Identification and characterization of peptides that bind human ErbB-2 selected from a bacteriophage display library

What are the key targeted delivery technologies of siRNA now?

Whilst significant advances have been made in the delivery of nucleic acids to mammalian cells, most of the used strategies do not distinguish between normal and cancer cells. The same challenge is also facing radioactive- and chemo-therapies which are highly toxic and poorly tolerated due to limited tumor specificity. Regardless of the nature of the drug, there is a need for developing a technology platform which targets drugs only to tumors cells, leaving normal cells undamaged. Among the targeting strategies, receptor-targeted delivery provides an innovative strategy to selectively direct therapeutics to cancer cells. Receptor-binding ligands (e.g., peptides, antibodies, aptamers) can be incorporated into gene delivery vesicles or directly conjugated to siRNA in the hope in promoting their localization in target cell expressing the cognate receptors. The present chapter discusses the current progress made in the specific delivery of siRNAs.

In-labeled KCCYSL peptide as an imaging probe for ErbB-2-expressing ovarian carcinomas

Aberrant expression of ErbB-2, a member of the epidermal growth factor family of receptors, has been implicated in the formation of various malignancies including ovarian cancer. The objective of this study was to determine if the bacteriophage (phage) display-selected ErbB-2 targeting peptide, KCCYSL, once radiolabeled with (111)In would serve as a tumor targeting and Single Photon Emission Computed Tomography (SPECT/CT) imaging agent in a mouse model of human ovarian carcinoma expressing ErbB-2. The KCCYSL peptide was synthesized with a chelator 1,4,7,10-tetra-azacyclododecane-N,N',N",N"'-tetraacetic acid (DOTA), and a Gly-Ser-Gly (GSG) spacer between DOTA and amino terminus of the peptide and radiolabeled with (111)InC1(3). In vitro cell binding studies indicated that (111)In-DOTA-GSG-KCCYSL bound to cultured ovcar-3 carcinoma cells. Biodistribution studies in scid mice bearing human ovcar-3 tumor xenografts revealed a tumor uptake of 0.50 ± 0.05 percent injected dose per gram (%ID/g) at 1 h, and 0.39 ± 0.1 %ID/g at 2 h. Blocking studies with non-radiolabeled counterpart indicated a partial inhibition (41%) (P = 0.04) in tumor uptake of (111)In-DOTA-GSG-KCCYSL. In vivo tumor uptake of (111)In-DOTA-GSG-KCCYSL was clearly evident through SPECT/CT images after 2 h post injection. These studies suggest the potential of this peptide as a radiopharmaceutical for imaging of ErbB-2-expressing ovarian tumors.

Targeted delivery of antisense oligonucleotides and siRNAs into mammalian cells

RNA interference (RNAi) is a natural mechanism for gene silencing that can be harnessed for the development of RNA-based drugs. Although synthetic small interfering RNA (siRNAs) can be delivered in vitro to virtually all cell types using lipid-based transfection agents or electroporation, efficient strategies for achieving either systemic or targeted delivery remains one of the major in vivo challenges. Among the targeting strategies, receptor-targeted delivery provides an innovative strategy to selectively direct therapeutics to cancer cells. Receptor-binding peptides can be incorporated into gene-delivery vesicles or directly conjugated to siRNAs in the hope of promoting their localization in target cells expressing the cognate receptors. This chapter discusses the current status of siRNA-targeting strategies using either peptides identified through iterative screening of random peptide phage libraries or naturally occurring peptides. Also, transcriptional targeting strategies and detailed protocols for the selection of cancer cell-binding peptide from random peptide libraries are described.

Phage peptide display

Molecular imaging is at the forefront in the advancement of in-vivo diagnosis and monitoring of cancer. New peptide-based molecular probes to facilitate cancer detection are rapidly evolving. Peptide-based molecular probes that target apoptosis, angiogenesis, cell signaling and cell adhesion events are in place. Bacteriophage (phage) display technology, a molecular genetic approach to ligand discovery, is commonly employed to identify peptides as tumor-targeting molecules. The peptide itself may perhaps have functional properties that diminish tumor growth or metastasis. More often, a selected peptide is chemically synthesized, coupled to a radiotracer or fluorescent probe, and utilized in the development of new noninvasive molecular imaging probes. A myriad of peptides that bind cancer cells and cancer-associated antigens have been reported from phage library selections. Phage selections have also been performed in live animals to obtain peptides with optimal stability and targeting properties in vivo. To this point, few in-vitro, in-situ, or in-vivo selected peptides have shown success in the molecular imaging of cancer, the notable exception being vascular targeting peptides identified via in-vivo selections. The success of vasculature targeting peptides, such as those with an RGD motif that bind alpha(v)beta(3)integrin, may be due to the abundance and expression patterns of integrins in tumors and supporting vasculature. The discovery of molecular probes that bind tumor-specific antigens has lagged considerably. One promising means to expedite discovery is through the implementation of selected phage themselves as tumor-imaging agents in animals.

Evaluation of an 111In-radiolabeled peptide as a targeting and imaging agent for ErbB-2 receptor expressing breast carcinomas

PURPOSE

The cellular targeting and tumor imaging properties of a novel ErbB-2-avid peptide, discovered from bacteriophage display, were evaluated in human breast carcinoma cells and in breast carcinoma-xenografted mice.

EXPERIMENTAL DESIGN

The affinity of the ErbB-2 targeting peptide KCCYSL and its alanine substituted counterparts for the extracellular domain (ECD) of purified recombinant ErbB-2 (ErbB-2-ECD) was assessed by fluorescence titration. Binding of the KCCYSL peptide to breast and prostate carcinoma cells was analyzed by confocal microscopy. A DOTA(GSG)-KCCYSL peptide conjugate was radiolabeled with 111In, and stability, target binding, and internalization were analyzed in vitro. In vivo biodistribution and single-photon emission computed tomography imaging studies were done with the radiolabeled peptide in MDA-MB-435 human breast tumor-bearing severe combined immunodeficient mice.

RESULTS

KCCYSL peptide exhibited high affinity (295 +/- 56 nmol/L) to ErbB-2-ECD. Substitution of alanine for lysine, tryptophan, and cysteine reduced the peptide affinity approximately 1- to 2.4-fold, whereas replacing leucine completely abolished binding. Both biotin-KCCYSL and 111In-DOTA(GSG)-KCCYSL were capable of binding ErbB-2-expressing human breast carcinoma cells in vitro. Approximately 11% of the total bound radioactivity was internalized in the carcinoma cells. Competitive binding studies indicated that the radiolabeled peptide exhibited an IC(50) value of 42.5 +/- 2.76 nmol/L for the breast carcinoma cells. 111In-DOTA(GSG)-KCCYSL was stable in serum and exhibited rapid tumor uptake (2.12 +/- 0.32 %ID/g) at 15 min postinjection and extended retention coupled with rapid whole body disappearance, as observed by biodistribution and single-photon emission computed tomography imaging studies, respectively.

CONCLUSIONS

The DOTA(GSG)-KCCYSL peptide has the potential to be used as a tumor-imaging agent and a vehicle for specific delivery of radionuclide or cytotoxic agents for tumors overexpressing ErbB-2.

In vivo selection of phage for the optical imaging of PC-3 human prostate carcinoma in mice

There is an increasing medical need to detect and spatially localize early and aggressive forms of prostate cancer. Affinity ligands derived from bacteriophage (phage) library screens can be developed to molecularly target prostate cancer with fluorochromes for optical imaging. Toward this goal, we used in vivo phage display and a newly described micropanning assay to select for phage that extravasate and bind human PC-3 prostate carcinoma xenografts in severe combined immune deficiency mice. One resulting phage clone (G1) displaying the peptide sequence IAGLATPGWSHWLAL was fluorescently labeled with the near-infrared fluorophore AlexaFluor 680 and was evaluated both in vitro and in vivo for its ability to bind and target PC-3 prostate carcinomas. The fluorescently labeled phage clone (G1) had a tumor-to-muscle ratio of approximately 30 in experiments. In addition, prostate tumors (PC-3) were readily detectable by optical-imaging methods. These results show proof of principle that disease-specific library-derived fluorescent probes can be rapidly developed for use in the early detection of cancers by optical means.

Point-of-care biosensor systems for cancer diagnostics/prognostics

With the growing number of fatalities resulting from the 100 or so cancer-related diseases, new enabling tools are required to provide extensive molecular profiles of patients to guide the clinician in making viable diagnosis and prognosis. Unfortunately with cancer-related diseases, there is not one molecular marker that can provide sufficient information to assist the clinician in making effective prognoses or even diagnoses. Indeed, large panels of markers must typically be evaluated that cut across several different classes (mutations in certain gene fragments--DNA; over/under-expression of gene activity as monitored by messenger RNAs; the amount of proteins present in serum or circulating tumor cells). The classical biosensor format (dipstick approach for monitoring the presence of a single element) is viewed as a valuable tool in many bioassays, but possesses numerous limitations in cancer due primarily to the single element nature of these sensing platforms. As such, if biosensors are to become valuable tools in the arsenal of the clinician to manage cancer patients, new formats are required. This review seeks to provide an overview of the current thinking on molecular profiling for diagnosis and prognosis of cancers and also, provide insight into the current state-of-the-art in the biosensor field and new strategies that must be considered to bring this important technology into the cancer field.

Designing scaffolds of peptides for phage display libraries

Phage display is a powerful method for the discovery of peptide ligands that are used for analytical tools, drug discovery, and target validations. Phage display technology can produce a huge number of peptides and generate novel peptide ligands. Recently, phage display technology has successfully managed to create peptide ligands that bind to pharmaceutically difficult targets such as the erythropoietin receptor. As a result of the structural analysis of their ligands, we found that the conformational design of peptides in library is important for selecting high-affinity ligands that bind to every target from a phage peptide library. Key issues concern constraints on the conformation of peptides on the phage and the development of chemically synthesized peptides derived from peptides on phage. This review discusses studies related to the conformation of peptides selected from phage display peptide libraries in addition to the conversion from peptides to non-peptides.

Cell-Specific Delivery of a Chemotherapeutic to Lung Cancer Cells

We report that lung cancer-targeting peptides isolated from a peptide library can be used to deliver an active chemotherapeutic in a cell-specific fashion. The peptides were removed from the context of the phage and placed on a pegylated tetrameric scaffold. The tetrameric peptides were shown to block uptake of their cognate phage. The tetrameric peptides were coupled to doxorubicin, and their cytotoxicity against a panel of different cell lines was tested. Our data demonstrate that these targeting peptides can deliver an active anticancer agent in a cell-specific fashion, resulting in an increase of the therapeutic index of the targeted drug compared to systemic delivery. The efficacy of the peptide conjugate correlates to the affinity of the targeting peptide for a particular cell line. As such, we have demonstrated that cell-specific targeted drugs can be synthesized, even when the cell surface target is unknown.

Phage display of random peptide libraries: applications, limits, and potential

The identification of ligands from large biological libraries by phage display has now been used for almost 15 years. Most of the successful reports on high-affinity ligand identification originated from work with different antibody libraries. In contrast, the progress of applying phage display to random peptide libraries was relatively slow. However, in the last few years several improvements have led to an increasing number of published peptide ligands identified by phage display from such libraries and which exhibited good biological activity and high affinity. This review summarizes the current state and the technical progress of the application of random peptide libraries using filamentous phage for ligand identification.

Combinatorial discovery of tumor targeting peptides using phage display

Peptides possess appropriate pharmacokinetic properties to serve as cancer imaging or therapeutic targeting agents. Currently, only a small number of rationally-derived, labeled peptide analogues that target only a limited subset of antigens are available. Thus, finding new cancer targeting peptides is a central goal in the field of molecular targeting. Novel tumor-avid peptides can be efficiently identified via affinity selections using complex random peptide libraries containing millions of peptides that are displayed on bacteriophage. In vitro and in situ affinity selections may be used to identify peptides with high affinity for the target antigen in vitro. Unfortunately, it has been found that peptides selected in vitro or in situ may not effectively target tumors in vivo due to poor peptide stability and other problems. To improve in vivo targeting, methodological combinatorial chemistry innovations allow selections to be conducted in the environment of the whole animal. Thus, new targeting peptides with optimal in vivo properties can be selected in vivo in tumor-bearing animals. In vivo selections have been proven successful in identifying peptides that target the vasculature of specific organs. In addition, in vivo selections have identified peptides that bind specifically to the surface of or are internalized into tumor cells. In the future, direct selection of peptides for cancer imaging may be expedited using genetically engineered bacteriophage libraries that encode peptides with intrinsic radiometal-chelation or fluorescent sequences.

Phage display-derived peptides as therapeutic alternatives to antibodies

Peptide-based drugs are now viable alternatives to biopharmaceuticals, such as antibodies. Most of the past limitations of peptides have been removed by new technologies, so that peptides now face similar hurdles to antibodies. Phage-display technology provides novel peptides that bind protein targets with high affinity and specificity. Most marketed peptide-based drugs are receptor agonists derived from natural peptides. To address the need for antagonists, novel strategies have been developed for inhibiting receptor-ligand interactions. We review results from phage display in finding peptide drug candidates and conclude with some business benefits of developing peptides.

Selective targeting of cancer cells using synthetic peptides

To establish efficient and reliable therapeutic delivery into cancer cells, a number of delivery agents and concepts have been investigated in the recent years. Among many improvements in targeted and controlled delivery of therapeutics, cell-targeting peptides have emerged as the most valuable non-immunogenic approach to target cancer cells. Peptides can be incorporated into multicomponent gene-delivery complexes for cell-specific targeting. In contrast to larger molecules such as monoclonal antibodies, peptides have an excellent tumor penetration, which make them ideal carriers of therapeutics to the site of primary tumor and the distant metastatic sites. Here we give an update on the progress made during the last two years on the identification and potential of specific synthetic tumor targeting peptides.

Identification of a small peptide that inhibits the phosphorylation of ErbB2 and proliferation of ErbB2 overexpressing breast cancer cells

ErbB2 is overexpressed in approximately 30% of breast cancer patients with a correlation to poor prognosis. ErbB2 has been identified as a useful receptor for molecular targeting. A cyclic 20 amino acid phage display random peptide library was constructed using the fUSE5 gene III system. The library was panned against 2 different purified forms of the external domain of ErbB2. This resulted in the identification of several ErbB2-binding phage clones with variable binding to different ErbB2 preparations. One clone (EC-1) bound all preparations of ErbB2 including live cells and fresh frozen human breast cancer specimens. The synthetic peptide based on the deduced sequence of the EC-1 clone and its biotin-conjugated form retained binding affinity for purified ErbB2 and ErbB2 overexpressing cell lysates. EC-1 peptide was able to effectively inhibit the phosphorylation of ErbB2 on residues Y1248 and Y877 in a dose- and time-dependent manner. Furthermore, EC-1 peptide selectively inhibits the proliferation of ErbB2 overexpressing breast cancer cells. The linear portion of the cyclic EC-1 peptide was shown to be essential for binding ErbB2. In addition, 4 biased phage libraries were constructed allowing 4 different regions of the EC-1 peptide to have random sequence. Screening these EC-1 biased libraries did not result in higher affinity peptides but did demonstrate the importance of amino acids at position 1-4 on the N-terminal flanking arm and 11-15 within the cyclic ring. Interestingly, EC-1 contains homologous motifs with known ErbB receptor family ligands. We have identified a small peptide that binds to the extracellular domain of ErbB2, inhibits ErbB2 autophosphorylation and inhibits the proliferation of ErbB2 overexpressing cells. This supports the notion that small peptides can bind to targets important in cancer therapy even if a target does not have a natural ligand. Continuing research with this peptide includes increasing its affinity to ErbB2, evaluation of pharmacokinetics and evaluation of anti-proliferative effects with conjugate anti-cancer agents.

Cyclic peptides selected by phage display mimic the natural epitope recognized by a monoclonal anti-colicin A antibody

A 10-mer random peptide library displayed on filamentous bacteriophage was used to determine the molecular basis of the interaction between the monoclonal anti-colicin A antibody 1C11 and its cognate epitope. Previous studies established that the putative epitope recognized by 1C11 antibody is composed of amino acid residues 19-25 (RGSGPEP) of colicin A. Using the phage display technique it was confirmed that the epitope of 1C11 antibody was indeed restricted to residues 19-25 and the consensus motif RXXXPEP was identified. Shorter consensus sequences (RXXPEP, RXXEP, KXXEP) were also selected. It was also demonstrated that the disulfide bond found in one group of the selected peptides was crucial for 1C11 antibody recognition. It was shown that cyclization of the peptides by disulfide bond formation could result in a structure that mimics the natural epitope of colicin A.