Anti-staphylococcal activity and mode of action of thioridazine photoproducts

Abstract

Antibiotic resistance became an increasing risk for population health threatening our ability to fight infectious diseases. The objective of this study was to evaluate the activity of laser irradiated thioridazine (TZ) against clinically-relevant bacteria in view to fight antibiotic resistance. TZ in ultrapure water solutions was irradiated (1-240 min) with 266 nm pulsed laser radiation. Irradiated solutions were characterized by UV-Vis and FTIR absorption spectroscopy, thin layer chromatography, laser-induced fluorescence, and dynamic surface tension measurements. Molecular docking studies were made to evaluate the molecular mechanisms of photoproducts action against Staphylococcus aureus and MRSA. More general, solutions were evaluated for their antimicrobial and efflux inhibitory activity against a panel of bacteria of clinical relevance. We observed an enhanced antimicrobial activity of TZ photoproducts against Gram-positive bacteria. This was higher than ciprofloxacin effects for methicillin- and ciprofloxacin-resistant Staphylococcus aureus. Molecular docking showed the Penicillin-binding proteins PBP3 and PBP2a inhibition by sulforidazine as a possible mechanism of action against Staphylococcus aureus and MRSA strains, respectively. Irradiated TZ reveals possible advantages in the treatment of infectious diseases produced by antibiotic-resistant Gram-positive bacteria. TZ repurposing and its photoproducts, obtained by laser irradiation, show accelerated and low-costs of development if compared to chemical synthesis.

Figure 1

(a) UV–Vis absorption spectra of unirradiated and 1–240 min irradiated TZ water solutions (dilution 0.2 mg/mL) in the 200–400 nm spectral range. Inset represents a close-up of the 280–400 nm range. (b) Peak wavelength behavior during irradiation. (c) Absorbance intensity behavior during irradiation.

Figure 2

(a) LIF spectra of 2 mg/ml TZ solution irradiated up to 240 min. (b) Peak wavelength behavior during irradiation. (c) LIF intensity behavior during irradiation.

Figure 3

FTIR spectra of unirradiated TZ and 120 min and 240 min irradiated TZ.

Figure 4

TLC plate of unirradiated and irradiated TZ visualized at 254 nm. Legend: Rf—retention factor.

Figure 5

The evolution of TZ and its photoproducts relative volumes, extracted from TLC plate via proper/selected software: (a) photoproducts with retention factor above 0.4; (b) photoproducts with retention factor below 0.4.

Figure 6

Docking simulations on MRSA PBP3 chain A ligand pocket for TZ and sulphoridazine, amino acid residues presented are at a VDW scaling factor 1.00 (atoms closer than distance = atom1 vdw + atom2 vdw) (a) TZ close to ligand pocket dotted green line H-bound: interaction between TZ and Leu 663 amino acid. (b) Sulforidazine in ligand pocket, dotted green line H-bound: interaction between sulforidazine and amino acids Thr 621, Ser 634 and Thr 603.