| Summary: | Vincristme is a widely used antineoplastic agent. It is a cell cycle specific drug
arresting cell growth during metaphase. Since tumors contain dividing cells distributed
throughout the cell cycle, it is important to achieve as long an exposure time as possible.
Encapsulation of drugs within liposomes can prolong circulation time in vivo as well as
reduce toxic side effects. This thesis examines the effect of liposomal encapsulation on
reducing toxicity as well as improving antineoplastic activity of vincristine by altering drug
pharmacokinetic and biodistribution behaviour.
The first focus of investigation demonstrates that encapsulation of vincristine within
liposomes greatly reduces soft-tissue toxicity of the drug. Subcutaneous injections of
liposomal vincristine are shown to demonstrate minimal toxic effects whereas similar
injections of free vincristine result in gross necrosis and ulceration. Free drug is rapidly
cleared from the area of injection. Liposomal drug remains at the area of injection much
longer, but remains trapped within the liposomes. Slow release rates presumably prevent it
from exerting cytotoxic effects.
The next topic concerns improving the retention of vincristine within liposomes. The
influence of lipid composition, internal pH and internal buffering capacity on the retention
properties of vincristine loaded into LUVs in response to transmembrane pH gradients has
been assessed. It is shown that increasing the (saturated) acyl chain length of the
phosphatidyicholine molecule, increasing the internal buffering capacity, and decreasing the
internal pH all result in increased drug retention. Further, a study of the pH dependence on the rates of accumulation indicate that uptake proceeds via the neutral form of the vincristine
molecule. This uptake is associated with an activation energy of 37 kcallmol for DSPCICho1
LUVs. It is shown that the major improvement in drug retention in vitro is achieved by
employing low initial internal pH values, where 90% retention is obtained over 24 h for an
initial internal pH of 2. Improved retention over the same system with an internal pH of 4
in viva was also observed where a drug-to-lipid ratio approximately 4-fold greater at 24 h
was maintained.
The third area of investigation concerns the incorporation of cationic lipids to further
improve vincristine retention within liposomes. The influence of both the incorporation of
10 mol% cationic lipid (AL-l, stearylamine, or sphingosine) into DSPC/Chol (55:45;
mol:mol) vesicles and lowering the internal pH to pH 2.0 on the circulation life-time and
antitumor activity of liposomal vincristine systems (drug-to-lipid ratio of 0.1:1) has been
examined. With an internal pH of 2.0, the incorporation of 10 mol% cationic lipid is shown
to significantly increase drug retention within the liposomes without affecting lipid clearance
times. The resulting increase in plasma drug concentration seen by the incorporation of 10
mol% sphingosine results in a significant increase in therapeutic activity against the P388
lymphocytic leukemia cell line in viva.
The final area of investigation examines two different methods for increasing the
circulation longevity of vincristine encapsulated in liposomes. The first involves
incorporation of the ganglioside GM₁, which acts to increase the circulation longevity of
liposomal carriers, while the second approach relies on modification of the vincristine
encapsulation procedure to enhance drug retention. It is shown that these approaches are
synergistic and increase the circulation half-life of vincristine from approximately one hour to greater than 12 hours. This results in a dramatic improvement in the therapeutic activity
of liposomal vincristirie as measured using a murine P388 lymphocytic leukemia model. At
doses above 2 mgfkg the optimized liposomal vincristine formulation cures greater than 50%
of mice bearing the P388 tumor, whereas free vincristine results in no cures. The optimized
formulation is also shown to significantly increase solid tumor uptake of vincristine within
the Lewis Lung tumor model and result in improved therapeutic activity. === Medicine, Faculty of === Biochemistry and Molecular Biology, Department of === Graduate
|
|---|
