Liposomal Formulations Of Vincristine Research Articles

Validation of an UPLC-MS-MS Method for Quantitative Analysis of Vincristine in Human Urine After Intravenous Administration of Vincristine Sulfate Liposome Injection

Vincristine sulfate liposome injection (VSLI) is a liposomal formulation of vincristine (VCR) sulfate, being developed for the systemic treatment of cancer. In this paper, we have developed and validated a method to quantify VCR in human urine to obtain the urinary excretion of VCR after intravenous administration of VSLI. The analyte was extracted from urine samples using liquid-liquid extraction after addition of vinblastine (VBL, used as internal standard) and chromatographed on an Acquity UPLC HSS T3 column with a gradient mobile phase at a flow rate of 0.4 mL/min. The multiple reactions monitoring transitions of m/z 413.2 → 353.2 and m/z 406.2 → 271.6 were used to quantify VCR and VBL, respectively. The lower limit of quantification was 0.5 ng/mL with a precision (RSD%) of 5.7% and an accuracy (RE%) of 6.7%. The calibration curve was linear up to 100.0 ng/mL. Intraday precision and accuracy ranged from 0.8 to 11.0% and from -12.4 to 11.3%, respectively. Interassay precision and accuracy ranged from 8.0 to 10.1% and from -7.7 to 3.6%, respectively. No significant matrix effect was observed for VCR. The method was successfully applied for pharmacokinetic study of VSLI to investigate the route and extent of VCR urinary excretion in Chinese subjects with lymphoma.

Development of Pegylated Liposomal Vincristine Using Novel Sulfobutyl Ether Cyclodextrin Gradient: Is Improved Drug Retention Sufficient to Surpass DSPE–PEG-Induced Drug Leakage?

The purpose of this study is to develop novel stable PEGylated liposome vincristine formulations with optimal antitumor efficacy. Vincristine could interact with negatively charged distearoylphosphatidylethanolamine-polyethylene glycol (DSPE-PEG), leading to rapid drug release from vesicles. To improve drug retention, vincristine was loaded into vesicles using sulfobutyl ether cyclodextrin (sbe-CD) as trapping agent. Despite that, vincristine could not form a precipitate with sbe-CD; the aggregation status of vincristine/sbe-CD inside vesicles must be complicated because drug retention was considerably improved in vivo. Theoretical consideration revealed that the release constant K equals to pA(m)k(1)k(2)/([H(+)](i)[sbe(-)](i)V(i) ), which can be used to expound why increasing drug/lipid ratio induced decreased vincristine circulation half-life. The stabilization effect afforded by sbe-CD was sufficient to surpass DSPE-PEG-induced drug leakage, so PEGylated liposomal vincristine formulations with prolonged circulation half-life (t(1/2): from 43.6 to 70.0 h) could be achieved, of which the formulation pLV-c-2.9-3 exhibited optimal antitumor effects and reduced toxicity. The strategy might be used to load other vinca alkaloids into PEGylated liposomes and improve their retention inside vesicles.

Characterization of highly stable liposomal and immunoliposomal formulations of vincristine and vinblastine

PurposeLiposome and immunoliposome formulations of two vinca alkaloids, vincristine and vinblastine, were prepared using intraliposomal triethylammonium sucroseoctasulfate and examined for their ability to stabilize the drug for targeted drug delivery in vivo.MethodsThe pharmacokinetics of both the encapsulated drug (vincristine or vinblastine) and liposomal carrier were examined in Sprague Dawley rats, and the in vivo drug release rates determined. Anti-HER2 immunoliposomal vincristine was prepared from a human anti-HER2/neu scFv and studied for targeted cytotoxic activity in cell culture, and antitumor efficacy in vivo.ResultsNanoliposome formulations of vincristine and vinblastine demonstrated similar pharmacokinetic profiles for the liposomal carrier, but increased clearance for liposome encapsulated vinblastine (t1/2 = 9.7 h) relative to vincristine (t1/2 = 18.5 h). Immunoliposome formulations of vincristine targeted to HER2 using an anti-HER2 scFv antibody fragment displayed a marked enhancement in cytotoxicity when compared to non-targeted liposomal vincristine control; 63- or 253-fold for BT474 and SKBR3 breast cancer cells, respectively. Target-specific activity was also demonstrated in HER2-overexpressing human tumor xenografts, where the HER2-targeted formulation was significantly more efficacious than either free vincristine or non-targeted liposomal vincristine.ConclusionsThese results demonstrate that active targeting of solid tumors with liposomal formulations of vincristine is possible when the resulting immunoliposomes are sufficiently stabilized.

In Vivo Anti-Tumor Efficacy of Vincristine Sulfate Liposomes Injection (VSLI, Marqibo®) in Multiple Myeloma

Multiple myeloma (MM) is a bone marrow-based malignancy characterized by expansion of plasma cells that produce monoclonal immunoglobulin. Although many new options are available to treat MM, patients develop resistance to these agents. Thus, new therapeutic options are necessary for patients. Vincristine has been used to treat MM but its neurotoxic side effects and low level of anti-MM activity have limited its clinical use. Pharmacokinetic studies on a different formulation of this vinca alkaloid, vincristine sulfate liposomes injection (VSLI, Marqibo®), have shown that the altered distribution and elimination phases with this drug may lead to increased exposure within the tumor versus traditional vincristine. Thus, this new agent offers the potential to both reduce the neurotoxicity and increase the anti-MM effects compared to standard vincristine. In the present study, we evaluated the anti-MM effects of Marqibo and vincristine using two severe combined immunodeficiency (SCID) murine models of human MM. These models were developed from intramuscular (i.m.) implantation of bone marrow biopsies from a MM patient before (LAGκ-1A) and following (LAGκ-1B) the development of both melphalan and bortezomib resistance clinically and have been successfully passaged. One week following implantation i.m., mice were treated with Marqibo, vincristine or vehicle alone. Marqibo was administered intravenously (i.v.) once weekly for three weeks at 0.5, 1, and 2.5 mg/kg and vincristine was given three times per week (M, W, F) i.v. at 0.3 mg/kg. Mice were bled in order to obtain sera to measure human immunoglobulin (Ig) G levels and tumor volume measured weekly. hIgG levels were measured by ELISA and tumor volume using standard calipers. Treatment of LAGk-1A–bearing mice with Marqibo at 0.5, 1 and 2.5 mg/kg inhibited paraprotein secretion as determined by hIgG levels (P = 0.0001; P = 0.0002; P = 0.0055, respectively) compared to mice receiving vehicle. A reduction in tumor volume, compared to control mice, was also observed in mice treated with Marqibo at the specified doses (P = 0.0001; P = 0.0001; P = 0.0001, respectively) and also when compared to vincristine (P = 0.0006; P = 0.0003; P = 0.0001, respectively). In fact, mice treated with vincristine showed no reduction in tumor volume compared to vehicle alone-treated mice. Marqibo at 0.5 mg/kg contained almost half the quantity of free vincristine compared to vincristine administered at 0.3 mg/kg, yet a significant inhibition of both human paraprotein secretion and reduction of tumor volume was observed in LAGκ-1A-bearing mice treated with this dose of Marqibo. The vincristine dose was administered three times more often than Marqibo, contained almost 100% more free vincristine compared to Marqibo at 0.5 mg/kg and most importantly was less effective. Mice bearing a slower growing MM tumor, LAGκ-1B, have recently begun treatment using the same doses and schedules as the LAGκ-1A study. This study is currently ongoing but preliminary data show inhibition of tumor volume growth and inhibition of IgG levels using Marqibo at 0.5, 1 and 2.5 mg/kg (hIgG; P = 0.011, P = 0.0045 and P = 0.0060, respectively). The results of this study show that Marqibo, a novel liposomal formulation of vincristine, produces anti-MM effects in this SCID-hu model of human MM whereas vincristine showed no anti-MM tumor volume growth activity. Further studies are currently being conducted to explore the optimal schedule and effects of this new agent in combination with other active MM agents. These studies should provide the rationale for further clinical development of Marqibo for the treatment of MM patients.

Characterization of the drug retention and pharmacokinetic properties of liposomal nanoparticles containing dihydrosphingomyelin

The drug retention and circulation lifetime properties of liposomal nanoparticles (LN) containing dihydrosphingomyelin (DHSM) have been investigated. It is shown that replacement of egg sphingomyelin (ESM) by DHSM in sphingomyelin/cholesterol (Chol) (55/45; mol/mol) LN results in substantially improved drug retention properties both in vitro and in vivo. In the case of liposomal formulations of vincristine, for example, the half-times for drug release ( T 1/2) were approximately 3-fold longer for DHSM/Chol LN as compared to ESM/Chol LN, both in vitro and in vivo. Further increases in T 1/2 could be achieved by increasing the drug-to-lipid ratio of the liposomal vincristine formulations. In addition, DHSM/Chol LN also exhibit improved circulation lifetimes in vivo as compared to ESM/Chol LN. For example, the half-time for LN clearance (Tc 1/2) at a low lipid dose (15 μmol lipid/kg, corresponding to 8 mg lipid/kg body weight) in mice was 3.8 h for ESM/Chol LN compared to 6 h for DHSM/Chol LN. In addition, it is also shown that DHSM/Chol LN exhibit much longer half-times for vincristine release as compared to LN with the “Stealth” lipid composition. It is anticipated that DHSM/Chol LN will prove useful as drug delivery vehicles due to their excellent drug retention and circulation lifetime properties.

Pharmacokinetics and Urinary Excretion of Vincristine Sulfate Liposomes Injection in Metastatic Melanoma Patients

Vincristine sulfate liposomes injection (VSLI) is a liposomal formulation of vincristine encapsulated in sphingosomes composed of sphinogomyelin and cholesterol (58/42; mol/mol). The pharmacokinetics and urinary excretion of VSLI were evaluated in 12 patients with metastatic melanoma after single-dose (2.0 mg/m2 every 2 weeks = 1 cycle) and multiple-dose (cycle 3, pharmacokinetics only) administrations (intravenous infusion over 1 hour). After VSLI infusion, total (released and encapsulated) vincristine concentrations in plasma remained relatively constant for 3 to 12 hours and thereafter declined, with interpatient variability seen in the rate of decline resulting in monoexponential or biexponential profiles. The area under the plasma concentration-time curve from time zero to infinity of total vincristine in plasma ranged from 4933 to 40495 h.ng/mL and total clearance ranged from 131 to 445 mL/h. The volume of distribution at steady state was 2650 +/- 731 mL, indicating VSLI was mainly confined within the plasma. The released vincristine concentrations in plasma were below the level of quantitation in 95% of samples. The pharmacokinetic parameters were similar between cycles 1 and 3, and trough plasma levels of total vincristine were below the level of quantitation of 1 ng/mL. Approximately 8% of the injected dose was excreted in the urine as unchanged vincristine (7%) or N-desformylvincristine (0.8%). Overall, VSLI exhibited a longer circulation half-life and higher area under the plasma concentration-time curve compared to conventional vincristine, whereas its route of elimination remained unchanged.

Therapeutically optimized rates of drug release can be achieved by varying the drug-to-lipid ratio in liposomal vincristine formulations

The anti-tumor efficacy of liposomal formulations of cell cycle dependent anticancer drugs is critically dependent on the rates at which the drugs are released from the liposomes. Previous work on liposomal formulations of vincristine have shown increasing efficacy for formulations with progressively slower release rates. Recent work has also shown that liposomal formulations of vincristine with higher drug-to-lipid ( D/ L) ratios exhibit reduced release rates. In this work, the effects of very high D/ L ratios on vincristine release rates are investigated, and the antitumor efficacy of these formulations characterized in human xenograft tumor models. It is shown that the half-times ( T 1/2) for vincristine release from egg sphingomyelin/cholesterol liposomes in vivo can be adjusted from T 1/2 = 6.1 h for a formulation with a D/ L of 0.025 (wt/wt) to T 1/2 = 117 h (extrapolated) for a formulation with a D/ L ratio of 0.6 (wt/wt). The increase in drug retention at the higher D/ L ratios appears to be related to the presence of drug precipitates in the liposomes. Variations in the D/ L ratio did not affect the circulation lifetimes of the liposomal vincristine formulations. The relationship between drug release rates and anti-tumor efficacy was evaluated using a MX-1 human mammary tumor model. It was found that the antitumor activity of the liposomal vincristine formulations increased as D/ L ratio increased from 0.025 to 0.1 (wt/wt) ( T 1/2 = 6.1–15.6 h respectively) but decreased at higher D/ L ratios ( D/ L = 0.6, wt/wt) ( T 1/2 = 117 h). Free vincristine exhibited the lowest activity of all formulations examined. These results demonstrate that varying the D/ L ratio provides a powerful method for regulating drug release and allows the generation of liposomal formulations of vincristine with therapeutically optimized drug release rates.

Vincristine Liposomal - INEX

INEX Pharmaceuticals is developing a liposomal formulation of vincristine [Onco TCS, vincacine, VSLI, Vincristine sulfate liposomes for injection] for the treatment of relapsed aggressive non-Hodgkin's lymphoma (NHL) and other cancers. It is being developed using INEX's proprietary drug-delivery technology platform called the transmembrane carrier systems (TCS), which enables the targeted intracellular delivery of various therapeutic agents. Liposomal vincristine is expected to have certain advantages over the existing standard preparation of vincristine because the use of TCS technology enables the vincristine to circulate in the blood for longer, accumulate in the tumour, and be released over an extended period of time at the tumour site. The application of TCS technology to any agent, including vincristine, has the potential to increase the efficacy and decrease the side effects of the agent. INEX decided in 1998 to focus on gaining approval for liposomal vincristine in the treatment of relapsed aggressive NHL because no standard therapy was approved for this indication. In 1999, liposomal vincristine was granted accelerated development status by the US FDA, which enables the FDA to approve it based on the surrogate endpoint of a single clinical trial. In addition, the FDA granted liposomal vincristine fast track status in August 2000. In April 2001, INEX and Elan Corporation formed a joint venture for the development and commercialisation of liposomal vincristine, with both companies contributing assets to the venture including worldwide rights to the product and intellectual property rights. The joint venture was called IE Oncology. However, in June 2002, Elan announced that it was going to focus its business strategy on three specific areas, which would not include cancer therapies. INEX announced it had regained 100% ownership of liposomal vincristine in April 2003, by reacquiring the 19.9% equity interest held by Elan and in addition retaining a fully paid-up licence to Elan's intellectual property pertaining to liposomal vincristine. All obligations to Elan under the agreement will be met through three milestone payments totalling $8 million. Some of the milestones may be paid in shares valued at the then current market price. In January 2004, INEX and Enzon Pharmaceuticals formed a strategic partnership to develop and commercialise liposomal vincristine. Under the terms of the agreement, Enzon receives the exclusive North American commercialisation rights for liposomal vincristine for all indications. INEX will receive upfront and milestone payments as well as a percentage of commercial sales. Additionally, the formation of this partnership triggered a US$3 million payment from INEX to the former joint venture partner, Elan Corporation. Nine clinical trials of liposomal vincristine are currently being conducted, including one phase I/II trial and eight phase II trials. A phase II/III trial was completed in December 2002. In September 2003, Inex commenced a 'rolling submission' for liposomal vincristine by submitting the first of three major sections of the NDA to the FDA. The second major section was submitted to the FDA in December 2003. INEX expects to complete the filing with the submission of the clinical section of the NDA in the first quarter of 2004. Dow Jones Newswires reported on 1 October 2001 that the CEO of INEX expects Onco TCS to achieve sales of between $US100 and $US400 million annually for the company. FDA approval was then predicted for late 2002 or early 2003.

Pharmacokinetics and antitumor activity of vincristine entrapped in vesicular phospholipid gels.

In vivo antitumoral activity, pharmacokinetics (PK) and biodistribution of a new liposomal formulation of vincristine (VCR-Lip) were compared to VCR in aqueous solution (VCR-Conv). VCR was entrapped into a vesicular phospholipid gel (VPG) consisting of densely packed liposomes. Redispersed VCR-containing VPG (VCR-Lip) consisted of 54% liposomally entrapped and 46% free VCR. In vivo efficacy of VCR-Lip versus VCR-Conv was tested using the s.c. growing human small cell lung carcinoma LXFS 650 and the human mammary carcinoma MX1. PK and biodistribution were evaluated using radiolabeled drug and lipid in LXFS 650 tumor-bearing mice. VCR-Lip at a dose of 1.0 mg/kg (dose near the maximum tolerated dose) led to partial remissions in the MX1 tumor xenograft model (T/C=3.9%). VCR-Conv at an equitoxic dose of 0.6 mg/kg produced only a tumor growth inhibition (T/C=7.0%). In LXFS 650 tumor-bearing mice, VCR-Lip was highly active at doses of 0.75 (T/C=0.7%) and 1.0 (T/C=0.0%) mg/kg, and complete tumor regressions were observed. In contrast, equitoxic doses of VCR-Conv (0.6 mg/kg) resulted only in less pronounced tumor remissions (T/C=4.1%). The PK study revealed that VCR-Lip achieved an over 10-fold higher plasma AUC (22.6 microg x h/ml) than VCR-Conv (2.16 microg x h/ml). Moreover, tumor drug levels were 2.3-fold higher when VCR was injected as VCR-Lip in comparison to VCR-Conv. In some cases, however, VCR-Lip as well as blank VPG appeared to be toxic. We conclude that VCR-Lip is an effective VCR delivery system with superior antitumor activity compared to VCR-Conv. The enhanced efficacy can be explained by sustained release and passive tumor targeting.

Clinical and Preclinical Pharmacology of Liposomal Vincristine

Vincristine is one of the most commonly administered anticancer drugs and is active in a wide range of indications including non-Hodgkin's lymphomas, acute lymphocytic leukemias and lung cancer. Administration of vincristine in long-circulating liposomes may be expected to result in increased accumulation of drug at tumor sites due to “passive targeting” or “disease-site targeting” effects arising from the more permeable vasculature in these regions. Further, for liposomes with appropriate drug release characteristics, extended exposure of tumor cells to vincristine would result from liposomal delivery. The combination of increased drug delivery and extended duration of drug exposure may be expected to result in increased efficacy, particularly because vincristine is a cell-cycle specific drug. It is shown that vincristine can be encapsulated in large unilamellar vesicles (diameter β 100 nm) using a pH gradient (interior acidic) approach. Further, the efficacy of liposomal formulations of vincristine in animal models is highly sensitive to the drug release rate in vivo. A liposomal formulation with drug retention characteristics such that more than 50% of the vincristine is retained in the carrier 24 h following i.v. injection exhibits significantly improved antitumor efficacy in A431 xenograft and P388 murine tumor models in comparison to either free drug or leakier liposomal formulations. The clinical activity of liposomal vincristine has been investigated in relapsed or refractory non-Hodgkin's lymphoma patients at a dose level of 2 mg/m2 every two weeks. Of 83 registered patients, there were 24 responses in 68 evaluable patients. The responses according to histology are: Indolent-13%; Transformed-42%; Aggressive-45%. There were no serious cases of myelosuppression or any toxic deaths. It is concluded that liposomal vincristine can be given at high doses, is active and well tolerated and is rarely neurotoxic or myelosuppressive in these heavily pretreated patients. It appears that the benefits of low toxicity and enhanced efficacy noted in the tumor models are also observed in the clinical setting. A multicenter pivotal Phase II trial of liposomal vincristine in relapsed and refractory non-Hodgkin's lymphoma has been approved by the US FDA and is ongoing.

Preclinical pharmacology, toxicology and efficacy of sphingomyelin/cholesterol liposomal vincristine for therapeutic treatment of cancer.

To establish the pharmacodynamic relationships between drug biodistribution and drug toxicity/efficacy, a comprehensive preclinical evaluation of sphingomyelin/cholesterol (SM/chol) liposomal vincristine and unencapsulated vincristine in mice was undertaken. Pharmaceutically acceptable formulations of unencapsulated vincristine and liposomal vincristine at drug/lipid ratios of 0.05 or 0.10 (wt/wt) were evaluated for toxicity, antitumor activity and pharmacokinetics following intravenous administration. Mice given liposomal vincristine at 2 mg/kg vincristine had concentrations of vincristine in blood and plasma at least two orders of magnitude greater then those achieved after an identical dose of unencapsulated drug. One day after administration of the liposomal vincristine, there were at least tenfold greater drug quantities, relative to unencapsulated vincristine, in the axillary lymph nodes, heart, inguinal lymph nodes, kidney, liver, skin, small intestines and spleen. Increased plasma and tissue exposure to vincristine as a result of encapsulation in SM/chol liposomes was not associated with increased drug toxicities. Treatment of the murine P388 ascitic tumor with a single intravenous dose of unencapsulated drug at 2, 3 and 4 mg/kg, initiated 1 day after tumor cell inoculation, resulted in a 33 to 38% increase in lifespan. In contrast, long-term survival rates of 50% or more were achieved in all groups treated with the SM/chol liposomal vincristine formulations at doses of 2, 3 and 4 mg/kg. At the 4 mg/kg dose, eight of ten and nine of ten animals survived past day 60 when treated with SM/chol liposomal vincristine prepared at the 0.05 and 0.1 drug/lipid ratios, respectively. Overall, increased and prolonged plasma concentrations of vincristine achieved by liposomal encapsulation were correlated with dramatically increased antitumor activity in comparison with the unencapsulated drug, but no correlations could be established between pharmacokinetic parameters and toxicity.

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

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.

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

This study reports on the development of a liposomal formulation of vincristine with significantly enhanced stability and biological properties. The in vitro and in vivo pharmacokinetic, tumour delivery and efficacy properties of liposomal vincristine formulations based on sphingomyelin (SM) and cholesterol were compared with liposomes composed of distearoylphosphatidylcholine (DSPC) and cholesterol. SM/cholesterol liposomes had significantly greater in vitro stability than did similar DSPC/cholesterol liposomes. SM/cholesterol liposomes also had significantly improved biological properties compared with DSPC/cholesterol. Specifically, SM/cholesterol liposomes administered intravenously retained 25% of the entrapped vincristine after 72 h in the circulation, compared with 5% retention in DSPC/cholesterol liposomes. The improved retention properties of SM/cholesterol liposomes resulted in plasma vincristine levels 7-fold higher than in DSPC/cholesterol liposomes. The improved circulation lifetime of vincristine in SM/cholesterol liposomes correlated with increased vincristine accumulation in peritoneal ascitic murine P388 tumours and in subcutaneous solid A431 human xenograft tumours. Increased vincristine delivery to tumours was also accompanied by increased anti-tumour efficacy. Treatment with SM/cholesterol liposomal formulations of vincristine resulted in greater than 50% cures in mice bearing ascitic P388 tumours, an activity that could not be achieved with the DSPC/cholesterol formulation. Similarly, treatment of mice with severe combined immunodeficiency (SCID) bearing solid human A431 xenograft tumours with SM/cholesterol vincristine formulations delayed the time required for 100% increase in tumour mass to > 40 days, compared with 5 days, 7 days and 14 days for mice receiving no treatment or treatment with free vincristine or DSPC/cholesterol formulations of vincristine respectively.

Pharmacology of liposomal vincristine in mice bearing L1210 ascitic and B16/BL6 solid tumours

Vincristine pharmacokinetic, tumour uptake and therapeutic characteristics were investigated here in order to elucidate the processes underlying the enhanced efficacy observed for vincristine entrapped in small (120 nm) distearoylphosphatidylcholine/cholesterol liposomes. Plasma vincristine levels after intravenous (i.v.) injection are elevated more than 100-fold in the liposomal formulation compared with free drug in tumour-bearing as well as non-tumour-bearing mice over 24 h. Biodistribution studies demonstrate that the extent and duration of tumour exposure to vincristine is dramatically improved when the drug is administered i.v. in liposomal form. Specifically, 72 h trapezoidal area under the curve values for liposomal vincristine in the murine L1210 ascitic and B16/BL6 solid tumours are 12.9- to 4.1-fold larger, respectively, than observed for free drug. Similar to previous results with the L1210 model, increased drug delivery to the B16 tumour results in significant inhibition of tumour growth, whereas no anti-tumour activity is observed with free vincristine. Comparisons of drug and liposomal lipid accumulation in tumour and muscle tissue indicate that the enhanced efficacy of liposomal vincristine is related predominantly to drug delivered by liposomes to the tumour site rather than drug released from liposomes in the circulation. Consequently, improvements in liposomal vincristine formulations must focus on factors that increase uptake of liposomes into tumour sites as well as enhance liposomal drug retention in the circulation.

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

Vincristine 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 sites. As a consequence of the improved accumulation of vincristine at tumor sites, liposomal formulations of vincristine exhibit dramatically improved efficacy against a variety of ascitic and solid murine and human tumors than does free vincristine. Liposomal vincristine is expected to be of wide utility in a variety of human malignancies.