Potentiation by potassium iodide reveals that the anionic porphyrin TPPS4 is a surprisingly effective photosensitizer for antimicrobial photodynamic inactivation

Abstract

We recently reported that addition of the non-toxic salt, potassium iodide can potentiate antimicrobial photodynamic inactivation of a broad-spectrum of microorganisms, producing many extra logs of killing. If the photosensitizer (PS) can bind to the microbial cells, then delivering light in the presence of KI produces short-lived reactive iodine species, while if the cells are added after light the killing is caused by molecular iodine produced as a result of singlet oxygen-mediated oxidation of iodide. In an attempt to show the importance of PS-bacterial binding, we compared two charged porphyrins, TPPS4 (thought to be anionic and not able to bind to Gram-negative bacteria) and TMPyP4 (considered cationic and well able to bind to bacteria). As expected TPPS4+light did not kill Gram-negative Escherichia coli, but surprisingly when 100mM KI was added, it was highly effective (eradication at 200nM+10J/cm of 415nm light). TPPS4 was more effective than TMPyP4 in eradicating the Gram-positive bacteria, methicillin-resistant Staphylococcus aureus and the fungal yeast Candida albicans (regardless of KI). TPPS4 was also highly active against E. coli after a centrifugation step when KI was added, suggesting that the supposedly anionic porphyrin bound to bacteria and Candida. This was confirmed by uptake experiments. We compared the phthalocyanine tetrasulfonate derivative (ClAlPCS4), which did not bind to bacteria or allow KI-mediated killing of E. coli after a spin, suggesting it was truly anionic. We conclude that TPPS4 behaves as if it has some cationic character in the presence of bacteria, which may be related to its delivery from suppliers in the form of a dihydrochloride salt.

Antimicrobial photodynamic inactivation is potentiated by the addition of selenocyanate: Possible involvement of selenocyanogen?

“…We recently reported that the addition of potassium iodide dramatically potentiated the aPDI killing of Gram-negative (and Gram-positive) bacteria by photoactivated MB [9], Photofrin [8], RB [10], and TPPS4 [6]. The singlet oxygen generated could oxidize iodide anion to produce enough iodine to eradicate both Gram-positive and Gram-negative bacteria.…”

“…The singlet oxygen generated could oxidize iodide anion to produce enough iodine to eradicate both Gram-positive and Gram-negative bacteria. Interestingly TPPS4, which we initially assumed to be an anionic porphyrin, turned out to be cationic in character due to the two positively charged pyrrole nitrogen atoms and could bind to E. coli although it could not penetrate into the interior [6]. The mechanism of the potentiation by KSeCN appears to depend on the photochemical production of an antibacterial species formed in a reaction between the photoactivated PS and the added KSeCN.…”

“…However its activity against Gram-negative species is very low to non-existent. We also used a tetrasulfonated porphyrin (TPPS4) that was previously considered to be anionic in character, but which we recently showed also has some cationic character [6]. We have recently reported that a surprisingly wide variety of inorganic salts can potentiate the aPDI killing of microbial cells [7].…”

Comparison of thiocyanate and selenocyanate for potentiation of antimicrobial photodynamic therapy

“…This strategy can be applied to gram‐positive bacteria, gram‐negative bacteria and also to fungi, and is effective in animal models of localized infections . A variety of different salts can be employed, including potassium iodide , potassium bromide , sodium azide , potassium thiocyanate (KSCN) and potassium selenocyanate (KSeCN) . Potassium iodide, potassium bromide and KSCN are nontoxic; KSeCN has low toxicity, while sodium azide has moderate toxicity.…”

“…We have also previously reported that KSeCN (at concentrations up to 100 mM) could potentiate aPDT mediated not only by MB, but also by Rose Bengal (RB) and also by 5,10,15,20‐tetrakis(4‐sulfonatophenyl)porphyrin dihydrochloride (TPPS4). We recently reported that the supposedly anionic porphyrin TPPS4 behaved more like a cationic porphyrin, and functioned as a surprisingly effective antimicrobial PS especially when potentiated by potassium iodide (KI). Considering the similarities between the chemical behaviors of thiocyanate and selenocyanate, we wanted to compare the relative activities of these two salts in potentiating aPDT bacterial killing, and investigate whether the mechanisms of action were similar or different.…”