Liposomal entrapment of the neutrophil-derived peptide indolicidin endows it with in vivo antifungal activity

Abstract

Indolicidin, a cationic tridecapeptide amide isolated from the granules of bovine neutrophils, has been found to possess potent antimicrobial activity in vitro but its nonselective toxicity could restrict its therapeutic utility. We found that the concentration at which indolicidin disrupts washed human red blood cell membranes coincided with the concentration at which indolicidin self associates. Because of a preponderance of hydrophobic residues, we believed that indolicidin would partition into liposomes which would restrict its exchange with biological tissues and consequently reduce its toxicity. Fluorescence spectroscopy of indolicidin added to 100 nm liposomes comprised of POPC, POPC/cholesterol (60:40 mol%), DPPC, or DPPC/cholesterol (60:40) revealed a large blue-shift and an increase in intensity of the emission profile indicating insertion into the bilayer. Of the lipids tested, POPC exhibited the highest degree of indolicidin binding as determined by fluorescence and encapsulation efficiency. By sequestering indolicidin within the lipid bilayer of 100 nm POPC liposomes we significantly reduced its toxicity to CHO/K1 cells. Likewise, the systemic toxicity of liposomal indolicidin in Balb/c mice was decreased dramatically relative to aqueous solutions; the maximum dose at which no deaths occurred was 0.4 mg/kg for free indolicidin versus 40 mg/kg for indolicidin-POPC. Because of this decrease in toxicity, we were able to administer liposomally encapsulated material at significantly higher concentrations than unencapsulated aqueous material and achieve efficacy in treating animals systemically infected with Aspergillus fumigatus. Liposomal but not free indolicidin was found to be effective in obtaining cures. This report is the first description of the in vivo therapeutic activity of a neutrophil-derived antimicrobial peptide and suggests that liposomal treatment modalities will provide effective strategies for endowing this class of compounds with pharmacological utility.