A Review of Current Cannabinoid Applications for Glioblastoma Multiforme 

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Natalie Pandher, B.S. [1]

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[1] University of California, Davis 

 

ABSTRACT

Glioblastoma multiforme, or GBM, is a highly resistant and lethal cancer that develops from tumorous growth in the brain. Gliomas arise from clusters of glial stem-like cells, GSCs, which are highly resistant to both chemo and radiotherapy. [1] Previous research has found that cannabinoids have anti-tumor growth effects in different kinds of malignant tumors. Seven experimental studies published within the last five years and gathered from peer reviewed medical journals explore the anti-tumor effects of CBD and THC, with other forms of treatment on glioblastomas. Studies conducted on cell cultures of human glioblastoma cells, xenografts in mice, and organotypic hippocampal slice cultures have shown significant results for the reduction of tumor growth via different combinations of THC, CBD, and other therapies. The analysis of research on cannabinoids in preclinical models provides evidence for its applications in further clinical studies to detect the effects of cannabinoid assisted treatment options for glioblastomas in inoperable brain regions. This review will outline the latest discoveries in cannabinoid research pertaining to gliomas and assess the promise of different experimental drug therapies in treating GBM.

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INTRODUCTION

Gliomas are tumors in the brain’s glial cells which are involved in maintaining neuron health and regulating synaptic activity. [1] Like most malignant tumors, gliomas are categorized into levels of severity. Glioblastoma multiforme is a stage IV glioma, one of the most aggressive kinds of glial tumor in humans. [2] GBM arises from astrocytomas, tumors in the star shaped glial cells called astrocytes, but can also arise de novo or without any kind of precursor. [3] Glioblastoma multiforme is the most lethal brain cancer, accounting for 35-40% of all malignant brain tumors arising in adults. [4] About 14,000 cases are diagnosed in the US each year. [4] With an average prognosis of 12 months or less after diagnosis, current treatment involves surgical removal of parts of the tumor that can be accessed and followed by chemotherapy or radiation. [2] Gliomas have low success rates with surgical removal and traditional cancer therapies because of high variability within the tumor. [3] The tumor landscape consists of various cell types, including neoplastic and p53-deficient cells, which have high mutation rates and varying regions of necrosis or hemorrhage which cannot all be targeted by current cancer treatment methods. [3] Even with treatment, the patient’s survival is only increased from a few months to a year. [2] Due to the poor outcomes of treatment in this type of brain tumor, many different approaches have been attempted to improve outlook.

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In recent research, the success of cannabinoids, most commonly delta 9-tetrahydrocannabinol (THC) and cannabidiol (CBD), in anti-inflammatory and anti-angiogenesis contexts highlights ways in which cannabinoids can be used in treating gliomas. The purpose of this review is to assess the different studies conducted about promising therapies involving the administration of cannabinoids in order to increase the effects of radiation and chemotherapy on the glioma. The discussion of promising cancer research will highlight the need to apply cannabinoids to research on larger mammals and in clinical settings, in order to bring this treatment option into the forefront of GBM therapy. The following articles explore the mechanisms by which cannabinoids are used to target gliomas, and how they enhance the results of chemotherapy and radiation on reducing tumor proliferation.

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First, the methods for selecting research will be outlined, followed by an assessment of therapies using two kinds of cannabinoids, THC and CBD together, to reduce the resistance of tumor cells. Next, a comparison will be made of therapies using just CBD along with other pharmacological interventions to target gliomas. Lastly, the different methods of cannabinoid administration will be discussed, along with a quick comparison of research exploring the molecular mechanisms by which cannabinoids impact tumor cells. Overall, this review aims to provide a discussion of different ways cannabinoids are used to prevent progression of glioblastomas, and assess the consensus reached by research on this topic thus far.

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SEARCH METHODS AND STUDY INCLUSION CRITERIA

Research articles were chosen from peer reviewed journals found through the PubMed and Elsevier databases. This search was first completed in August of 2019, but the references were updated in September of 2020. Another more recent search was done for the latest articles on cannabinoid therapies in December of 2020. The search criteria focused on cannabinoid uses in glioblastoma multiforme. Key words used while searching included the MeSH terms “cannabinoids,” “delta-9-tetrahyrocannabinol,” “cannabidiol,” and “glioblastoma multiforme.” The discovery of additional research articles used the relevant references included in other review articles or meta-analyses. Only articles published in the last five years were chosen. International publishings were not excluded from the search. In addition, only those articles were chosen that were publicly available and free of cost or available through the University of California library databases.

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THC AND CBD USED TOGETHER TO INCREASE CHEMO AND RADIOSENSITIVITY IN GLIOMAS

Among the articles discussing cannabinoids in cancer treatment, the articles exploring both the uses of THC and CBD in glioma treatment were isolated to construct a cohesive explanation of their relevance in GBM therapy. Research conducted by López-Valero et al. [5] demonstrated that cannabinoids used in conjunction with the chemotherapeutic drug temozolomide (TMZ) aided in the reduction of tumor proliferation. THC and CBD were administered in a 1:1 dosage ratio with a regular chemotherapy course of TMZ. [5] Effect on tumor growth was measured by planting orthotopic and intracranial xenografts in male mice to determine changes in tumor volume and number of tumor cells as the treatment progressed. [5] The researchers learned that systemic administration of a 1:1 dose of THC and CBD with TMZ greatly reduced tumor growth, and higher doses of THC also reduced likelihood of tumor growth relapse. [5] A greater than 50% tumor regression in mice was discovered after administration of the drug cocktail, even when the tumor was intracranial. [5] The 1:1 ratio tested by López-Valero et al. [5] in the above study had not been used before in combination with TMZ. Researchers devised this new therapy based on previous research showing CBD and TMZ to be efficacious in GBM control. The promise of the 1:1 THC to CBD ratio in reducing tumor growth led to further research by López-Valero et al. [6] which tested a 1:5 THC to CBD ratio in combination with TMZ. Continuing to use mice xenografts, the researchers discovered that a 1:5 THC to CBD ratio had increased effects on reducing tumor growth when used in conjunction with TMZ. [6] The higher proportion of CBD inhibits activity of glioma initiating cells or GICs which are highly resistant to cancer treatments, while THC triggers the autophagic pathways in the tumor cells [6]. Similar to the research proving the efficacy of THC + CBD + chemotherapy conducted by López-Valero et al. [5] and López-Valero et al. [6], the researchers Scott, Dalgleish, and Liu [7] demonstrated that THC and CBD together also enhanced the effects of radiation therapy on gliomas. The researchers used six female mice for xenografts and discovered that radiation alone did not decrease tumor size as much as the combination of THC, CBD, and radiation. [7] Results of CBD and THC used to enhance radiation showed that treatment with cannabinoids prevented repair of DNA damage in tumor cells after radiation. [7] All of the above research conducted by López-Valero et al. [5], López-Valero et al. [6], and Scott et al. [7] investigated methods of using THC and CBD in varying ratios in order to increase the efficacy of either radiation or chemotherapy on GBM. Whereas López-Valero et al. [5] and López-Valero et al. [6] focused on the effect of THC and CBD in 1:1 and 1:5 ratios on TMZ using male mice, Scott et al. [7] used THC and CBD on increasing the potency of radiation on gliomas using female mice xenografts. Despite these key differences in method and drug combination, all three studies showed that THC and CBD increased the sensitivity of tumor cells toward treatment by chemotherapy and radiation. [5-7] This discussion of results thus highlights the promise of THC and CBD combination therapy in the context of reducing tumor growth for GBM.

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CBD USED ALONE OR IN CONJUNCTION WITH OTHER DRUGS FOR GLIOMA CELL DEATH 

Glioblastomas are very resistant to chemotherapy because of glial stem-like cells or GSCs that proliferate rapidly. [8] These cells have unique molecular pathways that contribute to resistance. Research by Nabissi et al. [8] outlines that CBD interacts with a CBD receptor in the brain called TRPV2 and with transcription factor Aml-1a to induce autophagy in GSCs. Autophagy promotes the differentiation of GSCs, which in turn makes the tumor more susceptible to chemotherapy or radiation. [8] The researchers exploited the fact that activating the TRPV2 receptor created cell vulnerability, and they used CBD to increase TRPV2 expression which then made the tumor less resistant. [8] By forcing the glial stem-like cells to differentiate, using CBD, Nabissi et al. [8] increased the success of the chemotherapy drug carmustine or BCNU in preventing tumor proliferation. The study used only CBD and BCNU in combination due to the ability of CBD to enhance the effects of existing chemotherapy drugs. This finding was directly contradicted by research from López-Valero et al. [5] that concluded THC and CBD used in a 1:1 ratio in conjunction with BCNU did not promote “glioma cell death” [6],  as it did when used in conjunction with TMZ. However, López-Valero et al. [5] emphasized the use of both THC and CBD together in combination with chemotherapy, while research by Nabissi et al. [8] highlights only CBD as a chemotherapy enhancer. The discrepancy between the cannabinoids utilized in the two studies may be the reason behind the different results obtained, highlighting the need for further research on cannabinoids in combination with BCNU. Recent research by Hohmann et al. [12] also investigating the use of cannabinoids on TMZ resistant glioblastomas, looked at the interaction between activation of cannabinoid receptors and miRNAs involved in tumor suppression. [12] Unfortunately, the researchers did not observe consistent tumor-suppression in glioblastoma cell lines when cannabinoids and TMZ were used together. [12] This conflicting result may also be due to the fact that López-Valero et al. [5] used THC and CBD together, a combination that was not tested in the research by Hohmann et al. [12] A different study by Scott, Dennis, Dalgleish, and Liu [9] explored the stand alone use of CBD with a pharmacological intervention called heat shock protein inhibitors to promote radiosensitivity. The researchers demonstrated that using CBD actually increased reactive oxygen species or ROS which are free radicals that damage cell nuclear material and can cause cell death. [9] The increase in ROS led to an upregulation of heat shock proteins (HSPs) which in turn make CBD less effective because HSPs work by protecting malignant cell growth. [9] By using CBD in conjunction with HSP inhibitors, the efficacy of CBD increases allowing CBD more freedom in improving the susceptibility of the tumor to radiation. [9] In summation, the research conducted thus far on the use of only CBD to improve the performance of chemotherapy and radiation on gliomas has been contradictory. Research by Nabbisi et al. [8] directly contradicts research by López-Valero et al. [5] in the findings of cannabinoids improving the effectiveness of BCNU, while the Scott et al. [9] study explores a molecular pathway not discussed in any of the other research. The findings of research by Hohmann et al. [12] further reinforce the variability in cannabinoid related suppression of GBM chemoresistance. The different studies’ results do not show an overarching agreement on CBD efficacy when used alone to improve chemotherapy and radiation outcomes. The lack of consensus in the research described demonstrates the need for further research on CBD used alone. While the results of research by Nabissi et al. [8], López-Valero et al. [5], Scott et al. [9], and Hohmann et al. [12], lacked general agreement, each study highlighted the positive effects of CBD as a cannabinoid and showed promise in elevating treatment of GBM.

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MOLECULAR PATHWAYS TARGETED BY CANNABINOIDS TO PREVENT GLIOMAS

Cancer research targets many different molecular and biochemical pathways, and cannabinoids affect various cellular processes in unforeseen ways. One novel approach to reducing tumor cell growth was studied by Hohmann et al. [10] who showed that glioblastoma cells have mechanical and migratory properties that affect the invasiveness of the cell type. Hohmann et al.’s [10] team used multiple human GBM cell lines grown in organotypic hippocampal slice cultures, or OHSC, which were treated with cannabinoids and assessed for cell movement speed, cell adhesion, and other properties determining invasiveness. Cannabinoids reduced microtubule protein levels, caused cytoskeletal reorganization, and decreased tumor invasiveness by reducing cell adhesions in one of the cell lines. [10] However, the two types of cannabinoid agonists used, ACEA and JWH, had varying effects that differed greatly between the cell lines. [10] ACEA was used as the CB1 receptor agonist, JWH as the CB2 receptor agonist, and significant results were not obtained for their effects on all of the cell lines. [10] The extreme variation in results reduced conclusivity and lowered credibility of a possible discussion of this pathway’s promise when it comes to cannabinoid research. However, the researcher’s discussion of using cannabinoids to alter cell adhesiveness and stiffness, thereby increasing or decreasing invasiveness or migration can be improved upon by further investigation. [10] More current research on this topic by Hohmann et al. [12] in 2019 using the same cell lines and cannabinoid agents again found variability in the molecular pathways by which cannabinoids affect GBM cells. This study differed from the previous study by Hohmann et al. [10] cited earlier in this section, in that the researchers investigated cannabinoid’s effects on microRNAs and certain apoptosis and proliferation cell markers that influence tumor growth. [12] The results on cannabinoid activation via proliferation, apoptosis, Akt, and microRNAs were nonsignificant, and the researchers reached a consensus that cannabinoids act on CB1 and CB2 receptors in a “cell-and-receptor dependent” manner to influence GBM cells. [12] Returning to other molecular pathways relevant to glioblastomas, research by Nabissi et al. [8]  investigated the autophagic process used to stimulate GSC differentiation in the context of CBD therapy. CBD was used to target Aml-1a which binds to TRPV2 and increases its transcription to promote differentiation in a gene expression cascade. [8] The presence of Aml-1a is needed to impair tumor proliferation and to promote differentiation in glial stem-like cells. [8] This protein cascade explains the role played by CBD in arresting cell division and cell growth, as well as sending tumor cells into G0/G1 phase of the cell cycle. [8] Lastly, Scott et al. [9] discussed the role of reactive oxygen species and heat shock protein systems in tumor cell growth. They outlined the production of ROS through mitochondrial reactions and intracellular signaling cascades. [9] ROS upregulated the HSPs which in turn have client proteins that promote cell survival, supporting the pathways of tumor cell growth. [9] In the ROS and HSP pathway, the properties of CBD were diminished, so the researchers demonstrated a way to use CBD more efficiently by inhibiting the HSPs. [9] Overall, the above studies show promise on the molecular level of cancer therapy. The pathways explored by Hohmann et al. [10], Hohmann et al. [12], Nabissi et al. [8], and Scott et al. [9], provide insight about where cannabinoids should be targeted and how they can be utilized best to increase the efficacy of chemotherapy and radiation on glioblastomas. Knowledge of only the drug combinations will not be enough to determine potential ways to use cannabinoids on GBM patients; knowing the exact molecular precursors for the mechanisms of glioma growth can increase the success of cannabinoids which in turn will improve potency of cancer treatment. [8, 9, 11, 12]

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A COMPARISON OF DIFFERENT ADMINISTRATION METHODS OF CANNABINOIDS

Previous methods of localizing cannabinoid doses in combination therapy to reduce tumor growth are not as effective as systemic administration of THC and CBD. [5] The researchers demonstrated that administering orally or intraperitoneally a 1:1 dose of THC and CBD with TMZ to the mice with orthotopic or intracranial xenografts significantly reduced tumor growth. [5] Their results explained that localized drug delivery is associated with clinical complications so systemic delivery was tested for results of significant reduction of glioma growth. [5] Administering the cannabinoids through systemic delivery not only prevented tumor cell growth, but also played a role in inducing autophagy and apoptosis, specifically when THC was delivered intraperitoneally. [5] Another method of drug administration was discovered in research conducted by Aparicio-Blanco, Sebastián, Benoit, and Torres-Suárez [11] using human glioblastoma cell lines grown in cultures. The researchers developed a new method of inserting CBD into lipid nanomolecules (LNCs) creating a prolonged drug release effect which improved CBD administration techniques and increased effects of the CBD on tumor cells. [11] Since CBD tends to dissolve in nonpolar solvents quite easily, putting CBD into the oily core of lipid nanomolecules increases efficacy by ensuring that the CBD is uptaken by the glial cells before it dissolves. [11] The researchers saw a greater uptake of the CBD nanomolecules by the glioma than free floating CBD molecules. [11] Although the research was conducted using only CBD LNCs, the study proposed the use of LNCs for THC or chemotherapy administration in the future. [11] López-Valero et al.’s [5] and Aparicio-Blanco et al.’s [11] research on effective ways to administer the cannabinoids in order to best target tumor cells investigated two different methods. Whereas López-Valero et al. [5] demonstrated the greater effect of 1:1 THC to CBD doses administered systemically in vivo using mice xenografts, the study by Aparicio-Blanco et al. [11] opted to test the effectiveness of their CBD infused lipid nanomolecules in vitro, using glioblastoma cell cultures. Lipid nanomolecules involve administration of cannabinoids through a vector, while the systemic delivery emphasizes the flow of cannabinoids through the entire body. [5, 11] Aparicio-Blanco et al.’s [11] approach reflects a more localized administration method when compared to López-Valero et al. [5] This discrepancy highlights the variety in administration methods and emphasizes the different methods available to be tested clinically or researched further. The matter of administering cannabinoids is essential because until research obtains full knowledge of the most effective ways to inject the drug therapies into the body, cannabinoid research cannot be applied to humans. [5, 11]  

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CONCLUSION

Current research investigating cannabinoid uses for glioblastoma multiforme yielded significant results for the anti-tumor effects of cannabinoid assisted therapies, like cannabinoid assisted chemotherapy or radiation. Each study’s unique results demonstrated either contradiction or consensus with others, providing evidence for the promise of certain research. Multiple combinations of THC with CBD were assessed in conjunction with chemotherapy drugs like TMZ and BCNU. [5, 8] The cannabinoids in a 1:1 or 1:5 THC to CBD ratios were found to increase the sensitivity of tumor cells to TMZ chemotherapy by weakening cell proliferation processes in the tumor. [5, 6] Researchers demonstrated that THC induces autophagy and apoptosis of tumor cells, thus CBD used with THC has greater efficacy than CBD used alone. [8, 9, 11, 12] Additionally, the role of heat shock proteins in tumor growth was researched, and the results showed that heat shock proteins reduce the impact of CBD on the tumor cells, creating the need for CBD use in conjunction with HSP inhibiting drugs to increase the anti-tumor effects. [9] In addition, the research investigated the use of LNCs for administering CBD that can better target the gliomas and increase the duration of drug release. [11] A comparison of the administration methods also found evidence for higher efficacy when the cannabinoids were administered systemically, by either oral or intraperitoneal administration. [5] Multiple biochemical pathways, including the ROS and HSP relationship and the TRPV2 and Aml-1a pathway, were targeted by the treatment cocktails. [8, 9]  

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Thus far, research conducted on the topic of cannabinoid combination therapies has used only cell cultures, OHSC, and orthotopic and intracranial xenografts in mice. Despite the limited means of researching cannabinoid efficacy on improving GBM treatments, the research has shown promise with positive results of reduction in tumor growth and induction of autophagy and other destructive processes in glioblastoma cells. [5, 6, 7, 8, 9] This promise validates the need for further research, expanding to experiments on larger mammals or humans, to determine the clinical relevance and optimal dosages of cannabinoids strictly in the context of GBM. The exact ratios needed for maximum effect need to be further explored along with treatment administration schedules and durations for cannabinoid administration mixed with chemotherapy or radiation. With further knowledge of the exact mechanisms of cannabinoid effects on glioblastomas, the therapies discussed in this review can be cleared for clinical trials and for hospital use to improve prognosis of GBM patients.

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Conflicts of Interest: The author of this review has no conflicts of interest to disclose. 

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References: 

1. AANS. Glioblastoma Multiforme. Available at: https://www.aans.org/Patients/Neurosurgical-Conditions-and-Treatments/Glioblastoma-Multiforme. Published 2019. Accessed September 23, 2020.

2. American Brain Tumor Association. Glioblastoma (GBM). Available at: https://www.abta.org/tumor_types/glioblastoma-gbm/. Published 2019. Accessed September 23, 2020.

3. Holland EC. Glioblastoma multiforme: The terminator. PNAS 2000;97(12):6242-6244. doi:10.1073/pnas.97.12.6242

4. National Cancer Institute. Glioblastoma—unraveling the threads: A Q&A with Drs. Mark Gilbert and Terri Armstrong of the NIH neuro-oncology branch. Available at: https://www.cancer.gov/news-events/cancer-currents-blog/2017/glioblastoma-research-making-progress. Accessed September 22, 2020.

5. López-Valero I, Saiz-Ladera C, Torres S, Hernández-Tiedra S, et al. Targeting glioma initiating cells with a combined therapy of cannabinoids and temozolomide. Biochem Pharmacol 2018;157:266-274. doi: 10.1016/j.bcp.2018.09.007

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7. Scott KA, Dagleish AG, & Liu WM. The combination of cannabidiol and delta 9-tetrahydrocannabinol enhances the anticancer effects of radiation in an orthotopic murine glioma model. Mol Cancer Ther 2014;13(12):2955-2967. doi: 10.1158/1535-7163.MCT-14-0402

8. Nabissi M, Morelli MB, Amantini C, et al. Cannabidiol stimulates Aml-1a-dependent glial differentiation and inhibits glioma stem-like cells proliferation by inducing autophagy in a TRPV2-dependent manner. Int J Cancer 2015;137(8):1855-1869. doi: 10.1002/ijc.29573

9. Scott KA, Dennis JL, Dalgleish AG, et al. Inhibiting heat shock proteins can potentiate the cytotoxic effect of cannabidiol in human glioma cells. Int J Cancer Res Treat 2015;35(11):5827-5838

10. Hohmann T, Grabiec U, Ghadban C, Feese K, & Dehghani F. The influence of biomechanical properties and cannabinoids on tumor invasion. Cell Adh Migr 2017;11(1):54-67. doi: 10.1080/19336918.2016.1183867

11. Aparicio-Blanco J, Sebastián V, Benoit JP, & Torres-Suárez AI. Lipid nanocapsules decorated and loaded with cannabidiol as targeted prolonged release carriers for glioma therapy: In vitro screening of critical parameters. Eur J Pharm Biopharm 2018;134:126-137. doi: 10.1016/j.ejpb.2018.11.020

12. Hohmann T, Feese K, Greither T, et al. Synthetic cannabinoids influence the invasion of glioblastoma cell lines in a cell- and receptor-dependent manner. Cancers 2019;11(2): 161. doi: 10.3390/cancers11020161