Synthesis and evaluation of multivalent M2pep peptides for targeting alternatively activated M2 macrophages

Progress in reeducating tumor-associated macrophages in tumor microenvironment

Malignant tumor, one of the most threatening diseases to human health, has been comprehensively treated with surgery, radiotherapy, chemotherapy and targeted therapy, but the prognosis has not always been ideal. In the past decade, immunotherapy has shown increased efficacy in tumor treatment; however, for immunotherapy to achieve its fullest potential, obstacles are to be conquered, among which tumor microenvironment (TME) has been widely investigated. In remodeling the tumor immune microenvironment to inhibit tumor progression, macrophages, as the most abundant innate immune population, play an irreplaceable role in the immune response. Therefore, how to remodel TME and alter the recruitment and polarization status of tumor-associated macrophages (TAM) has been of wide interest. In this context, nanoparticles, photodynamic therapy and other therapeutic approaches capable of affecting macrophage polarization have emerged. In this paper, we categorize and organize the existing means and methods for reprogramming TAM to provide ideas for clinical application of novel tumor-related therapies.

Immunopeptides: immunomodulatory strategies and prospects for ocular immunity applications

Immunopeptides have low toxicity, low immunogenicity and targeting, and broad application prospects in drug delivery and assembly, which are diverse in application strategies and drug combinations. Immunopeptides are particularly important for regulating ocular immune homeostasis, as the eye is an immune-privileged organ. Immunopeptides have advantages in adaptive immunity and innate immunity, treating eye immune-related diseases by regulating T cells, B cells, immune checkpoints, and cytokines. This article summarizes the application strategies of immunopeptides in innate immunity and adaptive immunity, including autoimmunity, infection, vaccine strategies, and tumors. Furthermore, it focuses on the mechanisms of immunopeptides in mediating ocular immunity (autoimmune diseases, inflammatory storms, and tumors). Moreover, it reviews immunopeptides' application strategies and the therapeutic potential of immunopeptides in the eye. We expect the immune peptide to get attention in treating eye diseases and to provide a direction for eye disease immune peptide research.

Reprogramming tumor-associated macrophages by a dually targeted milk exosome system as a potent monotherapy for cancer

Tumor-associated macrophages (TAMs) play a key role in inducing an immunosuppressive tumor microenvironment (TME) and cancer immune escape. We previously revealed that PDL1 (a key immune checkpoint) was upregulated in TAMs and induced M2 polarization, highlighting PDL1 in TAMs as a promising cancer therapeutic target. In this study, we developed an engineered milk exosome (mExo) system decorated with M2pep (an M2 macrophage binding peptide) and 7D12 (an anti-EGFR nanobody) (7D12-mExo-M2pep-siPDL1) to specifically deliver siPDL1 into M2 TAMs. A series of in vitro and in vivo assays showed that the dually targeted engineered mExos efficiently delivered siPDL1 into M2 TAMs and repolarized them into M1 macrophages, restoring CD8+ T cell immune activity and remodeling TME. Importantly, systemically administered 7D12-mExo-M2pep-siPDL1 showed efficient single-agent antitumor activity, resulting in nearly 90% tumor growth inhibition in a mouse model of orthotopic epidermal growth factor receptor (EGFR) cancer. Collectively, our study indicates that PDL1 is a promising target for TAM-based cancer immunotherapy, and our engineered mExo-based nanomedicine represents a novel tool for specifically targeting M2 TAMs, distinguishing this novel therapeutic method from other TAM-targeting therapies and highlighting its promising clinical potential.

Tumor-Associated Macrophage Targeting of Nanomedicines in Cancer Therapy

The tumor microenvironment (TME) is pivotal in tumor growth and metastasis, aligning with the "Seed and Soil" theory. Within the TME, tumor-associated macrophages (TAMs) play a central role, profoundly influencing tumor progression. Strategies targeting TAMs have surfaced as potential therapeutic avenues, encompassing interventions to block TAM recruitment, eliminate TAMs, reprogram M2 TAMs, or bolster their phagocytic capabilities via specific pathways. Nanomaterials including inorganic materials, organic materials for small molecules and large molecules stand at the forefront, presenting significant opportunities for precise targeting and modulation of TAMs to enhance therapeutic efficacy in cancer treatment. This review provides an overview of the progress in designing nanoparticles for interacting with and influencing the TAMs as a significant strategy in cancer therapy. This comprehensive review presents the role of TAMs in the TME and various targeting strategies as a promising frontier in the ever-evolving field of cancer therapy. The current trends and challenges associated with TAM-based therapy in cancer are presented.

Exploiting innate immunity for cancer immunotherapy

Immunotherapies have revolutionized the treatment paradigms of various types of cancers. However, most of these immunomodulatory strategies focus on harnessing adaptive immunity, mainly by inhibiting immunosuppressive signaling with immune checkpoint blockade, or enhancing immunostimulatory signaling with bispecific T cell engager and chimeric antigen receptor (CAR)-T cell. Although these agents have already achieved great success, only a tiny percentage of patients could benefit from immunotherapies. Actually, immunotherapy efficacy is determined by multiple components in the tumor microenvironment beyond adaptive immunity. Cells from the innate arm of the immune system, such as macrophages, dendritic cells, myeloid-derived suppressor cells, neutrophils, natural killer cells, and unconventional T cells, also participate in cancer immune evasion and surveillance. Considering that the innate arm is the cornerstone of the antitumor immune response, utilizing innate immunity provides potential therapeutic options for cancer control. Up to now, strategies exploiting innate immunity, such as agonists of stimulator of interferon genes, CAR-macrophage or -natural killer cell therapies, metabolic regulators, and novel immune checkpoint blockade, have exhibited potent antitumor activities in preclinical and clinical studies. Here, we summarize the latest insights into the potential roles of innate cells in antitumor immunity and discuss the advances in innate arm-targeted therapeutic strategies.

Advancements in Macrophage-Targeted Drug Delivery for Effective Disease Management

Macrophages play a crucial role in tissue homeostasis and the innate immune system. They perform essential functions such as presenting antigens, regulating cytokines, and responding to inflammation. However, in diseases like cancer, cardiovascular disorders, and autoimmune conditions, macrophages undergo aberrant polarization, which disrupts tissue regulation and impairs their normal behavior. To address these challenges, there has been growing interest in developing customized targeted drug delivery systems specifically designed for macrophage-related functions in different anatomical locations. Nanomedicine, utilizing nanoscale drug systems, offers numerous advantages including improved stability, enhanced pharmacokinetics, controlled release kinetics, and precise temporal drug delivery. These advantages hold significant promise in achieving heightened therapeutic efficacy, specificity, and reduced side effects in drug delivery and treatment approaches. This review aims to explore the roles of macrophages in major diseases and present an overview of current strategies employed in targeted drug delivery to macrophages. Additionally, this article critically evaluates the design of macrophage-targeted delivery systems, highlighting limitations and discussing prospects in this rapidly evolving field. By assessing the strengths and weaknesses of existing approaches, we can identify areas for improvement and refinement in macrophage-targeted drug delivery.

The effect of m2 peptide targeted nanoliposomes containing crocin on induction of phenotypic change in tumor macrophages to M1 state

AIMS

Crocin has immunomodulatory and anticancer effects. In this study, crocin was used to induce the M1 phenotype in mouse tumor macrophages.

MAIN METHODS

A targeted liposomal formulation with m2 peptide was prepared and characterized to deliver crocin to the M2 macrophages present in the tumor environment. RT-qPCR and IHC were performed for in vitro and in vivo (in C26 colon carcinoma mouse model at a dose of 50 mg/kg) assessment of M1 induction, respectively.

KEY FINDINGS

In vitro results indicated that liposome modified with m2 peptide was non-toxic to macrophages and had an improved uptake by macrophages compared to the non-targeted formulation and induced M1 phenotype through an IL6-independent pathway. M2 peptide- modified liposome showed considerable tumor accumulation and anti-tumor effects and significantly shifted the phenotype of tumor macrophages towards an anti-tumor M1 phenotype.

SIGNIFICANCE

Probably the remarkable anti-tumor responses observed in this study with m2 peptide-targeted liposomal formulations containing crocin were due to the enhanced delivery of crocin to the tumor macrophage and the subsequent initiation of anti-tumor immune responses.

Cell microparticles loaded with tumor antigen and resiquimod reprogram tumor-associated macrophages and promote stem-like CD8+ T cells to boost anti-PD-1 therapy

The durable response rate to immune checkpoint blockade such as anti-programmed cell death-1 (PD-1) antibody remains relatively low in hepatocellular carcinoma (HCC), mainly depending on an immunosuppressive microenvironment with limited number of CD8+ T cells, especially stem-like CD8+ T cells, in tumor tissues. Here we develop engineered microparticles (MPs) derived from alpha-fetoprotein (AFP)-overexpressing macrophages to load resiquimod (R848@M2pep-MPsAFP) for enhanced anti-PD-1 therapy in HCC. R848@M2pep-MPsAFP target and reprogram immunosuppressive M2-like tumor-associated macrophages (TAMs) into M1-like phenotype. Meanwhile, R848@M2pep-MPsAFP-reprogrammed TAMs act as antigen-presenting cells, not only presenting AFP antigen to activate CD8+ T cell-mediated antitumor immunity, but also providing an intra-tumoral niche to maintain and differentiate stem-like CD8+ T cells. Combination immunotherapy with anti-PD-1 antibody generates strong antitumor immune memory and induces abundant stem-like CD8+ T cell proliferation and differentiation to terminally exhausted CD8+ T cells for long-term immune surveillance in orthotopic and autochthonous HCC preclinical models in male mice. We also show that the R848-loaded engineered MPs derived from macrophages overexpressing a model antigen ovalbumin (OVA) can improve anti-PD-1 therapy in melanoma B16-OVA tumor-bearing mice. Our work presents a facile and generic strategy for personalized cancer immunotherapy to boost anti-PD-1 therapy.

PEG-Sheddable Nanodrug Remodels Tumor Microenvironment to Promote Effector T Cell Infiltration and Revise Their Exhaustion for Breast Cancer Immunotherapy

Low infiltration of cytotoxic T lymphocytes and their exhaustion manifest the two concurrent main hurdles for achieving effective tumor immunotherapy of triple-negative breast cancer. It is found that Galectin-9 blockage can revise the exhaustion of effector T cells, meanwhile the repolarization of protumoral M2 tumor-associated macrophages (TAMs) into tumoricidal M1-like ones can recruit effector T cells infiltrating into tumor to boost immune responses. Herein, a sheddable PEG-decorated and M2-TAMs targeted nanodrug incorporating Signal Transducer and Activator of Transcription 6 inhibitor (AS) and anti-Galectin-9 antibody (aG-9) is prepared. The nanodrug responds to acidic tumor microenvironment (TME) with the shedding of PEG corona and the release of aG-9, exerting local blockade of PD-1/Galectin-9/TIM-3 interaction to augment effector T cells via exhaustion reversing. Synchronously, targeted repolarization of M2-TAMs into M1 phenotype by AS-loaded nanodrug is achieved, which promotes tumor infiltration of effector T cells and thus synergizes with aG-9 blockade to boost the therapeutic efficacy. Besides, the PEG-sheddable approach endows nanodrug with stealth ability to reduce immune-related adverse effects caused by AS and aG-9. This PEG sheddable nanodrug holds the potential to reverse the immunosuppressive TME and increase effector T cell infiltration, which dramatically enhances immunotherapy in highly malignant breast cancer.

Cell-targeted vaccines: implications for adaptive immunity

Recent advancements in immunology and chemistry have facilitated advancements in targeted vaccine technology. Targeting specific cell types, tissue locations, or receptors can allow for modulation of the adaptive immune response to vaccines. This review provides an overview of cellular targets of vaccines, suggests methods of targeting and downstream effects on immune responses, and summarizes general trends in the literature. Understanding the relationships between vaccine targets and subsequent adaptive immune responses is critical for effective vaccine design. This knowledge could facilitate design of more effective, disease-specialized vaccines.

A tetramethylpyrazine-loaded hyaluronic acid-based hydrogel modulates macrophage polarization for promoting wound recovery in diabetic mice

The failure of wound healing often causes lower limb disability and amputation of diabetic patients. Current strategies for diabetic wound management often fail to achieve the expected outcomes, and emerging alternatives are urgently needed. Recent advances in the identification of active compounds from traditional herbal medicines provide promising therapeutics for tissue repair and regeneration. In this study, the pro-healing effects of tetramethylpyrazine (TMP, a natural alkaloid found in Ligusticum chuanxiong Hort) for diabetic wounds were for the first time demonstrated. The cutaneous healing was mainly achieved by TMP-mediated macrophage polarization from pro-inflammatory to pro-healing phenotype. In addition, the topical administration of TMP was facilitated by the hyaluronic acid (HA) hydrogel for promoting the full-thickness wounds in the experimental diabetic mice. Consequently, TMP-loaded HA hydrogel (TMP-HA) profoundly accelerated the wound closure in comparison with TMP-loaded INTRASITE Gel (it is a commercial hydrogel), which was evident with the inflammation mitigation, the angiogenesis enhancement, and the collagen deposition. Our work reveals the macrophage-modulatory function of TMP for diabetic wound healing and demonstrates great potential of TMP-HA for clinical application.

d-lactate modulates M2 tumor-associated macrophages and remodels immunosuppressive tumor microenvironment for hepatocellular carcinoma

The polarization of tumor-associated macrophages (TAMs) from M2 to M1 phenotype demonstrates great potential for remodeling the immunosuppressive tumor microenvironment (TME) of hepatocellular carcinoma (HCC). d-lactate (DL; a gut microbiome metabolite) acts as an endogenous immunomodulatory agent that enhances Kupffer cells for clearance of pathogens. In this study, the potential of DL for transformation of M2 TAMs to M1 was confirmed, and the mechanisms underlying such polarization were mainly due to the modulation of phosphatidylinositol 3-kinase/protein kinase B pathway. A poly(lactide-co-glycolide) nanoparticle (NP) was used to load DL, and the DL-loaded NP was modified with HCC membrane and M2 macrophage-binding peptide (M2pep), forming a nanoformulation (DL@NP-M-M2pep). DL@NP-M-M2pep transformed M2 TAMs to M1 and remodeled the immunosuppressive TME in HCC mice, promoting the efficacy of anti-CD47 antibody for long-term animal survival. These findings reveal a potential TAM modulatory function of DL and provide a combinatorial strategy for HCC immunotherapy.

The role of metabolic reprogramming of tumor-associated macrophages in shaping the immunosuppressive tumor microenvironment

Macrophages are potent immune effector cells in innate immunity and exert dual-effects in the tumor microenvironment (TME). Tumor-associated macrophages (TAMs) make up a significant portion of TME immune cells. Similar to M1/M2 macrophages, TAMs are also highly plastic, and their functions are regulated by cytokines, chemokines and other factors in the TME. The metabolic changes in TAMs are significantly associated with polarization towards a protumour or antitumour phenotype. The metabolites generated via TAM metabolic reprogramming in turn promote tumor progression and immune tolerance. In this review, we explore the metabolic reprogramming of TAMs in terms of energy, amino acid and fatty acid metabolism and the potential roles of these changes in immune suppression.

PTN-PTPRZ1 signaling axis blocking mediates tumor microenvironment remodeling for enhanced glioblastoma treatment

Glioblastoma (GBM) is a malignant brain tumor with a poor prognosis that is highly heterogeneous and invasive. One of the most major challenges of GBM treatment in the clinic is the blood-brain barrier (BBB). Additionally, the tumor microenvironment (TME) is highly enriched with immunosuppressed M2-like tumor-associated macrophages (M2 TAMs) and glioblastoma stem cells (GSCs), which promoted the malignancy of GBM through the PTN-PTPRZ1 signaling axis. Here, we developed a self-assembled dual-targeted hybrid micelle (DT-GM1) as a nanocarrier to deliver the chemotherapeutic agent doxorubicin (DOX). We demonstrated that this DT-GM1/DOX can cross the BBB using in vitro and in vivo GBM models, and that M2pep and PTPRZ1 antibodies allow it to precisely target the tumor microenvironment where M2 TAMs and GSCs are enriched, increasing intracellular drug accumulation via multiple internalization pathways. Additionally, simultaneous elimination of M2 TAMs and GSCs blocked the PTN-PTPRZ1 signaling axis, resulting in less M2 TAM infiltration and increased polarization to the M1 phenotype, reshaping the immune microenvironment. Overall, we have established a nanocarrier that can penetrate the BBB and target the TME while also synergizing with GBM chemotherapeutic agents, providing a promising new strategy for GBM treatment.

Turning adversity into opportunity: Small extracellular vesicles as nanocarriers for tumor-associated macrophages re-education

Currently, small extracellular vesicles (sEV) as a nanoscale drug delivery system, are undergoing biotechnological scaling and clinical validation. Nonetheless, preclinical pharmacokinetic studies revealed that sEV are predominantly uptaken by macrophages. Although this "sEV-macrophage" propensity represents a disadvantage in terms of sEV targeting and their bioavailability as nanocarriers, it also represents a strategic advantage for those therapies that involve macrophages. Such is the case of tumor-associated macrophages (TAMs), which can reprogram/repolarize their predominantly immunosuppressive and tumor-supportive phenotype toward an immunostimulatory and anti-tumor phenotype using sEV as nanocarriers of TAMs reprogramming molecules. In this design, sEV represents an advantageous delivery system, providing precision to the therapy by simultaneously matching their tropism to the therapeutic cell target. Here, we review the current knowledge of the role of TAMs in the tumoral microenvironment and the effect generated by the reprogramming of these phagocytic cells fate using sEV. Finally, we discuss how these vesicles can be engineered by different bioengineering techniques to improve their therapeutic cargo loading and preferential uptake by TAMs.

Origin, production and molecular determinants of macrophages for their therapeutic targeting

Macrophages, the most heterogeneous cells of the hematopoietic system and the giant eaters of the immune system that present either as tissue-resident cells or infiltrated immune cells, eliminate foreign pathogens and microbes and also play different physiological roles to maintain the body's immune response. In this review, we basically provide a broad overview of macrophages from their origin, functional diversity to M1-M2 polarization, specialized markers, and their role as important therapeutic targets in different diseases based on the current research and evidence. Apart from this, we have precisely discussed about tumor-associated macrophages (TAMs) and their role in tumor progression and newly discovered lesser-known markers of TAMs that could be used as potential therapeutic targets to treat life-threatening diseases. It is really very important to understand the diversity of macrophages to develop TAM-modulating strategies to activate our own immune system against diseases and to overcome immune resistance.

Targeted Immunoimaging of Tumor-Associated Macrophages in Orthotopic Glioblastoma by the NIR-IIb Nanoprobes

Developing dynamic and highly sensitive methods for imaging M2-type tumor-associated macrophages (TAMs) is vital for monitoring the tumor progression and assessing the therapeutic efficacy. Here, the fabrication and application of rationally designed Er-based rare-earth nanoprobes for the targeted imaging of M2-type TAMs in glioblastoma (GBM) through the second near-infrared (NIR-II) fluorescence beyond 1500 nm is reported. The NIR-IIb fluorescence of Er-based rare-earth nanoparticles can be remarkably enhanced by optimizing their core-shell structures and the shell thickness, which allows for in vivo imaging under excitation by a 980 nm laser with the lowest power density (40 mW cm^-2 ). These bright Er-based nanoparticles functionalized with M2pep polypeptide show notable targeting ability to M2-type macrophages, which has been well tested in both in vitro and in vivo experiments by their up-conversion (UC) fluorescence (540 nm) and down-shifting (DS) fluorescence (1525 nm), respectively. The targeting capability of these nanoprobes in vivo is also demonstrated by the overlap of immunofluorescence of M2-type TAMs and Arsenazo III staining of rare-earth ions in tumor tissue. It is envisioned that these nanoprobes can serve as a companion diagnostic tool to dynamically assess the progression and prognosis of GBM.

Anti-cancer peptide-based therapeutic strategies in solid tumors

Background

Nowadays, conventional medical treatments such as surgery, radiotherapy, and chemotherapy cannot cure all types of cancer. A promising approach to treat solid tumors is the use of tumor-targeting peptides to deliver drugs or active agents selectively.

Result

Introducing beneficial therapeutic approaches, such as therapeutic peptides and their varied methods of action against tumor cells, can aid researchers in the discovery of novel peptides for cancer treatment. The biomedical applications of therapeutic peptides are highly interesting. These peptides, owing to their high selectivity, specificity, small dimensions, high biocompatibility, and easy modification, provide good opportunities for targeted drug delivery. In recent years, peptides have shown considerable promise as therapeutics or targeting ligands in cancer research and nanotechnology.

Conclusion

 This study reviews a variety of therapeutic peptides and targeting ligands in cancer therapy. Initially, three types of tumor-homing and cell-penetrating peptides (CPPs) are described, and then their applications in breast, glioma, colorectal, and melanoma cancer research are discussed.

Emerging Nanoparticle Strategies for Modulating Tumor-Associated Macrophage Polarization

Immunotherapy has made great progress in recent years, yet the efficacy of solid tumors remains far less than expected. One of the main hurdles is to overcome the immune-suppressive tumor microenvironment (TME). Among all cells in TME, tumor-associated macrophages (TAMs) play pivotal roles because of their abundance, multifaceted interactions to adaptive and host immune systems, as well as their context-dependent plasticity. Underlying the highly plastic characteristic, lots of research interests are focused on repolarizing TAMs from M2-like pro-tumor phenotype towards M1-like antitumoral ones. Nanotechnology offers great opportunities for targeting and modulating TAM polarization to mount the therapeutic efficacy in cancer immunotherapy. Here, this mini-review highlights those emerging nano-approaches for TAM repolarization in the last three years.

Micelle nanovehicles for co-delivery of Lepidium meyenii Walp. (maca) polysaccharide and chloroquine to tumor-associated macrophages for synergistic cancer immunotherapy

Here, we fabricated amphiphilic polysaccharide micelles for synergistic cancer immunotherapy targeting tumor-associated macrophages (TAMs). Lepidium meyenii Walp. (maca) polysaccharide (MP), a naturally derived macromolecule with a strong TAM-remodeling effect, was grafted on a hydrophobic poly(lactic-co-glycolic acid) (PLGA) segment, with a disulfide bond for redox-sensitive linkage. The amphiphilic polysaccharide derivatives could self-assemble into core (PLGA)-shell (MP)-structured micelles and encapsulate chloroquine (CQ) into the hydrophobic core. By using a 4T1-M2 macrophage co-culture model and a 4T1 tumor xenograft mouse model, we showed that the prepared micelles could co-deliver MP and CQ to the tumor sites and selectively accumulate at TAMs because of the specific properties of MP. Furthermore, the nanoparticles exerted synergistic tumor immunotherapeutic and antimetastatic effects, which might be attributable to the enhanced cell internalization of the micelles and the multiple regulatory mechanisms of MP and CQ. Thus, immunomodulatory MP may be a promising biomaterial for cancer immunotherapy.

Tackling TAMs for Cancer Immunotherapy: It's Nano Time

The tumor microenvironment (TME) is a highly complex environment that surrounds tumors. Interactions between cancer cells/non-cancerous cells and cells/non-cell components in the TME support tumor initiation, development, and metastasis. Of the cell types in the TME, tumor-associated macrophages (TAMs) have gained attention owing to their crucial roles in supporting tumors and conferring therapy resistance. Recent developments in nanotechnology raise opportunities for the application of nano targeted drug-delivery systems (Nano-TDDS) in cancer therapy. We focus our discussion on current knowledge of TAMs, and describe recent examples of Nano-TDDS-based TAM modulation, highlighting strategies to overcome in vivo delivery barriers associated with the TME and their potential for clinical translation.

The Use of Alternative Strategies for Enhanced Nanoparticle Delivery to Solid Tumors

Nanomaterial (NM) delivery to solid tumors has been the focus of intense research for over a decade. Classically, scientists have tried to improve NM delivery by employing passive or active targeting strategies, making use of the so-called enhanced permeability and retention (EPR) effect. This phenomenon is made possible due to the leaky tumor vasculature through which NMs can leave the bloodstream, traverse through the gaps in the endothelial lining of the vessels, and enter the tumor. Recent studies have shown that despite many efforts to employ the EPR effect, this process remains very poor. Furthermore, the role of the EPR effect has been called into question, where it has been suggested that NMs enter the tumor via active mechanisms and not through the endothelial gaps. In this review, we provide a short overview of the EPR and mechanisms to enhance it, after which we focus on alternative delivery strategies that do not solely rely on EPR in itself but can offer interesting pharmacological, physical, and biological solutions for enhanced delivery. We discuss the strengths and shortcomings of these different strategies and suggest combinatorial approaches as the ideal path forward.

Macrophages as a Double-Edged Weapon: The Use of Macrophages in Cancer Immunotherapy and Understanding the Cross-Talk Between Macrophages and Cancer

Macrophages (Mϕs) play an essential role in maintaining body homeostasis. They perform dual functions produced by different subtypes. Mϕs not only fight against pathogens and foreign bodies such as bacteria or cancer cells but also participate in healing and repairing damaged tissue since they maintain both proinflammatory and anti-inflammatory effects sequentially. Tumors possess the ability to polarize Mϕs from proinflammatory M1 subtype to anti-inflammatory M2-like Mϕs called tumor-associated macrophages, which, in turn, help the tumors to acquire cancer hallmarks. Consequently, this polarization allows tumors to grow and spread. In this light, Mϕs have been a subject of intense study, and researchers have developed protocols to derive different Mϕs subtypes either as a new state-of-the-art therapeutic approach or to understand the cross-talk between cancer and Mϕs. In this review, we present the use of primary Mϕs in adoptive immunotherapy for cancer, illustrate the reciprocating interplay between cancer and Mϕs, and the resulting structural and functional change on both cell types. Furthermore, we summarize the recent cutting-edge approaches of using Mϕs in cancer immunotherapy.

Tumor-associated macrophages: Role in the pathological process of tumorigenesis and prospective therapeutic use (Review)

The aim of the present study was to evaluate the current body of knowledge regarding tumor-associated macrophages (TAMs) and their potential use in antitumor therapy, based on their role in the pathological process of tumorigenesis. For this purpose, a critical analysis of published data and summarization of the findings available from original studies, focusing on the role of TAMs in the pathological process, and their potential therapeutic application was performed. Promising key avenues of research were identified in this field. The following issues seem the most promising and thus worth further investigation: i) The process of M1/M2 macrophage polarization, macrophage characteristics at intermediate polarization steps and their role in the tumor process; ii) determining the conditions necessary for transitions between the M1 and M2 macrophage phenotypes and the role of signals from the microenvironment in this process; iii) cause-and-effect associations between the quantity and quality of macrophages, and the prognosis and outcome of the pathological process; iv) modulation of macrophages and stimulation of their phagocytic activity with drugs; v) targeted vector-based systems for drug delivery to macrophages; and vi) targeted drug delivery systems with macrophages as carriers, thus potentially combining chemotherapy and immunotherapy.

Targeting Tumors Using Peptides

To penetrate solid tumors, low molecular weight (Mw < 10 KDa) compounds have an edge over antibodies: their higher penetration because of their small size. Because of the dense stroma and high interstitial fluid pressure of solid tumors, the penetration of higher Mw compounds is unfavored and being small thus becomes an advantage. This review covers a wide range of peptidic ligands—linear, cyclic, macrocyclic and cyclotidic peptides—to target tumors: We describe the main tools to identify peptides experimentally, such as phage display, and the possible chemical modifications to enhance the properties of the identified peptides. We also review in silico identification of peptides and the most salient non-peptidic ligands in clinical stages. We later focus the attention on the current validated ligands available to target different tumor compartments: blood vessels, extracelullar matrix, and tumor associated macrophages. The clinical advances and failures of these ligands and their therapeutic conjugates will be discussed. We aim to present the reader with the state-of-the-art in targeting tumors, by using low Mw molecules, and the tools to identify new ligands.

Peptide-based materials for cancer immunotherapy

Peptide-based materials hold great promise as immunotherapeutic agents for the treatment of many malignant cancers. Extensive studies have focused on the development of peptide-based cancer vaccines and delivery systems by mimicking the functional domains of proteins with highly specific immuno-regulatory functions or tumor cells fate controls. However, a systemic understanding of the interactions between the different peptides and immune systems remains unknown. This review describes the role of peptides in regulating the functions of the innate and adaptive immune systems and provides a comprehensive focus on the design, categories, and applications of peptide-based cancer vaccines. By elucidating the impacts of peptide length and formulations on their immunogenicity, peptide-based immunomodulating agents can be better utilized and dramatic breakthroughs may also be realized. Moreover, some critical challenges for translating peptides into large-scale synthesis, safe delivery, and efficient cancer immunotherapy are posed to improve the next-generation peptide-based immunotherapy.

Novel therapeutic interventions in cancer treatment using protein and peptide-based targeted smart systems

Cancer, being the most prevalent and resistant disease afflicting any gender, age or social status, is the ultimate challenge for the scientific community. The new generation therapeutics for cancer management has shifted the approach to personalized/precision medicine, making use of patient- and tumor-specific markers for specifying the targeted therapies for each patient. Peptides targeting these cancer-specific signatures hold enormous potential for cancer therapy and diagnosis. The rapid advancements in the combinatorial peptide libraries served as an impetus to the development of multifunctional peptide-based materials for targeted cancer therapy. The present review outlines benefits and shortcomings of peptides as cancer therapeutics and the potential of peptide modified nanomedicines for targeted delivery of anticancer agents.

Tumor-associated macrophages in tumor metastasis: biological roles and clinical therapeutic applications

Tumor metastasis is a major contributor to the death of cancer patients. It is driven not only by the intrinsic alterations in tumor cells, but also by the implicated cross-talk between cancer cells and their altered microenvironment components. Tumor-associated macrophages (TAMs) are the key cells that create an immunosuppressive tumor microenvironment (TME) by producing cytokines, chemokines, growth factors, and triggering the inhibitory immune checkpoint proteins release in T cells. In doing so, TAMs exhibit important functions in facilitating a metastatic cascade of cancer cells and, meanwhile, provide multiple targets of certain checkpoint blockade immunotherapies for opposing tumor progression. In this article, we summarize the regulating networks of TAM polarization and the mechanisms underlying TAM-facilitated metastasis. Based on the overview of current experimental evidence dissecting the critical roles of TAMs in tumor metastasis, we discuss and prospect the potential applications of TAM-focused therapeutic strategies in clinical cancer treatment at present and in the future.

Tumor-associated macrophages in tumor metastasis: biological roles and clinical therapeutic applications

Tumor metastasis is a major contributor to the death of cancer patients. It is driven not only by the intrinsic alterations in tumor cells, but also by the implicated cross-talk between cancer cells and their altered microenvironment components. Tumor-associated macrophages (TAMs) are the key cells that create an immunosuppressive tumor microenvironment (TME) by producing cytokines, chemokines, growth factors, and triggering the inhibitory immune checkpoint proteins release in T cells. In doing so, TAMs exhibit important functions in facilitating a metastatic cascade of cancer cells and, meanwhile, provide multiple targets of certain checkpoint blockade immunotherapies for opposing tumor progression. In this article, we summarize the regulating networks of TAM polarization and the mechanisms underlying TAM-facilitated metastasis. Based on the overview of current experimental evidence dissecting the critical roles of TAMs in tumor metastasis, we discuss and prospect the potential applications of TAM-focused therapeutic strategies in clinical cancer treatment at present and in the future.

Surface Modification of Polymeric Nanoparticles with M2pep Peptide for Drug Delivery to Tumor-Associated Macrophages

PURPOSE

Tumor-associated macrophages (TAMs) with immune-suppressive M2-like phenotype constitute a significant part of tumor and support its growth, thus making an attractive therapeutic target for cancer therapy. To improve the delivery of drugs that control the survival and/or functions of TAMs, we developed nanoparticulate drug carriers with high affinity for TAMs.

METHODS

Poly(lactic-co-glycolic acid) nanoparticles were coated with M2pep, a peptide ligand selectively binding to M2-polarized macrophages, via a simple surface modification method based on tannic acid-iron complex. The interactions of M2pep-coated nanoparticles with macrophages of different phenotypes were tested in vitro and in vivo. PLX3397, an inhibitor of the colony stimulating factor-1 (CSF-1)/CSF-1 receptor (CSF-1R) pathway and macrophage survival, was delivered to B16F10 tumors via M2pep-modified PLGA nanoparticles.

RESULTS

In bone marrow-derived macrophages polarized to M2 phenotype, M2pep-coated nanoparticles showed greater cellular uptake than those without M2pep. Consistently, M2pep-coated nanoparticles showed relatively high localization of CD206⁺ macrophages in B16F10 tumors. PLX3397 encapsulated in M2pep-coated nanoparticles attenuated tumor growth better than the free drug counterpart.

CONCLUSION

These results support that M2pep-coating can help nanoparticles to interact with M2-like TAMs and facilitate the delivery of drugs that control the tumor-supportive functions of TAMs.

Multiscale Agent-Based and Hybrid Modeling of the Tumor Immune Microenvironment

Multiscale systems biology and systems pharmacology are powerful methodologies that are playing increasingly important roles in understanding the fundamental mechanisms of biological phenomena and in clinical applications. In this review, we summarize the state of the art in the applications of agent-based models (ABM) and hybrid modeling to the tumor immune microenvironment and cancer immune response, including immunotherapy. Heterogeneity is a hallmark of cancer; tumor heterogeneity at the molecular, cellular, and tissue scales is a major determinant of metastasis, drug resistance, and low response rate to molecular targeted therapies and immunotherapies. Agent-based modeling is an effective methodology to obtain and understand quantitative characteristics of these processes and to propose clinical solutions aimed at overcoming the current obstacles in cancer treatment. We review models focusing on intra-tumor heterogeneity, particularly on interactions between cancer cells and stromal cells, including immune cells, the role of tumor-associated vasculature in the immune response, immune-related tumor mechanobiology, and cancer immunotherapy. We discuss the role of digital pathology in parameterizing and validating spatial computational models and potential applications to therapeutics.

SPIO-loaded nanostructured lipid carriers as liver-targeted molecular T2-weighted MRI contrast agent

Background

Superparamagnetic iron oxide (SPIO) acts as a negative contrast agent in magnetic resonance imaging (MRI), and is widely used in clinical applications, including the diagnosis of hepatic diseases. Hepatocyte-targeted magnetic resonance contrast agents (MRCAs) can provide useful information for evaluating hepatic diseases. We prepared targeted magnetic nanostructured lipid carriers (MNLCs) to enhance the hepatocytes targeting efficiency.

Methods

In vitro characterizations of MNLCs were determined by transmission electron microscopy (TEM). The cytotoxicity assay of the MNLCs was measured by methyl tetrazolium (MTT) method. The uptaken study was measured by confocal microscopy, flow cytometry and MRI in vitro. The enhanced liver-targeting efficiency of MNLCs was measured by fluorescence imaging and MRI in vivo.

Results

Gal-NLC-SPIO was prepared successfully. The cytotoxicity assay of the MNLCs demonstrated that the MNLC had relatively low cytotoxicity and high biocompatibility for LO2 cells. More importantly, we confirmed that Gal-NLC-SPIO had greater uptake by LO2 cells than Gal-NLC-SPIO/PEG and free Gal in vitro. A liver distribution study of MNLCs in normal mice demonstrated that the fluorescent signal values to livers of the Gal-NLC-SPIO were significantly stronger than those of NLC-SPIO and Gal-NLC-SPIO/PEG. The liver targeting efficiency of Gal-NLC-SPIO was confirmed both in vitro and in vivo.

Conclusions

We successfully developed liver-targeting MNLCs, which showed accurate hepatocytes targeting, and thus have the potential to be a new MRI contrast agent to help the diagnosis of liver diseases.

Combination Glioma Therapy Mediated by a Dual-Targeted Delivery System Constructed Using OMCN-PEG-Pep22/DOX

Accumulating studies have investigated the efficacy of receptor-mediated delivery of hydrophobic drugs in glioma chemotherapy. Here, a delivery vehicle comprising polyethylene glycol (PEG) and oxidized nanocrystalline mesoporous carbon particles (OMCN) linked to the Pep22 polypeptide targeting the low-density lipoprotein receptor (LDLR) is designed to generate a novel drug-loaded system, designated as OMCN-PEG-Pep22/DOX (OPPD). This system effectively targets glioma cells and the blood-brain barrier and exerts therapeutic efficacy through both near-infrared (NIR) photothermal and chemotherapeutic effects of loaded doxycycline (DOX). Pathological tissue microarrays show an association of LDLR overexpression in human glioma tissue with patient survival.NIR irradiation treatment and magnetic resonance imaging results show that OPPD reaches the effective glioma-killing temperature in a glioma-bearing rat with a skull bone removal model and considerably reduces glioma sizes relative to the drug-loaded system without the Pep22 peptide modification and the control respectively. Thus, OPPD not only effectively targets LDLR-overexpressing glioma but also exerts a dual therapeutic effect by transporting DOX into the glioma and generating thermal effects with near-infrared irradiation to kill tumor cells. These collective findings support the utility of the novel OPPD drug-loaded system as a promising drug delivery vehicle for clinical application in glioma therapy.

Pro-apoptotic peptides-based cancer therapies: challenges and strategies to enhance therapeutic efficacy

Cancer is a leading cause of death worldwide. Despite many advances in the field of cancer therapy, an effective cure is yet to be found. As a more potent alternative for the conventional small molecule anti-cancer drugs, pro-apoptotic peptides have emerged as a new class of anticancer agents. By interaction with certain members in the apoptotic pathways, they could effectively kill tumor cells. However, there remain bottleneck challenges for clinical application of these pro-apoptotic peptides in cancer therapy. In this review, we will overview the developed pro-apoptotic peptides and outline the widely adopted molecular-based and nanoparticle-based strategies to enhance their anti-tumor effects.

Seek & Destroy, use of targeting peptides for cancer detection and drug delivery

Accounting for 16 million new cases and 9 million deaths annually, cancer leaves a great number of patients helpless. It is a complex disease and still a major challenge for the scientific and medical communities. The efficacy of conventional chemotherapies is often poor and patients suffer from off-target effects. Each neoplasm exhibits molecular signatures - sometimes in a patient specific manner - that may completely differ from the organ of origin, may be expressed in markedly higher amounts and/or in different location compared to the normal tissue. Although adding layers of complexity in the understanding of cancer biology, this cancer-specific signature provides an opportunity to develop targeting agents for early detection, diagnosis, and therapeutics. Chimeric antibodies, recombinant proteins or synthetic polypeptides have emerged as excellent candidates for specific homing to peripheral and central nervous system cancers. Specifically, peptide ligands benefit from their small size, easy and affordable production, high specificity, and remarkable flexibility regarding their sequence and conjugation possibilities. Coupled to imaging agents, chemotherapies and/or nanocarriers they have shown to increase the on-site delivery, thus allowing better tumor mass contouring in imaging and increased efficacy of the chemotherapies associated with reduced adverse effects. Therefore, some of the peptides alone or in combination have been tested in clinical trials to treat patients. Peptides have been well-tolerated and shown absence of toxicity. This review aims to offer a view on tumor targeting peptides that are either derived from natural peptide ligands or identified using phage display screening. We also include examples of peptides targeting the high-grade malignant tumors of the central nervous system as an example of the complex therapeutic management due to the tumor's location. Peptide vaccines are outside of the scope of this review.

Peripheral Nerve Nanoimaging: Monitoring Treatment and Regeneration

Accidental and iatrogenic trauma are major causes of peripheral nerve injury. Healing after nerve injury is complex and often incomplete, which can lead to acute or chronic pain and functional impairment. Current assessment methods for nerve regeneration lack sensitivity and objectivity. There is a need for reliable and reproducible, noninvasive strategies with adequate spatial and temporal resolution for longitudinal evaluation of degeneration or regeneration after injury/treatment. Methods for noninvasive monitoring of the efficacy and effectiveness of neurotherapeutics in nerve regeneration or of neuropathic pain are needed to ensure adequacy and responsiveness to management, especially given the large variability in the patient populations, etiologies, and complexity of nerve injuries. Surrogate biomarkers are needed with positive predictive correlation for the dynamics and kinetics of neuroregeneration. They can provide direct real-time insight into the efficacy and mechanisms of individualized therapeutic intervention. Here, we review the state-of-the-art tools, technologies, and therapies in peripheral nerve injury and regeneration as well as provide perspectives for the future. We present compelling evidence that advancements in nanomedicine and innovation in nanotechnology such as nanotheranostics hold groundbreaking potential as paradigm shifts in noninvasive peripheral nerve imaging and drug delivery. Nanotechnology, which revolutionized molecular imaging in cancer and inflammatory disease, can be used to delineate dynamic molecular imaging signatures of neuroinflammation and neuroregeneration while simultaneously monitoring cellular or tissue response to drug therapy. We believe that current clinical successes of nanotechnology can and should be adopted and adapted to the science of peripheral nerve injury and regeneration.

Multivalent polymers displaying M2 macrophage-targeting peptides improve target binding avidity and serum stability

Tumor-associated macrophages (TAMs) display a spectrum of phenotypes ranging from pro-tumoral/anti-inflammatory "M2-like" to anti-tumoral/pro-inflammatory "M1-like" subtypes and, consequently, high intratumoral M2-to-M1 ratios are typically indicative of poor disease prognosis. Cancer immunotherapies that selectively modulate M2-like TAMs, enabling reversal of the M2-to-M1 ratio, represent a promising anti-cancer intervention but are difficult to implement due to the lack of effective targeting systems. In this study, we report the development of high avidity, M2 macrophage-selective targeted drug delivery platforms based on M2 macrophage-targeting peptides (M2pep) grafted onto poly(N-(2-hydroxypropyl) methacrylamide). Furthermore, these M2pep-grafted polymers also exhibit improved serum stability along with M2 macrophage-selective toxicity.

Tumor-Associated Macrophages as Target for Antitumor Therapy

It is well known that the microenvironment of solid tumors is rich in inflammatory cells that influence tumor growth and development. Macrophages, called tumor-associated macrophages (TAMs), are the most abundant immune cell population present in tumor tissue. Several studies have demonstrated that the density of TAMs is associated with a poor prognosis and positively correlates with tumor growth. Several studies have proved that TAMs may activate and protect tumor stem cells, stimulate their proliferation as well as promote angiogenesis and metastasis. Furthermore, TAMs-derived cytokines and other proteins, such as CCL-17, CCL-22, TGF-β, IL-10, arginase 1, and galectin-3, make a significant contribution to immunosuppression. Since TAMs influence various aspects of cancer progression, there are many attempts to use them as a target for immunotherapy. The numerous studies have shown that the primary tumor growth and the number of metastatic sites can be significantly decreased by decreasing the population of macrophages in tumor tissue, for example, by blocking recruitment of monocytes or eliminating TAMs already present in the tumor tissue. Moreover, there are attempts at reprogramming TAMs into proinflammatory M1 macrophages or neutralizing the protumoral products of TAMs. Another approach uses TAMs for anticancer drug delivery into the tumor environment. In this review, we would like to summarize the clinical and preclinical trials that were focused on macrophages as a target for anticancer therapies.

Surface chemistry governs cellular tropism of nanoparticles in the brain

Nanoparticles are of long-standing interest for the treatment of neurological diseases such as glioblastoma. Most past work focused on methods to introduce nanoparticles into the brain, suggesting that reaching the brain interstitium will be sufficient to ensure therapeutic efficacy. However, optimized nanoparticle design for drug delivery to the central nervous system is limited by our understanding of their cellular deposition in the brain. Here, we investigated the cellular fate of poly(lactic acid) nanoparticles presenting different surface chemistries, after administration by convection-enhanced delivery. We demonstrate that nanoparticles with ‘stealth' properties mostly avoid internalization by all cell types, but internalization can be enhanced by functionalization with bio-adhesive end-groups. We also show that association rates measured in cultured cells predict the extent of internalization of nanoparticles in cell populations. Finally, evaluating therapeutic efficacy in an orthotopic model of glioblastoma highlights the need to balance significant uptake without inducing adverse toxicity.

There have been numerous attempts to develop nanomaterials to reach cells of the central nervous system for drug delivery. Here, the authors investigate the cellular fate of polymer-based nanoparticles with varying surface chemistries after administration directly into the brain.

Progress in tumor-associated macrophage (TAM)-targeted therapeutics

As an essential innate immune population for maintaining body homeostasis and warding off foreign pathogens, macrophages display high plasticity and perform diverse supportive functions specialized to different tissue compartments. Consequently, aberrance in macrophage functions contributes substantially to progression of several diseases including cancer, fibrosis, and diabetes. In the context of cancer, tumor-associated macrophages (TAMs) in tumor microenvironment (TME) typically promote cancer cell proliferation, immunosuppression, and angiogenesis in support of tumor growth and metastasis. Oftentimes, the abundance of TAMs in tumor is correlated with poor disease prognosis. Hence, significant attention has been drawn towards development of cancer immunotherapies targeting these TAMs; either depleting them from tumor, blocking their pro-tumoral functions, or restoring their immunostimulatory/tumoricidal properties. This review aims to introduce readers to various aspects in development and evaluation of TAM-targeted therapeutics in pre-clinical and clinical stages.

Mononuclear phagocytes as a target, not a barrier, for drug delivery

Mononuclear phagocytes have been generally recognized as a barrier to drug delivery. Recently, a new understanding of mononuclear phagocytes (MPS) ontogeny has surfaced and their functions in disease have been unveiled, demonstrating the need for re-evaluation of perspectives on mononuclear phagocytes in drug delivery. In this review, we described mononuclear phagocyte biology and focus on their accumulation mechanisms in disease sites with explanations of monocyte heterogeneity. In the 'MPS as a barrier' section, we summarized recent studies on mechanisms to avoid phagocytosis based on two different biological principles: protein adsorption and self-recognition. In the 'MPS as a target' section, more detailed descriptions were given on mononuclear phagocyte-targeted drug delivery systems and their applications to various diseases. Collectively, we emphasize in this review that mononuclear phagocytes are potent targets for future drug delivery systems. Mononuclear phagocyte-targeted delivery systems should be created with an understanding of mononuclear phagocyte ontogeny and pathology. Each specific subset of phagocytes should be targeted differently by location and function for improved disease-drug delivery while avoiding RES clearance such as Kupffer cells and splenic macrophages.

Assessment of the Number and Phenotype of Macrophages in the Human BMB Samples of CML

Macrophages have emerged as a key player in tumor biology. However, their number and phenotype in human bone marrow of biopsy (BMB) samples of chronic myeloid leukemia (CML) and their association with disease progression from an initial chronic phase (CP) to accelerated phase (AP) to advanced blast phase (BP) are still unclear. BMB samples from 127 CML patients and 30 patients with iron-deficiency anemia (IDA) as control group were analyzed by immunohistochemistry. The expression levels of CD68, CD163, and CD206 in BMB samples of CML patients were significantly higher than those in the patients of control group (P < 0.01), and we observed that their positive expression was gradually elevated during the transformation of CML-CP to AP to BP (P < 0.01). However, the expressions of CD68, CD163, and CD206 in released group were downregulated and contrasted to these in control group; there exists statistical significance (P < 0.01). The percentage ratio of CD163 and CD206 to CD68 was pronounced to be increasing from CML-CP to AP to BP (P < 0.01). Hence, the higher proportion of CD68⁺, CD163⁺ and CD206⁺ macrophages in BMB samples can be considered a key factor for disease progression of CML patients. Targeting macrophages, especially the M2 phenotype may help in designing therapeutic strategies for CML.

Gazelles, unicorns, and dragons battle cancer through the Nanotechnology Startup Challenge

On March 4th, 2016, Springer’s Cancer Nanotechnology office promoted the launch of the Nanotechnology Startup Challenge in Cancer (NSC²). This innovation-development model is a partnership among our company, the Center for Advancing Innovation (CAI), MedImmune, the global biologics arm of AstraZeneca, and multiple institutes at the National Institutes of Health (NIH). NSC² “crowdsources” talent from around the world to launch startups with near-term, commercially viable cancer nanotechnology inventions, which were developed by the National Cancer Institute (NCI), the National Heart, Lung and Blood Institute (NHLBI), and the National Institute of Biomedical Imaging and Bioengineering (NIBIB). Crowdsourcing is a process in which one uses the internet to engage a large group of people in an activity, such as NSC². For this initiative, CAI engaged universities, industry professionals, foundations, investors, relevant media outlets, seasoned entrepreneurs, and life sciences membership organizations to request that they participate in the challenge. From this outreach, fifty-six key thought leaders have enrolled in NSC² as judges, mentors, and/or advisors to challenge teams (http://www.nscsquared.org/judges.html). Along with crowdsourcing talent to bolt startups around NIH inventions, NSC² will also catalyze the launch of companies around “third-party” cancer nanotechnology inventions, which were conceived and developed outside of the NIH. Twenty-eight robust teams were accepted to the challenge on March 14th, 2016.

Serum Stability and Affinity Optimization of an M2 Macrophage-Targeting Peptide (M2pep)

Tumor associated macrophages (TAMs) are a major stromal component of the tumor microenvironment in several cancers. TAMs are a potential target for adjuvant cancer therapies due to their established roles in promoting proliferation of cancer cells, angiogenesis, and metastasis. We previously discovered an M2 macrophage-targeting peptide (M2pep) which was successfully used to target and deliver a pro-apoptotic KLA peptide to M2-like TAMs in a CT-26 colon carcinoma model. However, the effectiveness of in vivo TAM-targeting using M2pep is limited by its poor serum stability and low binding affinity. In this study, we synthesized M2pep derivatives with the goals of increasing serum stability and binding affinity. Serum stability evaluation of M2pepBiotin confirmed its rapid degradation attributed to exolytic cleavage from the N-terminus and endolytic cleavages at the W10/W11 and S16/K17 sites. N-terminal acetylation of M2pepBiotin protected the peptide against the exolytic degradation while W10w and K(17,18,19)k substitutions were able to effectively protect endolytic degradation at their respective cleavage sites. However, no tested amino acid changes at the W10 position resulted in both protease resistance at that site and retention of binding activity. Therefore, cyclization of M2pep was investigated. Cyclized M2pep better resisted serum degradation without compromising binding activity to M2 macrophages. During the serum stability optimization process, we also discovered that K9R and W10Y substitutions significantly enhanced binding affinity of M2pep. In an in vitro binding study of different M2pep analogs pre-incubated in mouse serum, cyclic M2pep with K9R and W10Y modifications (cyclic M2pep(RY)) retained the highest binding activity to M2 macrophages over time due to its improved serum stability. Finally, we evaluated the in vivo accumulation of sulfo-Cy5-labeled M2pep and cyclic M2pep(RY) in both the CT-26 and 4T1 breast carcinoma models. Cyclic M2pep(RY) outperformed M2pep in both tumor localization and selective accumulation in M2-like TAMs. In conclusion, we report cyclic M2pep(RY) as our lead M2pep analog with improved serum stability and M2 macrophage-binding activity. Its enhanced utility as an in vivo M2-like-TAM-targeting agent was demonstrated in two tumor models, and is expected to be applicable for other tumor models or in models of M2 macrophage-related diseases.