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REVIEW ARTICLE OPEN
Cellular and Molecular Biology
Cannabinoids as anticancer drugs: current status of preclinical
research
Burkhard Hinz
1
✉and Robert Ramer
1
© The Author(s) 2022
Drugs that target the endocannabinoid system are of interest as pharmacological options to combat cancer and to improve the life
quality of cancer patients. From this perspective, cannabinoid compounds have been successfully tested as a systemic therapeutic
option in a number of preclinical models over the past decades. As a result of these efforts, a large body of data suggests that the
anticancer effects of cannabinoids are exerted at multiple levels of tumour progression via different signal transduction
mechanisms. Accordingly, there is considerable evidence for cannabinoid-mediated inhibition of tumour cell proliferation, tumour
invasion and metastasis, angiogenesis and chemoresistance, as well as induction of apoptosis and autophagy. Further studies
showed that cannabinoids could be potential combination partners for established chemotherapeutic agents or other therapeutic
interventions in cancer treatment. Research in recent years has yielded several compounds that exert promising effects on tumour
cells and tissues in addition to the psychoactive Δ
9
-tetrahydrocannabinol, such as the non-psychoactive phytocannabinoid
cannabidiol and inhibitors of endocannabinoid degradation. This review provides an up-to-date overview of the potential of
cannabinoids as inhibitors of tumour growth and spread as demonstrated in preclinical studies.
British Journal of Cancer; https://doi.org/10.1038/s41416-022-01727-4
BACKGROUND
History
The use of the cannabis plant for medicinal and ritual purposes
dates back several thousand years. Accordingly, the psychoactive
effect of cannabis is already mentioned in the Pen-ts’ao ching, the
oldest pharmacopoeia in the world [1,2]. Further evidence for a
millennia-old use is based on cannabis-containing grave goods
found in archaeological investigations of an Ukok ‘princess’
from the Pazyryk culture [3] or the remains of cannabis fruits
identified in archaeobotanical investigations at the Laoguanshan
cemetery from the Han dynasty in Chengdu, South China [4]. The
introduction of cannabis into European medicine can be
attributed to the Irish physician William B. O’Shaughnessy, who
published a groundbreaking study on hemp in 1839 with his work
‘On the preparations of the Indian hemp or gunjah’[5]. Although
the isolation of a substance called ‘cannabinol’from the exuded
resin of Indian hemp dates back to a communication published in
1899 [6], it was not until the 1960s that Raphael Mechoulam and
his collaborators published a series of studies that elucidated the
chemical structure and activity of Δ
9
-tetrahydrocannabinol (THC),
cannabidiol (CBD), and other cannabinoids [7–10].
Endocannabinoid system
The entry of the endocannabinoid (EC) system into modern
research as a potential target of pharmacotherapeutic interven-
tion began with the discovery and cloning of specificG
i/o
protein-
coupled cannabinoid receptors, termed CB
1
and CB
2
[11,12].
While CB
1
receptors are primarily localised in the central nervous
system, CB
2
receptors are mostly expressed on cells of the
immune system. Other components of the EC system discovered
in the 1990s are N-arachidonoylethanolamine (anandamide, AEA)
and 2-arachidonoylglycerol (2-AG), two endogenously synthe-
sised agonists at cannabinoid receptors [13,14]. Scientific
attention has also been paid in recent years to substances with
structural similarities to the aforementioned ECs, such as the
cannabinoid receptor ligands 2-arachidonoyl glycerol ether
(noladin ether), O-arachidonoylethanolamine (virodhamine),
N-arachidonoyldopamine and oleic acid amide (oleamide)
(reviewed in ref. [15]). However, the available data on their
biological role are very limited. Furthermore, a number of
N-acylethanolamines structurally similar to ECs, so-called EC-like
substances, such as N-palmitoylethanolamine (PEA) and
N-oleylethanolamine (OEA), have been described, which use the
biosynthesis and degradation enzymes of ECs, but do not trigger
cannabinoid receptor activation (reviewed in ref. [15]).
Around the turn of the millennium, the non-selective cation
channel transient receptor potential vanilloid 1 (TRPV1) was
described as an additional receptor target for several cannabi-
noids such as AEA [16] and the non-psychoactive phytocannabi-
noid CBD [17]. Among the phytocannabinoids, THC exhibits the
properties of an agonist at the CB
2
receptor and a partial
agonist at the CB
1
receptor [18], as well as an agonist at the G
Received: 19 August 2021 Revised: 9 December 2021 Accepted: 28 January 2022
1
Institute of Pharmacology and Toxicology, Rostock University Medical Centre, Schillingallee 70, 18057 Rostock, Germany. ✉email: burkhard.hinz@med.uni- rostock.de
www.nature.com/bjc
British Journal of Cancer
Published on Behalf of CRUK
1234567890();,:
protein-coupled receptor (GPR) 55 [19]. In contrast to the high-
affinity CB
1
and CB
2
receptor binding of THC with K
i
values in the
low nanomolar range, CBD has been demonstrated to have
weaker affinities with K
i
values in the micromolar range and non-
competitive antagonistic effects at both CB
1
and CB
2
receptor [18].
In addition, CBD shows a binding preference to the CB
2
receptor
[20] and an antagonistic effect at GPR55 [19]. Among a variety of
receptor interactions of CBD, the compound has been shown to
increase the transcriptional activity of peroxisome proliferator-
activated receptor γ(PPARγ)[21].
Other important elements affecting the tone of the EC system are
EC-synthesising and -degrading enzymes. In this context, N-acyl-
phosphatidylethanolamine-specificphospholipaseD(NAPE-PLD),α/β-
hydrolase domain-containing 4 (ABHD4), glycerophosphodiesterase-1
(GDE1), and tyrosine protein phosphatase non-receptor type 22
(PTPN22) have been described to contribute to AEA biosynthesis.
Diacylglycerol lipase αand -β(DAGLαand -β) have been identified as
2-AG-producing enzymes (reviewed in ref. [22]). The degradation of
AEA and 2-AG is endogenously mediated by the enzyme fatty acid
amide hydrolase (FAAH) [23], whereas the hydrolysis of 2-AG
proceeds mainly via monoacylglycerol lipase (MAGL) [24]with
contribution of several other hydrolytic enzymes (ABHD6, ABHD12,
FAAH). In addition, ECs can be degraded by enzymes of the
arachidonic acid cascade, i.e. cyclooxygenase-2 (COX-2) and lipox-
ygenases (reviewed in ref. [22]).
It is worth noting that the definition of the EC system has been
subject to evolution over time. Thus, the classical EC system only
includes the two ECs AEA and 2-AG, their anabolic and catabolic
enzymes, and the two cannabinoid receptors CB
1
and CB
2
.
However, as more became known about its complex networks
over time, the term and concept of the EC system was expanded
accordingly, leading to the definition of an “endocannabinoi-
dome”[25]. This now includes also other EC-like lipid mediators,
metabolic enzymes and novel cannabinoid targets such as GPRs
(GPR18, GPR55, GPR119) and members of the transient receptor
potential cation channel subfamily (TRPV1, TRPV2, TRPV4)
(reviewed in ref. [15,25]).
Anticancer effect of cannabinoids—pioneering work and
research strategies
The discovery of the anticarcinogenic effect of cannabinoid
compounds can be dated back to Munson et al. [26], who were
able to show in the mid-1970s that THC, Δ
8
-THC and cannabinol
inhibit Lewis lung adenocarcinoma growth in mice. However, the
discovery of cannabinoid receptors years later was only
the starting point for an extensive and detailed investigation of
the anticarcinogenic mechanisms of cannabinoid action. For
example, a study at the turn of the millennium showed that
intratumoural administration of THC and the synthetic cannabi-
noid agonist WIN 55,212-2 induced a considerable regression of
malignant gliomas in Wistar rats and in mice deficient in
recombination activating gene 2 [27]. A growth inhibitory effect
of cannabinoids against glioma cells was confirmed shortly
thereafter for the selective CB
2
agonist JWH-133 [28]. With the
discovery of a possible role of the EC system as a factor in cancer
progression, numerous studies have also addressed a possible link
between the regulation of cannabinoid receptors, ECs, and EC-
synthesising and -degrading enzymes on the one side and disease
severity and survival on the other (reviewed in ref. [15]). However,
no clear correlation between the regulation of these parameters in
tumour tissue and disease severity could be found, so that the
aforementioned factors therefore do not show suitability as
reliable biomarkers.
In addition to the activation of cannabinoid receptors by
exogenously applied agonists, newer approaches to pharma-
cotherapeutic intervention by blocking EC turnover have been
pursued and further developed in recent years, such as the
blockade of MAGL by the selective MAGL inhibitor JZL184 [29]. In
the context of potential cancer-preventive effects, knockdown or
pharmacological inhibition of MAGL have been associated with
decreased tumour cell invasion [30–32], metastasis [32] and
tumour growth [30,31]. This inhibitory effect on cancer
progression by MAGL inhibition appears to be due to a dual-
mode effect in prostate cancer cells involving both activation of
CB
1
receptor by increasing 2-AG levels and reduction of
protumorigenic free fatty acids resulting from MAGL activity
[30], whereas for the anti-invasive effect on lung cancer cells, only
CB
1
receptor involvement has been demonstrated [32]. On the
other hand, a recent publication concluded that inhibition of
MAGL promotes rather than retards cancer progression in mice
and that knockout of MAGL is associated with increased incidence
of lung adenocarcinoma [33]. With regard to the tumour
suppressor function of MAGL, it was shown in this context that
MAGL can inhibit the transactivation of epidermal growth factor
receptor (EGFR)-associated signalling pathways as well as proin-
flammatory proteins such as COX-2 and tumour necrosis factor
(TNF)-α. Accordingly, it is reasonable to assume that MAGL acts as
a tumour suppressor or oncoprotein depending on the tissue
type. Finally, inhibition of FAAH has also been shown to mediate
cancer-limiting effects with growth inhibitory [34,35] as well as
anti-invasive and antimetastatic properties [36]. The preclinical
work on the inhibition of FAAH and MAGL in cancer has been
summarised elsewhere (reviewed in ref. [15]).
Recently, further substances from Cannabis sativa L., which are
also found in other plants, have been investigated for their anti-
cancer effects. This concerns, for example, the sesquiterpene β-
caryophyllene, a potent CB
2
agonist [37]. In the hitherto
performed studies, β-caryophyllene showed antiproliferative and
proapoptotic properties on various cancer cell lines [38,39] and
enhanced the cytostatic effects of classical chemotherapeutic
agents such as doxorubicin [40] and sorafenib [41].
The first pilot clinical trial of cannabinoids as cancer treatment in 9
glioblastoma patients was published in 2006 and found that
intracranially administered THC was safe [42]. Recently, the results
of a randomised, placebo-controlled phase 1b study with nabiximols
oromucosal spray (standardised extract of Cannabis sativa L. with an
approximate 1:1 ratio of THC and CBD) in combination with dose-
intense temozolomide in patients with recurrent glioblastoma
multiforme were published [43]. Of the 21 patients in this study,
survival at one year was 83% in the 12 patients treated with
nabiximols, while it was significantly lower at 44% in the 9 patients
treated with placebo, leading the authors to recommend further
exploration in an adequately powered randomised controlled trial. So
far, however, randomised, placebo-controlled studies with a larger
number of cancer patients are lacking at all.
The following chapters provide an overview of the preclinical
evidence on the effect of cannabinoids on cancer growth and
metastasis, with the most relevant cannabinoid targets sum-
marised in Fig. 1(tumour cell proliferation, apoptosis and
autophagy) and Fig. 2(tumour cell invasion, angiogenesis).
TUMOUR CELL PROLIFERATION, APOPTOSIS AND AUTOPHAGY
Publications on growth inhibitory effects of cannabinoids have
accumulated over the last two decades. The dominant cellular
model used for this purpose in the 2000s was glioma cells.
However, over time, a wide range of different tumour cell lines of
various entities have been tested. Whereas early work focused on
mechanisms leading to cannabinoid-induced apoptosis and cell
cycle arrest of cancer cells, these investigations were comple-
mented later by studies dealing with autophagy effects. Due to
the large amount of data, only a selection could be made here.
Tumour cell proliferation
As mentioned earlier, the description of the tumour regressive
effects of THC and WIN 55,212-2 on rat and mouse glioma
B. Hinz and R. Ramer
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British Journal of Cancer
xenografts represents the first comprehensive study in this
century investigating cannabinoids as potential anticancer drugs
[27]. Since then, several mechanisms by which cannabinoids
inhibit tumour growth have been elucidated. Among these,
inhibition of protein kinase B (Akt) appears to be an important
mechanism. Accordingly, the antiproliferative action of cannabi-
noids on melanoma cells has been linked to cell cycle arrest at the
G
1
/S transition via inhibition of the prosurvival protein Akt and
hypophosphorylation of the retinoblastoma protein (pRb) tumour
suppressor protein [44]. Furthermore, CB
2
receptor-mediated
inhibition of Akt has been reported in ErbB2-positive breast
cancer progression in mouse mammary tumour virus (MMTV)/neu
transgenic mice as a syngeneic tumour model [45]. Meanwhile,
publications have confirmed that inactivation of the Akt pathway
is also involved in the antitumour activity of cannabinoids on
human gastric cancer [46], non-small cell lung cancer [47] and
hepatocellular carcinoma (HCC) cells [48]. In breast cancer cells,
features of cell cycle arrest induced by cannabinoid treatment
include blockade of the G
1
/S transition through downregulation of
cyclin-dependent kinase 1 (CDK1), induction of p21 [49], or
induction of p27kip1, decrease in cyclin (CCN) A and E,
degradation of cell division cycle 25 A (CDC25A) and inactivation
of CDK2 [50]. Furthermore, in prostate cancer cells, WIN 55,212-2
was shown to reduce proliferation and to arrest cells in the G
0
/G
1
phase via CB
2
receptor-dependent signalling [51]. Studies of the
underlying mechanism showed an upregulation of p27 and a
reduction in CDK4 expression and phosphorylated pRb (P-pRb)
when cells were treated with the cannabinoid.
CBD appears to inhibit cancer cell proliferation primarily via
apoptosis signalling. Thus, in a recent study [52], CBD suppressed
the proliferation and growth of head and neck squamous cell
carcinomas by inducing the apoptotic and autophagy activity of
DUSP1 (Dual Specificity Phosphatase 1), which is known to
interfere as a negative regulator with EGFR-initiated mitogen-
activated protein kinase (MAPK) signalling and associated
proliferation. Furthermore, increased expression levels of p21,
Kruppel-like factor 6 (KFL6) and growth arrest and DNA damage-
inducible protein α(GADD45A) were observed, for which
antiproliferative effects were also described. Another recent study
investigating the in vitro effects of CBD on human gastric cancer
cells revealed an antiproliferative effect accompanied by sig-
nificant upregulation of ataxia telangiectasia mutated (ATM) gene
and p21 protein expression and downregulation of p53 protein
expression, which subsequently resulted in decreased levels of
CDK2 and CCNE and cell cycle arrest in the G
0
/G
1
phase [53].
Finally, a contribution of the eicosanoid system was also
supported by the observation that CBD reduced the activity and
content of 5-lipoxygenase (5-LOX) and its end product leukotriene
B
4
in the tumour tissue of nude mice xenografted with human
glioma cells [54]. In addition, a synergism of the viability-reducing
effect of CBD and the 5-LOX inhibitor MK-886 could be shown
in vitro, which also argues for a modulatory effect of 5-LOX on
glioma cell fate.
Conversely, a recent study has demonstrated that the CB
1
agonist arachidonyl-2’-chloroethylamide (ACEA), the selective CB
2
agonist HU308 and THC promoted rather than inhibited the
progression of human papillomavirus-positive squamous cell
carcinomas of the head and neck, with THC-treated tumour
xenografts growing faster than controls [55]. This work is in line
with previous in vitro studies by other authors [56–58], who also
found proliferative effects on cancer cells triggered by submicro-
molar concentrations of various cannabinoids, with corresponding
THC effects associated with transactivation of EGFR in one paper
[56]. Some other studies, in contrast, failed to observe mitogenic
Fig. 1 Mechanisms of antiproliferative, proapoptotic and proautophagic effects of cannabinoids on cancer cells. The black arrows
emanating from the cannabinoid show the respective modulated structures or levels. Coloured arrows indicate inhibitory (red) and
stimulatory (blue) effects of cannabinoids on the indicated targets. Blue dashed arrows indicate reduced stimulation of the respective effect
by cannabinoid treatment. The grey arrows indicate a shift in a parameter. The black dashed arrow indicates a functional relationship between
autophagy and apoptosis. All abbreviations are explained in the text.
B. Hinz and R. Ramer
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British Journal of Cancer
effects of THC when nanomolar concentrations were tested
[42,59,60]. Overall, however, these results seem worth reconsider-
ing, as peak plasma concentrations of THC after inhalation or oral
administration do not indeed exceed 1 μM[55]. In fact, many
experimental studies investigating proapoptotic effects of canna-
binoids have been conducted with concentrations above 1 µM,
which, on the other hand, could well be achieved with intratumoral
administration [27]. Interestingly, hardly any mitogenic effects can
be observed in the case of CBD (reviewed in ref. [61]).
Tumour cell apoptosis
An important role in cannabinoid-induced apoptosis is played by
the proapoptotic sphingolipid ceramide. Initial studies showed
that THC and other cannabinoids cause the death of glioma
cells via cannabinoid receptor-dependent de novo synthesis of
ceramide [27,28,62]. The importance of ceramide for
cannabinoid-induced cytotoxic autophagy was discovered later
in 2009 [63]. Further apoptosis mechanisms were described with
CB
2
receptor-dependent induced apoptosis of glioma cells
associated with an increase in the stress-associated transcriptional
coactivator p8 as an upstream regulator of the endoplasmic
reticulum (ER) stress-related proteins activating transcription
factor (ATF)-4 and tribbles pseudokinase 3 (TRB3) [64]. A similar
apoptotic effect was found for CB
1
receptor-induced growth
inhibition in response to Akt inhibition in translocation-positive
rhabdomyosarcoma cells [65]. Here, cannabinoid-induced loss of
viability was suppressed by transfection of p8 siRNA.
Depending on the substance and cell type, the eicosanoid
system also revealed to play an important role in the proapoptotic
effect of cannabinoids. Thus, in temporal relation to the first
mechanistic findings on cannabinoid-induced glioma cell death,
ceramide synthesised de novo by R(+)-methanandamide (Met-
AEA) in neuroglioma cells was shown to lead to an induction of
COX-2 expression [66], which contributes to Met-AEA-induced cell
death via proapoptotic prostaglandin (PG) E
2
[67]. Thereby, Met-
AEA-triggered apoptosis was found to be independent of
cannabinoid receptor and TRPV1 activation. The influence of
COX-2 on cancer cell fate was also addressed in follow-up work on
the effect of Met-AEA [68] and the established chemotherapeutic
agents paclitaxel, cisplatin and 5-fluorouracil [69] on human
cervical carcinoma cells. Here, the PPARγ-activating eicosanoids
PGD
2
and 15-deoxy-Δ
12,14
-PGJ
2
could be identified as apoptosis
mediators, whereby post-transcriptional knockdown of COX-2,
downstream lipocalin-type PGD synthase (L-PGDS) and transcrip-
tion factor PPARγled to inhibition of apoptosis triggered by Met-
AEA and chemotherapeutic agents. A functional role of the
aforementioned PPARγ-activating PGs could also be proven for
the CBD-triggered cannabinoid receptor and TRPV1-independent
apoptotic death of lung cancer cells [70]. CBD also led to an
upregulation of COX-2 and PPARγin tumour tissue in A549-
xenografted nude mice and to tumour regression, which was
reversed by a PPARγantagonist [70].
As already mentioned above, cannabinoid receptors CB
1
and
CB
2
do not necessarily have to be involved in the apoptosis
Fig. 2 Mechanisms of anti-invasive, antimetastatic, anti-epithelial-to-mesenchymal-transition and anti-angiogenic effects of cannabi-
noids on cancer cells. The black arrows emanating from the cannabinoid show the respective modulated structures or levels. Coloured arrows
indicate the inhibitory (red) and stimulatory (blue) effects of cannabinoids on targets involved in cancer cell invasion/metastasis, angiogenesis
and epithelial-to-mesenchymal transition. Blue dashed arrows indicate reduced stimulation of each effect by cannabinoid treatment. Black lines
with circles at both ends indicate binding and dimer formation between the respective parameters. Black dashed arrow indicates functional
relationship between epithelial-to-mesenchymal transition and invasion/metastasis. All abbreviations are explained in the text.
B. Hinz and R. Ramer
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British Journal of Cancer
induction of a cannabinoid. In a previous study, AEA was
demonstrated to induce apoptotic death of human neuroblas-
toma and lymphoma cells via TRPV1, which was paralleled by an
increase in intracellular calcium, mitochondrial uncoupling and
cytochrome c (Cyt c) release [71]. In other studies, AEA has been
shown to induce receptor-independent apoptosis of non-
melanoma cancer cells via ER stress [72] and to elicit reduced
viability of cholangiocarcinoma cells via activation of GPR55, with
the latter eliciting increased recruitment of the death receptor Fas
in membrane lipid rafts [73]. Furthermore, activation of TRPV2 has
been revealed to be involved in the proapoptotic effect of CBD on
human bladder cancer cells [74]. Mostly, however, the proapop-
totic effects of CBD have been associated with receptor-
independent mechanisms, although corresponding inhibitor
experiments with receptor antagonists have not always been
performed. This applies, for example, to some studies in which
CBD elicited apoptosis of mammary carcinoma [75] and human
gastric cancer cells [53] via the formation of reactive oxygen
species (ROS). Interestingly, however, CBD induced a CB
2
receptor-
dependent mitochondrial apoptosis in human leukaemia cells,
which was accompanied by ROS production, increased expression
of the NAD(P)H oxidases Nox4 and p22phox and release of Cyt c
[76]. In a recently published study [77], CBD was also shown to
promote apoptosis of gastric cancer cells by suppressing X-linked
inhibitor apoptosis (XIAP), a member of the IAP protein family.
Thereby, CBD reduced XIAP protein levels while increasing
ubiquitination of the protein. In addition, CBD increased the
interaction between XIAP and Smac by inducing the release of
Smac from mitochondria into the cytosol and promoted
mitochondrial dysfunction. Finally, recent evidence suggests that
CBD at a relatively high concentration of 30 µM switches the
mitochondrial voltage-dependent anion channel (VDAC) from fully
open to major subconductance state, thereby arresting the
Ca
2+
-permeable state of this channel and inducing a severe
oxidative stress, mitochondrial Ca
2+
overload, Cyt c release into
the cytosol as well as induction of LC3-phosphatidylethanolamine
conjugate (LC3-II) and caspase activation in leukaemia cells [78].
Tumour cell autophagy
Autophagy can lead to survival or death of cells. Research in
recent years has shown that autophagy signalling pathways may
also play an important role in the toxicity of cannabinoid
compounds on cancer cells. Initial experiments conducted by
Salazar et al. [63] demonstrated that THC induces death of human
glioma cells via stimulation of autophagy. In this process, THC led
to ceramide accumulation and phosphorylation of eukaryotic
translation initiation factor 2α(eIF2α), a subsequent ER stress
response and, via this, finally to the induction of autophagy via the
TRB3-dependent inhibition of the Akt/Mammalian Target of
Rapamycin Complex 1 (mTORC1) axis. In the processes described,
autophagy preceded apoptosis. In another work, proautophagic
effects of THC and the CB
2
agonist JWH-015 in human HCC cells
were reported [79]. Here, autophagy was due to upregulation of
TRB3 and subsequent inhibition of the Akt/mTORC1 axis and
adenosine monophosphate-activated kinase (AMPK) stimulation,
with Ca
2+
/calmodulin-activated kinase kinase β(CaCMKKβ) being
responsible for cannabinoid-induced AMPK activation and au-
tophagy. Importantly, the tumour regressive effects of THC and
JWH-015 on subcutaneous HCC xenografts were abolished in vivo
when autophagy was genetically or pharmacologically inhibited
[79]. A functional link between autophagic signalling pathways
and cannabinoid-induced apoptosis was confirmed by several
other studies. Accordingly, in melanoma cells, co-treatment of THC
with the autophagic flux inhibitor chloroquine or knockdown of
autophagy-related 7 (Atg7), an essential molecule for induction of
autophagy, resulted in suppression of THC-induced autophagy as
well as cell death [80]. Consistent with this, inhibition of CBD-
induced autophagy by chloroquine also led to a significant
increase in the viability of CBD-treated human squamous cell
carcinomas of the head and neck [52].
On the other side and in contrast to previous studies on glioma
cells, a recent study reported that knockdown of autophagy genes
led to enhancement of WIN 55,212-2-induced apoptotic cell death
of human glioblastoma cells [81]. The authors concluded that
autophagy induced by cannabinoid treatment is a protective
mechanism and autophagy inhibitors may be potential agents to
enhance cannabinoid action. Evidence of cannabinoid-protective
autophagy was also provided by a glioblastoma cell culture study
in which the addition of chloroquine led to an increase in CBD-
induced cell death [82].
In glioma stem cells, a cell subpopulation of glioblastoma
multiforme implicated in chemoresistance, CBD was further
shown to induce a TRPV2-dependent autophagic process that
stimulates glioma stem cell differentiation via induction of a splice
variant of the acute myeloid leukaemia transcription factor
(Aml-1), thereby abrogating their chemoresistance to carmustine
[83]. In breast cancer cells, CBD-induced intrinsic apoptosis was
associated with autophagy and decreased levels of mTOR, its
downstream effector eukaryotic initiation factor 4E binding
protein 1 (4EBP1) and CCND1 [75]. Finally, in a recent study, a
significant increase in LC3-II levels as an autophagy marker was
observed in two out of three medulloblastoma cell lines after CBD
and THC exposure, which was accompanied by an increase in poly
(ADP-ribose) polymerase (PARP) cleavage [84]. However, these
findings did not translate to in vivo models in mice, as the survival
rate of the animals did not change.
As another cannabinoid-triggered upstream ion channel a
recent investigation found TRPV4 and a signalling pathway
including ATF4, DNA Damage Inducible Transcript 3 (DDIT3),
TRB3, Akt and mTOR as mediators of CBD-induced mitophagy in
glioma cells [85]. Mitophagy involves the selective degradation of
mitochondria by autophagy and is manifested in mitophagy-
related proteins such as BCL2 interacting protein 3 (BNIP3), Cyt c,
Parkin and engulfed mitochondria, with mitochondrial dysfunc-
tion and overactivation of heme oxygenase-1 (HO-1) acting
synergistically in lethal mitophagy [86].
TUMOUR CELL INVASION AND METASTASIS
Tumour cell invasion
A large number of publications also point to an inhibitory effect of
cannabinoid compounds on tumour cell migration, invasion and
in vivo metastasis (Fig. 2). The first report on this referred to the
CB
1
receptor-dependent inhibition of prostate cancer cell invasion
by the EC 2-AG [87]. Anti-invasive actions were also later proven
for AEA in glioma [88] and lung cancer cells [36]. A few years
earlier, the AEA derivative Met-AEA had already been shown to
exhibit anti-invasive properties on human cervical and lung cancer
cells via cannabinoid receptors and TRPV1 [89].
In one of the first mechanistic studies in this field, induction of
tissue inhibitor of metalloproteinase-1 (TIMP-1) expression was
shown to be a cause of the anti-invasive effect of THC, Met-AEA
and CBD in cervical and lung cancer cells, with cannabinoid
receptors (THC, Met-AEA, CBD) and TRPV1 (Met-AEA, CBD)
mediating this response [89,90]. Crucially, TIMP-1 inhibits
collagen-degrading enzymes such as matrix metalloproteinase
(MMP)-2 and MMP-9, which play an important role in promoting
cancer metastasis (reviewed in ref. [91]). Accordingly, overexpres-
sion of TIMP-1 has been associated with a reduction in tumour
growth, cancer cell invasiveness and metastasis (reviewed in ref.
[92]). An anti-invasive effect based on TIMP-1 induction in lung
cancer cells was also confirmed for the FAAH inhibitors
N-arachidonoyl-serotonin (AA-5HT) and URB597, AEA and OEA
[36] as well as for the MAGL inhibitor JZL184 and the MAGL
substrate 2-AG [32]. For the anti-invasive effect of THC, CBD and
Met-AEA on lung tumour cells, a cannabinoid-mediated increased
B. Hinz and R. Ramer
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British Journal of Cancer
expression of the intercellular adhesion molecule-1 (ICAM-1) as an
upstream inducer of TIMP-1 expression could be demonstrated
[93].
In support of the importance of modulation of extracellular
matrix proteolysis in anti-invasive cannabinoid action, other
groups have demonstrated downregulation of MMP-2 by THC in
glioma cells [94] and downregulation of MMP-2 and -9 in HCC cells
treated with CB
1
(ACEA) or CB
2
agonists (CB65) [95]. Notably, the
antimigratory effect of the synthetic cannabinoid WIN 55,212-2 on
osteosarcoma cells was associated with downregulation of MMP-2
and -9 and a 700-fold upregulation of miR-29b1, a key miRNA that
downregulates MMP-2 and -9 [96]. A recent study also focused on
the role of cancer-associated fibroblasts as a crucial element of the
stromal compartments in the tumour microenvironment. Here,
conditioned media from WIN 55,212-2-treated cancer-associated
fibroblasts were shown to impair the invasive properties of
prostate cancer cells due to a cannabinoid-mediated reduction in
MMP-2 release [97].
In breast cancer, glioblastoma and salivary gland cancer cells, a
CBD-mediated downregulation of Id-1, an inhibitor of basic helix-
loop-helix transcription factors, has been reported as the cause of
the anti-invasive effect of this compound [98–100]. CBD also led to
a downregulation of the sex-determining region Y (SRY)-Box 2
(Sox-2), a critical determinant of glioma tumour initiating cell
growth and downstream target of Id-1, in glioblastoma cells [99].
Finally, the antimetastatic effect on breast cancer cells demon-
strated for CBD in a mouse model [101] was directly linked to a
downregulation of Id-1 in a later work [102]. In another
investigation, siRNA, inhibitor and add-back experiments showed
that the cannabinoid receptor- and TRPV1-dependent down-
regulation of plasminogen activator inhibitor (PAI)-1 in CBD-
exposed lung cancer cells [103] is part of the anti-invasive effect of
this cannabinoid, in addition to the upregulation of TIMP-1
mentioned above. Still other work revealed that the anti-invasive
effect of THC on cholangiocarcinoma cells is associated with
reduced activation of Akt and p42/44 MAPK [104]. Furthermore, a
very recent work found that treatment with CBD in combination
with THC or CBD alone inhibited bladder urothelial carcinoma cell
migration independently of cannabinoid receptors [105].
Contradictory results have been published on the role of the CB
2
receptor in the invasion process. While studies with selective CB
2
agonists [95,102] as well as inhibitor studies [36,89,90,93,103]
suggest a role for CB
2
in the anti-invasive effects of various
cannabinoids, a recent investigation reported that silencing of the
CB
2
receptor reduced proliferation, migration and invasion of lung
cancer cells, which was associated with reduced levels of phospho-
Akt, phospho-mTOR and decreased expression of 70-kDa ribosomal
protein S6 kinase (p70S6K), a mitogen-activated Ser/Thr protein
kinase that promotes cell survival and growth [106]. With respect to
the CB
2
receptor, it has also been shown that the latter can form an
induced heterodimer with the G protein-coupled chemokine
receptor CXCR4 in human breast and prostate cancer cells [107].
Here, simultaneous agonist-dependent activation of CXCR4 and CB
2
resulted in reduced CXCR4-mediated formation of phosphorylated
p42/44 MAPK (P-p42/44) and ultimately diminished cancer cell
chemotaxis. Therefore, cannabinoids could also negatively modulate
tumour progression by interfering with CXCR4 receptor function.
A number of other publications addressed the role of epithelial-
mesenchymal transition (EMT)-lowering properties of cannabinoid
compounds as a contributing mechanism of action to reduce
cancer aggressiveness (Fig. 2). An early study on this topic showed
that the AEA derivative 2-methyl-2′-F-anandamide (Met-F-AEA)
reduced cytoplasmic and nuclear protein levels of β-catenin, one
of the key factors involved in the EMT transition [108]. In this
publication, Met-F-AEA further caused significant inhibition of
mesenchymal markers such as vimentin, N-cadherin, fibronectin
and EMT markers (Snail1, Slug and Twist) and upregulation of
epithelial markers such as E-cadherin and cytokeratin 18. Recently,
CBD was also shown to reverse interleukin (IL)-1β-induced EMT of
human breast cancer cells by reprogramming invasive cells into
cells with a non-invasive phenotype [109]. Here, CBD induced the
relocalisation of E-cadherin and β-catenin at adherens junctions,
thereby preventing nuclear translocation of β-catenin and
inhibited the expression of the EMT marker ΔNp63, an isoform
of tumour protein 63 (TP63), baculoviral IAP repeat-containing
protein 3 (BIRC3) and Id-1. In the same work, studying the
malignant phenotype of breast cancer cells, CB
1
receptor-
mediated inhibition of viability by CBD was registered, as well as
an antimigratory effect of the cannabinoid associated with
inhibition of Akt phosphorylation [109]. Others observed a reversal
of transforming growth factor (TGF)-β-induced spindle-shaped
morphology of lung cancer cells corresponding to the reorganisa-
tion of the stress fibre F-actin when the cells were treated with a
combination of THC and CBD [110]. In this study, TGF-β-induced
inhibition of E-cadherin expression and upregulation of
N-cadherin and vimentin were significantly reversed as a
characteristic EMT regulatory pattern in the presence of CBD,
THC or the combination of both. Finally, these cannabinoid-
mediated regulations were functionally linked to a reduced
migration potential of lung cancer cells.
Metastasis
The effect of cannabinoids in experimental metastasis models has
been described in detail elsewhere (reviewed in ref. [111]). In
short, phytocannabinoids (THC, CBD), ECs and EC analogue (AEA,
2-AG, Met-F-AEA), EC-like substances (OEA, PEA) and EC degrada-
tion inhibitors (AA-5HT, URB597, JZL184) showed an inhibitory
effect on metastatic infiltration of the lung with previously
injected lung carcinoma cells [32,36,90,93,112,113]. In addition,
inhibitory effects of cannabinoids on breast cancer [101,102,114],
salivary gland cancer [100] and melanoma cell metastasis [44]
have been described. Finally, further work showed that JZL184
impairs bone metastasis of osteotropic prostate and breast cancer
cells in mice and inhibits metastasis of osteosarcoma cells [115]
and that knockdown of MAGL is associated with reduced lymph
node metastasis in MAGL-overexpressing nasopharygeal carci-
noma cells [116].
Regarding the underlying mechanisms, one investigation
demonstrated increased ICAM-1 expression as a cause of the
antimetastatic effect of CBD on lung cancer cells, with the effect
being reversed by an ICAM-1 neutralising antibody [93]. Another
work showed that CBD inhibited lung metastasis of breast cancer
cells expressing a control vector but not an Id-1-containing vector
[102]. Further studies have identified the CB
1
receptor as an initial
platform of the antimetastatic effect of Met-F-AEA [112] and
JZL184 [32], as the inhibition of metastasis induced by these
compounds was counteracted by CB
1
receptor antagonists.
TUMOUR ANGIOGENESIS
Several studies have shown that cannabinoid compounds inhibit
tumour neovascularisation in mouse models with xenografts
(reviewed in ref. [117]). However, data on the exact mechanisms
underlying these effects, especially with regard to tumour-stroma
interactions, are still scarce. Early reports on the effect of cannabinoid
compounds on tumour formation found a regressive effect based on
anti-angiogenic effects through the downregulation of a number of
proangiogenic parameters such as vascular endothelial growth factor
(VEGF), placental growth factor (PlGF), angiopoietin-2 (Ang-2)
[118,119] and MMP-2 [119].Thesamegrouplaterrevealedthat
the CB
2
receptor agonist JWH-133 modulates several hypoxia-related
angiogenesis markers, most of which are associated with the VEGF
pathway [120]. Thus, downregulation of VEGF-A and -B, hypoxia-
inducible factor-1α(HIF-1α), connective tissue growth factor (CTGF),
midkine, Id-3, Ang-2 and its receptor tyrosine kinase with
immunoglobulin-like and EGF-like domains 1 (Tie-1), and HO-1 was
B. Hinz and R. Ramer
6
British Journal of Cancer
detected, while upregulation was found for the type I procollagen α1
chain (COL1A1) [120]. Remarkably, the authors of the latter study
found VEGF receptor (VEGFR)-2 downregulation in response to JWH-
133 in experimental glioma xenografts in mice. In another paper,
inhibition of MMP-2 expression in human umbilical vein endothelial
cells (HUVEC) was confirmed as part of the anti-angiogenic effects of
CBD [121]. Consistent with the aforementioned direct anti-angiogenic
effects, a further investigation identified the hexahydrocannabinol
analogues LYR-7 [(9S)-3,6,6,9-tetramethyl-6a,7,8,9,10,10a-hexahydro-
6H-benzo[c]chromen-1-ol] and LYR-8 [(1-((9S)-1-hydroxy-6,6,9-tri-
methyl-6a,7,8,9,10,10a-hexahydro-6H-benzo[c]chromen-2-yl)etha-
none)] as inhibitors of VEGF-induced proliferation, migration and
capillary-like tube formation of human endothelial cells as well as
VEGF-induced blood vessel formation in the chorioallantoic mem-
brane assay and VEGF release from cancer cells [122]. Thereby, the
effects of the test substances on cell proliferation and tube formation
were not abolished by cannabinoid receptor antagonists, suggesting
a cannabinoid receptor-independent mechanism.
A number of studies have also demonstrated indirect effects of
cannabinoids on endothelial cells through cannabinoid-mediated
modulation of the secretome of tumour or other non-endothelial
cells. Thus, conditioned media from AEA-treated breast cancer cells
showed an inhibitory effect on endothelial cell proliferation associated
with an approximate halving of the VEGF concentration, but also in a
significant reduction of other proangiogenic factors such as leptin and
thrombopoietin (THPO) [123]. A further indirect effect, here mediated
by cannabinoid-induced increased formation of anti-angiogenic TIMP-
1, was reported for the effect of conditioned media from CBD-, THC-,
Met-AEA- and JWH-133-treated lung cancer cells on tube and sprout
formation and endothelial cell migration [124]. A recent report also
dealt with the effect of cannabinoids on human neutrophils with
special regard to the regulation of angiogenic factors. Thereby,
submicromolar concentrations of ACEA and JWH-133 inhibited
lipopolysaccharide (LPS)-induced VEGF-A release, which was accom-
panied by a decrease in LPS-induced angiogenic effects on bovine
aortic endothelial cell tube formation [125].
Divergent findings have been published regarding the direct
effects of cannabinoid compounds on endothelial cells, which
depend on the cellular system, the substance and the concentration
used. Thus, in a previous study, inactivation of the CB
1
receptor with
the antagonist SR141716 as well as CB
1
knockdown using siRNA
caused inhibition of basic fibroblast growth factor (bFGF)-induced
proliferation, migration and capillary-like tube formation of HUVEC
[126] through pathways involving decreased activation of focal
adhesion kinase, c-Jun N-terminal kinase, RhoA and MMP-2.
Moreover, CB
1
antagonism was confirmed in vivo to inhibit bFGF-
induced neovascularisation in the rabbit cornea [126]. As for the
direct effects of ECs on endothelial cells, it has been shown that
bFGF-stimulated proliferation of HUVEC is upregulated by nanomo-
lar concentrations of AEA [126], while in another work of the same
group, micromolar proapoptotic concentrations of the AEA analo-
gue Met-F-AEA inhibited bFGF-stimulated proliferation of porcine
endothelial cells in a CB
1
receptor-dependent manner [127]. In a
further study, no anti-angiogenic effects of ECs on endothelial
proliferation were observed and proangiogenic effects of THC and
CBD were found at nanomolar concentrations [128]. Others reported
that CBD, THC, Met-AEA and JWH-133 (3 µM each) exhibited pro-
rather than anti-angiogenic effects in HUVEC directly exposed to
these substances [124]. Finally, AEA was demonstrated to induce
endothelial cell tube formation and proliferation, with both effects
being reversed by inactivation of TRPV1 [129]. The authors assumed
a TRPV1-dependent uptake of AEA into endothelial cells, which
subsequently causes a proangiogenic effect on endothelial cells via
the activation of intracellular cannabinoid receptors.
In terms of the direct influence of CBD on the endothelium, a
recent study should also be mentioned which showed that CBD
promotes ROS-dependent HO-1 expression in HUVEC, followed by
HO-1-dependent cytoprotective autophagy [130]. This protection
was maintained up to a certain CBD concentration (in the cited
study up to 6 µM), but was then no longer sufficient to protect the
cells from apoptotic cell death, which was also HO-1-dependent. The
proapoptotic concentration of 10 µM CBD registered in this work is
within the concentration range of the previously shown anti-
angiogenic effects of CBD at concentrations ≥9µM [121]. In the
latter study, however, cytostasis, but not the induction of apoptosis,
was associated with the decrease in metabolic activity. In this
context, the use of different supplements leading to phenotypic
heterogeneity [131] could explain the study-dependent varying
sensitivity of HUVEC to high CBD concentrations.
INTERACTIONS WITH THE IMMUNE SYSTEM
As stromal cells in the tumour microenvironment are targets for
therapeutic intervention, the role of immune cells as an important
part of the tumour stroma has been brought into focus in recent
years in relation to cannabinoid effects on cancer progression
(Fig. 3). In this context, single-cell transcriptome profiling revealed
that THC significantly affects transcriptomic subclusters in immune
cell types [132]. A recent study showed inhibition of Kras-activated
signalling pathways by p21 activated kinase 1 (PAK1) after treatment
of cells with CBD and THC. Cannabinoids were found to reduce
expression of programmed death ligand 1 (PD-L1) via decreasing
PAK1 activity, thereby enhancing immune checkpoint blockade of
pancreatic cancer cells [133]. For other peripheral cells, such as
neutrophil granulocytes, their ability to produce proangiogenic
factors such as VEGF after exposure to LPS was shown to be blocked
by treatment with CB
1
(ACEA) and CB
2
agonists (JWH-133), which in
turn led to decreased tumour growth via inhibition of angiogenesis
[125].
Regarding the effect of ECs, one study investigated the
influence of 2-AG on different subpopulations of immune cells
involved in the progression of pancreatic ductal adenocarcinoma
using an orthotopic mouse model. Here, 2-AG increased the
proportion of CD83
+
, CD86
+
and MHCII
+
cells in CD11C
+
cell
populations in the spleen of mice [134]. Dendritic cells promoted
to maturation by 2-AG exhibited higher expression of proin-
flammatory cytokines (IL-6, IL-12, interferon-α) mediated by
activation of the CB
1
receptor and subsequent upregulation of
the phosphorylated form of signal transducer and activator of
transcription 6 (STAT6). A concomitant activation of T cells in the
spleen was not observed. On the other hand, in spleen and
tumour tissue of mice, 2-AG also induced the proliferation of
myeloid-derived suppressor cells, which are known to suppress
the T cell response thereby promoting an immunosuppressive
microenvironment. Nevertheless, the antiproliferative effect of
2-AG on cancer cells ultimately led to an overall tumour-regressive
effect in vivo [134].
In vivo experiments with murine melanoma models indicated that
WIN 55,212-2 inhibited cancer growth more efficiently in immuno-
competent compared to immunodeficient mice [44]. A further in vivo
study reported reduced infiltration of macrophages and neutrophils
into experimental skin tumours after treatment of mice with THC
[135]. As THC did not inhibit cancer cell proliferation in this work, the
authors concluded that THC causes inhibition of cancer growth
in vivo due to its cannabinoid receptor-activating properties in the
tumour microenvironment and not in the tumour itself. In a further
study the in vitro interaction of lung cancer cells with lymphokine-
activated killer cells was addressed [136]. Here, prior treatment of
cancer cells with CBD, THC and Met-AEA led to increased killer cell-
mediated lysis of lung cancer cells. As a cause, an ICAM-1
upregulation on tumour cells induced by the tested cannabinoids
could be identified with the consequence of an increased cross-
linking with the lymphocyte function-associated antigen-1 (LFA-1) on
the surface of natural killer cells [136]. Yet another work showed that
inhibition of endogenous AEA degradation enhanced interaction
with the tumour immune system. Accordingly, the FAAH inhibitor
B. Hinz and R. Ramer
7
British Journal of Cancer
URB597 suppressed the shedding of the proteins major histocompat-
ibility complex class I polypeptide-related sequence A (MICA) and B
(MICB) on the surface of human HCC cells [137]. Since MICA/B
proteins are recognised by cytotoxic lymphocytes expressing the
natural killer group 2D (NKG2D) receptor and tumour cells are
subsequently eliminated, preventing the shedding of MICA/B proteins
can improve antitumour immunity. In the paper presented here, the
effect was indirect, and was shown to be due to increased expression
of TIMP-3. The authors further found AEA, 2-AG and the CB
2
receptor
agonist AM1241 to enhance TIMP-3 expression accompanied be a
reduction of MICA/B shedding. Noteworthy, the TIMP-3 inducing and
inhibitory action on MICA/B shedding upon treatment with URB597
was partially prevented by the CB
1
receptor antagonist rimonabant.
In contrast to these positive effects, however, another early study
on this topic reported that THC increases breast cancer growth and
promotes tumour cell spread by inhibiting the antitumour immune
response via enhancement of Th2-associated cytokines [138].
However, the study design used here was to exclude the direct
growth inhibitory effect of cannabinoids on tumour cells in order to
focus on the cannabinoid effect on the immune system. For this
reason, tumour cells whose expression of cannabinoid receptors was
low to undetectable were used in the experiments. This report was
consistent with another, even earlier study that had observed
tumour growth accelerating effects of THC based on reduced
tumour immunogenicity [139]. A recent publication further reported
that as a result of CB
2
receptor-induced microglial M2 polarisation,
conditioned media of cannabinoid-treated microglial cells increased
rather than inhibited the angiogenic capacities of human brain
microvascular endothelial cells [140]. The M2 polarisation induced
by JWH-133 was demonstrated here by mRNA analyses showing
increased levels of the M2 subtype markers CD206, arginase-1 (Arg1)
and the chitinase-like protein Ym1 and decreased levels of the
M1 subtype markers CD68, CD86 and inducible nitric oxide synthase
(iNOS). M2 polarisation also proved to be the mechanism by which
the CB
2
agonist JHW-133 attenuated its own tumour-regressive
effect in nude mice with intracranial glioma xenografts, which was
only fully expressed by knockdown of cytochrome P450 2J2
(CYP2J2), which mediates the proangiogenic effect [140]. As a
functional mediator of CB
2
receptor-dependent CYP2J2-induced M2
polarisation, the authors pointed to increased synthesis of 11,12-
epoxyeicosatrienoic acid (11,12-EET) by CYP2J2.
COMBINATION PARTNERS IN ANTICANCER TREATMENTS
Although some newly approved anticancer drugs are also used as
monotherapy for certain indications, it seems more likely that
cannabinoid compounds will be used as a combination and add-on
option with currently employed cytostatics, assuming successful
clinical trials. Against this background, THC and CBD, which are
currently being tested in some studies as combination, have been
preclinically shown to enhance the effect of various cytostatics, such
as for vinca alkaloids, cytarabine, doxorubicin, mitoxantrone,
carmustine, temozolomide, bortezomib, carfilzomib and cisplatin
(reviewed in ref. [111,141]). Thereby, combined administration of
CBD and temozolomide in patient-derived neurosphere cultures and
orthotopic mouse models was demonstrated to exert a significant
synergistic effect in both reducing tumour size and prolonging
survival [85]. Of particular importance for the use of cannabinoids in
the treatment of glioblastoma is that the aforementioned booster
effect on temozolomide action was previously confirmed in
Fig. 3 Effects of cannabinoid compounds on tumour-immune interaction. Green arrows indicate the specific site of action of the indicated
cannabinoids. Black lines with circles at the end indicate receptor interactions. Black arrows indicate a functional or regulatory consequence of
cannabinoid treatment. The indication “CB
1
/CB
2
”means that the substances listed here act via both cannabinoid receptors. All abbreviations
are explained in the text.
B. Hinz and R. Ramer
8
British Journal of Cancer
elaborate in vivo mouse models [59]. In glioblastoma cells, CBD has
also been shown to enhance the effect of cisplatin [142]. Recently, a
synergistic effect was also confirmed for the tumour regressive
effect of CBD and cisplatin in a murine model of squamous cell
carcinoma of the head and neck as well as for the in vitro
cytotoxicity of CBD in combination with cisplatin, 5-fluorouracil or
paclitaxel on human squamous cell carcinoma cells of the head and
neck [52]. However, the mechanisms of these synergies are not yet
fully understood. In this context, one study has shown that
cannabinoid-mediated enhancement of the effect of vinblastine in
resistant leukaemia cells was accompanied by THC- and CBD-
induced downregulation of P-glycoprotein [143], while the syner-
gistic cannabinoid effect over mitoxantrone in embryonic fibroblasts
occurred via inhibition of ATP-binding cassette transporters (ABC)G2
[144]. Another study focusing on the effect of THC on the
sensitisation of leukaemia cells to treatment with cytarabine,
doxorubicin and vincristine showed reduced p42/44 MAPK activity
as the underlying mechanism of THC-induced enhancement of
therespectivecytostaticeffect[145]. In addition, a number of
mechanistic studies have found a CBD-mediated increase in tumour
cell susceptibility to the proteasome inhibitor bortezomib [146,147],
doxorubicin [148,149] as well as temozolomide and carmustine
[148]. There has also been a report that CBD increases the uptake
into and toxicity on glioma cells of doxorubicin, temozolomide and
carmustine via an increase in TRPV2 activity and associated
increased calcium influx [148], with these results also confirmed
for doxorubicin in triple negative breast cancer cells [149]. With
regard to the synergistic effect with bortezomib, it was also shown
that the combination of CBD and THC inhibits the expression of the
immunoproteasome subunit β5i in multiple myeloma cells [147]. In
addition, the synergistic effect of the combination of CBD and THC
should also be mentioned here, which for example induces
autophagy-dependent necrosis in multiple myeloma cells and
inhibits cellular migration by downregulating the expression of the
chemokine receptor CXCR4 and the plasma membrane glycoprotein
CD147 [147]. However, in contrast to the results of these studies,
which showed an enhancement of cytostatic effects when
combined with cannabinoid compounds, a recently published
paper did not find a survival benefit in the cannabinoid treatment
group in combination with cyclophosphamide in an in vivo
medulloblastoma model [84].
In addition, several studies suggest that cannabinoid treatment
causes glioma cells to become more sensitive to ionising radiation,
as shown for the combination of THC and CBD [60] and the
combination of CBD with heat shock inhibitors [150]. Increased
radiosensitivity was confirmed in CBD-treated glioma cells in
another study [151], with the same group later reporting the
underlying mechanism to involve inhibition of ATM kinase, a
serine/threonine protein kinase that is recruited and activated by
DNA double-strand breaks [152].
CONCLUSION
Given a considerable number of in vitro and animal studies showing
that cannabinoid compounds exert tumour growth inhibitory and
antimetastatic effects, cannabinoid compounds may represent a
useful additional therapeutic option to currently used cytostatic
drugs. This view is also supported by studies indicating a synergistic
effect of cannabinoids in combination with currently used
chemotherapeutic agents and other therapeutic options. Further-
more, data increasingly suggest that cannabinoids may additionally
function as antimetastatic and anti-angiogenic tumour therapy and
support the immune system in its defence against tumours.
In addition to the partially divergent preclinical studies already
mentioned, some epidemiological studies should also be included in
the critical assessment of the potential use of cannabinoids as
systemic therapy options. Thus, a recently published prospective
observational study showed that cannabis use significantly shortens
the time to tumour progression and overall survival of cancer
patients [153]. This study illustrates that cannabis use, in this case via
modulation of the immune system, can lead to negative and thus
life-threatening effects for cancer patients. In addition, a retro-
spective observational study showed a reduction in the response
rate to nivolumab, although the addition of cannabis here had no
effect on progression-free survival or overall survival [154]. However,
it is worth noting that the administration of cannabinoids in these
studies conducted in relatively small patient groups was through the
consumption of cannabis oil or smoked/inhaled cannabis, and in
many cases the cannabis products were also changed during the
course of the study.
On the other hand, these data are counterbalanced by an
overwhelming number of studies that clearly show that activation of
the EC system is an important factor in tumour defence and thus
could serve as a promising target for pharmacological anticancer
interventions. In this context, a collection of case reports involving
119 patients also presented impressive examples of breast cancer
and glioma patients treated with pharmaceutical-grade synthetic
CBD, demonstrating a reduction in circulating tumour cells or a
reduction in tumour size by repeat scans [155]. Furthermore, the
results of the well-conducted but very small randomised, placebo-
controlled phase 1b trial in patients with recurrent glioblastoma
multiforme mentioned at the beginning of this article and the
higher survival rate shown here in patients taking nabiximols instead
of placebo in combination with dose-intense temozolomide [43]
provide the rationale for larger and thus adequately powered
randomised placebo-controlled trials.
In summary, the property of cannabinoids, in particular, to induce
inhibition of tumour growth and spread at multiple levels of tumour
progression argues for the use of these substances as an add-on
option in tumour treatment. However, it should also be noted that
research into the efficacy, dosage and drug safety of cannabinoids in
tumour therapy still has a long way to go, especially with regard to
clinical trials to be conducted, through which alone the benefits and
advantages for cancer patients but also possible risks can be
defined.
DATA AVAILABILITY
Not applicable.
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B. Hinz and R. Ramer
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British Journal of Cancer
ACKNOWLEDGEMENTS
The corresponding author’s preclinical experimental work on the effects of cannabinoids
and chemotherapeutic agents is currently funded by the German Research Foundation
(DFG, HI 813/9-1), the joint research project ONKOTHER-H (supported by the European
Social Fund, reference: ESF/14-BM-A55-0002/18, and the Ministry of Education, Science
and Culture of Mecklenburg-Vorpommern, Germany) and by the joint research projects
RESPONSE FV18 (reference: 03ZZ0928A) and RESPONSE TV3 (reference: 03ZZ0933A) of
the Federal Ministry of Education and Research (BMBF), Germany.
AUTHOR CONTRIBUTIONS
BH and RR: conceptualisation, formal analysis, methodology, writing and editing.
FUNDING
Open Access funding enabled and organized by Projekt DEAL.
ETHICS APPROVAL AND CONSENT TO PARTICIPATE
Not applicable.
CONSENT TO PUBLISH
Not applicable.
COMPETING INTERESTS
The authors declare no competing interests.
ADDITIONAL INFORMATION
Correspondence and requests for materials should be addressed to Burkhard Hinz.
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B. Hinz and R. Ramer
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British Journal of Cancer
