Antimicrobial photodynamic therapy using tetra-cationic porphyrins with peripheral platinum(II) and palladium(II) complexes against multidrug-resistant bacteria isolated from canine otitis

This study aimed to evaluate the cytotoxicity and photodynamic antimicrobial activity of tetra-cationic porphyrins containing peripheral platinum(II) (3-PtTPyP and 4-PtTPyP) and palladium(II) (3-PdTPyP and 4-PdTPyP) complexes against multidrug-resistant (MDR) bacteria isolates obtained from canine otitis. Cytotoxicity was assessed using the MTT test on a canine kidney cell line to determine the non-toxic dose for canine species. The antibacterial activity against MDR isolates of Staphylococcus spp. and Escherichia coli, along with their respective reference strains, was evaluated using minimum inhibitory concentration, bactericidal methods, and time-kill curve under white light irradiation for 120 min. The non-cytotoxic concentration observed for 3-PtTPyP (0.05 µM) and 3-PdTPyP (0.06 µM) was lower than those for 4-PtTPyP and 4-PdTPyP (30.25 and 34.75 µM, respectively). Only 3-PtTPyP and 3-PdTPyP were effective against MDR Staphylococcus pseudintermedius at non-cytotoxic concentrations. For E. coli strains, 4-PdTPyP and 4-PtTPyP exhibited low activity only at the minimum bactericidal concentration (MBC). All porphyrins evaluated showed higher MBC than the non-cytotoxic concentration identified by the MTT test. This study demonstrated that the porphyrins 3-PtTPyP and 3-PdTPyP are promising photosensitizers for the inactivation of MDR S. pseudintermedius isolated from canine otitis, showing bactericidal effects under white light with low cytotoxicity. Conversely, porphyrins linked at the para-position were ineffective at non-cytotoxic concentrations, limiting their isolated use in in vivo applications.