Proteolytically Stable Cyclic Decapeptide for Breast Cancer Cell Targeting

Molecular farming for sustainable production of clinical-grade antimicrobial peptides

Antimicrobial peptides (AMPs) are emerging as next-generation therapeutics due to their broad-spectrum activity against drug-resistant bacterial strains and their ability to eradicate biofilms, modulate immune responses, exert anti-inflammatory effects and improve disease management. They are produced through solid-phase peptide synthesis or in bacterial or yeast cells. Molecular farming, i.e. the production of biologics in plants, offers a low-cost, non-toxic, scalable and simple alternative platform to produce AMPs at a sustainable cost. In this review, we discuss the advantages of molecular farming for producing clinical-grade AMPs, advances in expression and purification systems and the cost advantage for industrial-scale production. We further review how 'green' production is filling the sustainability gap, streamlining patent and regulatory approvals and enabling successful clinical translations that demonstrate the future potential of AMPs produced by molecular farming. Finally, we discuss the regulatory challenges that need to be addressed to fully realize the potential of molecular farming-based AMP production for therapeutics.

Peptide-Based Therapeutics in Cancer Therapy

A monster called cancer is still one of the most challenging human problems and one of the leading causes of death in the world. Different types of treatment methods are used for cancer therapy; however, there are challenges such as high cost and harmful side effects in using these methods. Recent years have witnessed a surge in the development of therapeutic peptides for a wide range of diseases, notably cancer. Peptides are preferred over antibiotics, radiation therapy, and chemotherapy in the treatment of cancer due to a number of aspects, including flexibility, easy modification, low immunogenicity, and inexpensive cost of production. The use of therapeutic peptides in cancer treatment is a novel and intriguing strategy. These peptides provide excellent prospects for targeted drug delivery because of their high selectivity, specificity, small dimensions, good biocompatibility, and simplicity of modification. Target specificity and minimal toxicity are benefits of therapeutic peptides. Additionally, peptides can be used to design antigens or adjuvants for vaccine development. Here, types of therapeutic peptides for cancer therapy will be discussed, such as peptide-based cancer vaccines and tumor-targeting peptides (TTP) and cell-penetrating peptides (CPP).

Peptide-Drug Conjugate Targeting Keratin 1 Inhibits Triple-Negative Breast Cancer in Mice

Selective delivery of chemotherapy to the tumor site while sparing healthy cells and tissues is an attractive approach for cancer treatment. Carriers such as peptides can facilitate selective tumor targeting and payload delivery. Peptides with specific affinity for the overexpressed cell-surface receptors in cancer cells are conjugated to chemotherapy to afford peptide-drug conjugates (PDCs) that show selective uptake by cancer cells. Using a 10-mer linear peptide (WxEAAYQrFL) called 18-4 that targets and binds breast cancer cells, we designed a peptide 18-4-doxorubicin (Dox) conjugate with high specific toxicity toward triple-negative breast cancer (TNBC) MDA-MB-231 cells and 30-fold lower toxicity to normal breast MCF10A epithelial cells. Here, we elucidate the in vivo activity of this potent and tumor-selective peptide 18-4-Dox conjugate in mice bearing orthotopic MDA-MB-231 tumors. Mice treated with four weekly injections of the conjugate showed significantly lower tumor volumes compared to mice treated with free Dox at an equivalent Dox dose. Immunohistochemical (IHC) analysis of mice tissues revealed that treatment with a low dose of PDC (2.5 mg/kg of Dox equiv) reduced the expression of proliferation markers (PCNA and Ki-67) and increased apoptosis (evidenced by increased caspase-3 expression). At the same dose of free Dox (2.5 mg/kg), the expression of these markers was similar to that of saline treatment. Accordingly, significantly more Dox accumulated in tumors of conjugate-treated mice (7-fold) compared to the Dox-treated mice, while lower levels of Dox were observed in the liver, heart, and lungs of peptide-Dox conjugate-treated mice (up to 3-fold less) than Dox-treated mice. The IHC analysis of keratin 1 (K1), the receptor for peptide 18-4, revealed K1 upregulation in tumors and low levels in normal mammary fat pad and liver tissues from mice, suggesting preferential uptake of PDCs by TNBC to be K1 receptor-mediated. Taken together, our data support the use of a PDC approach to deliver chemotherapy selectively to the TNBC to inhibit tumor growth.

Current therapeutic approaches and promising perspectives of using bioengineered peptides in fighting chemoresistance in triple-negative breast cancer

Breast cancer is a collation of malignancies that manifest in the mammary glands at the early stages. Among breast cancer subtypes, triple-negative breast cancer (TNBC) shows the most aggressive behavior, with apparent stemness features. Owing to the lack of response to hormone therapy and specific targeted therapies, chemotherapy remains the first line of the TNBC treatment. However, the acquisition of resistance to chemotherapeutic agents increase therapy failure, and promotes cancer recurrence and distant metastasis. Invasive primary tumors are the birthplace of cancer burden, though metastasis is a key attribute of TNBC-associated morbidity and mortality. Targeting the chemoresistant metastases-initiating cells via specific therapeutic agents with affinity to the upregulated molecular targets is a promising step in the TNBC clinical management. Exploring the capacity of peptides as biocompatible entities with the specificity of action, low immunogenicity, and robust efficacy provides a principle for designing peptide-based drugs capable of increasing the efficacy of current chemotherapy agents for selective targeting of the drug-tolerant TNBC cells. Here, we first focus on the resistance mechanisms that TNBC cells acquire to evade the effect of chemotherapeutic agents. Next, the novel therapeutic approaches employing tumor-targeting peptides to exploit the mechanisms of drug resistance in chemorefractory TNBC are described.

A Small Peptide Increases Drug Delivery in Human Melanoma Cells

Melanoma is the most fatal type of skin cancer and is notoriously resistant to chemotherapies. The response of melanoma to current treatments is difficult to predict. To combat these challenges, in this study, we utilize a small peptide to increase drug delivery to melanoma cells. A peptide library array was designed and screened using a peptide array-whole cell binding assay, which identified KK-11 as a novel human melanoma-targeting peptide. The peptide and its D-amino acid substituted analogue (VPWxEPAYQrFL or D-aa KK-11) were synthesized via a solid-phase strategy. Further studies using FITC-labeled KK-11 demonstrated dose-dependent uptake in human melanoma cells. D-aa KK-11 significantly increased the stability of the peptide, with 45.3% remaining detectable after 24 h with human serum incubation. Co-treatment of KK-11 with doxorubicin was found to significantly enhance the cytotoxicity of doxorubicin compared to doxorubicin alone, or sequential KK-11 and doxorubicin treatment. In vivo and ex vivo imaging revealed that D-aa KK-11 distributed to xenografted A375 melanoma tumors as early as 5 min and persisted up to 24 h post tail vein injection. When co-administered, D-aa KK-11 significantly enhanced the anti-tumor activity of a novel nNOS inhibitor (MAC-3-190) in an A375 human melanoma xenograft mouse model compared to MAC-3-190 treatment alone. No apparent systemic toxicities were observed. Taken together, these results suggest that KK-11 may be a promising human melanoma-targeted delivery vector for anti-melanoma cargo.

Biodistribution and Activity of EGFR Targeted Polymeric Micelles Delivering a New Inhibitor of DNA Repair to Orthotopic Colorectal Cancer Xenografts with Metastasis

The disruption of polynucleotide kinase/phosphatase (PNKP) in colorectal cancer (CRC) cells deficient in phosphatase and tensin homolog (PTEN) is expected to lead to the loss of cell viability by a process known as synthetic lethality. In previous studies, we have reported on the encapsulation of a novel inhibitor of PNKP, namely, A83B4C63, in polymeric micelles and its activity in slowing the growth of PTEN-deficient CRC cells as well as subcutaneous xenografts. In this study, to enhance drug delivery and specificity to CRC tumors, the surface of polymeric micelles carrying A83B4C63 was modified with GE11, a peptide targeting epidermal growth factor receptor (EGFR) overexpressed in about 70% of CRC tumors. Using molecular dynamics (MD) simulations, we assessed the binding site and affinity of GE11 for EGFR. The GE11-modified micelles, tagged with a near-infrared fluorophore, showed enhanced internalization by EGFR-overexpressing CRC cells in vitro and a trend toward increased primary tumor homing in an orthotopic CRC xenograft in vivo. In line with these observations, the GE11 modification of polymeric micelles was shown to positively contribute to the improved therapeutic activity of encapsulated A83B4C63 against HCT116-PTEN^-/- cells in vitro and that of orthotopic CRC xenograft in vivo. In conclusion, our results provided proof of principle evidence for the potential benefit of EGFR targeted polymeric micellar formulations of A83B4C63 as monotherapeutics for aggressive and metastatic CRC tumors but at the same time highlighted the need for the development of EGFR ligands with improved physiological stability and EGFR binding.

On-Resin Synthesis of Linear Aryl Thioether Containing Peptides and in-Solution Cyclization via Cysteine SNAr Reaction

Cyclic peptides represent one of the most promising therapeutic agents in drug discovery due to their good affinity and selectivity. Herein, an on-resin synthesis of aryl thioether containing peptides and a concise cyclization strategy via chemoselective cysteine SNAr reaction was developed. The arylation group could be incorporated into a series of amino acids and used for standard SPPS and peptides cyclization. Constructed cyclic peptides showed increased cellular uptakes compared to their linear peptides.

Keratin 1 as a cell-surface receptor in cancer

Keratins are fibrous proteins that take part in several important cellular functions, including the formation of intermediate filaments. In addition, keratins serve as epithelial cell markers, which has made their role in cancer progression, diagnosis, and treatment an important focus of research. Keratin 1 (K1) is a type II keratin whose structure is comprised of a coiled-coil central domain flanked by flexible, glycine-rich loops in the N- and C-termini. While the structure of cytoplasmic K1 is established, the structure of cell-surface K1 is not known. Several transformed cells, such as cancerous cells and cells that have undergone oxidative stress, display increased levels of overall and/or cell-surface K1 expression. Cell-surface keratins (CSKs) may be modified or truncated, and their role is yet to be fully elucidated. Current studies suggest that CSKs are involved in receptor-mediated endocytosis and immune evasion. In this Review, we discuss findings relating to K1 structure, overexpression, and cell-surface expression in the context of utilizing CSK1 as a receptor for targeted drug delivery to cancer cells, and other strategies to develop novel treatments for cancer.

Novel Peptide Therapeutic Approaches for Cancer Treatment

Peptides are increasingly being developed for use as therapeutics to treat many ailments, including cancer. Therapeutic peptides have the advantages of target specificity and low toxicity. The anticancer effects of a peptide can be the direct result of the peptide binding its intended target, or the peptide may be conjugated to a chemotherapy drug or radionuclide and used to target the agent to cancer cells. Peptides can be targeted to proteins on the cell surface, where the peptide–protein interaction can initiate internalization of the complex, or the peptide can be designed to directly cross the cell membrane. Peptides can induce cell death by numerous mechanisms including membrane disruption and subsequent necrosis, apoptosis, tumor angiogenesis inhibition, immune regulation, disruption of cell signaling pathways, cell cycle regulation, DNA repair pathways, or cell death pathways. Although using peptides as therapeutics has many advantages, peptides have the disadvantage of being easily degraded by proteases once administered and, depending on the mode of administration, often have difficulty being adsorbed into the blood stream. In this review, we discuss strategies recently developed to overcome these obstacles of peptide delivery and bioavailability. In addition, we present many examples of peptides developed to fight cancer.

Methionine epimerization in cyclic peptides

Bioactive flax cyclic octa- and nona-peptides containing single methionine (Met) and its oxidized forms were treated under mild alkaline conditions to perform regio-selective epimerization. Cyclic peptide epimerization at the Met α-proton in a single chemical step has not been reported previously. The epimerization rate varies among Met oxidation states and ring size. These d-amino isomers along with the developed Met alkylation strategy will enable an approach to novel chemical functionalization of biomolecules. The amino acid configurations were confirmed by Marfey derivatizations, and cytotoxicity studies show the difference among the isomers. These d-amino analogs can act as a potential biomarker in plant protein processing and biomedical applications.

One step regioselective methionine epimerization in cyclic peptides followed by selective functionalization leads to chemical novelty.

Evaluation of a Keratin 1 Targeting Peptide-Doxorubicin Conjugate in a Mouse Model of Triple-Negative Breast Cancer

Chemotherapy is the main treatment for triple-negative breast cancer (TNBC), a subtype of breast cancer that is aggressive with a poor prognosis. While chemotherapeutics are potent, these agents lack specificity and are equally toxic to cancer and nonmalignant cells and tissues. Targeted therapies for TNBC treatment could lead to more safe and efficacious drugs. We previously engineered a breast cancer cell targeting peptide 18-4 that specifically binds cell surface receptor keratin 1 (K1) on breast cancer cells. A conjugate of peptide 18-4 and doxorubicin (Dox) containing an acid-sensitive hydrazone linker showed specific toxicity toward TNBC cells. Here, we report the in vivo evaluation of the K1 targeting peptide-Dox conjugate (PDC) in a TNBC cell-derived xenograft mouse model. Mice treated with the conjugate show significantly improved antitumor efficacy and reduced off-target toxicity compared to mice treated with Dox or saline. After six weekly treatments, on day 35, the mice treated with PDC (2.5 mg Dox equivalent/kg) showed significant reduction (1.5 times) in tumor volume compared to mice treated with Dox (2.5 mg/kg). The mice treated with the conjugate showed significantly higher (1.4 times) levels of Dox in tumors and lower (1.3-2.2 times) levels of Dox in other organs compared to mice treated with Dox. Blood collected at 15 min showed 3.6 times higher concentration of the drug (PDC and Dox) in mice injected with PDC compared to the drug (Dox) in mice injected with Dox. The study shows that the K1 targeting PDC is a promising novel modality for treatment of TNBC, with a favorable safety profile, and warrants further investigation of K1 targeting conjugates as TNBC therapeutics.

Impact of non-proteinogenic amino acids in the discovery and development of peptide therapeutics

With the development of modern chemistry and biology, non-proteinogenic amino acids (NPAAs) have become a powerful tool for developing peptide-based drug candidates. Drug-like properties of peptidic medicines, due to the smaller size and simpler structure compared to large proteins, can be changed fundamentally by introducing NPAAs in its sequence. While peptides composed of natural amino acids can be used as drug candidates, the majority have shown to be less stable in biological conditions. The impact of NPAA incorporation can be extremely beneficial in improving the stability, potency, permeability, and bioavailability of peptide-based therapies. Conversely, undesired effects such as toxicity or immunogenicity should also be considered. The impact of NPAAs in the development of peptide-based therapeutics is reviewed in this article. Further, numerous examples of peptides containing NPAAs are presented to highlight the ongoing development in peptide-based therapeutics.

Cyclic Derivative of Host-Defense Peptide IDR-1018 Improves Proteolytic Stability, Suppresses Inflammation, and Enhances In Vivo Activity

Host-defense peptides have drawn significant attention as new drugs or drug adjuvants to combat multidrug-resistant bacteria. In this study, we report the development of cyclic derivatives of the immunomodulatory and antibiofilm innate defense regulator peptide (IDR)-1018 based on three different synthetic strategies including head-to-tail cyclization (C1), side-chain-to-tail cyclization (C2), and a disulfide bond cross-linkage (C3). The generated mimetics showed enhanced proteolytic stability and reduced aggregation in vitro and in vivo. The C2 derivative exhibited exceptional ability to suppress inflammation and significantly reduce bacterial loads in a high-density Staphylococcus aureus murine skin infection model. The findings describe different routes to the creation of enzymatically stable mimetics of IDR-1018 and identify a promising new cyclic analogue against bacterial infections.

Antitumor Activity of Ohmyungsamycin A through the Regulation of the Skp2-p27 Axis and MCM4 in Human Colorectal Cancer Cells

Ohmyungsamycin A (1), a novel cyclic peptide discovered from a marine Streptomyces sp., was previously reported with antibacterial and anticancer activities. However, the antitumor activities and the underlying molecular mechanisms of 1 remain to be elucidated. Compound 1 inhibited the proliferation and tumor growth of HCT116 human colorectal cancer cells based on both in vitro cell cultures and an in vivo animal model. A cDNA microarray analysis revealed that 1 downregulated genes involved in cell cycle checkpoint control. Compound 1 also induced G₀/G₁ cell cycle arrest that was mediated by the regulation of S-phase kinase-associated protein 2 (Skp2)-p27 axis and minichromosome maintenance protein 4 (MCM4). Furthermore, a longer exposure of 1 exhibited an accumulation of a sub-G₁ phase cell population, which is characteristic of apoptotic cells. The induction of apoptosis by 1 was also associated with the modulation of caspase family proteins. Compound 1 effectively suppressed tumor growth in a xenograft mouse model subcutaneously implanted with HCT116 cells. In addition, analysis of tumors revealed that 1 upregulated the expression of the CDK inhibitor p27 but downregulated the expression of Skp2 and MCM4. These findings demonstrate the involvement of 1 in cell cycle regulation and the induction of apoptosis in human colorectal cancer cells.

Fluorogenic Trp(redBODIPY) cyclopeptide targeting keratin 1 for imaging of aggressive carcinomas

Keratin 1 (KRT1) is overexpressed in squamous carcinomas and associated with aggressive pathologies in breast cancer. Herein we report the design and preparation of the first Trp-based red fluorogenic amino acid, which is synthetically accessible in a few steps and displays excellent photophysical properties, and its application in a minimally-disruptive labelling strategy to prepare a new fluorogenic cyclopeptide for imaging of KRT1+ cells in whole intact tumour tissues.

Targeting Triple Negative Breast Cancer Cells with Novel Cytotoxic Peptide-Doxorubicin Conjugates

In this study, we have designed and synthesized two novel peptide-drug conjugates (PDCs) where the drug, doxorubicin (Dox), is linked to the peptide via a succinimidyl thioether bond or a hydrazone linker. A highly specific and proteolytically stable breast cancer cell targeting peptide (WxEAAYQrFL) is conjugated to Dox to synthesize peptide-Dox thioether (1) or hydrazone (2) conjugate. The evaluation of the stability in water, media, and human serum showed that the conjugate 1 with the succinimidyl thioether linkage is more stable compared to the acid-sensitive hydrazone containing conjugate 2. The cytotoxicity studies showed that the two PDCs were as toxic as free Dox toward the triple negative breast cancer (TNBC) cells and were 7-30 times less toxic (IC₅₀ 1.2-4.7 μM for TNBC cells versus 15-39 μM for noncancerous cells) toward the noncancerous breast cells compared to the free doxorubicin (IC₅₀ 0.35-1.5 μM for TNBC cells versus 0.24 μM for noncancerous cells). The results from the comparative study of the two PDCs suggest that both may have translational potential for TNBC treatment.

Small Peptide Ligands for Targeting EGFR in Triple Negative Breast Cancer Cells

The efficacy of chemotherapy for cancer treatment can be increased by targeted drug delivery to the cancer cells. This is particularly important for triple negative breast cancer (TNBC) for which chemotherapy is a major form of treatment. Here we designed and screened a library of 30 peptides starting with a previously reported epidermal growth factor receptor (EGFR) targeting peptide GE11 (YHWYGYTPQNVI). A direct peptide array-whole cell binding assay, where the peptides are conjugated to a cellulose membrane, was used to identify four peptides with enhanced binding to TNBC cells. Next, the four peptides were synthesized as FITC-labelled soluble peptides to study their direct uptake by TNBC cells using flow cytometry. The results showed that peptide analogue 22 had several fold higher uptake by the TNBC cells compared to the lead peptide GE11. The specific uptake of the peptide analogue 22 was confirmed by competition experiment using pure EGF protein. Further, peptide 22 showed dose dependent uptake by the TNBC MDA-MB-231 cells (10⁵) with uptake saturating at around 2 μM peptide concentration. Thus, peptide 22 is a promising EGFR specific TNBC cell binding peptide that can be conjugated directly to a chemotherapeutic drug or to nanoparticles for targeted drug delivery to enhance the efficacy of chemotherapy for TNBC treatment.