Liposome-encapsulated ISMN: a novel nitric oxide-based therapeutic agent against Staphylococcus aureus biofilms

Staphylococcus aureus in its biofilm form has been associated with recalcitrant chronic rhinosinusitis with significant resistance to conventional therapies. This study aims to determine if liposomal-encapsulation of a precursor of the naturally occurring antimicrobial nitric oxide (NO) enhances its desired anti-biofilm effects against S. aureus, in the hope that improving its efficacy can provide an effective topical agent for future clinical use.

S. aureus ATCC 25923 biofilms were grown in-vitro using the Minimum Biofilm Eradication Concentration (MBEC) device and exposed to 3 and 60 mg/mL of the NO donor isosorbide mononitrate (ISMN) encapsulated into different anionic liposomal formulations based on particle size (unilamellar ULV, multilamellar MLV) and lipid content (5 and 25 mM) at 24 h and 5 min exposure times. Biofilms were viewed using Live-Dead Baclight stain and confocal scanning laser microscopy and quantified using the software COMSTAT2.

At 3 and 60 mg/mL, ISMN-ULV liposomes had comparable and significant anti-biofilm effects compared to untreated control at 24 h exposure (p = 0.012 and 0.02 respectively). ULV blanks also had significant anti-biofilm effects at both 24 h and 5 min exposure (p = 0.02 and 0.047 respectively). At 5 min exposure, 60 mg/mL ISMN-MLV liposomes appeared to have greater anti-biofilm effects compared to pure ISMN or ULV particles. Increasing liposomal lipid content improved the anti-biofilm efficacy of both MLV and ULVs at 5 min exposure.

Liposome-encapsulated "nitric oxide" is highly effective in eradicating S. aureus biofilms in-vitro, giving great promise for use in the clinical setting to treat this burdensome infection. Further studies however are needed to assess its safety and efficacy in-vivo before clinical translation is attempted.