Phytocannabinoids, beyond THC: a brief survey

Phytocannabinoids are exclusively produced in cannabis (vide infra for exception), but their evolutionary and ecological raisons d’être were obscure until recently.

Phytocannabinoids are exclusively produced in cannabis (vide infra for exception), but their evolutionary and ecological raisons d’être were obscure until recently. THC production is maximized with increased light energy (Potter, 2009). It has been known for some time that CBG and CBC are mildly antifungal (ElSohly et al., 1982), as are THC and CBD against a cannabis pathogen (McPartland, 1984). More pertinent, however, is the mechanical stickiness of the trichomes, capable of trapping insects with all six legs (Potter, 2009). Tetrahydrocannabinolic acid (THCA) and cannabichromenic acid (Morimoto et al., 2007), as well as cannabidiolic acid and cannabigerolic acid (CBGA; Shoyama et al., 2008) produce necrosis in plant cells. Normally, the cannabinoid acids are sequestered in trichomes away from the flower tissues. Any trichome breakage at senescence may contribute to natural pruning of lower fan leaves that otherwise utilize energy that the plant preferentially diverts to the flower, in continued efforts to affect fertilization, generally in vain when subject to human horticulture for pharmaceutical production. THCA and CBGA have also proven to be insecticidal in their own right (Sirikantaramas et al.,2005).

Over 100 phytocannabinoids have been identified (Brenneisen, 2007; Mehmedic et al., 2010), but many are artefacts of analysis or are produced in trace quantities that have not permitted thorough investigation.

The pharmacology of the more accessible phytocannabinoids has received excellent recent reviews (Pertwee et al., 2007; Izzo et al., 2009; De Petrocellis and Di Marzo, 2010; De Petrocellis et al., 2011), and will be summarized here, with emphasis on activities with particular synergistic potential.

Table 1 | Phytocannabinoid activity table

THC (Table 1) is the most common phytocannabinoid in cannabis drug chemotypes, and is produced in the plant via an allele co-dominant with CBD (de Meijer et al., 2003).

THC is a partial agonist at CB1 and cannabinoid receptor 2 (CB2) analogous to AEA, and underlying many of its activities as a psychoactive agent, analgesic, muscle relaxant and antispasmodic (Pacher et al., 2006). Additionally, it is a bronchodilator (Williams et al., 1976), neuroprotective antioxidant (Hampson et al., 1998), antipruritic agent in cholestatic jaundice (Neff et al., 2002) and has 20 times the antiinflammatory power of aspirin and twice that of hydrocortisone (Evans, 1991). THC is likely to avoid potential pitfalls of either COX-1 or COX-2 inhibition, as such activity is only noted at concentrations far above those attained therapeutically (Stott et al., 2005).

CBD is the most common phytocannabinoid in fibre (hemp) plants, and second most prevalent in some drug chemotypes.

It has proven extremely versatile pharmacologically (Table 1) (Pertwee, 2004; Mechoulam et al., 2007), displaying the unusual ability to antagonize CB1 at a low nM level in the presence of THC, despite having little binding affinity (Thomas et al., 2007), and supporting its modulatory effect on THC-associated adverse events such as anxiety, tachycardia, hunger and sedation in rats and humans (Nicholson et al., 2004; Murillo-Rodriguez et al., 2006; Russo and Guy, 2006). CBD is an analgesic (Costa et al., 2007), is a neuroprotective antioxidant more potent than ascorbate tocopherol (Hampson et al., 1998), without COX inhibition (Stott et al., 2005), acts as a TRPV1 agonist analogous to capsaicin but without noxious effect (Bisogno et al., 2001), while also inhibiting uptake of AEA and weakly inhibiting its hydrolysis. CBD is an antagonist on GPR55, and also on GPR18, possibly supporting a therapeutic role in disorders of cell migration, notably endometriosis (McHugh et al., 2010). CBD is anticonvulsant (Carlini and Cunha, 1981; Jones et al., 2010), anti-nausea (Parker et al., 2002), cytotoxic in breast cancer (Ligresti et al., 2006) and many other cell lines while being cyto-preservative for normal cells (Parolaro and Massi, 2008), antagonizes tumour necrosis factor-alpha (TNF-a) in a rodent model of rheumatoid arthritis (Malfait et al., 2000), enhances adenosine receptor A2A signalling via inhibition of an adenosine transporter (Carrier et al., 2006), and prevents prion accumulation and neuronal toxicity (Dirikoc et al., 2007). A CBD extract showed greater anti-hyperalgesia over pure compound in a rat model with decreased allodynia, improved thermal perception and nerve growth factor levels and decreased oxidative damage (Comelli et al., 2009). CBD also displayed powerful activity against methicillin-resistant Staphylococcus aureus (MRSA), with a minimum inhibitory concentration (MIC) of 0.5–2 mg·mL-1 (Appendino et al., 2008). In 2005, it was demonstrated that CBD has agonistic activity at 5-hydroxytryptamine (5-HT)1A at 16 mM (Russo et al., 2005), and that despite the high concentration, may underlie its anti-anxiety activity (Resstel et al., 2009; Soares Vde et al., 2010), reduction of stroke risk (Mishima et al., 2005), anti-nausea effects (Rock et al., 2009) and ability to affect improvement in cognition in a mouse model of hepatic encephalopathy (Magen et al., 2009). A recent study has demonstrated that CBD 30 mg·kg-1 i.p. reduced immobility time in the forced swim test compared to imipramine (P < 0.01), an effect blocked by pre-treatment with the 5-HT1A antagonist WAY100635 (Zanelati et al., 2010), supporting a prospective role for CBD as an antidepressant. CBD also inhibits synthesis of lipids in sebocytes, and produces apoptosis at higher doses in a model of acne (vide infra). One example of CBD antagonism to THC would be the recent observation of lymphopenia in rats (CBD 5 mg·kg-1) mediated by possible CB2 inverse agonism (Ignatowska-Jankowska et al., 2009), an effect not reported in humans even at doses of pure CBD up to 800 mg (Crippa et al., 2010), possibly due to marked interspecies differences in CB2 sequences and signal transduction. CBD proved to be a critical factor in the ability of nabiximols oromucosal extract in successfully treating intractable cancer pain patients unresponsive to opioids (30% reduction in pain from baseline), as a high-THC extract devoid of CBD failed to distinguish from placebo (Johnson et al., 2010). This may represent true synergy if the THC–CBD combination were shown to provide a larger effect than a summation of those from the compounds separately (Berenbaum, 1989).

CBC (Table 1) was inactive on adenylate cyclase inhibition (Howlett, 1987), but showed activity in the mouse cannabinoid tetrad, but only at 100 mg·kg-1, and at a fraction of THC activity, via a non-CB1, non-CB2 mechanism (Delong et al., 2010). More pertinent are anti-inflammatory (Wirth et al., 1980) and analgesic activity (Davis and Hatoum, 1983), its ability to reduce THC intoxication in mice (Hatoum et al.,1981), antibiotic and antifungal effects (ElSohly et al., 1982), and observed cytotoxicity in cancer cell lines (Ligresti et al., 2006). A CBC-extract displayed pronounced antidepressant effect in rodent models (Deyo and Musty, 2003). Additionally, CBC was comparable to mustard oil in stimulating TRPA1- mediated Ca++ in human embryonic kidney 293 cells (50–60 nM) (De Petrocellis et al., 2008). CBC recently proved to be a strong AEA uptake inhibitor (De Petrocellis et al., 2011). CBC production is normally maximal, earlier in the plant’s life cycle (de Meijer et al., 2009a). An innovative technique employing cold water extraction of immature leaf matter from selectively bred cannabis chemotypes yields a high-CBC ‘enriched trichome preparation’ (Potter, 2009).

CBG (Table 1), the parent phytocannabinoid compound, has a relatively weak partial agonistic effect at CB1 (Ki 440 nM) and CB2 (Ki 337 nM) (Gauson et al., 2007). Older work supports gamma aminobutyric acid (GABA) uptake inhibition greater than THC or CBD (Banerjee et al., 1975) that could suggest muscle relaxant properties. Analgesic and anti-erythemic effects and the ability to block lipooxygenase were said to surpass those of THC (Evans, 1991). CBG demonstrated modest antifungal effects (ElSohly et al., 1982).

More recently, it proved to be an effective cytotoxic in high dosage on human epithelioid carcinoma (Baek et al., 1998), is the next most effective phytocannabinoid against breast cancer after CBD (Ligresti et al., 2006), is an antidepressant in the rodent tail suspension model (Musty and Deyo, 2006) and is a mildly anti-hypertensive agent (Maor et al., 2006). Additionally, CBG inhibits keratinocyte proliferation suggesting utility in psoriasis (Wilkinson andWilliamson, 2007), it is a relatively potent TRPM8 antagonist for possible application in prostate cancer (De Petrocellis and Di Marzo, 2010) and detrusor over-activity and bladder pain (Mukerji et al., 2006). It is a strong AEA uptake inhibitor (De Petrocellis et al., 2011) and a powerful agent against MRSA (Appendino et al., 2008; vide infra). Finally, CBG behaves as a potent a-2 adrenorecep-tor agonist, supporting analgesic effects previously noted (Formukong et al., 1988), and moderate 5-HT1A antagonist suggesting antidepressant properties (Cascio et al., 2010).

Normally, CBG appears as a relatively low concentration intermediate in the plant, but recent breeding work has yielded cannabis chemotypes lacking in downstream enzymes that express 100% of their phytocannabinoid content as CBG (de Meijer and Hammond, 2005; de Meijer et al., 2009a).

THCV (Table 1) is a propyl analogue of THC, and can modulate intoxication of the latter, displaying 25% of its potency in early testing (Gill et al., 1970; Hollister, 1974). A recrudescence of interest accrues to this compound, which is a CB1 antagonist at lower doses (Thomas et al., 2005), but is a CB1 agonist at higher doses (Pertwee, 2008). THCV produces weight loss, decreased body fat and serum leptin concentrations with increased energy expenditure in obese mice (Cawthorne et al., 2007; Riedel et al., 2009). THCV also demonstrates prominent anticonvulsant properties in rodent cerebellum and pyriform cortex (Hill et al., 2010). THCV appears as a fractional component of many southern African cannabis chemotypes, although plants highly predominant in this agent have been produced (de Meijer, 2004). THCV recently demonstrated a CB2-based ability to suppress carageenan-induced hyperalgesia and inflammation, and both phases of formalin-induced pain behaviour via CB1 and CB2 in mice (Bolognini et al., 2010). CBDV (Table 1), the propyl analogue of CBD, was first isolated in 1969 (Vollner et al., 1969), but formerly received little investigation. Pure CBDV inhibits diacylglycerol lipase [50% inhibitory concentration (IC50) 16.6 mM] and might decrease activity of its product, the endocannabinoid, 2-AG (De Petrocellis et al., 2011). It is also anticonvulsant in rodent hippocampal brain slices, comparable to phenobarbitone and felbamate (Jones et al., 2010). Finally, CBN is a non-enzymatic oxidative by-product of THC, more prominent in aged cannabis samples (Merzouki and Mesa, 2002). It has a lower affinity for CB1 (Ki 211.2 nM) and CB2 (Ki 126.4 nM) (Rhee et al., 1997); and was judged inactive when tested alone in human volunteers, but produced greater sedation combined with THC (Musty et al., 1976). CBN demonstrated anticonvulsant (Turner et al., 1980), anti-inflammatory (Evans, 1991) and potent effects against MRSA (MIC 1 mg·mL-1). CBN is a TRPV2 (highthreshold thermosensor) agonist (EC 77.7 mM) of possible Interest in treatment of burns (Qin et al., 2008). Like CBG, it inhibits keratinocyte proliferation (Wilkinson and Williamson, 2007), independently of cannabinoid receptor effects. CBN stimulates the recruitment of quiescent mesenchymal stem cells in marrow (10 mM), suggesting promotion of bone formation (Scutt and Williamson, 2007) and inhibits breast cancer resistance protein, albeit at a very high concentration (IC50 145 mM) (Holland et al., 2008).