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RESEARCH FRONT
CSIRO PUBLISHING
Aust. J. Chem. 2021, 74, 369–387
Primer Review
https://doi.org/10.1071/CH21006
The Chemistry of Cannabis and Cannabinoids
Peter J. Duggan
CSIRO Manufacturing, Research Way, Clayton, Vic. 3168, Australia; and College of
Science and Engineering, Flinders University, Adelaide, SA 5042, Australia.
Email: peter.duggan@csiro.au
The science of cannabis and cannabinoids encompasses a wide variety of scientific disciplines and can appear daunting
to newcomers to the field. The encroachment of folklore and ‘cannabis culture’ into scientific discussions can cloud the
situation further. This Primer Review is designed to give a succinct overview of the chemistry of cannabis and
cannabinoids. It is hoped that it will provide a useful resource for chemistry undergraduates, postgraduates and their
instructors, and experienced chemists who require a comprehensive and up to date summary of the field. The Review
begins with a brief overview of the history and botany of cannabis, then goes on to detail important aspects of the
chemistry of phytocannabinoids, endocannabinoids and synthetic cannabinomimetics. Other natural constituents of the
cannabis plant are then described including terpenes and terpenoids, polyphenolics, alkaloids, waxes and triglycerides,
and important toxic contaminants. A discussion of key aspects of the pharmacology associated with cannabinoids and
the endocannabinoid system then follows, with a focus on the cannabinoid receptors, CB1 and CB2. The medicinal
chemistry of cannabis and cannabinoids is covered, highlighting the range of diseases targeted with cannabis and
phytocannabinoids, as well as key aspects of phytocannabinoid metabolism, distribution, and delivery. The modulation
of endocannabinoid levels through the inhibition of key endocannabinoid-degrading enzymes fatty acid amide
hydrolase (FAAH) and monoacylglycerol lipase (MAGL) is then discussed. The Review concludes with an assessment
of the much touted ‘entourage effect’. References to primary literature and more specialised reviews are provided
throughout.
Received 5 January 2021, accepted 17 February 2021, published online 18 March 2021
Introduction campsites. Cannabis plants readily emerged along tracks and
With the recent relaxations in the legal restrictions on medicinal roads, and thrived in nitrogen-rich, organic waste heaps close to
cannabis and hemp in many jurisdictions around the world, there settlements.[2]
has been a huge surge in interest in cannabis-related science, Over many years, early farmers used selective breeding and
including all aspects of cannabis chemistry. There is still much other agricultural practices to produce taller varieties – opti-
to be learnt about the chemistry of the cannabis plant, its derived mised for fibre production – and cultivars that yielded larger
products, and the biological effects of its constituents. The lack seeds, more suitable as sources of food.[3] This led to the
of fundamental knowledge about some aspects of cannabis varieties collectively known today as hemp. Other phenotypes,
science stems from the fact that worldwide prohibition has selected for their ability to express high levels of resin and that
greatly hindered scientific investigations in the field. The pur- often induced feelings of euphoria when consumed, are usually
pose of this primer review is to provide an overview of the referred to today as marijuana or medicinal cannabis.
current knowledge and future directions of cannabis chemistry The taxonomy of cannabis is still subject to much debate. The
research relevant to the commercial and medicinal applications forms rich in psychoactive secondary metabolites, i.e. phyto-
of cannabis products. cannabinoids, are commonly divided into three hypothetical
species, Cannabis sativa L. (C. sativa), Cannabis indica Lam.
(C. indica), and Cannabis ruderalis Janisch. (C. ruderalis). The
History and Botany latter variety tends to exist mainly as wild populations growing
Cannabis has been cultivated by humans since ancient times, in central and eastern Europe and central Asia. By contrast,
with many researchers suggesting that it was one of the first C. sativa, originally bred in India and south-east Asia, and
crops to be domesticated.[1] There is debate about its natural C. indica, originating in Afghanistan, have been intensively
geographic origin, with central and south-east Asia being most cultivated. The taxonomic confusion stems from the fact that these
commonly suggested. However, there is strong evidence that its three hypothetical species do not appear to possess any genetic or
cultivation and use developed independently in Europe and east physiological features that limit cross-fertilisation,[4] and usually
Asia.[2] From early times, the plant became a source of fibre, yield fully fertile hybrids.[3] This has been used to argue that
food, and herbal medicine, and was incorporated into religious marijuana-type cannabis should be considered to be one species,
practices. Cannabis became a ‘camp follower’, with seeds and C. sativa, with variations classified in terms of subspecies and
perhaps a limited number of plants being carried between varieties.[2,4] Regardless, widespread interbreeding has meant
Journal compilation Ó CSIRO 2021 Open Access CC BY-NC-ND www.publish.csiro.au/journals/ajc370 P. J. Duggan
that divisions between the ‘species’ are becoming increasingly biological effects through this association. Cannabinoids are
blurred. Genetic analysis of representative samples of marijuana usually divided into three main groups: phytocannabinoids† –
and hemp, however, revealed that these broad cultivar classes cannabinoids found in plants; endocannabinoids – endogenous
are genetically distinct.[4] European hemp and east Asian hemp compounds found in animals that modulate the same receptors
are also thought to be genetically different.[2] as those affected by certain phytocannabinoids (the cannabinoid
Cannabis is dioecious, meaning that it exists as separate male receptors); and synthetic cannabinomimetics – synthetic com-
and female plants. Small glandular projections, known as tri- pounds that may or may not be structurally related to the
chomes, are expressed on its leaves, stems and especially bracts – phytocannabinoids that also produce agonistic effects in
small leaf-like structures associated with buds and flowers. The cannabinoid receptors.
trichomes are where the majority of the interesting secondary
metabolites are found, including phytocannabinoids and ter- Phytocannabinoids
penes. The female buds tend to be the richest source of these The chemical structure of the most well-known phytocannabi-
compounds. It is believed that the plant produces the metabolites noid, D9-tetrahydrocannabinol, or D9-THC (10a), was reported
for defence and to limit competition from other plants. In by Gaoni and Mechoulam in 1964.[6] This is the compound,
particular, terpenes and phytocannabinoids can protect the plant often simply referred to as ‘THC’, that is mainly responsible for
from microbial infestations and deter predation by insects. the euphoric effects induced by marijuana-type cannabis.
Terpenes can also be used by the plant to attract insects that Allegedly, cannabis cultivars have now been bred to produce
attack the larvae and eggs of predatory insects. The resinous buds with up to 30 wt-% D9-THC equivalent when dried. D9-
nature of the mixtures expressed in the trichomes can also serve as Tetrahydrocannabinol is just one of over 100 phytocannabinoids
a form of defence, ensnaring insects in a way similar to that used that have so far been found in cannabis. The most abundant ones
by some carnivorous plants to trap their prey. The more volatile bear an n-pentyl side chain, and their routes of formation are
terpenes produced by cannabis, responsible for its characteristic shown in Scheme 1.
aroma, may also suppress the growth of proximal vegetation.[1] Cannabigerolic acid (CBGA, 3a) is the common biosynthetic
While it might seem surprising that humans have found uses for precursor for all of the most abundant phytocannabinoids. It is
the secondary metabolites in cannabis that are totally unrelated produced by linking two fragments, the geranyl portion of
to the purpose for which they originally evolved, this is not geranylpyrophosphate (1), which is generated via the terpene
unique. There are actually many other examples of humans biosynthetic pathway, and olivetolic acid (2), which is formed
consuming defensive secondary metabolites produced by plants, via the polyketide biosynthetic pathway. The production of
including caffeine, the opiates, cocaine, and nicotine. CBGA (3a) is catalysed by geranylpyrophosphate:olivetolic
Cannabis is wind-pollinated and is highly susceptible to acid geranyltransferase (GOT).[7] Other plant enzymes catalyse
cross-breeding. Even in a controlled environment, genetic purity three different types of cyclisation reactions of CBGA. Single
is difficult to maintain if sexual reproduction is used. For these ring closures produce cannabichromenic acid (CBCA, 6) and
reasons, commercial medicinal cannabis growers tend to propa- cannabidiolic acid (CBDA, 4a), and a double cyclisation leads
gate their production plants through cuttings from vegetative to D9-tetrahydrocannabinolic acid (D9-THCA, 7a).[8] Techni-
‘mother plants’. The architecture of the female buds is influ- cally, these three carboxylic acids, together with CBGA, are the
enced by day length. As the number of daylight hours begins to only true cannabinoid natural products shown in Scheme 1. The
decrease, following a period of growth during longer days, buds other cannabinoids are effectively degradation products result-
undergo intensive branching,[5] ultimately leading to higher ing from the exposure of the acids to heat, light, and oxidants.
trichome yield. Phytocannabinoid content can also be influ- The four cannabinoid acids can all undergo thermally induced
enced by growing conditions including temperature, humidity, decarboxylation to the corresponding ‘neutral’ forms cannabi-
and soil nutrients.[1] As a consequence of these attributes of gerol (CBG, 5), cannabichromene (CBC, 9), D9-THC (10a), and
cannabis, a lot of medicinal cannabis production occurs indoors cannabidiol (CBD, 8a). This process begins spontaneously
where growing conditions, including light exposure, can be following harvest, but happens very slowly at room temperature.
carefully controlled and incidental cross-pollination can be The loss of carbon dioxide occurs much more rapidly at high
prevented. Many commercially grown cultivars have been bred temperatures, for example when the dried crop is burnt or heated
to produce large, dense, and tightly packed female buds, giving in other ways. Most of the biological effects resulting from
higher phytocannabinoid yield per plant, but the moist condi- cannabis administration are induced by the neutral cannabi-
tions within the buds makes them highly susceptible to fungal noids, which explains why D9-THC is often delivered by
infestations including grey mould (Botrytis cinerea Pers.). This smoking when cannabis is used recreationally. D9-Tetrahydro-
is the same pathogen that causes ‘noble rot’ in grapes. Unfortu- cannabinol (10a) can also undergo several chemical reactions,
nately, the natural resistance to fungal attack possessed by most notably isomerisation to D8-tetrahydrocannabinol
varieties originating in warm, humid climates such as south- (D8-THC, 11) through relocation of the D9-double bond, and
east Asia has been inadvertently bred out by crossing with non- oxidation to the fully aromatic cannabinol (CBN, 13). Cannabi-
resistant plants emanating from arid zones like Afghanistan, nol was actually the first cannabinoid to be fully characterised,
where fungal attack poses little threat.[3] following a series of investigations undertaken in Cahn’s
laboratory in the 1930s.[9] An alternative degradation product,
cannabicyclol (CBL, 12), is formed via a 2þ2 photocyclisation
Cannabinoid Chemistry of CBC, promoted by natural light.
The term cannabinoid generally refers to any chemical substance The main cannabinoid produced in hemp-type cannabis is
that associates with mammalian cannabinoid receptors and elicits CBDA, whereas the marijuana-type cannabis cultivars were
†
A more accurate definition of a phytocannabinoid is: a chemical found in a plant that is structurally related to D9-THC and its biosynthetic precursors (back to
CBGA), and their derivatives. Note that not all of these compounds interact with cannabinoid receptors.Chemistry of Cannabis and Cannabinoids 371
OH
OH CO2H
O GOT CBDA synthase OH
O P O CO2H
O + CO2H
O P O HO
HO
O HO
1 2 3a CBGA 4a CBDA
'
CBCA THCA '
OH synthase synthase
OH OH OH
HO H
CO2H CO2H
H
5 CBG O O HO
6 CBCA 7a '9-THCA 8a CBD
'
'
OH
OH
H isomerisation OH
H
O H
H
O
O
photocyclisation 9 CBC
10a '9-THC 11 '8-THC
[o]
OH
OH
O
O
12 CBL 13 CBN
Scheme 1. The formation of the major phytocannabinoids.
originally bred to produce high levels of D9-THCA. Since CBD Endocannabinoids
and mixtures of CBD and D9-THC are of significant interest for It was not until the early 1990s that the mammalian receptors for
their potential in the treatment of a spectrum of disorders and the phytocannabinoids, CB1 and CB2, were characterised (see
diseases in humans and companion animals (see below), medic- below), and it was after those discoveries that the endogenous
inal cannabis manufacturers often cultivate a series of varieties ligands for the receptors, termed the endocannabinoids, began to
that range from high CBDA producers through to cultivars that be identified. Examples are shown Fig. 1. Anandamide (14) and
yield high levels of D9-THCA. 2-arachidonoylglycerol (2-AG, 15) were the first to be identified
Phytocannabinoids that have different numbers of carbons in and their biological roles are still the subject of intensive
their alkyl side chains have also been isolated from cannabis. In research. These arachidonic acid derivatives appear to be
addition to the major phytocannabinoids that bear n-pentyl side members of a large class of lipophilic amides, esters, and ethers
chains (Scheme 1), D9-THC and CBD equivalents with that make up what has become known as the endocannabinoi-
methyl,[10] n-propyl,[10] n-butyl,[11] and n-heptyl[12] moieties dome.[15] Virodhamine (16)[16] is the O-acyl analogue of
have been found in cannabis (see Scheme 2). These metabolites anandamide and 2-arachidonylglycerol ether (2-AGE, 17) is a
occur at much lower levels than the major phytocannabinoids, reduced form of 2-AG. Other endocannabinoids include
with the ‘varinoids’ (tetrahydrocannabivarinic acid (D9- N-acylamino acids (eg N-arachidonoylserine, 18) and N-acyl-
THCVA, 7c), tetrahydrocannabivarin (D9-THCV, 10c), canna- ated neurotransmitters (eg N-arachidonoyldopamine, 19).
bidivarinic acid (CBDVA, 4c), and cannabidivarin (CBDV, 8c)) The endocannabinoidome is also believed to include con-
being the only analogues that are currently routinely quantified geners of several of the endocannabinoids shown in Fig. 1.[15]
in samples of medicinal cannabis. Interestingly, cis-perrotteti- For example, postulated congeners of anandamide are ethano-
nene and perrottetinenic acid, which are structurally closely lamides derived from other endogenous long chain fatty acids
related to D9-THC and D9-THCA respectively, have been such as the highly unsaturated docosahexaenoic acid, the doubly
isolated from liverworts of the Radula genus. They represent a unsaturated linoleic acid, the monounsaturated oleic acid, and
different class of phytocannabinoid to that found in cannabis, the fully saturated palmitic acid. Similarly, congeners of 2-AG
having different stereochemistry and a side chain bearing a include 2-oleoylglycerol and 2-linoleoylglycerol. Participating
phenyl substituent.[13] Other types of cannabinoids, including N-acylated amino acid analogues of N-arachidonoylserine (18)
CBG, CBGA, CBC, CBCA, and CBL analogues, have been include those derived from glycine and 2-aminoethanesulfonic
isolated from a range of other flowering plants.[14] acid (taurine). Together with the N-acyldopamine congeners,372 P. J. Duggan
OH OH
O O
CO2H
OH OH
N O
H
HO R
3b, R = CH3
3c, R = n-C3H7
3d, R = n-C4H9
3e, R = n-C7H15 14 15
anandamide 2-arachidonoylglycerol (2-AG)
synthase synthase
OH
O
NH3 OH
O O
OH OH
H
CO2H CO2H
H
O R HO R
7b, R = CH3, '9-THCA-C1 4b, R = CH3, CBDA-C1
7c, R = n-C3H7, '9-THCVA 4c, R = n-C3H7, CBDVA 16 17
7d, R = n-C4H9, '9-THCBA 4d, R = n-C4H9, CBDBA virodhamine 2-arachidonylglycerol ether
4e, R = n-C7H15, CBDPA (2-AGE)
7e, R = n-C7H15, '9-THCPA
OH OH
' O O
'
N CO2 N OH
H H
OH OH
H
H
O R HO R
18 19
10b, R = CH3, '9-THC-C1 8b, R = CH3, CBD-C1
N-arachidonoylserine N-arachidonoyldopamine
10c, R = n-C3H7, '9-THCV 8c, R = n-C3H7, CBDV
10d, R = n-C4H9, '9-THCB 8d, R = n-C4H9, CBDB
10e, R = n-C7H15, '9-THCP
8e, R = n-C7H15, CBDP Fig. 1. Chemical structures of important endocannabinoids.
Scheme 2. The side chain variants of the major phytocannabinoids.
HO CO2H
OH OH
N-acylserotonins are also thought to be part of the H H
endocannabinoidome. H H
O O
Synthetic Cannabinomimetics
20 HU-210 21 HU-239, ajulemic acid
The earliest class of cannabinomimetics is sometimes referred to
as ‘classical cannabinoids’, and largely emanated from the
laboratory of Raphael Mechoulam at Hebrew University. O CO2H
Mechoulam is a pioneer in the field of cannabinoid chemistry
OH
and reported the first synthesis of D9-THC in 1972.[17] Using H OH
methods developed during the synthesis of such phytocannabi- H
noids, Mechoulam’s group prepared a range of analogues and O HO
tested them for biological activity.[18] Most of these compounds
are often referred to by their ‘HU’ codes, HU being short for 22 nabilone 23 HU-320
Hebrew University. Results obtained from variation in the ali-
phatic side chain, inspired by much earlier work undertaken OH
separately in the 1940s by Adams[19] and Todd,[20] reinforced
the finding that stronger and longer-lasting activity in animals O
H
was obtained with THC analogues bearing heptyl side chains
with attached methyl groups. The 1,10 -dimethylheptyl (DMH) H
substituent became prominent and can now be found in several O O
important cannabinomimetics (Fig. 2) including HU-210
(20),[21] ajulemic acid (21),[22] and nabilone (22).[23] The for- 24 HU-308 25 JWH-133
mer two (20 and 21) resemble metabolites of D8-THC and
nabilone is a drug developed by Eli Lilly and used for the Fig. 2. Chemical structures of synthetic cannabinomimetics bearing 1,10 -
treatment of anorexia and chemotherapy-induced nausea and dimethyl side chains.
vomiting. Other cannabinomimetics that bear the DMH side
chain are HU-320 (23),[24] which is analogous to a metabolite ofChemistry of Cannabis and Cannabinoids 373
OH OH OH
OH OH OH
H H H
H H
R
O
26 HHC 27a, R = H, CP 47,497 28, (C8)-CP 47,497
27b, R = –(CH2)3OH, CP 55,940 cannabicyclohexanol
Fig. 3. Chemical structures of synthetic cannabinomimetic cyclohexanols.
O
O O O
O
N N O N
N O O
N O N N
29 pravadoline 30 WIN 52,212-2 31 JWH-200 32 JWH-015
X O
X O X
O
N
N N
R Y
33a R = n-propyl, X = H, JWH-072
33b R = n-butyl, X = H, JWH-073 34a X = 2-Me, JWH-251 35a X = 2-I, Y = H, AM-679
33c R = n-pentyl, X = H, JWH-018 34b X = 2-OMe, JWH-250 35b X = 2-I, Y = F, AM-694
33d R = n-hexyl, X = H, JWH-019 34c X = 2-Cl, JWH-203 35c X = 4-OMe, Y = H, RCS-4
33e R = n-pentyl, X = Me, JWH-122 34d X = 4-OMe, JWH-201
33f R = n-pentyl, X = ethyl, JWH-210 34e X = 4-F, JWH-313
33g R = n-pentyl, X = Cl, JWH-398
Fig. 4. Chemical structures of synthetic cannabinomimetic indoles.
CBD, and the more elaborate HU-308 (24).[25] A much simpler regularly referred to as ‘cannabicyclohexanol’, was the first
mimetic is JWH-133 (25),[26] which is closely related to D8- synthetic cannabinomimetic to be found in a commercial ‘syn-
THC. JWH stands for John W. Huffman, whose laboratory at thetic cannabis’, along with a stereoisomer, CP 47,497 (27a) and
Clemsen University produced a large number of cannabinomi- JWH-018 (33c, Fig. 4).[29]
metics in the latter part of the 20th century. The discovery in the late 1980s that pravadoline (29), a non-
A series of cannabinomimetic cyclohexanols, often referred steroidal anti-inflammatory drug (NSAID), and the related
to as ‘non-classical cannabinoids’, have also been developed. indole derivative, WIN 55,212-2 (30), produce pharmacological
They began with HHC (26, Fig. 3), which is a direct analogue of responses similar to those induced by phytocannabinoids like
THC, and was reported by the US National Institutes of Health D9-THC,[30,31] opened up a whole new field of synthetic
in 1976.[27] Ring-opened versions, CP 47,497 (27a) and CP cannabinomimetics. These were the first cannabinomimetics
55,26 940 (27b), which also incorporated the DMH side chain, with no obvious structural similarity to the phytocannabinoids
were later invented by Pfizer in the early 1980s.[28] The prefix and structure–activity relationships (SARs) for these com-
‘CP’ refers to Karl Pfizer, also known as Charles Pfizer, the pounds have been intensively investigated. Huffman’s labora-
founder of Pfizer Pharmaceuticals. The compound bearing the tory has been a major source of new indole-based
propyl alcohol substituent, 27b, is still commonly used as a cannabinomimetics and hence many important compounds in
pharmacological tool in cannabinoid receptor studies (see this series bear the ‘JWH’ label.[32] The examples 29–35 shown
below). in Fig. 4 demonstrate how the original ‘hits’, 29 and 30, were
Any discussion of synthetic cannabinomimetics inevitably progressively simplified, leading to cannabinomimetics that
leads to the subject of ‘synthetic cannabis’, herbal blends that are synthetically are readily accessible. The benzoyl indoles (35a
adulterated with cannabinomimetics and sold over the counter and 35b) were reported by Makriyannis and Deng, then at The
or the internet. These products have been of significant concern University of Connecticut, in a patent,[33] whereas RCS-4 (35c)
to health authorities, regulators, and law enforcement world- is an example of a ‘designer drug’. This exceedingly simple
wide. The octyl analogue of 27a, (C8)-CP 47,497 (28), also compound was first identified as an adulterant in illicit products374 P. J. Duggan
available online by researchers at the Tokyo Metropolitan
Institute of Public Health in 2011,[34] before any report in the
scientific literature of its synthesis or biological activity.[35]
Many more synthetic cannabinomimetics have been reported
but a majority of them are analogues of the structures described 36 37 38 39 40
here, particularly the indoles. They have been varied by the D-pinene E-pinene E-myrcene R-limonene E-ocimene
replacement of all heteroatoms with carbon, the incorporation of
additional heteroatoms, such as nitrogen and oxygen, into the
indole core, or the replacement of the aromatic substituents with
aliphatic sidechains.
The biological activity of synthetic cannabinomimetics is
stereochemistry-dependent. For example, HU-211, which is the
enantiomer of HU-210 (20), does not interact with cannabinoid 41 42 43 44
D-terpinene terpinolene D-phellandrene '3-carene
receptors but instead blocks N-methyl-D-aspartate (NMDA)
receptors,[36] and the R enantiomer of WIN 52,212-2 (30) is
Fig. 5. Some monoterpenes commonly found in cannabis.
much more active than the S enantiomer.[37]
Terpenes and Terpenoids
Terpenes and terpenoids are largely responsible for the char-
acteristic odour of cannabis and are thought by some to con-
tribute to the potential therapeutic benefits of cannabis. They
would undoubtedly play a prominent role in any possible pla- 45 46 47
cebo effects resulting from cannabis consumption. For these E-farnesene D-bergamotene D-guaiene
reasons, and their relative abundance in cannabis, terpenes and
terpenoids are considered by consumers and medical scientists
and practitioners to be the second-most important class of sec-
ondary metabolites produced by the cannabis plant. Terpenes
identified to be present in cannabis consist of monoterpenes, 10-
carbon hydrocarbons biosynthesised from geranylpyropho- 48 49
sphate (1), and sesquiterpenes. The latter are 15-carbon hydro- E-caryophyllene D-humulene
carbons biosynthesised from the 15-carbon analogue of
geranylpyrophosphate, farnesylpyrophosphate. Several terpene Fig. 6. Some sesquiterpenes commonly found in cannabis.
synthases present in cannabis are multi-product enzymes,
meaning that a single enzyme can catalyse the production of a
range of different terpenes from the same precursor.[38] were usually only reported in a single or small number of
The term terpenoid refers to terpenes that have been further publications, and detected at low to very low levels.
modified by the incorporation of oxygen. This may occur as a Of the monoterpenes, the pinenes (a-pinene 36, and to a
result of enzyme action within the plant or by a range of non- lesser extent b-pinene 37), b-myrcene 38, and R-limonene 39
enzymatic hydration or oxidation processes. These include ther- are most often found in high levels in fresh cannabis, with b-
mal and photochemical rearrangements. Many of the non- ocimene 40 and terpinolene 42 also commonly present in
enzymatic processes can occur within or on the crop following significant amounts. These volatile compounds are primarily
harvest. Terpene and terpenoid profiles can show considerable responsible for the characteristic odour of cannabis and varia-
variation, even in plant material obtained from the same cannabis tions in their expression is the most likely cause of different
variety, and vary depending on the way the harvested material has cannabis cultivars smelling slightly differently. Of the sesqui-
been handled and stored. This is because monoterpenes can be terpenes, b-caryophyllene 48 and a-humulene 49 are by far the
quite volatile and hence easily lost after harvest, and the terpenoid most abundant in cannabis and the terpenoids linalool 50 and
content can continue to change over time as a result of the a-terpineol 52 can usually be easily detected.
abovementioned non-enzymatic chemical processes. Given the widespread confusion regarding the taxonomy of
Many terpene profiles of cannabis varieties have been cannabis, originally based on the Cannabis sativa, C. indica, and
described, and while the identification of some of the common, C. ruderalis classifications that are largely related to growth
major components is usually well founded, the reporting of habit, there has been a move to define varieties by their
some of the minor components can be less reliable. There is a cannabinoid, and especially, terpene profiles.[40,51] As a result,
lack of authentic samples with which to confirm identification, cannabis scientists now tend to refer to cannabis varieties as
especially for sesquiterpenes and related terpenoids. Mass ‘chemovars’ rather than using the traditional term, cultivar.
spectral library matching is often used, which is valid as long A small number of triterpenoids, specifically friedelin 58 and
as a close match is found; however, the quality of the library epifriedelanol 59, as well structurally related phytosterols such
matches is rarely reported. An examination of analytical results as b-sitosterol 60 (Fig. 8), have also been found in cannabis.
recently obtained by the CSIRO (data not published) as well as a These are more likely to be detected in the leaves, bark, and
selection of recently published cannabis terpene and terpenoid particularly roots of the plant.[52]
profiles[39–50] found that the following monoterpenes, 36–44
(Fig. 5), sesquiterpenes, 45–49 (Fig. 6), and terpenoids, 50–57 Polyphenolics
(Fig. 7) are most often found in cannabis varieties. Over 50 other As with the terpenes and terpenoids, cannabis contains a wide
cannabis terpenes and terpenoids have been described, but these range of polyphenolic compounds that are also common in otherChemistry of Cannabis and Cannabinoids 375
OH OH
H
OH HO
50 51 52 53
linalool fenchol D-terpineol D-bisabolol
OH
HO H HO
H
O
54 55 56 57
guaiol E-eudesmol J-eudesmol E-caryophyllene
oxide
Fig. 7. Some terpenoids commonly found in cannabis.
O OH
H H
H
H H H
H H H
H
HO
58 59 60
friedelin epifriedelanol E-sitosterol
Fig. 8. Triterpenoids and a phytosterol found in cannabis.
OMe
OH
HO O
OH O
61
cannflavin A
OMe OMe
OH OH
HO O HO O
OH O OH O
62 63
cannflavin B cannflavin C
Fig. 9. Flavonoids found in cannabis.
plants. However, there are some polyphenolics that appear to be Examples of phenanthrene derivatives found in cannabis (64–
mainly found in cannabis or can even be unique to the spe- 67) are shown in Fig. 10.
cies.[53] Many have been given common names with the ‘cann’ Some of the dihydrostilbenes found in cannabis (68–72),
prefix, pointing to their original discovery being in the extracts including the prenylated canniprene 69 and cannabistilbene I 70,
of cannabis. These polyphenolics do not tend to be found in the are shown in Fig. 11. In the original investigation of the structure
trichomes and usually occur in other parts of the plant. These of cannabistilbene II,[54] the exact methylation pattern could not
include the geranylated and prenylated flavonoids cannflavin A, be determined, with both cannabistilbene IIa 71 and cannabis-
B, and C (61–63, Fig. 9), found in the leaves, twigs, flowers, and tilbene IIb 72 being likely possibilities.
pollen of the cannabis plant. A range of spiroindans has been detected in cannabis, with
Another class of polyphenolics, which are found in cannabis cannabispirone 73, cannabispiradienone 74, cannabispirenone
leaves, stems, buds, flowers, and resin is the stilbenoids. These A 75, a-cannabispiranol 76, and b-cannabispiranol 77 (Fig. 12)
include phenanthrenes, dihydrostilbenes, and spiroindans. being representative.376 P. J. Duggan
Finally, a large number of members of a class of polyphe- Waxes and Triglycerides
nolics known as lignanamides have been isolated from Involatile, higher molecular weight, hydrophobic compounds,
the roots, fruits, and seeds of cannabis. This has modern collectively known as waxes, are produced by plants to protect
relevance, given the increased popularity of the consumption their leaves and stems against moisture loss and pathogens. They
of hemp seeds in food. Representative examples are shown in are also expressed on plant inflorescence (flower heads) and
Fig. 13 and include cannabisin A 78, cannabisin G 79, canna- potentially have the additional purpose of stabilising defensive
bisin E 80, cannabisin N 81, and cannabisin O 82, as well as compounds, which in the case of cannabis include the phyto-
N-trans-caffeoyltyramine 83 and N-trans-feruloyltyramine 84, cannabinoids and terpenes. Cannabis inflorescences can have
the monomeric compounds from which most of the larger triple the levels of waxes compared with cannabis leaf materi-
lignanamides are biosynthesised. als.[59] Extraction of cannabis inflorescence with organic sol-
vents or supercritical carbon dioxide is often the first step in the
Alkaloids production of medicinal cannabis products and this usually
Cannabis is a rare example of a plant with psychoactive prop- results in ‘resin’ with a high wax content. Waxes are therefore an
erties in which the main compounds that cause the effect, the important class of phytochemical that typically need to be
cannabinoids, are not alkaloids.[55] There have, however, been a considered during such manufacture.[60] Cannabis waxes are
small number of alkaloids isolated from cannabis (Fig. 14). mainly composed of straight-chain hydrocarbons, with n-pen-
These include the spermidine alkaloids cannabisativine[56] 85 tacosane (C25H52), n-heptacosane (C27H56), n-nonacosane
and anhydrocannabisativine[57] 86, obtained from leaves and (C29H60), and n-hentriacontane (C31H64) being the most abun-
roots, and the diketopiperazine indole alkaloids neoechinulin A dant. Of these, n-nonacosane and n-heptacosane appear to be the
87, (12S,22R)-dihydroxyisoechinulin A 88, and (12S,22S)- major contributors to cannabis wax.[59,60]
dihydroxyisoechinulin A 89, found in hemp seeds.[58] The latter Hulled hemp seeds consist of almost 50 % fat (triglycerides)
compounds have previously been identified as fungal metabo- and the derived oil is unusual among food oils in that its
lites but, after a thorough investigation, Fan and coworkers triglycerides contain very low levels of saturated fatty acids
concluded that neoechinulin A and the two stereoisomers of (,9 %) and very high levels of polyunsaturated fatty acids
dihydroxyisoechinulin A were constituents of hemp seeds and
did not result from fungal contamination of the crop.
MeO MeO
OH OH
MeO MeO
O O
OH OH OH OH OMe 73 74
cannabispirone cannabispiradienone
64 65
cannithrene 1 cannithrene 2
MeO MeO
O
OH
MeO OH OH OH
OMe
O Y
OH OMe O X
MeO OH
75
66 67 cannabispirenone A 76 X = H, Y = OH, D-cannabispiranol
denbinobin 77 X = OH, Y = H, E-cannabispiranol
Fig. 10. Phenanthrene derivatives found in cannabis. Fig. 12. Spiroindans found in cannabis.
OH
MeO MeO MeO
OMe
OH
HO HO OH
OMe HO
68 69 70
canniprene cannabistilbene I
MeO MeO
OMe OH
OH OMe
HO HO
OMe OMe
71 72
cannabistilbene IIa cannabistilbene IIb
Fig. 11. Dihydrostilbenes found in cannabis.Chemistry of Cannabis and Cannabinoids 377
(,80 %). The three main fatty acids present in hemp seed oil are The bio-accumulation properties of cannabis also mean that
linoleic acid (18:2v6, 54–60 %), a-linolenic acid (18:3v3, 18– regulators routinely require cannabis products destined for
23 %), and oleic acid (18:1v9, 7–12 %).[61–64] human consumption to be tested for the presence of heavy
metals. A less common but still concerning class of con-
Important Contaminants taminants are the polyaromatic hydrocarbons, which can
Medicinal cannabis producers are required to ensure that their sometimes be found in crops exposed to diesel fumes.
products do not contain a range of dangerous contaminants.
Apart from residual pesticide levels, the propensity for cannabis Pharmacology and the Endocannabinoid System
plants to become infested with an array of fungal pathogens[65] In mammals, cannabinoids interact with two main receptors,
means that cannabis products are routinely tested for the pres- cannabinoid receptor 1 (CB1) and cannabinoid receptor 2 (CB2).
ence of harmful mycotoxins, such as aflatoxins B1, B2, G1, G2, These were discovered in a period from the mid-1980s to the
and ochratoxin A. The ingestion of these fungal metabolites can early 1990s when pharmacologists were searching for the
cause cancer, liver damage, and kidney damage. In addition, receptors for the phytocannabinoids and synthetic cannabino-
cannabis has long been known to be a heavy metal bio- mimetics.[68,69] The use of a tritium-labelled form of the can-
accumulator, meaning that it can absorb heavy metals from nabinomimetic 27b, [3H]-CP 55,940, was very important in
the soil.[66] Its ability to tolerate and store heavy metals has led to these studies.[70] Both CB1 and CB2 are members of a large class
interest in its use in soil bioremediation and bioprospecting.[67] of transmembrane proteins known as G-protein coupled
OH OH
O O
HO MeO
NH NH
O O
HO HO
HN HN
OH OH MeO OH
OH OH
78 79
cannabisin A cannabisin G
OH
OH OH
OH
OH O O
MeO MeO
NH
O NH
O O
HO NH O
NH
MeO
O
80 HO 81
cannabisin E cannabisin N
OH
O
MeO
MeO N
H
O O
HO H
N
MeO O O
NH
OH
82
HO cannabisin O
OH OH
O O
HO MeO
N N
H H
HO HO
83 84
N-trans-caffeoyltyramine N-trans-feruloyltyramine
Fig. 13. A selection of lignanamides found in cannabis.378 P. J. Duggan
receptors (GPCRs), that respond to extra-cellular signalling (PPARa), and the 5-hydroxytryptamine receptor 1A (5-HT1A)
molecules by binding them, undergoing conformational chan- serotonin receptor. There are also two ‘orphan’ GPCRs, GPR55
ges, and eliciting intracellular responses through coupling with and GPR18, that are being investigated as potential cannabinoid
G-proteins (guanine nucleotide-binding proteins). Recently, receptors.[15] If one considers all of these receptors, together
X-ray crystal and cryo-electron microscopy investigations have with the abovementioned endocannabinoid system, it is clear
more accurately defined the molecular structures of the canna- that cannabinoids can influence a diverse array of physiological
binoid receptors and how they associate with G-proteins.[71] processes, and it becomes obvious why cannabinoids are asso-
The CB1 receptors are found in a wide range of tissue types ciated with a highly diverse group of potential therapeutic
but are more highly concentrated in the central and peripheral interventions.
nervous systems. By contrast, CB2 receptors tend to be found in The main compound derived from cannabis that induces
tissues and organs associated with the immune system,[72] and euphoric effects in humans is D9-THC (10a). This phytocanna-
can be strongly upregulated in response to inflammation or binoid is an agonist (activator) of both CB1 and CB2 recep-
injury. As indicated above, the natural endogenous ligands for tors.[73] Stimulation of CB1 by 10a in animals is associated with
the CB receptors were discovered soon after the receptors the suppression of locomotor activity, hypothermia, catalepsy,
themselves, beginning with anandamide (14) in 1992.[70] This pain desensitisation, and appetite enhancement, whereas activa-
relatively simple amide is now thought to be a member of a huge tion of CB2 is associated with pain relief and anti-inflammatory
collection of endogenous signalling compounds – the endocan- effects. It should be noted that many synthetic cannabinomi-
nabinoidome. The levels of these important biological messen- metics are significantly more potent agonists of cannabinoid
gers are controlled by a complex system of biosynthetic and receptors than 10a or anandamide (14), but interestingly, the
degrading enzymes, the latter being mainly amide hydrolases or initial D9-THC metabolite produced in the liver, the 11-hydroxy
lipases.[15] Together, the network of cannabinoid receptors, their derivative (90, Fig. 15) is also a more effective agonist than
endogenous ligands, and their associated metabolic enzymes is D9-THC itself. This alcohol (90) is readily metabolised to
known as the endocannabinoid system or endocannabinoid the corresponding carboxylic acid (91), which is non-
signalling system. psychoactive.[74]
It has become apparent that there are several other receptors The other most common cannabinoid derived from cannabis,
and channels, apart from CB1 and CB2, that are modulated by CBD (8a), is not a cannabinoid receptor agonist and rather may
cannabinoids. Prominent amongst these are the transient recep- be a CB1 antagonist, particularly in the presence of D9-THC
tor potential cation channel subfamily V member 1 (TRPV1), a (10a). It has, however, been found to be an agonist for TRPV1
heat-sensing ion channel, the proliferator-activated receptor-a and 5-HT1A receptors, and is described as having anticonvul-
sive, anti-inflammatory, and anti-psychotic effects. It has been
shown to counteract some of the intoxicating and unwanted side
O O
NH NH
effects of D9-THC, including anxiety, tachycardia (elevated
pulse), increased appetite, and sedation,[73] and is widely
N N believed to limit or even reverse the psychotic effects experi-
HN HN enced by some heavy D9-THC users.
HO
OH
O There have been many medical claims associated with
terpenes and terpenoids, and particularly those present in can-
85 86
cannabisativine anhydrocannabisativine nabis,[73] and the most compelling relate to b-caryophyllene 48.
This sesquiterpene, which is also found in cloves, is a potent CB2
agonist and has been shown to interact with a wide range of other
H H
mammalian receptors. It has been investigated for its anti-
N
O N
O
inflammatory effects, and as a possible treatment for depression
O O and anxiety, among other indications. There are also suggestions
N
N
H HO H that b-caryophyllene could act synergistically with both D9-
THC and CBD.[73]
N X Y
H N
87
H Medicinal Cannabis and Cannabinoids
neoechinulin A 88 X = OH, Y = H, (12S,22R )-dihydroxyisoechinulin A
89 X = H, Y = OH, (12S,22S )-dihydroxyisoechinulin A
Diseases and Conditions Targeted
The breadth of conditions that clinicians are currently consid-
Fig. 14. Alkaloids isolated from cannabis. ering, or have considered, as promising opportunities for the
HO OH
O OH
HO
CO2H O
HO
OH OH O O
H H
H H OH
H
O O
H
90 91 92 O
Fig. 15. Metabolites of D9-THC.Chemistry of Cannabis and Cannabinoids 379
application of medicinal cannabis and cannabinoids is truly evidence to support or refute the use of cannabinoids for the
vast.[75] They include paediatric epilepsy, particularly Dravet treatment of epilepsy, or for any other conditions considered.
and Lennox–Gastaut syndromes; chemotherapy-induced nausea Interestingly, in 2018, the US Food and Drug Administration
and vomiting; cachexia (a wasting disorder associated with late (FDA) approved the use of EpidolexÒ, an oral liquid CBD
stages of diseases like cancer and HIV/AIDS); loss of appetite, formulation, for the treatment of Dravet and Lennox–Gastaut
obesity, and eating disorders; chronic pain including neuro- syndromes. Epidolex is the first FDA-approved drug that con-
pathic pain and fibromyalgia; insomnia, anxiety and depression, tains a purified drug substance derived from cannabis and has
post-traumatic stress disorder (PTSD); multiple sclerosis and since been approved in Europe in 2019 and by the TGA in
other autoimmune or autoinflammatory diseases like irritable September 2020 for the treatment of the same epilepsy
bowel and Crohn’s disease, psoriasis and rheumatoid arthritis; syndromes.
autism, attention deficit hyperactivity disorder (ADHD), Tour- With laws allowing the medical use of cannabis having been
ette’s syndrome; cancer including neuroblastoma, glioblastoma, introduced around the world, many careful clinical trials with a
breast cancer, leukemia, osteosarcoma, and nephroblastoma; range of cannabis products have begun, albeit usually with quite
Parkinson’s disease; motor neurone disease; dementia and in small treatment groups. It is expected that a clearer picture of the
palliative care. The inclusion of cannabis in Schedule IV of the efficacy of medicinal cannabis products will come to light in the
1961 United Nations Single Convention on Narcotic Drugs[76] coming years, as the results of these trials are analysed and
has, however, severely limited the proper investigation of the released.
efficacy and safety of cannabis and phytocannabinoids for the
treatment of these diseases.[77] Until recently, legitimate Metabolism and Delivery of Phytocannabinoids
placebo-controlled, random double-bind clinical trials, the gold As mentioned above, D9-THC (10a) undergoes oxidation in the
standard required to prove drug safety and efficacy, have been liver in two stages, first to the 11-hydoxy derivative (90) and
virtually impossible with cannabis and phytocannabinoids in the then on to the carboxylic acid (91). The latter can be glucur-
signatory countries. The development of suitable placebo for- onidated, mainly to the glucuronide ester (92, Fig. 15).[81]
mulations is also particularly challenging for cannabis, espe- Cannabidiol (8a) has been shown to be metabolised in similar
cially when inhalation is the route of delivery, as the placebo ways to D9-THC.[74] The D9-THC-derived alcohol (90) can also
effect can be quite prominent in such trials. be conjugated with long chain fatty acids and stored in adipose
In 2017, the Australian Therapeutic Goods Administration tissue,[81] which partially accounts for the fact that D9-THC
(TGA) released guidance for the use of cannabis and cannabi- metabolites can be present in the human system for a surprising
noids in the treatment of several conditions for which cannabis length of time after administration.
or cannabinoid treatment was often sought.[78] These were: The bioavailability of D9-THC from a cannabis cigarette
epilepsy in paediatric and young adult patients, the prevention typically ranges from ,10 to 35 %, with considerable variability
or management of nausea and vomiting, multiple sclerosis, between experienced and inexperienced smokers.[74] Approxi-
chronic non-cancer pain, and in palliative care. The develop- mately 30 % of the D9-THCA in the cigarette is thought to be
ment of the recommendations was hindered by a lack of destroyed through burning.[81] After inhalation, plasma D9-THC
evidence from rigorous clinical trials; however, patients with levels rise rapidly and peak within ,10 min,[81,82] after which
some of these conditions were thought likely to gain benefit the drug is rapidly distributed to many tissues around the body,
from treatment with medicinal cannabis or cannabinoids. It was eventually partitioning to the body fat, where it can be stored for
found that there was evidence for paediatric and young adult extended periods.[81,83] This is consistent with D9-THC’s high
patients with epilepsy to have an improved chance of a reduction lipophilicity and low total polar surface area.[84] The half-life of
of seizure frequency, an increased likelihood of complete D9-THC in plasma is hard to estimate because it is released from
freedom from seizures and improved quality of life, when adipose and other tissues back into the blood only slowly and at
treated with oral CBD, relative to placebo. Dronabinol such low concentrations that it is difficult to quantify.[81] While
(synthetic D9-THC) prescribed for the treatment of nausea and levels of the initial D9-THC metabolite, the alcohol (90), never
vomiting was found to be as effective as traditional antiemetics, seem to reach high concentrations in plasma, the concentration
and with some conditions, more effective at improving appetite of the corresponding acid (91) tends to rise over the first 30 min
than placebo. At the time, no proper comparisons had been made following inhalation and drops off very slowly. For example, it
with newer types of antiemetics. Medicinal cannabis treatments could be detected in the plasma of test subjects up to 7 days after
were also found to provide moderate relief from various types of smoking a D9-THCA-containing cigarette.[74] The observed
chronic pain, including that associated with multiple sclerosis. latency of D9-THC and its metabolites in humans, apparently
Some multiple sclerosis patients also reported improvement in long after any observable impairment, presents significant
spasticity symptoms with nabiximols (oral spray containing challenges for law enforcement agencies when attempting to
,1:1 D9-THC:CBD), and in fact this drug has been approved prosecute road users for driving under the influence of canna-
by the TGA, and in Canada and some European countries, for bis,[85] and consequently serves as a disincentive for patients to
this indication. In no cases was medicinal cannabis recom- take up medicinal cannabis products that contain D9-THC. The
mended by the TGA as a first-line treatment, and concerns were bioavailability of smoked CBD is similar to D9-THC, ranging
expressed about potentially negative interactions between CBD from 11 to 35 %,[81] with a plasma half-life of 27 to 35 h.[79] The
and existing prescribed drugs, resulting from the inhibition of metabolic profile and pattern of plasma distribution is also
cytochrome P450 metabolic enzymes in the liver.[79] A review apparently similar to D9-THC, although unlike D9-THC, a high
undertaken for the US National Academies of Sciences[80] proportion of CBD is excreted unchanged.[81]
conducted around the same time as the TGA reviews came to Cannabinoids can be delivered to the body in various ways.
similar conclusions regarding the treatment of chemotherapy- Historically, inhalation has been the most common method,
induced nausea and vomiting, chronic pain, and multiple through the pyrolysis of plant material, but there are now several
sclerosis-related spasticity, but could not find sufficient alternative delivery techniques that use this route of delivery.380 P. J. Duggan
O
These include devices that electrically heat plant material while O
O O H OH
avoiding complete combustion, e-cigarettes that either wick OH N
cannabinoid-containing liquids across a heating coil or allow O
O O
the user to manually place the liquid on the hot coil, and dabbing H H
in which a small amount of cannabis extract is placed on a H H
preheated surface such as a nail.[86] All of these methods present O O
concerns related to airway and lung damage caused by pyrolysis 93 94
products of cannabis, and those of the carrier oils and their
additives.[86] Devices that deliver a metered dose of inhaled
HO
cannabinoids that have been formulated into a powder are also OH
under development. HO OH
Oral administration via oils, tablets or capsules has several OH
O
O CO2Me
advantages over inhalation, including well-controlled dosing, an H
uncomplicated route of administration, and wide patient accep-
H HO
tance. The oral delivery of cannabinoids in medium-chain O
triglyceride oil, derived for example from coconut oil, is now
one of the most common ways by which medicinal cannabis 95 96
products are administered. Dosing via the oral route has,
Fig. 16. Phytocannabinoid prodrugs.
however, a major disadvantage. It is subject to first-pass
metabolism in the gut and liver before entry into the blood,
and absorption can be slow, highly variable, and patient- Prodrugs Derived from Phytocannabinoids
dependent.[81,82] The oral bioavailability of D9-THC ranges
from 2 to 14 %,[81] and 6 to 10 % for CBD.[87] The latter was As mentioned above, key reasons for the low bioavailability of
found to have a plasma half-life of 2 to 5 days,[81,87] much longer D9-THC and CBD are their relatively high hydrophobicity and
than when it is delivered via inhalation. In the case of D9-THC, low total polar surface area,[84] which results in them being
maximal plasma concentrations are commonly encountered 2– rapidly distributed to adipose tissue, from which they are only
3 h after administration and tend to be only a fraction of those slowly released. It can therefore be difficult to achieve thera-
achieved through inhalation. At the same time, the levels of the peutic levels in plasma and at the required sites of action with
D9-THC acid metabolite (91) in plasma rise steeply over the first these phytocannabinoids. One approach to this problem has
1–3 h and then trail away relatively slowly over many hours.[81] involved the development of more hydrophilic prodrugs –
Plasma levels of the D9-THC alcohol metabolite (90) tend to be covalently modified derivatives that are converted to the actual
higher than those observed through inhalation.[81] For orally cannabinoid drug within the body. One of the first examples was
delivered CBD, the time to maximum plasma concentration was the D9-THC-hemiglutarate (93, Fig. 16), which was investigated
found to be 2.5 to 5 h,[87] only slightly longer than D9-THC. for systemic delivery of D9-THC via the oral transmucosal route,
Oral mucosal delivery via lozenges, patches placed on the through incorporation into a polyethylene oxide matrix.[93] A
gums, oral spray or sublingual (under the tongue) is thought to slightly more elaborate modification involved the attachment of
largely avoid many of the problems with first-pass metabolism, valine and succinic acid to the phenol functionality of D9-THC
as drugs can be absorbed through the oral mucosa (the mucous (94), for the purpose of developing topical treatments for glau-
membrane that lines the inside of the mouth) directly into the coma.[94] The CB1 and CB2 antagonistic activity of D9-THC is
blood stream. Nabiximols is an example of a cannabinoid drug thought to decrease intraocular pressure. More recently, gly-
that uses the oral mucosal route and the development of other cosylated versions of D9-THC, such as the D9-THC-glucoside
medicinal cannabis products of the types mentioned here is (95), were developed by Vitality Biopharma for the treatment of
currently a very active area of commercial pharmaceutical drug-resistant inflammatory bowel disease.[82]
research. The natural phytocannabinoid acid CBDA (4a) has recently
Transdermal delivery via creams and patches also avoids been investigated for its ability to suppress nausea and vomiting,
first-pass metabolism, and owing to its convenience, is also of including ‘anticipatory nausea’ experienced by chemotherapy
great interest for the delivery of phytocannabinoids. Despite patients when they return to the chemotherapy clinic, and
there being several cannabinoid patches available on the recrea- anxiety under conditions of high stress. These effects are
tional market in North America, however, the potential of apparently mediated by CBDA’s activity at the 5-HT1A recep-
phytocannabinoids to penetrate skin has been the subject of tor, which appear to be superior to those of CBD. The spontane-
only a small number of rigorous scientific investigations. The ous decarboxylation of CBDA (4a) to CBD (8a, Scheme 1)
majority of this work has been with CBD (8a),[88] and promising limits the suitability of CBDA as a mainstream drug, but this
results for the treatment of muscle tension and pain[89] and issue has been addressed by a simple modification, namely the
peripheral neuropathy[90] with topical CBD (8a) preparations formation of the methyl ester (96).[95] This methyl ester was
have been recently described. An earlier report of an investiga- found to be more stable than CBD and, fortuitously, was also
tion into the transdermal delivery of D8-THC (11)[91] suggested found to be more effective at suppressing both acute and
that effective delivery of this compound and, by analogy, D9- anticipatory nausea, and stress-induced anxiety in rats.
THC (10a), is likely to be very difficult. This appears to be
supported by findings from a recent study of the application of Modulation of Endocannabinoid Levels
cannabis extract to beagles,[92] which revealed much better An alternative to the control of the endocannabinoid system
penetration of CBD (8a), CBDA (4a), and D9-THCA (7a) than through the activation and inhibition of cannabinoid receptors
D9-THC (10a). The high lipophilicity of D9-THC (10a) appears with phytocannabinoids and their derivatives is the modulation
to be a major impediment to its penetration of the skin. of endocannabinoid levels through the inhibition ofChemistry of Cannabis and Cannabinoids 381
O
N O
O N N
N O
O
OL-135
98
97
O NH2
N
HN N
H
O N F3C
N N O
O O
X
99a R = H, URB597 PF-04457845
99b R = OH, URB937 100
Fig. 17. FAAH inhibitors based on a-ketoheterocycle (97 and 98), carbamate (99a and 99b), and urea (100)
functional groups.
endocannabinoid-degrading and biosynthetic enzymes. Some This simple and quite compact compound was found to inhibit
medical conditions are thought to result from imbalances in FAAH with an IC50 value of 4.6 nM and has demonstrated
some aspect of the patients’ endocannabinoid systems and analgesic and anti-inflammatory effects in animal models.
adjusting the endogenous concentrations of certain endo- There is strong evidence to show that URB597 acts as a covalent
cannabinoids by slowing their degradation or limiting their inhibitor of FAAH, with the phenol part of the carbamate
formation has hence been a very active area of research. Can- displaced by an active site serine. Interestingly, the closely
nabinoid dependence, Tourette syndrome, schizophrenia, and related phenol URB937 (99b) is excluded from the CNS through
fear conditioning such as that associated with PTSD have been the action of an active membrane transporter,[100] and has been
considered for such pharmaceutical interventions. Much of the investigated for its modulation of the peripheral endocannabi-
work on endocannabinoid modulators in the pharmaceutical noid system in animal models. The discovery of these ‘URB’
industry has, however, been focussed on the development of compounds has stimulated wide-ranging searches in academia
new pain medications. and the pharmaceutical industry for even more therapeutically
By far the most attention has been given to the development effective carbamates that target FAAH, which have continued
of inhibitors of the hydrolases that catalyse the degradation unabated for over a decade.
endocannabinoids like anandamide (14), particularly fatty acid Related to the carbamates are the arylurea FAAH inhibitors.
amide hydrolase (FAAH) and monoacylglycerol lipase Somewhat surprisingly, given the hydrolytic stability of the urea
(MAGL), an enzyme that catalyses the degradation of glycerol functional group, these inhibitors have also been shown to attach
derivatives like 2-AG (15). The inhibition of these enzymes covalently to an active site serine, with the (hetero)aryl amine
allows the corresponding endocannabinoids to accumulate, fragment becoming the leaving group. The first FAAH inhibitor
increasing their agonistic effects on cannabinoid receptors. This to reach Phase II clinical trials was Pfizer’s urea, PF-04457845
appears to be supported by the interesting case of a woman with (100), which has been assessed for its suitability to treat
reduced expression and activity of FAAH in her central nervous inflammatory and non-inflammatory pain. It was unfortunately
system (CNS), and consequent high circulating levels of anan- found to be ineffective against osteoarthritis pain. Many other
damide and other endocannabinoids, who was found to have classes of FAAH inhibitors have been developed in recent years,
remarkable pain insensitivity, a highly optimistic outlook, and including reversible covalent inhibitors based on boronic acids
very low anxiety and depression scores.[96] and propanone derivatives, and a range of non-covalent hetero-
An enormous number of inhibitors have been developed for cyclic compounds. It has also been speculated that some
FAAH, and these have been thoroughly reviewed else- anaesthetics and NSAIDs may in part derive some of their
where.[97–99] The enzyme employs an amidase serine–serine– efficacy from FAAH inhibition. For example, propofol, a very
lysine catalytic triad and early inhibitors were electrophilic common anaesthetic, has been shown to competitively inhibit
analogues of anandamide, either being competitive reversible rat FAAH with an IC50 of 14 mM. The best evidence that FAAH
or irreversible inhibitors. After extensive investigations, partic- inhibition may contribute to its activity comes from the study of
ularly in Boger’s laboratories, a-ketoheterocycles like OL-135 plasma levels of anandamide in patients undergoing orthopaedic
(97, Fig. 17) were developed. This class of inhibitor has been surgery. Anandamide levels in the patients who had been
shown to reversibly form a covalent hemiketal intermediate with anesthetised with propofol were found to be up to 1.5 to 2 times
an active site serine residue in FAAH. The flexible OL-135 (97) higher than in the control group.
was found to inhibit FAAH with a half maximal inhibitory Compared with FAAH, the development of inhibitors of
concentration (IC50) value of 4.7 nM, and briefly induced MAGL has received much less attention,[101,102] presumably
analgesia in several in vivo preclinical pain models. The appar- because it was not until 2002 that the natural degradation of
ent rapid metabolism of 97 in rats was not seen with the analogue 2-AG (15) was discovered to occur primarily through the action
98, a compound with similar potency, which blocked pain in rats of this enzyme. Monoacylglycerol lipase employs a serine–
over more prolonged periods. histidine–aspartate catalytic triad similar to that employed by
Another class of FAAH inhibitors are the carbamates, one of serine proteases and, as with FAAH, covalent inhibitors have
the earliest and most thoroughly studied being URB597 (99a). played a key role in gaining an understanding of the enzyme’sYou can also read
