A Novel Dicationic Boron Dipyrromethene-based Photosensitizer for Antimicrobial Photodynamic Therapy against Methicillin-Resistant Staphylococcus aureus

Abstract

Background: We report herein the synthesis of a novel dicationic boron dipyrromethene derivative (compound 3) which is symmetrically substituted with two trimethylammonium styryl groups.

Methods: The antibacterial photodynamic activity of compound 3 was determined against sixteen methicillin-resistant Staphylococcus aureus (MRSA) strains, including four ATCC type strains (ATCC 43300, ATCC BAA-42, ATCC BAA-43, and ATCC BAA-44), two mutant strains [AAC(6')-APH(2") and RN4220/pUL5054], and ten nonduplicate clinical strains of hospital- and community-associated MRSA. Upon light irradiation, the minimum bactericidal concentrations of compound 3 were in the range of 1.56-50 μM against all the sixteen MRSA strains. Interestingly, compound 3 was not only more active than an analogue in which the ammonium groups are not directly connected to the n-conjugated system (compound 4), but also showed significantly higher (p < 0.05) antibacterial potency than the clinically approved photosensitizer methylene blue. The skin irritation of compound 3 during topical application was tested on human 3-D skin constructs and proven to be non-irritant in vivo at concentrations below 1.250 mM. In the murine MRSA infected wound study, the colony forming unit reduction of compound 3 + PDT group showed significantly (p < 0.05) higher value (>2.5 log₁₀) compared to other test groups except for the positive control.

Conclusion: In conclusion, the present study provides a scientific basis for future development of compound 3 as a potent photosensitizer for photodynamic therapy for MRSA wound infection.

Keywords: Methicillin resistant Staphylococcus aureus; antibacterial potency.; antimicrobial photodynamic therapy; boron dipyrromethene; murine wound infection model; photosensitizer.

Fig. (1)

Timeline for in vivo aPDT study. Four treatment cycles were performed at Day 2, 3, 5 and 9. After the 4^th treatment, the mice were sacrificed for the CFU count.

Fig. (2)

Synthetic scheme of Compound 3.

Fig. (3)

Structure of Compound 4.

Fig. (4)

Cell viability of EPI-200 cells under different treatments. They were treated with DPBS [Negative control (NC)], 5% SDS [Positive control (PC)], 12.5 μM of Compound 3 [equivalent to MBC against MRSA RN 4220/pUL5054 (Compound 3 (MBC)], 125 μM of Compound 3 [equivalent to 10 x MBC against MRSA RN 4220/pUL5054 (Compound 3 (10 x MBC)] or 1250 μM of Compound 3 [equivalent to 100 x MBC against MRSA RN 4220/pUL5054 (Compound 3 (100 x MBC)]. Experimental data are expressed as mean ± SD (n=3). Means that do not share a letter are significantly different. Mean cell viability > 50% for the Compound 3 (MBC, 10 x MBC or 100 x MBC) implies that Compound 3 did not pose any skin irritation on human 3-D skin construct at concentrations below 1250 μM. (A higher resolution / colour version of this figure is available in the electronic copy of the article).

Fig. (5)

In vivo aPDT efficiency against MRSA RN4220/pUL5054 infected wound mediated by 1250 μM concentration of Compound 3. Experimental data are expressed as mean ± SD (n=7). Means that do not share a letter are significantly different. Compound 3 + PDT cohort showed significantly lower (p < 0.05) bacterial load after 4 treatment cycles, compared to all other treated groups, except positive control (2% Fusidic cream). (A higher resolution / colour version of this figure is available in the electronic copy of the article).