A screening of curcumin derivatives for antibacterial phototoxic effects studies on curcumin and curcuminoids. XLIII.

Curcumin, bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione, has potential as a photosensitiser for photodynamic treatment of localised superficial infections in e.g., the mouth or skin. The aim of the present study was to evaluate the in vitro antibacterial phototoxic potential of a series of five curcumin derivatives. The gram-positive Enterococcus faecalis and the gram-negative Escherichia coli were used as bacterial models. The bacteria were exposed to curcuminoid preparations in two concentrations (2.5 and 25.0 microM) in combination with a constant irradiation dose (5 J/cm2). The UV-VIS absorption spectrum of the curcuminoids was in the same range as curcumin, 300-500 nm. Compound 1 (dimethoxycurcumin; C1) and compound 3 (bisdemethoxycurcumin; C3) were strongly phototoxic towards E. faecalis (no surviving bacteria) and showed a lower but significant effect towards E. coli (< or = 0.5 log reductions and 1 - 4 log reductions, respectively). Compound 2 (C2) and compound 4 (C4) in combination with blue light reduced the colony forming ability of E. faecalis (approximately 1-4 log reductions). The phototoxic effect of the curcuminoids varied with concentration, and for compounds C1, C2 and C3 it was further influenced by the addition of polyethylene glycol 400 (PEG 400) to the preparations. 2,6-Divanillylidenecyclohexanone (C5) showed very low phototoxic potential (< 0.2 log reductions) under the conditions used in the present study. The addition of polyethylene glycol 400 (PEG 400) seemed to increase the solubility of compound C1, C3 and C5 in phosphate buffered saline (PBS). This investigation demonstrates the importance and influence of the substituents on the phenolic rings and the keto-enol moiety for the phototoxic potential of curcumin and its derivatives.