Nancy L. Boman

Identification of vesicle properties that enhance the antitumour activity of liposomal vincristine against murine L1210 leukemia

The influence of vesicle lipid composition, size and drug-to-lipid ratio on the antitumour activity of liposomal vincristine was assessed in the murine L1210 ascitic leukemia model. A pH gradient-dependent entrapment procedure was used to encapsulate vincristine and allowed such vesicle properties to be independently varied. Free vincristine delivered i.v. at the maximum tolerated dose (2.0 mg/kg) resulted in a 27.8% increase in the life span (ILS) of mice inoculated i.p. with L1210 cells. Encapsulation of the drug in egg phosphatidylcholine/cholesterol vesicles did not significantly increase the antitumour efficacy of vincristine (ILS, 38.9%). In contrast, administration of vincristine entrapped in vesicles composed of distearoylphosphatidylcholine (DSPC)/cholesterol resulted in ILS values as high as 133%. This enhanced antitumour activity of the DSPC/cholesterol formulations was sensitive to the size of the liposomes; increasing the vesicle size from 100 nm to 1 μm decreased the ILS from 133.3% to 55.6% at a drug dose of 2.0 mg/kg. Decreasing the drug-to-lipid ratio from 0.1∶1 to 0.05∶1 (w/w) had negligible effects on the activity of liposomal vincristine; however, a further decrease in the drug-to-lipid ratio to 0.01∶1 (w/w) decreased the antitumour potency at all drug doses studied. Pharmacology studies indicated that the antitumour activities of free and various liposomal forms of vincristine correlated well with the residence time of the drug in the circulation. These studies indicate that efforts to enhance the therapeutic activity of vincristine through liposome encapsulation must address not only the circulation life-time of the vesicle systems but also the capacity of the liposomes to retain entrapped drug in vivo.

Optimization of the retention properties of vincristine in liposomal systems

The influence of lipid composition, internal pH and internal buffering capacity on the retention properties of vincristine loaded into large unilamellar vesicle (LUV) systems in response to transmembrane pH gradients has been assessed. It is shown that increasing the (saturated) acyl chain length of the phosphatidylcholine 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 kcal/mol for DSPC/Chol 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 in vivo was also observed where a drug-to-lipid ratio approx. 4-fold greater at 24 h was maintained.

Ionophore-mediated uptake of ciprofloxacin and vincristine into large unilamellar vesicles exhibiting transmembrane ion gradients

A new method, based on the ion-translocating properties of the ionophores nigericin and A23187, is described for loading large unilamellar vesicles (LUVs) with the drugs vincristine and ciprofloxacin. LUVs composed of distearoylphosphatidylcholine/cholesterol (DSPC/Chol) (55:45 mol/mol) or sphingomyelin (SPM)/Chol (55:45 mol/mol) exhibiting a transmembrane salt gradient (for example, internal solution 300 mM MnSO4 or K2SO4; external solution 300 mM sucrose) are incubated in the presence of drug and, for experiments involving divalent cations, the chelator EDTA. The addition of ionophore couples the outward movement of the entrapped cation to the inward movement of protons, thus acidifying the vesicle interior. External drugs that are weak bases can be taken up in response to this induced transmembrane pH gradient. It is shown that both nigericin and A23187 facilitate the rapid uptake of vincristine and ciprofloxacin, with entrapment levels approaching 100% and excellent retention in vitro. Following drug loading, the ionophores can be removed by gel exclusion chromatography, dialysis, or treatment with biobeads. In vitro leakage assays (addition of 50% mouse serum) and in vivo pharmacokinetic studies (in mice) reveal that the A23187/Mn2+ system exhibits superior drug retention over the nigericin/K+ system, and compares favorably with vesicles loaded by the standard DeltapH or amine methods. The unique features of this methodology and possible benefits are discussed.

Encapsulation of Vincristine in Liposomes Reduces its Toxicity and Improves its Anti-Tumor Efficacy

AbstractVincristine is a potent therapeutic agent with activity against a variety of tumor types. It is a cell-cycle specific agent which has exhibited enhanced anti-tumor activity when delivered in liposomal form. Vincristine can be encapsulated into large unilamellar vesicles in response to a transmembrane pH gradient with trapping efficiencies approaching 100%. The extent of vincristine encapsulation, and the subsequent retention of the drug within the liposomes, both in vitro and in vivo, are strongly dependent on the lipid composition of the liposome and on the magnitude of the transmembrane pH gradient. Liposomal formulations of vincristine have been optimized for both liposome circulation longevity, drug retention characteristics and in vivo antitumor activity. When compared to free vincristine, these formulations significantly increase the levels of vincristine remaining in the plasma after i.v. administration. These formulations also significantly increase the delivery of vincristine to tumor sit...

Vincristine-induced dermal toxicity is significantly reduced when the drug is given in liposomes.

Abstract A problem associated with the intravenous delivery of vincristine concerns drug extravasation at the site of injection or infusion. This can result in extensive local soft-tissue damage. A new formulation of vincristine has recently been developed based on encapsulation of the drug in liposomes. The liposomal drug is somewhat less toxic and substantially more efficacious than free drug. The studies described here assessed, using a murine model of drug extravasation, whether vincristine encapsulation in liposomes influences drug-induced dermal toxicity. It was shown that subcutaneous injection of vincristine in liposomes does not result in the gross skin necrosis and ulceration observed following injection of free drug. Histological analysis of the dermal tissue surrounding the injection site suggests that free drug induces a pronounced inflammatory reaction as judged by the presence of infiltrating leukocytes. In contrast, the liposomal formulation of vincristine engenders a mild prolonged inflammatory condition. These toxicological studies were correlated with an evaluation of drug retention at the site of administration. It was shown using radiolabelled vincristine as a drug marker, that free vincristine is rapidly eliminated from the injection site. In contrast, the level of drug at the site of injection was far greater when the drug was given in liposomal form.