Selective Targeting and Enhanced Photodynamic Inactivation of Methicillin-Resistant Staphylococcus aureus (MRSA) by a Decacationic Vancomycin–Mesochlorin Conjugate

<b>Background/Objectives:</b> Covalent conjugation of an antibiotic vancomycin (VCM) moiety and a photosensitizing mesochlorin (<i>m</i>Chl_Pd) unit into one molecular entity may present the potential to produce the combinatorial effect of both antibacterial photodynamic therapeutic (aPDT) and antibiotic activities. Our recent study indicated that a short linkage of <4 (C-C/or C-N) bond distances between these two moieties resulted in significant steric hindrance due to the bulky VCM, which greatly reduces the accessibility of the agent to the cell surface of methicillin-resistant <i>Staphylococcus aureus</i> (MRSA). The observed aPDT efficacy was found to be minimal. Here, we report that the revision of this linkage, via an EG₁₀ unit using identical synthetic procedures, was able to resolve the issue. <b>Methods:</b> Accordingly, the corresponding combinatorial aPDT-antibiotic compound, consisting of two covalently bonded quaternary ammonium pentacationic arms on the mesochlorin chromophore core, designated as VCMe-<i>m</i>Chl_Pd-N₁₀⁺ (LC40e⁺), was prepared for applications in antibacterial photodynamic inactivation (aPDI) activity. It was selected to investigate its enhanced binding and targeting ability to the surface of Gram-positive MRSA cells. Subsequent antibacterial photodynamic therapeutic (aPDT) activity to inactivate MRSA was investigated to substantiate the corresponding cell-surface binding effect on the efficacy of aPDT. <b>Results:</b> We found that the covalent combination of 10 positive charges and an MRSA-targeting vancomycin (VCM) moiety in a conjugated structure, functioning as an antibiotic-decacationic photosensitizing agent (Abx-dcPS), was capable of largely improving the MRSA cell-targeting efficiency. Importantly, variation in the chain length of the oligo(ethylene glycol) linker of VCMe-<i>m</i>Chl_Pd-N₁₀⁺, which was sufficiently long enough to properly separate the photoactive mesochlorin ring moiety from the VCM moiety within the molecular structure, resulted in significantly enhanced aPDT activity. The new conjugate provided nearly complete eradication (>6.5-log₁₀ colony-forming units (CFU) reduction) of MRSA cells in vitro. The aPDT efficacy followed the order Abx-dcPS (combinatorial decacationic) > dcPS (decacationic) >> nPS (nonionic). This order was also verified by the relative physical binding trend of these PSs using either nPS-, dcPS-, or Abx-dcPS-pretreated and pre-fixed MRSA cells in investigations of fluorescent confocal microscopy, UV-vis fluorescence spectroscopy, and transmission electron microscopy (TEM). <b>Conclusions:</b> Furthermore, the molecular conjugate of Abx-dcPS may provide covalent co-delivery of two drug components concurrently, which might also serve as an effective antibiotic agent after aPDT and potentially prevent the reoccurrence of MRSA-induced infection.