Mapping of vascular ZIP codes by phage display

Synthesis of linear and cyclic peptide-PEG-lipids for stabilization and targeting of cationic liposome-DNA complexes

Because nucleic acids (NAs) have immense potential value as therapeutics, the development of safe and effective synthetic NA vectors continues to attract much attention. In vivo applications of NA vectors require stabilized, nanometer-scale particles, but the commonly used approaches of steric stabilization with a polymer coat (e.g., PEGylation; PEG=poly(ethylene glycol)) interfere with attachment to cells, uptake, and endosomal escape. Conjugation of peptides to PEG-lipids can improve cell attachment and uptake for cationic liposome-DNA (CL-DNA) complexes. We present several synthetic approaches to peptide-PEG-lipids and discuss their merits and drawbacks. A lipid-PEG-amine building block served as the common key intermediate in all synthetic routes. Assembling the entire peptide-PEG-lipid by manual solid phase peptide synthesis (employing a lipid-PEG-carboxylic acid) allowed gram-scale synthesis but is mostly applicable to linear peptides connected via their N-terminus. Conjugation via thiol-maleimide or strain-promoted (copper-free) azide-alkyne cycloaddition chemistry is highly amenable to on-demand preparation of peptide-PEG-lipids, and the appropriate PEG-lipid precursors are available in a single chemical step from the lipid-PEG-amine building block. Azide-alkyne cycloaddition is especially suitable for disulfide-bridged peptides such as iRGD (cyclic CRGDKGPDC). Added at 10 mol% of a cationic/neutral lipid mixture, the peptide-PEG-lipids stabilize the size of CL-DNA complexes. They also affect cell attachment and uptake of nanoparticles in a peptide-dependent manner, thereby providing a platform for preparing stabilized, affinity-targeted CL-DNA nanoparticles.

Neuropilin Functions as an Essential Cell Surface Receptor

The Neuropilins (Nrps) are a family of essential cell surface receptors involved in multiple fundamental cellular signaling cascades. Nrp family members have key functions in VEGF-dependent angiogenesis and semaphorin-dependent axon guidance, controlling signaling and cross-talk between these fundamental physiological processes. More recently, Nrp function has been found in diverse signaling and adhesive functions, emphasizing their role as pleiotropic co-receptors. Pathological Nrp function has been shown to be important in aberrant activation of both canonical and alternative pathways. Here we review key recent insights into Nrp function in human health and disease.

RNA as a stable polymer to build controllable and defined nanostructures for material and biomedical applications

The value of polymers is manifested in their vital use as building blocks in material and life sciences. Ribonucleic acid (RNA) is a polynucleic acid, but its polymeric nature in materials and technological applications is often overlooked due to an impression that RNA is seemingly unstable. Recent findings that certain modifications can make RNA resistant to RNase degradation while retaining its authentic folding property and biological function, and the discovery of ultra-thermostable RNA motifs have adequately addressed the concerns of RNA unstability. RNA can serve as a unique polymeric material to build varieties of nanostructures including nanoparticles, polygons, arrays, bundles, membrane, and microsponges that have potential applications in biomedical and material sciences. Since 2005, more than a thousand publications on RNA nanostructures have been published in diverse fields, indicating a remarkable increase of interest in the emerging field of RNA nanotechnology. In this review, we aim to: delineate the physical and chemical properties of polymers that can be applied to RNA; introduce the unique properties of RNA as a polymer; review the current methods for the construction of RNA nanostructures; describe its applications in material, biomedical and computer sciences; and, discuss the challenges and future prospects in this field.

Systemically Administered, Target Organ-Specific Therapies for Regenerative Medicine

Growth factors and other agents that could potentially enhance tissue regeneration have been identified, but their therapeutic value in clinical medicine has been limited for reasons such as difficulty to maintain bioactivity of locally applied therapeutics in the protease-rich environment of regenerating tissues. Although human diseases are treated with systemically administered drugs in general, all current efforts aimed at enhancing tissue repair with biological drugs have been based on their local application. The systemic administration of growth factors has been ruled out due to concerns about their safety. These concerns are warranted. In addition, only a small proportion of systemically administered drugs reach their intended target. Selective delivery of the drug to the target tissue and use of functional protein domains capable of penetrating cells and tissues could alleviate these problems in certain circumstances. We will present in this review a novel approach utilizing unique molecular fingerprints (“Zip/postal codes”) in the vasculature of regenerating tissues that allows target organ-specific delivery of systemically administered therapeutic molecules by affinity-based physical targeting (using peptides or antibodies as an “address tag”) to injured tissues undergoing repair. The desired outcome of targeted therapies is increased local accumulation and lower systemic concentration of the therapeutic payload. We believe that the physical targeting of systemically administered therapeutic molecules could be rapidly adapted in the field of regenerative medicine.

Selection strategies for anticancer antibody discovery: searching off the beaten path

Antibody-based drugs represent one of the most successful and promising therapeutic approaches in oncology. Large combinatorial phage antibody libraries are available for the identification of therapeutic antibodies and various technologies exist for their further conversion into multivalent and multispecific formats optimized for the desired pharmacokinetics and the pathological context. However, there is no technology for antigen profiling of intact tumors to identify tumor markers targetable with antibodies. Such constraints have led to a relative paucity of tumor-associated antigens for antibody targeting in oncology. Here we review novel approaches aimed at the identification of antibody-targetable, accessible antigens in intact tumors. We hope that such advanced selection approaches will be useful in the development of next-generation antibody therapies for cancer.

Fluorescence microscopy colocalization of lipid-nucleic acid nanoparticles with wildtype and mutant Rab5-GFP: A platform for investigating early endosomal events

Endosomal entrapment is known to be a major bottleneck to successful cytoplasmic delivery of nucleic acids (NAs) using cationic liposome-NA nanoparticles (NPs). Quantitative measurements of distributions of NPs within early endosomes (EEs) have proven difficult due to the sub-resolution size and short lifetime of wildtype EEs. In this study we used Rab5-GFP, a member of the large family of GTPases which cycles between the plasma membrane and early endosomes, to fluorescently label early endosomes. Using fluorescence microscopy and quantitative image analysis of cells expressing Rab5-GFP, we found that at early time points (t<1h), only a fraction (≈35%) of RGD-tagged NPs (which target cell surface integrins) colocalize with wildtype EEs, independent of the NP's membrane charge density. In comparison, a GTP-hydrolysis deficient mutant, Rab5-Q79L, which extends the size and lifetime of EEs yielding giant early endosomes (GEEs), enabled us to resolve and localize individual NPs found within the GEE lumen. Remarkably, nearly all intracellular NPs are found to be trapped within GEEs implying little or no escape at early time points. The observed small degree of colocalization of NPs and wildtype Rab5 is consistent with recycling of Rab5-GDP to the plasma membrane and not indicative of NP escape from EEs. Taken together, our results show that endosomal escape of PEGylated nanoparticles occurs downstream of EEs i.e., from late endosomes/lysosomes. Our studies also suggest that Rab5-Q79L could be used in a robust imaging assay which allows for direct visualization of NP interactions with the luminal membrane of early endosomes.

Etchable plasmonic nanoparticle probes to image and quantify cellular internalization

There is considerable interest in using nanoparticles as labels or to deliver drugs and other bioactive compounds to cells in vitro and in vivo. Fluorescent imaging, commonly used to study internalization and subcellular localization of nanoparticles, does not allow unequivocal distinction between cell surface-bound and internalized particles, as there is no methodology to turn particles 'off'. We have developed a simple technique to rapidly remove silver nanoparticles outside living cells, leaving only the internalized pool for imaging or quantification. The silver nanoparticle (AgNP) etching is based on the sensitivity of Ag to a hexacyanoferrate-thiosulphate redox-based destain solution. In demonstration of the technique we present a class of multicoloured plasmonic nanoprobes comprising dye-labelled AgNPs that are exceptionally bright and photostable, carry peptides as model targeting ligands, can be etched rapidly and with minimal toxicity in mice, and that show tumour uptake in vivo.

Targeting osteosarcoma vasculature with peptide obtained by phage display

Aim of the study

Tumour endothelial cells have been proven to have molecular markers distinct from normal endothelial cells. These specific molecular markers allow for targeting of the tumour vasculature with specific pharmacological vehicles to direct diagnostic or therapeutic modalities at the endothelial cells. By performing a phage display-based screening, this study aimed to identify a certain short peptide that could specifically bind to osteosarcoma vasculature.

Material and methods

We performed in vivo screening in the murine models of osteosarcoma with annular Ph.D.-C7C library in the present study. To explore the in vivo binding specificity of the retrieved peptide, we conjugated the peptide with fluorescein isothiocyanate (FITC) and injected it intravenously into osteosarcoma-bearing BALB-c mice.

Results

CTKPDKGYC was the dominant sequence isolated from in vivo screening and was named as NF-1. Fluorescence staining found that FITC-NF-1 peptide could be specifically homed to osteosarcoma vasculature while being almost undetectable in the heart, brain, lung and liver. Simultaneously, a small amount of fluorescence could also be detected in the renal glomerulus and renal tubule but not in renal vascular endothelium, indicating that FITC-NF-1 peptide might be excreted mainly through the renal-urinary route.

Conclusions

Our data suggest that, with high binding specificity to osteosarcoma vasculature, peptide NF-1 may have potential value in early diagnosis or targeted therapy for osteosarcoma.

Pharmacophore generation from a drug-like core molecule surrounded by a library peptide via the 10BASEd-T on bacteriophage T7

We have achieved site-specific conjugation of several haloacetamide derivatives into designated cysteines on bacteriophage T7-displayed peptides, which are fused to T7 capsid protein gp10. This easiest gp10 based-thioetherification (10BASE_d-T) undergoes almost quantitatively like a click reaction without side reaction or loss of phage infectivity. The post-translational modification yield, as well as the site-specificity, is quantitatively analyzed by a fluorescent densitometric analysis after gel electrophoresis. The detailed structure of the modified peptide on phage is identified with tandem mass spectrometry. Construction of such a peptide-fused phage library possessing non-natural core structures will be useful for future drug discovery. For this aim, we propose a novel concept of pharmacophore generation from a drug-like molecule (i.e., salicylic acid) conjugated with surrounding randomized peptides. By using the hybrid library, streptavidin-specific binders are isolated through four rounds of biopanning.

Caspase-8 as a regulator of tumor cell motility

The caspases are a family of ubiquitously expressed cysteine proteases best known for their roles in programmed cell death. However, caspases play a number of other roles in vertebrates. In the case of caspase-8, loss of expression is an embryonic lethal phenotype, and caspase-8 plays roles in suppressing cellular necrosis, promoting differentiation and immune signaling, regulating autophagy, and promoting cellular migration. Apoptosis and migration require localization of caspase-8 in the periphery of the cells, where caspase-8 acts as part of distinct biosensory complexes that either promote migration in appropriate cellular microenvironments, or cell death in inappropriate settings. In the cellular periphery, caspase-8 interacts with components of the focal adhesion complex in a tyrosine-kinase dependent manner, promoting both cell migration in vitro and metastasis in vivo. Mechanistically, caspase-8 interacts with components of both focal adhesions and early endosomes, enhancing focal adhesion turnover and promoting rapid integrin recycling to the cell surface. Clinically, this suggests that the expression of caspase-8 may not always be a positive prognostic sign, and that the role of caspase-8 in cancer progression is likely context-dependent.

Tumor Targeting via Integrin Ligands

Selective and targeted delivery of drugs to tumors is a major challenge for an effective cancer therapy and also to overcome the side-effects associated with current treatments. Overexpression of various receptors on tumor cells is a characteristic structural and biochemical aspect of tumors and distinguishes them from physiologically normal cells. This abnormal feature is therefore suitable for selectively directing anticancer molecules to tumors by using ligands that can preferentially recognize such receptors. Several subtypes of integrin receptors that are crucial for cell adhesion, cell signaling, cell viability, and motility have been shown to have an upregulated expression on cancer cells. Thus, ligands that recognize specific integrin subtypes represent excellent candidates to be conjugated to drugs or drug carrier systems and be targeted to tumors. In this regard, integrins recognizing the RGD cell adhesive sequence have been extensively targeted for tumor-specific drug delivery. Here we review key recent examples on the presentation of RGD-based integrin ligands by means of distinct drug-delivery systems, and discuss the prospects of such therapies to specifically target tumor cells.

Tumor-penetrating peptides

Tumor-homing peptides can be used to deliver drugs into tumors. Phage library screening in live mice has recently identified homing peptides that specifically recognize the endothelium of tumor vessels, extravasate, and penetrate deep into the extravascular tumor tissue. The prototypic peptide of this class, iRGD (CRGDKGPDC), contains the integrin-binding RGD motif. RGD mediates tumor-homing through binding to αv integrins, which are selectively expressed on various cells in tumors, including tumor endothelial cells. The tumor-penetrating properties of iRGD are mediated by a second sequence motif, R/KXXR/K. This C-end Rule (or CendR) motif is active only when the second basic residue is exposed at the C-terminus of the peptide. Proteolytic processing of iRGD in tumors activates the cryptic CendR motif, which then binds to neuropilin-1 activating an endocytic bulk transport pathway through tumor tissue. Phage screening has also yielded tumor-penetrating peptides that function like iRGD in activating the CendR pathway, but bind to a different primary receptor. Moreover, novel tumor-homing peptides can be constructed from tumor-homing motifs, CendR elements and protease cleavage sites. Pathologies other than tumors can be targeted with tissue-penetrating peptides, and the primary receptor can also be a vascular "zip code" of a normal tissue. The CendR technology provides a solution to a major problem in tumor therapy, poor penetration of drugs into tumors. The tumor-penetrating peptides are capable of taking a payload deep into tumor tissue in mice, and they also penetrate into human tumors ex vivo. Targeting with these peptides specifically increases the accumulation in tumors of a variety of drugs and contrast agents, such as doxorubicin, antibodies, and nanoparticle-based compounds. Remarkably the drug to be targeted does not have to be coupled to the peptide; the bulk transport system activated by the peptide sweeps along any compound that is present in the blood.

Proteasome activator complex PA28 identified as an accessible target in prostate cancer by in vivo selection of human antibodies

Antibody cancer therapies rely on systemically accessible targets and suitable antibodies that exert a functional activity or deliver a payload to the tumor site. Here, we present proof-of-principle of in vivo selection of human antibodies in tumor-bearing mice that identified a tumor-specific antibody able to deliver a payload and unveils the target antigen. By using an ex vivo enrichment process against freshly disaggregated tumors to purge the repertoire, in combination with in vivo biopanning at optimized phage circulation time, we have identified a human domain antibody capable of mediating selective localization of phage to human prostate cancer xenografts. Affinity chromatography followed by mass spectrometry analysis showed that the antibody recognizes the proteasome activator complex PA28. The specificity of soluble antibody was confirmed by demonstrating its binding to the active human PA28αβ complex. Whereas systemically administered control phage was confined in the lumen of blood vessels of both normal tissues and tumors, the selected phage spread from tumor vessels into the perivascular tumor parenchyma. In these areas, the selected phage partially colocalized with PA28 complex. Furthermore, we found that the expression of the α subunit of PA28 [proteasome activator complex subunit 1 (PSME1)] is elevated in primary and metastatic human prostate cancer and used anti-PSME1 antibodies to show that PSME1 is an accessible marker in mouse xenograft tumors. These results support the use of PA28 as a tumor marker and a potential target for therapeutic intervention in prostate cancer.

Using shape effects to target antibody-coated nanoparticles to lung and brain endothelium

Vascular endothelium offers a variety of therapeutic targets for the treatment of cancer, cardiovascular diseases, inflammation, and oxidative stress. Significant research has been focused on developing agents to target the endothelium in diseased tissues. This includes identification of antibodies against adhesion molecules and neovascular expression markers or peptides discovered using phage display. Such targeting molecules also have been used to deliver nanoparticles to the endothelium of the diseased tissue. Here we report, based on in vitro and in vivo studies, that the specificity of endothelial targeting can be enhanced further by engineering the shape of ligand-displaying nanoparticles. In vitro studies performed using microfluidic systems that mimic the vasculature (synthetic microvascular networks) showed that rod-shaped nanoparticles exhibit higher specific and lower nonspecific accumulation under flow at the target compared with their spherical counterparts. Mathematical modeling of particle-surface interactions suggests that the higher avidity and specificity of nanorods originate from the balance of polyvalent interactions that favor adhesion and entropic losses as well as shear-induced detachment that reduce binding. In vivo experiments in mice confirmed that shape-induced enhancement of vascular targeting is also observed under physiological conditions in lungs and brain for nanoparticles displaying anti-intracellular adhesion molecule 1 and anti-transferrin receptor antibodies.

Sequence dependence of C-end rule peptides in binding and activation of neuropilin-1 receptor

Neuropilin-1 (NRP-1) is a hub receptor that plays an essential role in angiogenesis and vascular permeability. It is over-expressed in the new blood vessels grown by tumor cells and is a target for anti-tumor treatments. Peptides that expose the consensus sequence R/K/XXR/K at the C-terminus (C-end rule or CendR peptides) bind to NRP-1 and are internalized into the cell. We used peptide phage display binding assays and molecular dynamics (MD) simulations to study the potential role of the central residues of CendR peptides in binding and activation of the NRP-1 receptor. The high stability of RPAR-receptor domain complex stems from the formation of a characteristic pattern of three hydrogen bonds between the peptide C-terminus and the residues in the NRP-1 loop III. Any changes in the peptide structure that fail to preserve this triad result in a less-stable complex. We performed a systematic study of RXXR mutants, where X=A/D/S/R/P, in order to test the effect of replacement of A or P on the binding capabilities. Our results, both experimental and computational, show that RRAR, RDAR, RPDR, RPRR and RPPR are capable of binding NRP-1. However, only RPPR and RPRR segments form an optimal organization around loop III with low potential energy. In other analogs, the absence of these stabilizing interactions always results in higher potential energy of the complexes. The binding of RPAR analogs does not guarantee receptor activation; only stable complexes that are properly stabilized via loop III appear able to trigger NRP-1 activation.

Identification of human embryonic progenitor cell targeting peptides using phage display

Human pluripotent stem (hPS) cells are capable of differentiation into derivatives of all three primary embryonic germ layers and can self-renew indefinitely. They therefore offer a potentially scalable source of replacement cells to treat a variety of degenerative diseases. The ability to reprogram adult cells to induced pluripotent stem (iPS) cells has now enabled the possibility of patient-specific hPS cells as a source of cells for disease modeling, drug discovery, and potentially, cell replacement therapies. While reprogramming technology has dramatically increased the availability of normal and diseased hPS cell lines for basic research, a major bottleneck is the critical unmet need for more efficient methods of deriving well-defined cell populations from hPS cells. Phage display is a powerful method for selecting affinity ligands that could be used for identifying and potentially purifying a variety of cell types derived from hPS cells. However, identification of specific progenitor cell-binding peptides using phage display may be hindered by the large cellular heterogeneity present in differentiating hPS cell populations. We therefore tested the hypothesis that peptides selected for their ability to bind a clonal cell line derived from hPS cells would bind early progenitor cell types emerging from differentiating hPS cells. The human embryonic stem (hES) cell-derived embryonic progenitor cell line, W10, was used and cell-targeting peptides were identified. Competition studies demonstrated specificity of peptide binding to the target cell surface. Efficient peptide targeted cell labeling was accomplished using multivalent peptide-quantum dot complexes as detected by fluorescence microscopy and flow cytometry. The cell-binding peptides were selective for differentiated hPS cells, had little or no binding on pluripotent cells, but preferential binding to certain embryonic progenitor cell lines and early endodermal hPS cell derivatives. Taken together these data suggest that selection of phage display libraries against a clonal progenitor stem cell population can be used to identify progenitor stem cell targeting peptides. The peptides may be useful for monitoring hPS cell differentiation and for the development of cell enrichment procedures to improve the efficiency of directed differentiation toward clinically relevant human cell types.

The non-peptidic part determines the internalization mechanism and intracellular trafficking of peptide amphiphiles

Background

Peptide amphiphiles (PAs) are a class of amphiphilic molecules able to self-assemble into nanomaterials that have shown efficient in vivo targeted delivery. Understanding the interactions of PAs with cells and the mechanisms of their internalization and intracellular trafficking is critical in their further development for therapeutic delivery applications.

Methodology/Principal Findings

PAs of a novel, cell- and tissue-penetrating peptide were synthesized possessing two different lipophilic tail architectures and their interactions with prostate cancer cells were studied in vitro. Cell uptake of peptides was greatly enhanced post-modification. Internalization occurred via lipid-raft mediated endocytosis and was common for the two analogs studied. On the contrary, we identified the non-peptidic part as the determining factor of differences between intracellular trafficking and retention of PAs. PAs composed of di-stearyl lipid tails linked through poly(ethylene glycol) to the peptide exhibited higher exocytosis rates and employed different recycling pathways compared to ones consisting of di-palmitic-coupled peptides. As a result, cell association of the former PAs decreased with time.

Conclusions/Significance

Control over peptide intracellular localization and retention is possible by appropriate modification with synthetic hydrophobic tails. We propose this as a strategy to design improved peptide-based delivery systems.

De novo design of a tumor-penetrating peptide

Poor penetration of antitumor drugs into the extravascular tumor tissue is often a major factor limiting the efficacy of cancer treatments. Our group has recently described a strategy to enhance tumor penetration of chemotherapeutic drugs through use of iRGD peptide (CRGDK/RGPDC). This peptide comprises two sequence motifs: RGD, which binds to αvβ3/5 integrins on tumor endothelia and tumor cells, and a cryptic CendR motif (R/KXXR/K-OH). Once integrin binding has brought iRGD to the tumor, the peptide is proteolytically cleaved to expose the cryptic CendR motif. The truncated peptide loses affinity for its primary receptor and binds to neuropilin-1, activating a tissue penetration pathway that delivers the peptide along with attached or co-administered payload into the tumor mass. Here, we describe the design of a new tumor-penetrating peptide based on the current knowledge of homing sequences and internalizing receptors. The tumor-homing motif in the new peptide is the NGR sequence, which binds to endothelial CD13. The NGR sequence was placed in the context of a CendR motif (RNGR), and this sequence was embedded in the iRGD framework. The resulting peptide (CRNGRGPDC, iNGR) homed to tumor vessels and penetrated into tumor tissue more effectively than the standard NGR peptide. iNGR induced greater tumor penetration of coupled nanoparticles and co-administered compounds than NGR. Doxorubicin given together with iNGR was significantly more efficacious than the drug alone. These results show that a tumor-specific, tissue-penetrating peptide can be constructed from known sequence elements. This principle may be useful in designing tissue-penetrating peptides for other diseases.

Tools from viruses: bacteriophage successes and beyond

Viruses are ubiquitous and can infect any of the three existing cellular lineages (Archaea, Bacteria and Eukarya). Despite the persisting negative public perception of these entities, scientists learnt how to domesticate some of them. The study of molecular mechanisms essential to the completion of viral cycles has greatly contributed to deciphering fundamental processes in biology. Nowadays, viruses have entered the biotechnological era and numerous applications have already been developed. Viral-derived tools are used to manipulate genetic information, detect, diagnose, control and cure infectious diseases, or even design new structural assemblies. With the recent advances in the field of metagenomics, an overwhelming amount of information on novel viruses has become available. As current tools have been historically developed from a limited number of viruses, the potential of discoveries from new archaeal, bacterial and eukaryotic viruses may be limited only by our understanding of the multiple facets of viral cycles.

Peptides as targeting elements and tissue penetration devices for nanoparticles

The use of nanoparticles in medicine (nanomedicine) has recently become an intensely studied field. Nanoparticles carrying drugs and imaging agents have already reached the clinic, but they are essentially passive delivery vehicles, not what are referred to as "smart" nanoparticles. An important function to add to make nanoparticles smarter is active homing to the target tissue. It makes nanoparticles accumulate in the target tissue at higher concentrations than would be the case without this feature, increasing therapeutic efficacy and reducing side effects. This review discusses the recent developments in the nanoparticle targeting field with emphasis on peptides that home to vascular "zip codes" in target tissues and provide a tissue- and cell-penetrating function.

Filamentous bacteriophage fd as an antigen delivery system in vaccination

Peptides displayed on the surface of filamentous bacteriophage fd are able to induce humoral as well as cell-mediated immune responses, which makes phage particles an attractive antigen delivery system to design new vaccines. The immune response induced by phage-displayed peptides can be enhanced by targeting phage particles to the professional antigen presenting cells, utilizing a single-chain antibody fragment that binds dendritic cell receptor DEC-205. Here, we review recent advances in the use of filamentous phage fd as a platform for peptide vaccines, with a special focus on the use of phage fd as an antigen delivery platform for peptide vaccines in Alzheimer's Disease and cancer.

Design of target-seeking antifibrotic compounds

Selective delivery of drugs and biotherapeutics to the site of disease (synaphic targeting) has a number of advantages. First, the enhanced accumulation of the therapeutic compound at the target tissue increases drug efficacy without increasing side effects. Alternatively, the dose of the drug can be lowered to reduce the side effects. On the practical level, when a drug is difficult or expensive to make, being able to lower the dose may be the key to commercial viability. Certain targeting systems can change the distribution of the drug in a beneficial way. Examples include wider distribution and deeper penetration of the drug in the target tissue, active intracellular targeting when desirable, and even targeting to a particular subcellular organelle. In this chapter, we illustrate these principles by describing the development of a targeting system for an antifibrotic biotherapeutic, decorin. The system is based on vascular homing peptide (sequence: CARSKNKDC; referred to as CAR) that specifically recognizes angiogenic blood vessels in injured (regenerating) and inflammatory tissues and can deliver a payload to such tissues with high selectivity. So far, the CAR-targeted decorin has been shown to promote tissue repair with reduced scarring in a skin wound model, but this biotherapeutic can potentially be used in other injuries and in various fibrotic diseases.