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.

Phenothiazinium photosensitisers VII: novel substituted asymmetric N-benzylphenothiaziniums as photoantimicrobial agents.

The synthesis of asymmetrical analogues of methylene blue, in which one of the dimethylamino groups is replaced by a diethylamino or di-n-propylamino group, and the other by benzylamino or 4-substituted benzylamino, is reported, the substituents being alkyl, alkoxyl or halogen. As expected, because of their longer alkyl chains these diethylamino- and di-n-propylamino derivatives proved to be considerably more lipophilic than the parent compound methylene blue, while maintaining suitable maximum absorption wavelengths and singlet oxygen efficiencies for photoantimicrobial use. Also as expected, in screening tests against Gram-positive and Gram-negative bacteria, the substituted benzylamino derivatives were highly active on illumination, presumably via singlet oxygen damage, and exhibited considerably increased activity against both classes relative to that of the standard, methylene blue. In addition, the more lipophilic derivatives exhibited greater activity against Escherichia coli. This may be due to increased interaction with the lipid-rich outer membrane of this Gram-negative bacterium. DNA binding of the derivatives was also increased, relative to methylene blue, showing large bathochromic shifts (>10nm) on binding typical of strong intercalators.

Phenothiazinium-fluoroquinolone drug conjugates.

Synthesis and antibacterial screening of a homologous series of 3-dialkylaminophenothiazinium-7-norfloxacin conjugates was carried out alongside a corresponding series of symmetrical methylene blue derivatives. The norfloxacin conjugates maintained typical methylene blue derivative photoproperties, such as long wavelength absorption, but produced no measurable singlet oxygen in the standard assay and provided no significant increase in the magnitude of photoantibacterial action, this being similar to the methylene blue homologues, although both the conjugates and homologues were considerably more active than methylene blue itself both against Staphylococcus aureus and Escherichia coli. DNA binding via intercalation was considerably greater for the series of norfloxacin conjugates than for the methylene blue homologues.