The ultraviolet derivative spectrophotometric determination of neutral liposome-entrapped β-lactam antibiotics

A simple, non-invasive spectrophotometric assay to measure the concentration of some β-lactam antibiotics in turbid solutions containing liposomes was carried out. Since zero-order spectra gave strong interference because of sample turbidity, derivative spectrophotometry was used to enhance the spectral details. Derivative spectra showed bands in the ultraviolet region due to the presence of the cephalosporin and penicillin β-lactams. A linear relationship between derivative amplitudes and antibiotic concentration was found when antibiotic-containing liposome solutions were measured. A saturative trend in the liposome-encapsulation was observed. The antibiotic entrapment was lowered by increasing the cholesterol-phospholipid ratio in the mixture used for liposome preparation. After treatment of antibiotic-loaded liposomes with β-lactamase, a hydrolytic enzyme specific for β-lactams, the remaining antibiotic concentration decreased significantly, showing that some of the antibiotic was retained on the outer surface of the vesicles. © 1989.