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UBC Theses and Dissertations

Therapeutic applications of ceramide lipids for apoptosis induction Shabbits, Jennifer A.

Abstract

The emerging role of ceramide lipids in apoptosis and an increased understanding of their involvement in multidrug resistance (MDR) has revealed new opportunities for manipulating ceramide levels in order to achieve specific therapeutic objectives. The research presented in this thesis focused on the relationship between ceramide, MDR and apoptosis. Direct and indirect approaches for modulating intracellular ceramide levels were investigated in an attempt to chemosensitize MDR tumors and induce apoptosis. Inhibition of pro-apoptotic ceramide conversion to its non-cytotoxic glucosylceramide metabolite was shown to sensitize two human MDR breast cancer cell lines to the cytotoxic effects of tubulin-binding chemotherapy drugs. Enhanced sensitization was correlated with increased ceramide, suggesting that therapeutic manipulations aimed at increasing endogenous ceramide should promote apoptosis. On the basis of these results, the feasibility of delivering therapeutic amounts of exogenous ceramides to cells was then investigated. After evaluating different chain length ceramides it was determined that synthetic C₆-ceramide was internalized and cytotoxic to cells whereas naturally occurring C₁₆-ceramide was neither internalized nor cytotoxic. This difference established the importance of intracellular delivery as a prerequisite to apoptosis induction by exogenous ceramides. Liposome-based delivery systems were then introduced in an attempt to overcome the limitations associated with intracellular delivery of natural ceramide. Physically stable liposomes containing up to 50 mole percent C₁₆-ceramide in the lipid bilayer were successfully formulated. These liposomes were internalized by J774 macrophage cells in vitro and induced apoptosis with similar potency to free C6-ceramide. In order to translate this encouraging data to an in vivo model it was first necessary to evaluate the behavior of these liposomes in the circulation. Pharmacokinetic studies demonstrated in vivo stability over 24 hours following iv bolus administration. The antitumor activity of these liposomes was then evaluated in the J774 ascites tumor model. Optimal antitumor activity was observed following intraperitoneal administration of C₁₆-ceramide liposomes on days 1, 5, and 9. This corresponded to a statistically significant increase in animal survival of 43.5% over non-ceramide control liposomes. Taken together, this research provides evidence for the rational design of ceramide-based liposomes as a novel approach for cancer chemotherapy.

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