1. Introduction: Cannabis sativa and Cannabinoids
) was known among ancient Asian, African, and European agricultural societies. Due to its hallucinogenic effects, Cannabis sativa
was applied in religious ceremonies, but it was also widely used in fiber manufacturing, nutrition and medicine. However, in the early part of the last century, C. sativa
lost its importance in industry and medicine [1
]. At present, application of C. sativa
in industry and medicine is experiencing a revival. Since 1990, C. sativa
became important as a source of compounds to treat cancer and life-threating diseases. The C. sativa
plant contains >500 chemical and biologically active compounds [3
]. So far, 60 structures have been identified as belonging to the family of cannabinoids (CBs). CBs share a lipid structure featuring alkylresorcinol and monoterpene moieties (terpenophenols) [2
Two CBs have been intensively investigated for their pharmacological properties: delta-9-tetrahydrocannabinol (THC) and cannabidiol (CBD); THC, but not CBD, exerts potent psychotropic effects (Figure 1
). A high THC/CBD ratio is responsible for the euphoric, relaxing, and anxiolytic effects of medical cannabis (marijuana), whereas, a high CBD/THC ratio has a rather sedating effect [5
Cultivation from different varieties of C. sativa
produces two main varieties with distinct concentrations of CBs, and the discrimination from the THC/CBD ratio divides commercial cannabis strains into three principal chemotypes. Chemotype I flowers have the highest THC content (18–23%). Industrial C. sativa
flowers (chemotype II and III flowers) contain less than 0.3% THC and CBD levels are 10–12% when calculated for dry weight [6
]. Since there are still systematic differences in reports on the CB content and the relative stability of CB levels from different laboratories, a better standardization of CB analysis is urgently required.
Based on the ability of CBs to inhibit inflammation and block cancer cell proliferation, plant-derived and synthetic CBs have been investigated for their applications as antitumor drugs. Indeed, a growing number of reports on the role of receptors for CBs in tumor cells suggest that CBs with different properties that can block or activate CB-receptors (CB-Rs) may be useful in cancer treatment [7
2. Mechanism of Cannabinoid Action
The term ‘endocannabinoid’ was invented in the mid-1990s after the discovery of membrane receptors for THC and their endogenous ligands. It now comprises a whole signaling system consisting of the ‘classical’ CB-Rs, their endogenous ligands, which are lipid signaling molecules called endocannabinoids, and the associated biochemical machinery, including precursor molecules, enzymes for synthesis and degradation, and transporter proteins, such as fatty acid binding protein and heat shock protein 70 [9
]. There is now a growing number of endocannabinoid molecules known, which share a similar structure and are natural ligands of the two CB-Rs, CB1-R and CB2-R. They seem to be involved in an increasing number of pathological conditions. Plant-derived CBs (phytocannabinoids, phyto-CBs) as well as synthetic CBs interfere with the endocannabinoid system, and a number of pharmacological effects of phyto-CBs can be explained by this interference [10
The most studied compounds of the endocannabinoid system are anandamide (N-arachidonoylethanolamine; AEA) and 2-arachidonoylglycerol (2-AG) (Figure 1
). Each can activate both CB-Rs and both are synthesized on demand in response to elevations of intracellular calcium [11
]. The biosynthesis of AEA, which was the first endocannabinoid identified, starts from the activation of N-acyltransferase (NAT), which transfers an acyl group to the membrane phospholipid phosphatidylethanolamine. In this way, N-acyl-phosphatidylethanolamine (NAPE) is generated. The NAPE-specific phospholipase D forms AEA from NAPE. The major biosynthetic pathway for 2-AG involves the sequential hydrolyses of inositol phospholipids via diacylglycerol (DAG) by phospholipase C and DAG lipase.
AEA and 2-AG are produced on demand by cells and work to maintain homeostasis [9
]. They have a short half-life and are quickly degraded through transport protein reuptake and hydroxylation by either fatty acid amide hydrolase (FAAH) for AEA or monoacylglycerol lipase (MAGL) for 2-AG. Finally, arachidonic acid (AA) and ethanolamine, from AEA, and AA and glycerol, from 2-AG, are formed. Endocannabinoids are responsible for retrograde synaptic signaling in the central nervous system. They move across the synaptic cleft in order to bind and activate the presynaptic CB1-R, causing an inhibition of neurotransmitter release.
These compounds serve as a new class of endogenous signaling molecules involved in a plethora of physiological functions related to behavior, memory, temper, addiction, and reward systems, as well as cellular metabolism and energy regulation. Their synthesis occurs ‘on demand’ (no storage) with a very short half-life. Drugs influencing the endocannabinoid system (e.g., inhibitors of FAAH and MAGL) were developed to treat neurological diseases and neuropathic pain in cancer patients [12
]. However, a tragic incidence at a phase I clinical trial with an FAAH inhibitor put its application into question [14
Endocannabinoids work via specific G-protein coupled receptors (GPRs) CB-Rs (CB1-R and CB2-R). While AEA acts as a partial CB1-R agonist and is a weak CB2-R agonist, 2-AG is a strong CB1-R agonist. CB1-R and CB2-R belong to the seven-transmembrane-spanning receptor superfamily. The distinct tissue distribution of CB1-R and CB2-R allows a selective and cell-specific effect of receptor activation. CB1-R is highly expressed in brain areas related to cognitive functions, memory, anxiety, pain, sensory and visceral perception, motor coordination, and endocrine functions. Low expression levels are observed in the peripheral nervous system, testicles, heart, small intestine, prostate, uterus, bone marrow and vascular endothelium. CB1-R activations inhibit forskolin-stimulated adenylyl cyclase through activation of a pertussis toxin-sensitive G-protein, to inhibit N-, P-, and Q-type calcium channels, and activate inwardly rectifying potassium channels.
CB2-R is present at high levels in cells of the immune system. In glial cells, the spleen and tonsils, CB1-R levels are low. CB2-Rs are also present at a lower level in the heart, endothelium, bones, liver, and pancreas. Furthermore, a functionally relevant expression of CB2-Rs was also found in the brain [15
]. Intracellular CB2-R dependent signaling pathways include Gi/o-dependent inhibition of adenylyl cyclase, stimulation of mitogen-activated protein kinase (MAPK), phosphoinositide 3-kinase (PI3K) and cyclooxygenase-2 (COX-2) signaling, and activation of de novo ceramide synthesis. Both CB-R types are highly expressed in a variety of cancerous tissues, and it is well established that CB2-R plays a crucial role in carcinogenesis and cancer progression. Therefore, CB2-R is now emerging as target for cancer treatment, although the exact role of CB2-R in cancer progression is still not completely elucidated [16
At molecular levels, the activation of CB-Rs confers signals of endo, phyto, and synthetic CBs (Figure 1
) via inhibition or activation of a variety of signaling pathways [17
] (Figure 2
). An important signal transduction pathway regulated by CB-R is linked to the synthesis of ceramide with palmitoyl-transferase as the rate-limiting enzyme in ceramide synthesis [18
]. Long-term treatment with ceramide, which activates the proto-oncogene serine/threonine-protein kinase (RAF1), leads to sustained activation of p42/p44 MAPK and induction of apoptosis, as demonstrated in a glioma cell line. This activation could be blocked by CB-R agonists, including THC, by the synthetic CB WIN55,212-2, and the endocannabinoids AEA and 2-AG. However, the duration of the activation of p42/p44 MAPK seems to be critical to the apoptotic response because a protective role of CBs against ceramide-induced apoptosis was also reported [19
Importantly, CBs also bind and activate several other receptors, including the GPRs, GPR18, GPR55, and GPR119. Of particular interest is GPR55, which is activated by lysophospholipid and also by the endocannabinoids AEA and 2-AG. Downstream targets of GPR55 include phospholipase C (PLC), transforming protein RhoA (RhoA), Rho-associated protein kinase (Rock), extracellular-signal-regulated kinase (ERK), and p38 MAPK [20
]. CB-Rs form heterodimers with other GPRs, e.g., GPR55, which consequently affects the functions of both receptors. Other GPRs, which are activated by CBs, are acetylcholine receptors and 2-alpha adrenoreceptors as well as opioid-, adenosine-, 5-hydroxytryptophan-, angiotensin-, prostanoid-, dopamine-, melatonin-, and tachykinin receptors. Furthermore, the peroxisome proliferator-activated receptors (PPARs) α and γ are also considered to be receptors for endocannabinoids [21
5. Current Therapeutic Application of Cannabinoids in Cancer Patients
At present, clinical trials on the effects of CBs from C. sativa
in cancer patients are rare [55
]. From the C. sativa
-derived CBs, non-psychoactive CBD has been studied as an anticancer agent based on its in vitro and in vivo activity against tumor cells. On the other hand, THC was applied for its valuable effects in the palliative care of patients with advanced stages of cancer. However, not all molecular mechanisms through which CBs exert antitumoral activities are fully elucidated. With an increasing number of legal changes in the different countries that now allow patients to take CBs for the management of cancer-related symptoms, further studies may be conducted, which will improve the knowledge of the antitumoral effects of CBs.
Clinical studies (available at: https://www.cannabis-med.org/studies/study.php
) monitoring the effects of CBs in patients with different late stages of cancer given a cannabinoid spray (Sativex®
containing THC and CBD at a ratio of 27:25 mg/mL) showed that this preparation is well tolerated and brought pain relief for ≤60% of patients suffering severe pain. The antiemetic, orexigenic, and anxiolytic effects of the CBs lead to an improved quality of life for cancer patients. Therefore, application of CBs in the palliative care of patients is well established. CBs are also successfully applied to treat muscle spasms and pain in patients with advanced multiple sclerosis, due to its analgesic and anticonvulsive effects. In these patients, dose-dependent adverse effects such as dizziness, gastrointestinal discomfort and confusion were reported [94
Clinical studies showed that application of the synthetic CBs dronabinol and nabilone are only moderately effective for relieving cancer-related pain, but they improve chemotherapy-induced nausea and anorexia in most patients. Their antiemetic effect of synthetic CBs is superior to that of many neuroleptics. In particular, the ability of synthetic CBs to reduce delayed emesis after chemotherapy is comparable to that of serotonin receptor antagonists. Therefore, both CBs have been recommended for therapy-resistant nausea and vomiting caused by chemotherapy. Moreover, dronabinol proved to be effective in improving anorexia in patients with AIDS and may also benefit patients with an advanced stage of cancer for proper nutrition [95
A study showed that a combination of CB drugs with opiates reduced chronic pain in ~27% of patients receiving oxycodone or morphine analgesics. No serious adverse effects were reported [96
]. To reach a significant reduction in opioid dependence and achieve a reduction in the use of prescription medication for pain and cancer-related malaise, a concomitant application of CBs and opioids might be considered. Further clinical studies are required to introduce a wider application of CBs for substituting opioids at least partly in palliative therapy [1
Many constituents of C. sativa, such as CBD and THC, exhibit beneficial anti-inflammatory or antitumoral properties. They act through the CB-Rs, CB1-R, and CB2-R. The latter receptor is highly expressed in cells of the immune system and both receptors are abundantly present in breast cancer cells. CB-R expression and activity determine the effects of CBs, but also of other drugs applied in the treatment of hormone-sensitive breast cancers. This was shown for the SERMs tamoxifen and raloxifene, which interfere with CB-R signaling. By influencing the tumor microenvironment and the immune system, blocking the expression of COX-2 and the proto-oncogene c-FOS and interfering with the EGF/EGFR pathway, they are able to reduce inflammation, inhibit tumor cell growth, induce apoptosis, and cause autophagy. This is important for HER2-positive tumors, where an increased CB2-R expression leads to activation of the HER2 pro-oncogenic signaling via the proto-oncogene tyrosine-protein kinase Src. On the other hand, CBs may enhance the proliferation of tumor cells by suppressing the immune system or by activating mitogenic factors.
Taken together, CBs are promising agents for inhibiting breast cancer progression. However, to develop safe therapeutic drugs, a further examination of the molecular pathways associated with CB activities is required.