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

Real-Time Quantitative Evaluation of a Drug during Liposome Preparation Using a Probe-Type Raman Spectrometer

During the manufacturing process of liposome formulations, it is considered difficult to evaluate their physicochemical properties and biological profiles due to the complexity of their structure and manufacturing process. Conventional quality evaluation is labor-intensive and time-consuming; therefore, there was a need to introduce a method that could perform in-line, real-time evaluation during the manufacturing process. In this study, Raman spectroscopy was used to monitor in real time the encapsulation of drugs into liposomes and the drug release, which are particularly important quality evaluation items. Furthermore, Raman spectroscopy combined with partial least-squares (PLS) analysis was used for quantitative drug evaluation to assess consistency with results from UV-visible spectrophotometry (UV), a common quantification method. The prepared various ciprofloxacin (CPFX) liposomes were placed in cellulose tubes, and a probe-type Raman spectrophotometer was used to monitor drug encapsulation, the removal of unencapsulated drug, and drug release characteristics in real time using a dialysis method. In the Raman spectra of the liposomes prepared by remote loading, the intensities of the CPFX-derived peaks increased upon drug encapsulation and showed a slight decrease upon removal of the unencapsulated drug. Furthermore, the peak intensity decreased more gradually during the drug release. In all Raman monitoring experiments, the discrepancy between quantified values of CPFX concentration in liposomes, as measured by Raman spectroscopy combined with partial least-squares (PLS) analysis, and those obtained through ultraviolet (UV) spectrophotometry was within 6.7%. The results revealed that the quantitative evaluation of drugs using a combination of Raman spectroscopy and PLS analysis was as accurate as the evaluation using UV spectrophotometry, which was used for comparison. These results indicate the promising potential of Raman spectroscopy as an innovative method for the quality evaluation of liposomal formulations.

What We Need to Know about Liposomes as Drug Nanocarriers: An Updated Review

Liposomes have been attracted considerable attention as phospholipid spherical vesicles, over the past 40 years. These lipid vesicles are valued in biomedical application due to their ability to carry both hydrophobic and hydrophilic agents, high biocompatibility and biodegradability. Various methods have been used for the synthesis of liposomes, so far and numerous modifications have been performed to introduce liposomes with different characteristics like surface charge, size, number of their layers, and length of circulation in biological fluids. This article provides an overview of the significant advances in synthesis of liposomes via active or passive drug loading methods, as well as describes some strategies developed to fabricate their targeted formulations to overcome limitations of the "first-generation" liposomes.

Metal complex-based liposomes: Applications and prospects in cancer diagnostics and therapeutics

Metal complexes are of increasing interest as pharmaceutical agents in cancer diagnostics and therapeutics, while some of them suffer from issues such as limited water solubility and severe systemic toxicity. These drawbacks severely hampered their efficacy and clinical applications. Liposomes hold promise as delivery vehicles for constructing metal complex-based liposomes to maximize the therapeutic efficacy and minimize the side effects of metal complexes. This review provides an overview on the latest advances of metal complex-based liposomal delivery systems. First, the development of metal complex-mediated liposomal encapsulation is briefly introduced. Next, applications of metal complex-based liposomes in a variety of fields are overviewed, where drug delivery, cancer imaging (single photon emission computed tomography (SPECT), positron emission tomography (PET), and magnetic resonance imaging (MRI)), and cancer therapy (chemotherapy, phototherapy, and radiotherapy) were involved. Moreover, the potential toxicity, action of toxic mechanisms, immunological effects of metal complexes as well as the advantages of metal complex-liposomes in this content are also discussed. In the end, the future expectations and challenges of metal complex-based liposomes in clinical cancer therapy are tentatively proposed.

Computational and Experimental Approaches to Investigate Lipid Nanoparticles as Drug and Gene Delivery Systems

Lipid nanoparticles (LNPs) have been widely applied in drug and gene delivery. More than twenty years ago, DoxilTM was the first LNPs-based drug approved by the US Food and Drug Administration (FDA). Since then, with decades of research and development, more and more LNP-based therapeutics have been used to treat diverse diseases, which often offer the benefits of reduced toxicity and/or enhanced efficacy compared to the active ingredients alone. Here, we provide a review of recent advances in the development of efficient and robust LNPs for drug/gene delivery. We emphasize the importance of rationally combining experimental and computational approaches, especially those providing multiscale structural and functional information of LNPs, to the design of novel and powerful LNP-based delivery systems.

What Drives Innovation: The Canadian Touch on Liposomal Therapeutics

Liposomes are considered one of the most successful drug delivery systems (DDS) given their established utility and success in the clinic. In the past 40–50 years, Canadian scientists have made ground-breaking discoveries, many of which were successfully translated to the clinic, leading to the formation of biotech companies, the creation of research tools, such as the Lipex Extruder and the NanoAssemblr™, as well as contributing significantly to the development of pharmaceutical products, such as Abelcet^®, MyoCet^®, Marqibo^®, Vyxeos^®, and Onpattro™, which are making positive impacts on patients’ health. This review highlights the Canadian contribution to the development of these and other important liposomal technologies that have touched patients. In this review, we try to address the question of what drives innovation: Is it the individual, the teams, the funding, and/or an entrepreneurial spirit that leads to success? From this perspective, it is possible to define how innovation will translate to meaningful commercial ventures and products with impact in the future. We begin with a brief history followed by descriptions of drug delivery technologies influenced by Canadian researchers. We will discuss recent advances in liposomal technologies, including the Metaplex technology from the author’s lab. The latter exemplifies how a nanotechnology platform can be designed based on multidisciplinary groups with expertise in coordination chemistry, nanomedicines, disease, and business to create new therapeutics that can effect better outcomes in patient populations. We conclude that the team is central to the effort; arguing if the team is entrepreneurial and well positioned, the funds needed will be found, but likely not solely in Canada.

The influence of trapping agents on the antitumor efficacy of irinotecan liposomes: head-to-head comparison of ammonium sulfate, sulfobutylether-β-cyclodextrin and sucrose octasulfate

Remote loading technology is an outstanding achievement in liposome-based drug delivery systems.

Arsenic trioxide encapsulated liposomes prepared via copper acetate gradient loading method and its antitumor efficiency

In this study, arsenic trioxide (ATO) was encapsulated in liposomes via copper acetate (Cu(OAc)₂) gradients and high entrapment efficiency of over 80% was obtained. The average particle size and the zeta-potential of the liposomes were detected to be 115.1 ± 29.1 nm and −21.97 ± 0.6 mV, respectively. The TEM images showed rod-like precipitates in the inner aqueous phase, which was supposed be due to the formation of insoluble ATO—Cu complex. The in vitro drug release of ATO—Cu liposomes exhibited a sustained release over 72 h, and the release rates decreased with the increase of the pH of release media. Pharmacokinetic and tissue distribution studies of ATO liposomes showed significantly reduced plasma clearance rate, increased AUC_0–12h and T_1/2, and improved tumor distribution of As compared to iv administration of ATO solution. The anti-tumor effect of ATO loaded liposomes to S180 tumor-bearing mice was significantly improved with a tumor inhibition rate of 61.2%, meanwhile the toxicity of encapsulated ATO was greatly decreased. In conclusion, ATO can be effectively encapsulated into liposomes by remote loading method via Cu(OAc)₂ gradients; the co-administration of ATO and Cu(II) via liposomal formulation may find wide applications in the treatment of various tumors.

Liposomal formulation of hypoxia activated prodrug for the treatment of ovarian cancer

In this work, a new sphingomyelin-cholesterol liposomal formulation (CPD100Li) for the delivery of a hypoxia activated prodrug of vinblastine, mon-N-oxide (CPD100), is developed. The optimized liposomal formulation uses an ionophore (A23187) mediated pH-gradient method. Optimized CPD100Li is characterized for size, drug loading, and stability. The in vitro toxicity of CPD100Li is assessed on different aspects of cell proliferation and apoptosis of ES2 ovarian cancer under normoxic and hypoxic conditions. The pharmacokinetics of CPD100Li in mice as well as the influence of A23187 on the retention of CPD100 are assessed. The dose limiting toxicity (DLT) and maximum tolerated dose (MTD) for CPD100Li are evaluated in nude mice. CPD100 is loaded in the liposome at 5.5 mg/mL. The sizes of CPD100Li using DLS, qNano and cryo-TEM techniques are 155.4 ± 4.2 nm, 132 nm, and 112.6 ± 19.8 nm, respectively. There is no difference between the in vitro characterization of CPD100Li with and without ionophore. Freshly prepared CPD100Li with ionophore are stable for 48 h at 4 °C, while the freeze-dried formulation is stable for 3 months under argon at 4 °C. The hypoxic cytotoxicity ratios (HCR) of CPD100 and CPD100Li are 0.16 and 0.11, respectively. CPD100Li under hypoxic conditions has a 9.2-fold lower IC₅₀ value as compared to CPD100Li under normoxic conditions, confirming the hypoxia dependent activation of CPD100. CPD100Li treated ES2 cells show a time dependent enhanced cell death, along with caspase production and an increase in the number of cells in G₀/G₁ and higher cell arrest. The blood concentration profile of CPD100Li in mice without A23187 has a 12.6-fold lower area under the curve (AUC) and 1.6-fold lower circulation time compared to the CPD100Li with A23187. The DLT for both CPD100 and CPD100Li is 45 mg/kg and the MTD is 40 mg/kg in nude mice. Based on the preliminary data obtained, we clearly show that the presence of ionophore affects the in vivo stability of CPD100. CPD100Li presents a unique opportunity to develop a first-in-kind chemotherapy product based on achieving selective drug activation through the hypoxic physiologic microenvironment of solid tumors.

Resolution V fractional factorial design for screening of factors affecting weakly basic drugs liposomal systems

This study aims to investigate factors affecting weakly basic drugs liposomal systems. Resolution V fractional factorial design (2_V^5-1) is used as an example of screening designs that would better be used as a wise step before proceeding with detailed factors effects or optimization studies. Five factors probable to affect liposomal systems of weakly basic drugs were investigated using Amisulpride as a model drug. Factors studied were; A: Preparation technique B: Phosphatidyl choline (PhC) amount (mg) C: Cholesterol: PhC molar ratio, D: Hydration volume (ml) and E: Sonication type. Levels investigated were; Ammonium sulphate-pH gradient technique or Transmembrane zinc chelation-pH gradient technique, 200 or 400 mg, 0 or 0.5, 10 or 20 ml and bath or probe sonication for A, B, C, D and E respectively. Responses measured were Particle size (PS) (nm), Zeta potential (ZP) (mV) and Entrapment efficiency percent (EE%). Ion selective electrode was used as a novel method for measuring unentrapped drug concentration and calculating entrapment efficiency without the need for liposomal separation. Factors mainly affecting the studied responses were Cholesterol: PhC ratio and hydration volume for PS, preparation technique for ZP and preparation technique and hydration volume for EE%. The applied 2_V^5-1 design enabled the use of only 16 trial combinations for screening the influence of five factors on weakly basic drugs liposomal systems. This clarifies the value of the use of screening experiments before extensive investigation of certain factors in detailed optimization studies.

Practical Liposomal Formulation for Taxanes with Polyethoxylated Castor Oil and Ethanol with Complete Encapsulation Efficiency and High Loading Efficiency

Taxanes including paclitaxel and docetaxel are effective anticancer agents preferably sufficient for liposomal drug delivery. However, the encapsulation of these drugs with effective amounts into conventional liposomes is difficult due to their high hydrophobicity. Therefore, an effective encapsulation strategy for liposomal taxanes has been eagerly anticipated. In this study, the mixture of polyethoxylated castor oil (Cremophor EL) and ethanol containing phosphate buffered saline termed as CEP was employed as a solvent of the inner hydrophilic core of liposomes where taxanes should be incorporated. Docetaxel-, paclitaxel-, or 7-oxacetylglycosylated paclitaxel-encapsulating liposomes were successfully prepared with almost 100% of encapsulation efficiency and 29.9, 15.4, or 29.1 mol% of loading efficiency, respectively. We then applied the docetaxel-encapsulating liposomes for targeted drug delivery. Docetaxel-encapsulating liposomes were successfully developed HER2-targeted drug delivery by coupling HER2-specific binding peptide on liposome surface. The HER2-targeting liposomes exhibited HER2-specific internalization and enhanced anticancer activity in vitro. Therefore, we propose the sophisticated preparation of liposomal taxanes using CEP as a promising formulation for effective cancer therapies.

A simple passive equilibration method for loading carboplatin into pre-formed liposomes incubated with ethanol as a temperature dependent permeability enhancer

A passive equilibration method which relies on addition of candidate drugs to pre-formed liposomes is described as an alternative method for preparing liposome encapsulated drugs. The method is simple, rapid and applicable to liposomes prepared with high (45mol%) or low (<20mol%) levels of cholesterol. Passive equilibration is performed in 4-steps: (i) formation of liposomes, (ii) addition of the candidate drug to the liposomes in combination with a permeability enhancing agent, (iii) incubation at a temperature that facilitates diffusion of the added compound across the lipid bilayer, and (iv) quenching the enhanced membrane permeability by reduction in temperature and/or removal of the permeabilization enhancer. The method is fully exemplified here using ethanol as the permeabilization enhancer and carboplatin (CBDCA) as the drug candidate. It is demonstrated that ethanol can be added to liposomes prepared with 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and Cholesterol (Chol) (55:45mol ratio) in amounts up to 30% (v/v) with no change in liposome size, even when incubated at temperatures>60°C. Super-saturated solutions of CBDCA (40mg/mL) can be prepared at 70°C and these are stable in the presence of ethanol even when the temperature is reduced to <30°C. maximum CBDCA encapsulation is achieved within 1h after the CBDCA solution is added to pre-formed DSPC/Chol liposomes in the presence of 30% (v/v) ethanol at 60°C. When the pre-formed liposomes are mixed with ethanol (30% v/v) at or below 40°C, the encapsulation efficiency is reduced by an order of magnitude. The method was also applied to liposomes prepared from other compositions include a cholesterol free formulations (containing 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[carboxy(polyethylene glycol)-2000] (DSPE-PEG₂₀₀₀)) and a low Chol (<20mol%) formulations prepared with the distearoyl-sn-glycero-3-phospho-(1'-rac-glycerol) DSPG)). The cytotoxic activity of CBDCA was unaffected when prepared in this manner and two of the resultant formulations exhibited good stability in vitro and in vivo. The cytotoxic activity of CBDCA was unaffected when prepared in this manner and the resultant formulations exhibited good stability in vitro and in vivo. Pharmacokinetics studies in CD-1 mice indicated that the resulting formulations increased the circulation half life of the associated CBDCA significantly (AUC_0-24h of CBDCA=0.016μg·hr/mL; AUC_0-24h of the DSPC/Chol CBDCA formulation=1014.0μg·hr/mL and AUC_0-24h of the DSPC/DSPG/Chol CBDCA formulation=583.96μg·hr/mL). Preliminary efficacy studies in Rag-2M mice with established subcutaneous H1975 and U-251 tumors suggest that the therapeutic activity of CBDCA is improved when administered in liposomal formulations. The encapsulation method described here has not been disclosed previously and will have broad applications to drugs that would normally be encapsulated during liposome manufacturing.

The influence of different long-circulating materials on the pharmacokinetics of liposomal vincristine sulfate

PURPOSE

This study was designed to improve the in vivo pharmacokinetics of long-circulating vincristine sulfate (VS)-loaded liposomes; three different long-circulating materials, chitosan, poly(ethylene glycol)-1,2-distearoyl sn-glycero-3-phosphatidylethanolamine (PEG-DSPE), and poly(ethylene glycol)-poly-lactide-co-glycolide (PEG-PLGA), were evaluated at the same coating molar ratio with the commercial product Marqibo(®) (vincristine sulfate liposome injection [VSLI]).

MATERIALS AND METHODS

VS-loaded liposomes were prepared by a pH gradient method and were then coated with chitosan, PEG-DSPE, or PEG-PLGA. Physicochemical properties, including the morphology, particle size, zeta potential, encapsulation efficiency (EE%), pH, drug loading, and in vitro release, were determined. Preservation stability and pharmacokinetic studies were performed to compare the membrane-coated liposomes with either commercially available liposomes or the VS solution.

RESULTS

The sphere-like morphology of the vesicles was confirmed by transmission electron microscope. Increased particle size, especially for the chitosan formulation, was observed after the coating process. However, the EE% was ~99.0% with drug loading at 2.0 mg/mL, which did not change after the coating process. The coating of long-circulation materials, except for chitosan, resulted in negatively charged and stable vesicles at physiological pH. The near-zero zeta potential exhibited by the PEG-DSPE formulation leads to a longer circulation lifetime and improved absorption for VS, when compared with the PEG-PLGA formulation. Compared with the commercial product, PEG was responsible for a higher plasma VS concentration and a longer half-life.

CONCLUSION

PEG-DSPE coating may be related to better absorption, based on the stability and a pharmacokinetic improvement in the blood circulation time.

Irinotecan Delivery by Lipid-Coated Mesoporous Silica Nanoparticles Shows Improved Efficacy and Safety over Liposomes for Pancreatic Cancer

Urgent intervention is required to improve the 5 year survival rate of pancreatic ductal adenocarcinoma (PDAC). While the four-drug regimen, FOLFIRINOX (comprising irinotecan, 5-fluorouracil, oxaliplatin, and leucovorin), has a better survival outcome than the more frequently used gemcitabine, the former treatment platform is highly toxic and restricted for use in patients with good performance status. Since irinotecan contributes significantly to FOLFIRINOX toxicity (bone marrow and gastrointestinal tract), our aim was to reduce the toxicity of this drug by a custom-designed mesoporous silica nanoparticle (MSNP) platform, which uses a proton gradient for high-dose irinotecan loading across a coated lipid bilayer (LB). The improved stability of the LB-coated MSNP (LB-MSNP) carrier allowed less drug leakage systemically with increased drug concentrations at the tumor sites of an orthotopic Kras-derived PDAC model compared to liposomes. The LB-MSNP nanocarrier was also more efficient for treating tumor metastases. Equally important, the reduced leakage and slower rate of drug release by the LB-MSNP carrier dramatically reduced the rate of bone marrow, gastrointestinal, and liver toxicity compared to the liposomal carrier. We propose that the combination of high efficacy and reduced toxicity by the LB-MSNP carrier could facilitate the use of irinotecan as a first-line therapeutic to improve PDAC survival.

Lipid nanoparticles for short interfering RNA delivery

The discovery of RNA interference (RNAi) in mammalian cells has created a new class of therapeutics based on the reversible silencing of specific disease-causing genes. This therapeutic potential depends on the ability to deliver inducers of RNAi, such as short-interfering RNA (siRNA) and micro-RNA (miRNA), to cells of target tissues. This chapter reviews various challenges and delivery strategies for siRNA, with a particular focus on the development of lipid nanoparticle (LNP) delivery technologies. Currently, LNP delivery systems are the most advanced technology for systemic delivery of siRNA, with numerous formulations under various stages of clinical trials. We also discuss methods to improve gene silencing potency of LNP-siRNA, as well as application of LNP technologies beyond siRNA to the encapsulation of other nucleic acids such as mRNA and clustered regularly interspaced short palindromic repeats (CRISPR).

Physicochemical aspects of the coformulation of colistin and azithromycin using liposomes for combination antibiotic therapies

Remote loading of azithromycin into liposomes, and subsequent release behavior in the presence of colistin, has been investigated with a view to understand the potential of liposomes to enable the coformulation of these two antibiotics for application in inhalation therapy. Azithromycin was successfully encapsulated into liposomes by remote loading (encapsulation efficiency > 98%). Slow release of azithromycin was achieved in the presence of cholesterol in a concentration-dependent manner, with a 4:1 mol ratio of phospholipid-cholesterol releasing 22% azithromycin in 24 h, whereas a 2:1 mol ratio released only 4.9% of azithromycin in 24 h. Addition of colistin to the formulation with increasing concentration did not change the loading behavior, but accelerated drug release, increasing the percentage of released azithromycin from 4.9% to 30% over 24 h. The permeabilizing ability of colistin on liposomes is consistent with its permeabilizing effect on bacterial cells. This behavior opens opportunities to tailor the release rate of drugs coformulated with colistin using liposomes as the carrier.

Nanotechnology-based approaches in anticancer research

Cancer is a highly complex disease to understand, because it entails multiple cellular physiological systems. The most common cancer treatments are restricted to chemotherapy, radiation and surgery. Moreover, the early recognition and treatment of cancer remains a technological bottleneck. There is an urgent need to develop new and innovative technologies that could help to delineate tumor margins, identify residual tumor cells and micrometastases, and determine whether a tumor has been completely removed or not. Nanotechnology has witnessed significant progress in the past few decades, and its effect is widespread nowadays in every field. Nanoparticles can be modified in numerous ways to prolong circulation, enhance drug localization, increase drug efficacy, and potentially decrease chances of multidrug resistance by the use of nanotechnology. Recently, research in the field of cancer nanotechnology has made remarkable advances. The present review summarizes the application of various nanotechnology-based approaches towards the diagnostics and therapeutics of cancer.

Active methods of drug loading into liposomes: recent strategies for stable drug entrapment and increased in vivo activity

INTRODUCTION

The use of liposomes increases the therapeutic index of many drugs, and also offers drug targeting and controlled release. The commercial impact of liposomes is strengthened by the invention of several active drug encapsulation methods, allowing the encapsulation of several weak base or weak acid drugs with very high drug-to-lipid ratios.

AREAS COVERED

In recent years, there have been reports on several new approaches to retain more hydrophobic drugs inside liposomes, in the circulation. Most of these methods apply drug precipitation inside preformed liposomes, as low soluble complexes with ions or chemicals. In some cases, drug derivatization was applied to enable active encapsulation of hydrophobic drugs, previously not reported to encapsulate, by active or remote loading. This review presents and compares most of the existing methods of active drug encapsulation and outlines recent strategies to achieve stable drug encapsulation in vivo.

EXPERT OPINION

At present, there is no single universal encapsulation method that offers stable encapsulation of most drugs; each drug requires a different approach to manage all of its properties. Now is the time to combine all these strategies to achieve the goal of a complex, but successful, anticancer therapy.

Novel sulfobutyl ether cyclodextrin gradient leads to highly active liposomal irinotecan formulation

OBJECTIVES

Liposomal delivery of irinotecan could provide protection against drug hydrolysis, deliver more active lactone form to tumours and prolong irinotecan exposure time. Nevertheless, conventional drug-loading technologies have typically resulted in undesired drug retention properties. To resolve the problem, a modified gradient loading method was developed and the resulting formulations were evaluated in a systemic manner.

METHODS

Irinotecan was loaded into liposomes using a novel sulfobutyl ether beta-cyclodextrin (sbe-CD) gradient. The effect of drug-to-lipid ratio (D/L) and polyethylene glycol (PEG) grafting density were investigated. Drug release experiments were performed in ammonium-containing medium based on the fluorescence dequenching phenomenon of irinotecan. Pharmacokinetic studies were performed in normal balb/c mice treated with different formulations. To compare the anti-tumour effect of different formulations, an RM-1 prostate cancer model was used. Acute toxicity studies were performed in healthy female c57 mice.

KEY FINDINGS

Irinotecan could be encapsulated into liposomes with >90% loading efficiency at a high drug-to-lipid mass ratio (>0.5). In-vitro release experiments revealed that sbe-CD anion was more able to retain irinotecan than sulfate. Moreover, the elevated D/L ratio elicited decreased drug release kinetics. Both trends had also been observed when the effects of anions and D/L ratio on half-life of irinotecan were assessed. Pegylated liposomal irinotecan loaded with sbe-CD/triethylammonium gradient had irinotecan half-life values ranging from 9.4 to 13.1 h, surpassing vesicles prepared by the triethylammonium sulfate method (∼4.5 h). In the RM-1 tumour model, all the liposomal irinotecan formulations were more therapeutically active than free irinotecan and the formulation with a high D/L ratio was the most efficacious. Moreover, the high D/L formulation might be less toxic than free irinotecan based on acute toxicity studies.

CONCLUSIONS

The novel sbe-CD gradient could mediate effective irinotecan loading and improve irinotecan retention, thus resulting in highly active liposomal irinotecan formulations. The improvement in drug retention might be associated with the formation of complicated aggregates inside vesicles.

Simultaneous liposomal delivery of quercetin and vincristine for enhanced estrogen-receptor-negative breast cancer treatment

Breast cancers are either estrogen receptor-positive (ER) or negative (ER). ER breast cancers are clinically more aggressive and have fewer effective treatment options. Quercetin and vincristine are both active against ER breast cancers and exhibit synergism in vitro. However, the clinical use of quercetin is hampered by its low water solubility. In addition, optimal synergism can only be achieved at a particular ratio of the drugs. Therefore, the objectives of this study are to develop a liposomal formulation to solubilize quercetin, and to co-encapsulate and coordinate the release of quercetin and vincristine in their synergistic ratios to maximize anticancer activity. The optimal synergistic molar ratio of quercetin/vincristine was found to be 1 : 2 by in-vitro MTT assay. Quercetin liposomes were prepared by the film hydration method followed by extrusion, and vincristine was subsequently loaded into the core of the liposomes by remote loading with manganese sulfate and the ionophore A23187. The optimal liposome formulation co-encapsulating quercetin and vincristine comprised egg sphingomyelin/cholesterol/PEG2000 ceramide/quercetin (72.5 : 17.5 : 5 : 5 mol ratio). This formulation was physically stable, enhanced quercetin solubility 8.6 times, co-encapsulated quercetin and vincristine with efficiencies of 78.3 and 78.5%, respectively, and displayed coordinated release of both drugs to maintain the synergistic molar ratio. In-vitro MTT assays of this liposomal formulation showed significant synergism, with a combination index of 0.113 and a dose-reduction index value of 115 at ED50 for vincristine. Therefore, liposomal delivery represents a strategy to solubilize poorly soluble drugs and coordinate the release of two drugs in their synergistic ratio for optimal anticancer effect.

Copper ion-mediated liposomal encapsulation of mitoxantrone: the role of anions in drug loading, retention and release

Besides pH gradient, other transmembrane gradients such as metal ion gradient could be also employed to load drugs into liposomes. In pH gradient method, anions have an important role since they could form specific aggregates with drugs, and then affect drug release kinetics from vesicles. To explore the role of anions in metal ion gradient method, copper ion-mediated mitoxantrone (MIT) loading was investigated systematically. When empty liposomes exhibiting a transmembrane copper ion gradient (300 mM) were mixed with MIT in a molar ratio of 0.2:1, after 5 min incubation at 60 degrees C, >95% MIT could be loaded into vesicles and the encapsulation was stable, regardless of the kinds of anions and initial intraliposomal pH values. The encapsulation ratio decreased with increased MIT/lipid molar ratio. But even when the molar ratio increased to 0.4, >90% encapsulation could still be achieved. In the presence of nigericin and ammonium, the drug loading profiles were affected to different degree with respect to both drug loading rate and encapsulation ratio. Relative to CuSO(4)-containing systems, CuCl(2) mediated MIT loading was unstable. Both nigericin and ammonium could alter the absorption spectra of liposomal MITs loaded with CuSO(4) gradient. In vitro release studies were performed in glucose/histidine buffer and in 50% human plasma using a dialysis method. In both of release media, CuCl(2)-containing vesicles displayed rapid release kinetics in comparison with CuSO(4) systems; and during the experiment period, MIT was lost from the vesicles continuously. When the formulations were injected into BDF1 mice at a dose of 4 mg/kg, all the liposomal formulations exhibited enhanced blood circulation time, with half-life values of 6.8-7.2h, significantly compared to the rapid clearance of free-MIT. In L1210 ascitic model, CuCl(2) formulation was more therapeutically active than CuSO(4) formulation. At a dose of 6 mg/kg, the treatment with CuCl(2) formulation resulted in a median survival time of 21 days, considerably larger than that of CuSO(4) groups (15 days). Based on these data, it was concluded that during the drug loading process, a dynamic transmembrane pH gradient is generated and intraliposomal pH might affect the complexation manner in which Cu(2+) binds MIT. Owing to the presence of pH gradient, after the accumulation within vesicles, a part of MIT will be protonated and precipitated by sulfate. Accordingly, the aggregation status of MIT inside CuSO(4) system was more complicated than that in CuCl(2) vesicles. The difference in physical status of MIT aggregates affects not only the drug release rate, but also their therapeutic effects.

Liposomal delivery of hydrophobic weak acids: enhancement of drug retention using a high intraliposomal pH

Clinical development of highly potent lipophilic neutral camptothecins has been impeded by the poor solubility, stability, and nonspecific toxicity of these compounds. Liposomal encapsulation offers a promising formulation route for tumor site-specific delivery of these novel drug candidates. However, the development of formulation strategies for liposomal loading and retention of hydrophobic drugs such as the neutral camptothecins has been lacking. In the studies presented here, we explored the potential of a trans-bilayer pH gradient strategy for prolonging the liposome retention of DB-67, a novel lipophilic camptothecin that can undergo lactone ring-opening to form a hydrophobic weak acid. The liposome membrane permeability of DB-67 was obtained as a function of pH in aqueous buffers. A permeability model was developed and liposome membrane permeability was shown to be controlled by the fraction of unbound neutral lactone entrapped in the vesicles. Liposome membrane permeability of DB-67 was also studied under physiological conditions. The high membrane partitioning of DB-67 in the intraliposomal microenvironment was found to shift the equilibrium between lactone and carboxylate towards the lactone species resulting in a faster than desired drug release under physiological conditions. The effectiveness of the pH gradient strategy was further reduced under physiological conditions by the rapid loss of trans-membrane pH gradients due to CO(2) uptake. Simulations were conducted to explore the role of membrane binding, intravesicular pH, and carbonate buffer concentration in successful utilization of the pH gradient strategy for hydrophobic weak acids.

Liposomal Nanomedicines: An Emerging Field

Liposomal nanoparticles (LNs) encapsulating therapeutic agents, or liposomal nanomedicines (LNMs), represent one of the most advanced classes of drug delivery systems, with several currently on the market and many more in clinical trials. During the past 20 years, a variety of techniques have been developed for encapsulating both conventional drugs and the new genetic drugs (plasmid DNA–containing therapeutic genes, anti-sense oligonucleotides, and small, interfering RNA [siRNA]) within LNs encompassing a very specific set of properties: a diameter centered on 100 nm, a high drug-to-lipid ratio, excellent retention of the encapsulated drug, and a long (> 6 hours) circulation lifetime. Particles with these properties tend to accumulate at sites of disease, such as tumors, where the endothelial layer is “leaky” and allows extravasation of particles with small diameters. Thus, LNs protect the drug during circulation, prevent it from reaching healthy tissues, and permit its accumulation at sites of disease. We will discuss recent advances in this field involving conventional anticancer drugs as well as gene-delivery, immunostimulatory, and gene-silencing applications involving the new genetic drugs. LNMs have the potential to offer new treatments in such areas as cancer therapy, vaccine development, and cholesterol management.

Long-circulating poly(ethylene glycol)-coated emulsions to target solid tumors

The purpose of this study was to develop oil-in-water emulsions (100-120 nm in diameter) and to correlate the surface properties of the emulsions with blood residence time and accumulation into neoplastic tissues by passive targeting. We investigated the effect of phospholipid and sphingolipid emulsifiers, hydrogenated soybean phosphatidylcholine (HSPC) and egg sphingomyelin (ESM), in combination with polysorbate 80 (PS-80) and 1,2-distearoyl-sn-glycero-3-phosphatidylethanolamine (DSPE)-PEG lipids of various PEG chain lengths and structures in prolonging circulation time and enhancing accumulation into B16 melanoma or C26 colon adenocarcinoma. The relationship between amphiphile molecular packing at the air/water interface on emulsion stability upon dilution in albumin and circulation longevity in vivo was also explored for non-PEGylated emulsions. PEGylation of the droplet surface with 10-15 mol% of DSPE-PEG 2000 or 5000 enhanced the circulation time of the emulsions, however, accumulation was only observed in the C26 tumor model. The tighter molecular packing observed with ESM/PS-80 monolayers at the air/water interface compared to HSPC/PS-80 correlated with improved emulsion stability in vitro, however, enhanced circulation time in vivo was not observed. A better understanding of the relationships between composition and performance will result in improved emulsion-based drug delivery vehicles for cancer therapy.

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 micromol 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.

Chemodosimetry of in vivo tumor liposomal drug concentration using MRI

Effective cancer chemotherapy depends on the delivery of therapeutic drugs to cancer cells at cytotoxic concentrations. However, physiologic barriers, such as variable vessel permeability, high interstitial fluid pressure, and heterogeneous perfusion, make it difficult to achieve that goal. Efforts to improve drug delivery have been limited by the lack of noninvasive tools to evaluate intratumoral drug concentration and distribution. Here we demonstrate that tumor drug concentration can be measured in vivo using T(1)-weighted MRI, following systemic administration of liposomes containing both drug (doxorubicin (DOX)) and contrast agent (manganese (Mn)). Mn and DOX concentrations were calculated using T(1) relaxation times and Mn:DOX loading ratios, as previously described. Two independent validations by high-performance liquid chromatography (HPLC) and histologic fluorescence in a rat fibrosarcoma (FSA) model indicate a concordant linear relationship between DOX concentrations determined using T(1) and those measured invasively. This method of imaging exhibits potential for real-time evaluation of chemotherapeutic protocols and prediction of tumor response on an individual patient basis.

Characterization and in vitro evaluation of spherulites as sequestering vesicles with potential application in drug detoxification

The aim of the present investigation was to prepare and characterize lecithin spherulites as parenteral drug sequestering agents with potential application in the treatment of drug overdose and chemical poisoning. The spherulites (approximately 200 nm) obtained by controlled hydration and shearing of lipid-alcohol mixtures, revealed unexpected differences in the physical properties of the bilayer when compared to liposomes. Differential scanning calorimetry, 31-phosphorus nuclear magnetic resonance, and pH-sensitive pyranine steady-state fluorescence studies indicated that although spherulites retained the typical bilayer conformation, the arrangement of the phospholipid molecules was perturbed relative to native liposome bilayer. The loosened packing of the phospholipids in bilayers was strongly supported by the relative ease with which spherulites lost the established pH-gradient. This permeability problem was overcome via incorporation of cholesterol in the bilayer. Subsequently, albumin/buffer components were encapsulated in these spherulites and the drug sequestration potential for detoxification application was examined. Citrate pH-gradient spherulites accumulated 75% of external haloperidol while those loaded with approximately 20% (w/w) albumin were able to take up 45% of haloperidol and 91-95% of taxanes (docetaxel and paclitaxel). In cytotoxicity studies, the competitive internalization of docetaxel by albumin-loaded spherulites resulted in an increase of the IC50 value for the free drug. Thus, the spherulite technology could be a versatile approach for actively sequestering toxins in the blood and for reducing the adverse effects by altering the pharmacokinetics and biodistribution of overdosed drugs.

Transition Metal-Mediated Liposomal Encapsulation of Irinotecan (CPT-11) Stabilizes the Drug in the Therapeutically Active Lactone Conformation

PURPOSE

To determine whether entrapped transition metals could mediate the active encapsulation of the anticancer drug irinotecan into preformed liposomes. Further, to establish that metal complexation could stabilize liposomal irinotecan in the therapeutically active lactone conformation.

MATERIALS AND METHODS

Irinotecan was added to preformed 1,2-distearoyl-sn-glycero-phosphocholine/cholesterol (DSPC/chol) liposomes prepared in CuSO4, ZnSO4, MnSO4, or CoSO4 solutions, and drug encapsulation was determined over time. The roles of the transmembrane pH gradient and internal pH were evaluated. TLC and HPLC were used to monitor drug stability and liposome morphology was assessed by cryo-TEM.

RESULTS

Irinotecan was rapidly and efficiently loaded into preformed liposomes prepared in unbuffered (approximately pH 3.5) 300 mM CuSO4 or ZnSO4. For Cu-containing liposomes, results suggested that irinotecan loading occurred when the interior pH and the exterior pH were matched; however, addition of nigericin to collapse any residual transmembrane pH gradient inhibited irinotecan loading. Greater than 90% of the encapsulated drug was in its active lactone form and cryo-TEM analysis indicated dark intravesicular electron-dense spots.

CONCLUSION

Irinotecan is stably entrapped in the active lactone conformation within preformed copper-containing liposomes as a result of metal-drug complexation.

In vitro and in vivo characterization of a combination chemotherapy formulation consisting of vinorelbine and phosphatidylserine

The purpose of these studies was to design an intravenous drug formulation consisting of two active agents having synergistic in vitro activity. Specifically, we describe a novel drug combination consisting of a cytotoxic agent (vinorelbine) with an apoptosis-inducing lipid (phosphatidylserine, PS). In vitro cytotoxicity screening of PS and vinorelbine, alone and in combination, against human MDA435/LCC6 breast cancer and H460 lung cancer cells was used to identify the molar ratio of these two agents required for synergistic activity. PS and vinorelbine were co-formulated in a lipid-based system at the synergistic molar ratio and the pharmacokinetic and antitumor characteristics of the combination assessed in mice bearing H460 tumors. The cytotoxicity of the lipid, and the synergy between the lipid and vinorelbine, were specific to PS; these effects were not observed using control lipids. A novel formulation of PS, incorporated as a membrane component in liposomes, and encapsulating vinorelbine using a pH gradient based loading method was developed. The PS to vinorelbine ratio in this formulation was 1/1, a ratio that produced synergistic in vitro cytotoxicity over a broad concentration range. The vinorelbine and PS dual-agent treatment significantly delayed the growth of subcutaneous human H460 xenograft tumors in Rag2M mice compared to the same dose of free vinorelbine given alone or given as a cocktail of the free vinorelbine simultaneously with empty PS-containing liposomes. These studies demonstrate the potential to develop clinically relevant drug combinations identified using in vitro drug-drug interactions combined with lipid-based delivery systems to co-formulate drugs at their synergistic ratios.

Pharmacokinetic and pharmacodynamic evaluation of a novel in situ forming poly(ethylene glycol)-based hydrogel for the controlled delivery of the camptothecins

Inadequate drug delivery, due to problems associated with achieving constant therapeutic blood levels, has hampered the use of anticancer agents of the camptothecin (CPT) class. The objective of the current studies was to develop a depot delivery system for the water-soluble analog of CPT, topotecan (TPT). In this study, a 2-phase drug depot consisting of TPT-loaded liposomes entrapped in a poly(ethylene glycol) hydrogel was designed. Physically entrapped unaltered TPT displayed a rapid release rate from the hydrogel. Controlled release was demonstrated in vitro and in vivo from the 2-phase system with constant blood levels being achieved for several days in rats. Cytotoxicity and antitumor activity were also evaluated in rats inoculated with syngeneic MAT B III breast cancer cells. Rats treated with the liposome-loaded hydrogel displayed significantly longer tumor growth suppression and did not exhibit body weight loss compared to those treated with other delivery modes. These experiments constitute a proof-of-principle of the 2-phase depot concept and its potential value for enhancing safety and efficacy in chemotherapy.

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.

Formation of drug-arylsulfonate complexes inside liposomes: a novel approach to improve drug retention

The development of procedures to enhance drug retention in liposomes is important in order to achieve therapeutically optimized rates of drug release from liposomal carriers. In this study, the ability of lipophilic weak base drugs to complex with arylsulfonates resulting in formation of intravesicular precipitates is investigated as a means to enhance drug retention. It is shown that the arylsulfonates benzenesulfonate and hydroxybenzenesulfonate (HBS) induce precipitation of ciprofloxacin and vinorelbine, two representative weak base drugs that are difficult to retain in liposomal systems. The most complete precipitation was observed at pH values corresponding to charge neutralization of the drug-arylsulfonate complex. HBS is shown to be a much more effective precipitating agent than benzenesulfonate. It is also shown that vinorelbine and ciprofloxacin can be loaded into large unilamellar vesicles (LUV) containing the calcium salt of HBS using an ionophore-based loading method. Following drug loading, the formation of intravesicular drug-arylsulfonate precipitates of vinorelbine and ciprofloxacin was observed by cryo-electron microscopy. In vitro release experiments showed substantial improvements in drug retention for both vinorelbine and ciprofloxacin when HBS was present as compared to standard loading procedures employing MgSO4 as the entrapped solute. In vivo release experiments for vinorelbine in NuNu mice indicated a half-time for release for HBS-containing LUV of approximately 30 h, compared to 6.4 h for LUV loaded employing MgSO4. It is suggested that encapsulation procedures employing HBS in the internal medium can improve the retention of drugs that are difficult to retain in liposomes, possibly leading to enhanced therapeutic properties.

Optimization and characterization of a sphingomyelin/cholesterol liposome formulation of vinorelbine with promising antitumor activity

Vinorelbine (VRL) is a particularly lipophilic member of the vinca alkaloids which, as a class of drugs, exhibit improved cytotoxicity and therapeutic activity through increased duration of exposure. Here, we describe and optimize a sphingomyelin/cholesterol (SM/Chol) liposome formulation of VRL to maximize in vivo drug retention, plasma circulation time, and therapeutic activity. VRL was efficiently encapsulated (>90%) into 100 nm liposomes using an ionophore-mediated loading method. VRL retention in SM/Chol liposomes after intravenous injection in mice was dependent on drug-to-lipid ratio (D/L), with higher D/L ratios exhibiting increased drug retention (0.3 > 0.2 > 0.1, wt/wt) and improved pharmacokinetics. Cryo-electron microscopic examination of a high D/L ratio formulation indicated that the intravesicular regions of these liposomes were electron dense compared with empty liposomes. The optimized, high D/L ratio SM/Chol VRL formulation showed promising activity against subcutaneous B16 melanoma tumors compared with VRL or SM/Chol formulations of vincristine or vinblastine. Finally, the stability of the formulation was excellent (<5% drug leakage, >99% intact VRL, no changes in liposome size after 1 year at 2-8 degrees C). The optimized drug retention properties of the SM/Chol formulation of VRL, combined with its promising antitumor activity and pharmaceutical stability, make this formulation an excellent candidate for future clinical development.

Liposome-encapsulated vincristine, vinblastine and vinorelbine: A comparative study of drug loading and retention

A comparative study of the loading and retention properties of three structurally very closely related vinca alkaloids (vincristine, vinorelbine and vinblastine) in liposomal formulations has been performed. All three vinca alkaloids showed high levels of encapsulation when accumulated into egg sphingomyelin/cholesterol vesicles in response to a transmembrane pH gradient generated by the use of the ionophore A23187 and encapsulated MgSO4. However, despite the close similarities of their structures the different vinca drugs exhibited very different release behavior, with vinblastine and vinorelbine being released faster than vincristine both in vitro and in vivo. The differences in loading and retention can be related to the lipophilicity of the drugs tested, where the more hydrophobic drugs are released more rapidly. It was also found that increasing the drug-to-lipid ratio significantly enhanced the retention of vinca alkaloids when the ionophore-based method was used for drug loading. In contrast, drug retention was not dependent on the initial drug-to-lipid ratio for vinca drugs loaded into liposomes containing an acidic citrate buffer. The differences in retention can be explained on the basis of differences in the physical state of the drug inside the liposomes. The drug-to-lipid ratio dependence of retention observed for liposomes loaded with the ionophore technique may provide a way to improve the retention characteristics of liposomal formulations of vinca drugs.

Encapsulation of doxorubicin into thermosensitive liposomes via complexation with the transition metal manganese

In the present study, doxorubicin was encapsulated into two thermosensitive liposome formulations which were composed of DPPC/MSPC/DSPE-PEG(2000) (90/10/4 mole ratio) or DPPC/DSPE-PEG(2000) (95/5 mole ratio). Doxorubicin loading was achieved through the use of a pH gradient or a novel procedure that involved doxorubicin complexation with manganese. Regardless of the initial drug-to-lipid ratios (D:L), the final D:L reached a maximum of 0.05 (w/w) when doxorubicin was encapsulated via a pH gradient for both thermosensitive liposome formulations. In contrast, the final maximum D:L achieved through manganese complexation was 0.2 (w/w), and this loading method did not affect temperature-induced drug release, with 85% of drug released from MSPC-containing liposomes within 10 min at 42 degrees C but <5% released over 60 min at 37 degrees C. When the thermosensitive liposomes prepared via the two different loading methods were injected into mice, similar plasma elimination profiles were observed. Cryo-transmission electron microscopy analysis indicated the presence of doxorubicin fiber bundles in liposomes loaded via pH gradient, compared to a stippled and diffuse morphology in those loaded via manganese complexation. To investigate the effect of intraliposomal pH on drug precipitate morphology, the A23187 ionophore (mediates Mn(2+)/H(+) exchange) was added to liposomes loaded with doxorubicin-manganese complex, and the stippled and diffuse appearance could be converted to one exhibiting fiber bundles after acidification of the liposome core. This suggests that the formation of doxorubicin-manganese complex is favored when the intraliposomal pH is >6.5. During the conversion to the fiber bundle morphology, no doxorubicin release was observed when A23187 was added to liposomes exhibiting a 0.05 (w/w), whereas a significant release was noted when the initial D:L was 0.2 (w/w). Following acidification of the liposomal interior and establishment of an apparent new D:L equilibrium, the measured D:L ratio was 0.05 (w/w). In conclusion, the manganese complexation loading method increased the encapsulation efficiency of doxorubicin in thermosensitive liposomes with no major impact on temperature-triggered drug release or pharmacokinetics.

Liposomal irinotecan: formulation development and therapeutic assessment in murine xenograft models of colorectal cancer

PURPOSE

The purpose is to demonstrate whether an appropriately designed liposomal formulation of irinotecan is effective in treating mice with liver-localized colorectal carcinomas.

EXPERIMENTAL DESIGN

Irinotecan was encapsulated in 1,2-distearoyl-sn-glycero-3-phosphocholine/cholesterol (55:45 molar ratio) liposomes using an ionophore (A23187)-generated transmembrane proton gradient. This formulation was evaluated in vivo by measuring plasma elimination of liposomal lipid and drug after i.v. administration. Therapeutic activity was determined in SCID/Rag-2M mice bearing s.c. LS180 tumors or orthotopic LS174T colorectal metastases.

RESULTS

Drug elimination from the plasma was significantly reduced when irinotecan was administered in the liposomal formulation. At 1 hour after i.v. administration, circulating levels of the liposomal drug were 100-fold greater than that of irinotecan given at the same dose. High-performance liquid chromatographic analysis of plasma samples indicated that liposomal irinotecan was protected from inactivating hydrolysis to the carboxylate form. This formulation exhibited substantially improved therapeutic effects. For the LS180 solid tumor model, it was shown that after a single injection of liposomal irinotecan at 50 mg/kg, the time to progress to a 400-mg tumor was 34 days (as compared with 22 days for animals treated with free drug at an equivalent dose). In the model of colorectal liver metastases (LS174T), a median survival time of 79 days was observed after treatment with liposomal irinotecan (50 mg/kg, given every 4 days for a total of three doses). Saline and free drug treated mice survived for 34 and 53 days, respectively.

CONCLUSIONS

These results illustrate that liposomal encapsulation can substantially enhance the therapeutic activity of irinotecan and emphasize the potential for using liposomal irinotecan to treat liver metastases.

Entrapment of small molecules and nucleic acid-based drugs in liposomes

In the past two decades there have been major advances in the development of liposomal drug delivery systems suitable for applications ranging from cancer chemotherapy to gene therapy. In general, an optimized system consists of liposomes with a diameter of approximately 100 nm that possess a long circulation lifetime (half-life >5 h). Such liposomes will circulate sufficiently long to take advantage of a phenomenon known as disease site targeting, wherein liposomes accumulate at sites of disease, such as tumors, as a result of the leaky vasculature and reduced blood flow exhibited by the diseased tissue. The extended circulation lifetime is achieved by the use of saturated lipids and cholesterol or by the presence of PEG-containing lipids. This chapter will focus on the methodology required for the generation of two very different classes of liposomal carrier systems: those containing conventional small molecular weight (usually anticancer) drugs and those containing larger genetic (oligonucleotide and plasmid DNA) drugs. Initially, we will examine the encapsulation of small, weakly basic drugs within liposomes in response to transmembrane pH and ion gradients. Procedures will be described for the formation of large unilamellar vesicles (LUVs) by extrusion methods and for loading anticancer drugs into LUVs in response to transmembrane pH gradients. Three methods for generating transmembrane pH gradients will be discussed: (1) the use of intravesicular citrate buffer, (2) the use of transmembrane ammonia gradients, and (3) ionophore-mediated generation of pH gradients via transmembrane ion gradients. We will also discuss the loading of doxorubicin into LUVs by formation of drug-metal ion complexes. Different approaches are required for encapsulating macromolecules within LUVs. Plasmid DNA can be encapsulated by a detergent-dialysis approach, giving rise to stabilized plasmid-lipid particles, vectors with potential for systemic gene delivery. Antisense oligonucleotides can be spontaneously entrapped upon electrostatic interaction with ethanol-destabilized cationic liposomes, giving rise to small multilamellar systems known as stabilized antisense-lipid particles (SALP). These vectors have the potential to regulate gene expression.

Preparation, characterization, and biological analysis of liposomal formulations of vincristine

Vincristine is a dimeric Catharanthus alkaloid derived from the Madagascan periwinkle that acts by binding to tubulin and blocking metaphase in actively dividing cells. While vincristine is widely used in the treatment of a number of human carcinomas, its use is associated with dose-limiting neurotoxicity, manifested mainly as peripheral neuropathy. It is known that the therapeutic activity of vincristine can be significantly enhanced after its encapsulation in appropriately designed liposomal systems. Enhanced efficacy is also associated with a slight decrease in drug toxicity. Thus, the therapeutic index of vincristine can be enhanced significantly through the use of a liposomal delivery system. Vincristine may be encapsulated into liposomes of varying lipid composition by several techniques, including passive loading, pH gradient loading, and ionophore-assisted loading. However, most research has focused on the encapsulation of vincristine in response to a transbilayer pH gradient, which actively concentrates the drug within the aqueous interior of the liposome. This chapter details the preparation and evaluation of liposomal vincristine. Specifically, we elaborate on the components (choice of lipids, molar proportions, etc.), methods (preparation of liposomes, drug loading methods, etc.), critical design features (size, surface charge, etc.), and key biological endpoints (circulation lifetime, bioavailability, efficacy measurements) important to the development of a formulation of vincristine with enhanced therapeutic properties.