A mosaic adenovirus possessing serotype Ad5 and serotype Ad3 knobs exhibits expanded tropism

Tropism and transduction of oncolytic adenovirus 5 vectors in cancer therapy: Focus on fiber chimerism and mosaicism, hexon and pIX

The cellular internalization (infection of cells) of adenovirus 5 (Ad5) is mediated by the initial attachment of the globular knob domain of the capsid fiber protein to the cell surface coxsackievirus and adenovirus receptor (CAR), then followed by the interaction of the virus penton base proteins with cellular integrins. In tumors, there is a substantial intra- and intertumoral variability in CAR expression. The CAR-negative cells generally exhibit very low infectability. Since the fiber knob is a primary mediator of Ad5 binding to the cell surface, improved infectivity of Ad5-based vectors as oncolytic agents may be achieved via genetic modifications of this domain. The strategies to modify or broaden tropism and increase transduction efficiency of Ad5-based vectors include: 1) an incorporation of a targeting peptide into the fiber knob domain (the HI loop and/or C-terminus); 2) fiber knob serotype switching, or pseudotyping, by constructing chimeric fibers consisting of the knob domain derived from an alternate serotype (e.g., Ad5/3 or Ad5/35 chimeras), which binds to receptor(s) other than CAR (e.g., desmoglein 2/DSG2 and/or CD46); 3) "fiber complex mosaicism", an approach of combining serotype chimerism with peptide ligand(s) incorporation (e.g., Ad5/3-RGD); 4) "dual fiber mosaicism" by expressing two separate fibers with distinct receptor-binding capabilities on the same viral particle (e.g., Ad5-5/3 or Ad5-5/σ1); 5) fiber xenotyping by replacing the knob and shaft domains of wild-type Ad5 fiber protein with fibritin trimerization domain of T4 bacteriophage or σ1 attachment protein of reovirus. Other genetic approaches to increase the CAR-independent transduction efficiency include insertion of a targeting peptide into the hypervariable region of the capsid protein hexon or fusion to the C-terminus of pIX. Finally, we consider a yet unsolved molecular mechanism of liver targeting by Ad5-based vectors (CAR-, integrin-, fiber shaft KKTK motif-, and hepatic heparan sulfate glycosaminoglycans-independent, but fiber-, hexon- and blood factor X-dependent).

Hitting the nail on the head: combining oncolytic adenovirus-mediated virotherapy and immunomodulation for the treatment of glioma

Glioblastoma is a highly aggressive malignant brain tumor with a poor prognosis and the median survival 14.6 months. Immunomodulatory proteins and oncolytic viruses represent two treatment approaches that have recently been developed for patients with glioblastoma that could extend patient survival and result in better treatment outcomes for patients with this disease. Together, these approaches could potentially augment the treatment efficacy and strength of these anti-tumor therapies. In addition to oncolytic activities, this combinatory approach introduces immunomodulation locally only where cancerous cells are present. This thereby results in the change of the tumor microenvironment from immune-suppressive to immune-vulnerable via activation of cytotoxic T cells or through the removal of glioma cells immune-suppressive capability. This review discusses the strengths and weaknesses of adenoviral oncolytic therapy, and highlights the genetic modifications that result in more effective and targeted viral agents. Additionally, the mechanism of action of immune-activating agents is described and the results of previous clinical trials utilizing these treatments in other solid tumors are reviewed. The feasibility, synergy, and limitations for treatments that combine these two approaches are outlined and areas for which more work is needed are considered.

CXCL12 retargeting of an adenovirus vector to cancer cells using a bispecific adapter

Ad vectors are promising delivery vehicles for cancer therapeutic interventions. However, their application is limited by promiscuous tissue tropism and hepatotoxicity. This limitation can be avoided by altering the native tropism of Ads so that they can be redirected to the target cells through alternate cellular receptors. The CXCR4 chemokine receptor belongs to a large superfamily of G-protein-coupled receptors and is known to be upregulated in a wide variety of cancers, including breast cancer and melanoma. These receptors have been associated with cancer cell survival, progression, and metastasis. In the current study, an Ad to cancer cells overexpressing CXCR4 by using a bispecific adapter, sCAR-CXCL12, was retargeted. The sCAR-CXCL12 adapter contained the soluble ectodomain form of the native Ad5 receptor (sCAR), which was fused to a mature human chemokine ligand, CXCL12, through a short peptide linker. A dramatic increase in the infectivity of cancer cells using a targeted Ad vector compared with an untargeted vector was observed. Furthermore, sCAR-CXCL12 attenuated Ad infection of liver ex vivo and in vivo and enhanced Ad vector infection of xenograft tumors implanted in immunodeficient SCID-bg mice. Thus, the sCAR-CXCL12 adapter could be used to retarget Ad vectors to chemokine receptor-positive tumors.

Fiber-modified adenovirus for central nervous system Parkinson's disease gene therapy

Gene-based therapies for neurological diseases continue to develop briskly. As disease mechanisms are elucidated, flexible gene delivery platforms incorporating transcriptional regulatory elements, therapeutic genes and targeted delivery are required for the safety and efficacy of these approaches. Adenovirus serotype 5 (Ad5)-based vectors can carry large genetic payloads to provide this flexibility, but do not transduce neuronal cells efficiently. To address this, we have developed a tropism-modified Ad5 vector with neuron-selective targeting properties for evaluation in models of Parkinson disease therapy. A panel of tropism-modified Ad5 vectors was screened for enhanced gene delivery in a neuroblastoma cell line model system. We used these observations to design and construct an unbiased Ad vector platform, consisting of an unmodified Ad5 and a tropism-modified Ad5 vector containing the fiber knob domain from canine Ad serotype 2 (Ad5-CGW-CK2). Delivery to the substantia nigra or striatum showed that this vector produced a neuronally-restricted pattern of gene expression. Many of the transduced neurons were from regions with afferent projections to the injection site, implicating that the vector binds the presynaptic terminal resulting in presynaptic transduction. We show that Ad5-CGW-CK2 can selectively transduce neurons in the brain and hypothesize that this modular platform is potentially adaptable to clinical use.

Retinal transduction profiles by high-capacity viral vectors

Retinal gene therapy with adeno-associated viral (AAV) vectors is safe and effective in humans. However, the limited cargo capacity of AAV prevents their use for therapy of those inherited retinopathies (IRs) due to mutations in large (>5kb) genes. Viral vectors derived from Adenovirus (Ad), Lentivirus (LV) and Herpesvirus (HV) can package large DNA sequences but do not target efficiently retinal photoreceptors (PRs) where the majority of genes responsible for IRs are expressed.

Here, we have evaluated the mouse retinal transduction profiles of vectors derived from 16 different Ad serotypes, 7 LV pseudotypes, and from a bovine HV. Most of the vectors tested transduced efficiently the retinal pigment epithelium (RPE). We found that LV-GP64 tends to transduce more PRs than the canonical LV-VSVG albeit this was restricted to a narrow region. We observed more extensive PR transduction with HdAd1, 2 and 5/F35++ than with LV, although none of them outperformed the canonical HdAd5 or matched the extension of PR transduction achieved with AAV2/8.

Experimental virotherapy of chemoresistant pancreatic carcinoma using infectivity-enhanced fiber-mosaic oncolytic adenovirus

Pancreatic cancer is a significant clinical problem and novel therapeutic approaches are desperately needed. Recent advances in conditionally replicative adenovirus-based (CRAd) oncolytic virus design allow the application of CRAd vectors as a therapeutic strategy to efficiently target and eradicate chemoresistant pancreatic cancer cells, thereby improving the efficacy of pancreatic cancer treatment. The goal of this study was to construct and validate the efficacy of an infectivity-enhanced, liver-untargeted, tumor-specific CRAd vector. A panel of CRAds has been derived that embodies the C-X-C chemokine receptor type 4 promoter for conditional replication, two-fiber complex mosaicism for targeting expansion and hexon hypervariable region 7 (HVR7) modification for liver untargeting. We evaluated CRAds for cancer virotherapy using a human pancreatic tumor xenograft model. Employment of the fiber mosaic approach improved CRAd replication in pancreatic tumor xenografts. Substitution of the HVR7 of the Ad5 hexon for Ad serotype 3 hexon resulted in decreased liver tropism of systemically administrated CRAd. Obtained data demonstrated that employment of complex mosaicism increased efficacy of the combination of oncolytic virotherapy with chemotherapy in a human pancreatic tumor xenograft model.

Oncolytic virus therapy for cancer

The use of oncolytic viruses to treat cancer is based on the selection of tropic tumor viruses or the generation of replication selective vectors that can either directly kill infected tumor cells or increase their susceptibility to cell death and apoptosis through additional exposure to radiation or chemotherapy. In addition, viral vectors can be modified to promote more potent tumor cell death, improve the toxicity profile, and/or generate host antitumor immunity. A variety of viruses have been developed as oncolytic therapeutics, including adenovirus, vaccinia virus, herpesvirus, coxsackie A virus, Newcastle disease virus, and reovirus. The clinical development of oncolytic viral therapy has accelerated in the last few years, with several vectors entering clinical trials for a variety of cancers. In this review, current strategies to optimize the therapeutic effectiveness and safety of the major oncolytic viruses are discussed, and a summary of current clinical trials is provided. Further investigation is needed to characterize better the clinical impact of oncolytic viruses, but there are increasing data demonstrating the potential promise of this approach for the treatment of human and animal cancers.

A new cancer cell detection method using an infectivity-enhanced adenoviral vector

Cytological examination is widely used as a diagnostic tool because of the ease of collecting cells from the involved area. However, the diagnostic yield of cytological examination is unsatisfactory; the reasons include sampling error, poorly prepared samples, small numbers of malignant cells, and low grades of cellular atypia. In this study, we focused on the high infectivity of adenovirus towards epithelial cells and applied the luciferase- expressing adenoviral vector to a new cancer cell detection tool. In addition, adenoviral infectivity was enhanced by modifying viral fiber proteins. The sensitivity of the diagnostic tool was tested using the NCI-H1299 lung cancer cell line, and validated in body fluid samples from cancer patients with a variety of etiology. Results showed that the adenovirus efficiently transfected NCI-H1299 with high sensitivity. Only 10 cancer cells were sufficient for detection of luciferase signals. In body fluid samples, the adenovirus confirmed the diagnosis for malignant and benign cancer, but not in non-epithelial cell derived samples. This study provides proof-of-concept for a more reliable and sensitive diagnostic tool for epithelium-derived cancer.

Characterization of malleability and immunological properties of human adenovirus type 3 hexon hypervariable region 1

Adenovirus (Ad) capsids that display exogenous epitopes can be potently immunogenic, eliciting a potent humoral response against components of the capsid. We used the epitopes flag, his(6)flag, his(6)lgsflag and AdV4HVR5 as model antigens to characterize the hexon hypervariable region (HVR) 1 as a site for epitope insertion. A peptide of up to 17 amino acids could be incorporated into HVR1 of the Ad3 hexon without adversely affecting the biological characteristics of the virus. Multiple vaccinations with capsid-modified Ad3 induced a humoral response against the epitope inserted in HVR1. However, antiserum against the his(6)flag or his(6)lgsflag epitope did not recognize glutathione S-transferase (GST)-his(6) and GST-flag fusion protein. Our study illustrates that there is an immune response against the new epitope within the amino acids of his(6)flag or his(6)lgsflag epitopes. This discovery could be a warning for the generation of multivalent vaccine vectors by incorporation of multiple epitopes into single HVRs.

Viral delivery for gene therapy against cell movement in cancer

Viral delivery for cancer gene therapy is a promising approach, where traditional radiotherapy or chemotherapy to limit proliferation and movement of cancer cells has met resistance. Based on the new understanding of the biology of the viral vectors, therapeutic viral vectors for cancer gene therapy have been improved for greater safety and efficacy as well as transitioned from being non-replicating to replication-competent. Traditional oncolytic vectors have focused on eliminating tumor growth, while novel vectors simultaneously target epithelial-to-mesenchymal transition (EMT) in cancer cells, which could further prevent and reverse the aggressive tumor progression. In this review, we highlight the illustrative examples of cancer gene therapy in clinical trials as well as preclinical data and include proposals on methods to further enhance the safety and efficacy of oncolytic viral vectors in cancer gene therapy.

Intracellular trafficking and gene expression of pH-sensitive, artificially enveloped adenoviruses in vitro and in vivo

Recombinant adenovirus (Ad) has shown great promise in gene therapy. Artificial envelopment of adenovirus within lipid bilayers has previously been shown to decrease the immunogenicity and hepatic affinity of naked Ad in vivo. Unfortunately, this also resulted in a significant reduction of gene expression, which we attributed to poor endosomal release of the Ad from its artificial lipid envelope. In this work, we explored the artificial envelopment of Ad within pH-sensitive DOPE:CHEMS bilayers and characterized this vector by TEM, AFM, dot blot, dynamic light scattering and zeta potential measurements. The artificially enveloped viral vectors exhibited good stability at physiological pH but immediately collapsed and released naked Ad virions at pH 5.5. Intracellular trafficking using confocal laser scanning microscopy (CLSM) revealed that Cy3-labelled Ad enveloped in DOPE:CHEMS bilayers exhibited the characteristic Ad distribution within the cytoplasm that led to virion accumulation around the nuclear membrane, indicating endosomal release of Ad. We obtained equivalent levels of gene expression as those of naked Ad in a series of CAR-positive (CAR+) and CAR-negative (CAR-) cell lines. This suggested that the mechanism of infection for the artificially enveloped Ad remained dependent on the presence of CAR receptors. Finally, the pH-sensitive enveloped Ad were injected intratumorally in human cervical carcinoma xenograft-bearing nude mice, also illustrating their capacity for efficient in vivo marker gene expression. This study is a step forward toward the engineering of functional, artificially enveloped adenovirus vectors for gene transfer applications.

Oncolytic adenovirus based on serotype 3

Oncolytic adenoviruses have been safe in clinical trials but the efficacy has been mostly limited. All published trials have been performed with serotype 5 based viruses. The expression level of the Ad5 receptor CAR may be variable in advanced tumors. In contrast, the Ad3 receptor remains unclear, but is known to be abundantly expressed in most tumors. Therefore, we hypothesized that a fully serotype 3 oncolytic adenovirus might be useful for treating cancer. Patients exposed to adenoviruses develop high titers of serotype-specific neutralizing antibodies, which might compromise re-administration. Thus, having different serotype oncolytic viruses available might facilitate repeated dosing in humans. Ad3-hTERT-E1A is a fully serotype 3 oncolytic adenovirus controlled by the promoter of the catalytic domain of human telomerase. It was effective in vitro on cell lines representing seven major cancer types, although low toxicity was seen in non-malignant cells. In vivo, the virus had anti-tumor efficacy in three different animal models. Although in vitro oncolysis mediated by Ad3-hTERT-E1A and wild-type Ad3 occurred more slowly than with Ad5 or Ad5/3 (Ad3 fiber knob in Ad5) based viruses, in vivo the virus was at least as potent as controls. Anti-tumor efficacy was retained in presence of neutralizing anti-Ad5 antibodies whereas Ad5 based controls were blocked. In summary, we report generation of a non-Ad5 based oncolytic adenovirus, which might be useful for testing in cancer patients, especially in the context of high anti-Ad5 neutralizing antibodies.

An adenoviral vector expressing human adenovirus 5 and 3 fiber proteins for targeting heterogeneous cell populations

Human adenovirus serotype 5 (HAdV-5) attaches to its primary receptor, the coxsackie and adenovirus receptor (CAR) as the first step of infection. However, CAR expression decreases as tumors progress, thereby diminishing the utility of HAdV-5-based vectors for cancer therapy. In contrast, many aggressive tumor cells highly express CD46, a cellular receptor for HAdV-3. We hypothesized that a mosaic HAdV vector, containing two kinds of fiber proteins, would provide extensive transduction in a heterogeneous population of tumor cells with varying expression levels of HAdV receptors. We therefore generated a fiber-mosaic HAdV vector displaying both a chimeric HAdV-3 fiber and the HAdV-5 fiber protein. We verified the structural integrity of purified viral particles and confirmed that the fiber-mosaic HAdV vector has expanded tropism. We conclude that the use of fiber-mosaic HAdV vectors is a promising approach for transducing a heterogeneous cell population with different expression levels of adenovirus receptors.

hTERT-promoter-dependent oncolytic adenovirus enhances the transduction and therapeutic efficacy of replication-defective adenovirus vectors in pancreatic cancer cells

Adenovirus-mediated gene therapy shows promise for cancer therapy, but transgene expression of replication-defective adenovirus may be low and transient in clinical settings. Recent reports have shown that the use of a conditionally replication-competent adenovirus (CRAd) enhanced the gene transduction of a replication-defective adenovirus vector. The control of tumor-stromal interactions has also been determined to be important in cancer therapy. In this study, we investigated the effect of the human telomerase reverse transcriptase (hTERT)-CRAd, Ad5/3hTERTE1, which possesses the tumor-specific hTERT promoter with the chimeric fiber 5/3, on the transgene expression and therapeutic efficacy of a replication-defective adenovirus vector expressing NK4 under the control of the CMV promoter, Ad-NK4. In addition, we established a new strategy to target both cancer cells and cancer-stromal interactions. Human pancreatic cancer cells were infected with Ad-NK4 and either Ad5/3hTERTE1 (CRAd-combination group) or Ad5/3hTERTLuc (control-combination group). In the CRAd-combination group, Ad-NK4-delivered transgene expression was increased, leading to an enhanced inhibitory effect on the invasion of cancer cells. In in vivo experiments, NK4 expression within tumors and its inhibitory effect on tumor growth, angiogenesis, and metastasis were enhanced in the CRAd-combination group. These results suggest that hTERT-CRAd enhances the transgene expression and therapeutic efficacies of Ad-NK4, possibly through the in-trans replication of Ad-NK4 induced by adenovirus E1 derived from co-infected hTERT-CRAd. This approach may be a promising combination therapy against advanced pancreatic cancer.

Outbreak of acute respiratory infection among infants in Lisbon, Portugal, caused by human adenovirus serotype 3 and a new 7/3 recombinant strain

Human adenoviruses (AdVs) typically cause mild illnesses in otherwise healthy hosts. We investigated a pediatric outbreak of acute respiratory infection with fatal outcomes that occurred in Lisbon, Portugal, in 2004. Biological specimens were collected from 83 children attending two nurseries, a kinesiotherapy clinic, and the household of a nanny. Adenovirus infection was confirmed in 48 children by PCR and virus isolation. Most (96%) isolates were classified as being of subspecies B1. Phylogenetic analysis of fiber and hexon gene sequences revealed that most infants were infected with AdV serotype 3 (AdV3) strains. Infants attending one nursery harbored a new recombinant strain containing an AdV serotype 7 hexon and serotype 3 fiber (AdV7/3). Both the AdV3 and the AdV7/3 strains caused fatal infections. Two different serotype 3 strains were circulating in Lisbon in 2004, and the new AdV7/3 recombinant type originated from only one of those strains. These results demonstrate that recombination leads to the emergence of new adenovirus strains with epidemic and lethal potential.

Derivation of a triple mosaic adenovirus for cancer gene therapy

A safe and efficacious cancer medicine is necessary due to the increasing population of cancer patients whose particular diseases cannot be cured by the currently available treatment. Adenoviral (Ad) vectors represent a promising therapeutic medicine for human cancer therapy. However, several improvements are needed in order for Ad vectors to be effective cancer therapeutics, which include, but are not limited to, improvement of cellular uptake, enhanced cancer cell killing activity, and the capability of vector visualization and tracking once injected into the patients. To this end, we attempted to develop an Ad as a multifunctional platform incorporating targeting, imaging, and therapeutic motifs. In this study, we explored the utility of this proposed platform by generating an Ad vector containing the poly-lysine (pK), the herpes simplex virus type 1 (HSV-1) thymidine kinase (TK), and the monomeric red fluorescent protein (mRFP1) as targeting, tumor cell killing, and imaging motifs, respectively. Our study herein demonstrates the generation of the triple mosaic Ad vector with pK, HSV-1 TK, and mRFP1 at the carboxyl termini of Ad minor capsid protein IX (pIX). In addition, the functionalities of pK, HSV-1 TK, and mRFP1 proteins on the Ad vector were retained as confirmed by corresponding functional assays, indicating the potential multifunctional application of this new Ad vector for cancer gene therapy. The validation of the triple mosaic Ad vectors also argues for the ability of pIX modification as a base for the development of multifunctional Ad vectors.

Current issues and future directions of oncolytic adenoviruses

Oncolytic adenoviruses (Ads) constitute a promising new class of anticancer agent. They are based on the well-studied adenoviral vector system, which lends itself to concept-driven design to generate oncolytic variants. The first oncolytic Ad was approved as a drug in China in 2005, although clinical efficacy observed in human trials has failed to reach the high expectations that were based on studies in animal models. Current obstacles to the full realization of efficacy of this class of anticancer agent include (i) limited efficiency of infection and specific replication in tumor cells, (ii) limited vector spread within the tumor, (iii) imperfect animal models and methods of in vivo imaging, and (iv) an incomplete understanding of the interaction of these agents with the host. In this review, we discuss recent advances in the field of oncolytic Ads and potential ways to overcome current obstacles to their clinical application and efficacy.

Engineering lymphocyte subsets: tools, trials and tribulations

Cell-based therapies with various lymphocyte subsets hold promise for the treatment of several diseases, including cancer and disease resulting from inflammation and infection. The ability to genetically engineer lymphocyte subsets has the potential to improve the natural immune response and correct impaired immunity. In this Review we focus on the lymphocyte subsets that have been modified genetically or by other means for therapeutic benefit, on the technologies used to engineer lymphocytes and on the latest progress and hurdles in translating these technologies to the clinic.

Adenoviral virotherapy for malignant brain tumors

Glioblastoma multiforme is the most common form of primary brain cancer. In the past decade, virotherapy of tumors has gained credence, particularly in glioma management, as these tumors are not completely resectable and tend to micro-metastasize. Adenoviral vectors have an advantage over other viral vectors in that they are relatively non-toxic and do not integrate in the genome. However, the lack of coxsackie and adenovirus receptors on surface of gliomas provides for inefficient transduction of wild-type adenoviral vectors in these tumors. By targeting receptors that are overexpressed in gliomas, modified adenoviral constructs have been shown to efficiently infect glioma cells. In addition, by taking advantage of tumor-specific promoter elements, oncolytic adenoviral vectors offer the promise of selective tumor-specific replication. This dual targeting strategy has enabled specificity in both laboratory and pre-clinical settings. This review examines current trends in adenoviral virotherapy of gliomas, with an emphasis on targeting modalities and future clinical applications.

Development of fiber-substituted adenovirus vectors containing foreign peptides in the adenovirus serotype 35 fiber knob

Fiber-substituted adenovirus (Ad) vectors containing fibers of Ad serotype 35 (AdF35) efficiently transduce a variety of human cells because their receptor, human CD46, is ubiquitously expressed on almost all nucleated cells. However, the ubiquitous expression of CD46 might lead to unexpected transduction in untargeted organs. In this study, we developed fiber-modified AdF35 vectors with an integrin-binding Arg-Gly-Asn (RGD) peptide incorporated into the FG, HI or IJ loop, which have been identified as important regions for binding to CD46. Incorporation of foreign peptides into these loops does not inhibit trimerization of the fibers. In CD46-negative cells, fiber-mutant AdF35 vectors containing an RGD peptide in the FG or HI loop showed 6- to 30-fold higher transduction efficiencies in an RGD-peptide-dependent manner than the unmodified AdF35 vectors. In contrast, in CD46-positive cells, insertion of foreign peptides markedly reduced the transduction efficiencies of the AdF35 vectors, indicating that insertion of foreign peptides significantly inhibits binding to CD46. In particular, CD46-mediated transduction was completely diminished by insertion of foreign peptides into the HI loop. Our findings indicate that HI loop is the most suitable domain to mediate a foreign peptide-dependent and CD46-independent transduction by incorporation of foreign peptides into the Ad35 fiber knob.

Adenoviral vector-based strategies for cancer therapy

Definitive treatment of cancer has eluded scientists for decades. Current therapeutic modalities like surgery, chemotherapy, radiotherapy and receptor-targeted antibodies have varied degree of success and generally have moderate to severe side effects. Gene therapy is one of the novel and promising approaches for therapeutic intervention of cancer. Viral vectors in general and adenoviral (Ad) vectors in particular are efficient natural gene delivery systems and are one of the obvious choices for cancer gene therapy. Clinical and preclinical findings with a wide variety of approaches like tumor suppressor and suicide gene therapy, oncolysis, immunotherapy, anti-angiogenesis and RNA interference using Ad vectors have been quite promising, but there are still many hurdles to overcome. Shortcomings like increased immunogenicity, prevalence of preexisting anti-Ad immunity in human population and lack of specific targeting limit the clinical usefulness of Ad vectors. In recent years, extensive research efforts have been made to overcome these limitations through a variety of approaches including the use of conditionally-replicating Ad and specific targeting of tumor cells. In this review, we discuss the potential strengths and limitations of Ad vectors for cancer therapy.

Improved adenovirus type 5 vector-mediated transduction of resistant cells by piggybacking on coxsackie B-adenovirus receptor-pseudotyped baculovirus

Taking advantage of the wide tropism of baculoviruses (BVs), we constructed a recombinant BV (BV(CAR)) pseudotyped with human coxsackie B-adenovirus receptor (CAR), the high-affinity attachment receptor for adenovirus type 5 (Ad5), and used the strategy of piggybacking Ad5-green fluorescent protein (Ad5GFP) vector on BV(CAR) to transduce various cells refractory to Ad5 infection. We found that transduction of all cells tested, including human primary cells and cancer cell lines, was significantly improved using the BV(CAR)-Ad5GFP biviral complex compared to that obtained with Ad5GFP or BV(CAR)GFP alone. We determined the optimal conditions for the formation of the complex and found that a high level of BV(CAR)-Ad5GFP-mediated transduction occurred at relatively low adenovirus vector doses, compared with transduction by Ad5GFP alone. The increase in transduction was dependent on the direct coupling of BV(CAR) to Ad5GFP via CAR-fiber knob interaction, and the cell attachment of the BV(CAR)-Ad5GFP complex was mediated by the baculoviral envelope glycoprotein gp64. Analysis of the virus-cell binding reaction indicated that the presence of BV(CAR) in the complex provided kinetic benefits to Ad5GFP compared to the effects with Ad5GFP alone. The endocytic pathway of BV(CAR)-Ad5GFP did not require Ad5 penton base RGD-integrin interaction. Biodistribution of BV(CAR)-Ad5Luc complex in vivo was studied by intravenous administration to nude BALB/c mice and compared to Ad5Luc injected alone. No significant difference in viscerotropism was found between the two inocula, and the liver remained the preferred localization. In vitro, coagulation factor X drastically increased the Ad5GFP-mediated transduction of CAR-negative cells but had no effect on the efficiency of transduction by the BV(CAR)-Ad5GFP complex. Various situations in vitro or ex vivo in which our BV(CAR)-Ad5 duo could be advantageously used as gene transfer biviral vector are discussed.

Targeting adenovirus to CD80 and CD86 receptors increases gene transfer efficiency to malignant glioma cells

OBJECT

Gene therapy protocols for malignant gliomas utilize adenoviral vectors that rely almost exclusively on the adenovirus serotype 5 (Ad5) backbone. The authors have previously shown that chimeric vectors that bind to the Ad3 receptor, or CD46, increase the transduction efficiency of malignant brain tumors. In light of the debate regarding the efficacy of CD46 compared with CD80/CD86 in binding Ad3 virions, the authors now examine the expression and transduction efficiency of Ad5/3 chimeras that bind via CD80/CD86.

METHODS

The authors first analyzed CD80/CD86 expression in glioma cell lines. They then used three replication-defective vectors containing a luciferase reporter gene: Ad5/3 (containing the tail and shaft domain of Ad5 and the knob domain of Ad3); Ad3/5 (containing the tail of Ad5, shaft of Ad3, and knob of Ad5); and Ad3/3 (containing the tail of Ad5, shaft of Ad3, and knob of Ad3). These vectors were analyzed both in vitro and in vivo against malignant glioma cells. To examine further the effect of Ad5/3 fiber modification, the authors created an oncolytic vector, conditionally replicative Ad5/3 (CRAd5/3).

RESULTS

The Ad5/3 vector showed a 10- to 100-fold enhanced transduction efficiency of malignant glioma compared with replication-defective wild-type adenovirus (reAd5) (p < 0.05). Moreover the use of Ad5/3 reduced transgene expression by more than 90% in normal human brain cells compared with reAd5. Finally, the use of CRAd5/3 inhibited tumor cell proliferation by 43% more than replication-competent wild-type virus in vitro (p < 0.05).

CONCLUSIONS

The results of this study demonstrate that the Ad5/3 vector offers superior transduction efficiency and low toxicity in the setting of brain tumors, and therefore represents a potential new approach to gene therapy for malignant gliomas.

Targeting mesothelioma using an infectivity enhanced survivin-conditionally replicative adenoviruses

Mesothelioma is a highly malignant neoplasm with no effective treatment. Conditionally replicative adenoviruses (CRAds) represent a promising new modality for the treatment of cancer in general. A key contribution in this regard is the introduction of tumor-selective viral replication for amplification of the initial inoculum in the neoplastic cell population. Under ideal conditions following cellular infection, the viruses replicate selectively in the infected tumor cells and kill the cells by cytolysis, leaving normal cells unaffected. However, to date there have been two limitations to clinical application of these CRAd agents; viral infectivity and tumor specificity have been poor. Herein we report on two CRAd agents, CRAd-S.RGD and CRAd-S.F5/3, in which the tumor specificity is regulated by a tumor-specific promoter, the survivin promoter, and the viral infectivity is enhanced by incorporating a capsid modification (RGD or F5/3) in the adenovirus fiber region. These CRAd agents effectively target human mesothelioma cell lines, induce strong cytoxicity in these cells in vitro, and viral replication in a H226 murine xenograft model in vivo. In addition, the survivin promoter has extremely low activity both in the non-transformed cell line, HMEC, and in human liver tissue. Our results suggest that the survivin-based CRAds are promising agents for targeting mesothelioma with low host toxicity. These agents should provide important insights into the identification of novel therapeutic strategies for mesothelioma.

Enhanced transduction of malignant glioma with a double targeted Ad5/3-RGD fiber-modified adenovirus

Malignant brain tumors remain refractory to adenovirus type 5 (Ad5)-based gene therapy, mostly due to the lack of the primary Ad5 receptor, the coxsackie and adenovirus receptor, on brain tumor cells. To bypass the dependence on coxsackie and adenovirus receptor for adenoviral entry and infectivity, we used a novel, double targeted Ad5 backbone-based vector carrying a chimeric Ad5/3 fiber with integrin-binding RGD motif incorporated in its Ad3 knob domain. We then tested the new virus in vitro and in vivo in the setting of malignant glioma. Ad5/3-RGD showed a 10-fold increase in gene expression in passaged cell lines and up to 75-fold increase in primary tumors obtained from patients relative to the control. These results were further corroborated in our in vivo human glioma xenograft model, where the Ad5/3-RGD vector showed a 1,000-fold increase in infectivity as compared with the control. Taken together, our findings indicate that Ad5/3-RGD may be a superior vector for applications in glioma gene therapy and therefore warrants further attention in the field of neuro-oncology.

Infectivity enhancement for adenoviral transduction of canine osteosarcoma cells

The full realization of conditionally replicative adenoviruses (CRAds) for cancer therapy has been hampered by the limited knowledge of CRAd function in vivo and particularly in an immunocompetent host. To address this issue, we previously proposed a canine adenovirus type 2 (CAV2)-based CRAd for clinical evaluation in canine patients with osteosarcoma (OS). In this study, we evaluated infectivity-enhancement strategies to establish the foundation for designing a potent CAV2 CRAd with effective transduction capacity in dog osteosarcoma cells. The results indicate that the native CAV2 fiber-knob can mediate increased binding, and consequently gene transfer, in both canine osteosarcoma immortalized and primary cell lines relative to previously reported Ad5 infectivity-enhancement strategies. Gene delivery was further enhanced by incorporating a polylysine polypeptide onto the carboxy terminus of the CAV2 knob. This vector demonstrated improved gene delivery in osteosarcoma xenograft tumors. These data provide the rationale for generation of infectivity-enhanced syngeneic CAV2 CRAds for clinical evaluation in a dog osteosarcoma model.

CD46 Represents a Target for Adenoviral Gene Therapy of Malignant Glioma

Malignant gliomas remain refractory to adenovirus serotype 5 (Ad5) gene therapy because of the lack of the primary adenoviral receptor, the coxsackie-adenovirus receptor (CAR), on tumor cells. To bypass the dependence on CAR, we investigated the expression of adenovirus serotype 3 (Ad3) receptor, or CD46, on glioma cells. First, we analyzed the expression of CD46 by RT-PCR on primary and passaged glioma cells. We then performed immunofluorescence studies to examine protein expression of CAR and CD46 on the same tumor lines. Finally, we constructed a replication-defective Ad vector that binds to CD46 and contains a luciferase transgenic cassette in place of the deleted E1 region: Ad5/3 (containing tail/shaft domain of Ad5 and knob domain of Ad3). These vectors were analyzed in vitro and in vivo against malignant glioma and compared with wild-type Ad5 or control vector Ad3/5 (containing tail of Ad5, shaft of Ad3, and knob of Ad5). The chimeric vector Ad5/3 showed a significant increase in the transduction efficiency of glioma tumor cells. At the same time, blocking the CD46 receptor caused a 65% inhibition of adenoviral infection when using Ad5/3. Taken together, these results indicate that CD46 is overexpressed by malignant glioma. Retargeting to the Ad3 receptor enhances gene transfer and offers a novel target for gene therapy of malignant brain tumors.

Adenoviral vectors--how to use them in cancer gene therapy?

Gene therapy is most often described as a technique for introducing the foreign genetic material into cells with a correction of a dysfunctional gene as its final goal. Today, it is well known that cancer is one of the leading causes of mortality in the world. Besides classical methods for cancer treatment new strategies against cancer are needed. Although originally being designed as a treatment for monogenetic illness, soon after, gene therapy appeared as a potential new strategy in cancer therapy. One of the widely used vectors for cancer gene therapy is adenovirus. In this review we have described molecular biology of adenoviruses and basis for construction of adenoviral vectors. We have also described concepts for cancer gene therapy including their in vitro and in vivo application. Special attention is drawn toward retargeting of adenovirus as a new approach in vector design for cancer gene therapy, in order to restrict transgene expression in tumor tissue. This approach uses biophysical as well as genetic characteristics of tumor itself and its supporting tissue, allowing new "bypass" in cancer gene therapy.

Transductional targeting of adenovirus vectors for gene therapy

Cancer gene therapy approaches will derive considerable benefit from adenovirus (Ad) vectors capable of self-directed localization to neoplastic disease or immunomodulatory targets in vivo. The ablation of native Ad tropism coupled with active targeting modalities has demonstrated that innate gene delivery efficiency may be retained while circumventing Ad dependence on its primary cellular receptor, the coxsackie and Ad receptor. Herein, we describe advances in Ad targeting that are predicated on a fundamental understanding of vector/cell interplay. Further, we propose strategies by which existing paradigms, such as nanotechnology, may be combined with Ad vectors to form advanced delivery vehicles with multiple functions.

Reovirus sigma1 fiber incorporated into adenovirus serotype 5 enhances infectivity via a CAR-independent pathway

Adenovirus serotype 5 (Ad5) has been used for gene therapy with limited success because of insufficient infectivity in cells with low expression of the primary receptor, the coxsackie and adenovirus receptor (CAR). To enhance infectivity in tissues with low CAR expression, tropism expansion is required via non-CAR pathways. Serotype 3 Dearing reovirus utilizes a fiber-like sigma1 protein to infect cells expressing sialic acid and junction adhesion molecule 1 (JAM1). We hypothesized that replacement of the Ad5 fiber with sigma1 would result in an Ad5 vector with CAR-independent tropism. We therefore constructed a fiber mosaic Ad5 vector, designated as Ad5-sigma1, encoding two fibers: the sigma1 and the wild-type Ad5 fiber. Functionally, Ad5-sigma1 utilized CAR, sialic acid, and JAM1 for cell transduction and achieved maximum infectivity enhancement in cells with or without CAR. Thus, we have developed a new type of Ad5 vector with expanded tropism, possessing fibers from Ad5 and reovirus, that exhibits enhanced infectivity via CAR-independent pathway(s).

Complex mosaicism is a novel approach to infectivity enhancement of adenovirus type 5-based vectors

The use of adenovirus type 5 (Ad5) for cancer therapy is limited by deficiency of its primary cell attachment receptor, coxsackie and adenovirus receptor (CAR), on cancer cells. Ad5 retargeting to alternate receptors through fiber genetic modification can be used to circumvent CAR dependence of its tropism, and thereby achieve infectivity enhancement. Here we propose and test a novel "complex mosaicism" approach for fiber modification, which combines serotype chimerism with peptide ligand(s) incorporation in a single-fiber molecule. We incorporated integrin-binding peptide RGD-4C in the HI-loop, at the carboxy (C)-terminus, or both locales of the Ad3 knob, in the context of Ad5/3 chimera fiber in order to retarget simultaneously the Ad vector to integrins and Ad3 receptors. The infectivity enhancement of the fiber modifications was assessed in various cancer cell lines as cancer-targeting models. Replication-defective complex mosaic Ad-luc vectors bearing chimeric fiber (F.5/3), with or without C-terminal RGD-modification of Ad3 knob, demonstrated up to 55-fold gene transfer increase in bladder cancer cell lines. Although this augmentation was primarily due to Ad3 receptor targeting, some contribution of RGD-mediated integrin-targeting was also observed, suggesting that complex mosaic modification can function in a dual-receptor targeting via a single Ad3 fiber knob.

Infectivity enhanced, hTERT promoter-based conditionally replicative adenoviruses are useful for SCLC treatment

Treatment of advanced small-cell lung cancer (SCLC) remains one of the major challenges in current medicine because of the high morbidity and mortality of the disease. Advanced stage lung cancer is refractory to conventional therapies and it also has an extremely poor prognosis. As a result, new therapeutic approaches are needed. Telomere maintenance to the regulation of replicative lifespan strongly implies that alterations in telomere biology play an important role during malignant transformation. Cancers that exhibit high levels of telomerase activity, such as all of the SCLC, were examined in a previous study. In this study, we turned the expression of human telomerase reverse transcriptase (hTERT) by tumors to a therapeutic advantage using a conditionally replication-competent adenovirus (CRAd) in which the expression of E1 (early region 1) is controlled by the hTERT promoter. This virus achieved good levels of viral replication in SCLC cells and induced a substantial anticancer effect in vitro and in vivo. As a further enhancement, the cancer cell killing effect was improved with a tropism modification of the virus to express the knob domain of Ad3 (serotype 3 adenovirus), and this improved infectivity for cancer cells. Conversely, the hTERT promoter has low activity in normal tissues, and the CRAd caused no damage to normal lung fibroblast cells. Since the telomerase activity is common in many types of cancers, these CRAds may be applicable to a wide range of tumors. We concluded that the use of hTERT promoter-based CRAds may be a potentially effective strategy for cancer treatment.

Approaches to improving the kinetics of adenovirus-delivered genes and gene products

Adenovirus (Ad) vectors have been expected to play a great role in gene therapy because of their extremely high transduction efficiency and wide tropism. However, due to the intrinsic deficiency of their immunogenic toxicities, Ad vectors are rapidly cleared from the host, transgene expression is transient, and readministration of the same serotype Ad vectors is problematic. As a result, Ad vectors are continually undergoing refinement to realize their potential for gene therapy application. Even after 1999, when a patient fatally succumbed to the toxicity associated with Ad vector administration at a University of Pennsylvania (U.S.) experimental clinic, enthusiasm of gene therapists for Ad vectors has not waned. With great efforts from various research groups, significant advances have been achieved through comprehensive approaches to improving the kinetics of Ad vector-delivered genes and gene products.

Fiber-mosaic adenovirus as a novel approach to design genetically modified adenoviral vectors

Genetic modification of the adenovirus (Ad) capsid is one of the successful strategies to achieve viral retargeting. However, it has been widely recognized that structural constraints imposed by viral proteins limit the number and nature of incorporated targeting ligands and often hamper viral propagation. To address this issue, we propose a genetic fiber-mosaic virus (having two distinct fibers in one viral particle) as a means to facilitate fiber modifications. Fiber-mosaic virus having tandem fibers: a wild type (wt) fiber and second adjunctive fiber, will utilize natural viral entry for the conventional propagation of the vectors whereas, adjunctive fiber will serve multiple potential purposes such as targeting, purification, or imaging of viral particles via genetic incorporation of the corresponding functional moieties. We generated the mosaic adenovirus vector encoding two fibers: wild-type and adjunctive fiber--Fiber-Fibritin (FF) and confirmed incorporation of FF in the mosaic viral particles. We investigated binding specificity of the mosaic virus and the possible interference of the two fibers during virus life cycle. Fiber-mosaic Ad attained new binding properties provided by the second fiber, while preserving the binding ability attributed to the wt fiber. Our results suggest that the two fibers being presented and structurally separated on the viral particle may also function separately as binding counterparts for virus attachment. Therefore, the mosaic setting will allow more flexibility in Ad retargeting approaches.

Genomic and bioinformatics analysis of HAdV-4, a human adenovirus causing acute respiratory disease: implications for gene therapy and vaccine vector development

Human adenovirus serotype 4 (HAdV-4) is a reemerging viral pathogenic agent implicated in epidemic outbreaks of acute respiratory disease (ARD). This report presents a genomic and bioinformatics analysis of the prototype 35,990-nucleotide genome (GenBank accession no. AY594253). Intriguingly, the genome analysis suggests a closer phylogenetic relationship with the chimpanzee adenoviruses (simian adenoviruses) rather than with other human adenoviruses, suggesting a recent origin of HAdV-4, and therefore species E, through a zoonotic event from chimpanzees to humans. Bioinformatics analysis also suggests a pre-zoonotic recombination event, as well, between species B-like and species C-like simian adenoviruses. These observations may have implications for the current interest in using chimpanzee adenoviruses in the development of vectors for human gene therapy and for DNA-based vaccines. Also, the reemergence, surveillance, and treatment of HAdV-4 as an ARD pathogen is an opportunity to demonstrate the use of genome determination as a tool for viral infectious disease characterization and epidemic outbreak surveillance: for example, rapid and accurate low-pass sequencing and analysis of the genome. In particular, this approach allows the rapid identification and development of unique probes for the differentiation of family, species, serotype, and strain (e.g., pathogen genome signatures) for monitoring epidemic outbreaks of ARD.

Infectivity-enhanced cyclooxygenase-2-based conditionally replicative adenoviruses for esophageal adenocarcinoma treatment

The employment of conditionally replicative adenoviruses (CRAd) constitutes a promising alternative for cancer treatment; however, in the case of esophageal adenocarcinoma (EAC) the lack of an appropriate tumor-specific promoter and relative resistance to adenovirus infection have hampered the construction of CRAds with clinically applicable specificity and efficacy. By combining transcriptional targeting with infectivity enhancement for CRAds, we generated novel cyclooxygenase-2 (Cox-2) promoter-controlled replicative viral agents for the treatment of EAC. We used infectivity enhancement based on incorporation of an RGD-4C motif into the HI loop of the adenoviral (Ad) fiber knob domain as well as replacement of the Ad5 knob with the Ad3 knob. The Cox-2 promoter was highly active in EAC, whereas showing no significant activity in Cox-2-negative cell lines and primary cells isolated from normal mouse esophagus and stomach. Evaluation of infectivity-enhanced vectors revealed that the transduction and virus-cell binding ability of Ad5/Ad3-chimera were significantly more efficient than that of unmodified and Arg-Gly-Asp (RGD)-modified vectors. All of the Cox-2 CRAds demonstrated replication and subsequent oncolysis in EAC cells but not in Cox-2-negative cells in vitro, thus confirming the dependence of their replication on the Cox-2 promoter activity. Ad5/Ad3 CRAds exhibited significantly improved oncolysis and progeny production compared with unmodified and RGD-modified vectors without sacrificing tumor selectivity. Whereas unmodified and RGD-modified CRAds showed insignificant therapeutic effect in vivo, Ad5/Ad3 CRAds remarkably suppressed tumor growth of established xenografts in mice. Thus, our studies have demonstrated that Ad5/Ad3-chimeric Cox-2 promoter-driven CRAds are selective and potent agents for the treatment of EAC.