Proteolytically Stable Cancer Targeting Peptides with High Affinity for Breast Cancer Cells

An all-in-one tetrazine reagent for cysteine-selective labeling and bioorthogonal activable prodrug construction

The prodrug design strategy offers a potent solution for improving therapeutic index and expanding drug targets. However, current prodrug activation designs are mainly responsive to endogenous stimuli, resulting in unintended drug release and systemic toxicity. In this study, we introduce 3-vinyl-6-oxymethyl-tetrazine (voTz) as an all-in-one reagent for modular preparation of tetrazine-caged prodrugs and chemoselective labeling peptides to produce bioorthogonal activable peptide-prodrug conjugates. These stable prodrugs can selectively bind to target cells, facilitating cellular uptake. Subsequent bioorthogonal cleavage reactions trigger prodrug activation, significantly boosting potency against tumor cells while maintaining exceptional off-target safety for normal cells. In vivo studies demonstrate the therapeutic efficacy and safety of this prodrug design approach. Given the broad applicability of functional groups and labeling versatility with voTz, we foresee that this strategy will offer a versatile solution to enhance the therapeutic range of cytotoxic agents and facilitate the development of bioorthogonal activatable biopharmaceuticals and biomaterials.

The pearl jubilee of microcin J25: thirty years of research on an exceptional lasso peptide

Covering: 1992 up to 2023Since their discovery, lasso peptides went from peculiarities to be recognized as a major family of ribosomally synthesized and post-translationally modified peptide (RiPP) natural products that were shown to be spread throughout the bacterial kingdom. Microcin J25 was first described in 1992, making it one of the earliest known lasso peptides. No other lasso peptide has since then been studied to such an extent as microcin J25, yet, previous review articles merely skimmed over all the research done on this exceptional lasso peptide. Therefore, to commemorate the 30th anniversary of its first report, we give a comprehensive overview of all literature related to microcin J25. This review article spans the early work towards the discovery of microcin J25, its biosynthetic gene cluster, and the elucidation of its three-dimensional, threaded lasso structure. Furthermore, the current knowledge about the biosynthesis of microcin J25 and lasso peptides in general is summarized and a detailed overview is given on the biological activities associated with microcin J25, including means of self-immunity, uptake into target bacteria, inhibition of the Gram-negative RNA polymerase, and the effects of microcin J25 on mitochondria. The in vitro and in vivo models used to study the potential utility of microcin J25 in a (veterinary) medicine context are discussed and the efforts that went into employing the microcin J25 scaffold in bioengineering contexts are summed up.

A Bio-Inspired Free-Standing Film with Versatility: From Heterogeneous CTCs Programed Isolating to Breast Cancer Molecular Typing

Developing a robust strategy for profiling heterogeneous circular tumor cells specifically, distinguishing the phenotypes of which in blood sample of cancer patient precisely, and releasing them sequentially, is significant for cancer management by liquid biopsy. Herein, a bio-inspired free-standing and flexible film composed of TiO2 nanotube and silk fibroin, fabricated with multiply dynamic bioactive surface (TSF/MDBS) by a simple and eco-friendly way including using polydopamine chemistry and dual dynamic covalent chemistry, is reported. The as-prepared TSF/MDBS binds specific peptides toward cells with epithelial biomarker and human epithelial growth factor receptor 2 (HER2) biomarker, and antifouling agents bovine serum albumin for obviating platelets and proteins adhering of blood, can capture heterogeneous CTCs with enhanced capability due to the cytocompatible soft film and exquisite surface design, and further release the captured cells as program, by specifically breaking down the covalent bonds in sequence via the action of adding biocompatible molecules fructose and glutathione. By applying the TSF/MDBS, it can be tailored into desired pieces for identifying CTCs with different phenotypes (HER2-high and HER2-low) from the unprocessed blood samples of breast cancer patients, and finally profiling these heterogeneous CTCs, to discriminate HER2 positive or negative of breast cancer patients in clinical applications.

Role of Functionalized Peptides in Nanomedicine for Effective Cancer Therapy

Peptide-functionalized nanomedicine, which addresses the challenges of specificity and efficacy in drug delivery, is emerging as a pivotal approach for cancer therapy. Globally, cancer remains a leading cause of mortality, and conventional treatments, such as chemotherapy, often lack precision and cause adverse effects. The integration of peptides into nanomedicine offers a promising solution for enhancing the targeting and delivery of therapeutic agents. This review focuses on the three primary applications of peptides: cancer cell-targeting ligands, building blocks for self-assembling nanostructures, and elements of stimuli-responsive systems. Nanoparticles modified with peptides improved targeting of cancer cells, minimized damage to healthy tissues, and optimized drug delivery. The versatility of self-assembled peptide structures makes them an innovative vehicle for drug delivery by leveraging their biocompatibility and diverse nanoarchitectures. In particular, the mechanism of cell death induced by self-assembled structures offers a novel approach to cancer therapy. In addition, peptides in stimuli-responsive systems enable precise drug release in response to specific conditions in the tumor microenvironment. The use of peptides in nanomedicine not only augments the efficacy and safety of cancer treatments but also suggests new research directions. In this review, we introduce systems and functionalization methods using peptides or peptide-modified nanoparticles to overcome challenges in the treatment of specific cancers, including breast cancer, lung cancer, colon cancer, prostate cancer, pancreatic cancer, liver cancer, skin cancer, glioma, osteosarcoma, and cervical cancer.

Peptides Targeting HER2-Positive Breast Cancer Cells and Applications in Tumor Imaging and Delivery of Chemotherapeutics

Breast cancer represents the most common cancer type and one of the major leading causes of death in the female worldwide population. Overexpression of HER2, a transmembrane glycoprotein related to the epidermal growth factor receptor, results in a biologically and clinically aggressive breast cancer subtype. It is also the primary driver for tumor detection and progression and, in addition to being an important prognostic factor in women diagnosed with breast cancer, HER2 is a widely known therapeutic target for drug development. The aim of this review is to provide an updated overview of the main approaches for the diagnosis and treatment of HER2-positive breast cancer proposed in the literature over the past decade. We focused on the different targeting strategies involving antibodies and peptides that have been explored with their relative outcomes and current limitations that need to be improved. The review also encompasses a discussion on targeted peptides acting as probes for molecular imaging. By using different types of HER2-targeting strategies, nanotechnology promises to overcome some of the current clinical challenges by developing novel HER2-guided nanosystems suitable as powerful tools in breast cancer imaging, targeting, and therapy.

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.

Multilayer gold nanoparticles as non-viral vectors for targeting MCF-7 cancer cells

Cargocomplexes play a vital role in non-viral delivery methods due to their capacity to target certain cells (or cells through the cell-division cycle) and inject their (macro)molecular "cargo" into them. The development of gene carriers that can efficiently transport and deliver genetic material into human-targeted cells with minimal toxicity is an important challenge in the field. The present study reports the straightforward preparation and testing of a modular non-viral gene carrier based on AuNPs. The design, synthesis, and in vitro evaluation of multilayer gold nanoparticles (AuNPs) as non-viral gene carriers with high transfection efficiency, reduced cytotoxicity for targeted therapeutic delivery of nucleic acids to MCF-7 cancer cells are presented. The developed non-viral vector is based on supramolecular "host-guest" inclusion complexes of β-cyclodextrin, positioned on the AuNPs surface over a layer of polyethyleneimine, and adamantyl moiety from polyethylene glycol conjugated decapeptide (WXEAAYQRFL). First, the β-CD functionalized PEI was utilized as the template for the synthesis of AuNPs of controlled sizes. The reaction produced small AuNPs with a cationic layer which is known for efficient condensation of genetic material and β-CD suitable for the decoration of the carrier with targeting moieties using "host-guest" inclusion complexation. Subsequently, adamantine-polyethylene glycol conjugated decapeptide was attached to the AuNPs. The in vitro results have validated the ability of the proposed systems to selectively target tumor cells with high efficacy and low toxicity due to the unique affinity of the aptamer-functionalized nanoparticles toward breast cancer cells. The findings of this work demonstrated that the proposed modular system may represent a very promising platform for the AuNP-based non-viral vectors mainly due to the versatility of the system, which allows for the facile exchange of several types of ligands for improving the targeting properties and transfection efficiency, or for providing better protection from the endocytotic systems.

2D MOF Nanosensor-Integrated Digital Droplet Microfluidic Flow Cytometry for In Situ Detection of Multiple miRNAs in Single CTC Cells

Current circulating tumor cells (CTCs) detection strategies based on surface epithelial markers suffer from low specificity in distinguishing between CTCs and epithelial cells in hematopoietic cell population. Tumor-associated miRNAs within CTCs are emerging as new biomarkers due to their high correlation with tumor development and progress. However, in-situ simultaneous analysis of multiple miRNAs in single CTC cell is still challenging. To overcome this limitation, a digital droplet microfluidic flow cytometry based on biofunctionalized 2D metal-organic framework nanosensor (Nano-DMFC) is developed for in situ detection of dual miRNAs simultaneously in single living breast cancer cells. Here, 2D MOF-based fluorescent resonance energy transfer (FRET) nanosensors are established by conjugating dual-color fluorescence dye-labeled DNA probes on MOF nanosheet surface. In the Nano-DMFC, 2D MOF-based nanoprobes are precisely microinjected into each single-cell encapsulated droplets to achieve dual miRNA characterization in single cancer cell. This Nano-DMFC platform successfully detects dual miRNAs at single-cell resolution in 10 mixed positive MCF-7 cells out of 10 000 negative epithelial cells in serum biomimic samples. Moreover, this Nano-DMFC platform shows good reproductivity in the recovery experiment of spiked blood samples, which demonstrate the high potential for CTC-based cancer early diagnosis and prognosis.

Fluorescence Analysis of Circulating Exosomes for Breast Cancer Diagnosis Using a Sensor Array and Deep Learning

Emerging liquid biopsy methods for investigating biomarkers in bodily fluids such as blood, saliva, or urine can be used to perform noninvasive cancer detection. However, the complexity and heterogeneity of exosomes require improved methods to achieve the desired sensitivity and accuracy. Herein, we report our study on developing a breast cancer liquid biopsy system, including a fluorescence sensor array and deep learning (DL) tool AggMapNet. In particular, we used a 12-unit sensor array composed of conjugated polyelectrolytes, fluorophore-labeled peptides, and monosaccharides or glycans to collect fluorescence signals from cells and exosomes. Linear discriminant analysis (LDA) processed the fluorescence spectral data of cells and cell-derived exosomes, demonstrating successful discrimination between normal and different cancerous cells and 100% accurate classification of different BC cells. For heterogeneous plasma-derived exosome analysis, CNN-based DL tool AggMapNet was applied to transform the unordered fluorescence spectra into feature maps (Fmaps), which gave a straightforward visual demonstration of the difference between healthy donors and BC patients with 100% prediction accuracy. Our work indicates that our fluorescent sensor array and DL model can be used as a promising noninvasive method for BC diagnosis.

Chiral separation of dipeptides on Cinchona-based zwitterionic chiral stationary phases under buffer-free reversed-phase conditions

Chiral zwitterion ion exchangers represent efficient chiral stationary phases for stereoselective resolution of various analytes including chiral acids, bases, and zwitterions. In this contribution, we have focused on utilization of chiral zwitterionic sorbents, denoted as ZWIX (+A) and ZWIX (-A). These are analogical chiral systems to commercially available columns, Chiralpak ZWIX (+) and Chiralpak ZWIX (-), which are usually operated with buffered mobile phases. In this contribution, we have studied the enantiorecognition power of the ZWIX (+A) and ZWIX (-A) columns on a series of dipeptides operated under buffer-free reversed-phase conditions. Retention characteristics of zwitterionic dipeptides are discussed using an electrostatically driven adsorption model, which provides a good fit with both monotonous and U-shaped curves.

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.

Synthesis and Evaluation of High Functionality and Quality Cell-penetrating Peptide Conjugated Lipid for Octaarginine Modified PEGylated Liposomes In U251 and U87 Glioma Cells

The use of peptide ligand modified PEGylated liposomes has been widely investigated for tumor targeting. Peptides are mainly inserted in the liposomal lipid bilayer using PEG_2K-lipid spacer (Peptide-PEG_2K-DSPE). However, a lower cellular uptake from longer nonlinear PEG_2K spacer was reported, we here synthesized a high functionality and quality (HFQ) lipid with a short, linear serine-glycine repeated peptide [(SG)₅] spacer. The objective of the current study is to develop novel octaarginine (R₈) peptide-HFQ lipid grafted PEGylated liposomes for glioma cells targeting. In vitro liposomes characterization showed that the mean particle size of all liposomal formulations was in the nano-scale range < 120 nm, with a small PDI value (i.e. ∼0.2) and had a spherical shape under Transmission Electron Microscope, indicating a homogenous particle size distribution. The flow cytometry in vitro cellular association data with U251 MG and U87 cells revealed that 1.5% R₈-(SG)₅-lipid-PEGylated liposomes exhibited significantly higher cellular association of ∼15.87 and 7.59-fold than the conventional R₈-PEG_2K-lipid-PEGylated liposomes (10.4 and 6.19-fold), respectively, relative to the unmodified PEGylated liposomes. Moreover, intracellular distribution studies using confocal laser scanning microscopy (CLSM) corroborated the results of the in vitro cell association. The use of ligand-HFQ-lipid liposomes could be a potential alternative to ligand-PEG_2K-lipid-modified liposomes as a drug delivery system for tumor targeting.

Modular Synthesis of Peptide-Based Single- and Multimodal Targeted Molecular Imaging Agents

A practical, modular synthesis of targeted molecular imaging agents (TMIAs) containing near-infrared dyes for optical molecular imaging (OMI) or chelated metals for magnetic resonance imaging (MRI) and single-photon emission correlation tomography (SPECT) or positron emission tomography (PET) has been developed. In the method, imaging modules are formed early in the synthesis by attaching imaging agents to the side chain of protected lysines. These modules may be assembled to provide a given set of single- or dual-modal imaging agents, which may be conjugated in the last steps of the synthesis under mild conditions to linkers and targeting groups. A key discovery was the ability of a metal such as gadolinium, useful in MRI, to serve as a protecting group for the chelator, 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA). It was further discovered that two lanthanide metals, La and Ce, can double as protecting groups and placeholder metals, which may be transmetalated under mild conditions by metals used for PET in the final step. The modular method enabled the synthesis of discrete targeted probes with two of the same or different dyes, two same or different metals, or mixtures of dyes and metals. The approach was exemplified by the synthesis of single- or dual-modal imaging modules for MRI-OMI, PET-OMI, and PET-MRI, followed by conjugation to the integrin-seeking peptide, c(RGDyK). For Gd modules, their efficacy for MRI was verified by measuring the NMR spin-lattice relaxivity. To validate functional imaging of TMIAs, dual-modal agents containing Cy5.5 were shown to target A549 cancer cells by confocal fluorescence microscopy.

Engineering and characterization of human β-defensin-3 and its analogues and microcin J25 peptides against Mannheimia haemolytica and bovine neutrophils

Mannheimia haemolytica-induced bovine respiratory disease causes loss of millions of dollars to Canadian cattle industry. Current antimicrobials are proving to be ineffective and leave residues in meat. Antimicrobial peptides (AMPs) may be effective against M. haemolytica while minimizing the risk of drug residues. Cationic AMPs can kill bacteria through interactions with the anionic bacterial membrane. Human β-Defensin 3 (HBD3) and microcin J25 (MccJ25) are AMPs with potent activity against many Gram-negative bacteria. We tested the microbicidal activity of wild-type HBD3, three HBD3 peptide analogues (28 amino acid, 20AA, and 10AA) derived from the sequence of natural HBD3, and MccJ25 in vitro against M. haemolytica. Three C-terminal analogues of HBD3 with all cysteines replaced with valines were manually synthesized using solid phase peptide synthesis. Since AMPs can act as chemoattractant we tested the chemotactic effect of HBD3, 28AA, 20AA, and 10AA peptides on bovine neutrophils in Boyden chamber. Minimum bactericidal concentration (MBC) assay showed that M. haemolytica was intermediately sensitive to HBD3, 28AA and 20AA analogues with an MBC of 50 µg/mL. The 10AA analogue had MBC 6.3 µg/mL which is likely a result of lower final inoculum size. MccJ25 didn't have significant bactericidal effect below an MBC < 100 µg/mL. Bovine neutrophils showed chemotaxis towards HBD3 and 20AA peptides (P < 0.05) but not towards 28AA analogue. Co-incubation of neutrophils with any of the peptides did not affect their chemotaxis towards N-formyl-L-methionyl-L-leucyl-phenylalanine (fMLP). The data show that these peptides are effective against M. haemolytica and are chemotactic for neutrophils in vitro.

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.

Design and evaluation of peptide-18-targeted nanoliposomes constructed by poly(2-oxazoline)-DOPE for doxorubicin delivery

AIMS

The aim of this study is to develop targeted nanoliposome formulations to provide efficient treatment for breast cancer. In this study, peptide 18-modified poly(2-ethyl-2-oxazoline)-dioleoylphosphatidylethanolamine (P18-PEtOx-DOPE), was synthesised to construct nanoliposomes.

METHODS

Doxorubicin (DOX) was encapsulated into the nanoliposomes by ethanol injection method. Particle size and polydispersity index were measured by dynamic light scattering. Zeta potential was determined by electrophoretic laser Doppler anemometry. The shape of the nanoliposomes was examined by transmission electron microscope. Specific bindings of P18-PEtOx-DOPE nanoliposomes were demonstrated on AU565 cells by confocal microscopy and flow cytometry studies.

RESULTS

DOX-loaded nanoliposomes with particle diameter of 150.00 ± 2.84 nm and PDI of 0.212 ± 0.013 were obtained. PEtOx-DOPE and PEtOx-DOPE nanoliposomes are non-toxic on HUVEC, HEK293 and hMSC cells for 48 h. Furthermore, P18-PEtOx-DOPE nanoliposomes demonstrated specificity towards AU565 cells with high binding affinity.

CONCLUSIONS

As a result, DOX-loaded P18-PEtOx-DOPE nanoliposomes can serve as favourable candidates in breast cancer targeted therapy.

Peptide 18-4/chlorin e6-conjugated polyhedral oligomeric silsesquioxane nanoparticles for targeted photodynamic therapy of breast cancer

Chlorin e6 (Ce6), with its high phototoxic potential, has wide applications in photodynamic therapy (PDT) for many human diseases. However, poor cancer cell localization of Ce6 has limited its direct application for PDT. Here, we developed cancer-targeting peptide p 18-4/chlorin e6 (Ce6)-conjugated polyhedral oligomeric silsesquioxane (PPC) nanoparticles for improving the targeting ability of Ce6 to breast cancer cells, thereby enhancing PDT efficacy. The synthesized PPC nanoparticles exhibited a spherical shape with an average diameter of 127.2 ± 11.3 nm in aqueous solution. Compared with free Ce6, the immobilization of p 18-4 enhanced the in vitro cellular uptake and targeting ability of PPC nanoparticles in breast cancer cell line MDA-MB-231. In addition, the intracellular uptake of PPC nanoparticles in MDA-MB-231 cells was dramatically increased compared with other cancer cells, indicating an obvious targeting ability of PPC nanoparticles on breast cancer cells. Upon light irradiation, PPC nanoparticles revealed significantly improved phototoxicity to MDA-MB-231 cells, mainly due to apoptotic cell death. In vivo PDT study suggested that PPC nanoparticles exhibited increased retention in tumor tissues and effectively inhibited the growth of MDA-MB-231 tumors in a target-specific manner. Overall, these results indicate that PPC nanoparticles are highly effective PDT agents for breast cancer therapy.

Evolving a Peptide: Library Platforms and Diversification Strategies

Peptides are widely used in pharmaceutical industry as active pharmaceutical ingredients, versatile tools in drug discovery, and for drug delivery. They find themselves at the crossroads of small molecules and proteins, possessing favorable tissue penetration and the capability to engage into specific and high-affinity interactions with endogenous receptors. One of the commonly employed approaches in peptide discovery and design is to screen combinatorial libraries, comprising a myriad of peptide variants of either chemical or biological origin. In this review, we focus mainly on recombinant peptide libraries, discussing different platforms for their display or expression, and various diversification strategies for library design. We take a look at well-established technologies as well as new developments and future directions.

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.

Modulation of Hypoxia-Induced Chemoresistance to Polymeric Micellar Cisplatin: The Effect of Ligand Modification of Micellar Carrier Versus Inhibition of the Mediators of Drug Resistance

Hypoxia can induce chemoresistance, which is a significant clinical obstacle in cancer therapy. Here, we assessed development of hypoxia-induced chemoresistance (HICR) against free versus polymeric cisplatin micelles in a triple negative breast cancer cell line, MDA-MB-231. We then explored two strategies for the modulation of HICR against cisplatin micelles: a) the development of actively targeted micelles; and b) combination therapy with modulators of HICR in MDA-MB-231 cells. Actively targeted cisplatin micelles were prepared through surface modification of acetal-poly(ethylene oxide)-poly(α-carboxyl-ε-caprolactone) (acetal-PEO-PCCL) micelles with epidermal growth factor receptor (EGFR)-targeting peptide, GE11 (YHWYGYTPQNVI). Our results showed that hypoxia induced resistance against free and cisplatin micelles in MDA-MB-231 cells. A significant increase in micellar cisplatin uptake was observed in MDA-MB-231 cells that overexpress EGFR, following surface modification of micelles with GE11. This did not lead to increased cytotoxicity of micellar cisplatin, however. On the other hand, the addition of pharmacological inhibitors of key molecules involved in HICR in MDA-MB-231 cells, i.e., inhibitors of hypoxia inducing factor-1 (HIF-1) and signal transducer and activator of transcription 3 (STAT3), substantially enhanced the cytotoxicity of free and cisplatin micelles. The results indicated the potential benefit of combination therapy with HIF-1 and STAT3 inhibitors in overcoming HICR to free or micellar cisplatin.

Synthesis of breast cancer targeting conjugate of temporin-SHa analog and its effect on pro- and anti-apoptotic protein expression in MCF-7 cells

The frog natural product temporin-SHa (FLSGIVGMLGKLF_amide) is a potent antimicrobial peptide, as is the analog [S3K]SHa. By solid-phase synthesis, we prepared temporin-SHa and several temporin-SHa analogs with one or more D-alanine residues incorporated. The natural product and the analog [G10a]SHa were found to be cytotoxic in mammalian cell lines and induce cell death. To achieve selectivity, we conjugated the analog [G10a]SHa with a breast cancer targeting peptide (BCTP). The resulting peptide temporin [G10a]SHa-BCTP conjugate was selectively active against the MCF-7 breast cancer cell line with no cytotoxicity in NIH-3T3 fibroblasts. Unlike the natural product or [G10a]SHa, the conjugated peptide induced apoptosis, downregulating the expression of Bcl-2 and survivin and upregulating Bax and caspase-3.

Traceable PEO-poly(ester) micelles for breast cancer targeting: The effect of core structure and targeting peptide on micellar tumor accumulation

Traceable poly(ethylene oxide)-poly(ester) micelles were developed through chemical conjugation of a near-infrared (NIR) dye to the poly(ester) end by click chemistry. This strategy was tried for micelles with poly(ε-caprolactone) (PCL) or poly(α-benzyl carboxylate-ε-caprolactone) (PBCL) cores. The surface of both micelles was also modified with the breast cancer targeting peptide, P18-4. The results showed the positive contribution of PBCL over PCL core on micellar thermodynamic and kinetic stability as well as accumulation in primary orthotopic MDA-MB-231 tumors within 4-96 h following intravenous administration in mice. This was in contrast to in vitro studies where better uptake of PEO-PCL versus PEO-PBCL micelles by MDA-MB-231 cells was observed. The presence of P18-4 enhanced the in vitro cell uptake and homing of both polymeric micelles in breast tumors, but only at early time points. In conclusion, the use of developed NIR labeling technique provided means for following the fate of PEO-poly(ester) based nano-carriers in live animals. Our results showed micellar stabilization through the use of PBCL over PCL cores, to have a more significant effect in enhancing the level and duration of nano-carrier accumulation in primary breast tumors than the modification of polymeric micellar surface with breast tumor targeting peptide, P18-4.

Identification of GXXXXG motif in Chrysophsin-1 and its implication in the design of analogs with cell-selective antimicrobial and anti-endotoxin activities

Marine fish antimicrobial peptide, chrysophsin-1 possesses versatile biological activities but its non-selective nature restricts its therapeutic possibilities. Often small alterations in structural motifs result in significant changes in the properties of concerned proteins/peptides. We have identified GXXXXG motif in chrysophsin-1. Glycine residue(s) of this motif in Chrysophsin-1 was/were replaced with alanine, valine and proline residue(s). Of these, proline-substituted Chrysophsin-1 analogs exhibited significantly reduced cytotoxicity towards mammalian cells. Further, these analogs showed broad-spectrum activity against Gram-positive, Gram-negative bacteria, Methicillin-resistant Staphylococcus aureus strains and fungi and also retained antibacterial activity in presence of physiological salts, serum and at elevated temperatures indicative of their therapeutic potential. These Chrysophsin-1 analogs also inhibited lipopolysaccharide (LPS) induced pro-inflammatory responses in THP-1 cells and in murine primary macrophages. One of these single proline-substituted Chrysophsin-1 analogs inhibited LPS-stimulated pro-inflammatory cytokine production in BALB/c mice and elicited appreciable survival of mice administered with a lethal dose of LPS in a model of severe sepsis. The data for the first time showed the implication of GXXXXG motifs in functional and biological properties of an antimicrobial peptide and could be useful to design novel anti-microbial and anti-endotoxin peptides by employing this motif.

Proteolytically Stable Cyclic Decapeptide for Breast Cancer Cell Targeting

Starting with a previously reported linear breast cancer targeting decapeptide WxEAAYQkFL, here we report the synthesis of a novel cyclic peptide analogue cyclic WXEAAYQkFL. The N- to C-terminus amide cyclized peptide with one d-amino acid (k) displayed higher uptake by breast cancer cells, with minimal uptake by the noncancerous cells compared to the linear peptide with two d-amino acids (x and k), and was stable toward proteolytic degradation. When immobilized on gold microcantilever surface, the cyclic peptide was able to capture breast cancer cells specifically and sense samples with ≥25 cancer cells/mL. Animal studies using mice carrying orthotopic breast MDA-MB-231 tumors showed that the cyclic peptide preferentially accumulates in tumor (2 h after injection) and is rapidly cleared from all other organs except kidneys and liver. The study highlights the discovery of a novel proteolytically stable cyclic peptide that can be used for targeted drug delivery or for enumerating circulating breast tumor cells.

Breast Cancer Targeting Peptide Binds Keratin 1: A New Molecular Marker for Targeted Drug Delivery to Breast Cancer

The biomarkers or receptors expressed on cancer cells and the targeting ligands with high binding affinity for biomarkers play a key role in early detection and treatment of breast cancer. The breast cancer targeting peptide p160 (12-mer) and its enzymatically stable analogue 18-4 (10-mer) showed marked potential for breast cancer drug delivery using cell studies and animal models. Herein, we used affinity purification, liquid chromatography-tandem mass spectrometry, and proteomics to identify keratin 1 (KRT1) as the target receptor highly expressed on breast cancer cells for p160 peptide(s). Western blot and immunocytochemistry in MCF-7 breast cancer cells confirmed the identity of KRT1. We demonstrate that the p160 or 18-4 binding to MCF-7 breast cancer cells is dependent on the expression of KRT1, and we confirm peptide-KRT1 binding specificity using SPR experiments (K_d ∼ 1.1 μM and 0.98 μM for p160 and 18-4, respectively). Furthermore, we assessed the ability of peptide 18-4 to improve the cellular uptake and anticancer activity of a pro-apoptotic antimicrobial peptide, microcin J25 (MccJ25), in breast cancer cells. A covalent conjugate of peptide 18-4 with MccJ25 showed preferential cytotoxicity toward breast cancer cells with minimal cytotoxicity against normal HUVEC cells. The conjugate inhibited the growth of MDA-MB-435 MDR multidrug-resistant cells with an IC₅₀ comparable to that of nonresistant cells. Conjugation improved selective cellular uptake of MccJ25, and the conjugate triggered cancer cell death by apoptosis. Our findings establish KRT1 as a new marker for breast cancer targeting. Additionally, it pinpoints the potential use of antimicrobial lasso peptides as a novel class of anticancer therapeutics.

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.

Aptamers: Promising Tools for the Detection of Circulating Tumor Cells

Circulating tumor cells (CTCs) are cells that shed from a primary tumor and freely circulate in the blood, retaining the ability to initiate metastasis and form a secondary tumor in distant organs in the body. CTCs reflect the molecular profile of the primary tumor, therefore studying CTCs can allow for an understanding of the mechanism of metastasis, and an opportunity to monitor the prognosis of cancer. Unfortunately, the detection of CTCs is a considerable challenge due to their low abundance in the bloodstream and the lack of consistent markers present to recognize these cells. The aim of this review is to summarize some of the aptamer-based affinity methods for the detection of CTCs. The basic biological concept of how metastasis occurs and the role of CTCs in this process are presented. Some methods of CTC detection employing antibodies or peptides are mentioned here for comparison. The review of present literature suggests that aptamers are emerging as competitive technology in the detection of CTCs, especially due to their unique properties, but there still remain several challenges to be met, including the need to improve the throughput and sensitivity of such methods.

Real-time Detection of Breast Cancer Cells Using Peptide-functionalized Microcantilever Arrays

Ligand-directed targeting and capturing of cancer cells is a new approach for detecting circulating tumor cells (CTCs). Ligands such as antibodies have been successfully used for capturing cancer cells and an antibody based system (CellSearch(®)) is currently used clinically to enumerate CTCs. Here we report the use of a peptide moiety in conjunction with a microcantilever array system to selectively detect CTCs resulting from cancer, specifically breast cancer. A sensing microcantilever, functionalized with a breast cancer specific peptide 18-4 (WxEAAYQrFL), showed significant deflection on cancer cell (MCF7 and MDA-MB-231) binding compared to when exposed to noncancerous (MCF10A and HUVEC) cells. The peptide-functionalized microcantilever allowed efficient capture and detection of cancer cells in MCF7 spiked human blood samples emulating CTCs in human blood. A detection limit of 50-100 cancer cells mL(-1) from blood samples was achieved with a capture yield of 80% from spiked whole blood samples. The results emphasize the potential of peptide 18-4 as a novel peptide for capturing and detecting cancer cells in conjunction with nanomechanical cantilever platform. The reported peptide-based cantilever platform represents a new analytical approach that can lead to an alternative to the various detection platforms and can be leveraged to further study CTCs.

Synthesis, characterization, and biological activity of poly(arginine)-derived cancer-targeting peptides in HepG2 liver cancer cells

The solid-phase synthesis, structural characterization, and biological evaluation of a small library of cancer-targeting peptides have been determined in HepG2 hepatoblastoma cells. These peptides are based on the highly specific Pep42 motif, which has been shown to target the glucose-regulated protein 78 receptors overexpressed and exclusively localized on the cell surface of tumors. In this study, Pep42 was designed to contain varying lengths (3-12) of poly(arginine) sequences to assess their influence on peptide structure and biology. Peptides were effectively synthesized by 9-fluorenylmethoxycarbonyl-based solid-phase peptide synthesis, in which the use of a poly(ethylene glycol) resin provided good yields (14-46%) and crude purities >95% as analyzed by liquid chromatography-mass spectrometry. Peptide structure and biophysical properties were investigated using circular dichroism spectroscopy. Interestingly, peptides displayed secondary structures that were contingent on solvent and length of the poly(arginine) sequences. Peptides exhibited helical and turn conformations, while retaining significant thermal stability. Structure-activity relationship studies conducted by flow cytometry and confocal microscopy revealed that the poly(arginine) derived Pep42 sequences maintained glucose-regulated protein 78 binding on HepG2 cells while exhibiting cell translocation activity that was contingent on the length of the poly(arginine) strand. In single dose (0.15 mM) and dose-response (0-1.5 mM) cell viability assays, peptides were found to be nontoxic in human HepG2 liver cancer cells, illustrating their potential as safe cancer-targeting delivery agents.

Structural similarity between β(3)-peptides synthesized from β(3)-homo-amino acids and aspartic acid monomers

Formation of stable secondary structures by oligomers that mimic natural peptides is a key asset for enhanced biological response. Here we show that oligomeric β(3)-hexapeptides synthesized from L-aspartic acid monomers (β(3)-peptides 1, 5a, and 6) or homologated β(3)-amino acids (β(3)-peptide 2), fold into similar stable 14-helical secondary structures in solution, except that the former form right-handed 14-helix and the later form left-handed 14-helix. β(3)-Peptides from L-Asp monomers contain an additional amide bond in the side chains that provides opportunities for more hydrogen bonding. However, based on the NMR solution structures, we found that β(3)-peptide from L-Asp monomers (1) and from homologated amino acids (2) form similar structures with no additional side-chain interactions. These results suggest that the β(3)-peptides derived from L-Asp are promising peptide-mimetics that can be readily synthesized using L-Asp monomers as well as the right-handed 14-helical conformation of these β(3)-peptides (such as 1 and 6) may prove beneficial in the design of mimics for right-handed α-helix of α-peptides.

Novel peptide-doxorubucin conjugates for targeting breast cancer cells including the multidrug resistant cells

The efficacy of chemotherapeutic doxorubucin (Dox) in cancer treatment is limited by two main factors, nonspecific toxicity and the emergence of tumor resistance. To overcome these hurdles, in this study peptide-Dox conjugates were prepared. A decapeptide 18-4a (NH₂-WxEAAYQkFL-CONH₂) [corrected] with high specificity for breast cancer cells and improved proteolytic stability was conjugated to Dox to give peptide-Dox ester (1) and amide (2) conjugates. Cell uptake studies showed that the conjugates were 6-10 times selective for breast cancerous cells (MCF-7 and MDA-MB-435) over noncancerous cells (HUVECs and MCF-10A). Conjugate 1 displayed similar toxicity as free Dox toward the breast cancerous cells and was about 40 times less toxic toward the noncancerous cells and 4-fold more toxic toward the Dox resistant MDA-MB-435-MDR cells than the free Dox. These data suggest that conjugate 1 can be used as a potential prodrug for improving the therapeutic index of Dox and potentially many other cytotoxic drugs.

Peptide modified polymeric micelles specific for breast cancer cells

The specific targeting ability of novel breast cancer targeting peptides as ligands coupled to polymeric micelles was evaluated in the present study. In this context, engineered breast cancer cell targeting peptides, denoted as peptide 11 (RGDPAYQGRFL) and peptide 18 (WXEAAYQRFL), were compared with the lead 12-mer p160 peptide and cyclic RGDfK peptide. All four peptides were conjugated individually to poly(ethylene oxide)-b-poly(caprolactone) (PEO-b-PCL) diblock polymeric micelles to obtain targeted carrier systems PM11, PM18, PM 160, and PM c-RGD. Physical blending of the peptides 11 and 18 with PEO-b-PCL was also done to yield combination micelles, comPM11 and comPM18. The structural confirmation of polymer was carried out using (1)H NMR and MALDI-TOF, and the size distribution and zeta potential of the micelles were determined using dynamic light scattering. Lipophilic cyanine fluorescent probe DiI was physically incorporated in the polymeric micelles to imitate the hydrophobic drug loaded in the micellar core. The cellular uptake of DiI-loaded peptide-containing polymeric micelles by MDA-MB-435, MDA-MB-231, and MCF7 breast cancer cell lines, as well as HUVEC and MCF10A noncancerous cells, were analyzed using flow cytometry and confocal microscopy techniques. Modification of polymeric micelles with peptide 11 or 18 led to an increase in micellar uptake specifically in breast cancer cells compared to p160, c-RGD modified, or naked micelles. The peptide-micelle combinations (comPM11 and comPM18) displayed better uptake by the cells compared to the covalently conjugated PM11 and PM18 micelles; however, the combinations were less selective toward cancer cells. The results point to a potential for peptides 11- and 18-micelle conjugates as attractive platforms for improved performance of a wide range of chemotherapeutic drugs and/or imaging agents in cancer therapy and diagnosis.