In vitro and in vivo antitumor activity of liposomal Fenretinide targeted to human neuroblastoma

GD2-targeting therapy: a comparative analysis of approaches and promising directions

Disialoganglioside GD2 is a promising target for immunotherapy with expression primarily restricted to neuroectodermal and epithelial tumor cells. Although its role in the maintenance and repair of neural tissue is well-established, its functions during normal organism development remain understudied. Meanwhile, studies have shown that GD2 plays an important role in tumorigenesis. Its functions include proliferation, invasion, motility, and metastasis, and its high expression and ability to transform the tumor microenvironment may be associated with a malignant phenotype. Structurally, GD2 is a glycosphingolipid that is stably expressed on the surface of tumor cells, making it a suitable candidate for targeting by antibodies or chimeric antigen receptors. Based on mouse monoclonal antibodies, chimeric and humanized antibodies and their combinations with cytokines, toxins, drugs, radionuclides, nanoparticles as well as chimeric antigen receptor have been developed. Furthermore, vaccines and photoimmunotherapy are being used to treat GD2-positive tumors, and GD2 aptamers can be used for targeting. In the field of cell therapy, allogeneic immunocompetent cells are also being utilized to enhance GD2 therapy. Efforts are currently being made to optimize the chimeric antigen receptor by modifying its design or by transducing not only αβ T cells, but also γδ T cells, NK cells, NKT cells, and macrophages. In addition, immunotherapy can combine both diagnostic and therapeutic methods, allowing for early detection of disease and minimal residual disease. This review discusses each immunotherapy method and strategy, its advantages and disadvantages, and highlights future directions for GD2 therapy.

The Pyrazolo[3,4-d]Pyrimidine Derivative Si306 Encapsulated into Anti-GD2-Immunoliposomes as Therapeutic Treatment of Neuroblastoma

Si306, a pyrazolo[3,4-d]pyrimidine derivative recently identified as promising anticancer agent, has shown favorable in vitro and in vivo activity profile against neuroblastoma (NB) models by acting as a competitive inhibitor of c-Src tyrosine kinase. Nevertheless, Si306 antitumor activity is associated with sub-optimal aqueous solubility, which might hinder its further development. Drug delivery systems were here developed with the aim to overcome this limitation, obtaining suitable formulations for more efficacious in vivo use. Si306 was encapsulated in pegylated stealth liposomes, undecorated or decorated with a monoclonal antibody able to specifically recognize and bind to the disialoganglioside GD2 expressed by NB cells (LP[Si306] and GD2-LP[Si306], respectively). Both liposomes possessed excellent morphological and physio-chemical properties, maintained over a period of two weeks. Compared to LP[Si306], GD2-LP[Si306] showed in vitro specific cellular targeting and increased cytotoxic activity against NB cell lines. After intravenous injection in healthy mice, pharmacokinetic profiles showed increased plasma exposure of Si306 when delivered by both liposomal formulations, compared to that obtained when Si306 was administered as free form. In vivo tumor homing and cytotoxic effectiveness of both liposomal formulations were finally tested in an orthotopic animal model of NB. Si306 tumor uptake resulted significantly higher when encapsulated in GD2-LP, compared to Si306, either free or encapsulated into untargeted LP. This, in turn, led to a significant increase in survival of mice treated with GD2-LP[Si306]. These results demonstrate a promising antitumor efficacy of Si306 encapsulated into GD2-targeted liposomes, supporting further therapeutic developments in pre-clinical trials and in the clinic for NB.

Retinoids Delivery Systems in Cancer: Liposomal Fenretinide for Neuroectodermal-Derived Tumors

Retinoids are a class of natural and synthetic compounds derived from vitamin A. They are involved in several biological processes like embryogenesis, reproduction, vision, growth, inflammation, differentiation, proliferation, and apoptosis. In light of their important functions, retinoids have been widely investigated for their therapeutic applications. Thus far, their use for the treatment of several types of cancer and skin disorders has been reported. However, these therapeutic agents present several limitations for their widespread clinical translatability, i.e., poor solubility and chemical instability in water, sensitivity to light, heat, and oxygen, and low bioavailability. These characteristics result in internalization into target cells and tissues only at low concentration and, consequently, at an unsatisfactory therapeutic dose. Furthermore, the administration of retinoids causes severe side-effects. Thus, in order to improve their pharmacological properties and circulating half-life, while minimizing their off-target uptake, various retinoids delivery systems have been recently developed. This review intends to provide examples of retinoids-loaded nano-delivery systems for cancer treatment. In particular, the use and the therapeutic results obtained by using fenretinide-loaded liposomes against neuroectodermal-derived tumors, such as melanoma, in adults, and neuroblastoma, the most common extra-cranial solid tumor of childhood, will be discussed.

Drug Delivery Systems: Application of Liposomal Anti-Tumor Agents to Neuroectodermal Cancer Treatment

Disseminated neuroectoderma-derived tumors, mainly neuroblastoma in childhood and melanoma in the adulthood, are refractory to most current therapeutic regimens and hence the prognosis remains very poor. Preclinical research studies have indicated several agents that show promising therapeutic potential for these neoplasms. However, there appears to be a limitation to their in vivo applicability, mainly due to unfavorable pharmacokinetic properties that lead to insufficient drug delivery to the tumor or metastatic sites or to high systemic or organ-specific toxicity. In this scenario, the focus is on targeted cancer therapy. Encapsulating anticancer drugs in liposomes enables targeted drug delivery to tumor tissue and prevents damage to the normal surrounding tissue. Indeed, sterically stabilized liposomes have been shown to enhance the selective localization of entrapped drugs to solid tumors, with improvements in therapeutic indices. The identification of tumor-associated antigens and/or genes and the relative ease of manipulating the physicochemical features of liposome hold promise for the development of novel therapeutic strategies that selectively target tumor cells. Combined targeting is still investigated, especially the availability to simultaneously target and kill both the cancer cells and the tumor vasculature. Animal models make it possible to link molecular genetics and biochemistry information to the physiological basis of disease and are important predictive tools that offer a frontline testing system for studying the involvement of specific genes and the efficacy of novel therapeutics approaches. Relevant experimental models of human neuroblastoma and melanoma, which better reflect the tumor behavior in patients, are required to evaluate the effectiveness of the various targeted liposomal formulations and their possible systemic and organ-specific toxicity. The most multifunctional targeted liposomes are herein described, with primary attention on testing their efficacy in clinically relevant animal models for the treatment of neuroblastoma and melanoma.

Anti-GD2 immunotherapy for neuroblastoma

INTRODUCTION

Current therapeutic approaches for high-risk neuroblastoma (HR-NB) include high-dose chemotherapy, surgery and radiotherapy; interventions that are associated with long and short-term toxicities. Effective immunotherapy holds particular promise for improving survival and quality of life by reducing exposure to cytotoxic agents. GD2, a surface glycolipid is the most common target for immunotherapy. Areas covered: We review the status of anti-GD2 immunotherapies currently in clinical use for neuroblastomas and novel GD2-targeted strategies in preclinical development. Expert commentary: Anti-GD2 monoclonal antibodies are associated with improved survival in patients in their first remission and are increasingly being used for chemorefractory and relapsed neuroblastoma. As protein engineering technology has become more accessible, newer antibody constructs are being tested. GD2 is also being targeted by natural killer cells and T-cells. Active immunity can be elicited by anti-GD2 vaccines. The rational combination of currently available and soon-to-emerge immunotherapeutic approaches, and their integration into conventional multimodality therapies will require further investigation to optimize their use for HR-NB.

Anti-GD2 mAbs and next-generation mAb-based agents for cancer therapy

Tumor-specific monoclonal antibodies (mAbs) have demonstrated efficacy in the clinic, becoming an important approach for cancer immunotherapy. Due to its limited expression on normal tissue, the GD2 disialogangloside expressed on neuroblastoma cells is an excellent candidate for mAb therapy. In 2015, dinutuximab (an anti-GD2 mAb) was approved by the US FDA and is currently used in a combination immunotherapeutic regimen for the treatment of children with high-risk neuroblastoma. Here, we review the extensive preclinical and clinical development of anti-GD2 mAbs and the different mechanisms by which they mediate tumor cell killing. In addition, we discuss different mAb-based strategies that capitalize on the targeting ability of anti-GD2 mAbs to potentially deliver, as monotherapy, or in combination with other treatments, improved antitumor efficacy.

Novel mechanisms of action of classical chemotherapeutic agents on sphingolipid pathways

The prevailing mechanisms of action of traditional chemotherapeutic agents have been challenged by sphingolipid cancer research. Many studies have shown that ceramide generation in response to cytotoxic agents is central to tumor cell death. Ceramide can be generated either via hydrolysis of cell-membrane sphingomyelin by sphingomyelinases, hydrolysis of cerebrosides, or via de novo synthesis by ceramide synthases. Ceramide can act as a second messenger for apoptosis, senescence or autophagy. Inherent or acquired alterations in the sphingolipid pathway can account for resistance to the classic chemotherapeutic agents. In particular, it has been shown that activation of the acid ceramidase can lead to the formation of sphingosine 1-phosphate, which then antagonizes ceramide signaling by initiating a pro-survival signaling pathway. Furthermore, ceramide glycosylation catalyzed by glucosylceramide synthase converts ceramide to glucosylceramide, thus eliminating ceramide and consequently protecting cancer cells from apoptosis. In this review, we describe the effects of some of the most commonly used chemotherapeutic agents on ceramide generation, with a particular emphasis on strategies used to enhance the efficacy of these agents.

GD2-targeted immunotherapy and radioimmunotherapy

Ganglioside GD2 is a tumor-associated surface antigen found in a broad spectrum of human cancers and stem cells. They include pediatric embryonal tumors (neuroblastoma, retinoblastoma, brain tumors, osteosarcoma, Ewing sarcoma, rhabdomyosarcoma), as well as adult cancers (small cell lung cancer, melanoma, soft tissue sarcomas). Because of its restricted normal tissue distribution, GD2 has been proven safe for antibody targeting. Anti-GD2 antibody is now incorporated into the standard of care for the treatment of high-risk metastatic neuroblastoma. Building on this experience, novel combinations of antibodies, cytokines, cells, and genetically engineered products all directed at GD2 are rapidly moving into the clinic. In this review, past and present immunotherapy trials directed at GD2 will be summarized, highlighting the lessons learned and the future directions.

Engineering anti-GD2 monoclonal antibodies for cancer immunotherapy

Ganglioside GD2 is highly expressed on neuroectoderm-derived tumors and sarcomas, including neuroblastoma, retinoblastoma, melanoma, small cell lung cancer, brain tumors, osteosarcoma, rhabdomyosarcoma, Ewing's sarcoma in children and adolescents, as well as liposarcoma, fibrosarcoma, leiomyosarcoma and other soft tissue sarcomas in adults. Since GD2 expression in normal tissues is restricted to the brain, which is inaccessible to circulating antibodies, and in selected peripheral nerves and melanocytes, it was deemed a suitable target for systemic tumor immunotherapy. Anti-GD2 antibodies have been actively tested in clinical trials for neuroblastoma for over the past two decades, with proven safety and efficacy. The main limitations have been acute pain toxicity associated with GD2 expression on peripheral nerve fibers and the inability of antibodies to treat bulky tumor. Several strategies have been developed to reduce pain toxicity, including bypassing complement activation, using blocking antibodies, or targeting of O-acetyl-GD2 derivative that is not expressed on peripheral nerves. To enhance anti-tumor efficacy, anti-GD2 monoclonal antibodies and fragments have been engineered into immunocytokines, immunotoxins, antibody drug conjugates, radiolabeled antibodies, targeted nanoparticles, T-cell engaging bispecific antibodies, and chimeric antigen receptors. The challenges of these approaches will be reviewed to build a perspective for next generation anti-GD2 therapeutics in cancer therapy.

Nanocarrier-mediated targeting of tumor and tumor vascular cells improves uptake and penetration of drugs into neuroblastoma

Neuroblastoma (NB) is the most common extracranial solid tumor in children, accounting for about 8% of childhood cancers. Despite aggressive treatment, patients suffering from high-risk NB have very poor 5-year overall survival rate, due to relapsed and/or treatment-resistant tumors. A further increase in therapeutic dose intensity is not feasible, because it will lead to prohibitive short-term and long-term toxicities. New approaches with targeted therapies may improve efficacy and decrease toxicity. The use of drug delivery systems allows site specific delivery of higher payload of active agents associated with lower systemic toxicity compared to the use of conventional (“free”) drugs. The possibility of imparting selectivity to the carriers to the cancer foci through the use of a targeting moiety (e.g., a peptide or an antibody) further enhances drug efficacy and safety. We have recently developed two strategies for increasing local concentration of anti-cancer agents, such as CpG-containing oligonucleotides, small interfering RNAs, and chemotherapeutics in NB. For doing that, we have used the monoclonal antibody anti-disialoganglioside (GD₂), able to specifically recognize the NB tumor and the peptides containing NGR and CPRECES motifs, that selectively bind to the aminopeptidase N-expressing endothelial and the aminopeptidase A-expressing perivascular tumor cells, respectively. The review will focus on the use of tumor- and tumor vasculature-targeted nanocarriers to improve tumor targeting, uptake, and penetration of drugs in preclinical models of human NB.

Enhanced anti-tumor and anti-angiogenic efficacy of a novel liposomal fenretinide on human neuroblastoma

Neuroblastoma is an embryonal tumor originating from the simpatico-adrenal lineage of the neural crest. It approximately accounts for about 15% of all pediatric oncology deaths. Despite advances in multimodal therapy, metastatic neuroblastoma tumors at diagnosis remain a clinical challenge. Retinoids are a class of compounds known to induce both terminal differentiation and apoptosis/necrosis of neuroblastoma cells. Among them, fenretinide (HPR) has been considered one of the most promising anti-tumor agent but it is partially efficacious due to both poor aqueous solubility and rapid metabolism. Here, we have developed a novel HPR formulation, by which the drug was encapsulated into sterically stabilized nanoliposomes (NL[HPR]) according to the Reverse Phase Evaporation method. This procedure led to a higher structural integrity of liposomes in organic fluids for a longer period of time, in comparison with our previous liposomal formulation developed by the film method. Moreover, NL[HPR] were further coupled with NGR peptides for targeting the tumor endothelial cell marker, aminopeptidase N (NGR-NL[HPR]). Orthotopically xenografted neuroblastoma-bearing mice treated with NGR-NL[HPR] lived statistically longer than mice untreated or treated with free HPR (NGR-NL[HPR] vs both control and HPR: P<0.0001). Also, NL[HPR] resulted in a statistically improved survival (NL[HPR] vs both control and HPR: P<0.001) but to a less extent if compared with that obtained with NGR-NL[HPR] (NGR-NL[HPR] vs NL[HPR]: P<0.01). Staining of tumor sections with antibodies specific for neuroblastoma and for either pericytes or endothelial cells evidenced that HPR reduced neuroblastoma growth through both anti-tumor and anti-angiogenic effects, mainly when delivered by NGR-NL[HPR]. Indeed, in this group of mice a marked reduction of tumor progression, of intra-tumoral vessel counts and VEGF expression, together with a marked down-modulation of matrix metalloproteinases MMP2 and MMP9, was observed. In conclusion, the use of this novel targeted delivery system for the apoptotic and antiangiogenic drug, fenretinide, could be considered as an adjuvant tool in the future treatment of neuroblastoma patients.

Building and characterizing antibody-targeted lipidic nanotherapeutics

Immunoliposomes provide a complementary, and in many instances advantageous, drug delivery strategy to antibody-drug conjugates. Their high carrying capacity of 20,000-150,000 drug molecules/liposome, allows for the use of a significantly broader range of moderate-to-high potency small molecule drugs when compared to the comparably few subnanomolar potency maytansinoid- and auristatin-based immunoconjugates. The multivalent display of 5-100 antibody fragments/liposome results in an avidity effect that can make use of even moderate affinity antibodies, as well as a cross-linking of cell surface receptors to induce the internalization required for intracellular drug release and subsequent activity. The underlying liposomal drug must be effectively engineered for long circulating pharmacokinetics and stable in vivo drug retention in order to allow for the drug to be efficiently delivered to the target tissue and take advantage of the site-specific bioavailability provided for by the targeting arm. In this chapter, we describe the rationale for engineering stable immunoliposome-based therapeutics, methods required for preparation of immunoliposomes, as well as for their physicochemical and in vivo characterization.

Enhanced anti-neuroblastoma activity of a fenretinide complexed form after intravenous administration

OBJECTIVES

The major limitation to successful chemotherapy of neuroblastoma (NB) is the toxicity and the poor bioavailability of traditional drugs.

METHODS

We synthesised an amphiphilic dextrin derivative (DX-OL) able to host fenretinide (4-HPR) by complexation. In this study, we have investigated the effects of 4-HPR-loaded amphipilic dextrin (DX-OL/4-HPR) in comparison with 4-HPR alone both in vitro on human NB cells and in vivo in pseudometastatic NB models. The haemolysis assay was used as a measure of the potential damage caused by the pharmaceutical formulation in vivo. Pharmacokinetic experiments were performed to assess drug plasma levels in mice treated with free or complexed 4-HPR.

KEY FINDINGS

DX-OL/4-HPR exerted a more potent cytotoxic activity on NB cells. Complexed 4-HPR significantly increased the proportion of sub-G1 cells with respect to free 4-HPR. Dextrin derivatives showed no haemolytic activity, indicating their suitability for parenteral administration. DX-OL/4-HPR increased the lifespan and the long-term survival of treated mice over controls. The analysis of drug plasma levels indicates that the complexed drug has a higher AUC due to a reduced clearance from the blood.

CONCLUSIONS

Our data suggest that DX-OL/4-HPR is an injectable formulation that is able to improve drug aqueous solubility and bioavailability.

Neuroblastoma-targeted nanoparticles entrapping siRNA specifically knockdown ALK

RNA interference molecules have some advantages as cancer therapeutics, including a proved efficacy on both wild-type (WT) and mutated transcripts and an extremely high sequence-specificity. The most significant hurdle to be overcome if exogenous small interfering RNAs (siRNA) is to be used therapeutically is the specific, effective, nontoxic delivery of siRNA to its intracellular site of action. At present, human applications are confined almost exclusively to targets within the liver, where the delivery systems naturally accumulate, and extra-hepatic targets remain a challenge. Anaplastic lymphoma kinase (ALK) is a receptor tyrosine kinase that has recently been shown to contribute to the cell growth and progression of human neuroblastoma (NB). We investigated its potential as a therapeutic target in NB by generating anti-GD₂-targeted nanoparticles that carry ALK-directed siRNA, which are specifically and efficiently delivered to GD₂-expressing NB cells. Relative to free ALK-siRNA, anti-GD₂-targeted liposomal formulations of ALK-siRNA had low plasma clearance, increased siRNA stability, and improved binding, uptake, silencing and induction of cell death, and specificity for NB cells. In NB xenografts, intravenous (i.v.) injection of the targeted ALK-siRNA liposomes showed gene-specific antitumor activity with no side effects. ALK-selective siRNA entrapped in anti-GD₂-targeted nanoparticles is a promising new modality for NB treatment.

Anti-GD2 antibody therapy for GD2-expressing tumors

In the development of novel immune therapies for high-risk cancers, one goal is to find tumor targets that are not widely shared by normal cells. One such target is the surface disialoganglioside GD2. This antigen is expressed on the surface of a variety of tumors for which no curative therapies exist for patients with advanced disease. In childhood, the most common GD2-expressing tumor is neuroblastoma. GD2 is also expressed on several other high-risk tumors, including those of neuroectodermal or epithelial origin, virtually all melanomas, and approximately 50% of tumor samples from osteosarcoma and soft-tissue sarcomas. Because of the tumor-selective expression of this molecule, it is an attractive target for tumor-specific therapies such as antibody therapy. Over the last 2 decades, several anti-GD2 antibodies have been developed. To reduce both the toxicity of the antibody and the development of human anti-mouse antibodies (HAMA), research efforts have primarily focused on exploring anti-GD2 antibodies that have progressively more human elements while at the same time reducing the mouse components. This review will examine antibodies currently undergoing clinical testing as well as the most recent advances to improve antibody therapy for patients with GD2-expressing tumors.

Regulation of endoplasmic reticulum stress-induced cell death by ATF4 in neuroectodermal tumor cells

The neuroectodermal tumors neuroblastoma and melanoma represent biologically aggressive and chemoresistant cancers. The chemotherapeutic agents fenretinide and bortezomib induce apoptosis through endoplasmic reticulum (ER) stress in these tumor types. The aim of this study was to test the hypothesis that the early events of ER stress signaling and response pathways induced by fenretinide and bortezomib are mediated by the eukaryotic initiation factor 2alpha (eIF2alpha)-ATF4 signaling pathway. Treatment of neuroblastoma and melanoma cell lines with fenretinide, bortezomib, or thapsigargin resulted in induction of eIF2alpha signaling, characterized by increased expression of phosphorylated eIF2alpha, ATF4, ATF3, and GADD34. These events correlated with induction of the pro-apoptotic protein Noxa. The cytotoxic response, characterized by up-regulation of Noxa and cell death, was dependent on ATF4, but not the ER-related pro-death signaling pathways involving GADD153 or IRE1. Although PERK-dependent phosphorylation of eIF2alpha enhanced ATF4 protein levels during ER stress, cell death in response to fenretinide, bortezomib, or thapsigargin was not abrogated by inhibition of eIF2alpha phosphorylation through PERK knockdown or overexpression of wild-type eIF2alpha. Furthermore, ATF4 induction in response to ER stress was dependent primarily on transcriptional activation, which occurred in a PERK- and phosphorylated eIF2alpha-independent manner. These results demonstrate that ATF4 mediates ER stress-induced cell death of neuroectodermal tumor cells in response to fenretinide or bortezomib. Understanding the complex regulation of cell death pathways in response to ER stress-inducing drugs has the potential to reveal novel therapeutic targets, thus allowing the development of improved treatment strategies to overcome chemoresistance.

Anti-IL-10R antibody improves the therapeutic efficacy of targeted liposomal oligonucleotides

High-risk Neuroblastoma (NB) has still a poor prognosis. Liposomes targeted to NB cells and encapsulating antisense CpG-containing oligonucleotides (TL-asCpG) had increased anti-tumour efficacy in NB xenografts compared to free asCpG. Interleukin 10 (IL-10) suppresses antigen presenting cell activation contributing to tumour-mediated immune suppression. In principle, combination of TL-asCpG and antibodies against IL-10 receptor (aIL-10R) could prolong immune system activation, leading to better therapeutic results. Mice treated with TL-asCpG 4 h after human NB cell inoculation survived significantly longer than controls. An increased life span was achieved also in mice receiving TL-asCpG 24 and 72 h after NB cell challenge. The addition of aIL-10R to TL-asCpG in the 4-h protocol significantly increased the percentage of long term survivors compared to TL-asCpG only. Surviving mice treated with the combined strategy were completely cured. In contrast, long term surviving mice treated only with TL-asCpG presented lymph node infiltration with NB cells. TL-asCpG plus aIL-10R treatment was significantly superior to TL-asCpG alone also for the 24-h protocol. Ex vivo experiments demonstrated that the combined therapy evoked a stronger and more prolonged immune system activation compared to monotherapy. These results support the feasibility of a clinical trial with TL-asCpG and aIL-10R in advanced NB patients.

Antibody-based immunotherapy in high-risk neuroblastoma

Although great advances have been made in the treatment of low- and intermediate-risk neuroblastoma in recent years, the prognosis for advanced disease remains poor. Therapies based on monoclonal antibodies that specifically target tumour cells have shown promise for treatment of high-risk neuroblastoma. This article reviews the use of monoclonal antibodies either as monotherapy or as part of a multifaceted treatment approach for advanced neuroblastoma, and explains how toxins, cytokines, radioactive isotopes or chemotherapeutic drugs can be conjugated to antibodies to enhance their effects. Tumour resistance, the development of blocking antibodies, and other problems hindering the effectiveness of monoclonal antibodies are also discussed. Future therapies under investigation in the area of immunotherapy for neuroblastoma are considered.

Fenretinide-polyvinylalcohol conjugates: new systems allowing fenretinide intravenous administration

N-(4-hydroxyphenyl)retinamide (fenretinide, 4-HPR) has been shown to be active toward many tumors without appreciable side effects. However its in vitro activity does not match a correspondent efficacy in vivo. The main reason is that the drug's hydrophobicity hinders its bioavailability in the body fluids. Even if the drug is previously dissolved in organic solvents, such as ethanol or DMSO, the subsequent dilution in body fluids trigger its precipitation in fine aggregates characterized by very low dissolution efficiency, never reaching amounts suitable for therapeutic response. To date no intravenous formulation of 4-HPR exists on the market. The 4-HPR linkage to a hydrophilic polymer by a covalent bond easily hydrolyzable in aqueous environment is expected to increase the drug's aqueous solubility, providing the free drug after hydrolysis of the covalent bond. This may be a useful tool for the preparation of aqueous intravenous formulations of 4-HPR. For this purpose, we linked 4-HPR to polyvinylalcohol (PVA) by a carbonate bond at different drug/hydroxy vinyl monomer molar ratios. We demonstrated that conjugation increased 4-HPR aqueous solubility and strongly inhibited neuroblastoma cell proliferation. In addition, in an in vivo neuroblastoma metastatic model, we obtained a significant antitumor effect as a consequence of the improved drug bioavailability.

Targeting liposomal chemotherapy via both tumor cell-specific and tumor vasculature-specific ligands potentiates therapeutic efficacy

Neuroblastoma, the most common solid tumor of infancy derived from the sympathetic nervous system, continues to present a formidable clinical challenge. Sterically stabilized immunoliposomes (SIL) have been shown to enhance the selective localization of entrapped drugs to solid tumors, with improvements in therapeutic indices. We showed that SIL loaded with doxorubicin (DXR) and targeted to the disialoganglioside receptor GD(2) [aGD(2)-SIL(DXR)] led to a selective inhibition of the metastatic growth of experimental models of human neuroblastoma. By coupling NGR peptides that target the angiogenic endothelial cell marker aminopeptidase N to the surface of DXR-loaded liposomes [NGR-SL(DXR)], we obtained tumor regression, pronounced destruction of the tumor vasculature, and prolonged survival of orthotopic neuroblastoma xenografts. Here, we showed good liposome stability, long circulation times, and enhanced time-dependent tumor accumulation of both the carrier and the drug. Antivascular effects against animal models of lung and ovarian cancer were shown for formulations of NGR-SL(DXR). In the chick embryo chorioallantoic assay, NGR-SL(DXR) substantially reduced the angiogenic potential of various neuroblastoma xenografts, with synergistic inhibition observed for the combination of NGR-SL(DXR) with aGD(2)-SIL(DXR). A significant improvement in antitumor effects was seen in neuroblastoma-bearing animal models when treated with the combined formulations compared with control mice or mice treated with either tumor- or vascular-targeted liposomal formulations, administered separately. The combined treatment resulted in a dramatic inhibition of tumor endothelial cell density. Long-term survivors were obtained only in animals treated with the combined tumor- and vascular-targeted formulations, confirming the pivotal role of combination therapies in treating aggressive metastatic neuroblastoma.

In vitro and In vivo Antitumor Activity of the Novel Derivatized Polyvinyl Alcohol-Based Polymer P10(4)

PURPOSE

The major limitation to successful chemotherapy of neuroblastoma is the toxicity of traditional antitumor drugs. Hence, less toxic and more effective drugs are to be found, and novel formulations of conventional compounds allowing a more favorable biodistribution should be sought for. In an attempt to pursue this task, we recently synthesized an amphiphilic polymer based on a polyvinyl alcohol backbone [P10(4)].

EXPERIMENTAL DESIGN

The cytotoxic activity of P10(4) was evaluated both in vitro on neuroblastoma and melanoma cell lines and in vivo in pseudometastatic neuroblastoma models. Apoptosis was assessed by morphology, cytofluorimetric analysis of DNA content, and DNA fragmentation assay. Caspases activation was investigated by kits specific for caspase-1, caspase-2, caspase-3, caspase-4, caspase-6, caspase-7, caspase-8, caspase-9, caspase-10, and caspase-13. Colony formation was evaluated by soft agar assay.

RESULTS

P10(4) exerted a potent cytotoxic activity on different neuroblastoma and melanoma cell lines through induction of both extrinsic and intrinsic caspase cascades and subsequent apoptosis. Moreover, the clonogenic potential of cells that survived P10(4) treatment was strongly reduced. Next, we tested the effects of P10(4) in nude mice injected with both a human and a murine neuroblastoma cell lines i.v. P10(4) significantly increased the life span and the long-term survival of treated mice over controls. No side effects were observed, even at doses higher than those used for therapeutic purposes.

CONCLUSIONS

Our data suggest that P10(4) holds promise as an anticancer compound and, because of its lack of interaction with DNA, is unlikely to give rise to drug resistance.

Development of ligand-targeted liposomes for cancer therapy

The continued evolution of targeted liposomal therapeutics has resulted in new agents with remarkable antitumour efficacy and relatively mild toxicity profiles. A careful selection of the ligand is necessary to reduce immunogenicity, retain extended circulation lifetimes, target tumour-specific cell surface epitopes, and induce internalisation and subsequent release of the therapeutic substance from the liposome. Methods for assembling targeted liposomes, including a novel micellar insertion technology, for incorporation of targeting molecules that efficiently transforms a non-targeted liposomal therapeutic to a targeted one, greatly assist the translation of targeted liposome technology into the clinic. Targeting strategies with liposomes directed at solid tumours and vascular targets are discussed. The authors believe the development of ligand-targeted liposomes is now in the advanced stage and offers unique and important advantages among other targeted therapies. Anti-HER2 immunoliposomal doxorubicin is awaiting Phase I clinical trials, the results of which should provide new insights into the promise of ligand-targeted liposomal therapies.

Immune cell-mediated antitumor activities of GD2-targeted liposomal c-myb antisense oligonucleotides containing CpG motifs

BACKGROUND

Expression of the c-myb proto-oncogene in neuroblastoma, the most common extracranial solid tumor of infancy, is linked with cell proliferation and differentiation. Neuroblastoma can be selectively targeted via monoclonal antibodies against the disialoganglioside (GD2) tumor-associated antigen. Liposomes coated with anti-GD2 antibodies (targeted liposomes) and entrapping a c-myb antisense oligonucleotide have antitumor activity. Because antisense oligonucleotides containing CpG motifs can stimulate immune responses, we evaluated the effect of CpG-containing c-myb antisense oligonucleotides encapsulated within targeted liposomes.

METHODS

Antisense (myb-as) and scrambled (myb-scr) control oligonucleotides with CpG motifs were encapsulated within GD2-targeted and non-targeted liposomes. Two murine (nude and SCID-bg) xenograft models of neuroblastoma were established. Mice (groups of 10) were injected intravenously with various oligonucleotide and liposome formulations, and life span, long-term survival, immune cell activation, and cytokine release were measured over time.

RESULTS

Tumor-bearing mice injected with targeted liposome-CpG-myb-as or targeted liposome-CpG-myb-scr lived longer than mice in any other group, although long-term survival (i.e., more than 120 days) was obtained only in mice injected with targeted liposome-CpG-myb-as. Splenocytes isolated from mice injected with targeted liposome-CpG-myb-as contained activated macrophages, B cells, and natural killer (NK) cells, but only activated NK cells were associated with antitumor cytotoxic activity. In vivo immune cell activation was accompanied by the time-dependent increases in plasma levels of the cytokines interleukin 12 (IL-12; maximum level reached by 2 hours) and interferon gamma (IFN-gamma; maximum level reached by 18 hours) and was dependent on the oligonucleotide CpG motif. Ablation of macrophages or NK cells resulted in a loss of in vivo antitumor activity.

CONCLUSION

Immune cell activation, involving the time-dependent activation of macrophages and NK cells, contributes to the antitumor activity of targeted liposome-CpG-myb-as against neuroblastoma and could improve the effectiveness of antitumor targeted liposomes.

Active targeting with particulate drug carriers in tumor therapy: fundamentals and recent progress

Drug therapy for the treatment of tumors is often limited by a narrow therapeutic index. One approach that overcomes this limitation is the active targeting of tumors with particulate drug carriers. The derivatization of particulate drug carriers with a ligand leads to the selective targeting of the particulate to selected cells, thereby focusing drug delivery. In addition, particulate drug carriers have a high loading capacity, do not need covalent conjugation of the drug and the formulation protects the entrapped drug from enzymatic inactivation. Despite these favorable properties, their therapeutic efficacy in animal models has been reported only in recent years. The use of internalizing ligands and the targeting of intravascular tumor cells and endothelial cells of tumor blood vessels have been instrumental in demonstrating the clinical effectiveness of particulate drug carriers in animal models. As a result, several actively targeted particulate carriers have now entered, or are about to enter, clinical investigation. Recent findings, for example, the identification of cell-penetrating peptides with restricted cell selectivity, suggest that further improvements in this approach are likely in the near future.

The chemopreventive agent N-(4-hydroxyphenyl)retinamide induces apoptosis through a mitochondrial pathway regulated by proteins from the Bcl-2 family

N-(4-hydroxyphenyl)retinamide (4-HPR, fenretinide) is a potent chemopreventive agent whose effect has been suggested to involve apoptosis induction. 4-HPR induces a loss of the mitochondrial transmembrane potential and the mitochondrial release of cytochrome c before caspase activation. Inhibition of mitochondrial membrane permeabilization (MMP) by transfection with Bcl-2 or the Cytomegalovirus UL37 gene product vMIA prevented caspase activation and cell death. In contrast to other retinoid derivatives, 4-HPR has no direct MMP-inducing effects when added to isolated mitochondria or when added to proteoliposomes containing the MMP-regulatory permeability transition pore complex (PTPC). Moreover, although reactive oxygen species (ROS) overproduction appears to be instrumental for 4-HPR-induced MMP and apoptosis, inhibition of the NF-kappaB or p53-mediated signal transduction pathways failed to modulate 4-HPR-induced apoptosis. 4-HPR was found to cause an antioxidant-inhibitable conformational change of both Bax and Bak, leading to the exposure of their N-termini and to the mitochondrial relocalization of Bax. Cells with a Bax(-/-) Bak(-/-) genotype were resistant against the 4-HPR-induced MMP, overproduction of ROS and cell death. Altogether, these data indicate that 4-HPR induces MMP through an ROS-mediated pathway that involves the obligatory contribution of the proapopotic Bcl-2 family members Bax and/or Bak.