In vitro photodynamic treatment of Fusarium oxysporum conidia through the action of thiopyridinium and methoxypyridinium chlorins

Abstract

Antimicrobial photodynamic inactivation (PDI) has been explored as an innovative therapeutic and phytosanitary approach because it can be used to inactivate microbial vegetative forms and spores, without significant development of resistance to photosensitization and collateral damage to host tissues. The objective of this study was to assess the photodynamic antifungal/sporicidal activity of quaternized chlorins (Chls) with thiopyridinium or methoxypyridinium substituents. Free-base and zinc(II) thiopyridinium or methoxypyridinium Chls (1a, 1b, 2a, and 2b, respectively) were prepared and tested on Fusarium oxysporum conidia. The PDI tests were conducted with photosensitizer (PS) concentrations of 100, 25, 15 and 5 µM, under white light exposure at an irradiance of 50 mW.cm−2, applied during 60 min (light dose of 180 J.cm−2). A high PDI efficiency, corresponding to inactivation to the detection limit, was observed with all the molecules. The free-base thiopyridinium chlorin 1a was the most effective PS, requiring the lowest concentration and the shortest irradiation time, for the complete inactivation of conidia (15 µM, 15 min, 45 J.cm−2). Complete inactivation was also achieved with the other chlorins but a higher concentration (100 µM) of PS and, in the case of chlorin 1b, a longer irradiation period (30 min, 90 J.cm−2), were required. Because of the low concentrations and small energy doses required to inactivate resistant biological forms such as fungal conidia, these chlorins can be regarded as potent antifungal photodynamic drugs.