The use of certain synthetic phospholipids to preserve pharmaceutical compositions from microbial contamination is described. The synthetic phospholipids have unique molecular arrangements wherein a phosphate group is linked to a quaternary ammonium functionality via a substituted-propenyl group, and the quaternary ammonium functionality is further linked to at least one long hydrocarbon chain. Such molecular arrangements are what make the phospholipids of formula (I) highly water soluble, e.g., the length of the hydrocarbon chain assists to maintain solubility and efficacy of the molecules for the uses described herein. The synthetic phospholipids described herein have been found to be particularly useful as antimicrobial preservatives for ophthalmic, otic and nasal pharmaceutical compositions, especially ophthalmic compositions. These compounds may also be utilized to disinfect contact lenses. The invention is based in-part upon a finding that the antimicrobial activity of the synthetic phospholipids is affected by the ionic strength of the compositions in which the compounds are contained. The provision of compounds having limited ionic strengths is therefore preferred.
