Preparation and characteristics of dichloromethylene diphosphonate-containing liposomes

Dichloromethylene diphosphonate (DMDP), encapsulated in liposomes and administered intravenously in mice, will eliminate all macrophages in spleen and liver. DMDP can be incorporated in liposomes to a maximum of 15 mM, if less than 12 mM is encapsulated not all macrophages will be eliminated (since the animals could not be injected with more liposomes). To determine DMDP content of the liposomes before in vivo administration, an in vitro test system was developed. This method is based on the competition for calcium binding by either DMDP or murexide. Murexide is a metallochromatic indicator which gives a distinct wavelength shift after binding of calcium. The decrease in absorbance at 510 nm of the murexide-calcium complex, due to the addition of DMDP, was used as a reliable (s.d. 5 per cent) value for measurement of DMDP concentrations. The possible use of this system in the study of calcium binding and transport over artificial biomembranes is discussed.