Binding of the fibronectin-mimetic peptide, PR_b, to alpha5beta1 on pig islet cells increases fibronectin production and facilitates internalization of PR_b functionalized liposomes

Delivery of Theranostic Nanoparticles to Various Cancers by Means of Integrin-Binding Peptides

Active targeting of tumors is believed to be the key to efficient cancer therapy and accurate, early-stage diagnostics. Active targeting implies minimized off-targeting and associated cytotoxicity towards healthy tissue. One way to acquire active targeting is to employ conjugates of therapeutic agents with ligands known to bind receptors overexpressed onto cancer cells. The integrin receptor family has been studied as a target for cancer treatment for almost fifty years. However, systematic knowledge on their effects on cancer cells, is yet lacking, especially when utilized as an active targeting ligand for particulate formulations. Decoration with various integrin-targeting peptides has been reported to increase nanoparticle accumulation in tumors ≥ 3-fold when compared to passively targeted delivery. In recent years, many newly discovered or rationally designed integrin-binding peptides with excellent specificity towards a single integrin receptor have emerged. Here, we show a comprehensive analysis of previously unreviewed integrin-binding peptides, provide diverse modification routes for nanoparticle conjugation, and showcase the most notable examples of their use for tumor and metastases visualization and eradication to date, as well as possibilities for combined cancer therapies for a synergetic effect. This review aims to highlight the latest advancements in integrin-binding peptide development and is directed to aid transition to the development of novel nanoparticle-based theranostic agents for cancer therapy.

Nanoparticles as Physically- and Biochemically-Tuned Drug Formulations for Cancers Therapy

Simple Summary

Conventional antitumor drugs have limitations, including poor water solubility and lack of targeting capability, with consequent non-specific distribution, systemic toxicity, and low therapeutic index. Nanotechnology promises to overcome these drawbacks by exploiting the physical properties of diverse nanocarriers that can be linked to moieties with binding selectivity for cancer cells. The use of nanoparticles as therapeutic formulations allows a targeted delivery and a slow, controlled release of the drug(s), making them tunable modules for applications in precision medicine. In addition, nanoparticles are also being developed as cancer vaccines, offering an opportunity to increase both cellular and humoral immunity, thus providing a new weapon to beat cancer.

Abstract

Malignant tumors originate from a combination of genetic alterations, which induce activation of oncogenes and inactivation of oncosuppressor genes, ultimately resulting in uncontrolled growth and neoplastic transformation. Chemotherapy prevents the abnormal proliferation of cancer cells, but it also affects the entire cellular network in the human body with heavy side effects. For this reason, the ultimate aim of cancer therapy remains to selectively kill cancer cells while sparing their normal counterparts. Nanoparticle formulations have the potential to achieve this aim by providing optimized drug delivery to a pathological site with minimal accumulation in healthy tissues. In this review, we will first describe the characteristics of recently developed nanoparticles and how their physical properties and targeting functionalization are exploited depending on their therapeutic payload, route of delivery, and tumor type. Second, we will analyze how nanoparticles can overcome multidrug resistance based on their ability to combine different therapies and targeting moieties within a single formulation. Finally, we will discuss how the implementation of these strategies has led to the generation of nanoparticle-based cancer vaccines as cutting-edge instruments for cancer immunotherapy.

Delivery of therapeutic agents and cells to pancreatic islets: Towards a new era in the treatment of diabetes

Pancreatic islet cells, and in particular insulin-producing beta cells, are centrally involved in the pathogenesis of diabetes mellitus. These cells are of paramount importance for the endocrine control of glycemia and glucose metabolism. In Type 1 Diabetes, islet beta cells are lost due to an autoimmune attack. In Type 2 Diabetes, beta cells become dysfunctional and insufficient to counterbalance insulin resistance in peripheral tissues. Therapeutic agents have been developed to support the function of islet cells, as well as to inhibit deleterious immune responses and inflammation. Most of these agents have undesired effects due to systemic administration and off-target effects. Typically, only a small fraction of therapeutic agent reaches the desired niche in the pancreas. Because islets and their beta cells are scattered throughout the pancreas, access to the niche is limited. Targeted delivery to pancreatic islets could dramatically improve the therapeutic effect, lower the dose requirements, and lower the side effects of agents administered systemically. Targeted delivery is especially relevant for those therapeutics for which the manufacturing is difficult and costly, such as cells, exosomes, and microvesicles. Along with therapeutic agents, imaging reagents intended to quantify the beta cell mass could benefit from targeted delivery. Several methods have been developed to improve the delivery of agents to pancreatic islets. Intra-arterial administration in the pancreatic artery is a promising surgical approach, but it has inherent risks. Targeted delivery strategies have been developed based on ligands for cell surface molecules specific to islet cells or inflamed vascular endothelial cells. Delivery methods range from nanocarriers and vectors to deliver pharmacological agents to viral and non-viral vectors for the delivery of genetic constructs. Several strategies demonstrated enhanced therapeutic effects in diabetes with lower amounts of therapeutic agents and lower off-target side effects. Microvesicles, exosomes, polymer-based vectors, and nanocarriers are gaining popularity for targeted delivery. Notably, liposomes, lipid-assisted nanocarriers, and cationic polymers can be bioengineered to be immune-evasive, and their advantages to transport cargos into target cells make them appealing for pancreatic islet-targeted delivery. Viral vectors have become prominent tools for targeted gene delivery. In this review, we discuss the latest strategies for targeted delivery of therapeutic agents and imaging reagents to pancreatic islet cells.

Culture on a native bone marrow-derived extracellular matrix restores the pancreatic islet basement membrane, preserves islet function, and attenuates islet immunogenicity

Islet transplantation in man is limited by multiple factors including islet availability, islet cell damage caused by collagenase during isolation, maintenance of islet function between isolation and transplantation, and allograft rejection. In this study, we describe a new approach for preparing islets that enhances islet function in vitro and reduces immunogenicity. The approach involves culture on native decellularized 3D bone marrow-derived extracellular matrix (3D-ECM), which contains many of the matrix components present in pancreas, prior to islet transplantation. Compared to islets cultured on tissue culture plastic (TCP), islets cultured on 3D-ECM exhibited greater attachment, higher survival rate, increased insulin content, and enhanced glucose-stimulated insulin secretion. In addition, culture of islets on 3D-ECM promoted recovery of vascular endothelial cells within the islets and restored basement membrane-related proteins (eg, fibronectin and collagen type VI). More interestingly, culture on 3D-ECM also selectively decontaminated islets of “passenger” cells (co-isolated with the islets) and restored basement membrane-associated type VI collagen, which were associated with an attenuation in islet immunogenicity. These results demonstrate that this novel approach has promise for overcoming two major issues in human islet transplantation: (a) poor yield of islets from donated pancreas tissue and (b) the need for life-long immunosuppression.

Bioinspired Liposomes for Oral Delivery of Colistin to Combat Intracellular Infections by Salmonella enterica

Bacterial invasion into eukaryotic cells and the establishment of intracellular infection has proven to be an effective means of resisting antibiotic action, as anti-infective agents commonly exhibit a poor permeability across the host cell membrane. Encapsulation of anti-infectives into nanoscaled delivery systems, such as liposomes, is shown to result in an enhancement of intracellular delivery. The aim of the current work is, therefore, to formulate colistin, a poorly permeable anti-infective, into liposomes suitable for oral delivery, and to functionalize these carriers with a bacteria-derived invasive moiety to enhance their intracellular delivery. Different combinations of phospholipids and cholesterol are explored to optimize liposomal drug encapsulation and stability in biorelevant media. These liposomes are then surface-functionalized with extracellular adherence protein (Eap), derived from Staphylococcus aureus. Treatment of HEp-2 and Caco-2 cells infected with Salmonella enterica using colistin-containing, Eap-functionalized liposomes resulted in a significant reduction of intracellular bacteria, in comparison to treatment with nonfunctionalized liposomes as well as colistin alone. This indicates that such bio-invasive carriers are able to facilitate intracellular delivery of colistin, as necessary for intracellular anti-infective activity. The developed Eap-functionalized liposomes, therefore, present a promising strategy for improving the therapy of intracellular infections.

RGD-containing elastin-like polypeptide improves islet transplantation outcomes in diabetic mice

Successful islet transplantation critically depends on the isolation of healthy islets. However, the islet isolation procedure itself contributes to islet death due to the destruction of intra- and peri-islet extracellular matrices (ECMs) during digestion. We investigated whether an RGD-containing elastin-like polypeptide (REP) could function as a self-assembling matrix to replenish ECMs and protects islets from cell death. Immediately following isolation, islets were coated with REP coacervate particles via isothermal adsorption of an REP solution followed by thermal gelation. REP-coated islets displayed increased viability and insulin secretory capacity in pretransplant culture compared to untreated islets. Co-transplantation of REP-treated islets and REP beneath the renal sub-capsule in streptozotocin-induced diabetic mice restored normoglycemia and serum insulin levels. Mice that received co-transplants maintained normoglycemia for a longer period of time than those receiving untreated islets without REP. Moreover, co-transplantation sites exhibited enhanced β-cell proliferation and vascularization. Thus, the REP-based coacervation strategy improve the survival, function and therapeutic potential of transplanted islets. STATEMENT OF SIGNIFICANCE: 1). An artificial matrix polypeptide comprised of thermoresponsive elastin-like peptides and integrin-stimulatory RGD ligands (REP) to reconstitute damaged or lost matrices. 2). Through body temperature-induced coacervation, REP reconstitutes intra-islet environment and enhances islet viability and insulin secretion by activating the pro-survival and insulin signaling pathways. 3). REP-coated islets were transplanted together with the matrix polypeptide under the kidney sub-capsule of mice, it develops a new peri-insular environment, which protects the islet grafts from immune rejection thus extending islet longevity. 4). Our data suggest that in situ self-assembly of biomimetic islet environments become a new platform allowing for improved islet transplantation at extrahepatic sites.

Extracellular matrix molecules and their potential contribution to the function of transplanted pancreatic islets

Extracellular matrix (ECM) molecules are responsible for structural and biochemical support, as well as for regulation of molecular signalling and tissue repair in many organ structures, including the pancreas. In pancreatic islets, collagen type IV and VI, and laminins are the most abundant molecules, but other ECM molecules are also present. The ECM interacts with specific combinations of integrin α/β heterodimers on islet cells and guides many cellular processes. More specifically, some ECM molecules are involved in beta cell survival, function and insulin production, while others can fine tune the susceptibility of islet cells to cytokines. Further, some ECM induce release of growth factors to facilitate tissue repair. During enzymatic isolation of islets for transplantation, the ECM is damaged, impacting islet function. However, restoration of the ECM in human islets (for example by adding ECM to the interior of immunoprotective capsules) has been shown to enhance islet function. Here, we provide current insight into the role of ECM molecules in islet function and discuss the clinical potential of ECM manipulation to enhance pancreatic islet function and survival.

The design of peptide-amphiphiles as functional ligands for liposomal anticancer drug and gene delivery

Liposomal nanomedicine has led to clinically useful cancer therapeutics like Doxil and DaunoXome. In addition, peptide-functionalized liposomes represent an effective drug and gene delivery vehicle with increased cancer cell specificity, enhanced tumor-penetrating ability and high tumor growth inhibition. The goal of this article is to review the recently published literature of the peptide-amphiphiles that were used to functionalize liposomes, to highlight successful designs that improved drug and gene delivery to cancer cells in vitro, and cancer tumors in vivo, and to discuss the current challenges of designing these peptide-decorated liposomes for effective cancer treatment.

Targeted iron-oxide nanoparticle for photodynamic therapy and imaging of head and neck cancer

Photodynamic therapy (PDT) is a highly specific anticancer treatment modality for various cancers, particularly for recurrent cancers that no longer respond to conventional anticancer therapies. PDT has been under development for decades, but light-associated toxicity limits its clinical applications. To reduce the toxicity of PDT, we recently developed a targeted nanoparticle (NP) platform that combines a second-generation PDT drug, Pc 4, with a cancer targeting ligand, and iron oxide (IO) NPs. Carboxyl functionalized IO NPs were first conjugated with a fibronectin-mimetic peptide (Fmp), which binds integrin β1. Then the PDT drug Pc 4 was successfully encapsulated into the ligand-conjugated IO NPs to generate Fmp-IO-Pc 4. Our study indicated that both nontargeted IO-Pc 4 and targeted Fmp-IO-Pc 4 NPs accumulated in xenograft tumors with higher concentrations than nonformulated Pc 4. As expected, both IO-Pc 4 and Fmp-IO-Pc 4 reduced the size of HNSCC xenograft tumors more effectively than free Pc 4. Using a 10-fold lower dose of Pc 4 than that reported in the literature, the targeted Fmp-IO-Pc 4 NPs demonstrated significantly greater inhibition of tumor growth than nontargeted IO-Pc 4 NPs. These results suggest that the delivery of a PDT agent Pc 4 by IO NPs can enhance treatment efficacy and reduce PDT drug dose. The targeted IO-Pc 4 NPs have great potential to serve as both a magnetic resonance imaging (MRI) agent and PDT drug in the clinic.

Transfection mechanisms of polyplexes, lipoplexes, and stealth liposomes in α₅β₁ integrin bearing DLD-1 colorectal cancer cells

Receptor targeted, PEGylated transfection agents can improve stability and delivery specificity of current cationic lipid and polymer based nonviral gene delivery vehicles, but their mode of transfection is poorly understood. We therefore investigated the transfection mechanisms of 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP)/1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) lipoplexes, branched polyethylenimine (bPEI) polyplexes, and bPEI encapsulated in either PEGylated (stealth) nontargeted liposomes or PR_b peptide (targeted to α5β1 integrin) functionalized stealth liposomes in DLD-1 colorectal cancer cells in vitro with gene expression assays, flow cytometry and confocal microscopy. DOTAP/DOPE and PR_b functionalized stealth liposomes mediated higher gene expression compared to nontargeted stealth liposomes and bPEI. However DOTAP/DOPE was internalized slowly leading to lower levels of DNA uptake. In contrast, despite high internalization of bPEI polyplexes, gene expression levels were low as DNA was unable to escape from the endosomes. Nontargeted stealth liposomes also mediated low gene expression due to low amounts of DNA internalized and slow internalization kinetics. PR_b functionalized stealth liposomes struck an optimal balance among these transfection agents with efficient transfection arising from fast integrin mediated internalization kinetics, high amounts of DNA uptake, and endosomal escape. We found α5β1 integrin to be a valuable target for gene delivery and that the caveolar endocytic pathway may offer an advantage to receptor targeted PEGylated transfection agents in DLD-1 cells.

Maintenance of ischemic β cell viability through delivery of lipids and ATP by targeted liposomes

Islet transplantation is a promising treatment for type 1 diabetes, but despite the successes, existing challenges prevent widespread application. Ischemia, occurring during pancreas preservation and isolation, as well as after islet transplantation, decreases islet viability and function. We hypothesized that the liposomal delivery of adenosine triphosphate (ATP) could prevent the loss of cell viability during an ischemic insult. In this work we use a model β cell line, INS-1 to probe the liposome/cell interactions and examined the ability of liposomes functionalized with the fibronectin-mimetic peptide PR_b to facilitate the delivery of ATP to ischemic β cells. We demonstrate that PR_b increases the binding and internalization of liposomes to the β cells. Unexpectedly, when comparing the ability of PR_b liposomes with and without ATP to protect INS-1 cells from ischemia we found that both formulations increased cell survival. By probing the functional activity of ischemic cells treated with PR_b functionalized liposomes with and without ATP we find that both lipids and ATP play a role in maintaining cell metabolic activity after an ischemic insult and preventing cell necrosis. This approach may be beneficial for preventing ischemia related damage to islet cells, especially in the organ preservation stage.

Bioresorbable polymersomes for targeted delivery of cisplatin

Nontoxic bioresorbable polymersomes have been developed that efficiently and site-selectively tether targeting peptides under mild conditions with no toxic catalysts. The binding and release properties of these polymersomes have been evaluated when targeting DLD-1 human colon cancer cells overexpressing the α(5)β(1) integrin. The delivery efficacy to these cells is markedly improved over commonly used RGD targeting peptides by use of an α(5)β(1)-specific targeting peptide, PR_b. Release profiles in buffered solution from pH 7.4 to 4.5 were evaluated and compared to release after binding to cells, and enzymatic degradation was identified as a major cause of rapid payload release in the cell. Intracellular trafficking and release were imaged via confocal microscopy in live cells and colocalization with organelles was evaluated quantitatively over time. Finally, the anticancer drug cisplatin was encapsulated in the PR_b functionalized polymersomes and the presence of PR_b greatly improved delivery efficacy, with increased cisplatin-induced losses to targeted DLD-1 colon cancer cell viability. When delivered to CACO-2 model human epithelial cells expressing low levels of α(5)β(1) integrin, low toxicity was maintained, suggesting that targeting was specific to α(5)β(1) overexpressing cells. These results demonstrate that PR_b-functionalized bioresorbable polymersomes may be an attractive route to minimizing the dose-limiting side effects associated with existing approaches to cisplatin chemotherapy.

Effects of Arg-Gly-Asp-modified elastin-like polypeptide on pseudoislet formation via up-regulation of cell adhesion molecules and extracellular matrix proteins

Extracellular matrix (ECM) plays an important role in controlling the β-cell morphology, survival and insulin secretary functions. An RGD-modified elastin-like polypeptide (RGD-ELP), TGPG[VGRGD(VGVPG)(6)](20)WPC, has been reported previously as a bioactive matrix. In this study, to investigate whether RGD-ELP affects β-cell growth characteristics and insulin secretion, β-TC6 cells were cultured on the RGD-ELP coatings prepared via thermally induced phase transition. On RGD-ELP, β-TC6 cells clustered into an islet-like architecture with high cell viability. Throughout 7days' culture, the proliferation rate of the cells within a pseudoislet was similar to that of monolayer culture. Under high glucose (25mM), β-TC6 pseudoislets showed up-regulated insulin gene expression and exhibited glucose-stimulated insulin secretion. Importantly, the mRNA and protein abundances of cell adhesion molecules (CAM) E-cadherin and connexin-36 were much higher in pseudoislets than in monolayer cells. The siRNA-mediated inhibition of E-cadherin or connexin-36 expression severely limited pseudoislet formation. In addition, the mRNA levels of collagen types I and IV, fibronectin and laminin were significantly elevated in pseudoislets. The results suggest that RGD-ELP promotes pseudoislet formation via up-regulation of the CAM and ECM components. The functional roles of RGD-ELP are discussed in respect of its molecular composition.

Peptide targeted lipid nanoparticles for anticancer drug delivery

Encapsulating anticancer drugs in nanoparticles has proven to be an effective mechanism to alter the pharmacokinetic and pharmacodynamic profiles of the drugs, leading to clinically useful cancer therapeutics like Doxil and DaunoXome. Underdeveloped tumor vasculature and lymphatics allow these first-generation nanoparticles to passively accumulate within the tumor, but work to create the next-generation nanoparticles that actively participate in the tumor targeting process is underway. Lipid nanoparticles functionalized with targeting peptides are among the most often studied. The goal of this article is to review the recently published literature of targeted nanoparticles to highlight successful designs that improved in vivo tumor therapy, and to discuss the current challenges of designing these nanoparticles for effective in vivo performance.

PEGylated liposomal doxorubicin targeted to α5β1-expressing MDA-MB-231 breast cancer cells

Targeting drugs selectively to cancer cells can potentially benefit cancer patients by avoiding side effects generally associated with several cancer therapies. One of the attractive approaches to direct the drug cargo to specific sites is to incorporate ligands at the surface of the delivery systems. Integrin α(5)β(1) is overexpressed in tumor vasculature and cancer cells, thus making it an attractive target for use in drug delivery. Our group has developed a fibronectin-mimetic peptide, PR_b, which has been shown to bind specifically to integrin α(5)β(1), thereby providing a tool to target α(5)β(1)-expressing cancer cells in vitro as well as in vivo. Our current work focuses on designing modified stealth liposomes (liposomes functionalized with polyethylene glycol, PEG) for combining the benefits associated with PEGylation, as well as imparting specific targeting properties to the liposomes. We have designed PEGylated liposomes that incorporate in their bilayer the fibronectin-mimetic peptide-amphiphile PR_b that can target several cancer cells that overexpress α(5)β(1), including the MDA-MB-231 breast cancer cells used in this study. We have encapsulated doxorubicin inside the liposomes to enhance its therapeutic potential via PEGylation as well as active targeting to the cancer cells. Our results show that PR_b-functionalized stealth liposomes were able to specifically bind to MDA-MB-231 cells, and the binding could be controlled by varying the peptide concentration. The intracellular trafficking of the doxorubicin liposomes was examined, and within minutes after delivery the majority of them were found to be in the early endosomes, whereas after a longer period of time they had accumulated in the late endosomes and lysosomes. The functionalized liposomes were found to be equally cytotoxic as the free doxorubicin, especially at higher doxorubicin concentrations, and provided higher cytotoxicity than the nontargeted and GRGDSP-functionalized stealth liposomes. Thus, the PR_b-functionalized PEGylated nanoparticles examined in this study offer a promising strategy to deliver their therapeutic payload directly to the breast cancer cells, in an efficient and specific manner.

Enhanced integrin mediated signaling and cell cycle progression on fibronectin mimetic peptide amphiphile monolayers

In recent years, a variety of biomimetic constructs have emerged which mimic the bioactive sequences found in the natural extracellular matrix (ECM) proteins such as fibronectin (FN) that promote cell adhesion as well as proliferation on artificially functionalized interfaces. Much interest lies in investigating the ability of the ECM mimetic materials in regulating a number of vital cell functions including differentiation, gene expression, migration, and proliferation. A peptide amphiphile PR_b containing both the cell adhesive GRGDSP and synergistic PHSRN peptide sequences was developed in our group that was shown to support enhanced cell proliferation and ECM FN secretion as compared to GRGDSP and FN functionalized interfaces. In this study, we have investigated the binding affinity of the PR_b peptide ligand with the FN cell surface receptor, the α(5)β(1) integrin. We compared PR_b functionalized surfaces with FN and BSA coated surfaces and GRGDSP functionalized surfaces in terms of promoting intracellular signaling cascades that are essential for enhanced cellular activity. Specifically, we studied the phosphorylation of focal adhesion kinase (FAK) at tyrosine residues Y397 and Y576 and the formation of cyclin D1, both of which are intracellular markers of integrin mediated attachment of cells, signaling pathways, and progression of cell cycle. FAK and cyclin D1 encourage enhanced cell proliferation, differentiation, and gene expression. Our results show that the PR_b peptide ligand has a specific and strong binding affinity for the α(5)β(1) integrin with a dissociation constant of 76.3 ± 6.3 nM. The PR_b peptide ligands supported enhanced FAK phosphorylation activity and increased cyclin D1 formation as compared to the widely used GRGDSP ligand, the native protein FN (positive control), and BSA nonadhesive surfaces (negative control). These results encourage the use of the FN mimetic PR_b peptide in functionalizing biomaterials for potential tissue engineering and therapeutic applications.