Rapid assessment of liposomal stability in blood by an aqueous nitroxide spin label.

An electron spin resonance method using an aqueous nitroxide spin label, 2,2,6,-tetramethyl-piperidine-N-oxyl-4-trimethyl-ammonium, for rapid assessment of liposome stability in blood is presented. The retention of the nitroxide in liposomes is measured by its electron spin resonance signal intensity, a procedure which does not require separation of the sample from the blood. Any nitroxide that is released from the liposomes is reduced by external ascorbic acid which is added to the sample. The method permits kinetic studies on the integrity of liposomes in the presence of destabilizing factors such as detergent, blood, or alteration in temperature.