Parallel in Vivo and in Vitro Selection Using Phage Display Identifies Protease-dependent Tumor-targeting Peptides

We recently developed activatable cell-penetrating peptides (ACPPs) that target contrast agents to in vivo sites of matrix metalloproteinase activity, such as tumors. Here we use parallel in vivo and in vitro selection with phage display to identify novel tumor-homing ACPPs with no bias for primary sequence or target protease. Specifically, phage displaying a library of ACPPs were either injected into tumor-bearing mice, followed by isolation of cleaved phage from dissected tumor, or isolated based on selective cleavage by extracts of tumor versus normal tissue. Selected sequences were synthesized as fluorescently labeled peptides, and tumor-specific cleavage was confirmed by digestion with tissue extracts. The most efficiently cleaved peptide contained the substrate sequence RLQLKL and labeled tumors and metastases from several cancer models with up to 5-fold contrast. This uniquely identified ACPP was not cleaved by matrix metalloproteinases or various coagulation factors but was efficiently cleaved by plasmin and elastases, both of which have been shown to be aberrantly overexpressed in tumors. The identification of an ACPP that targets tumor expressed proteases without rational design highlights the value of unbiased selection schemes for the development of potential therapeutic agents.     

Identification of High-Risk Plaques by MRI and Fluorescence Imaging in a Rabbit Model of Atherothrombosis

IntroductionThe detection of atherosclerotic plaques at risk for disruption will be greatly enhanced by molecular probes that target vessel wall biomarkers. Here, we test if fluorescently-labeled Activatable Cell Penetrating Peptides (ACPPs) could differentiate stable plaques from vulnerable plaques that disrupt, forming a luminal thrombus. Additionally, we test the efficacy of a combined ACPP and MRI technique for identifying plaques at high risk of rupture.ConclusionsOur targeted fluorescence ACPP probes distinguished disrupted plaques from stable plaques with high sensitivity and specificity. The combination of anatomic, MRI-derived predictors for disruption and ACPP uptake can further improve the power for identification of high-risk plaques and suggests future development of ACPPs with molecular MRI as a readout.     

Improved mitochondrial targeting effect of HPMA copolymer by SS20 peptide mediation and nonendocytosis pathway

Mitochondrion plays an important role in executing cell programmed death pathway. Therefore, drugs designed to target mitochondria are supposed to make superior contributions to cancer therapy. However, the problem that drugs or drug delivery systems being sequestrated in endosomes/lysosomes needs to be solved for effective drug delivery. Here, mitochondrial targeting and nonendocytic cell entry peptide SS20 modified HPMA copolymer (P‐FITC‐SS20) was synthesized. With SS20 peptide modification, the uptake behavior of HPMA copolymers changed remarkably compared with unmodified ones. The internalization of P‐FITC‐SS20 was not influenced by endocytic inhibitors and temperature. Further, the internalized copolymers were not trapped in endosomes/lysosomes. Although cellular uptake of HPMA copolymer was decreased after SS20 peptide modification, SS20 peptide significantly improved mitochondrial accumulation of HPMA copolymers due to its outstanding mitochondrial targeting ability. Moreover, owing to lower susceptibility to macrophagocyte in blood, P‐SS20‐Cy5 showed longer blood circulation time and enhanced tumor accumulation. The current study validated that SS20 peptide modification is a promising strategy for mitochondrial targeting drug delivery systems and can be further applied to mitochondria associated diseases to improve therapeutic efficacy.     

Tat-conjugated synthetic macromolecules facilitate cytoplasmic drug delivery to human ovarian carcinoma cells

We have synthesized N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer-cell penetrating peptide Tat conjugates and evaluated their subcellular distribution in A2780 human ovarian carcinoma cells by confocal fluorescence microscopy and subcellular fractionation. Our data indicate the transport of these conjugates by a single Tat molecule to both the cytoplasm and nucleus via a nonendocytotic and concentration independent process. The uptake was observed to occur within 3 min, as confirmed by live cell microscopy. In contrast, HPMA copolymers lacking the Tat peptide were internalized solely by endocytosis. For the first time, Tat-mediated cytoplasmic delivery of a polymer bound anticancer drug, doxorubicin, was also demonstrated. These findings establish the feasibility of overcoming major cellular and subcellular obstacles to intracellular macromolecular delivery and hold great promise for the development of polymer-based systems for the cytoplasmic delivery of therapeutic molecules.     

Photochemically enhanced transduction of polymer-complexed adenovirus targeted to the epidermal growth factor receptor

BACKGROUND: The development of methods for specific delivery of genes into target tissues is an important issue for the further progress of gene therapy. Biological and physical targeting techniques may be combined to redirect gene therapy vectors to specific cells and enhance gene transfer. METHODS: The polymer poly(2-(dimethylamino)ethyl methacrylate) (pDMAEMA) was conjugated with avidin or poly(ethylene glycol) (PEG) and complexed with adenovirus serotype 5 (Ad5). Targeting of polymer-coated Ad5 to the epidermal growth factor receptor (EGFR) was accomplished by the binding of biotin-EGF to pDMAEMA-avidin. A photochemical treatment procedure using photosensitizer and light was applied to increase transduction with EGFR-targeted viral complexes. RESULTS: pDMAEMA-avidin efficiently enhanced transduction through unspecific viral uptake into cells, while pDMAEMA-PEG provided charge shielding of the complexes and increased the specificity to EGFR when biotin-EGF ligands were used. Transduction of PEG-containing, EGFR-targeted viral complexes was inhibited by 66% in coxsackie and adenovirus receptor (CAR)-deficient RD cells and by 47% in CAR-expressing DU 145 cells in receptor antibody experiments. The photochemical treatment had a substantial effect on transduction, enhancing the percentage of reporter gene positive cells from 20% to 75% of the total viable RD cell population and from 10% to 70% in DU 145 cells. CONCLUSION: Photochemical treatment of cells infected with targeted viral vectors exhibiting a neutral surface charge is a potent method for enhancing transgene expression.     

Aptamer modification improves the adenoviral transduction of malignant glioma cells

Adenovirus has shown increasing promise in the gene-viral therapy for glioblastoma, a treatment strategy that relies on the delivery of viruses or transgenes into tumor cells. However, targeting of adenovirus to human glioblastoma remains a challenge due to the low expression level of coxsackie and adenovirus receptor (CAR) in glioma cells. Aptamers are small and highly structured single-stranded oligonucleotides that bind at high affinity to a target molecule, and are good candidates for targeted imaging and therapy. In this study, to construct an aptamer-modified Ad5, we first genetically modified the HVR5 of Ad hexon by biotin acceptor peptide (BAP), which would be metabolically biotinylated during production in HEK293 cells, and then attached the biotin labeled aptamer to the modified Ad through avidin–biotin binding. The aptamers used in this study includes AS1411 and GBI-10. The former is a DNA aptamer that can bind to nucleolin, a nuclear matrix protein found on the surface of cancer cells. The latter is a DNA aptamer that can recognize the extracellular matrix protein tenascin-C on the surface of human glioblastoma cells. To examine if aptamer-modification of the hexon protein could improve the adenoviral transduction efficiency, a glioblastoma cell line, U251, was transduced with aptamer-modified Ads. The transduction efficiency of AS1411- or GBI-10-modified Ad was approximately 4.1-fold or 5.2-fold higher than that of the control. The data indicated that aptamer modified adenovirus would be a useful tool for cancer gene therapy.     

Polymerizable Fab' antibody fragments for targeting of anticancer drugs.

We have designed a new pathway for the synthesis of targeted polymeric drug delivery systems, using polymerizable antibody Fab' fragments (MA-Fab'). The targeted systems can be directly prepared by copolymerization of the MA-Fab', N-(2-hydroxypropyl)methacrylamide (HPMA) and drug-containing monomers. Both MA-Fab' and the Fab'-targeted copolymers can effectively bind to target cells. An MA-Fab' (from OV-TL 16 Ab) targeted HPMA copolymer containing mesochlorin e6 (Mce6) was synthesized by copolymerization of MA-Fab', HPMA, and MA-GFLG-Mce6. The targeted copolymer exhibited a higher cytotoxicity toward OVCAR-3 human ovarian carcinoma cells than the nontargeted Mce6-containing copolymer or free Mce6. The targeted copolymer was internalized more efficiently by OVCAR-3 cells than the nontargeted copolymer.     

Traceless Bioresponsive Shielding of Adenovirus Hexon with HPMA Copolymers Maintains Transduction Capacity In Vitro and In Vivo

Capsid surface shielding of adenovirus vectors with synthetic polymers is an emerging technology to reduce unwanted interactions of the vector particles with cellular and non-cellular host components. While it has been shown that attachment of shielding polymers allows prevention of undesired interactions, it has become evident that a shield which is covalently attached to the vector surface can negatively affect gene transfer efficiency. Reasons are not only a limited receptor-binding ability of the shielded vectors but also a disturbance of intracellular trafficking processes, the latter depending on the interaction of the vector surface with the cellular transport machinery. A solution might be the development of bioresponsive shields that are stably maintained outside the host cell but released upon cell entry to allow for efficient gene delivery to the nucleus. Here we provide a systematic comparison of irreversible versus bioresponsive shields based on synthetic N-(2-hydroxypropyl)methacrylamide (HPMA) copolymers. In addition, the chemical strategy used for generation of the shield allowed for a traceless bioresponsive shielding, i.e., polymers could be released from the vector particles without leaving residual linker residues. Our data demonstrated that only a bioresponsive shield maintained the high gene transfer efficiency of adenovirus vectors both in vitro and in vivo. As an example for bioresponsive HPMA copolymer release, we analyzed the in vivo gene transfer in the liver. We demonstrated that both the copolymer''s charge and the mode of shielding (irreversible versus traceless bioresponsive) profoundly affected liver gene transfer and that traceless bioresponsive shielding with positively charged HPMA copolymers mediated FX independent transduction of hepatocytes. In addition, we demonstrated that shielding with HPMA copolymers can mediate a prolonged blood circulation of vector particles in mice. Our results have significant implications for the future design of polymer-shielded Ad and provide a deeper insight into the interaction of shielded adenovirus vector particles with the host after systemic delivery.     

Adenovirus with hexon Tat-protein transduction domain modification exhibits increased therapeutic effect in experimental neuroblastoma and neuroendocrine tumors

Adenovirus serotype 5 (Ad5) is widely used as an oncolytic agent for cancer therapy. However, its infectivity is highly dependent on the expression level of coxsackievirus-adenovirus receptor (CAR) on the surfaces of tumor cells. Furthermore, infected cells overproduce adenovirus fiber proteins, which are released prior to cell lysis. The released fibers block CAR on noninfected neighboring cells, thereby preventing progeny virus entry. Our aim was to add a CAR-independent infection route to Ad5 to increase the infectivity of tumor cells with low CAR expression and prevent the fiber-masking problem. We constructed Ad5 viruses that encode the protein transduction domain (PTD) of the HIV-1 Tat protein (Tat-PTD) in hypervariable region 5 (HVR5) of the hexon protein. Tat-PTD functions as a cell-penetrating peptide, and Tat-PTD-modified Ad5 showed a dramatic increased transduction of CAR-negative cell lines compared to unmodified vector. Moreover, while tumor cell infectivity was severely reduced for Ad5 in the presence of fiber proteins, it was only marginally reduced for Tat-PTD-modified Ad5. Furthermore, because of the sequence alteration in the hexon HVR, coagulation factor X-mediated virus uptake was significantly reduced. Mice harboring human neuroblastoma and neuroendocrine tumors show suppressed tumor growths and prolonged survival when treated with Tat-PTD-modified oncolytic viruses. Our data suggest that modification of Ad5 with Tat-PTD in HVR5 expands its utility as an oncolytic agent.     

Screening and characterization of a novel high‐efficiency tumor‐homing cell‐penetrating peptide from the buffalo cathelicidin family

Targeted delivery of antitumor drugs is especially important for tumor therapy. Cell‐penetrating peptides (CPPs) have been shown to be very effective drug carriers for tumor therapy. However, most CPPs lack tumor cell specificity. Here, we identified a highly efficient CPP, CAT, from the newly identified buffalo‐derived cathelicidin family, which exhibits a preferential binding capacity for multiple tumor cell lines and delivers carried drug molecules into cells. CAT showed an approximately threefold to sixfold higher translocation efficiency than some reported cell‐penetrating antimicrobial peptides, including the well‐known classical CPP TAT. Moreover, the delivery efficiency of CAT was greater in a variety of tested tumor cells than in normal cells, especially for the human hepatoma cell line SMMC‐7721, for which delivery was 7 times more efficient than the normal human embryonic lung cell line MRC‐5, according to fluorescent labeling experiment results. CAT was conjugated to the Momordica charantia‐derived type‐I ribosome‐inactivating protein MAP 30, and the cytotoxicity of the MAP 30‐CAT fusion protein in the tumor cell line SMMC‐7721 was significantly enhanced compared with that of the unconjugated MAP 30. The IC50 value of MAP 30‐CAT was approximately 83 times lower than the IC50 value of the original MAP 30. Interestingly, the IC50 value of MAP 30 alone for MRC‐5 was approximately twofold higher than the value for SMMC‐7721, showing a small difference. However, when MAP 30 was conjugated to CAT, the difference in IC50 values between the two cell lines was significantly increased by 38‐fold. The results of the flow cytometric detection of apoptosis revealed that the increase in cytotoxicity after CAT conjugation was mainly caused by the increased induction of apoptosis by the fusion protein. These results suggest that CAT, as a novel tumor‐homing CPP, has great potential in drug delivery applications in vivo and will be beneficial to the development of tumor therapeutics.     

Adenovirus-Mediated Expression of the p14 Fusion-Associated Small Transmembrane Protein Promotes Cancer Cell Fusion and Apoptosis In Vitro but Does Not Provide Therapeutic Efficacy in a Xenograft Mouse Model of Cancer

Adenoviruses (Ads) are used in numerous preclinical and clinical studies for delivery of anti-cancer therapeutic genes. Unfortunately, Ad has a poor ability to distribute throughout a tumor mass after intratumoral injection, and infects cells primarily within the immediate area of the injection tract. Thus, Ad-encoded transgene expression is typically limited to only a small percentage of cells within the tumor. One method to increase the proportion of the tumor impacted by Ad is through expression of fusogenic proteins. Infection of a single cell with an Ad vector encoding a fusogenic protein should lead to syncytium formation with adjacent cells, effectively spreading the effect of Ad and Ad-encoded therapeutic transgenes to a greater percentage of the tumor mass. Moreover, syncytium formation can be cytotoxic, suggesting that such proteins may be effective sole therapeutics. We show that an early region 1 (E1)-deleted Ad expressing reptilian reovirus p14 fusion-associated small transmembrane (FAST) protein caused extensive cell fusion in the replication-permissive 293 cell line and at high multiplicity of infection in non-permissive human lung adenocarcinoma A549 cells in vitro. FAST protein expression in the A549 cancer cell line led to a loss of cellular metabolic activity and membrane integrity, which correlated with induction of apoptosis. However, in an A549 xenograft CD-1 nude mouse cancer model, Ad-mediated FAST gene delivery did not induce detectable cell fusion, reduce tumor burden nor enhance mouse survival compared to controls. Taken together, our results show that, although AdFAST can enhance cancer cell killing in vitro, it is not effective as a sole therapeutic in the A549 tumor model in vivo.     

Genetically targeted adenovirus vector directed to CD40-expressing cells

The success of gene therapy depends on the specificity of transgene delivery by therapeutic vectors. The present study describes the use of an adenovirus (Ad) fiber replacement strategy for genetic targeting of the virus to human CD40, which is expressed by a variety of diseased tissues. The tropism of the virus was modified by the incorporation into its capsid of a protein chimera comprising structural domains of three different proteins: the Ad serotype 5 fiber, phage T4 fibritin, and the human CD40 ligand (CD40L). The tumor necrosis factor-like domain of CD40L retains its functional tertiary structure upon incorporation into this chimera and allows the virus to use CD40 as a surrogate receptor for cell entry. The ability of the modified Ad vector to infect CD40-positive dendritic cells and tumor cells with a high efficiency makes this virus a prototype of choice for the derivation of therapeutic vectors for the genetic immunization and targeted destruction of tumors.     

Preparation of a cathepsin D sensitive near-infrared fluorescence probe for imaging.

A variety of proteases are overexpressed or activated during pathogenesis and represent important targets for therapeutic drugs. We have previously shown that optical imaging probes sensitive in the near-infrared fluorescence (NIRF) spectrum can be used for in vivo imaging of enzyme activity. In the current study, we show that these probes can be designed with specificity for specific enzymes, for example, cathepsin D which is known to be overexpressed in many tumors. A NIR cyanine fluorochrome served as the optical reporter and was attached to the amino terminal of an 11 amino acid peptide sequence with specificity for cathepsin D. The peptides were subsequently attached to a synthetic graft copolymer for efficient tumoral delivery. The close spatial proximity of the multiple fluorochromes resulted in quenching of fluorescence in the bound state. A 350-fold signal amplification was observed post cleavage during in vitro testing. Cell culture experiments using a rodent tumor cell line stably transfected with human cathepsin D confirmed enzyme specific activation within cells. This sequence but not a scrambled control sequence showed enzyme specificity in vitro. We conclude that activatable NIRF optical probes can be synthesized to potentially probe for specific enzymes in living organisms.     

Temperature-sensitive poly(N-(2-hydroxypropyl)methacrylamide mono/dilactate)-coated liposomes for triggered contents release

We prepared thermosensitive poly( N-(2-hydroxypropyl)methacrylamide mono/dilactate) (pHPMA mono/dilactate) polymer and studied temperature-triggered contents release from polymer-coated liposomes. HPMA mono/dilactate polymer was synthesized with a cholesterol anchor suitable for incorporation in the liposomal bilayers and with a cloud point (CP) temperature of the polymer slightly above normal body temperature (42 degrees C). Dynamic light scattering (DLS) measurements showed that whereas the size of noncoated liposomes remained stable upon raising the temperature from 25 to 46 degrees C, polymer-coated liposomes aggregated around 43 degrees C. Also, noncoated liposomes loaded with calcein showed hardly any leakage of the fluorescent marker when heated to 46 degrees C. However, polymer-coated liposomes showed a high degree of temperature-triggered calcein release above the CP of the polymer. Likely, liposome aggregation and bilayer destabilization are triggered because of the precipitation of the thermosensitive polymer above its CP onto the liposomal bilayers, followed by permeabilization of the liposomal membrane. This study demonstrates that liposomes surface-modified with HPMA mono/dilactate copolymer are attractive systems for achieving temperature-triggered contents release.     

Coxsackievirus and adenovirus receptor expression in non-malignant lung tissues and clinical lung cancers

Adenoviral vector mediated gene delivery has been applied in clinical trials and mechanistic studies to explore new treatment approaches for lung cancers. The expression of coxsackievirus adenovirus receptor (CAR), the primary receptor for the most commonly used adenovirus serotype 5 (Ad5)-based vectors, predominantly determines the permissiveness of lung cancer cells. CAR expression is also suggested to modulate tumor cell proliferation capacity. Here, we studied CAR expression in archival lung cancer specimens by using well-characterized CAR 72 antibodies. High levels of CAR expression were observed in most of the 32 cases of squamous cell carcinoma lung cancers and in all the five cases of small cell lung cancers investigated. In contrast, high levels of CAR expression were detected only in 6 of 22 adenocarcinoma lung cancers. The relative levels of CAR expression did not correlate with the pathologic grade in lung cancers, and was thus inconsistent with a role of modulating cancer cell proliferation. Of note, CAR expression was not detected in non-malignant alveolar cells. Our data suggest a preferred utility of Ad5 vector mediated gene delivery to squamous cell carcinoma lung cancers, small cell lung cancers, but not to the majority of adenocarcinoma lung cancers.     

Improved targeting of ligand-modified adenovirus as a new near infrared fluorescence tumor imaging probe

E1/E3-deleted Adenovirus 5 (Ad.5) possesses a great potential in gene therapy because of its high efficacy in gene transfer and low toxicity. Studies have shown that Coxsackie-Adenovirus receptor (CAR) is the determinant factor for the targeting of Adenovirus vectors. To extend the natural targeting of Ad to low CAR expressing tumors, we covalently attached folic acid (FA) to E1/E3-deleted Ad.5 capsids. Near-infrared (NIR) fluorescent dye ICG-Der-02 was subsequently conjugated with FA-Ad particles for in vivo imaging. The cell experiments and acute toxicity studies demonstrated the low toxicity of FA-Ad-ICG02 to normal cell/tissues. The dynamic behavior and targeting ability of FA-Ad-ICG02 to different tumors were investigated by NIR fluorescence imaging. In vitro and in vivo studies demonstrated its high targeting capability to CAR or FR positive tumors. The results support the potential of using ligand-modified Ad probe for tumor diagnosis and targeted therapy.     

Evaluation of protein-N-(2-hydroxypropyl)methacrylamide copolymer conjugates as targetable drug carriers. 1. Binding, pinocytic uptake and intracellular distribution of transferrin and anti-transferrin receptor antibody conjugates

The transferrin receptor of human skin fibroblasts was studied as an in vitro model target antigen receptor for interaction with protein-polymer conjugates having potential for targeted drug delivery. Pinocytic uptake of 125I-labelled N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer conjugated to monoclonal antibody B3/25 (specific for the transferrin receptor) or transferrin was up to 9-fold greater than uptake of the parent HPMA copolymer. The ability of these conjugates to bind specifically was confirmed by Scatchard analysis. Pinocytic internalisation was dependent on the molecular mass of the conjugate. Intracellular routing following internalisation was evaluated using density-gradient centrifugation. Unmodified HPMA copolymer was transferred via the endosomal compartment into secondary lysosomes, where, being resistant to degradation, it accumulated. Although the majority of endocytosed transferrin is recycled via the endosome, it was shown that any transferrin reaching the lysosomes was rapidly degraded and low-molecular-weight degradation products were released. Monoclonal antibody B3/25 showed a subcellular distribution consistent with prolongation on the cell surface, followed by internalisation and subcellular trafficking, via endosomes, into the lysosomal compartment, with subsequent degradation. Conjugation of protein to HPMA copolymer increased lysosomal accumulation of polymer up to 9-fold, with no detectable degradation of conjugate. The data presented here have implications regarding clinical potential of protein-HPMA copolymer conjugates designed for lysosomotropic drug delivery.     

Mitochondrial targeting of a cationic amphiphilic polyproline helix

The development of cell penetrating peptides (CPPs) for the cellular delivery of attached cargo is an area of growing interest. Many CPPs, however, are found trapped within endosomes, thereby limiting their use as drug delivery agents with sub-cellular applications. Herein, we detail the properties of a highly efficient class of CPPs, cationic amphiphilic polyproline helices (CAPHs), that are found localized to the mitochondria by direct transport into cells.     

Comparative biodistribution of PAMAM dendrimers and HPMA copolymers in ovarian-tumor-bearing mice

The biodistribution profile of a series of linear N-(2-hydroxylpropyl)methacrylamide (HPMA) copolymers was compared with that of branched poly(amido amine) dendrimers containing surface hydroxyl groups (PAMAM-OH) in orthotopic ovarian-tumor-bearing mice. Below an average molecular weight (MW) of 29 kDa, the HPMA copolymers were smaller than the PAMAM-OH dendrimers of comparable molecular weight. In addition to molecular weight, hydrodynamic size and polymer architecture affected the biodistribution of these constructs. Biodistribution studies were performed by dosing mice with (125)iodine-labeled polymers and collecting all major organ systems, carcass, and excreta at defined time points. Radiolabeled polymers were detected in organ systems by measuring gamma emission of the (125)iodine radiolabel. The hyperbranched PAMAM dendrimer, hydroxyl-terminated, generation 5 (G5.0-OH), was retained in the kidney over 1 week, whereas the linear HPMA copolymer of comparable molecular weight was excreted into the urine and did not show persistent renal accumulation. PAMAM dendrimer, hydroxyl-terminated, generation 6.0 (G6.0-OH), was taken up by the liver to a higher extent, whereas the HPMA copolymer of comparable molecular weight was observed to have a plasma exposure three times that of this dendrimer. Tumor accumulation and plasma exposure were correlated with the hydrodynamic sizes of the polymers. PAMAM dendrimer, hydroxyl-terminated, generation 7.0 (G7.0-OH), showed extended plasma circulation, enhanced tumor accumulation, and prolonged retention with the highest tumor/blood ratio for the polymers under study. Head-to-head comparative study of HPMA copolymers and PAMAM dendrimers can guide the rational design and development of carriers based on these systems for the delivery of bioactive and imaging agents.     

Adenovirus vector pseudotyping in fiber-expressing cell lines: improved transduction of Epstein-Barr virus-transformed B cells

While adenovirus (Ad) gene delivery vectors are useful in many gene therapy applications, their broad tropism means that they cannot be directed to a specific target cell. There are also a number of cell types involved in human disease which are not transducible with standard Ad vectors, such as Epstein-Barr virus (EBV)-transformed B lymphocytes. Adenovirus binds to host cells via the viral fiber protein, and Ad vectors have previously been retargeted by modifying the fiber gene on the viral chromosome. This requires that the modified fiber be able to bind to the cell in which the vector is grown, which prevents truly specific vector targeting. We previously reported a gene delivery system based on a fiber gene-deleted Ad type 5 (Ad5) vector (Ad5.betagal.DeltaF) and packaging cells that express the viral fiber protein. Expression of different fibers in packaging cells will allow Ad retargeting without modifying the viral chromosome. Importantly, fiber proteins which can no longer bind to the producer cells can also be used. Using this approach, we generated for the first time pseudotyped Ad5.betagal.DeltaF particles containing either the wild-type Ad5 fiber protein or a chimeric fiber with the receptor-binding knob domain of the Ad3 fiber. Particles equipped with the chimeric fiber bound to the Ad3 receptor rather than the coxsackievirus-adenovirus receptor protein used by Ad5. EBV-transformed B lymphocytes were infected efficiently by the Ad3-pseudotyped particles but poorly by virus containing the Ad5 fiber protein. The strategy described here represents a broadly applicable method for targeting gene delivery to specific cell types.