Effect of 1-hydroxyethylidene-1,1-bisphosphonate on membrane-mediated calcium phosphate formation in model liposomal suspensions

Abstract

The bisphosphonate, 1-hydroxyethylidene-1,1-bisphosphonate (HEBP), was examined for its effect on calcium phosphate precipitation in pH 7.4, 22°C suspensions of 7∶2∶1 phosphatidylcholine (PC):dicetylphosphate (DCP):cholesterol (Choi) and 7∶1∶1 PCphosphatidylserine (PS).Chol liposomes. HEBP (0.5–50 μmol/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-537A) driven 2.25 mmol/l Ca²⁺ influxes from the medium. On the other hand, HEBP had a significant negative impact on the subsequent spread of the precipitate into the surrounding medium when the latter was made metastable with 1.5 mmol/l total inorganic phosphate (PO₄). The inhibitory effect of HEBP was more strongly felt in the 7PC∶1PS∶1Chol liposomal suspensions, with only 1 μmol/l HEBP needed to effectively block extraliposomal precipitation compared to 7.5 μmol/l for 7PC∶2DCP∶1Chol suspensions. Direct encapsulation of HEBP (1–1000 μmol/l) together with PO₄ in the aqueous cores of 7PC∶2DCP∶Cho1 liposomes reduced somewhat (~ 30%) intraliposomal yields and delayed but did not block extraliposomal precipitate development. These results provide a possible physicochemical explanation for the suppression of matrix vesicle initiated mineralization in ectopically-induced osteoid tissue of HEBP treated mice [1]. In particular, the liposome results suggest that membrane phosphatidylserine interactions with mineral may enhance HEBP's effectiveness in vivo.