Engineering lipid vesicles of enhanced intratumoral transport capabilities: correlating liposome characteristics with penetration into human prostate tumor spheroids

Effect of stroma on the behavior of temoporfin-loaded lipid nanovesicles inside the stroma-rich head and neck carcinoma spheroids

Background

Despite the highly expected clinical application of nanoparticles (NPs), the translation of NPs from lab to the clinic has been relatively slow. Co-culture 3D spheroids account for the 3D arrangement of tumor cells and stromal components, e.g., cancer-associated fibroblasts (CAFs) and extracellular matrix, recapitulating microenvironment of head and neck squamous cell carcinoma (HNSCC). In the present study, we investigated how the stroma-rich tumor microenvironment affects the uptake, penetration, and photodynamic efficiency of three lipid-based nanoformulations of approved in EU photosensitizer temoporfin (mTHPC): Foslip^® (mTHPC in conventional liposomes), drug-in-cyclodextrin-in-liposomes (mTHPC-DCL) and extracellular vesicles (mTHPC-EVs).

Results

Collagen expression in co-culture stroma-rich 3D HNSCC spheroids correlates with the amount of CAFs (MeWo cells) in individual spheroid. The assessment of mTHPC loading demonstrated that Foslip^®, mTHPC-DCL and mTHPC-EVs encapsulated 0.05 × 10^− 15 g, 0.07 × 10^− 15 g, and 1.3 × 10^− 15 g of mTHPC per nanovesicle, respectively. The mid-penetration depth of mTHPC NPs in spheroids was 47.8 µm (Foslip^®), 87.8 µm (mTHPC-DCL), and 49.7 µm (mTHPC-EVs), irrespective of the percentage of stromal components. The cellular uptake of Foslip^® and mTHPC-DCL was significantly higher in stroma-rich co-culture spheroids and was increasing upon the addition of serum in the culture medium. Importantly, we observed no significant difference between PDT effect in monoculture and co-culture spheroids treated with lipid-based NPs. Overall, in all types of spheroids mTHPC-EVs demonstrated outstanding total cellular uptake and PDT efficiency comparable to other NPs.

Conclusions

The stromal microenvironment strongly affects the uptake of NPs, while the penetration and PDT efficacy are less sensitive to the presence of stromal components. mTHPC-EVs outperform other lipid nanovesicles due to the extremely high loading capacity. The results of the present study enlarge our understanding of how stroma components affect the delivery of NPs into the tumors.

Response of breast cancer carcinoma spheroids to combination therapy with radiation and DNA-PK inhibitor: growth arrest without a change in α/β ratio

PURPOSE

Agents that increase tumor radiosensitivity are of interest in improving outcomes in radiotherapy (XRT). DNA-PK inhibitors radiosensitize and alter cell adhesion proteins. We investigated combination radiation and a DNA-PK inhibitor in monolayers vs spheroids.

MATERIALS AND METHODS

Using HER2 positive mammary carcinoma cells, we investigated the impact of NU7441, a DNA-PK inhibitor, on irradiated monolayer and spheroid cultures. Colony formation assays were performed with monolayer culture cells and spheroids after irradiation with/without NU7441 (5 μM).

RESULTS

In monolayer culture cells, α/β increased from 3.0 ± 0.2 Gy (XRT alone) to 6.9 ± 0.2 Gy (XRT+NU7441). Corresponding α/β values for cells obtained by disaggregating treated spheroids were 3.6 ± 0.7 Gy (XRT alone) and 3.5 ± 0.2 Gy (XRT+NU7441). However, spheroid survival was highly sensitive to NU7441 incubation. After 4 Gy XRT alone 75% of the irradiated spheroids remained intact; when NU7441 treatment was involved, 13% remained intact. No spheroids survived to 3 weeks at 6 Gy or more. The discrepancy between the minimal change in α/β from cells derived from spheroids and the spheroid growth response was not related to poor penetration of NU7441.

CONCLUSIONS

DNA-PK inhibitor NU7441 radiosensitized monolayer cells but not cells obtained from spheroids. NU7441 and radiation increased spheroid fragmentation.

Active Transportation of Liposome Enhances Tumor Accumulation, Penetration, and Therapeutic Efficacy

Liposomes are the first and mostly explored nanocarriers for cancer drug delivery, which have shown great promise in clinical applications, but their limited accumulation and penetration into the tumor interstitial space, significantly reduce the therapeutic efficacy. Here, a γ-glutamyltranspeptidase (GGT)-triggered charge-switchable approach is reported that can trigger the fast endocytosis and transcytosis of the liposome in tumor microenvironments to overcome the harsh biological barriers in tumor tissues. The active transporting liposomal nanocarrier (GCSDL) is prepared by surface modification with a glutathione (GSH) moiety and encapsulated with doxorubicin (DOX). When the GCSDL contacts with tumor vascular endothelial cells, the overexpressed GGT enzyme on cytomembrane catalyzes the hydrolysis of GSH to generate cationic primary amines. The cationic GCSDL triggers fast caveolae-mediated endocytosis and vesicle-mediated transcytosis, resulting in sequential transcytosis to augment its tumor accumulation and penetration. Along with continual intercellular transportation, GCSDL can release DOX throughout the tumor to induce cancer cell apoptosis, resulting in complete eradication of hepatocellular carcinoma and cessation of pancreatic ductal adenocarcinoma's progression. This study develops an efficient strategy to realize high tumor accumulation and deep penetration for the liposomal drug delivery system via active transcytosis.

Transport of trans-activator of transcription (TAT) peptide in tumour tissue model: evaluation of factors affecting the transport of TAT evidenced by flow cytometry

OBJECTIVES

Trans-activator of transcription (TAT), a cell penetrating peptide, has been explored to overcome resistance to penetration and transport inside the cell, therefore, suggested to be used as drug delivery vector into drug-resistant tumours. The generosity of this study was to evaluate modifiable factors (concentration, temperature, incubation time and spheroid age) on the penetration of TAT.

METHODS

Multicellular tumour spheroids (MCTS) used as tumour tissue models to mimic some characteristics with in-vivo tumors. Cell monolayer and 3-, 5-, 7-day-old MCTS were incubated with TAT and effects of modifiable factors were determined quantitatively through flow cytometry, based on TAT-positive cell count (%) and mean fluorescence intensity.

KEY FINDINGS

Enhancing TAT concentration (1, 5 and 25 µm), transport significantly increased (ANOVA, P < 0.0001) in cell monolayer and spheroids. However, rising temperature from 7 to 37°C (t, P > 0.05) and increasing incubation time; 20 min, 1 h and 3 h; (ANOVA, P > 0.05) were statistically non-significant. Moreover, TAT penetration declines as spheroids get older (ANOVA, P < 0.01).

CONCLUSION

While exploiting MCTS as tumour tissue model, older spheroids could be preferred to target penetration-resistant cells and mimic the in-vivo microenvironment.

The therapeutic potential of polymersomes loaded with 225Ac evaluated in 2D and 3D in vitro glioma models

Alpha emitters have great potential in targeted tumour therapy, especially in destroying micrometastases, due to their high linear energy transfer (LET). To prevent toxicity caused by recoiled daughter atoms in healthy tissue, alpha emitters like ²²⁵Ac can be encapsulated in polymeric nanocarriers (polymersomes), which are capable of retaining the daughter atoms to a large degree. In the translation to a (pre-)clinical setting, it is essential to evaluate their therapeutic potential. As multicellular tumour spheroids mimic a tumour microenvironment more closely than a two-dimensional cellular monolayer, this study has focussed on the interaction of the polymersomes with U87 human glioma spheroids. We have found that polymersomes distribute themselves throughout the spheroid after 4 days which, considering the long half-life of ²²⁵Ac (9.9 d) (Vaidyanathan and Zalutsky, 1996), allows for irradiation of the entire spheroid. A decrease in spheroidal growth has been observed upon the addition of only 0.1 kBq ²²⁵Ac, an effect which was more pronounced for the ²²⁵Ac in polymersomes than when only coupled to DTPA. At higher activities (5 kBq), the spheroids have been found to be destroyed completely after two days. We have thus demonstrated that ²²⁵Ac containing polymersomes effectively inhibit tumour spheroid growth, making them very promising candidates for future in vivo testing.

Application of 3D cultured multicellular spheroid tumor models in tumor-targeted drug delivery system research

Tumor-targeted drug delivery systems are promising for their advantages in enhanced tumor accumulation and reduced toxicity towards normal organs. However, few nanomedicines have been successfully translated into clinical application. One reason is the gap between current pre-clinical and clinical studies. The prevalent in vitro models utilized in pre-clinical phase are mainly based on the two-dimensional (2D) cell culture and are limited by the difficulty of simulating three-dimensional physiological conditions in human body, such as three-dimensional (3D) architecture, cell heterogeneity, nutrient gradients and the interaction between cells and the extracellular matrix (ECM). In addition, traditional animal models have drawbacks such as high-cost, long periods and physiological differences between animal and human. On the other hand, the employment of 3D tumor cell culture models, especially multicellular tumor spheroids (MCTS), has increased significantly in recent decades. These models have been shown to simulate 3D structures of tumors in vitro with relatively low cost and simple protocols. Currently, MCTS have also been widely exploited in drug delivery system research for comprehensive study of drug efficacy, drug penetration, receptor targeting, and cell recruitment abilities. This review summarizes the delivery barriers for nano-carriers presented in tumor microenvironment, the characteristics and formation methods for applicable multicellular tumor spheroid culture models and recent studies related to their applications in tumor-targeted drug delivery system research.

pH-triggered chitosan nanogels via an ortho ester-based linkage for efficient chemotherapy

We report on new types of chitosan-based nanogels via an ortho ester-based linkage, used as drug carriers for efficient chemotherapy. First, we synthesized a novel diacrylamide containing ortho ester (OEAM) as an acid-labile cross-linker. Subsequently, methacrylated succinyl-chitosan (MASCS) was prepared and polymerized with OEAM at different molar ratios to give a series of pH-triggered MASCS nanogels. Doxorubicin (DOX) as a model anticancer drug was loaded into MASCS nanogels with a loading content of 16.5%. As expected, with the incorporation of ortho ester linkages, these nanogels showed pH-triggered degradation and drug release at acidic pH values. In vitro cellular uptake shows that the DOX-loaded nanogels could be preferentially internalized by two-dimensional (2D) cells and three-dimensional (3D) multicellular spheroids (MCs), resulting in higher inhibition of the proliferation of tumor cells. In vivo biodistribution and anti-tumor effect were determined in H22 tumor-bearing mice, and the results demonstrate that the acid-labile MASCS nanogels can significantly prolong the blood circulation time of DOX and improve the accumulation in tumor areas, leading to higher therapeutic efficacy.

STATEMENT OF SIGNIFICANCE

We designed new pH-triggered chitosan nanogels via an ortho ester-based cross-linker for efficient drug-loading and chemotherapy. These drug-loaded nanogels exhibit excellent pH-triggered drug release behavior due to the degradation of ortho ester linkages in mildly acidic environments. In vitro and in vivo results demonstrate that the nanogels could be efficiently internalized by 2D cells and 3D-MCs, improve drug concentration in solid tumors, and lead to higher therapeutic efficacy. To the best of our knowledge, this is the first report on using an ortho ester-based cross-linker to prepare pH-triggered chitosan nanogels as tumor carriers, which may provide a potential route for improved safety and to increase the therapeutic efficacy of anticancer therapy.

Nonviral cancer gene therapy: Delivery cascade and vector nanoproperty integration

Gene therapy represents a promising cancer treatment featuring high efficacy and limited side effects, but it is stymied by a lack of safe and efficient gene-delivery vectors. Cationic polymers and lipid-based nonviral gene vectors have many advantages and have been extensively explored for cancer gene delivery, but their low gene-expression efficiencies relative to viral vectors limit their clinical translations. Great efforts have thus been devoted to developing new carrier materials and fabricating functional vectors aimed at improving gene expression, but the overall efficiencies are still more or less at the same level. This review analyzes the cancer gene-delivery cascade and the barriers, the needed nanoproperties and the current strategies for overcoming these barriers, and outlines PEGylation, surface-charge, size, and stability dilemmas in vector nanoproperties to efficiently accomplish the cancer gene-delivery cascade. Stability, surface, and size transitions (3S Transitions) are proposed to resolve those dilemmas and strategies to realize these transitions are comprehensively summarized. The review concludes with a discussion of the future research directions to design high-performance nonviral gene vectors.

Control of Liposomal Penetration into Three-Dimensional Multicellular Tumor Spheroids by Modulating Liposomal Membrane Rigidity

Effective penetration of drug-carrying nanoparticles into solid tumors is a major challenge in cancer therapy. Exploration of the physicochemical properties of nanoparticles that affect penetration efficiency is required to achieve maximum therapeutic effects. Here, we used confocal laser scanning microscopy to evaluate the efficiencies of penetration of fluorescently labeled liposomes into three-dimensional spheroids composed of HeLa cells. The prepared liposomes were composed of phosphatidylcholines and varying contents of cholesterol and/or a polyethylene glycol-modified phospholipid. We demonstrated that the efficiency of penetration into spheroids increased with the bending modulus (i.e., membrane rigidity) of the liposome, as determined by atomic force microscopy (correlation coefficient, 0.84). To clarify the mechanism by which membrane rigidity contributes to the penetration behavior of liposomes, we also analyzed the cellular uptake using monolayer cells. We showed that penetration efficiency was explained partially by cellular uptake efficiency, but that other factors such as liposome diffusion efficiency in the intercellular space of tumor spheroids contributed. Our results quantitatively demonstrate that the bending modulus of the liposomal membrane is a major determinant of liposomal penetration into three-dimensional spheroids. The present study will contribute to the understanding and control of tumor penetration of liposomal formulations.

Inhibition of hexokinase-2 with targeted liposomal 3-bromopyruvate in an ovarian tumor spheroid model of aerobic glycolysis

BACKGROUND

The objective of this study was to evaluate the expression levels of glycolytic markers, especially hexokinase-2 (HK2), using a three-dimensional multicellular spheroid model of human ovarian adenocarcinoma (SKOV-3) cells and to develop an epidermal growth factor receptor-targeted liposomal formulation for improving inhibition of HK2 and the cytotoxicity of 3-bromopyruvate (3-BPA).

METHODS

Multicellular SKOV-3 tumor spheroids were developed using the hanging drop method and expression levels of glycolytic markers were examined. Non-targeted and epidermal growth factor receptor-targeted liposomal formulations of 3-BPA were formulated and characterized. Permeability and cellular uptake of the liposomal formulations in three-dimensional SKOV-3 spheroids was evaluated using confocal microscopy. The cytotoxicity and HK2 inhibition potential of solution form of 3-BPA was compared to the corresponding liposomal formulation by using cell proliferation and HK2 enzymatic assays.

RESULTS

SKOV-3 spheroids were reproducibly developed using the 96-well hanging drop method, with an average size of 900 µm by day 5. HK2 enzyme activity levels under hypoxic conditions were found to be higher than under normoxic conditions (P<0.0001, Student's t-test, unpaired and two-tailed). Liposomal formulations (both non-targeted and targeted) of 3-BPA showed a more potent inhibitory effect (P<0.001, Student's t-test, unpaired and two-tailed) at a dose of 50 µM than the aqueous solution form at 3, 6, and 24 hours post administration. Similarly, the cytotoxic activity 3-BPA at various concentrations (10 µM-100 µM) showed that the liposomal formulations had an enhanced cytotoxic effect of 2-5-fold (P<0.0001, Student's t-test, unpaired and two-tailed) when compared to the aqueous solution form for both 10 µM and 25 µM concentrations.

CONCLUSION

SKOV-3 spheroids developed by the hanging drop method can be used as a tumor aerobic glycolysis model for evaluation of therapies targeting the glycolytic pathway in cancer cells. Encapsulation of 3-BPA in a liposomal formulation improved permeability, HK2 inhibition, and cytotoxicity in the multicellular spheroid model.

Shock waves and DNA-cationic lipid assemblies: a synergistic approach to express exogenous genes in human cells

Cationic lipid/DNA complexes (lipoplexes) represent a powerful tool for cell transfection; however, their use is still limited by important concerns, including toxicity and poor internalization into deep tissues. In this work, we investigated the use of shock wave-induced acoustic cavitation in vitro for the transfection of lipoplexes in human embryo kidney 293 cells. We selected shock waves with the ability to internalize 10-kDa fluorescein isothiocyanate-dextran into cells while maintaining survival rates above 50%. Cell transfection was tested using the green fluorescent protein-encoding plasmid pCX::GFPGPI2. Confocal microscopy and fluorescence-assisted cell sorting analyses revealed successful transfection after treatments ranging from 1 to 3 min using 60 to 180 shock waves at peak amplitudes of 12.3 ± 1.5 MPa. Interestingly, the combination of shock waves and lipoplexes induced a 3.1- and 3.8-fold increase in the expression of the reporter gene compared with the use of lipoplexes or shock waves alone, respectively. These results indicate that cationic DNA assembly and shock waves act in a synergistic manner to promote transfection of human cells, revealing a potential approach for non-invasive site-specific gene therapy.

Sequential intra-intercellular nanoparticle delivery system for deep tumor penetration

To achieve deep tumor penetration of large-sized nanoparticles (NPs), we have developed a reversible swelling-shrinking nanogel in response to pH variation for a sequential intra-intercellular NP delivery. The nanogel had a crosslinked polyelectrolyte core, consisting of N-lysinal-N'-succinyl chitosan and poly(N-isopropylacrylamide), and a crosslinked bovine serum albumin shell, which was able to swell in an acidic environment and shrink back under neutral conditions. The swelling resulted in a rapid release of the encapsulated chemotherapeutics in the cancer cells for efficient cytotoxicity. After being liberated from the dead cells, the contractive nanogel could infect neighboring cancer cells closer to the center of the tumor tissue.

Opportunities and challenges for use of tumor spheroids as models to test drug delivery and efficacy

Multicellular spheroids are three dimensional in vitro microscale tissue analogs. The current article examines the suitability of spheroids as an in vitro platform for testing drug delivery systems. Spheroids model critical physiologic parameters present in vivo, including complex multicellular architecture, barriers to mass transport, and extracellular matrix deposition. Relative to two-dimensional cultures, spheroids also provide better target cells for drug testing and are appropriate in vitro models for studies of drug penetration. Key challenges associated with creation of uniformly sized spheroids, spheroids with small number of cells and co-culture spheroids are emphasized in the article. Moreover, the assay techniques required for the characterization of drug delivery and efficacy in spheroids and the challenges associated with such studies are discussed. Examples for the use of spheroids in drug delivery and testing are also emphasized. By addressing these challenges with possible solutions, multicellular spheroids are becoming an increasingly useful in vitro tool for drug screening and delivery to pathological tissues and organs.

Nanoscale drug delivery systems for enhanced drug penetration into solid tumors: current progress and opportunities

Poor penetration of anticancer drags into solid tumors significantly limits their efficacy. This phenomenon has long been observed for small-molecule chemotherapeutics, and it can be even more pronounced for nanoscale therapies. Nanoparticles have enormous potential for the treatment of cancer due to their wide applicability as drug delivery and imaging vehicles and their size-dependent accumulation into solid tumors by the enhanced permeability and retention (EPR) effect. Further, synthetic nanoparticles can be engineered to overcome barriers to drag delivery. Despite their promise for the treatment of cancer, relatively little work has been done to study and improve their ability to diffuse into solid tumors following passive accumulation in the tumor vasculature. In this review, we present the complex issues governing efficient penetration of nanoscale therapies into solid tumors. The current methods available to researchers to study nanoparticle penetration into malignant tumors are described, and the most recent works studying the penetration of nanoscale materials into solid tumors are summarized. We conclude with an overview of the important nanoparticle design parameters governing their tumor penetration, as well as by highlighting critical directions in this field.

Binding and transport of PAMAM-RGD in a tumor spheroid model: the effect of RGD targeting ligand density

The mechanisms governing the efficient tumor spheroid penetration and transport by poly(amidoamine) (PAMAM) dendrimers displaying varying numbers of cyclic RGD targeting peptides (2, 3, 7, or 10) were evaluated in this work. The cell-free binding affinities and cellular internalization kinetics of PAMAM-RGD conjugates to malignant glioma cells were determined experimentally, and the results were incorporated into a mathematical model to predict the transport of these materials through a multicellular tumor spheroid. The theoretical analysis demonstrated that greater RGD crosslinking may improve transport through tumor spheroids due to their decreased integrin-binding affinity. This study provides evidence that altering the density of tumor-targeting ligands from a drug delivery platform is a feasible way to optimize the tumor-penetration efficiency of an anticancer agent, and provides insight into the physicochemical mechanisms governing the relative effectiveness of these conjugates.

Longitudinal investigation of permeability and distribution of macromolecules in mouse malignant transformation using PET

PURPOSE

We apply positron emission tomography (PET) to elucidate changes in nanocarrier extravasation during the transition from premalignant to malignant cancer, providing insight into the use of imaging to characterize early cancerous lesions and the utility of nanoparticles in early disease.

EXPERIMENTAL DESIGN

Albumin and liposomes were labeled with (64)Cu (half-life 12.7 hours), and longitudinal PET and CT imaging studies were conducted in a mouse model of ductal carcinoma in situ. A pharmacokinetic model was applied to estimate the tumor vascular volume and permeability.

RESULTS

From early time points characterized by disseminated hyperproliferation, the enhanced vascular permeability facilitated lesion detection. During disease progression, the vascular volume fraction increased 1.6-fold and the apparent vascular permeability to albumin and liposomes increased ∼2.5-fold to 6.6 × 10(-8) and 1.3 × 10(-8) cm/s, respectively, with the accumulation of albumin increasing earlier in the disease process. In the malignant tumor, both tracers reached similar mean intratumoral concentrations of ∼6% ID/cc but the distribution of liposomes was more heterogeneous, ranging from 1% to 18% ID/cc compared with 1% to 9% ID/cc for albumin. The tumor-to-muscle ratio was 17.9 ± 8.1 and 7.1 ± 0.5 for liposomes and albumin, respectively, indicating a more specific delivery of liposomes than with albumin.

CONCLUSIONS

PET imaging of radiolabeled particles, validated by confocal imaging and histology, detected the transition from premalignant to malignant lesions and effectively quantified the associated changes in vascular permeability.

The pH-dependent association with cancer cells of tunable functionalized lipid vesicles with encapsulated doxorubicin for high cell-kill selectivity

To enable selective cell-kill, we designed functionalized lipid vesicles with pH-triggered heterogeneous membranes and encapsulated doxorubicin that exhibit tunable surface topography. These vesicles "hide" (mask) the targeting ligands from their surface during circulation in the blood, and only progressively "expose" these ligands as they gradually penetrate deeper into the tumor interstitium, where after endocytosis they burst release their contents. The stimulus to activate the binding reactivity is the pH gradient between the blood stream (pH 7.4-7.0) and the increasingly acidic pH inside the tumor interstitium (pH 6.7-6.5). Doxorubicin release is activated at the endosomal pH 5.5-5.0. We show that tunable functionalized vesicles exhibit environmentally-dependent (pH-dependent) association with cancer cells resulting in high cell-kill selectivity. When lowering the extracellular pH from 7.4 to 6.5, tunable functionalized vesicles deliver doxorubicin to cancer cells that increases from 41% to 93% of maximum resulting in cancer cell killing that increases from 23 to 71% of maximum, respectively. This proof-of-concept shows the potential of tunable targeted liposomal chemotherapy to selectively kill cancer cells in an environmentally-dependent way.

Enhanced loading efficiency and retention of 225Ac in rigid liposomes for potential targeted therapy of micrometastases

Targeted alpha-particle emitters are promising therapeutics for micrometastatic disease. Actinium-225 has a 10-day half-life and generates a total of four alpha-particles per parent decay rendering (225)Ac an attractive candidate for alpha-therapy. For cancer cells with low surface expression levels of molecular targets, targeting strategies of (225)Ac using radiolabeled carriers of low specific radioactivities (such as antibodies) may not deliver enough alpha-particle emitters at the targeted cancer cells to result in killing. We previously proposed and showed using passive (225)Ac entrapment that liposomes can stably retain encapsulated (225)Ac for long time periods, and that antibody-conjugated liposomes (immunoliposomes) with encapsulated (225)Ac can specifically target and become internalized by cancer cells. However, to enable therapeutic use of (225)Ac-containing liposomes, high activities of (225)Ac need to be stably encapsulated into liposomes. In this study, various conditions for active loading of (225)Ac in preformed liposomes (ionophore-type, encapsulated buffer solution, and loading time) were evaluated, and liposomes with up to 73 +/- 9% of the initial activity of (225)Ac (0.2-200 microCi) were developed. Retention of radioactive contents by liposomes was evaluated at 37 degrees C in phosphate buffer and in serum-supplemented media. The main fraction of released (225)Ac from liposomes occurs within the first two hours of incubation. Beyond this two hour point, the encapsulated radioactivity is released from liposomes slowly with an approximate half-life of the order of several days. In some cases, after 30 days, (225)Ac retention as high as 81 +/- 7% of the initially encapsulated radioactivity was achieved. The (225)Ac loading protocol was also applied to immunoliposome loading without significant loss of targeting efficacy. Liposomes with surface-conjugated antibodies that are loaded with (225)Ac overcome the limitations of low specific activity for molecular carriers and are expected to be therapeutically useful against tumor cells having a low antigen density.

Prodrug cancer gene therapy

There is no effective treatment for late stage and metastatic cancers of colorectal, prostate, pancreatic, breast, glioblastoma and melanoma cancers. Novel treatment modalities are needed for these late stage patients because cytotoxic chemotherapy offers only palliation, usually accompanied with systemic toxicities and poor quality of life. Gene directed enzyme prodrug therapy (GDEPT), which concentrates the cytotoxic effect in the tumor site may be one alternative. This review provides an explanation of the GDEPT principle, focusing on the development, application and potential of various GDEPTs. Current gene therapy limitations are in efficient expression of the therapeutic gene and in tumor-specific targeting. Therefore, the current status of research related to the enhancement of in situ GDEPT delivery and tumor-specific targeting of vectors is assessed. Finally, GDEPT versions of stem cell based gene therapy as another potential treatment modality for progressed tumors and metastases are discussed. Combinations of traditional, targeted, and stem cell directed gene therapy could significantly advance the treatment of cancer.

Methotrexate loaded polyether-copolyester dendrimers for the treatment of gliomas: enhanced efficacy and intratumoral transport capability

Therapeutic benefit in glial tumors is often limited due to low permeability of delivery systems across the blood-brain barrier (BBB), drug resistance, and poor penetration into the tumor tissue. In an attempt to overcome these hurdles, polyether-copolyester (PEPE) dendrimers were evaluated as drug carriers for the treatment of gliomas. Dendrimers were conjugated to d-glucosamine as the ligand for enhancing BBB permeability and tumor targeting. The efficacy of methotrexate (MTX)-loaded dendrimers was established against U87 MG and U 343 MGa cells. Permeability of rhodamine-labeled dendrimers and MTX-loaded dendrimers across the in vitro BBB model and their distribution into avascular human glioma tumor spheroids was also studied. Glucosylated dendrimers were found to be endocytosed in significantly higher amounts than nonglucosylated dendrimers by both the cell lines. IC 50 of MTX after loading in dendrimers was lower than that of the free MTX, suggesting that loading MTX in PEPE dendrimers increased its potency. Similar higher activity of MTX-loaded glucosylated and nonglucosylated dendrimers was found in the reduction of tumor spheroid size. These MTX-loaded dendrimers were able to kill even MTX-resistant cells highlighting their ability to overcome MTX resistance. In addition, the amount of MTX-transported across BBB was three to five times more after loading in the dendrimers. Glucosylation further increased the cumulative permeation of dendrimers across BBB and hence increased the amount of MTX available across it. Glucosylated dendrimers distributed through out the avascular tumor spheroids within 6 h, while nonglucosylated dendrimers could do so in 12 h. The results show that glucosamine can be used as an effective ligand not only for targeting glial tumors but also for enhanced permeability across BBB. Thus, glucosylated PEPE dendrimers can serve as potential delivery system for the treatment of gliomas.

Surface-active liposomes for targeted cancer therapy

An overview of liposome-based drug-delivery carriers to cancer cells is presented. Properties related to interfacial interactions between liposomes and the biological milieu that determine the fate of liposomes in vivo are discussed. Original approaches to improve specificity for the target and to control the structural responsiveness of liposomes, depending on their immediate environment, with the aim of enhancing the delivered therapeutic doses, are summarized. This review is not exhaustive on research examples of liposomes as carriers for cancer therapy but, rather, aims to describe major directions of designs and strategies over recent years. The current therapeutic trends that exhibit increasingly higher complexity in structures and responses are also discussed.

Enhanced retention of the alpha-particle-emitting daughters of Actinium-225 by liposome carriers

Targeted alpha-particle emitters hold great promise as therapeutics for micrometastatic disease. Because of their high energy deposition and short range, tumor targeted alpha-particles can result in high cancer-cell killing with minimal normal-tissue irradiation. Actinium-225 is a potential generator for alpha-particle therapy: it decays with a 10-day half-life and generates three alpha-particle-emitting daughters. Retention of (225)Ac daughters at the target increases efficacy; escape and distribution throughout the body increases toxicity. During circulation, molecular carriers conjugated to (225)Ac cannot retain any of the daughters. We previously proposed liposomal encapsulation of (225)Ac to retain the daughters, whose retention was shown to be liposome-size dependent. However, daughter retention was lower than expected: 22% of theoretical maximum decreasing to 14%, partially due to the binding of (225)Ac to the phospholipid membrane. In this study, Multivesicular liposomes (MUVELs) composed of different phospholipids were developed to increase daughter retention. MUVELs are large liposomes with entrapped smaller lipid-vesicles containing (225)Ac. PEGylated MUVELs stably retained over time 98% of encapsulated (225)Ac. Retention of (213)Bi, the last daughter, was 31% of the theoretical maximum retention of (213)Bi for the liposome sizes studied. MUVELs were conjugated to an anti-HER2/neu antibody (immunolabeled MUVELs) and were evaluated in vitro with SKOV3-NMP2 ovarian cancer cells, exhibiting significant cellular internalization (83%). This work demonstrates that immunolabeled MUVELs might be able to deliver higher fractions of generated alpha-particles per targeted (225)Ac compared to the relative fractions of alpha-particles delivered by (225)Ac-labeled molecular carriers.