Light, Dyes, and Action: Photodynamic Inactivation of Leishmania amazonensis Using Methylene Blue, New Methylene...

Photodynamic therapy (PDT), which combines light and dyes, has emerged as a cost-effective, selective, and less toxic alternative to conventional chemotherapy for cutaneous leishmaniasis (CL), offering potential benefits for millions, especially those who are socioeconomically vulnerable. Therefore, this study investigated the in vitro effects of methylene blue (MB), a widely used photosensitizer with proven clinical efficacy, along with its derivatives-new methylene blue-NMB, NMB-B, and NMB-P-in PDT against L. amazonensis promastigotes, using a red LED device. Inhibitory concentrations (ICs) and 168 h proliferation curves were obtained. The production of reactive oxygen species (ROS) and the mechanism of cell death induction were analyzed by flow cytometry. PDT enhanced leishmanicidal effects compared to non-PDT conditions, reducing ICs by up to 85% and outperforming miltefosine, reaching the submicromolar range (IC_25NMB-P = 0.73 ± 0.16 µM, p < 0.05). The proliferation curve showed a consistent inhibitory effect, with MB exhibiting a greater decline than miltefosine, a pattern also observed with MB derivatives. PDT also increased ROS production by up to 5-fold and induced apoptosis-like cell death, characterized by AV⁺ parasites (up to 51.49 ± 2.90%, p < 0.0001). The results demonstrated that the tested dyes effectively eliminated L. amazonensis promastigotes, highlighting the potential of the NMB derivatives as photosensitizers and supporting further investigations.