Cannabis sativa L. has long been used as a herbal remedy and is a rich source of phytocannabinoids, which interact with the endocannabinoid system (ECS) to influence various physiological and pathological processes. In cancer patients, cannabinoids have been utilized in palliative care to alleviate pain, nausea, and stimulate appetite. Additionally, numerous studies have demonstrated the potential anticancer effects of cannabinoids in different cancer types using cell culture and animal models. This review aims to explore the mechanisms of action underlying the anticancer effects of plant-derived and synthetic cannabinoids and assess their role in cancer treatment. Moreover, we examine the current legislative landscape governing the medical and therapeutic use of cannabinoids in EU countries. While cannabinoids have shown promising results in modulating tumor growth, their efficacy appears to vary depending on cancer type and drug dosage. Understanding how cannabinoids regulate crucial cellular processes involved in tumorigenesis and their interactions with the immune system is pivotal for developing effective therapeutic approaches. However, it is worth noting that the existing EU legislation on cannabinoid use may not fully align with the current scientific knowledge.
Cannabis sativa L., the first discovered and most significant source of cannabinoids, has been utilized as an herbal remedy for centuries. Historical evidence from ancient China and India demonstrates the early medical uses of cannabis for various conditions. In Western medicine, cannabis gained recognition in the mid-19th century, with physicians reporting positive effects on pain, vomiting, convulsions, and mental abilities. However, due to recreational use, abuse potential, lack of standardized quality, and alternative medications, cannabis fell out of favor in the early 20th century. Legal restrictions further hindered the exploration of its medical uses for several decades. Recent changes in legislation in the European Union (EU), US, and Canada, along with scientific advancements and increased public awareness, have revived interest in the therapeutic potential of cannabinoids.
Cannabinoids have been extensively studied for their potential anticancer effects and management of cancer-related symptoms. Early research in 1975 described the antineoplastic activity of cannabinoids, and current studies focus on the antitumor potential of plant-derived or synthetic cannabinoids. Notable examples include (−)-trans-∆9-tetrahydrocannabinol (THC), cannabinol (CBN), ∆8-THC, cannabidiol (CBD), cannabicyclol (CBL), and synthetic cannabinoid WIN-55,212-2.
In the 1990s, the components of the endocannabinoid system (ECS) were identified, including CB1 and CB2 receptors, endocannabinoids (anandamide and 2-arachidonoylglycerol), and metabolic enzymes (FAAH and MAG lipase). The ECS has been found to play a crucial role in various physiological and pathological processes, including CNS regulation, food intake, pain signaling, immune modulation, inflammation, and cancer cell signaling.
Cannabinoids have primarily been used in cancer patients as part of palliative care to alleviate pain, nausea, and stimulate appetite. Additionally, cell culture and animal studies have demonstrated the potential antitumor effects of cannabinoids, opening new therapeutic possibilities for cancer patients. However, it is essential to consider safety precautions, particularly regarding cognitive function impairment, especially in adolescents.
This article aims to review the existing literature on the anticancer effects of plant-derived and synthetic cannabinoids, shedding light on their mechanisms of action and potential role in cancer treatment. Furthermore, it examines the current legislative updates concerning the medical and therapeutic use of cannabinoids, primarily focusing on the EU countries.
Molecular Basis for Cannabinoid Treatment of Cancer
The Endocannabinoid System (ECS) plays a crucial role in cancer. Endocannabinoids interact with various receptors, including the CB1 and CB2 receptors, which are Gi/o-coupled receptors. CB1 receptors are predominantly found in the central nervous system (CNS) and to a lesser extent in some peripheral tissues. On the other hand, CB2 receptors are primarily expressed on immune cells. The low expression of CB2 receptors in the CNS makes them an attractive pharmacological target, as selective CB2 ligands may not have psychotropic effects. Additionally, there are other receptor types, isoforms, and alternative pharmacological targets of cannabinoids, such as TRPV1, GPR55, PPARs, TRPM8, TRPV2, and TRPA1 channels.
It’s important to note that cannabinoids can exert their antitumor effects independently of CB receptors. For example, studies have shown that cannabinoids can demonstrate antitumor effects in human pancreatic cancer cells (MIA PaCa-2) without involving CB receptors.
The precise biological role of the ECS in cancer pathophysiology is not fully understood. However, research suggests that CB receptors and their endogenous ligands are upregulated in tumor tissue. Furthermore, the overexpression of ECS components, including receptors, ligands, and enzymes, is associated with tumor aggressiveness. Nevertheless, some studies have also indicated a tumor-suppressive role of the ECS, as the upregulation of endocannabinoid-degrading enzymes has been observed in aggressive human cancers and cancer cell lines.
Experimental studies have demonstrated that the activation of CB receptors by cannabinoids generally exhibits antitumorigenic effects. This includes inhibiting tumor cell proliferation, inducing apoptosis in vitro, and blocking angiogenesis, tumor invasion, and metastasis in vivo. The effects of CB receptor expression or overexpression in specific human tumor cell lines are further detailed in Table 1.
Image Source: (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6387667/)
Antitumor Effects of Cannabinoids
Cannabinoids exert their antitumor effects by targeting the ECS, impacting various essential cellular processes and signaling pathways crucial for tumor development. They have been shown to induce cell cycle arrest, promote apoptosis (programmed cell death), and inhibit proliferation, migration, and angiogenesis in tumor cells (Figure 1).
These effects of cannabinoids are not limited to CB receptor-mediated actions through CB1 and CB2 receptors. It has been observed that cannabinoids can also act independently of CB receptors, engaging other receptors such as TRPV1, 5-hydroxytryptamine (5-HT)3, or nicotinic acetylcholine receptors (nAChR), among others. This suggests that the molecular mechanisms underlying the antitumor activity of cannabinoids are even more intricate than initially understood. Additionally, ongoing research is expected to unveil novel molecular targets of cannabinoids, further expanding our understanding of their anticancer properties.
Image Source: (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6387667/)
Signaling Pathways Induced by Cannabinoids in Cancer Cells
Cannabinoids, by targeting the endocannabinoid system (ECS), impact various cellular processes and signaling pathways crucial for tumor development. They can modulate multiple pathways involved in cell cycle regulation, apoptosis, proliferation, migration, and angiogenesis in tumor cells. Here are examples of different signaling pathways influenced by cannabinoids in cancer cells:
- Cell Cycle Regulation:
- Induction of cell cycle arrest
- Modulation of cyclin-dependent kinases (CDKs) such as Cdk 2
- Regulation of cyclins, including Cyclin D and Cyclin E
- Inhibition of Akt (Protein Kinase B) signaling
- Activation of AMPK (5′ adenosine monophosphate-activated protein kinase)
- Promotion of apoptosis (programmed cell death)
- Activation of pro-apoptotic factors like Bax
- Inhibition of anti-apoptotic factors like Bcl-2
- Induction of reactive oxygen species (ROS) production
- Activation of p8 (Candidate of metastasis 1) and CHOP (C/EBP homologous protein)
- Proliferation and Survival:
- Suppression of proliferation signaling pathways such as PI3K (Phosphoinositide 3-kinase)/Akt/mTORC1 (mammalian target of rapamycin complex 1)
- Inhibition of mTORC2 (mammalian target of rapamycin complex 2)
- Modulation of growth factor signaling pathways, including ERK (Extracellular-signal-regulated kinase)
- Regulation of TRIB3 (Tribbles homolog 3) expression
- Migration and Invasion:
- Inhibition of cell migration and invasion
- Modulation of calcium signaling pathways involving TRPV1 (Transient receptor potential vanilloid receptor 1), TRPV2 (Transient receptor potential vanilloid receptor 2), and TRPM8 (Transient receptor potential melastatin 8)
- Interaction with GPR55 (Orphan G-protein coupled receptor 55) and other receptors involved in migration and invasion processes
- Inhibition of angiogenesis, the formation of new blood vessels
- Regulation of factors involved in angiogenesis, including VEGF (Vascular endothelial growth factor)
- Modulation of endothelial cell functions and signaling pathways
It is important to note that the effects of cannabinoids on these signaling pathways can vary depending on the specific cannabinoids used, the cancer type, and other factors. Ongoing research aims to further elucidate the complex molecular mechanisms underlying the anticancer effects of cannabinoids.
PLANT-DERIVED CANNABINOIDS AND THEIR ANTITUMOR ACTIVITY
Phytocannabinoids are a group of C21 terpenophenolic compounds primarily found in plants from the Cannabis genus. There are over 90 different cannabinoids identified, although some sources suggest a more conservative estimate of over 60 compounds. The most abundant phytocannabinoids include THC, CBD, CBN, and cannabichromene (CBC), followed by ∆8-THC, cannabidiolic acid (CBDA), cannabidivarin (CBDV), and cannabigerol (CBG). The highest concentrations of cannabinoids are typically found in the flowering tops of the plant and the young leaves surrounding the flowers.
Pharmacologically, THC acts as a partial agonist at CB1 and CB2 receptors, with inhibitory constants (Ki) of 40.7 nM for CB1 and 36.4 nM for CB2. ∆8-THC, a stable isomer of THC, exhibits similar Ki values. CBD, the most extensively studied non-psychotropic phytocannabinoid, lacks psychotomimetic activity. It has low affinity for CB1 and CB2 receptors and has been proposed to act as an antagonist of CB1/CB2 agonists and a CB2 inverse agonist. Additionally, CBD exerts its effects through other mechanisms independent of CB receptors, such as inhibition of FAAH, inhibition of anandamide (AEA) reuptake, activation of PPARγ, and modulation of TRPV1, TRPA1, GPR55, and TRPM8 receptors (Table 2). CBN acts as a weak partial agonist at CB1 (Ki of 308 nM) and CB2 (Ki of 96.3 nM) receptors. CBG functions as a potent TRPM8 antagonist, TRPV1 and TRPA1 agonist, and partial agonist at CB receptors. CBC acts as a potent TRPA1 agonist and a weak inhibitor of AEA reuptake.
Image Source: (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6387667/)
Plant-derived cannabinoids have shown promise in various therapeutic applications, both approved and off-label. For instance, nabiximols, a standardized cannabis extract containing THC and CBD in a 1:1 ratio, is approved in Germany for treating refractory spasticity in multiple sclerosis. Off-label use includes the management of chronic pain and symptomatic treatment of neuropsychological disorders like anxiety and sleep disturbances. Common side effects of cannabinoids include tiredness, dizziness, dry mouth, and psychoactive effects, although tolerance to these effects develops quickly in most cases. Withdrawal symptoms are rare in therapeutic settings.
Advancements in pharmaceutical formulations have led to the development of new cannabis-based extracts. For example, a CO2-extracted cannabis extract with a high content of Δ9-tetrahydrocannabivarin (THCV) has shown potential in modulating inflammatory responses. Another study compared the antioxidant activity and gene expression of antioxidant enzymes in ethanol and supercritical fluid extracts of hemp seed. The supercritical fluid extract exhibited higher antioxidant activity and upregulated the expression of antioxidant enzymes in human hepatoma cells challenged with H2O2.
Research has focused on investigating the antitumor activity of different plant-derived cannabinoids and cannabis-based pharmaceutical drugs, particularly in cancer cells that overexpress CB1 and/or CB2 receptors. However, the results can be contradictory, as cannabinoids have shown both antitumor and protumorigenic effects depending on their concentration and other factors. For example, low concentrations of THC have been associated with increased cell proliferation in certain cancer cell lines. Therefore, concentration-dependent proliferative potential must be considered when using cannabinoids in cancer therapy.
CBD, another plant-derived cannabinoid, has been extensively studied for its potential antitumor effects. It has been shown to inhibit cancer cell growth in prostate, breast, and glioma cell lines. CB1 and CB2 receptors are also expressed on immune cells, indicating their role in immune system regulation. Studies have demonstrated that cannabinoids can suppress inflammatory responses, downregulate cytokine and chemokine production, and upregulate T-regulatory cells. However, cannabinoids have also been found to stimulate cell proliferation in certain cancer models and inhibit antitumor immunity, potentially through CB2 receptor-mediated cytokine pathways. Further research on cannabinoid-immune cell interactions is crucial for improving the efficacy and safety of cannabinoid therapy in oncology.
SYNTHETIC CANNABINOIDS WITH POTENTIAL ANTITUMOR EFFECTS
Synthetic cannabinoids, including dronabinol, nabilone, and synthetic CBD, are CB1 and CB2 receptor ligands. They exhibit similar physiological, psychoactive, analgesic, anti-inflammatory, and anticancer effects as plant-derived cannabinoids but can be up to 100 times more potent than THC. In vitro studies have demonstrated the anticancer effects of synthetic cannabinoid agonists in certain cancer cell lines.
Dronabinol and nabilone, synthetic THC, as well as synthetic CBD in the form of oil and alcohol-based drops or capsules, are approved for treating cytostatic-induced nausea/vomiting in cancer patients and stimulating appetite in AIDS patients.
A new class of compounds has emerged that targets metabolic enzymes involved in ECS regulation, such as inhibitors of FAAH. These compounds aim to treat neurological diseases, chronic pain, obesity, and cancer. Combination studies using synthetic AEA analogues and FAAH inhibitors have shown synergistic inhibition of proliferative and chemotactic activity in lung cancer cell lines, as well as antimetastatic effects.
However, there have been tragic incidents in clinical trials, such as the case of the experimental FAAH inhibitor BIA 10-2474, which resulted in one death and irreversible brain damage in four patients. Other FAAH inhibitors, such as Merck’s MK-4409, Pfizer’s PF-04457845, and Vernalis’ V158866, have been considered safe in humans.
In summary, synthetic cannabinoids exhibit similar antitumor effects to plant-derived cannabinoids, including inhibition of cell growth, viability, proliferation, and invasion, enhanced apoptosis, and suppression of specific proinflammatory cytokines. Synthetic cannabinoids have the potential to be more selective and potent, making them a promising therapeutic approach. However, the safety and targeting of synthetic cannabinoids require further research and careful evaluation.
INTERNATIONAL AND NATIONAL LEGAL BASIS FOR THE USE OF CANNABINOIDS
The legislation surrounding the use, cultivation, and marketing of cannabinoids is evolving to accommodate the growing research on their medical and therapeutic potential. In the Republic of Slovenia, significant progress was made in 2017, which will be discussed below.
The development of legislation on cannabinoid use in European Union (EU) Member States is guided by international conventions, including the United Nations Single Convention on Narcotic Drugs, 1961, the 1972 Protocol amending the Single Convention on Narcotic Drugs, the Convention on Psychotropic Substances 1971, and the United Nations Convention against Illicit Traffic in Narcotic Drugs and Psychotropic Substances 1988.
The United Nations Convention against Illicit Traffic in Narcotic Drugs and Psychotropic Substances provides legal mechanisms to enforce the 1961 Single Convention on Narcotic Drugs and the 1971 Convention on Psychotropic Substances. While the treaty primarily focuses on combating organized crime, it also prohibits the possession of drugs for personal use, including the cultivation of opium poppy, coca bush, and cannabis plants for the production of narcotic drugs.
The United Nations Single Convention on Narcotic Drugs, 1961 categorizes controlled substances into four Schedules. Cannabis, cannabis resin, extracts, and tinctures are listed in Schedule IV. Tetrahydrocannabinol (THC) is included in Schedule I of the Convention on Psychotropic Substances, and its stereoisomers, including dronabinol, are listed in Addendum 2. Nabilone is not controlled under international law.
Within the EU regulatory framework, Directive 2001/83/EC regulates medicinal products for human use. The directive exempts medicinal products prepared in a pharmacy with a medical prescription, those prepared in accordance with pharmacopoeia prescriptions, and medicinal products intended for research and development trials. Medicinal products based on cannabinoids can be made available in EU Member States, provided they are permitted by national legislation.
In the Republic of Slovenia, illicit drugs, including cannabis, are governed by various regulations, including the Production of and Trade in Illicit Drugs Act, Act Regulating the Prevention of the Use of Illicit Drugs and the Treatment of Drug Users, Criminal Code, Decree on the classification of illicit drugs, Rules on method and form of record-keeping and reports on illicit drugs, and Rules governing the procedures for the issue of licenses for illicit drug marketing.
In 2017, an amendment to the Decree on the classification of illicit drugs in Slovenia was adopted. This amendment moved cannabis from Schedule I to Schedule II and permitted its use for medicinal purposes in accordance with the Medicinal Products Act and Pharmacy Services Act. The amendment expanded the scope of medicinal cannabis to include the cannabis plant, cannabis resin, and tinctures containing adjusted and harmonized delta-9-THC, as long as they meet the conditions specified in the Medicinal Products Act.
Changes in legislation regarding the use of cannabinoids for medical purposes should also consider related areas such as labor law and workplace drug testing regulations. Any changes should be implemented in accordance with work, health, and safety regulations to ensure a smooth workflow for employees.