The transferrin receptor and the targeted delivery of therapeutic agents against cancer

Iron deprivation in cancer--potential therapeutic implications

Iron is essential for normal cellular function. It participates in a wide variety of cellular processes, including cellular respiration, DNA synthesis, and macromolecule biosynthesis. Iron is required for cell growth and proliferation, and changes in intracellular iron availability can have significant effects on cell cycle regulation, cellular metabolism, and cell division. Perhaps not surprisingly then, neoplastic cells have been found to have higher iron requirements than normal, non-malignant cells. Iron depletion through chelation has been explored as a possible therapeutic intervention in a variety of cancers. Here, we will review iron homeostasis in non-malignant and malignant cells, the widespread effects of iron depletion on the cell, the various iron chelators that have been explored in the treatment of cancer, and the tumor types that have been most commonly studied in the context of iron chelation.

Transferrin and the transferrin receptor for the targeted delivery of therapeutic agents to the brain and cancer cells

The potential use of many promising novel drugs is limited by their inability to specifically reach their site of action after intravenous administration, without secondary effects on healthy tissues. In order to remediate this problem, the protein transferrin (Tf) has been extensively studied as a targeting molecule for the transport of drug and gene delivery systems to the brain and cancer cells. A wide range of delivery approaches have been developed to target the Tf receptor and they have already improved the specific delivery of Tf-bearing therapeutic agents to their site of action. This review provides a summary of the numerous delivery strategies used to target the Tf receptor and focuses on recent therapeutic advances.

Structure and dynamics of drug carriers and their interaction with cellular receptors: focus on serum transferrin

Highly proliferative cells have a dramatically increased need for iron which results in the expression of an increased number of transferrin receptors (TFR). This insight makes the transferrin receptor on these cells an excellent candidate for targeted therapeutics. In this regard, it is critical to understand at a molecular level exactly how the TFR interacts with its ligand, hTF. Understanding of the hTF/TFR pathway could, in theory, maximize the use of this system for development of more effective small molecules or toxin-conjugates to specifically target cancer cells. Many strategies have been attempted with the objective of utilizing the hTF/TFR system to deliver drugs; these include conjugation of a toxin or drug to hTF or direct targeting of the TFR by antibodies. To date, in spite of all of the effort, there is a conspicuous absence of any successful candidate drugs reaching the clinic. We suggest that a lack of quantitative data to determine the basic biochemical properties of the drug carrier and the effects of drug-conjugation on the hTF-TFR interaction may have contributed to the failure to realize the full potential of this system. This review provides some guidelines for developing a more quantitative approach for evaluation of current and future hTF-drug conjugates.

Perspectives of nanotechnology in minimally invasive therapy of breast cancer

Breast cancer, the most common type of cancer among women in the western world, affects approximately one out of every eight women over their lifetime. In recognition of the high invasiveness of surgical excision and severe side effects of chemical and radiation therapies, increasing efforts are made to seek minimally invasive modalities with fewer side effects. Nanoparticles (<100 nm in size) have shown promising capabilities for delivering targeted therapeutic drugs to cancer cells and confining the treatment mainly within tumors. Additionally, some nanoparticles exhibit distinct properties, such as conversion of photonic energy into heat, and these properties enable eradication of cancer cells. In this review, current utilization of nanostructures for cancer therapy, especially in minimally invasive therapy, is summarized with a particular interest in breast cancer.

Receptor-based targeting of therapeutics

Receptor-based targeting of therapeutics may be a fascinating proposition to improve the therapeutic efficacy of encapsulated drugs. The development of safe and effective nanomedicines is a prerequisite in the current nanotechnological scenario. Currently, the surface engineering of nanocarriers has attracted great attention for targeted therapeutic delivery by selective binding of targeting ligand to the specific receptors present on the surface of cells. In this review, we have discussed the current status of various receptors such as transferrin, lectoferrin, lectin, folate, human EGF receptor, scavenger, nuclear and integrin, which are over-expressed on the surface of cancer cells; along with the relevance of targeted delivery systems such as nanoparticles, polymersomes, dendrimers, liposomes and carbon nanotubes. The review also focuses on the effective utilization of receptor-based targeted delivery systems for the management of cancer in effective ways by minimizing the drug-associated side effects and improving the therapeutic efficacy of developed nano-architectures.

Iron and cancer: more ore to be mined

Iron is an essential nutrient that facilitates cell proliferation and growth. However, iron also has the capacity to engage in redox cycling and free radical formation. Therefore, iron can contribute to both tumour initiation and tumour growth; recent work has also shown that iron has a role in the tumour microenvironment and in metastasis. Pathways of iron acquisition, efflux, storage and regulation are all perturbed in cancer, suggesting that reprogramming of iron metabolism is a central aspect of tumour cell survival. Signalling through hypoxia-inducible factor (HIF) and WNT pathways may contribute to altered iron metabolism in cancer. Targeting iron metabolic pathways may provide new tools for cancer prognosis and therapy.

Phase I study of a systemically delivered p53 nanoparticle in advanced solid tumors

Selective delivery of therapeutic molecules to primary and metastatic tumors is optimal for effective cancer therapy. A liposomal nanodelivery complex (scL) for systemic, tumor-targeting delivery of anticancer therapeutics has been developed. scL employs an anti-transferrin receptor (TfR), scFv as the targeting molecule. Loss of p53 suppressor function, through mutations or inactivation of the p53 pathway, is present in most human cancers. Rather than being transiently permissive for tumor initiation, persistence of p53 dysfunction is a continuing requirement for maintaining tumor growth. Herein, we report results of a first-in-man Phase I clinical trial of restoration of the normal human tumor suppressor gene p53 using the scL nanocomplex (SGT-53). Minimal side effects were observed in this trial in patients with advanced solid tumors. Furthermore, the majority of patients demonstrated stable disease. One patient with adenoid cystic carcinoma had his status changed from unresectable to resectable after one treatment cycle. More significantly, we observed an accumulation of the transgene in metastatic tumors, but not in normal skin tissue, in a dose-related manner. These results show not only that systemically delivered SGT-53 is well tolerated and exhibits anticancer activity, but also supply evidence of targeted tumor delivery of SGT-53 to metastatic lesions.

Human ferritin for tumor detection and therapy

Ferritin, a major iron storage protein found in most living organisms, is composed of a 24-subunit protein cage with a hollow interior cavity. Serum ferritin serves as a critical marker to detect total body iron status. However, recent research reveals a number of novel functions of ferritin besides iron storage; for example, a ferritin receptor, transferrin receptor 1 (TfR1), has been identified and serum ferritin levels are found to be elevated in tumors. A particular new finding is that magnetoferritin nanoparticles, biomimetically synthesized using H-chain ferritin to form a 24-subunit cage with an iron oxide core, possess intrinsic dual functionality, the protein shell specifically targeting tumors and the iron oxide core catalyzing peroxidase substrates to produce a color reaction allowing visualization of tumor tissues. Here we attempt to summarize current research on ferritin, particularly newly identified functions related to tumors, in order to address current challenges and highlight future directions.

Application of liposomes in drug development--focus on gastroenterological targets

Over the past decade, liposomes became a focal point in developing drug delivery systems. New liposomes, with novel lipid molecules or conjugates, and new formulations opened possibilities for safely and efficiently treating many diseases including cancers. New types of liposomes can prolong circulation time or specifically deliver drugs to therapeutic targets. This article concentrates on current developments in liposome based drug delivery systems for treating diseases of the gastrointestinal tract. We will review different types and uses of liposomes in the development of therapeutics for gastrointestinal diseases including inflammatory bowel diseases and colorectal cancer.

Exploring polymeric micelles for improved delivery of anticancer agents: recent developments in preclinical studies

As versatile drug delivery systems, polymeric micelles have demonstrated particular strength in solubilizing hydrophobic anticancer drugs while eliminating the use of toxic organic solvents and surfactants. However, the true promise of polymeric micelles as drug carriers for cancer therapy resides in their potential ability to preferentially elevate drug exposure in the tumor and achieve enhanced anticancer efficacy, which still remains to be fully exploited. Here, we review various micellar constructs that exhibit the enhanced permeation and retention effect in the tumor, the targeting ligands that potentiate the anticancer efficacy of micellar drugs, and the polyplex micelle systems suitable for the delivery of plasmid DNA and small interference RNA. Together, these preclinical studies in animal models help us further explore polymeric micelles as emerging drug carriers for targeted cancer therapy.

In vivo performance of polymer nanocarriers dually-targeted to epitopes of the same or different receptors

Modification of drug delivery nanomaterials with affinity molecules that facilitate targeting, has rendered a new class of ligands for cell receptors, which often possess valency and dimensions different from natural counterparts. Designing strategies to target multiple receptors or, never explored, multiple epitopes on the same receptor may modulate the biodistribution properties of these nanomaterials. We examined this using antibody-directed targeting of polymer nanocarriers to transferrin receptor (TfR) and intercellular adhesion molecule 1 (ICAM-1). Regarding epitopes on one receptor, nanocarriers addressed with anti-TfR-R17 maintained brain and lung targeting in mice, compared with "free" antibody, while anti-TfR-8D3 nanocarriers lost specificity. Coating nanocarriers with both antibodies decreased targeting in brain and liver, not lungs, modulating biodistribution. Regarding different receptors, nanocarriers coated with both anti-ICAM and anti-TfR displayed intermediate specific accumulation in lungs and higher in liver, compared to single-targeted nanocarriers, while brain targeting was comparable to TfR- and lower than ICAM-1-targeted nanocarriers. Tracing a model therapeutic cargo, acid sphingomyelinase (enzyme replacement for Niemann-Pick Disease A-B), showed that combined-targeted anti-ICAM/TfR nanocarriers enhanced enzyme delivery versus "free" enzyme, with biodistribution patterns different from single-targeted nanocarriers. Hence, targeting nanocarriers to multiple epitopes or receptors holds promise to control distribution of drug delivery nanomaterials in the body.

TfR1 interacts with the IKK complex and is involved in IKK-NF-κB signalling

The IKK [inhibitor of NF-κB (nuclear factor κB) kinase] complex has an essential role in the activation of the family of NF-κB transcription factors in response to a variety of stimuli. To identify novel IKK-interacting proteins, we performed an unbiased proteomics screen where we identified TfR1 (transferrin receptor 1). TfR1 is required for transferrin binding and internalization and ultimately for iron homoeostasis. TfR1 depletion does not lead to changes in IKK subunit protein levels; however, it does reduce the formation of the IKK complex, and inhibits TNFα (tumour necrosis factor α)-induced NF-κB-dependent transcription. We find that, in the absence of TfR1, NF-κB does not translocate to the nucleus efficiently, and there is a reduction in the binding to target gene promoters and consequentially less target gene activation. Significantly, depletion of TfR1 results in an increase in apoptosis in response to TNFα treatment, which is rescued by elevating the levels of RelA/NF-κB. Taken together, these results indicate a new function for TfR1 in the control of IKK and NF-κB. Our data indicate that IKK–NF-κB responds to changes in iron within the cell.

Iron in multiple myeloma

Multiple myeloma is a non-curable B-cell malignancy in which iron metabolism plays an important role. Patients with this disorder almost universally suffer from clinically significant anemia, which is often symptomatic, and which is due to impaired iron utilization. Recent studies have indicated that the proximal cause of dysregulated iron metabolism and anemia in these patients is cytokine-induced upregulation of hepcidin expression. Malignant myeloma cells are dependent on an increased influx of iron, and therapeutic efforts are being made to target this requirement. The studies detailing the characteristics and biochemical abnormalities in iron metabolism causing anemia and the initial attempts to target iron therapeutically are described in this review.

Cellular iron metabolism in prognosis and therapy of breast cancer

Despite many recent advances, breast cancer remains a clinical challenge. Current issues include improving prognostic evaluation and increasing therapeutic options for women whose tumors are refractory to current frontline therapies. Iron metabolism is frequently disrupted in breast cancer, and may offer an opportunity to address these challenges. Iron enhances breast tumor initiation, growth and metastases. Iron may contribute to breast tumor initiation by promoting redox cycling of estrogen metabolites. Up-regulation of iron import and down-regulation of iron export may enable breast cancer cells to acquire and retain excess iron. Alterations in iron metabolism in macrophages and other cells of the tumor microenvironment may also foster breast tumor growth. Expression of iron metabolic genes in breast tumors is predictive of breast cancer prognosis. Iron chelators and other strategies designed to limit iron may have therapeutic value in breast cancer. The dependence of breast cancer on iron presents rich opportunities for improved prognostic evaluation and therapeutic intervention.

Targeting receptor-mediated endocytotic pathways with nanoparticles: rationale and advances

Targeting of drugs and their carrier systems by using receptor-mediated endocytotic pathways was in its nascent stages 25 years ago. In the intervening years, an explosion of knowledge focused on design and synthesis of nanoparticulate delivery systems as well as elucidation of the cellular complexity of what was previously-termed receptor-mediated endocytosis has now created a situation when it has become possible to design and test the feasibility of delivery of highly specific nanoparticle drug carriers to specific cells and tissue. This review outlines the mechanisms governing the major modes of receptor-mediated endocytosis used in drug delivery and highlights recent approaches using these as targets for in vivo drug delivery of nanoparticles. The review also discusses some of the inherent complexity associated with the simple shift from a ligand-drug conjugate versus a ligand-nanoparticle conjugate, in terms of ligand valency and its relationship to the mode of receptor-mediated internalization.

Targeted drug delivery for cancer therapy: the other side of antibodies

Therapeutic monoclonal antibody (TMA) based therapies for cancer have advanced significantly over the past two decades both in their molecular sophistication and clinical efficacy. Initial development efforts focused mainly on humanizing the antibody protein to overcome problems of immunogenicity and on expanding of the target antigen repertoire. In parallel to naked TMAs, antibody-drug conjugates (ADCs) have been developed for targeted delivery of potent anti-cancer drugs with the aim of bypassing the morbidity common to conventional chemotherapy. This paper first presents a review of TMAs and ADCs approved for clinical use by the FDA and those in development, focusing on hematological malignancies. Despite advances in these areas, both TMAs and ADCs still carry limitations and we highlight the more important ones including cancer cell specificity, conjugation chemistry, tumor penetration, product heterogeneity and manufacturing issues. In view of the recognized importance of targeted drug delivery strategies for cancer therapy, we discuss the advantages of alternative drug carriers and where these should be applied, focusing on peptide-drug conjugates (PDCs), particularly those discovered through combinatorial peptide libraries. By defining the advantages and disadvantages of naked TMAs, ADCs and PDCs it should be possible to develop a more rational approach to the application of targeted drug delivery strategies in different situations and ultimately, to a broader basket of more effective therapies for cancer patients.

Improved eIF4E binding peptides by phage display guided design: plasticity of interacting surfaces yield collective effects

Eukaryotic initiation factor (eIF)4E is over-expressed in many types of cancer such as breast, head and neck, and lung. A consequence of increased levels of eIF4E is the preferential translation of pro-tumorigenic proteins (e.g. c-Myc and vascular endothelial growth factor) and as a result is regarded as a potential therapeutic target. In this work a novel phage display peptide has been isolated against eIF4E. From the phage sequence two amino acids were delineated which improved binding when substituted into the eIF4G1 sequence. Neither of these substitutions were involved in direct interactions with eIF4E and acted either via optimization of the helical capping motif or restricting the conformational flexibility of the peptide. In contrast, substitutions of the remaining phage derived amino acids into the eIF4G1 sequence disrupted binding of the peptide to eIF4E. Interestingly when some of these disruptive substitutions were combined with key mutations from the phage peptide, they lead to improved affinities. Atomistic computer simulations revealed that the phage and the eIF4G1 derivative peptide sequences differ subtly in their interaction sites on eIF4E. This raises the issue, especially in the context of planar interaction sites such as those exhibited by eIF4E, that given the intricate plasticity of protein surfaces, the construction of structure-activity relationships should account for the possibility of significant movement in the spatial positioning of the peptide binding interface, including significant librational motions of the peptide.

Insights into the mechanism of cell death induced by saporin delivered into cancer cells by an antibody fusion protein targeting the transferrin receptor 1

We previously developed an antibody-avidin fusion protein (ch128.1Av) that targets the human transferrin receptor 1 (TfR1) and exhibits direct cytotoxicity against malignant B cells in an iron-dependent manner. ch128.1Av is also a delivery system and its conjugation with biotinylated saporin (b-SO6), a plant ribosome-inactivating toxin, results in a dramatic iron-independent cytotoxicity, both in malignant cells that are sensitive or resistant to ch128.1Av alone, in which the toxin effectively inhibits protein synthesis and triggers caspase activation. We have now found that the ch128.1Av/b-SO6 complex induces a transcriptional response consistent with oxidative stress and DNA damage, a response that is not observed with ch128.1Av alone. Furthermore, we show that the antioxidant N-acetylcysteine partially blocks saporin-induced apoptosis suggesting that oxidative stress contributes to DNA damage and ultimately saporin-induced cell death. Interestingly, the toxin was detected in nuclear extracts by immunoblotting, suggesting the possibility that saporin might induce direct DNA damage. However, confocal microscopy did not show a clear and consistent pattern of intranuclear localization. Finally, using the long-term culture-initiating cell assay we found that ch128.1Av/b-SO6 is not toxic to normal human hematopoietic stem cells suggesting that this critical cell population would be preserved in therapeutic interventions using this immunotoxin.

Using antibodies to target cancer therapeutics

INTRODUCTION

Over a half a century ago, radiolabeled antibodies were shown to localize selectively in tissues based on the expression of unique antigens. Antibodies have since become the de facto targeting agent, even inspiring the development of non-antibody compounds for targeting purposes.

AREAS COVERED

In this article, we review various aspects of how antibodies are transforming the way cancer is being detected and treated, with the growing demand for unconjugated and many new antibody conjugates. While unconjugated antibodies continue to garner most of the attention, interest in new antibody drug conjugates and immunotoxins has expanded over the past few years. However, there continues to be active research with new radioimmunoconjugates for imaging and therapy, particularly with α-emitters, as well as antibody-targeted cytokines and other biological response modifiers.

EXPERT OPINION

The increasing number of new agents being developed and tested clinically suggests that antibody-targeted compounds will have an expanding role in the future.

Transferrin-mediated cellular iron delivery

Essential to iron homeostasis is the transport of iron by the bilobal protein human serum transferrin (hTF). Each lobe (N- and C-lobe) of hTF forms a deep cleft which binds a single Fe(3+). Iron-bearing hTF in the blood binds tightly to the specific transferrin receptor (TFR), a homodimeric transmembrane protein. After undergoing endocytosis, acidification of the endosome initiates the release of Fe(3+) from hTF in a TFR-mediated process. Iron-free hTF remains tightly bound to the TFR at acidic pH; following recycling back to the cell surface, it is released to sequester more iron. Efficient delivery of iron is critically dependent on hTF/TFR interactions. Therefore, identification of the pH-specific contacts between hTF and the TFR is crucial. Recombinant protein production has enabled deconvolution of this complex system. The studies reviewed herein support a model in which pH-induced interrelated events control receptor-stimulated iron release from each lobe of hTF.