Abstract

INTRODUCTION

Preparations of Cannabis sativa L. (marijuana) have been used for many centuries both medicinally and recreationally. However, the chemical structures of their unique active components””the cannabinoids””were not elucidated until the 1960s. Three decades later, the first solid clues to cannabinoid molecular action were established, which led to an impressive expansion of basic cannabinoid research and a renaissance in the study of the therapeutic effects of cannabinoids in various fields, including oncology. Today, it is widely accepted that, of the approximately 108 cannabinoids produced by C. sativa, Î”⁹-tetrahydrocannabinol (thc) is the most relevant because of its high potency and abundance in plant preparations^1,2.

Tetrahydrocannabinol exerts a wide variety of biologic effects by mimicking endogenous substances””the endocannabinoids anandamide³ and 2-arachidonoylglycerol^4,5””that engage specific cell-surface cannabinoid receptors⁶. So far, two major cannabinoid-specific receptors””cb1 and cb2””have been cloned from mammalian tissues and characterized^7,8. In addition, other receptors such as the transient receptor potential cation channel subfamily V, member 1, and the orphan G protein-coupled receptor 55 have been proposed to act as endocannabinoid receptors⁶. Most of the effects produced by cannabinoids in the nervous system and in non-neural tissues rely on cb1 receptor activation. In contrast, the cb2 receptor was initially described to be present in the immune system⁶, but was more recently shown to also be expressed in cells from other origins^9,10. Notably, expression of the cb1 and cb2 receptors has been found in many types of cancer cells, but not necessarily correlating with the expression of those receptors in the tissue of origin^9,11,12.

The endocannabinoids, together with their receptors and the proteins responsible for their synthesis, transport, and degradation, constitute the endocannabinoid system. Aside from its pivotal neuromodulatory activity¹³, the endocannabinoid system exerts other regulatory functions in the body such as control of cardiovascular tone, energy metabolism, immunity, and reproduction^14,15. This miscellaneous activity makes the pharmacologic manipulation of the endocannabinoid system a promising strategy for the management of many diseases. Specifically, cannabinoids are well known to exert palliative effects in cancer patients^14,15. Their best-established use is the inhibition of chemotherapy-induced nausea and vomiting^15,16. Today, capsules of thc (Marinol: AbbVie, North Chicago, IL, U.S.A.) and its synthetic analogue nabilone (Cesamet: Meda Pharmaceuticals, Somerset, NJ, U.S.A.) are approved for that purpose. Cannabinoids also inhibit pain, and thus a standardized cannabis extract (Sativex: GW Pharmaceuticals, Salisbury, U.K.) has already been approved in Canada and is currently the subject of large-scale phase iiiclinical trials for managing cancer-associated pain. Another potential palliative effect of cannabinoids in oncology, supported by phase iii clinical trials, includes appetite stimulation and attenuation of wasting. In that respect, Marinol can currently be prescribed for anorexia associated with weight loss in aids patients.

The therapeutic potential of cannabinoids in oncology might not be restricted to their aforementioned palliative actions. Thus, numerous studies have provided evidence that thc and other cannabinoids exhibit antitumour effects in a wide array of animal models of cancer^12,16,17. Moreover, those observations led to the development of several clinical studies to investigate the antitumour activity of thc in humans (see “Clinical Antitumour Effects of Cannabinoids” later in this article). Nonetheless, a few studies have shown that, under certain conditions, cannabinoid treatment can stimulate cancer cell proliferation in vitro^18,19and interfere with the tumour-suppressor role of the immune system^20,21. Likewise, reports about the role of the endocannabinoid system in cancer (tumour-suppressor or oncogenic) are conflicting²².

Reports concerning the biologic role of the endocannabinoid system in cancer physiopathology are sparse. Although some exceptions that might be tumour-type-specific are known, cannabinoid receptors and their endogenous ligands are both generally upregulated in tumour tissue compared with non-tumour tissue^16,22–24. Additionally, various studies have associated the expression levels of cannabinoid receptors, endocannabinoids, or endocannabinoid-metabolizing enzymes with tumour aggressiveness^22,25,26, which suggests that the endocannabinoid system might be overactivated in cancer and hence pro-tumourigenic²². In support of that hypothesis, reports showed that genetic ablation of cb1 and cb2 receptors in mouse models of cancer reduces ultraviolet light-induced skin carcinogenesis²⁷, that overexpression of cb2 receptor enhances the predisposition to leukemia after leukemia viral infection²⁸, and that cb2 promotes her2 (human epidermal growth factor receptor 2) pro-oncogenic signalling in breast cancer²⁹.

Conversely, and in line with the evidence supporting the hypothesis that pharmacologic activation of cannabinoid receptors decreases tumour growth^12,16, upregulation of endocannabinoid-degrading enzymes has been observed in aggressive human tumours and cancer cell lines^25,26, indicating that endocannabinoid signalling can also have a tumour-suppressive role. In support of that observation, deletion of cb1 receptors was noted to accelerate intestinal tumour growth in a genetic mouse model of colon cancer³⁰, increased endocannabinoid levels were observed to diminish azoxymethane-induced precancerous lesions in the mouse colon³¹, and reduction in the expression of the endocannabinoid-degrading enzyme monoacylglycerol lipase was seen to decrease tumour growth in xenografted mice²⁵.

Further studies””including analyses of the activation of the precise signalling mechanisms involved in the regulation of cannabinoid-induced cell death, and of cell proliferation upon genetic or pharmacologic manipulation of the endocannabinoid system””are therefore needed to clarify the contextual determinants that result in the system acting as either a guardian or an inducer of tumourigenesis or tumour progression. The present review summarizes such observations and provides an integrated view of the molecular mechanisms responsible for cannabinoid antitumour activity. It also discusses the experimental evidence supporting the existence of mechanisms of resistance to the cell death-promoting actions of thc in certain types of cancer cells, the strategies that could possibly be used to overcome such resistance, and the preclinical data supporting the potential usefulness of the combined administration of cannabinoids and other drugs in anticancer therapies.

The clinical approval of cannabinoids is largely restricted to palliative uses in various diseases, but since the emergence of promising preclinical data, the antitumour effects of cannabinoids are beginning to be clinically assessed.

In a pilot phase i clinical study, 9 patients with actively- growing recurrent glioblastoma for whom standard therapy had previously failed underwent intracranial thc administration¹¹. Under those conditions, cannabinoid delivery was safe and could be achieved without significant unwanted effects. In addition, although no statistically significant conclusions can be extracted from a cohort of 9 patients, the results obtained in the study suggest that some patients responded””at least partially””to thc treatment in terms of a decreased tumour growth rate as evaluated by magnetic resonance imaging¹¹. Importantly, analyses of samples obtained from 2 study patients before and after thc administration indicated that the molecular mechanism of cannabinoid antitumour action””namely, p8 and trib3 upregulation^35,37, mtorc1 inhibition³⁷, stimulation of autophagy and apoptosis^11,35,37, inhibition of cell proliferation¹¹, decreased vegf signalling³², and matrix metalloproteinase 2 downregulation⁶⁴ (delineated here earlier)””also operates in vivo.

Those findings were encouraging and reinforced interest in the potential use of cannabinoids in cancer therapies. However, they also highlighted the need for further research aimed at optimizing the use of cannabinoids in terms of patient selection, combination with other anti-cancer agents, and use of other routes of administration.

Administration of endocannabinoids or inhibitors of endocannabinoid-degrading enzymes has been shown to reduce the growth of various tumour xenograft types^95,96 and could therefore be a reasonable strategy for targeting cannabinoid receptors for anticancer purposes. However, as discussed here earlier, the role of the endocannabinoid system, including the endocannabinoid-degrading enzymes, in the control of tumour generation and progression is not well understood. Because enhancing endocannabinoid tone only has mild antitumour effects in mice and because no inhibitor of endocannabinoid degradation has yet been approved for use in humans, clinical studies aimed at analyzing the efficacy of cannabinoids as antitumour agents should be based on the use of plant-derived or synthetic agonists of cannabinoid receptors rather than on endocannabinoids or inhibitors of endocannabinoid degradation.

The long-known therapeutic properties of C. sativa””including amelioration of symptoms associated with cancer and its chemotherapy””have led to the authorization of the medical use of the plant and its extracts in several countries. As already mentioned, C. sativa produces approximately 108 different cannabinoids, including thc and cbd. Some of the other cannabinoids present in marijuana might contribute to the attenuation of thc‘s psychoactive side effects or even to the production of other therapeutic benefits¹⁵. However, pure drugs are more prone to standardization than complex molecular cocktails. Thus, it would be ideal for studies aiming to investigate the anticancer actions of cannabinoids in patients to be performed comparatively with both pure substances and cannabis extracts containing controlled amounts of thc, cbd, and other cannabinoids.

The most widely used route of administration for recreational and self-medicating marijuana is smoking. Although thc and other phytocannabinoids are rapidly absorbed by inhalation, smoking is an unattractive clinical option. Preclinical work in animal models has typically used peri-tumoural administration of cannabinoids. Likewise, in the only clinical trial in which a cannabinoid was assayed as an antitumour agent, thc was administered locally (intracranial delivery to patients with glioblastoma multiforme)¹¹. Nevertheless, this route of administration has many obvious limitations. Currently available cannabis-based medicines are administered as capsules or using an oromucosal spray¹⁵. Preclinical animal models have yielded data indicating that systemic (oral or intraperitoneal) administration of cannabinoids effectively decreases tumour growth (GV, CS, and MG. Unpublished observations), and so it seems reasonable that future clinical studies with the goal of determining the efficacy of cannabinoids as antitumour agents use oral or oromucosal routes of administration.

Two currently ongoing clinical trials could shed some light on these issues. One of the studies is a phase i/iitrial aimed at evaluating the combined effect of Sativex (an oromucosal cannabis extract whose main active components are thc and cbd in a c. 1:1 ratio) and temozolomide in patients with recurrent glioblastoma multiforme (https://clinicaltrials.gov/ct2/show/NCT01812603 and https://clinicaltrials.gov/ct2/show/NCT01812616). The other is a phase ii trial aimed at evaluating the effect of cbd as single treatment in patients with solid tumours (https://clinicaltrials.gov/ct2/show/NCT02255292). Hopefully, in the near future, new clinical trials will start, helping to determine whether cannabinoids can be used, for other than their palliative effects, in the treatment of cancer patients

CONCLUSIONS AND FUTURE DIRECTIONS

It is widely believed that strategies aimed at reducing mortality from cancer should consist of targeted therapies capable of providing the most efficacious and selective treatment for each individual tumour and patient. Thus, the major focus of anticancer drug development has progressively moved from nonspecific chemotherapies to molecularly-targeted inhibitors. However, despite the huge amount of preclinical literature on how these rationally designed compounds work, their use in clinical practice is still limited.

How do cannabinoid-based medicines fit into this ongoing scenario? Consider glioma, the type of cancer in which the most detailed cannabinoid research has been conducted to date. As discussed here, engagement of a molecular target (the cb receptors) by a family of selective drugs (thc and other cannabinoid agonists) inhibits tumour growth in animal models through a well-established mechanism of action that also seems to operate in human patients. Moreover, cannabinoids potentiate the antitumour efficacy of temozolomide and alk inhibitors in mice harbouring gliomas. Those findings provide preclinical proof-of-concept that “cannabinoid sensitizers” could improve the clinical efficacy of classical cytotoxic drugs in glioblastoma (Figure 2) and perhaps other highly malignant tumours such as pancreatic cancer, melanoma, and hepatocellular carcinoma. However, further research is required to define the precise molecular cross-talk between cannabinoids and chemotherapeutic drugs and to optimize the pharmacology of preclinical cannabinoid-based combination therapies.

With respect to patient stratification, the particular individuals that are potentially responsive to cannabinoid administration should be unequivocally determined. To that end, high-throughput approaches should be implemented to find cannabinoid therapy-associated biomarkers in tumour biopsies or, ideally, in easily acquired fluids containing circulating cancer cells or enhanced levels of resistance factors that might have been released by cancer cells. Such biomarkers would conceivably relate to cannabinoid pharmacodynamics””namely, expression and activity of cannabinoid receptors and their downstream cell-death-inducing effectors. The approach would be analogous to the biochemical evaluation of estrogen and ErbB2 receptors, which respectively predict benefit from endocrine therapies and trastuzumab in breast cancer. Predictive markers to define the sensitivity of a particular tumour to cannabinoid-based therapies could also include the status of growth factors, such as mdk in gliomas, and their receptors and signalling partners.

To summarize, cannabinoids induce tumour cell death and inhibit tumour angiogenesis and invasion in animal models of cancer, and there are indications that they act similarly in patients with glioblastoma. Given that cannabinoids show an acceptable safety profile, clinical trials testing them as single drugs or, ideally, in combination therapies in glioblastoma and other types of cancer are both warranted and urgently needed.