Curcumin [1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione] is a natural yellow pigment isolated from the rhizomes of the plant Curcuma longa L. (turmeric). Its chemical structure is described in Fig. 1.1) The pharmacological effects of curcumin include anti-inflammatory, 2) antioxidant, 3) antiprotozoal, 4) antibacterial, 5) anticancer, 6) antihepatotoxic, 7) hypolipidemic, 8) and antifibrotic effects. 9) Curcumin showed no toxicity in acute toxicity studies in rats, guinea pigs, and monkeys, 10) as well as in humans.
11)Several prior studies have examined the effects of curcumin on allergic diseases. An ethyl acetate extract of Curcuma longa L., which contains curcuminnoids, exhibited a preventive effect in allergic type I and IV models.12) Furthermore, both the ethyl acetate extract of Curcuma longa L. and curcumin were found to decrease histamine release from mast cells. However the ethyl acetate extract of Curcuma longa L. had somewhat stronger activity than that of curcumin, which may have been due to the presence of curcumin derivatives.13) Several studies have also examined the anti-allergic activity of curcumin and curcumin-related compounds in relation to their antioxidant effects. Most of these compounds were shown to inhibit histamine release from rat basophilic leukemia (RBL-2H3) cells; however, histamine release was decreased by antioxidant compounds and by those that lacked antioxidant activity. The results suggested that the hydroxy moiety of curcumin is responsible for antioxidant activity and may also contribute to the inhibition of histamine release. 14) In a guinea pig model of airway hyperresponsiveness, curcumin at a dose of 20 mg/kg body weight significantly inhibited ovalbumin (OVA)-induced airway constriction and airway hyperactivity.
15)Although curcumin has a wide range of potentially beneficial pharmacological effects, its use is limited by the compounds instability. The stability of curcumin is strongly affected by factors such as pH and exposure to light.16) The kinetics of the pH-dependent degradation of curcumin in aqueous medium at various pH levels was studied by Tonnesen and Karlsen. 17) They found that curcumin in aqueous solution at pH less than 7 is quite stable, but with increasing pH, curcumin decomposes. The main decomposition products are ferulic acid and feruloylmethane, which rapidly undergo retro-aldol condensation to vanillin and acetone. The instability of curcumin at alkaline pH is caused by the active methylene moiety. Decomposition of curcumin can also occur as a result of light exposure, and this process is also mediated by the active methylene moiety; the degradation products include ferulic aldehyde, ferulic acid, dihydroxynaphthalene product, 4-vinylguaiacol, vanillin, and vanillic acid. 16,17) The modification of the middle site of curcumin to compounds such as the 1,4-pentadiene-3-ones, cyclopentanone, and cyclohexanone which lack the active methylene moiety and one carbonyl moiety of curcumin may show improved stability. These compounds which st...
