Phenothiazinium photoantimicrobials with basic side chains.

Derivatives of the standard cationic photosensitiser, methylene blue, were synthesised, having extra amino (basic) functionality in the auxochromic side-chain. The resulting analogues were profiled for photodynamic activity in vitro, and screened against standard Gram-positive and Gram-negative bacteria for photobactericidal activity. The substitution pattern of the derivatives was such that ionisation of the amino groups in situ, via protonation, provided a range of charge distribution and degree of charge across the molecular framework. While most examples exhibited greater activity than the lead compound, in addition to similar activity to the known, but more powerful, phenothiazinium photoantimicrobial, dimethyl methylene blue, this was also associated with relatively high dark toxicity, inferring that these compounds were targeting crucial structures before illumination. One derivative having an asymmetrical structure, with separation between a lipophilic and a hydrophilic region exhibited a combination of very high phototoxicity coupled with very low dark effects, against both the standard screen and an additional one containing further, relevant pathogen species, including Candida albicans. It is suggested that the great activity of this analogue is due to efficient membrane targeting.