Sphingomyelin-cholesterol liposomes significantly enhance the pharmacokinetic and therapeutic properties of vincristine in murine and human tumour models

Contact hypersensitivity: a simple model for the characterization of disease-site targeting by liposomes

A murine model of delayed-type hypersensitivity (DTH) is characterized with respect to liposome accumulation at a site of inflammation. Mice were sensitized by painting the abdominal region with a solution of 2,4-dinitrofluorobenzene (DNFB) and inflammation was induced 5 days later by challenging the ear with a dilute solution of DNFB. The inflammatory response was readily monitored by measuring ear thickness (edema) and radiolabeled leukocyte infiltration. Maximum ear swelling and cellular infiltration occurred 24 h after the epicutaneous challenge with the ear returning to normal size after approximately 72 h. We demonstrate that large unilamellar vesicles (LUV) accumulate at the site of inflammation to a level more than 20-fold higher than that measured in the untreated ear. Vesicle delivery to the ear correlated with increased vascular leakage resulting from endothelium remodeling in response to DNFB challenge, and was not a consequence of increased local tissue blood volume. Extravasation occurred only during the first 24 h after ear challenge; after this time the permeability of the endothelium to vesicles returned to normal. We further showed that LUV with a diameter of 120 nm exhibit maximum levels of accumulation, that a polyethylene glycol surface coating does not increase delivery, and that the process can be inhibited by the application of topical corticosteroids at the time of induction. These data and the inflammation model are discussed with respect to developing lipid-based drug delivery vehicles designed to accumulate at inflammatory disease sites.

Phase I study of liposomal vincristine

PURPOSE

A phase I study of vincristine encapsulated inside 120-nm-diameter distearoylphosphatidylcholine-cholesterol liposomes was performed. The primary objectives were to determine the maximum-tolerated dose (MTD), recommended phase II dose, toxicity, and pharmacokinetics of liposomal vincristine (ONCO-TCS).

PATIENTS AND METHODS

Twenty-five patients with histologically confirmed malignancies were enrolled and assessable. Vincristine doses were increased from 0.5 mg/m2 to 1.0, 1.5, 2.0, 2.4, and 2.8 mg/m2 with cohorts of three or more patients per dose level. A total of 64 courses of ONCO-TCS were administered intravenously once every 3 weeks. The pharmacokinetics of total vincristine content in plasma were determined using a high-performance liquid chromatography method.

RESULTS

Patients were treated with vincristine doses up to 2.8 mg/m2; however, 2.4 mg/m2 was defined as the MTD and 2.0 mg/m2 as the phase II recommended dose. Pain and obstipation were the dose-limiting toxicites. Other toxicities were fever, rigors, fatigue, myalgias, and peripheral neuropathy. Hematologic toxicity was mild. All patients who were treated with doses above 1.5 mg/m2 received in excess of 2.0 mg of vincristine, with doses as high as 6.2 mg. One partial response was seen in a patient with pancreatic cancer. Tumor response not meeting partial response criteria was seen in two other patients. Pharmacokinetic studies revealed significantly elevated concentrations of total vincristine, but parameters varied and were not directly correlated with toxicity or response.

CONCLUSION

The ability to administer elevated doses of vincristine, as well as indications of efficacy, suggests that ONCO-TCS warrants further clinical investigation in a phase II setting.

Intravenous pretreatment with empty pH gradient liposomes alters the pharmacokinetics and toxicity of doxorubicin through in vivo active drug encapsulation

Liposomes have been used widely to improve the therapeutic activity of pharmaceutical agents. The traditional approach for such applications has been to formulate the pharmaceutical agent in liposomes prior to administration in vivo. In this report we demonstrate that liposomes exhibiting a transmembrane pH gradient injected intravenously (iv) can actively encapsulate doxorubicin in the circulation after iv administration of free drug. Small (110 nm) liposomes composed of phosphatidylcholine (PC)/cholesterol (Chol, 55:45 mol:mol) exhibiting a pH gradient (inside acidic) were administered iv 1 h prior to free doxorubicin, and plasma drug levels as well as toxicity and efficacy were evaluated. Predosing with egg PC/Chol pH gradient liposomes increased the plasma concentration of doxorubicin as much as 200-fold compared to free drug alone as well as to predosing with dipalmitoyl PC/Chol pH gradient liposomes or EPC/Chol liposomes without a pH gradient. The ability of the liposomes to alter the pharmacokinetics of doxorubicin was dependent on the presence of a transmembrane pH gradient and correlated with the extent of doxorubicin uptake into the liposomes at 37 degreesC in pH 7.5 buffer, indicating that doxorubicin was being actively accumulated in the circulating liposomes. This in vivo drug loading was achieved over a range of doxorubicin doses (5 mg/kg-40 mg/kg) and was dependent on the dose of EPC/Chol liposomes administered prior to free doxorubicin injection. The altered pharmacokinetic properties of doxorubicin associated with in vivo doxorubicin encapsulation were accompanied by a decrease in drug toxicity and maintained antitumor potency. These results suggest that pretreatment with empty liposomes exhibiting a pH gradient may provide a versatile and straightforward method for enhancing the pharmacological properties of many drugs that can accumulate into such vesicle systems at physiological temperatures.

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.

Loading of doxorubicin into liposomes by forming Mn2+-drug complexes

A new procedure for loading doxorubicin into large unilamellar vesicles (LUVs) is characterized. It is shown that doxorubicin can be loaded into LUVs composed of sphingomyelin/cholesterol (55:45 mole/mole) in response to a transmembrane MnSO4 gradient in the absence of a transmembrane pH gradient. Complex formation between doxorubicin and Mn2+ is found to be a driving force for doxorubicin uptake. Uptake levels approaching 100% can be achieved up to a drug-to-lipid molar ratio of 0.5 utilizing an encapsulated MnSO4 concentration of 0.30 M. In vitro leakage assays show excellent retention properties over a 24 h period. The possible advantages of a liposomal formulation of doxorubicin loaded in response to entrapped MnSO4 are discussed.

Comparison of different hydrophobic anchors conjugated to poly(ethylene glycol): effects on the pharmacokinetics of liposomal vincristine

Poly(ethylene glycol) (PEG) conjugated lipids have been used to increase the circulation longevity of liposomal carriers encapsulating therapeutic compounds. PEG is typically conjugated to distearoylphosphatidylethanolamine (DSPE) via a carbamate linkage that results in a net negative charge on the phosphate moiety at physiological pH. It was anticipated that the presence of this negative charge could have deleterious effects on liposome pharmacokinetic characteristics. We describe here the synthesis of a new class of neutrally charged PEG-lipid conjugates in which the PEG moiety was linked to ceramide (CER). These PEG-CER conjugates were compared with PEG-DSPE conjugates for their effects on the pharmacokinetics of liposomal vincristine. PEG-CER (78% palmitic acid, C16) and PEG-DSPE achieved comparable increases in the circulation lifetimes of sphingomyelin/cholesterol (SM/chol) liposomes. However, PEG-DSPE significantly increased the in vitro and in vivo leakage rates of vincristine from SM/chol-based liposomes compared to vincristine leakage observed when PEG-CER was used. The increase in drug leakage observed in vitro that was due to the presence of PEG-DSPE was likely due to the presence of a negative surface charge. Analysis of the electrophoretic mobilities of these formulations suggested that the negative surface charges were shielded by approx. 80% by the PEG layer extending from the membrane surface. In contrast, formulations containing PEG-CER had no surface charge and no electrophoretic mobility. A comparison of the effects of the ceramide acyl chain length (C8 through C24) on the pharmacokinetics of SM/chol/PEG-CER formulations of vincristine demonstrated that longer acyl chains on the PEG-CER were associated with longer circulation lifetimes of the liposomal carriers and, consequently, higher plasma vincristine concentrations. These data suggest that the short chain PEG-ceramides underwent rapid partitioning from the vesicles after i.v. administration, whereas the longer chain PEG-ceramides had stronger anchoring properties in the liposome bilayers and partitioned slowly from the administered vesicles. These data demonstrate the utility of ceramide-based steric stabilizing lipids as well as the potential for developing controlled release formulations by manipulating the retention of the PEG-ceramide conjugate in liposome bilayers.

Recent advances in liposome technologies and their applications for systemic gene delivery

The recent clinical successes experienced by liposomal drug delivery systems stem from the ability to produce well-defined liposomes that can be composed of a wide variety of lipids, have high drug-trapping efficiencies and have a narrow size distribution, averaging less than 100 nm in diameter. Agents that prolong the circulation lifetime of liposomes, enhance the delivery of liposomal drugs to specific target cells, or enhance the ability of liposomes to deliver drugs intracellularly can be incorporated to further increase the therapeutic activity. The physical and chemical requirements for optimum liposome drug delivery systems will likely apply to lipid-based gene delivery systems. As a result, the development of liposomal delivery systems for systemic gene delivery should follow similar strategies.

Antibacterial efficacy against an in vivo Salmonella typhimurium infection model and pharmacokinetics of a liposomal ciprofloxacin formulation

The fluoroquinolone antibiotic ciprofloxacin has been encapsulated into large unilamellar vesicles (LUV) at efficiencies approaching 100%. Drug accumulation proceeded in response to a transmembrane gradient of methylammonium sulfate and occurred concomitantly with the efflux of methylamine. A mechanism for the encapsulation process is described. LUV composed of dipalmitoylphosphatidylcholine-cholesterol (DPPC/chol), distearoylphosphatidylcholine-cholesterol (DSPC/chol), or sphingomyelin-cholesterol (SM/chol) increased the circulation lifetime of ciprofloxacin after intravenous (i.v.) administration by > 15-fold. The retention of ciprofloxacin in liposomes in the circulation decreased in the sequence SM/chol > DSPC/chol > DPPC/chol. Increased circulation lifetimes were associated with enhanced delivery of the drug to the livers, spleens, kidneys, and lungs of mice. Encapsulation of ciprofloxacin also conferred significant increases in the longevity of the drug in the plasma after intraperitoneal administration and in the lungs after intratracheal administration in comparison to free ciprofloxacin. The efficacy of a single i.v. administration of an SM/chol formulation of ciprofloxacin was measured in a Salmonella typhimurium infection model. At 20 mg of ciprofloxacin per kg of body weight, the encapsulated formulation resulted in 10(3)- to 10(4)-fold fewer viable bacteria in the livers and spleens of infected mice than was observed for animals treated with free ciprofloxacin. These results show the utility of liposomal encapsulation of ciprofloxacin in improving the pharmacokinetics, biodistribution, and antibacterial efficacy of the antibiotic. In addition, these formulations are well suited for i.v., intraperitoneal, and intratracheal or aerosol administration.

Validation of a high-performance liquid chromatographic assay method for quantification of total vincristine sulfate in human plasma following administration of vincristine sulfate liposome injection

The validation of a high performance liquid chromatographic (HPLC) assay method for quantitation of total vincristine sulfate (VINC) in human plasma is described. VINC was extracted from plasma using BondElut CBA solid phase cartridges with vinblastine as the internal standard. Chromatography was accomplished using a Waters Symmetry C8 (250 mm x 4.6 mm i.d.) analytical column, a Waters Delta-Pak ODS guard column with a mobile phase of 34.9% water-0.1% diethylamine (pH 7.0)-40% acetonitrile-25% methanol pumped isocratically at 1.0 ml min(-1) with ultraviolet detection at 297 nm. Above the limit of quantitation of 28.6 ng ml(-1), the area ratio precision (R.S.D. range 3.33-11.6%) and accuracy of predicted values (R.S.D. range 8.56-23.8% with the limit of quantitation being the only value above 20%) were acceptable. The assay was linear from 28.6-2860 ng ml(-1) VINC in plasma. Recovery of VINC from plasma and VINC from plasma spiked with vincristine sulfate liposome injection ranged from 74.9-87.1%. Stability of VINC in plasma stored at -20 degrees C for at least 49 days and of extracted plasma samples was demonstrated. Potential interference in quantitation of VINC from commonly co-administered drugs was evaluated along with day-to-day variability. The assay procedure was found suitable for evaluation of VINC clinical pharmacokinetics in plasma following administration of vincristine sulfate liposome injection prepared using distearoylphosphatidylcholine (DSPC)/cholesterol liposomes for injection.

Recent advances in liposomal drug-delivery systems

Liposomal drug-delivery systems have come of age in recent years, with several liposomal drugs currently in advanced clinical trials or already on the market. It is clear from numerous pre-clinical and clinical studies that drugs, such as antitumor drugs, packaged in liposomes exhibit reduced toxicities, while retaining, or gaining enhanced, efficacy. This results, in part, from altered pharmacokinetics, which lead to drug accumulation at disease sites, such as tumors, and reduced distribution to sensitive tissues. Fusogenic liposomal systems that are under development have the potential to deliver drugs intracellularly, and this is expected to markedly enhance therapeutic activity. Advances in liposome design are leading to new applications for the delivery of new biotechnology products, such as recombinant proteins, antisense oligonucleotides and cloned genes.