Development of improved bendamustin-liposomes

Abstract

Bendamustin is an alkylating anticancer agent which is currently in routine use for the treatment of different types of cancer. The drug is very unstable in serum due to hydrolysis; the half life of the first part of the serum elimination curve is about 6-10 minutes. The rapid degradation of the drug in serum impairs its cytostatic action within a short period of time, and frequent application of relatively high doses is required. This, in turn, leads to dose-limiting systemic toxicity. Incorporation of bendamustin into liposomes might be a promising way to prolong its half life in plasma, and thus improve the efficiency and toxicity profile of the drug. Up to now only a few attempts to incorporate bendamustin into liposomes are found in literature. However, none of these have been successful and reached clinical practice. Recently, a new technique for liposome preparation, dual asymmetric centrifugation (DAC), has been suggested which is suitable for making liposomes immidiatly prior to application (bed-side preparation). In a previous study a protocol for liposomes made of phosphatidyl choline and cholesterol was developed and used for direct entrapment of bendamustin. However, the formulation turned out to be unstable in terms of rapid efflux of bendamustin out of the liposomes.

In this study, a new liposomal formulation of bendamustin was developed using the DAC. The intention was to improve the stability of the liposome formulation by obtaining a reduced leakage of drug. Liposome release of incorporated drug was investigated by incubation of liposomes under physiological conditions; 37 °C, with further assay of the samples with respect to loss of incorporated drug over time. For this, cation-exchange chromatography and RP-HPLC was used. It turned out that bendamustin in its zwitterionic form tended to diffuse through the lipid-membrane more readily than both the cationic and anionic forms. In order to reduce the amount of zwitterionic molecules, and hence efflux, a buffered system with a pH of 2.0 in the liposome interior was chosen. Compared to the previous formulation, the new formulation showed an increase in encapsulation efficiency as well as a slower efflux of drug when incubated in phosphate buffered saline pH 7.4 at 37 ºC; EE= 61% ± 2 as compared to 44 % ± 3 and t1/2= 3 h as compared to 1.5 h.

Stability of the new formulation was assayed with respect to intact bendamustin as well as lipid at 23 °C over 24 hours. For this RP-HPLC and HPTLC were used, respectively. The results showed that the formulation is stable enough to be used within the same day as a bed-side preparation. Furthermore, stability of the new bendamustin-liposomes was compared to the free drug in cell culture medium at 37 ºC. Unfortunately, the bendamustin-liposomes showed a minor improvement in stability as compared to the free bendamustin only; the half life was prolonged to 20 minutes for the liposomes (14 minutes for the solution). Finally, an attempt of active loading of bendamustin was performed by the means of a pH gradient between the liposome interior and exterior. An EE of 14 % was observed. Further optimization of the protocol will be needed to render active loading an attractive alternative.