Antibacterial phototoxic effects of synthetic asymmetric and glycosylated curcuminoids in aqueous formulations: Studies on curcumin and curcuminoids. LIV

•Aromatic OH-substituents in p-position essential for aPDT effect of curcuminoids.•Reduced aPDT effect by increased interaction with solubilizer.•Natural curcuminoids show more potent aPDT effect than synthetic derivatives.

The aim of this study was to evaluate the in vitro phototoxic potential of synthetic asymmetric and glycosylated curcuminoids on planktonic model bacteria by counting the colony forming units. The Gram-positive Enterococcus faecalis and the Gram-negative Escherichia coli were exposed to aqueous solutions of the curcuminoids (⩽2.5 μM) in the presence or absence of selected pharmaceutical excipients (Pluronic® F127, PEG 400 and HPγCD) in combination with a low irradiation dose (5 J/cm2; λmax: 450 nm) of constant irradiance and time. All the asymmetric curcuminoids, but only one of the glycosylated curcuminoids demonstrated substantial phototoxic effect on E.faecalis (⩾4.7 log reduction). Only two of the asymmetric curcuminoids showed a moderate to low phototoxic effect on the more persistent E.coli. This study emphasized that aromatic hydroxyl substituents in the para-position are important to maintain the phototoxic potential of curcuminoids independent of molecular symmetry. Glycosylation of the aromatic substituents resulted in a substantial loss in phototoxicity towards planktonic bacteria, an apparent change in the non-radiative S1-decay process and a weaker interaction with Pluronic® F127 compared to the non-glycosylated curcuminoids. The selected excipients Pluronic® F127, PEG 400 and HPγCD strongly influenced the phototoxic potential of the unsymmetrical, non-glycosylated compounds.