Liquorice for pain?

Liquorice has a long history of use in traditional Chinese (TCM), Ayurvedic and herbal medicine as a treatment for kidney, lung and liver ailments, gastric discomfort, arthritis and infections. 1 , 2 Liquorice is also used in TCM to alleviate pain. 3 In addition, the 10th edition of the European Pharmacopoeia contains monographs on liquorice root ( Liquiritiae radix ) and liquorice extract for flavouring purposes ( Liquiritiae extractum siccum ad saporandum ), a herbal drug and herbal preparation, respectively. 4 The European Pharmacopoeia, the Committee on Herbal Medicinal Products of the European Medicines Agency and TCM use three plants as liquorice: Glycyrrhiza glabra L. ( Figure 1 ), Glycyrrhiza uralensis Fisch. and Glycyrrhiza inflata Bat., with roots and rhizomes of the first two being used most commonly. All three plants contain numerous bioactive secondary metabolites. Glycyrrhiza glabra has the highest concentration of triterpenoids and Glycyrrhiza uralensis has the highest concentration of flavonoids. 5 Glycyrrhiza species have been found to contain more than 20 triterpenes and 300 flavonoids, with 73 bioactive plant secondary metabolites being identified. 3 A recent untargeted metabolomic analysis of the three liquorice species identified 101 significant metabolites. 6 Glycyrrhiza glabra . L. M. Alcade illustrator Europeana source: Real Jardin Botánico Madrid Licensed with CC BY-NC-SA 4.0. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-sa/4.0/ . Glycyrrhizin (glycyrrhizinic acid, GLA, 1 ) ( Figure 2 ) is the main active compound of liquorice. GLA constitutes approximately 10% of the liquorice root dry weight and is present as a mixture of potassium, calcium and magnesium salts of GLA. 7 GLA is a β-amyrin-type triterpenoid saponin, which numerous preclinical and cell studies report has antiviral, neuroprotective and potent anti-inflammatory properties. 3 , 8 – 11 In Japan, GLA has been used as a medical drug in humans for more than 60 years to treat chronic hepatitis. 12 Due to its antiviral and anti-inflammatory effects, GLA 1 was suggested as a treatment for severe acute respiratory syndrome (SARS), 13 while most recently it has been designated a potential treatment for coronavirus disease 2019, COVID-19. 14 , 15 Liquorice root constituents, including GLA 1 have also been shown to have anticancer effects and, since GLA 1 is both inexpensive and easily available, it has been proposed as a scaffold for the synthesis of new anti-tumour drugs. 16 – 18 Terpenoids, flavonoids and chalcones from liquorice. Liquorice root extract, its flavones and flavonones exert potent antioxidant effects, with lipid peroxidation inhibition being the most pronounced effect. 19 For this reason, it is a source of antioxidants for the pharmaceutical, cosmetic and wellness industries. 19 In TCM, liquorice is used to treat painful ailments. Glycyrrhiza uralensis , in combination in equal amounts with white peony root ( Radix paeoniae alba ), is a well-known Chinese herbal formula (Shaoyao-Gancao Decoction, Shakuyaku-Kanzo-to in Japanese), which is commonly used to relieve myalgia, arthralgia and neuropathic pain, including paclitaxel-induced pain. 20 , 21 The liquorice root O -glycosidic trans -chalcone isoliquiritin 2 ( Figure 2 ) has also been shown to have analgesic properties. 22 This paper will explore the mechanisms by which glycyrrhizin 1 , the main active compound of liquorice, its major metabolite glycyrrhetic (glycyrrhetinic) acid (enoxolone, GRA 3 ) and its synthetic analogue, carbenoxolone 4 , could influence nociception, analgesia, depression and morphine tolerance/ dependency. In addition, relevant actions of some of the major isoflavonoids isolated from liquorice root such as glabridin 5 , as well as the main trans -chalcones isoliquiritin 2 and licochalcone A 6 , the main Glycyrrhiza trans- chalcone ( Figure 2 ) will be addressed. The compound structures with respective numbering are depicted in Figure 2 and a summary of their therapeutic actions and potential clinical use is portrayed in Table 1 . Liquorice compounds, mechanisms of action and potential clinical use. 11β-HSD = 11β-hydroxysteroid dehydrogenase; HMGB1, high mobility group box 1; NA, noradrenaline; NP, neuropathic pain, OIH, opioid-induced hyperalgesia; mPGE, microsomal prostaglandin E. *Already in clinical use/positive RCTs licenced drug for Diabetes type 2 in the UK

The scientific literature contains many case reports on the adverse effects of liquorice, most recently a report of cardiac arrest due to liquorice-associated mineralocorticoid excess with profound hypokalaemia, 73 hypertension and hypokalaemia attributed to chronic ingestion of herbal tea containing liquorice root and hypokalaemia attributed to liquorice-flavoured vaping. 74 , 75 The adverse effects of liquorice seem to be related to dose, duration and individual susceptibility and are usually reversed after discontinuation of liquorice. It has long been established that liquorice can cause potentially serious dose-dependent adverse effects in the form of pseudohyperaldosteronism (high blood pressure, sodium retention, hypokalaemia and oedema). This is due to GLA 1 /GRA 3 -related inhibition of 11β-HSD2 – the enzyme that converts the active cortisol to locally inactive cortisone – leading to increased levels of cortisol with activation of renal mineralocorticosteroid receptors. A systematic review found that chronic consumption of liquorice, or products containing GLA 1 , in healthy subjects increases systolic and diastolic blood pressure and decreases plasma potassium levels, concluding that these changes, while modest, may be particularly relevant for persons with pre-existing cardiovascular or renal disease. 76 GLA 1 is known to be a potent inhibitor of placental 11β-HSD2 and may lead to higher levels of foetal cortisol exposure. An epidemiological study found that high maternal consumption of GLA 1 in pregnant women may adversely affect the developing offspring. 77 Considering the potential for adverse effects, the long half-life of glycyrrhizin 1 and the enterohepatic recirculation (see below), the efficacy and safety of intermittent dose regimens should be considered. According to one author, when treating gastrointestinal inflammation with carbenoxolone 4 , the common dosage is ‘100 mg for 1 week, three times a week, followed by a reduction to 50 mg every other day, to prevent potential side effects such as sodium retention, hypokalaemia, increased blood pressure and edema’. 78 Both GLA 1 and GRA 3 exhibit antithrombotic properties, inhibiting coagulation factor Xa and thrombin. A case of haemorrhagic stroke and cerebral microbleeds attributed to high intake of liquorice root supplement has been reported. 79 Liquorice also has endocrinological properties, with oestrogen-like effects due mainly to the root isoflavonoids glabrene 10 ( Figure 2 ) and glabridin 5 as well as trans -chalcone isoliquiritigenin 7 , while the triterpenoids GLA 1 and GRA 3 exhibit weak anti-androgen effects. 7 A study in nine healthy women found that daily ingestion of 3.5 g of commercially prepared liquorice for 2 months resulted in slight, but statistically significant, increases in serum parathyroid hormone, 25-hydroxycholecalciferol and urinary calcium levels, compared with baseline. 80 All parameters returned to baseline values by 1 month after discontinuation of liquorice. The same group found that total testosterone is reduced slightly in healthy males and females after 1 week of daily liquorice intake. 81 , 82 The maximum dose of GLA 1 recommended by the European Commission Scientific Committee on Food is 100 mg per day, although the committee acknowledged the limited amount of research on human toxicity of this compound. 83 A comprehensive review on the toxicology of liquorice, which acknowledged the difficulty of assessing safety due to the differences in the administered substances tested, estimated that, based on rodent studies, a no-observed-effects-level (NOEL) for purified GLA 1 would be in the range of 15–229 mg/kg/day and, by applying uncertainty factors for intraspecies and interspecies differences, an acceptable daily intake (ADI) for GLA 1 would be 0.015–0.229 mg/kg/day. 84 This estimate is in line with a previous toxicological assessment of an ADI for GLA 1 of 0.2 mg/kg/day. 85

The liquorice plant has a long tradition of use in traditional Chinese medicine and has also been used extensively in herbal medicine and folk remedies in other countries. 7 Liquorice is commonly used in modern medicine as a laxative and as an expectorant in cough syrups. There are a number of clinical studies documenting long-term therapy with GLA 1 for chronic hepatitis, due to its anti-viral, anti-inflammatory and hepatoprotective effects, 65 with it recently being proposed as a possible secondary chemoprevention strategy for hepatocellular carcinoma. 66 GLA 1 is also used as oral, intravenous or topical adjuvant therapy for dermatological disorders such as psoriasis and eczema. 67 , 68 Carbenoxolone 4 is a licenced drug in the United Kingdom (UK). GRA 3 is reported to be 200–1000 times more potent inhibitor of 11β-HSD, compared with GLA 1. 69 Carbenoxolone 4 was used primarily to accelerate the healing of gastric and duodenal ulcers, 70 and is currently a therapeutic agent for diabetes 2, due to its inhibitory effect on 11β-HSD type 1, with the effect of lowering intracellular cortisol concentrations and enhancing insulin sensitivity. 71 It should be noted that carbenoxolone 4 also blocks 11β-HSD type 2 and binds to mineralocorticoid receptors as an agonist inducing pseudo-hyperaldosteronism. 71

Apart from pharmaceuticals, GLA 1 is used as a sweetener and flavouring agent in the confectionery, 2 food and tobacco industry. It was approved for use as a food additive in 1985 in the United States (US) and has been given the status of Generally Recognized as Safe (GRAS) by the US Food and Drug Administration (FDA). 14 Glycyrrhizin 1 is reputed to be approximately 50 times sweeter than sucrose and has no effect on the glycaemic index; however, its use as a sweetener is limited by the intense liquorice flavour. The isoflavane-type isoflavonoid glabridin 5 is used by the cosmetic industry in antioxidant and anti-inflammatory creams and as a whitening agent. Liquorice plant extracts are also used as a feed additive for livestock and poultry, due primarily to their immunomodulatory, anti-inflammatory and antiviral effects. 72

A recent paper by Thomford and colleagues addressed the challenges involved in evaluating and using plant products and the difficulty in accepting their therapeutic efficacy. 91 The authors acknowledged the difficulty of assessing complex molecular mixtures such as liquorice, emphasising that the therapeutic activity of plant extracts is usually because of the synergistic and simultaneous action of several constituents, rather than single compounds. Design and synthesis of novel semisynthetic derivatives, analogues and mimetics of GLA 1 and other constituents of Glycyrrhiza species will be mandatory to produce libraries of chemically purified single compounds from such extracts for additional biological and pharmacological studies of plants belonging to the bean family (Fabaceae).

Liquorice plant and its herbal preparations have had centuries of use in traditional Chinese, Ayurvedic and herbal medicine for a range of ailments, including pain. Liquorice constituents have been found to have anti-inflammatory, antioxidant, antiviral, anticancer, hepatoprotective and neuroprotective properties. In addition, they appear to have antidepressant actions and effects on morphine tolerance. Liquorice constituents such as glycyrrhizin (GLA) 1 , its metabolite glycyrrhetic (glycyrrhetinic) acid (GRA) 3 and other liquorice-derived compounds such as glabridin 5 and isoliquiritigenin 7 exert these effects via a range of mechanisms including HMGB1 inhibition, gap junction blockade and α 2A -adrenoceptor antagonism. A large body of preclinical studies and a long history of use in herbal medicine suggest that liquorice constituents may be useful for pain management. However, whether liquorice plant-derived compounds represent a novel class of analgesics is yet to be established. Regular administration of liquorice can cause serious adverse effects and this could prove a limiting factor with regard to chronic pain management. Clinical research examining available preparations such as liquorice root extract or single compounds such as glycyrrhizin 1 , carbenoxolone 4 , liquiritin 8 , isoliquiritigenin 7 , isoliquiritin 2 , or glabridin 5 or new liquorice-derived drugs in clearly defined models of acute and chronic pain, with regard to efficacy, tolerability and safety are required. Having a host of bioactive compounds with a broad range of mechanisms of effect, liquorice is a plant, which in the future may give rise to new therapies for pain.

The immune system appears to play a role in the pathology of psychiatric disorders and there is clear evidence of correlations between immune dysfunction and psychiatric disease. 57 The immune system engages directly with central nervous system glia, 58 and neuroinflammation can be caused by excessive production of proinflammatory cytokines in microglia, which are the resident immune cells of the brain. HMGB1 acting directly on microglia appears to be a major mediator in various neuroinflammatory diseases. HMGB1 has been shown to mediate depressive behaviour induced by chronic stress through activating the kynurenine pathway. 59 GLA 1 blockage of HMGB1 in partial sciatic nerve ligation (PSNL) mice reduced microglia activation and anxiodepressive-like behaviour. 60 A recent clinical study found that GLA 1 as an adjuvant to selective serotonin reuptake inhibitors (SSRI) had better effect in depressed patients than SSRI and placebo and that the effect was more pronounced in those who had a higher grade of inflammation [cut-off concentration C-reactive protein (CRP) ⩾3 mg/l]. 61 While acknowledging the limitations of the study due to small size and short duration, the authors proposed that patients with depression should be classified according to grade of inflammation and that those with a high grade of inflammation should receive adjuvant treatment with an anti-inflammatory agent, such as GLA 1 . The liquorice plant-derived O -glycosidic trans -chalcone isoliquiritin 2 and O -glycosidic flavanone liquiritin 8 have been found to have antidepressant-like effects in rodent models, 62 , 63 with the effects most likely due to increased serotonin and noradrenaline in the hippocampus, hypothalamus and cortex. 62 , 64 In addition, isoliquiritin 2 appears to have antihyperalgesic and antiallodynic effects in neuropathic pain. A murine study demonstrated these effects and found them to be dependent on spinal serotonergic mechanisms. 22

Studies in rodents and humans have demonstrated that GLA 1 is poorly absorbed by the gastrointestinal tract, but extensively metabolized by the intestinal microflora to GRA 3 and monoglucuronyl glycyrrhetic (glycyrrhetinic) acid, both of which are readily absorbed. GLA 1 and GRA 3 pharmacokinetics are characterised by a biphasic elimination from the central compartment with a dose-dependent second elimination phase. 86 An enterohepatic recycling of glycyrrhetinic acid 3 can occur, requiring several days for complete elimination. 87 A pharmacokinetic study in three healthy subjects found that, after oral administration of 100 mg of GLA 1 , the major metabolite GRA 3 appeared in plasma. 88 However, GLA 1 corresponding to 1.1–2.5% of the oral dose was found in urine, suggesting that GLA 1 is at least partially absorbed intact from the gastrointestinal tract. When GLA 1 at 40, 80 or 120 mg was administered intravenously, its concentration in plasma showed biexponential profiles during the 24 h after administration, but GRA 3 and glycyrrhetic acid-3- O -glucuronide were not detected in either plasma or urine. Three dosing experiments found that the terminal half-life of GLA 1 was 2.7–4.8 h, the apparent volume of the central compartment was 37–64 ml/kg, the steady-state distribution volume was 59–98 ml/kg and the total body clearance was 16–25 ml/kg/h. GLA 1 was not detected in plasma after oral administration of the usual therapeutic dose and no dose dependency was observed in the dose range of 40–120 mg. 88 The constituents of liquorice have complex effects on CYP enzymes, and further research is required to elucidate these effects. 89 For example, liquorice extract has been shown to be a potent inhibitor of CYP3A4 activity. This effect appears to be due not to the major active component GLA 1 but to other constituents of the extract, such as flavanone-class liquiritigenin 11 ( Figure 2 ). 89 Both GLA 1 and GRA 3 appear to increase CYP3A4 activity, whereas GRA 3 and isoflavane-class glabridin 5 inhibit CYP2C9. Liquorice may therefore potentiate the effects of warfarin therapy. 89 GLA 1 has been shown to have effects on the pharmacokinetics of prednisolone. A pharmacokinetic study in six healthy men treated with intravenous prednisolone (prednisolone hemisuccinate, PSL-HS) found that pretreatment with oral administration of GLA 1 significantly increased the concentrations of total prednisolone (PSL) at 6, 8 h, and of free PSL at 4, 6 and 8 h after PSL-HS infusion. 90 Oral administration of GLA 1 was also found to modify the pharmacokinetics of both total and free PSL. The authors concluded that oral GLA 1 increases plasma prednisolone concentration and influences its pharmacokinetics by inhibiting its metabolism, but without affecting its distribution.

GLA 1 and derivatives appear to reduce/inhibit the development of morphine tolerance and physical dependence via several mechanisms. One possible mechanism is through HMGB1 inhibition. A recent study in rats found that morphine mediates upregulation of spinal HMGB1, contributing to analgesic tolerance and hyperalgesia via activation of TLR4/NF-κB signaling. 54 The study also found that the HMGB1 inhibitor GLA 1 reduced morphine tolerance and alleviated morphine-induced withdrawal hyperalgesia. The authors concluded that GLA 1 may be a promising adjuvant to morphine in the treatment of intractable pain. Another possible mechanism by which liquorice constituents could influence morphine tolerance is via α 2A -adrenoreceptor antagonism. Low doses of competitive α 2 -adrenoreceptor agonists can potentiate acute morphine analgesia and block or reverse tolerance to spinal administration of morphine in experimental animals. 55 Nakagawa et al. investigated the effects of a traditional Japanese medicine, Yokukansan (YKS), on morphine tolerance and physical dependence in mice. 56 YKS consists of seven medicinal herbs, including the root and stolon of Chinese liquorice ( Glycyrrhiza uralensis Fisch). The authors found that the membrane expression of α 2A -adrenoreceptors in the pons/medulla of mice was decreased in morphine-withdrawn animals. Three compounds of Chinese liquorice in YKS, namely isoliquiritin 2 , isoliquiritigenin 7 and glycyrrhetic acid 3 , had specific binding affinity for and antagonist activity against the α 2A -adrenoreceptor. Repeated administration of Chinese liquorice extract and its active substance GLA 1 inhibited morphine withdrawal signs significantly. 56