Quantitative determination of hydrophobic compound entrapment in dipalmitoylphosphatidylcholine liposomes by differential scanning calorimetry

Abstract

In this paper the feasibility of determing the percentage of drug entrapment (PDE) in dipalmitoylphosphatidylcholine (DPPC) liposomes by using differential scanning calorimetry (DSC) was assessed. Three different lipophilic compounds, namely testosterone, vitamin E acetate and retinoic acid, were encapsulated in multilamellar liposomes and their PDE values were determined by DSC and by conventional separative techniques, such as dialysis and centrifugation. PDE values of testosterone, vitamin E acetate and retinoic acid determined by DSC analysis, applying Van't Hoff equation, were 6.3 ± 0.1, 68.9 ± 0.7 and 87.8 ± 0.6, respectively. Dialysis was performed at different time intervals, 3, 6, 9 and 12 h, on DPPC liposomes entrapping the compounds tested. The duration of the dialysis process strongly affected the quantification of testosterone incorporated in DPPC liposomes while it slightly influenced the determination of vitamin E acetate and retinoic acid PDE values. Centrifugation of DPPC liposomes entrapping the compounds tested was carried out at different rpm (6000, 9000 and 12000 rpm). No significant difference was observed comparing PDE values obtained centrifugating at different rpm. Comparing PDE values obtained by dialysis (at 9 and 12 h) and centrifugation for each compound with the corresponding PDE values determined by DSC, linear relationships were observed. These results suggest that DSC analysis could be regarded as a suitable technique for the quantitative determination of the percentage of hydrophobic compound entrapped in DPPC liposomes.