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In recent years, considerable interest has been generated by findings that cannabinoids not only have useful palliative effects, but also can affect the viability and invasivity of a variety of different cancer cells. In the present review, the potential of targeting the cannabinoid system for the treatment of cancer is considered from a practical, rather than a mechanistic viewpoint, addressing questions such as whether human tumour cells express CB receptors; whether the potencies of action of cannabinoids in vitro match the potencies expected on the base of receptor theory; what is known about the in vivo effects of cannabinoids and cancer, and how relevant the experiments undertaken are to the clinical situation; and finally, what approaches can be taken to minimise unwanted effects of cannabinoid treatment. It is concluded that cannabinoids (or agents modulating the endogenous cannabinoid system) are an attractive target for drug development in the cancer area, but that more in vivo studies, particularly those investigating the potential of cannabinoids as an addition to current treatment strategies, are needed.

Cannabinoids (CBs) are compounds that activate the CB₁ and CB₂ receptors. CB receptors mediate many different physiological functions, and cannabinoids have been reported to decrease tumor cell viability, proliferation, migration, as well as to modulate metastasis.

In this thesis, the effects of cannabinoids on human colorectal carcinoma Caco-2 cells (Paper I) and mouse P19 embryonal carcinoma (EC) cells (Paper III) were studied.  In both cell lines, the compounds examined produced a concentration- and time-dependent decrease in cell viability. In Caco-2-cells, HU 210 and the pyrimidine antagonist 5-fluorouracil produced synergistic effects upon cell viability. The mechanisms behind the cytocidal effects of cannabinoids appear to be mediated by other than solely the CB receptor, and a common mechanism in Caco-2 and P19 EC cells was oxidative stress. However, in P19 EC cells the CB receptors contribute to the cytocidal effects possibly via ceramide production.

In paper II, the association between CB₁ receptor immunoreactivity (CB₁IR) and different histopathological variables and disease-specific survival of colorectal cancer (CRC) was investigated. In microsatellite stable (MSS) cases there was a significant positive association of the tumor grade with the CB₁IR intensity. A high CB1IR is indicative of a poorer prognosis in MSS with stage II CRC patients.

Paper IV focused on the cytotoxic effects of cannabinoids during neuronal differentiation. HU 210 affected the cell viability, neurite formation and produced a decreased intracellular AChE activity. The effects of cannabinoids on embryonic development and survival were examined in Paper V, by repeated injection of cannabinoids in fertilized chicken eggs. After 10 days of incubation, HU 210 and cannabidiol (without CB receptor affinity), decreased the viability of chick embryos, in a manner that could be blocked by α-tocopherol (antioxidant) and attenuated by AM251 (CB₁ receptor antagonist).

In conclusion, based on these studies, the cannabinoid system may provide a new target for the development of drugs to treat cancer such as CRC. However, the CBs also produce seemingly unspecific cytotoxic effects, and may have negative effects on the neuronal differentiation process. This may be responsible for, at least some of, the embryotoxic effects found in ovo, but also for the cognitive and neurotoxic effects of cannabinoids in the developing and adult nervous system.

Cannabinoids (CBs) have been found to exert antiproliferative effects upon a variety of cancer cells, including colorectal carcinoma cells. However, little is known about the signalling mechanisms behind the antitumoural effect in these cells, whether the effects are shared by endogenous lipids related to endocannabinoids, or whether such effects are synergistic with treatment paradigms currently used in the clinic. The aim of this preclinical study was to investigate the effect of synthetic and endogenous CBs and their related fatty acids on the viability of human colorectal carcinoma Caco-2 cells, and to determine whether CB effects are synergistic with those seen with the pyrimidine antagonist 5-fluorouracil (5-FU). The synthetic CB HU 210, the endogenous CB anandamide, the endogenous structural analogue of anandamide, N-arachidonoyl glycine (NAGly), as well as the related polyunsaturated fatty acids arachidonic acid and eicosapentaenoic acid showed antiproliferative and cytotoxic effects in the Caco-2 cells, as measured by using [³H]-thymidine incorporation assay, the CyQUANT proliferation assay and calcein-AM fluorescence. HU 210 was the most potent compound examined, followed by anandamide, whereas NAGly showed equal potency and efficacy as the polyunsaturated fatty acids. Furthermore, HU 210 and 5-FU produced synergistic effects in the Caco-2 cells, but not in the human colorectal carcinoma cell lines HCT116 or HT29. The compounds examined produced cytotoxic, rather than antiproliferative effects, by a mechanism not involving CB receptors, since the CB receptor antagonists AM251 and AM630 did not attenuate the effects, nor did pertussis toxin. However, α-tocopherol and the nitric oxide synthase inhibitor L-NAME attenuated the CB toxicity, suggesting involvement of oxidative stress. It is concluded that the CB system may provide new targets for the development of drugs to treat colorectal cancer.

Background: There is good evidence in the literature that the cannabinoid system is disturbed in colorectal cancer. In the present study, we have investigated whether CB(1) receptor immunoreactive intensity (CB(1)IR intensity) is associated with disease severity and outcome.

Methodology/Principal Findings: CB(1)IR was assessed in formalin-fixed, paraffin-embedded specimens collected with a consecutive intent during primary tumour surgical resection from a series of cases diagnosed with colorectal cancer. Tumour centre (n = 483) and invasive front (n = 486) CB(1)IR was scored from 0 (absent) to 3 (intense staining) and the data was analysed as a median split i.e. CB(1)IR <2 and >= 2. In microsatellite stable, but not microsatellite instable tumours (as adjudged on the basis of immunohistochemical determination of four mismatch repair proteins), there was a significant positive association of the tumour grade with the CB1IR intensity. The difference between the microsatellite stable and instable tumours for this association of CB(1)IR was related to the CpG island methylation status of the cases. Cox proportional hazards regression analyses indicated a significant contribution of CB(1)IR to disease-specific survival in the microsatellite stable tumours when adjusting for tumour stage. For the cases with stage II microsatellite stable tumours, there was a significant effect of both tumour centre and front CB(1)IR upon disease specific survival. The 5 year probabilities of event-free survival were: 8565 and 66+/-8%; tumour interior, 86+/-4% and 63+/-8% for the CB(1)IR<2 and CB(1)IR >= 2 groups, respectively.

Conclusions/Significance: The level of CB(1) receptor expression in colorectal cancer is associated with the tumour grade in a manner dependent upon the degree of CpG hypermethylation. A high CB(1)IR is indicative of a poorer prognosis in stage II microsatellite stable tumour patients.

Cannabinoids and cannabinoid receptors play an important role in development and differentiation of the nervous system, but the mechanisms behind that role have not been fully elucidated. We have examined the effects of synthetic and endogenous cannabinoids and related polyunsaturated fatty acids upon mouse embryonal carcinoma P19 stem cell viability - before, during and after retinoic acid (RA)-induced neural differentiation. Experiments were also performed to investigate whether the cannabinoids affect the differentiation of P19-derived neurons by measuring the development and growth of neurites and intracellular acetylcholinesterase activity.

Both synthetic and endogenous cannabinoids as well as related fatty acids produced a concentration-dependent decrease in undifferentiated P19 cell viability, but induction of the neural pathway reduced the sensitivity to the cytotoxic effects, and in differentiated neurons anandamide and related fatty acids showed no cytotoxicity. However, synthetic cannabinoids such as HU 210, HU 211 and WIN 55,212-2 produced cytotoxicity in both undifferentiated and differentiated cells, but there was a right-shifted concentration-effect curve in RA-induced cells and differentiated neurons compared with the undifferentiated cells.

HU 210 produced a time- and concentration-dependent decrease in cell number, percentage of cells expressing neurites, number of neurites per cell and neurite length. Statistically significant inhibition was seen at a concentration of 1 µM to 3 µM, and this was confirmed by the measurement of intracellular acetylcholinesterase activity, an enzyme that is dramatically increased during the differentiation process, where HU 210 significantly decreased the activity after six and nine days of exposure. However, these effects of HU 210 could only be observed in the same concentration range as those affecting neuronal viability. Anandamide, on the other hand, had modest effect on measured markers of neuronal differentiation but decreased the fraction of neurite expressing cells and neurite length after nine days of exposure at a concentration ≥ 10 µM. No effect on the acetylcholinesterase activity was observed.

It is concluded that cannabinoids and related fatty acids have cytotoxic effects in undifferentiated P19 embryonal carcinoma cells, but induction of the neuronal pathway reduces the sensitivity to the cytotoxic effects. The synthetic cannabinoids are more potent than the endogenous cannabinoids and fatty acids in causing cytotoxicity in differentiated neurons, but the CB-induced decrease in neurite formation and acetylcholinesterase activity in RA-induced P19-derived neurons occurs only at concentrations that cause measurable neuronal cell death. 

We have examined the effects of the synthetic cannabinoids HU 210 and HU 211, the plant-derived cannabidiol and the endogenous cannabinoid anandamide on the viability and development of chick embryos. Fertilized White Leghorn chicken eggs were injected with the test compounds or carrier vehicle, via a drilled small hole in the egg, directly into the egg yolk. After nine days of exposure, the embryonal viability, length and wet weight of embryos, and wet weight of brains were measured, and the development stages were assessed according to the Hamburger and Hamilton (HH) scale. The potent synthetic cannabinoid receptor agonist HU 210 and the non-psychotropic cannabidiol were embryotoxic at the highest concentrations examined (10µM and 50µM, respectively), with no viable embryos after the HU 210 injection, and 20% viability after the cannabidiol injections. The efects of HU 210 on the chick embryo were attenuated by α-tocopherol and the cannabinoid receptor antagonist AM251, whereas only α-tocopherol gave a statistically signifcant protection against the embryotoxic efects of cannabidiol. This study shows that exposure to plant-derived or synthetic cannabinoids during early embryonal development decreases embryonal viability. Extrapolation of data across species is of course difcult, but the data would argue against the use of cannabinoids, be it recreationally or therapeutically, during pregnancy

Compounds acting on the cannabinoid (CB) receptors are involved in the control of cell fate, and there is an emerging consensus that CBs have anticancer effects. However, the CB-mediated effects are contradictory since some studies suggest stimulatory effects on cancer cell proliferation, and CBs have been shown to stimulate both proliferation and differentiation of other mitotic cells such as stem and progenitor cells. In this study, the concentration-dependent effects of synthetic and endogenous CBs on the viability of mouse P19 embryonal carcinoma (EC) cells have been examined by using fluorescence assays of cell membrane integrity, cell proliferation, oxidative stress, and detection of apoptosis and necrosis. All compounds examined produced a concentration-dependent decrease in cell viability in the micromolar range, with the potent CB receptor agonist HU 210 and the enantiomer HU 211(with no CB receptor activity) being the most potent compounds examined with apparent IC₅₀ values of 1 µM and 0.6 µM, respectively. The endogenous CB anandamide showed similar potency and efficacy as structurally related polyunsaturated fatty acids with no reported activity at the CB receptors. The rapid (within hours) decrease in cell viability induced by the examined CBs suggests cytocidal rather than antiproliferative effects, and is dependent on the plating cell population density with the highest toxicity around 100 cells/mm². The CB-induced cytotoxicity, that appears to involve CB receptors and the sphingomyelin-ceramide pathway, is a mixture of both apoptosis and necrosis that can be blocked by the antioxidants α-tocopherol and N-acetylcysteine. In conclusion, both synthetic and endogenous CBs, produce seemingly unspecific cytotoxic effects in the P19 EC cells.