomyelitis virus (TMEV) (186). Respiratory syncytial virus (RSV) used in the mouse model has indicated that CB2 activation reduced infection symptoms, and CB1 antagonist administration alleviated pulmonary complications (176).

Research has shown that CBD is a reasonably safe molecule (187). In the case of COVID-19 patients, it is essential to establish the toxicity profile of CBD when administered concomitantly with other drugs used in the current anti-COVID-19 protocols.

First, pharmacokinetic (PK) and pharmacodynamic (PD) interactions between cannabinoids and experimental COVID-19 drugs must be determined. Both Δ⁹-THC and CBD are lipophilic, highly protein-bound molecules. They have a long half-life, undergo bioaccumulation, and share metabolic pathways with cytochrome P450, drug transporters (e.g., breast cancer resistance protein), and substrates that bind to plasma proteins (127). Furthermore, PK (e.g., warfarin and clobazam) and PD (e.g., valproic acid) interactions were described for THC and CBD (188). Land et al. (2020) (127) compiled a table with 16 compounds examined for their potential use in COVID-19 treatment and their possible pharmacokinetic or pharmacodynamic interactions with cannabinoids.

They found that most candidate drugs could interact with THC and CBD. The authors propose that COVID-19 patients should be asked about their use of substances containing cannabinoids, as these may significantly affect their reaction to the selected treatment method (127).

Esposito points to the possibility of testing the therapeutic potential of CBD in COVID-19 patients at the beginning of the disease to suppress the cytokine storm, prevent the danger of respiratory failure, or assess the effect of CBD on pulmonary fibrosis. The central aspect of being clarified is dosing. In the case of HIV and post-Ebola syndrome, CBD was used as an agent controlling immune activation at doses of 10–20 mg · kg⁻¹ · day⁻¹ and 1.7–10 mg · kg⁻¹ · day⁻¹ (100 mg · day⁻¹ titrating up to 600 mg · day⁻¹) (189, 190).

The results of pre-clinical studies are encouraging. However, evidence is needed to approve cannabidiol as a supportive drug in COVID-19 treatment.