Cannabinoids in cancer treatment: Therapeutic potential and legislation
The plant Cannabis sativa L. has been used as an herbal remedy for centuries and is the most important source of phytocannabinoids. The endocannabinoid system (ECS) consists of receptors, endogenous ligands (endocannabinoids) and metabolizing enzymes, and plays an important role in different physiological and pathological processes. Phytocannabinoids and synthetic cannabinoids can interact with the components of ECS or other cellular pathways and thus affect the development/progression of diseases, including cancer. In cancer patients, cannabinoids have primarily been used as a part of palliative care to alleviate pain, relieve nausea and stimulate appetite. In addition, numerous cell culture and animal studies showed antitumor effects of cannabinoids in various cancer types. Here we reviewed the literature on anticancer effects of plant-derived and synthetic cannabinoids, to better understand their mechanisms of action and role in cancer treatment. We also reviewed the current legislative updates on the use of cannabinoids for medical and therapeutic purposes, primarily in the EU countries. In vitro and in vivo cancer models show that cannabinoids can effectively modulate tumor growth, however, the antitumor effects appear to be largely dependent on cancer type and drug dose/concentration. Understanding how cannabinoids are able to regulate essential cellular processes involved in tumorigenesis, such as progression through the cell cycle, cell proliferation and cell death, as well as the interactions between cannabinoids and the immune system, are crucial for improving existing and developing new therapeutic approaches for cancer patients. The national legislation of the EU Member States defines the legal boundaries of permissible use of cannabinoids for medical and therapeutic purposes, however, these legislative guidelines may not be aligned with the current scientific knowledge.
The first discovered and most important source of cannabinoids was the plant Cannabis sativa L., which has been used as an herbal remedy for centuries. The earliest archaeological evidence of cannabis medical use dates back to the Han Dynasty in ancient China, where it was recommended for rheumatic pain, constipation, disorders of the female reproductive tract, and malaria among other conditions. In traditional Indian Ayurvedic medicine, cannabis was used to treat neurological, respiratory, gastrointestinal, urogenital, and various infectious diseases . The plant was also cultivated in other countries in Asia as well as in Europe, especially for making ropes, clothes/fibres, food and paper . In Western medicine, the use of cannabis was notably introduced by the work of William B. O’Shaughnessy (an Irish physician) and Jacques-Joseph Moreau (a French psychiatrist) in the mid-19 th century, who described positive effects of cannabis preparations, including hashish (the compressed stalked resin glands), on pain, vomiting, convulsions, rheumatism, tetanus and mental abilities. Cannabis was recognized as a medicine in the United States (US) Pharmacopoeia from 1851, in the form of tinctures, extracts and resins. However, in the beginning of the 20 th century, cannabis use decreased in Western medicine due to several reasons: increased use as a recreational drug, abuse potential, variability in the quality of herbal material, individual (active) compounds were not identified and alternative medications, with known efficacy, were introduced to treat the same symptoms [2,3]. In 1941, as the result of many legal restrictions, cannabis was removed from the American Pharmacopoeia and considered to be in the same group as other illicit drugs . Consequently, the exploration of medical uses of cannabis has been significantly slowed down for more than a half of century. In 2013, a step forward was made with the inclusion of a monograph of Cannabis spp. in the American Herbal Pharmacopoeia . Moreover, the current legislative changes in the European Union (EU), US and Canada that allow cannabis for medical and/or recreational use, the progress in scientific research and public awareness on the benefits of medical cannabis all contributed to the rising interest in the therapeutic potential of cannabinoids [5,6].
In recent years, cannabinoids have been extensively studied for their potential anticancer effects and symptom management in cancer patients [7-9]. One of the first studies describing antineoplastic activity of cannabinoids was published in 1975 . Potential antitumor activity of plant-derived or phytocannabinoids, e.g., (−)-trans-∆9-tetrahydrocannabinol (THC), cannabinol (CBN), ∆8-THC, cannabidiol (CBD) and cannabicyclol (CBL), as well as of synthetic cannabinoids, such as WIN-55,212-2, is the focus of current research [7,8,11].
In the 1990s, the main components of the endocannabinoid system (ECS) were identified as follows: (i) two types of cannabinoid (CB) receptors, CB1 and CB2 receptor; (ii) two main endogenous ligands (endocannabinoids) in mammals, anandamide or N-arachidonoyl ethanolamine (AEA) and 2-arachidonoylglycerol (2-AG); and (iii) endocannabinoid metabolic enzymes, fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAG lipase). FAAH is the primary catabolic enzyme for fatty acid amides (FAAs), a class of bioactive lipids including AEA, while MAG lipase is a key enzyme in the hydrolysis of 2-AG [12-16]. Subsequent studies demonstrated the important role of the ECS and endocannabinoids in different physiological and pathological processes, such the regulation of excitatory and inhibitory synaptic transmission in the central nervous system (CNS), food intake, nociceptive signaling, analgesia, immunomodulation, inflammation, and cancer cell signaling [17-19].
In cancer patients, cannabinoids have primarily been used as a part of palliative care to alleviate pain, relieve nausea and stimulate appetite [8,20]. In addition, numerous cell culture and animal studies showed antitumor effects of cannabinoids and suggested new therapeutic opportunities for cancer patients . However, recent research also emphasizes the importance of safety measures when using cannabinoids, since these compounds can potentially impair cognitive functions, especially in adolescents .
The aim of this article is to review the relevant literature on anticancer effects of plant-derived and synthetic cannabinoids, to increase our understanding of their potential mechanisms of action and possible role in cancer treatment. We also reviewed the current legislative updates on the use of cannabinoids for medical and therapeutic purposes, primarily in the EU countries.
MOLECULAR BASIS FOR CANNABINOID TREATMENT OF CANCER
The role of the endocannabinoid system in cancer
Endocannabinoids interact with different types of receptors, including the two Gi/o-coupled CB receptors, CB1 and CB2 . While CB1 receptors are mainly located in the CNS and, to a lesser degree, in some peripheral tissues, CB2 receptors are primarily expressed on the surface of immune cells . Due to the low expression of CB2 receptors in the CNS they represent a promising pharmacological target, as selective CB2 ligands potentially would not have psychotropic effects . In addition, other CB receptor types and isoforms or completely different pharmacological targets of cannabinoids have been described, for example transient receptor potential vanilloid receptor 1 (TRPV1), orphan G-protein coupled receptor (GPR)55, peroxisome proliferator-activated receptors (PPARs) [24,25], transient receptor potential melastatin 8 (TRPM8), TRP vanilloid 2 (TRPV2) and TRP ankyrin 1 (TRPA1) channel . It is important to note that cannabinoids may also exert their antitumor effects independent of the CB receptors, for example as demonstrated in human pancreatic cancer cell line MIA PaCa-2 .
The biological role of the ECS in cancer pathophysiology is not completely clear  but most studies suggest that CB receptors and their endogenous ligands are upregulated in tumor tissue [28,29,31,34-39,41,48] and that the overexpression of ECS components (i.e., receptors, ligands, and enzymes) correlates with tumor aggressiveness [49-51]. However, a tumor-suppressive role of ECS was also indicated by some studies, e.g., the upregulation of endocannabinoid-degrading enzymes was observed in aggressive human cancers and cancer cell lines . Moreover, experimental studies showed that the activation of CB receptors by cannabinoids is antitumorigenic in most cases, i.e., it inhibits tumor cell proliferation, induces apoptosis in vitro, and blocks angiogenesis and tumor invasion/metastasis in vivo [35,46,51,52]. The effects of CB receptor (over)expression in selected human tumor cell lines are described in more detail in Table 1 .
Expression of cannabinoid (CB) receptors in selected human cancer types
Antitumor effects of cannabinoids
By targeting the ECS, cannabinoids affect many essential cellular processes and signaling pathways which are crucial for tumor development [51,53,54]. For example, they can induce cell cycle arrest, promote apoptosis, and inhibit proliferation, migration and angiogenesis in tumor cells ( Figure 1 ) [53,54]. In addition to CB receptor-mediated (CB1 and CB2 receptors) cannabinoid effects, it appears that these processes can also be CB receptor-independent (e.g., through TRPV1, 5-hydroxytryptamine [5-HT]3, or nicotinic acetylcholine receptor [nAChR] among others) , suggesting that molecular mechanisms underlying the antitumor activity of cannabinoids are even more complex than originally thought. Moreover, it is expected that future studies will discover novel molecular targets of cannabinoids .
Example of different signaling pathways induced by cannabinoids in cancer cells [46,51,53-55]. By targeting the endocannabinoid system (ECS), cannabinoids affect many essential cellular processes and signaling pathways which are crucial for tumor development. For example, they can induce cell cycle arrest, promote apoptosis, and inhibit proliferation, migration and angiogenesis in tumor cells. AEA: Anandamide; 2-AG: 2-Arachidonoylglycerol; Akt: Protein Kinase B; AMPK: 5’ adenosine monophosphate-activated protein kinase; Bad: Bcl-2-associated death promoter; Bax: Apoptosis regulator; CaMKK: Calcium/calmodulin-dependent protein kinase kinase; Cdk 2: Cyclin-dependent kinase 2; CHOP: C/EBP homologous protein; CycD: Cyclin D; Cyc E: Cyclin E; ELK1: ETS domain-containing protein; ERK: Extracellular-signal-regulated kinase; FAAH: Fatty acid amide hydrolase; GPR55: Orphan G-protein coupled receptor 55; MAG lipase: Monoacylglycerol lipase; MAPK: Mitogen-activated protein kinase; p8: Candidate of metastasis 1; p21: Cyclin-dependent kinase inhibitor 1; p27: Cyclin-dependent kinase inhibitor 1B; PI3K: Phosphoinositide 3-kinase; PKA: Protein kinase A; ROS: Reactive oxygen species; TRPV1: Transient receptor potential vanilloid receptor 1; TRPV2: Transient receptor potential vanilloid receptor 2; TRPM8: Transient receptor potential melastatin 8; mTORC1: Mammalian target of rapamycin complex 1; mTORC2: Mammalian target of rapamycin complex 2; TRIB3: Tribbles homolog 3.
The ability of plant-derived and synthetic cannabinoids to control cancer cell growth, invasion, and death has been demonstrated in numerous experimental studies using cancer cell lines and genetically engineered mouse models. Also, different types of cannabinoids may have different modes of action. For example, a phytocannabinoid THC promotes apoptosis in a CB-receptor dependent manner, while CBD exerts this effect independently of CB1/CB2 receptors and possibly includes the activation of TRPV2 receptor, at least in some cancer types. Also, some CB receptor agonists are less efficient in promoting cancer cell death although they demonstrate higher affinity for CB receptors than THC, such as synthetic CB receptor agonist WIN-55,212-2. Better understanding of homo- or hetero-oligomerization of CB receptors, their interactions with lipid rafts for example, and mechanisms of selective G-protein coupling may clarify these differences . Finally, because molecular changes are tumor-specific in most cases (i.e., the presence of intra- and inter-tumor heterogeneity), CB-receptor mediated antitumor effects largely depend on the type of cancer that is being investigated and characteristics of derived tumor cell line, including the donor characteristics, tumor site of origin and hormonal responsiveness [53-55].
PLANT-DERIVED CANNABINOIDS AND THEIR ANTITUMOR ACTIVITY
Phytocannabinoids are a group of C21 terpenophenolic compounds predominately produced by the plants from the genus Cannabis. Different resources indicate that there are more than 90 different cannabinoids together with their breakdown products, although some report that > 60 compounds is a more accurate estimation. Among these, the most abundant are THC, CBD, CBN and cannabichromene (CBC) followed by ∆8-THC, cannabidiolic acid (CBDA), cannabidivarin (CBDV) and cannabigerol (CBG). The highest content of cannabinoids is located in the flowering tops of the plant and small, young leaves around the flowers .
Pharmacologically, THC is a partial agonist at CB1 and CB2 receptor with inhibitory constant (Ki) of 40.7 nM for CB1 and 36.4 nM for CB2 . ∆8-THC is a stable isomer of THC with similar Ki . The most studied non-psychotropic phytocannabinoid is CBD which does not have psychotomimetic activity. CBD has a low affinity for CB1 and CB2; it was suggested that it acts as an antagonist of CB1/CB2 agonists but also as a CB2 inverse agonist (an inverse agonist binds to the same receptor-binding site as an agonist and it does not only antagonize the effects of the agonist but exerts the opposite effect). Other mechanisms of action of CBD, that are independent of CB receptors, include FAAH inhibition, inhibition of AEA reuptake, it acts as an agonist at PPARγ, TRPV1, TRPA1 and an antagonist at GPR55 and TRPM8 ( Table 2 ). CBN is a weak partial agonist at CB1 (Ki of 308 nM) and CB2 (Ki of 96.3 nM); CBG is a potent TRPM8 antagonist, TRPV1 and TRPA1 agonist, and CB partial agonist; while CBC is a potent TRPA1 agonist and weak inhibitor of AEA reuptake .
Antitumor activity of selected plant-derived cannabinoids in different cancer cell lines
Plant-derived cannabinoids are approved only for some indications, but additionally have been used off-label. For example, a standardized alcoholic cannabis extract nabiximols, which has the THC: CBD ratio of 1:1 and is available as an oromucosal spray, was approved in Germany for the treatment of moderate to severe refractory spasticity in multiple sclerosis. Examples of off-label use of this medication are of chronic pain in several medical conditions and symptomatic treatment of selected neuropsychological disorders (e.g., anxiety and sleeping disturbances). Common side effects of cannabinoids are tiredness and dizziness (in more than 10% of patients), dry mouth, and psychoactive effects among others. Nevertheless, tolerance to these side effects develops within a short time in almost all cases. Withdrawal symptoms are rarely observed in the therapeutic setting .
An exciting area of research is the technological improvement of existing pharmaceutical formulations, especially the development of new cannabis-based extracts. Romano et al.  found that a CO2 extracted cannabis extract, with a high content (64.8%) in Δ9-tetrahydrocannabivarin (THCV), inhibits nitrite production induced by lipopolysaccharides (LPS) in murine peritoneal macrophages, and thus may have a potential to modulate the inflammatory response in different disease conditions . Another study compared in vitro antioxidant activity and gene expression of antioxidant enzymes between ethanol and supercritical fluid (SF) extracts of dehulled hemp seed. SF extract exhibited higher radical scavenging activities compared to ethanol extract. Both extracts upregulated the expression of the antioxidant enzymes superoxide dismutase (SOD), glutathione peroxidase (GPx), and catalase (CAT) in human hepatoma (HepG2) cells challenged with H2O2, and this effect was greater for SF extracts at the concentration of 500 µg/mL .
Different plant-derived cannabinoids and cannabis-based pharmaceutical drugs have been the subject of intensive research for their potential antitumor activity, especially in cancer cells that overexpress CB1 and/or CB2 receptors compared to normal tissues . Many studies were conducted in different cell lines with cannabis extracts or individual isolated compounds and the results are sometimes confounding, because efficient anticancer effects, such as decreased proliferation of cancer cells, activation of apoptosis, inhibition of cell migration and decreased tumor vascularization are mainly recorded in breast, prostate and glioma cancer cell lines. In contrast, protumorigenic activity of natural cannabinoids, i.e., increased cell proliferation, has been reported in lung, breast, and hepatoma cell lines . It appears that the balance between protumorigenic and antitumor effects of cannabinoids critically depends on their concentration, among other factors. For example, Hart et al.  showed that the treatment of glioblastoma U373-MG and lung carcinoma NCI-H292 cell line with nanomolar concentrations of THC (instead of commonly used micromoral concentrations) led to increased cell proliferation. The authors also emphasized that nanomolar concentrations of THC are more likely to be detected in the serum of patients after drug treatment . Therefore, in cancer therapy, it is very important to consider the risk of acceleration of tumor growth due to the concentration-dependent proliferative potential of cannabinoids .
In addition to THC, CBD is another plant-derived cannabinoid that has been extensively studied for its potential antitumor effects [39,65-68]. In a panel of human prostate cancer cell lines, Sharma et al.  showed that CBD is a potent inhibitor of cancer cell growth, while this potency was significantly lower in non-cancer cells. Moreover, CBD downregulated CB1, CB2, vascular endothelial growth factor (VEGF) and prostate-specific antigen (PSA) in prostate cancer cells, as well as pro-inflammatory interleukin (IL)-6 and IL-8 in LPS-stimulated dermal fibroblasts, suggesting its anti-inflammatory properties . Other studies showed that CBD preferentially inhibited the survival of breast cancer cells by inducing apoptosis and autophagy  and inhibited proliferation and cell invasion in human glioma cell lines .
The expression of CB1 and CB2 receptors on immune cells suggests their important role in the regulation of the immune system. Recently, it was demonstrated that the administration of THC into mice induced apoptosis in T cells and dendritic cells, leading to immunosuppression. Several studies suggested that cannabinoids are able to suppress inflammatory responses by downregulating cytokine and chemokine production and upregulating T-regulatory cells. Similar results were obtained with endocannabinoids, i.e., the administration of these compounds or the use of inhibitors of enzymes that break down endocannabinoids had an immunosuppressive effect and resulted in the recovery from immune-mediated injury to organs, e.g., in the liver . As indicated in previous paragraphs, cannabinoids were able to stimulate cell proliferation in in vitro and/or in vivo models of several types of cancer. For example, a treatment with THC in the mouse mammary carcinoma 4T1 expressing low levels of CB1 and CB2 led to enhanced growth of tumor and metastasis, due to the inhibition of the antitumor immune response, primarily via CB2. Moreover, THC led to an increased production of IL-4 and IL-10 in these mice, indicating that it suppresses the Th1 response by enhancing Th2-associated cytokines as confirmed by their microarray data (Th2-related genes were upregulated and Th1-related genes downregulated). Lastly, the injection of anti-IL-4 and anti-IL-10 monoclonal antibodies partially reversed the THC-induced suppression of the immune response . In another study, THC promoted tumorigenicity in two weakly immunogenic murine lung cancer models by inhibiting their antitumor immunity; namely, the inhibitory cytokines IL-10 and transforming growth factor beta (TGF-β) were upregulated, while interferon gamma (IFN-γ) was downregulated at the tumor site and in the spleens of the mice treated with THC . These findings suggest that THC could decrease tumor immunogenicity and promote tumor growth by inhibiting antitumor immunity, probably via CB2 receptor-mediated, cytokine-dependent pathway. Additional studies on the interactions between cannabinoids and immune cells will provide crucial data to improve the efficacy and safety of cannabinoid therapy in oncology .
SYNTHETIC CANNABINOIDS WITH POTENTIAL ANTITUMOR EFFECTS
Most synthetic cannabinoids, including dronabinol, nabilone, and synthetic CBD are CB1 and CB2 receptor ligands . Studies in cells and animals show that they produce similar qualitative physiological, psychoactive, analgesic, anti-inflammatory, and anticancer effects to plant-derived cannabinoids, but they can be up to 100× more potent than THC [73,74]. Similar to naturally occurring cannabinoids, synthetic cannabinoid agonists also demonstrated anticancer effects in certain cancer cell lines in vitro [17,75]. Oil and alcohol-based drops or capsules of dronabinol and nabilone (synthetic THC) as well as synthetic CBD are approved to treat cytostatic-induced nausea/vomiting in cancer patients and to stimulate appetite in patients with acquired immune deficiency syndrome .
Recently, a subclass of compounds emerged that act on metabolic enzymes involved in the regulation of ECS activity, such as inhibitors of FAAH which increase the levels of endogenous cannabinoid AEA. They were developed with the purpose to treat a variety of neurological diseases, chronic pain, obesity, and cancer . A study investigating the combination of the synthetic analogue of AEA Met-F-AEA and the selective irreversible carbamate-based FAAH inhibitor URB597 showed that they synergistically inhibited epidermal growth factor (EGF)-induced proliferative and chemotactic activity of non-small cell lung cancer cell lines A549 and H460 . Moreover, the two FAAH inhibitors URB597 and arachidonoyl serotonin (AA-5HT) had antimetastatic effects on A549 lung cancer cell metastasis . However, recently in France, the first-in-human phase I clinical trial of an experimental FAAH inhibitor BIA 10-2474, for neuropathic pain treatment, ended up tragically; one person died and other four had irreversible brain damage [79,80]. The magnetic resonance imaging (MRI) showed evidence of deep cerebral hemorrhage and necrosis in the affected patients . Other clinical trials conducted on FAAH inhibitors are Merck’s MK-4409, Pfizer’s PF-04457845, and Vernalis’ V158866; no adverse effects were reported with these agents and they were considered safe in humans [79,81,82]. Thus, it could be speculated that the negative effects of BIA 10-2474 occurred because the drug may have interacted with a wrong and unexpected molecular target . Nevertheless, no FAAH inhibitor is yet approved for therapeutic use.
To summarize, the antitumor effects of synthetic cannabinoids such as the inhibition of cell growth, viability, proliferation and invasion, enhanced apoptosis, and suppression of specific proinflammatory cytokines are generally similar to the antitumor effects of plant-derived cannabinoids. Moreover, synthetic cannabinoids have the potential to be even more selective and potent than their natural counterparts and, thus, represent a promising therapeutic approach [73,74].
INTERNATIONAL AND NATIONAL LEGAL BASIS FOR THE USE OF CANNABINOIDS
As the number of studies investigating the medical and therapeutic potential of cannabinoids has increased in recent years, it is necessary to change the legislation on the use, cultivation, and marketing of cannabinoids. This should, however, be done with extreme care. In the Republic of Slovenia, the legislator made a significant progress in this area in 2017, which will be elaborated below.
In the EU Member States, the basis for developing and passing the legislation on cannabinoid use is provided by international conventions, including: i) the United Nations Single Convention on Narcotic Drugs, 1961  and the 1972 Protocol amending the Single Convention on Narcotic Drugs, ii) the Convention on Psychotropic Substances 1971 , and iii) 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 additional legal mechanisms for enforcing the 1961 Single Convention on Narcotic Drugs and the 1971 Convention on Psychotropic Substances. Much of the treaty is devoted to fighting organized crime, but it also prohibits possession of drugs for personal use saying that “Subject to its constitutional principles and the basic concepts of its legal system, each Party shall adopt such measures as may be necessary to establish as a criminal offence under its domestic law, when committed intentionally, the possession, purchase or cultivation of narcotic drugs or psychotropic substances for personal consumption contrary to the provisions of the Conventions”, and this includes the cultivation of opium poppy, coca bush and cannabis plant for the production of narcotic drugs .
The United Nations Single Convention on Narcotic Drugs, 1961 sets out four Schedules. Substances controlled by the state are set out in Schedule I and Schedule II, preparations in Schedule III, whereas Schedule IV defines drugs, such as heroin. The Single Convention’s Schedules range from most restrictive to least restrictive, as follows: Schedule IV, Schedule I, Schedule II, Schedule III. Cannabis, cannabis resin, extracts and tinctures are included in the Schedule IV of The Single Convention on Narcotic Drugs. Tetrahydrocannabinol (THC, synonym delta-9-THC) is included in the Schedule I of the Convention on Psychotropic Substances. Delta-9-THC and its stereoisomers, including dronabinol, are listed in the Addendum 2 to the Convention on Psychotropic Substances. Nabilone is not controlled under international law .
Under the EU regulatory framework, the subject matter is regulated by Directive 2001/83/EC of the European Parliament and of the Council of 6 November 2001 on the Community code relating to medicinal products for human use . Pursuant to the Article 3 of Directive 2001/83/EC, this Directive shall not apply to medicinal products prepared in a pharmacy in accordance with a medical prescription, medicinal products prepared in a pharmacy in accordance with the prescriptions of a pharmacopoeia, and medicinal products intended for research and development trials. This Directive also allowed the use of medicinal products for human use, intended to be placed on the market in the Member States and either prepared industrially or manufactured by a method involving an industrial process. This made cannabinoid-based medicinal products available in all the Member States, provided they are permitted by the national legislation .
In the Republic of Slovenia, illicit drugs including cannabis, are governed by the following regulations: i) Production of and Trade in Illicit Drugs Act , ii) Act Regulating the Prevention of the Use of Illicit Drugs and the Treatment of Drug Users , iii) Criminal Code of the Republic of Slovenia , iv) Decree on the classification of illicit drugs , v) Rules on method and form of record-keeping and of reports on illicit drugs , and vi) the Rules governing the procedures for the issue of licenses for illicit drugs marketing .
As previously mentioned, in 2017 the adoption of the Decree amending the Decree on the classification of illicit drugs  was made. This Decree removed cannabis from Schedule I and placed it under Schedule II, with the note that the use of cannabis for medicinal purposes is permitted in accordance with the Medicinal Products Act  and Pharmacy Services Act , and in accordance with the rules and regulations governing the prescribing of cannabinoid-based drugs.
The aforementioned amendment to the Decree on the classification of illicit drugs now allows patients to use medicinal cannabis as a means of treatment, including the cannabis plant and cannabis resin. Medicinal products are thus not limited anymore to products containing nabilone or cannabis extracts, but also extend to tinctures adjusted and harmonized to delta-9-THC, as long as they meet the conditions laid down in the Medicinal Products Act.
Changes in the legislation on the use of cannabinoids for medical purposes and inclusion of these compounds in the list of medicinal products needs to be coordinated with the changes in both labor law and the regulation of workplace drug testing. Naturally, any change should be adopted in strict agreement with work, health, and safety regulations and ensure smooth workflow for the employees.
Cannabinoids are a large and important class of complex compounds that have a promising therapeutic potential for the treatment of variety of diseases, including cancer. In this review, we focused on studies that provided evidence for anticancer effects of plant-derived and synthetic cannabinoids and their potential mechanisms of action. Cannabinoids were able to effectively modulate tumor growth in different in vitro and in vivo cancer models, however, these anticancer effects appears to be dependent on cancer type and drug dose. Understanding how cannabinoids are able to modulate essential cellular processes involved in tumorigenesis, such as the progression through the cell cycle, cell proliferation and cell death, as well as the interactions between cannabinoids and immune system are crucial for improving existing medications and developing new therapeutic approaches.
Although still strict, the legislation on the use of cannabis-based medications has been improved, especially following the promising results of related basic research. The Republic of Slovenia established a legal basis for the use of cannabinoids in the years 2016 and 2017. The increasing popularity of cannabis and cannabis-based medication should lead to clear regulatory guidelines on their use, in the near future.
The authors acknowledge Jan Schmidt for his initial help in preparing this manuscript.
What Is Cannabigerol (CBG)?
Toketemu has been multimedia storyteller for the last four years. Her expertise focuses primarily on mental wellness and women’s health topics.
Steven Gans, MD is board-certified in psychiatry and is an active supervisor, teacher, and mentor at Massachusetts General Hospital.
Verywell / Alex Dos Diaz
What Is Cannabigerol (CBG)?
Cannabigerol (CBG) is a type of cannabinoid obtained from the cannabis plant. It’s often referred to as the mother of all cannabinoids. This is because other cannabinoids are derived from cannabigerolic acid (CBGA), an acidic form of CBG.
Other more common cannabinoids obtained from cannabis plants include cannabidiol (CBD) and tetrahydrocannabinol (THC).
CBG is found in smaller quantities than other cannabinoids in cannabis plants. In most strains of the plant, only 1% of CBG can be found compared to 20 to 25% of CBD or 25 to 30% of THC.
This makes consumer products derived from the cannabinoid rare and often expensive. However, CBG is growing in popularity as a result of the host of potential benefits the cannabinoid has to offer.
How CBG Is Made
CBG is derived from young cannabis plants because they contain higher amounts of CBG than fully developed plants.
Some strains of cannabis like White CBG, Super Glue CBG, and Jack Frost CBG also have higher CBG content than other strains. These strains are specifically cultivated to produce higher quantities of CBG.
Both CBD and THC start as CBGA, an acidic form of CBG. This is why younger cannabis plants contain higher concentrations of CBG.
In fully developed plants with high concentrations of THC and CBD, you’ll find very low concentrations of CBG. This happens because most of the CBG has already been converted to CBD and THC as the plant developed.
Due to the difficulty of getting CBG, cannabis growers have been experimenting with cross-breeding and genetic manipulation to help cannabis plants produce more CBG.
How CBG Works
CBG is processed by the body’s endocannabinoid system. The endocannabinoid system is made up of molecules and receptors in our bodies that are responsible for keeping our bodies in an optimal state regardless of what’s going on in our external environment.
In our bodies, CBG imitates endocannabinoids , the natural compounds our body makes.
Cannabinoid Receptors in the Body
Our body contains two types of cannabinoid receptors—CB1 and CB2. CB1 receptors are found in the nervous system and brain, while CB2 receptors are located in the immune system and other areas of the body.
CBG works by binding to both receptors where it’s thought to strengthen the function of anandamide, a neurotransmitter that plays a role in enhancing pleasure and motivation, regulating appetite and sleep, and alleviating pain. Unlike THC, CBG has no psychotropic effects, so it will not give you a high.
Potential Benefits of CBG
Like CBD, CBG has been used to combat pain without having the intoxicating effect of cannabinoids like THC.
Research shows that CBG can also have therapeutic effects. However, human studies on this are sparse and more research needs to be done in this area.
Some promising animal studies show that CBG might ultimately be found useful for the following therapeutic benefits listed below.
Inflammatory Bowel Disease (IBD)
Inflammatory bowel disease is a condition that causes chronic inflammation in the bowel. It affects millions of people across the globe and is incurable.
An experimental animal study conducted in 2013 observed the beneficial effects of CBG on inflammatory bowel disease.
Researchers induced inflammations similar to IBD in the colons of mice and then administered CBG. CBG was found to reduce the inflammation and the production of nitric oxide. It also reduced the formation of reactive oxygen species (ROS) in the intestines. They concluded that CBG should be considered for clinical experimentation in IBD patients.
In an animal study, researchers found that CBG has therapeutic potential for the treatment of glaucoma.
Reseachers administered CBG to cats with glaucoma and noticed a reduction in eye pressure and an increase in aqueous humor outflow, a fluid produced by the eye which maintains eye pressure and provides the eye with nutrition.
Huntington's disease is a condition that causes a breakdown of nerve cells in the brain. In a 2015 study, researchers examined the potential neuroprotective properties of CBG and other cannabinoids in mice who had an experimental model of Huntington’s disease.
It was observed that CBG acted as a neuroprotectant, protecting the nerve cells in your brain from damage. It also improves motor deficits and preserves striatal neurons against 3-nitropropionic acid toxicity.
A 2020 study on the antibiotic potential of cannabis, found that CBG has antibacterial properties. Especially against methicillin-resistant strains of Staphylococcus aureus (MRSA), a bacteria which causes staph infections and is drug-resistant.
Fighting Cancer Cells
In a 2014 study, researchers observed the effects of CBG on rats with colon cancer. They observed that CBG showed some promise in blocking the receptors that cause cancer cell growth and inhibiting the growth of colorectal cancer cells.
They suggested that the use of CBG should be considered translationally in the cure and prevention of colon cancer.
How to Use CBG
The most common way CBG is produced for consumers is as an oil. You can get the benefits of CBG by using pure CBG oil. However, CBG oils are rare and expensive.
The good news is that you can also get some of the benefits of CBG from using broad-spectrum CBD oils. Broad-spectrum CBD oils contain all the cannabinoids found in a cannabis plant including CBG, but it doesn’t include THC.
When cannabinoids are used together, they can increase the effectiveness of each other by a phenomenon called the entourage effect.
CBG vs. CBD
CBG is often compared to CBD because it shares many similarities and they both act on the endocannabinoid system.
Both CBG and CBD are non-psychoactive which means they will not alter your state of mind in the way THC will.
They can however reduce the psychotropic effect of THC if you consume a cannabis plant. One of the biggest differences between CBD and CBG is the quantity which is found in most cannabis plants. Most cannabis plants contain only 1% of CBG, but up to 25% of CBD.
The way CBG interacts with our endocannabinoid system is different from CBD. CBG binds directly to both CB1 and CB2 receptors and might be more efficient at delivering its benefits into our systems.
The production difficulties of CBG makes it very scarce. It’s much harder to produce than other cannabinoids like THC and CBD. Since CBG shares many similarities with CBD, manufacturers would rather produce CBD.
When CBG is produced, products derived from it are very expensive. However, CBG has a host of promising potential benefits and more research is being done into easing the production and availability of the cannabinoid.
Medical Cannabis: Is It Good For Our Dogs?
A Bulldog who spent two years either lying down or throwing up plays like a puppy thanks to a daily dose of medical marijuana. A Boxer’s skin cancer begins to disappear following topical applications of cannabis oil. A 12-year-old Lab mix diagnosed with liver and lung cancer regains his appetite and becomes more himself after his owner gives him a cannabis tincture purchased from a licensed medical marijuana dispensary.
These stories offer hope to those of us who live with aging and/or infirm dogs, hope that we can improve the quality of their lives and perhaps even extend them.
Even more hopeful is the fact that these aren’t isolated incidents, but rather, three in an ever-increasing narrative of companion animals and cannabis- assisted healing. Yet, veterinarians played little to no official role in them. Why? Because Cannabis sativa (aka marijuana, grass, pot, hash, ganja, et al.)— a plant cultivated for literally thousands of years for its seeds, fibers and medicinal value—is a federally designated Schedule 1 controlled substance, a “drug with no currently accepted medical use and a high potential for abuse.”
So, even if vets believe that medical marijuana could or would relieve a dog’s pain, nausea or seizures, their hands are tied, including (as of 2015) in the 23 states and the District of Columbia where cannabis is legal for human medical use. Physicians in those states are exempt from prosecution, but veterinarians don’t have the same protection. Prescribing, or even recommending, cannabis for medicinal use exposes them to the loss of their license to practice.
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It’s a difficult place for a vet to find him- or herself: to have a remedy that has been shown to have very real benefits but not be able to use it, or even mention it, without career-ending consequences. Nonetheless, some have put their livelihoods at risk by challenging that prohibition, usually for the same reasons given by the late Doug Kramer, DVM, of Chatsworth, Calif., in a 2013 interview with Julia Szabo: compassion, and to prevent owners from accidentally overdosing their animals in well-intentioned efforts to relieve their pain.
And that’s part of the veterinary quandary. Medical marijuana has been described as the new “dot.com” boom, fueled by a growing body of research that seems to be validating cannabis’s beneficial effects for people. When people are helped by a particular treatment, they tend to want to share it with their ailing companion animals.
With medical marijuana, they’re doing this in increasing numbers, acting on the belief that if it works for them, it can also work for their dog or cat … or horse, for that matter. In doing so, they’re not necessarily curing incurable conditions but rather, are helping their animals enjoy daily life with better appetite and less pain until age or disease ultimately catches up.
The plant world has given us some of our oldest and most trusted—and, it’s true, sometimes abused—remedies. Pain relievers like codeine and morphine (poppy); colchicine, an antitumor drug (autumn crocus); the cardiac drug digitalin (purple foxglove); antimalarial quinine (quinine tree); and salicin, the chemical precursor to aspirin (white willow). The list is long.
When that plant has a cultural backstory like marijuana’s, however— “demon weed” in the ’50s, counterculture toke of choice in the ’60s, DEA Schedule 1 drug in the ’70s and onward —empirical evidence is harder to come by. Many barriers are placed in the path of those who want to find answers to questions about marijuana’s potential healing powers. Consequently, there’s a scarcity of rigorous research on the topic, particularly for veterinary application.
Determining whether or not to bring medical marijuana into general and legal use nationwide for humans and animals alike—and how to do it in a way that maximizes its benefits and minimizes its risks—requires this research. Stories, no matter how compelling and promising, are not science, and anecdotal evidence isn’t evidence in the scientific sense. Rather, hypotheses need to be tested in randomized, placebo-controlled studies, the results analyzed and conclusions drawn. The results are then retested and found to be replicable (or not) by others.
Until relatively recently, claims for cannabis’s medicinal values haven’t been supported in this way. As Hampton Sides notes in “High Science,” the June 2015 National Geographic cover story, “for nearly 70 years, the plant went into hiding, and medical research largely stopped … In America, most people expanding knowledge about cannabis were, by definition, criminals.”
Now for the more technical aspects of the topic, greatly simplified and synthesized.
The first published research related to cannabis and companion animals appeared in 1899 in the British Medical Journal. Written by English physician and pharmacologist Walter E. Dixon, the article included Dixon’s observations on dogs’ response to cannabis. However, it would be almost 100 years before we understood where the response originated: in the endocannabinoid system (ECS).
All vertebrates, from sea squirts to humans, have an endocannabinoid system, which scientists estimate evolved more than 600 million years ago. This ancient system, unknown until the late 20th century, is named for the botanical that most dramatically affects it, Cannabis sativa. Cannabinoids are the ECS’s messengers. The system’s purpose is to maintain internal balance— to “Relax, Eat, Sleep, Forget and Protect.”
Marijuana, a complex botanical with more than 400 known natural compounds, contains at least 64 phytocannabinoids (plant-based cannabinoids). The two produced in greatest abundance are cannabidiol (CBD) and tetrahydrocannabinol (THC).
How do they work? According to the National Cancer Society, cannabinoids “activate specific receptors found throughout the body to produce pharmacologic effects, particularly in the central nervous system and the immune system.” The effects depend on the receptors to which they bind.
Robert J. Silver, DVM and veterinary herbalist of Boulder, Colo., provides another way to look at it. “Receptors are like locks, and cannabinoids are like keys. They fit together perfectly. Once the cannabinoid connects to the receptor and ‘turns that lock,’ a series of actions in the cell membrane occur; these actions are responsible for some of the cannabinoid’s effects.”
In his book, Medical Marijuana and Your Pet, Dr. Silver notes that the ECS is unique in the world of neurotransmitters. Instead of releasing signals across a synapse (gap) in a forward direction, “the body’s naturally occurring endocannabinoids travel backward from the post- to the presynaptic nerve cell, inhibiting its ability to fire a signal. This is one way the ECS helps modulate and influence the nervous system.”
Research has revealed two distinct cannabinoid receptors, CB1 and CB2. As in other vertebrates, canine CB1 receptors are primarily found in the brain, but also appear in dogs’ salivary glands and hair follicles, while CB2 receptors are localized in canine skin, immune system, peripheral nervous system and some organs, such as the liver and kidneys.
Of the currently known cannabinoids, only one—THC—provokes a “mind-bending” response. CBD, on the other hand, has several well-documented biological effects, including antianxiety, anticonvulsive, antinausea, anti-inf lammatory and antitumor properties.
Terpenoids, components that give plants their distinctive odors, also play a role, helping cannabis cross the bloodbrain barrier and work synergistically. Ethan B. Russo, MD, associated with GW Pharmaceuticals in the UK, calls this the “entourage effect.” In an article in the British Journal of Pharmacology, Russo notes that terpenoids may make a meaningful contribution to cannabisbased medicinal extracts “with respect to treatment of pain, inf lammation, depression, anxiety, addiction, epilepsy, cancer, fungal and bacterial infections (including methicillin-resistant Staphylococcus aureus [MRSA]).” The entourage effect also suggests that in general, the whole plant, with all of its phytocannabinoids, is likely to be most effective for medicinal purposes.
Those who choose to treat their companion animals with medical marijuana generally give it to them in one of two ways: as an oil or as an edible —a food item made with marijuana or infused with its oil. While edibles intended for human consumption usually contain THC, those for dogs and cats more commonly use CBD from industrial hemp, strains of cannabis cultivated for non-drug use, which has almost no THC.
In 1996, California became the first state in the nation to legalize medical marijuana. It now has the largest legal medical marijuana market in the U.S. —not to mention an almost clichéd historical relationship with the herb— so it’s no surprise that many who are pushing the boundaries of its use with companion animals are based there.
Constance Finley, founder of Constance Pure Botanical Extracts (a Northern California legal medical cannabis collective) became involved in cannabis use with dogs when her 10-year-old service dog was diagnosed with hemangiosarcoma and given six weeks to six months to live.
Finley had been using cannabis oil herself to treat the effects of a debilitating autoimmune disease that began when she was in her mid-40s. The prescription medication she took almost killed her, she says, an experience that inspired her to set aside her long-held bias against marijuana and give it a try. The oil provided both pain- and symptom relief, and Finley went on to study cannabis cultivation and the complicated laws around its use. She eventually developed proprietary blends of highly concentrated oils from multiple strains of cannabis, extracted with organic, food-grade solvents.
So, when her much-loved dog was struggling with cancer, she says she dithered, then began giving the dog small amounts of cannabis oil, wiping it on her gums. Within days, the dog started to move around normally and eat; after three weeks of treatment with the oil, her vet could find no signs of the cancer. Unfortunately, she didn’t completely understand how cannabis worked; she figured her dog was cured and stopped using the oil. Within six months, the cancer was back, and ultimately it claimed her dog’s life.
Auntie Dolores has been making cannabis-infused edibles for California’s medical marijuana users since 2008. It launched Treatibles, locally manufactured product for dogs and cats. The active ingredients are CBD, CBN (cannabinol) and CBG (cannabigerol) distilled from European industrial hemp, which, founder and CEO Julianna Carella notes, is “non-toxic, 100 percent safe and non-psychoactive. Even dogs who do not have health problems can use the product as a preventive measure.”
Each bag of Treatibles, about 40 pieces, contains 54.6 mg of CBD; each t reat contains about 1 mg. Carella says that the company guarantees 40 mg per bag, but often the consumer gets a bit more. “We feel that all products purporting the health benefits of CBD should have at least enough of the material in the product to warrant the price, as well as to provide a medicinal dose. Even so, dogs are more sensitive to cannabinoids and generally need less than humans.”
Carella says that she was inspired to develop edibles for companion animals by cannabinoid science and research into the endocannabinoid system as it relates to all animals. Like others in the field, she is dismayed by cannabis’s current federal legal status. “Unfortunately, research on cannabinoids and animals is delayed due to the status of cannabis and the Controlled Substance Act, which has disallowed research into its medicinal value. CBD has become part of this controversy, even when derived from hemp.”
Initially, Treatibles was sold only through the company’s Treatibles website, but Auntie Dolores has recently been making it available in California cannabis dispensaries and local pet retail outlets. Holistic Hound in Berkeley, Calif., is one of the first stores to carry the product. While its name includes the word “treats,” store owner Heidi Hill considers Treatibles to be more closely aligned with supplements— i.e., to have health benefits. She says her customers have given Treatibles an enthusiastic reception, with most reportedly using the edible to alleviate their dogs’ anxiety and, in some cases, pain.
Hill says she gives Treatibles to Pearl, her aging, arthritic Siberian Husky, and has observed an improvement in her appetite and energy level. The quality of its other ingredients—among them, organic, gluten-free oat flour; pumpkin; peanut butter; organic coconut oil and coconut nectar; organic brown rice flour; applesauce; turmeric; and cinnamon— also recommends it, she says.
FAQ for members. The American Holistic VMA is the first, and so far only, veterinary organization to officially encourage research into the safety, dosing and uses of cannabis in animals. In 2014, the group released a statement that said in part, “There is a growing body of veterinary evidence that cannabis can reduce pain and nausea in chronically ill or suffering animals, often without the dulling effects of narcotics. This herb may be able to improve the quality of life for many patients, even in the face of life-threatening illnesses.”
In March of this 2015, Nevada state senator Tick Segerblom D-District 3) introduced Senate Bill 372, which makes a variety of changes related to medical marijuana in the state. Among its provisions is one that would allow officials to issue medical marijuana cards to companion animals whose owners are Nevada residents and whose vet is willing to certify that the animal has an illness that might be helped by marijuana (the illness does not need to be fatal).
In the June 4, 2015, edition of the Sacramento Bee, reporter Jeremy White summarized California Assembly Bill 266: “[It] would create what’s called a dual-licensure system, with cannabis entrepreneurs needing to secure permits both from local authorities and from one of a few state agencies. The Department of Public Health would oversee testing, the Department of Food and Agriculture would deal with cultivation and the Board of Equalization would handle sales and transportation—all under the auspices of a new Governor’s Office of Marijuana Regulation.”
According to Constance Finley, the fact that the marijuana industry is unregulated has been part of the problem regarding access. But next year may be the tipping point. If California’s AB 266 is passed and the marijuana industry comes out of the shadows into effective regulation, particularly in terms of verifiable cannabinoid content and freedom from contaminants, the rest of the nation could follow. The state’s size, market potential, and trailblazing environmental and technology industries have historically inf luenced trends nationwide, and that dynamic is likely to drive the discussion in this case as well.
Veterinary professionals are generally in agreement that more study is needed. In a 2013 interview with R. Scott Nolen, Dawn Boothe, DVM and director of the Clinical Pharmacology Laboratory at Auburn University’s College of Veterinary Medicine, commented: “Veterinarians do need to be part of the dialogue. I can see a welldesigned, controlled clinical trial looking at the use of marijuana to treat cancer pain in animals. That would be a wonderful translational study, with relevance to both pets and their people.” (In translational research, laboratory science and clinical medicine combine their efforts to develop new treatments and bring them to market.)
Narda G. Robinson, DVM, director of Colorado State University’s Center for Comparative and Integrative Pain Medicine, agrees. In an email exchange, Dr. Robinson said, “There is a big gap that needs to be addressed between those who are already using hemp products and finding value for their animal and science-based practitioners who want to make sure that their patients are receiving safe and effective treatment. Research will help bridge that gap.”
Clearly, veterinarians—our partners in keeping our animals healthy—need a voice in this debate. While interested in the herb’s potential, many are leery about trying it, not only because of the legal consequences but also, because there’s so little evidence-based information. On the other hand, dog owners who have found it useful for themselves feel that not including it in the vet-med repertoire is a missed opportunity.
Although the tide is slowly turning in its favor, the debate about the utility of medical marijuana and its related components for both people and their pets is often mired in personal bias and opinion. Regardless of what position we take, it would seem that the best way to come to a resolution is to focus on the science. Controlled studies that determine cannabis’s therapeutic and toxic ranges in veterinary use and standardization of THC and/or CBD content have the potential to make a potent natural ally legally and safely available to our four-legged companions.
In transforming anecdote to evidence, we can move from what we think, what we believe and what we imagine to what we actually know. That would be a very good thing for us and for our co-pilots as well.
When Marijuana and Dogs Don’t Mix
As is often the case, if people consume something, dogs are likely to do so as well, either deliberately or on the sly. In states where marijuana is legal, an uptick has been reported in the number of vet visits for dogs who’ve ingested pot (as an FYI, they also show up in states where it’s illegal). Two dogs, a Schipperke and a Cocker Spaniel, died after filching and eating baked goods made with unusually large amounts of THC butter (as well as rich in chocolate and raisins, two known canine toxins). An ASPCA Animal Poison Control Center study lists the top five symptoms of marijuana toxicity as ataxia—loss of control of body movements—depression, vomiting, urinary incontinence and bradycardia, or abnormally slow heart rate.
The takeaway? While death by pot isn’t common, it’s not unheard of. Dogs who get into a private stash or eat marijuana- enhanced edibles intended for people require veterinary attention.