A novel set of symmetric methylene blue derivatives exhibits effective bacteria photokilling — a structure — response study

Abstract

This study focuses on the structure-response relationship of symmetrically substituted phenothiazinium dyes. Four hydrophilic derivatives with the ability of additional hydrogen bonding (, ) or additional electrostatic interaction (, ) were synthesized, photophysically characterized and compared to the parent compound methylene blue (MB, ) and a lipophilic derivative () without additional coordination sites. Derivative was most effective against Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli reaching a maximum photodynamic efficacy of >5log10 steps (≥99.999%) of bacteria killing in 10 minutes (5 μM, 30 J cm(-2)) without inherent dark toxicity after one single treatment with the incoherent light source PDT1200 (λmax = 660 nm, 50 mW cm(-2)). Interestingly, one derivative with two additional primary positive charges () showed selective killing of Escherichia coli (5 μM, 30 J cm(-2), 4log10 steps inactivation (≥99.99%)) and no antimicrobial effect on Staphylococcus aureus. This might allow the development of a new generation of photosensitizers with higher antimicrobial efficacy and selectivity for future applications.

Antibacterial photodynamic therapy: overview of a promising approach to fight antibiotic-resistant bacterial infections

“…Methylene blue derivatives (structures are summarized in Table 2) such as new methylene blue, dimethyl methylene blue, and methylene green were developed with different functional substituents and shown to have improved effectiveness of killing bacteria. As indicated previously, increasing the positive charge on methylene blue correlated positively with increased APDT efficacy than native methylene blue [19,20], mainly because substitution with cationic functional groups improved the binding and uptake of the PS by bacteria relative to the parent structure [19][20][21]. For example, Felgentrager et al revealed that derivatization of methylene blue with tertiary ammonium substituents increased the uptake by microbial cells due to the fact that these substituents imposed a greater cationic charge in a pH-equilibrated aqueous solution, and therefore more avid bacterial cell binding, compared to primary or secondary substituents [19].…”

A Perspective on the Trends and Challenges Facing Porphyrin‐Based Anti‐Microbial Materials

“…In 2015, they further observed that increasing polarity of these MB derivatives through the ammonium groups would promote better uptake by the bacteria. A maximum photodynamic efficacy of > 5log 10 steps (99.999%) against S. aureus and E. coli has been reached in 10 minutes at 5 μM concentration . Phenothiazines are increasing utilized in PDI due to their absorption band between 600 and 800 nm.…”

“…A maximum photodynamic efficacy of > 5log 10 steps (99.999%) against S. aureus and E. coli has been reached in 10 minutes at 5 µM concentration. [ 88 ] Phenothiazines are increasing utilized in PDI due to their absorption band between 600 and 800 nm. The disadvantage of phenothiazine dyes is their inherent toxicity, which reduces their therapeutic effectiveness.…”

Strategies to optimize photosensitizers for photodynamic inactivation of bacteria

“…The authors stated better polarity characteristics of these MB derivatives by the use of these functional groups facilitating a better attachment of even uptake by the bacteria. In another study additional positive charges or hydrogen bond acceptors on symmetrically substituted phenothiazinium molecules were investigated [42]. The focus in this study was that positive charges were added on both sides of the core group of methylene blue.…”