Rendering conventional liposomes sensitive to mild acidity

Purpose: Liposomes were among the earliest nano drug carriers to gain wide acceptance in the clinic. This was mostly attributed to advances in liposome technology for active drug loading and being able to modify the vesicle size to suit tumour accumulation while evading rapid in vivo clearance through surface coating. The formidable challenge that still remains is to target their action in response to specific stimuli or disease state. Purpose of this study was to show proof of concept that membrane active peptides can be engineered and inserted (post vesicle formation) into conventional unilamellar liposomes for achieving specific release of payload with superior pH sensitivity than traditional approaches.Methods: Lipopeptides were made by solid phase synthesis combining t-Boc and Fmoc chemistries. SUVs were prepared by drying a lipid film, hydration with payload (dyes or enzymes) and extrusion through polycarbonate membranes. The release of payload in vitro was followed fluorometrically for dyes or colourimetrically for the enzymes. Antimicrobial assays were conducted by colony counting.Results: Peptide quality was confirmed by HPLC (>95%) and mass spectrometry. Once integrated with liposomes pH dependent release of payload was evident in response to as little as 1.0 unit change in pH. Quantitative measurements using intracellular infection model and tumour xenograft models revealed significantly higher unloading for peptide treated liposomes and against controls. We were able to observe peptide dependent intracellular release (corresponding to endosomal pH) of an antibiotic which resulted in almost 10 fold reduction in CFU when recovered from macrophage borne infection. In extension of this study pH change could be detected from growth of as few as 100 microorganisms. Conclusion: Conventional liposomes can be rendered bioresponsive by mixing with modified membrane active peptides. However a careful design to fine tune the pH triggering window is essential.