Antimicrobial Photodynamic Activity of the Zn(II) Phthalocyanine RLP068/Cl Versus Antimicrobial-Resistant Priority Pathogens

Abstract

The emergence and spread of antimicrobial resistance among pathogens are significantly reducing available therapeutic options, highlighting the urgent need for novel and complementary treatment strategies. Antimicrobial photodynamic therapy (aPDT) is a promising alternative approach that can overcome antimicrobial resistance through a multitarget mechanism of action, exerting direct bactericidal and fungicidal effects with minimal risk of resistance development. Although aPDT has shown efficacy against a variety of pathogens, data on its activity against large collections of clinical multidrug-resistant strains are still limited. In this study, we assessed the antimicrobial activity of the photosensitizer RLP068/Cl combined with a red light-emitting LED source at 630 nm (Molteni Farmaceutici, Italy) against a large panel of Gram-negative and Gram-positive bacterial strains harboring relevant resistance traits and Candida species. Our results demonstrated the significant microbicidal activity of RLP068/Cl against all of the tested strains regardless of their resistance phenotype, with particularly prominent activity against Gram-positive bacteria (range of bactericidal concentrations 0.05-0.1 µM), which required significantly lower exposure to photosensitizer compared to Candida and Gram-negative species (range 5-20 µM). Overall, these findings support the potential use of RLP068/Cl-mediated aPDT as an effective therapeutic strategy for the management of localized infections caused by MDR organisms, particularly when conventional therapeutic options are limited.

Keywords: Zn(II) phthalocyanine; multidrug-resistant pathogens; photoinactivation.

Figure 1

Antimicrobial activity of compound RLP068/Cl against Gram-negative strains (A,B: Enterobacterales strains, C,D: P. aeruginosa strains, E,F: A. baumannii strains). Graphs plot in log₁₀ scale the means of replicates of viable cell counts (CFU/mL) for each strain and represent photoinactivation after exposure to RLP068/Cl and to a light source (A,C,E panels) or without light (B,D,F panels), respectively. Differences between means from strains exposed to different concentrations of RLP068/Cl and unexposed strains were statistically analyzed by one-way ANOVA test and significance has been reported on the graph for p-values < 0.05. ****: p < 0.0001.

Figure 2

Antimicrobial activity of compound RLP068/Cl against Gram-positive strains (A,B: Staphylococcus species, C,D: Enterococcus species). Graphs plot in log₁₀ scale the means of replicates of viable cell counts (CFU/mL) for each strain and represent photoinactivation after exposure to RLP068/Cl and to a light source (A,C panels) or without light (B,D panels), respectively. Differences between means from strains exposed to different concentrations of RLP068/Cl and unexposed strains were statistically analyzed by one-way ANOVA test and significance has been reported on the graph for p-values < 0.05. *: p < 0.05, ****: p < 0.0001.

Figure 3

Antimicrobial activity of compound RLP068/Cl against Candida species. Graphs plot in log₁₀ scale the means of replicates of viable cell counts (CFU/mL) for each strain. Photoinactivation after exposure to RLP068/Cl and to a light source or without light is represented in panel (A,B), respectively. Differences between means from strains exposed to different concentrations of RLP068/Cl and unexposed strains were statistically analyzed by one-way ANOVA test and significance has been reported on the graph for p-values < 0.05. *: p < 0.05, ****: p < 0.0001.