³¹P-NMR and differential scanning calorimetry studies for determining vesicle's drug physical state and fraction in alendronate liposomes

Background: A liposomal delivery system requires a complete understanding of the physicochemical characteristics of the drug-liposome system in order to predict their behavior and stability in-vitro and in-vivo. Objectives: Develop a rapid and simple experimental method to determine the fractions of the drug, alendronate (ALN), encapsulated and as a free form distributed in the liposomal suspension, and the physical state of the encapsulated drug. Methods: ³¹P-NMR measurements utilizing Ga⁺³ as a shifting reagent in comparison to HPLC determinations, theoretical calculations and differential scanning calorimetry (DSC) studies of various liposomal ALN formulations. Results: The ³¹P-NMR demonstrated that titrating liposomal ALN with increasing amounts of Ga⁺³ induced a significant shift in the exterior fraction without changing the interior fraction. Quantitative determination of the encapsulated and non-encapsulated fractions of ALN has been achieved at Ga⁺³ concentrations of 3.2-25mM. The DSC study revealed that none of the formulation ingredients is in a solid phase. Conclusions: ³¹P-NMR was found to be sensitive enough to allow accurate differentiation of the distributed fractions of ALN, encapsulated and the non-encapsulated free form. Based on theoretical calculations and DSC analysis it can be concluded that ALN is dissolved in the aqueous core of the liposome.