EFFECT OF 1-HYDROXYETHYLIDENE-1,1-BISPHOSPHONATE ON MEMBRANE-MEDIATEDCALCIUM-PHOSPHATE FORMATION IN MODEL LIPOSOMAL SUSPENSIONS

The phosphonate,1-hydroxyethylidene-1,1-bisphosphonate (HEBP), was exa mined for its effect on calcium phosphate precipitation in pH 7.4, 22 degrees C suspensions of 7:2:1 phosphatidylcholine (PC):dicetylphospha te (DCP):cholesterol (Chol) and 7:1:1 PC:phosphatidylserine (PS):Chol liposomes. HEBP (0.5-50 mu mol/l) in the suspending medium had little, if any, effect on precipitation that formed inside phosphate-rich (50 mmol/l) aqueous interiors of liposomes as a result of ionophore (X-53 7A) driven 2.25 mmol/l Ca2+ influxes from the medium. On the other han d, HEBP had a significant negative impact on the subsequent spread of the precipitate into the surrounding medium when the latter was made m etastable with 1.5 mmol/l total inorganic phosphate (PO4). The inhibit ory effect of HEBP was more strongly felt in the 7PC:1PS:1Chol liposom al suspensions, with only 1 mu mol/l HEBP needed to effectively block extraliposomal precipitation compared to 7.5 mu mol/l for 7PC:2DCP:1Ch ol suspensions. Direct encapsulation of HEBP (1-1000 mu mol/l) togethe r with PO4 in the aqueous cores of 7PC:2DCP:1Chol liposomes reduced so mewhat (similar to 30%) intraliposomal yields and delayed but did not block extraliposomal precipitate development. These results provide a possible physicochemical explanation for the suppression of matrix ves icle initiated mineralization in ectopically-induced osteoid tissue of HEBP treated mice [1]. In particular, the liposome results suggest th at membrane phosphatidylserine interactions with mineral may enhance H EBP's effectiveness in vivo.