Affinity Maturation of an ERBB2-Targeted SPECT Imaging Peptide by In Vivo Phage Display

Theranostic applications of peptide-based nanoformulations for growth factor defective cancers

Growth factors play a pivotal role in orchestrating cellular growth and division by binding to specific cell surface receptors. Dysregulation of growth factor production or activity can contribute to the uncontrolled cell proliferation observed in cancer. Peptide-based nanoformulations (PNFs) have emerged as promising therapeutic strategies for growth factor-deficient cancers. PNFs offer multifaceted capabilities including targeted delivery, imaging modalities, combination therapies, resistance modulation, and personalized medicine approaches. Nevertheless, several challenges remain, including limited specificity, stability, pharmacokinetics, tissue penetration, toxicity, and immunogenicity. To address these challenges and optimize PNFs for clinical translation, in-depth investigations are warranted. Future research should focus on elucidating the intricate interplay between peptides and nanoparticles, developing robust spectroscopic and computational methodologies, and establishing a comprehensive understanding of the structure-activity relationship governing peptide-nanoparticle interactions. Bridging these knowledge gaps will propel the translation of peptide-nanoparticle therapies from bench to bedside. While a few peptide-nanoparticle drugs have obtained FDA approval for cancer treatment, the integration of nanostructured platforms with peptide-based medications holds tremendous potential to expedite the implementation of innovative anticancer interventions. Therefore, growth factor-deficient cancers present both challenges and opportunities for targeted therapeutic interventions, with peptide-based nanoformulations positioned as a promising avenue. Nonetheless, concerted research and development endeavors are essential to optimize the specificity, stability, and safety profiles of PNFs, thereby advancing the field of peptide-based nanotherapeutics in the realm of oncology research.

Evaluation of [89Zr]Zr-DFO-2Rs15d Nanobody for Imaging of HER2-Positive Breast Cancer

One of the most aggressive forms of breast cancer involves the overexpression of human epidermal growth factor receptor 2 (HER2). HER2 is overexpressed in ∼25% of all breast cancers and is associated with increased proliferation, increased rates of metastasis, and poor prognosis. Treatment for HER2-positive breast cancer has vastly improved since the development of the monoclonal antibody trastuzumab (Herceptin) as well as other biological constructs. However, patients still commonly develop resistance, illustrating the need for newer therapies. Nanobodies have become an important focus for potential development as HER2-targeting imaging agents and therapeutics. Nanobodies have many favorable characteristics, including high stability in heat and nonphysiological pH, while maintaining their low-nanomolar affinity for their designed targets. Specifically, the 2Rs15d nanobody has been developed for targeting HER2 and has been evaluated as a diagnostic imaging agent for single-photon emission computed tomography (SPECT) and positron emission tomography (PET). While a construct of 2Rs15d with the positron emitter 68Ga is currently in phase I clinical trials, the only PET images acquired in preclinical or clinical research have been within 3 h postinjection. We evaluated our in-house produced 2Rs15d nanobody, conjugated with the chelator deferoxamine (DFO), and radiolabeled with 89Zr for PET imaging up to 72 h postinjection. [89Zr]Zr-DFO-2Rs15d demonstrated high stability in both phosphate-buffered saline (PBS) and human serum. Cell binding studies showed high binding and specificity for HER2, as well as prominent internalization. Our in vivo PET imaging confirmed high-quality visualization of HER2-positive tumors up to 72 h postinjection, whereas HER2-negative tumors were not visualized. Subsequent biodistribution studies quantitatively supported the significant HER2-positive tumor uptake compared to the negative control. Our studies fill an important gap in understanding the imaging and binding properties of the 2Rs15d nanobody at extended time points. As many therapeutic radioisotopes have single or multiday half-lives, this information will directly benefit the potential of the radiotherapy development of 2Rs15d for HER2-positive breast cancer patients.

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.

Recent advances in peptide-based therapeutic strategies for breast cancer treatment

Breast cancer is the leading cause of cancer-related fatalities in female worldwide. Effective therapies with low side effects for breast cancer treatment and prevention are, accordingly, urgently required. Targeting anticancer materials, breast cancer vaccines and anticancer drugs have been studied for many years to decrease side effects, prevent breast cancer and suppress tumors, respectively. There are abundant evidences to demonstrate that peptide-based therapeutic strategies, coupling of good safety and adaptive functionalities are promising for breast cancer therapy. In recent years, peptide-based vectors have been paid attention in targeting breast cancer due to their specific binding to corresponding receptors overexpressed in cell. To overcome the low internalization, cell penetrating peptides (CPPs) could be selected to increase the penetration due to the electrostatic and hydrophobic interactions between CPPs and cell membranes. Peptide-based vaccines are at the forefront of medical development and presently, 13 types of main peptide vaccines for breast cancer are being studied on phase III, phase II, phase I/II and phase I clinical trials. In addition, peptide-based vaccines including delivery vectors and adjuvants have been implemented. Many peptides have recently been used in clinical treatments for breast cancer. These peptides show different anticancer mechanisms and some novel peptides could reverse the resistance of breast cancer to susceptibility. In this review, we will focus on current studies of peptide-based targeting vectors, CPPs, peptide-based vaccines and anticancer peptides for breast cancer therapy and prevention.

Evaluation of 68Ga-Radiolabeled Peptides for HER2 PET Imaging

One in eight women will be diagnosed with breast cancer in their lifetime and approximately 25% of those cases will be HER2-positive. Current methods for diagnosing HER2-positive breast cancer involve using IHC and FISH from suspected cancer biopsies to quantify HER2 expression. HER2 PET imaging could potentially increase accuracy and improve the diagnosis of lesions that are not available for biopsies. Using two previously discovered HER2-targeting peptides, we modified each peptide with the chelator DOTA and a PEG2 linker resulting in DOTA-PEG2-GSGKCCYSL (P5) and DOTA-PEG2-DTFPYLGWWNPNEYRY (P6). Each peptide was labeled with 68Ga and was evaluated for HER2 binding using in vitro cell studies and in vivo tumor xenograft models. Both [68Ga]P5 and [68Ga]P6 showed significant binding to HER2-positive BT474 cells versus HER2-negative MDA-MB-231 cells ([68Ga]P5; 0.68 ± 0.20 versus 0.47 ± 0.05 p < 0.05 and [68Ga]P6; 0.55 ± 0.21 versus 0.34 ± 0.12 p < 0.01). [68Ga]P5 showed a higher percent injected dose per gram (%ID/g) binding to HER2-positive tumors two hours post-injection compared to HER2-negative tumors (0.24 ± 0.04 versus 0.12 ± 0.06; p < 0.05), while the [68Ga]P6 peptide showed significant binding (0.98 ± 0.22 versus 0.51 ± 0.08; p < 0.05) one hour post-injection. These results lay the groundwork for the use of peptides to image HER2-positive breast cancer.

Review: Radionuclide Molecular Imaging Targeting HER2 in Breast Cancer with a Focus on Molecular Probes into Clinical Trials and Small Peptides

As the most frequently occurring cancer worldwide, breast cancer (BC) is the leading cause of cancer-related death in women. The overexpression of HER2 (human epidermal growth factor receptor 2) is found in about 15% of BC patients, and it is often associated with a poor prognosis due to the effect on cell proliferation, migration, invasion, and survival. As a result of the heterogeneity of BC, molecular imaging with HER2 probes can non-invasively, in real time, and quantitatively reflect the expression status of HER2 in tumors. This will provide a new approach for patients to choose treatment options and monitor treatment response. Furthermore, radionuclide molecular imaging has the potential of repetitive measurements, and it can help solve the problem of heterogeneous expression and conversion of HER2 status during disease progression or treatment. Different imaging probes of targeting proteins, such as monoclonal antibodies, antibody fragments, nanobodies, and affibodies, are currently in preclinical and clinical development. Moreover, in recent years, HER2-specific peptides have been widely developed for molecular imaging techniques for HER2-positive cancers. This article summarized different types of molecular probes targeting HER2 used in current clinical applications and the developmental trend of some HER2-specific peptides.

Peptide Based Imaging Agents for HER2 Imaging in Oncology

Breast cancer continues to be the most lethal cancer type in women and one of the most diagnosed. Understanding Breast cancer receptor status is one of the most vital processes for determining treatment options. One type of breast cancer, human epidermal growth factor receptor 2 (HER2) positive, has approved receptor-based therapies including trastuzumab and pertuzumab that can significantly increase the likelihood of survival. Current methods to determine HER2 status include biopsies with immunohistochemical staining and/or fluorescence in situ hybridization. However, positron emission tomography (PET) imaging techniques using ⁸⁹Zr-trastuzumab or ⁸⁹Zr-pertuzumab are currently in clinical trials for a non-invasive, full body diagnostic approach. Although the antibodies have strong specificity to the HER2 positive lesions, challenges involving long post-injection time for imaging due to the blood circulation of the antibodies and matching of long-live isotopes leading to increased dose to the patient leave opportunities for alternative PET imaging probes. Peptides have been shown to allow for shorter injection-to-imaging time and can be used with shorter lived isotopes. HER2 specific peptides under development will help improve the diagnosis and potentially therapy options for HER2 positive breast cancer. Peptides showing specificity for HER2 could start widespread development of molecular imaging techniques for HER2 positive cancers.

Radiolabeled Peptides for SPECT and PET Imaging in the Detection of Breast Cancer: Preclinical and Clinical Perspectives

Breast cancer is the most common cancer in women worldwide. Due to the heterogeneous nature of breast cancer, the optimal treatment and expected response for each patient may not necessarily be universal. Molecular imaging techniques could play an important role in the early detection and targeted therapy evaluation of breast cancer. This review focuses on the development of peptides labeled with SPECT and PET radionuclides for breast cancer imaging. We summarized the current status of radiolabeled peptides for different receptors in breast cancer. The characteristics of radionuclides and major techniques for peptide labeling are also briefly discussed.

The emerging role of radionuclide molecular imaging of HER2 expression in breast cancer

Targeting of human epidermal growth factor type 2 (HER2) using monoclonal antibodies, antibody-drug conjugates and tyrosine kinase inhibitors extends survival of patients with HER2-expressing metastatic breast cancer. High expression of HER2 is a predictive biomarker for such specific treatment. Accurate determination of HER2 expression level is necessary for stratification of patients to targeted therapy. Non-invasive in vivo radionuclide molecular imaging of HER2 has a potential of repetitive measurements, addressing issues of heterogeneous expression and conversion of HER2 status during disease progression or in response to therapy. Imaging probes based of several classes of targeting proteins are currently in preclinical and early clinical development. Both preclinical and clinical data suggest that the most promising are imaging agents based on small proteins, such as single domain antibodies or engineered scaffold proteins. These agents permit a very specific high-contrast imaging at the day of injection.

Application of Phage-Displayed Peptides in Tumor Imaging Diagnosis and Targeting Therapy

Phage display is an effective and powerful technique that provides a route to discovery unique peptides targeting to tumor cells. Specifically binding peptides are considered as the valuable target directing molecule fragments with potential efficiency to improve the current tumor clinic, and offer new approaches for tumor prevention, diagnosis and treatment. We focus on the recent advances in the isolation of tumor-targeting peptides by biopanning methods, with particular emphasis on molecular imaging, and pharmaceutical targeting therapy.

Phage Display Selection, Identification, and Characterization of Novel Pancreatic Cancer Targeting Peptides

Pancreatic cancer is characterized by a 5-year survival rate of 3%, in part due to inadequate detection methods. The small size of peptides offers advantages regarding molecular targeting. Thus, peptides may be used in detection of pancreatic cancer. Here, peptides that target pancreatic cancer cells were selected using phage display technology using a 15-mer fUSE5 library. Phage were pre-cleared against immortalized pancreatic cells (hTERT-HPNE), followed by selections against pancreatic cancer (Mia Paca-2) cells. Next-generation sequencing identified two peptides, MCA1 and MCA2, with a Log2 fold change (Mia Paca-2/ hTERT-HPNE) >1.5. Modified ELISA and fluorescent microscopy showed that both peptides bound significantly higher to Mia Paca-2 cells, and not to hTERT-HPNE, embryonic kidney (HEK 293), ovarian (SKOV-3) and prostate cancer (LNCaP) cell lines. Further characterization of MCA1 and MCA2 revealed EC₅₀ values of 16.11 µM (95% CI [9.69, 26.31 µM]) and 97.01 µM (95% CI [58.64, 166.30 µM]), respectively. Based on these results, MCA1 was selected for further studies. A competitive dose response assay demonstrated specific binding and an IC₅₀ value of 2.15 µM (95% CI [1.28, 3.62 µM]). Taken together, this study suggests that MCA1 may be used as a pancreatic cancer targeting ligand for detection of the disease.

Phage nanofibers in nanomedicine: Biopanning for early diagnosis, targeted therapy, and proteomics analysis

Display of a peptide or protein of interest on the filamentous phage (also known as bacteriophage), a biological nanofiber, has opened a new route for disease diagnosis and therapy as well as proteomics. Earlier phage display was widely used in protein-protein or antigen-antibody studies. In recent years, its application in nanomedicine is becoming increasingly popular and encouraging. We aim to review the current status in this research direction. For better understanding, we start with a brief introduction of basic biology and structure of the filamentous phage. We present the principle of phage display and library construction method on the basis of the filamentous phage. We summarize the use of the phage displayed peptide library for selecting peptides with high affinity against cells or tissues. We then review the recent applications of the selected cell or tissue targeting peptides in developing new targeting probes and therapeutics to advance the early diagnosis and targeted therapy of different diseases in nanomedicine. We also discuss the integration of antibody phage display and modern proteomics in discovering new biomarkers or target proteins for disease diagnosis and therapy. Finally, we propose an outlook for further advancing the potential impact of phage display on future nanomedicine. This article is categorized under: Biology-Inspired Nanomaterials > Protein and Virus-Based Structures.

Evolution of a Landscape Phage Library in a Mouse Xenograft Model of Human Breast Cancer

Peptide-displayed phage libraries are billion-clone collections of diverse chimeric bacteriophage particles, decorated by genetically fused peptides built from a random combination of natural amino acids. Studying the molecular evolution of peptide-displayed libraries in mammalian model systems, using in vivo phage display techniques, can provide invaluable knowledge about the underlying physiology of the vasculature system, allow recognition of organ- and tissue-specific networks of protein-protein interactions, and provide ligands for targeted diagnostics and therapeutics. Recently, we discovered that landscape phage libraries, a specific type of multivalent peptide phage display library, expose on their surface comprehensive collections of elementary binding units (EBUs), which can form short linear motifs (SLiMs) that interact with functional domains of physiologically relevant proteins. Because of their unique structural and functional features, landscape phages can use an alternative mechanism of directed molecular evolution, i.e., combinatorial avidity selection. These discoveries fueled our interest in revisiting the in vivo evolution of phage displayed libraries using another format of display, i.e., landscape phages. In this study, we monitored the evolution of a landscape phage library in a mouse model with and without an implanted human breast cancer tumor xenograft. As expected, the multivalent architecture of landscape phage displayed proteins provided strong tissue selectivity and resulted in a huge diversity of tissue penetrating, chimeric phage particles. We identified several types of EBU interactions that evolved during the course of tissue distribution, which included interactions of EBUs with all tissue types, those EBUs that interacted selectively with specific organs or tissues with shared gene expression profiles or functionalities, and other EBUs that interacted in a tissue-selective manner. We demonstrated that landscape phage libraries are a rich collection of unique nanobioparticles that can be used to identify functional organ and tissue-binding elements after the evolution of a phage display library in vivo.

Phage Display Selection, In Vitro Characterization, and Correlative PET Imaging of a Novel HER3 Peptide

PURPOSE

HER3 (ERBB3) is a receptor tyrosine kinase that is implicated in treatment resistance across multiple cancers, including those of the breast, lung, and prostate. Overexpression of HER3 following targeted therapy can occur rapidly and heterogeneously both within a single lesion and across sites of metastasis, making protein quantification by biopsy highly challenging. A global, non-invasive methodology such as positron emission tomography (PET) imaging can permit serial quantification of HER3, providing a useful approach to monitor HER3 expression across the entire tumor burden both prior to and following treatment. PET imaging of HER3 expression may permit a more personalized approach to targeted therapy by allowing for detection of HER3-mediated resistance, in addition to informing clinical trial patient selection for novel therapies targeting HER3.

PROCEDURES

Phage display selection targeting the HER3 extracellular domain was performed in order to develop a peptide with optimal blood clearance and highly accurate HER3 quantification.

RESULTS

The selection converged to a consensus peptide sequence that was subsequently found to bind HER3 with an affinity of 270 ± 151 nM. The peptide, termed HER3P1, was bound with high selectivity to HER3 over other similar receptor tyrosine kinases such as EGFR and HER2. Furthermore, HER3P1 was able to distinguish between high and low HER3-expressing cells in vitro. The peptide was radiolabeled with Ga-68 and demonstrated to specifically bind HER3 by in vivo PET imaging. Uptake of [⁶⁸Ga]HER3P1 was highly specific for HER3-positive tumors, with tumor-to-background ratios ranging from 1.59-3.32, compared to those of HER3-negative tumors, ranging from 0.84-0.93. The uptake of [⁶⁸Ga]HER3P1 also demonstrated high (P < 0.001) correlation with protein expression as quantified by Western blot and confirmed by biodistribution.

CONCLUSIONS

HER3P1 accurately quantifies expression of HER3 by PET imaging and has potential utility as a clinical imaging agent.

In vivo near-infrared imaging of ErbB2 expressing breast tumors with dual-axes confocal endomicroscopy using a targeted peptide

ErbB2 expression in early breast cancer can predict tumor aggressiveness and clinical outcomes in large patient populations. Accurate assessment with physical biopsy and conventional pathology can be limited by tumor heterogeneity. We aim to demonstrate real-time optical sectioning using a near-infrared labeled ErbB2 peptide that generates tumor-specific contrast in human xenograft breast tumors in vivo. We used IRDye800CW as the fluorophore, validated performance characteristics for specific peptide binding to cells in vitro, and investigated peak peptide uptake in tumors using photoacoustic tomography. We performed real-time optical imaging using a handheld dual-axes confocal fluorescence endomicroscope that collects light off-axis to reduce tissue scattering for greater imaging depths. Optical sections in either the vertical or horizontal plane were collected with sub-cellular resolution. Also, we found significantly greater peptide binding to pre-clinical xenograft breast cancer in vivo and to human specimens of invasive ductal carcinoma that express ErbB2 ex vivo. We used a scrambled peptide for control. Peptide biodistribution showed high tumor uptake by comparison with other organs to support safety. This novel integrated imaging strategy is promising for visualizing ErbB2 expression in breast tumors and serve as an adjunct during surgery to improve diagnostic accuracy, identify tumor margins, and stage early cancers.

Targeting aggressive prostate cancer-associated CD44v6 using phage display selected peptides

There is a crucial need to identify new biomarkers associated with aggressive prostate cancer (PCa) including those associated with cancer stem cells (CSCs). CD44v6, generated by alternative splicing of CD44, has been proposed as a CSC biomarker due to its correlation with aggressive PCa disease. We hypothesized that phage display selected peptides that target CD44v6 may serve as theranostic agents for aggressive PCa. Here, a 15 amino acid peptide ("PFT") was identified by affinity selection against a peptide derived from the v6 region of CD44v6. Synthesized PFT exhibited specific binding to CD44v6 with an equilibrium dissociation constant (Kd) of 743.4 nM. PFT also bound CD44v6 highly expressed on human PCa cell lines. Further, an aggressive form of PCa cells (v6A3) was isolated and tagged by a novel CSC reporter vector. The v6A3 cells had a CSC-like phenotype including enriched CD44v6 expression, enhanced clonogenicity, resistance to chemotherapeutics, and generation of heterogeneous offspring. PFT exhibited preferential binding to v6A3 cells compared to parental cells. Immunohistofluorescence studies with human PCa tissue microarrays (TMA) indicated that PFT was highly accurate in detecting CD44v6-positive aggressive PCa cells, and staining positivity was significantly higher in late stage, metastatic and higher-grade samples. Taken together, this study provides for the first time phage display selected peptides that target CD44v6 overexpressed on PCa cells. Peptide PFT may be explored as an aid in the diagnosis and therapy of advanced PCa disease.

High-Throughput Approaches to the Development of Molecular Imaging Agents

Molecular imaging allows for the visualization of changes at the cellular level in diseases such as cancer. A successful molecular imaging agent must rely on disease-selective targets and ligands that specifically interact with those targets. Unfortunately, the translation of novel target-specific ligands into the clinic has been frustratingly slow with limitations including the complex design and screening approaches for ligand identification, as well as their subsequent optimization into useful imaging agents. This review focuses on combinatorial library approaches towards addressing these two challenges, with particular focus on phage display and one-bead one-compound (OBOC) libraries. Both of these peptide-based techniques have proven successful in identifying new ligands for cancer-specific targets and some of the success stories will be highlighted. New developments in screening methodology and sequencing technology have pushed the bounds of phage display and OBOC even further, allowing for even faster and more robust discovery of novel ligands. The combination of multiple high-throughput technologies will not only allow for more accurate identification, but also faster affinity maturation, while overall streamlining the process of translating novel ligands into clinical imaging agents.

Ectopic expression of the ATP synthase β subunit on the membrane of PC-3M cells supports its potential role in prostate cancer metastasis

Metastatic prostate cancer is associated with high mortality rates. Identification of metastasis-related proteins may facilitate the development of novel therapies for the treatment of metastatic disease. In the present study, we aimed to identify prostate cancer metastasis-associated membrane proteins. We developed a phage-displayed 7-mer peptide library to screen the target peptides that were specifically bound to PC-3M cells with subtractive panning from normal prostate cells and PC-3 prostate cancer cells. A novel short peptide (B04) was found to have high affinity to highly metastatic PC-3M cells. ATP synthase β subunit (ATP5B) was then identified as a binding partner of B04 on the PC-3M cell surface. ATP5B was expressed on the PC-3M cell membrane and on highly malignant human prostate cancer specimens, as shown using multiple methodologies. Furthermore, ATP5B-positive gold particles were detected on the cellular and mitochondrial membranes by immunoelectromicroscopy. These results implied the possibility that ATP5B may translocate from the inner mitochondrial membrane to the outer surface of PC-3M cells. Additional analysis showed that incubation of B04 with PC-3M cells reduced the detection of ATP5B by western blotting and flow cytometry and significantly inhibited the proliferation, invasion and metastasis of PC-3M cells. In conclusion, ATP5B, as a binding partner of a metastasis-related short peptide (B04) on prostate cancer cells, is involved in promoting prostate cancer metastasis. In conclusion, ATP5B may be a promising biomarker and therapeutic target for highly metastatic malignancies.

Engineering an Affinity-Enhanced Peptide through Optimization of Cyclization Chemistry

Peptide cyclization is a strategy used to improve stability and activity of peptides. The most commonly used cyclization method is disulfide bridge formation of cysteine-containing peptides, as is typically found in nature. Over the years, an increasing number of alternative chemistries for peptide cyclization with improved efficiency, kinetics, orthogonality, and stability have been reported. However, there has been less appreciation for the opportunity to fine-tune peptide activity via the diverse chemical entities introduced at the site of linkage by different cyclization strategies. Here, we demonstrate how cyclization optimization of an M2 "anti-inflammatory" macrophage-binding peptide (M2pep) resulted in a significant increase in binding affinity of the optimized analog to M2 macrophages while maintaining binding selectivity compared to M1 "pro-inflammatory" macrophages. In this study, we report synthesis and evaluation of four cyclic M2pep(RY) analogs with diverse cyclization strategies: (1) Asp-[amide]-Lys, (2) azido-Lys-[triazole(copper(I)-catalyzed alkyne-azide cycloaddition (CuAAC))]-propargyl-Gly, (3) Cys-[decafluorobiphenyl (DFBP)]-Cys, and (4) Cys-[decafluorobiphenyl sulfone (DFS)]-Cys, whereby the chemical entity or linker at the linkage site is shown in the square bracket and is between the residues involved in cyclization. These peptides are compared to a disulfide-cyclized M2pep(RY) that we previously reported as a serum-stable, affinity-enhanced analog to the original linear M2pep. DFBP-cyclized M2pep(RY) exhibits the highest binding activity to M2 macrophages with apparent dissociation constant (K_D) about 2.03 μM compared to 36.3 μM for the original disulfide-cyclized M2pep(RY) and 220 μM for the original linear peptide. DFS-cyclized M2pep(RY) also binds more strongly than the original cyclized analog, whereas amide- and triazole-cyclized M2pep(RY) analogs bind less strongly. We verified that DFBP alone has negligible binding to M2 macrophages and the incorporation of diphenylalanine to the original sequence improves binding activity at the expense of solubility and increased toxicity. In conclusion, we report development of cyclic M2pep(RY) analogs with diverse cyclization strategies leading to the discovery of DFBP-cyclized M2pep(RY) with enhanced M2 macrophage-binding activity.

Identification and Tumour-Binding Properties of a Peptide with High Affinity to the Disialoganglioside GD2

Neuroectodermal tumours are characterized by aberrant processing of disialogangliosides concomitant with high expression of GD2 or GD3 on cell surfaces. Antibodies targeting GD2 are already in clinical use for therapy of neuroblastoma, a solid tumour of early childhood. Here, we set out to identify peptides with high affinity to human disialoganglioside GD2. To this end, we performed a combined in vivo and in vitro screen using a recombinant phage displayed peptide library. We isolated a phage displaying the peptide sequence WHWRLPS that specifically binds to the human disialoganglioside GD2. Binding specificity was confirmed by mutational scanning and by comparative analyses using structurally related disialogangliosides. In vivo, significant enrichment of phage binding to xenografts of human neuroblastoma cells in mice was observed. Tumour-specific phage accumulation could be blocked by intravenous coinjection of the corresponding peptide. Comparative pharmacokinetic analyses revealed higher specific accumulation of 68Ga-labelled GD2-binding peptide compared to 111In-labelled peptide in xenografts of human neuroblastoma. In contrast to 124I-MIBG, which is currently evaluated as a neuroblastoma marker in PET/CT, 68Ga-labelled GD2-specific peptide spared the thyroid but was enriched in the kidneys, which could be partially blocked by infusion of amino acids.In summary, we here report on a novel tumour-homing peptide that specifically binds to the disialoganglioside GD2, accumulates in xenografts of neuroblastoma cells in mice and bears the potential for tumour detection using PET/CT. Thus, this peptide may serve as a new scaffold for diagnosing GD2-positive tumours of neuroectodermal origin.

Design and Synthesis of Near-Infrared Peptide for in Vivo Molecular Imaging of HER2

We report the development, characterization, and validation of a peptide specific for the extracellular domain of HER2. This probe chemistry was developed for molecular imaging by using a structural model to select an optimal combination of amino acids that maximize the likelihood for unique hydrophobic and hydrophilic interactions with HER2 domain 3. The sequence KSPNPRF was identified and conjugated with either FITC or Cy5.5 via a GGGSK linker using Fmoc-mediated solid-phase synthesis to demonstrate flexibility for this chemical structure to be labeled with different fluorophores. A scrambled sequence was developed for control by altering the conformationally rigid spacer and moving both hydrophobic and hydrophilic amino acids on the C-terminus. We validated peptide specificity for HER2 in knockdown and competition experiments using human colorectal cancer cells in vitro, and measured a binding affinity of kd = 21 nM and time constant of k = 0.14 min(-1) (7.14 min). We used this peptide with either topical or intravenous administration in a preclinical model of colorectal cancer to demonstrate specific uptake in spontaneous adenomas and to show feasibility for real time in vivo imaging with near-infrared fluorescence. We used this peptide in immunofluorescence studies of human proximal colon specimens to evaluate specificity for sessile serrated and sporadic adenomas. Improved visualization can be used endoscopically to guide tissue biopsy and detect premalignant lesions that would otherwise be missed. Our peptide design for specificity to HER2 is promising for clinical translation in molecular imaging methods for early cancer detection.

Humoral immune responses against gonadotropin releasing hormone elicited by immunization with phage-peptide constructs obtained via phage display

Phage display is based on genetic engineering of phage coat proteins resulting in fusion peptides displayed on the surface of phage particles. The technology is widely used for generation of phages with novel characteristics for numerous applications in biomedicine and far beyond. The focus of this study was on development of phage-peptide constructs that stimulate production of antibodies against gonadotropin releasing hormone (GnRH). Phage-peptide constructs that elicit production of neutralizing GnRH antibodies can be used for anti-fertility and anti-cancer applications. Phage-GnRH constructs were generated via selection from a phage display library using several types of GnRH antibodies as selection targets. Such phage constructs were characterized for sequence similarities to GnRH peptide and frequency of their occurrence in the selection rounds. Five of the constructs with suitable characteristics were tested in mice as a single dose 5×10(11) virions (vir) vaccine and were found to be able to stimulate production of GnRH-specific antibodies, but not to suppress testosterone (indirect indicator of GnRH antibody neutralizing properties). Next, one of the constructs was tested at a higher dose of 2×10(12) vir per mouse in combination with a poly(lactide-co-glycolide) (PLGA)-based adjuvant. This resulted in multifold increase in GnRH antibody production and significant reduction of serum testosterone, indicating that antibodies produced in response to the phage-GnRH immunization possess neutralizing properties. To achieve optimal immune responses for desired applications, phage-GnRH constructs can be modified with respect to flanking sequences of GnRH-like peptides displayed on phage. Anticipated therapeutic effects also might be attained using optimized phage doses, a combination of several constructs in a single treatment, or application of adjuvants and advanced phage delivery systems.

Microarray based screening of peptide nano probes for HER2 positive tumor

Peptides are excellent biointerface molecules and diagnostic probes with many advantages such as good penetration, short turnover time, and low cost. We report here an efficient peptide screening strategy based on in situ single bead sequencing on a microarray. Two novel peptides YLFFVFER (H6) and KLRLEWNR (H10) specifically binding to the tumor biomarker human epidermal growth factor receptor 2 (HER2) with aKD of 10(-8) M were obtained from a 10(5) library. Conjugated to nanoparticles, both the H6 and H10 probes showed specific accumulation in HER2-positive tumor tissues in xenografted mice by in vivo imaging.

Multiple Bacteriophage Selection Strategies for Improved Affinity of a Peptide Targeting ERBB2

Due to the heterogeneity of ERBB2-expression between tumors and over the course of treatment, a non-invasive molecular imaging agent is needed to accurately detect overall ERBB2 status. Peptides are a highly advantageous platform for molecular imaging, since they have excellent tumor penetration and rapid pharmacokinetics. One limitation of peptides however, is their traditionally low target affinity, and consequently, tumor uptake. The peptide KCCYSL was previously selected from a bacteriophage (phage) display library to bind ERBB2 and did so with moderate affinity of 295 nM. In order to enhance tumor uptake and clinical utility of the peptide, a novel phage microlibrary was created by flanking the parent sequence with random amino acids, followed by reselection using parallel strategies for high affinity and specific ERBB2 binding in an attempt to affinity maturate the peptide. One limitation of traditional phage display selections is difficulty in releasing the highest affinity phages from the target by incubation of acidic buffer. In an attempt to recover high affinity second-generation peptides from the ERBB2 microlibrary, two elution strategies, sonication and target elution, were undertaken. Sonication resulted in an approximately 50-fold enhancement in recovered phage per round of selection in comparison to target elution. Despite the differences in elution efficiency, the affinities of phage-displayed peptides selected from either strategy were relatively similar. Although both selections yielded peptides with significantly improved affinity in comparison to KCCYSL, the improvements were modest, most likely because the parental peptide binding cannot be improved by additional amino acids.

Screening of a specific peptide binding to esophageal squamous carcinoma cells from phage displayed peptide library

To select a specifically binding peptide for imaging detection of human esophageal squamous cell carcinoma (ESCC), a phage-displayed 12-mer peptide library was used to screen the peptide that bind to ESCC cells specifically. After four rounds of bio-panning, the phage recovery rate gradually increased, and specific phage clones were effectively enriched. The 60 randomly selected phage clones were tested using cellular enzyme-linked immunosorbent assay (ELISA), and 41 phage clones were identified as positive clones with the over 2.10 ratio of absorbance higher than other clones, IRP and PBS controls. From the sequencing results of the positive clones, 14 peptide sequences were obtained and ESCP9 consensus sequence was identified as the peptide with best affinity to ESCC cells via competitive inhibition, fluorescence microscopy, and flow cytometry. The results indicate that the peptide ESCP9 can bind to ESCC cells specifically and sensitively, and it is a potential candidate to be developed as an useful molecule to the imaging detection and targeting therapy for ESCC.

Vascular-homing peptides for targeted drug delivery and molecular imaging: meeting the clinical challenges

The vasculature of each organ expresses distinct molecular signatures critically influenced by the pathological status. The heterogeneous profile of the vascular beds has been successfully unveiled by the in vivo phage display, a high-throughput tool for mapping normal, diseased, and tumor vasculature. Specific challenges of this growing field are targeted therapies against cancer and cardiovascular diseases, as well as novel bioimaging diagnostic tools. Tumor vasculature-homing peptides have been extensively evaluated in several preclinical and clinical studies both as targeted-therapy and diagnosis. To date, results from several Phase I and II trials have been reported and many other trials are currently ongoing or recruiting patients. In this review, advances in the identification of novel peptide ligands and their corresponding receptors on tumor endothelium through the in vivo phage display technology are discussed. Emphasis is given to recent findings in the clinical setting of vascular-homing peptides selected by in vivo phage display for the treatment of advanced malignancies and their altered vascular beds.