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

Superior infectivity of the fiber chimeric oncolytic adenoviruses Ad5/35 and Ad5/3 over Ad5-delta-24-RGD in primary glioma cultures

Ad5-delta-24-RGD is currently the most clinically advanced recombinant adenovirus (rAd) for glioma therapy. We constructed a panel of fiber-modified rAds (Ad5RGD, Ad5/3, Ad5/35, Ad5/3RGD, and Ad5/35RGD, all harboring the delta-24 modification) and compared their infectivity, replication, reproduction, and cytolytic efficacy in human and rodent glioma cell lines and short-term cultures from primary gliomas. In human cells, both Ad5/35-delta-24 and Ad5/3-delta-24 displayed superior infectivity and cytolytic efficacy over Ad5-delta-24-RGD, while Ad5/3-delta-24-RGD and Ad5/35-delta-24-RGD did not show further improvements in efficacy. The expression of the adenoviral receptors/coreceptors CAR, DSG2, and CD46 and the integrins αVβ3/αVβ5 did not predict the relative cytolytic efficacy of the fiber-modified rAds. The cytotoxicity of the fiber-modified rAds in human primary normal cultures of different origins and in primary glioma cultures was comparable, indicating that the delta-24 modification did not confer tumor cell selectivity. We also revealed that CT-2A and GL261 glioma cells might be used as murine cell models for the fiber chimeric rAds in vitro and in vivo. In GL261 tumor-bearing mice, Ad5/35-delta-24, armed with the immune costimulator OX40L as the E2A/DBP-p2A-mOX40L fusion, produced long-term survivors, which were able to reject tumor cells upon rechallenge. Our data underscore the potential of local Ad5/35-delta-24-based immunovirotherapy for glioblastoma treatment.

A new insight into aggregation of oncolytic adenovirus Ad5-delta-24-RGD during CsCl gradient ultracentrifugation

Two-cycle cesium chloride (2 × CsCl) gradient ultracentrifugation is a conventional approach for purifying recombinant adenoviruses (rAds) for research purposes (gene therapy, vaccines, and oncolytic vectors). However, rAds containing the RGD-4C peptide in the HI loop of the fiber knob domain tend to aggregate during 2 × CsCl gradient ultracentrifugation resulting in a low infectious titer yield or even purification failure. An iodixanol-based purification method preventing aggregation of the RGD4C-modified rAds has been proposed. However, the reason explaining aggregation of the RGD4C-modified rAds during 2 × CsCl but not iodixanol gradient ultracentrifugation has not been revealed. In the present study, we showed that rAds with the RGD-4C peptide in the HI loop but not at the C-terminus of the fiber knob domain were prone to aggregate during 2 × CsCl but not iodixanol gradient ultracentrifugation. The cysteine residues with free thiol groups after the RGD motif within the inserted RGD-4C peptide were responsible for formation of the interparticle disulfide bonds under atmospheric oxygen and aggregation of Ad5-delta-24-RGD4C-based rAds during 2 × CsCl gradient ultracentrifugation, which could be prevented using iodixanol gradient ultracentrifugation, most likely due to antioxidant properties of iodixanol. A cysteine-to-glycine substitution of the cysteine residues with free thiol groups (RGD-2C2G) prevented aggregation during 2 × CsCl gradient purification but in coxsackie and adenovirus receptor (CAR)-low/negative cancer cell lines of human and rodent origin, this reduced cytolytic efficacy to the levels observed for a fiber non-modified control vector. However, both Ad5-delta-24-RGD4C and Ad5-delta-24-RGD2C2G were equally effective in the murine immunocompetent CT-2A glioma model due to a primary role of antitumor immune responses in the therapeutic efficacy of oncolytic virotherapy.

Oncolytic Adenoviruses: Strategies for Improved Targeting and Specificity

Cancer is a major health problem. Most of the treatments exhibit systemic toxicity, as they are not targeted or specific to cancerous cells and tumors. Adenoviruses are very promising gene delivery vectors and have immense potential to deliver targeted therapy. Here, we review a wide range of strategies that have been tried, tested, and demonstrated to enhance the specificity of oncolytic viruses towards specific cancer cells. A combination of these strategies and other conventional therapies may be more effective than any of those strategies alone.

Efficient gene transfer into T lymphocytes by fiber-modified human adenovirus 5

Background

The gene transduction efficiency of adenovirus to hematopoietic cells, especially T lymphocytes, is needed to be improved. The purpose of this study is to improve the transduction efficiency of T lymphocytes by using fiber-modified human adenovirus 5 (HAdV-5) vectors.

Results

Four fiber-modified human adenovirus 5 (HAdV-5) vectors were investigated to transduce hematopoietic cells. F35-EG or F11p-EG were HAdV-35 or HAdV-11p fiber pseudotyped HAdV-5, and HR-EG or CR-EG vectors were generated by incorporating RGD motif to the HI loop or to the C-terminus of F11p-EG fiber. All vectors could transduce more than 90% of K562 or Jurkat cells at an multiplicity of infection (MOI) of 500 viral particle per cell (vp/cell). All vectors except HR-EG could transduce nearly 90% cord blood CD34+ cells or 80% primary human T cells at the MOI of 1000, and F11p-EG showed slight superiority to F35-EG and CR-EG. Adenoviral vectors transduced CD4+ T cells a little more efficiently than they did to CD8+ T cells. These vectors showed no cytotoxicity at an MOI as high as 1000 vp/cell because the infected and uninfected T cells retained the same CD4/CD8 ratio and cell growth rate.

Conclusions

HAdV-11p fiber pseudotyped HAdV-5 could effectively transduce human T cells when human EF1a promoter was used to control the expression of transgene, suggesting its possible application in T cell immunocellular therapy.

Enhanced Delivery of Oncolytic Adenovirus by Neural Stem Cells for Treatment of Metastatic Ovarian Cancer

Oncolytic virotherapy is a promising approach for treating recurrent and/or drug-resistant ovarian cancer. However, its successful application in the clinic has been hampered by rapid immune-mediated clearance or neutralization of the virus, which reduces viral access to tumor foci. To overcome this barrier, patient-derived mesenchymal stem cells have been used to deliver virus to tumors, but variability associated with autologous cell isolations prevents this approach from being broadly clinically applicable. Here, we demonstrate the ability of an allogeneic, clonal neural stem cell (NSC) line (HB1.F3.CD21) to protect oncolytic viral cargo from neutralizing antibodies within patient ascites fluid and to deliver it to tumors within preclinical peritoneal ovarian metastases models. The viral payload used is a conditionally replication-competent adenovirus driven by the survivin promoter (CRAd-S-pk7). Because the protein survivin is highly expressed in ovarian cancer, but not in normal differentiated cells, viral replication should occur selectively in ovarian tumor cells. We found this viral agent was effective against cisplatin-resistant ovarian tumors and could be used as an adjunct treatment with cisplatin to decrease tumor burden without increasing toxicity. Collectively, our data suggest NSC-delivered CRAd-S-pk7 virotherapy holds promise for improving clinical outcome, reducing toxicities, and improving quality of life for patients with advanced ovarian cancer.

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).

Adenovirus armed with VGLL4 selectively kills hepatocellular carcinoma with G2/M phase arrest and apoptosis promotion

The Vestigial-Like Family Member 4 (VGLL4) functions as a native inhibitor of cell proliferation and tumor growth through multiple signaling pathways. We first discovered that VGLL4 causes G2/M phase arrest in hepatocellular carcinoma (HCC) cells. Then, we designed a novel survivin-regulated oncolytic adenovirus Ad-sp-VGLL4 carrying the VGLL4 gene. Ad-sp-VGLL4 exerted high HCC-targeting-selectivity but is less harmful to normal cells. This adenovirus construction enhanced antitumor activity due to G2/M phase arrest and enhanced apoptosis. It's also indicated that Ad-sp-VGLL4 could suppress the growth of transplanted tumor of HCC in vivo experiment. Taken together, our results suggest that Ad-sp-VGLL4 possesses strong antitumor capacity and has great potential use for HCC therapy.

Autophagy in glioma cells: An identity crisis with a clinical perspective

Over the last decade, autophagy has emerged as one of the critical cellular systems that control homeostasis. Besides management of normal homeostatic processes, autophagy can also be induced by tissue damage stress or by rapidly progressing tumors. During tumor progression, autophagy mediates a cellular reaction to the changes inside and outside of cells, which leads to tumor adaptation. Even though the regulation of autophagy seems universal and is a well-described process, its dysregulation and role in glioma progression remain an important topic of investigation. In this review, we summarize recent evidence of autophagy regulation in brain tumor tissues and possible interconnection between signaling pathways that govern cellular responses. This perspective may help to assess the qualitative differences and various outcomes in response to autophagy stimulation.

Combining Oncolytic Virotherapy with p53 Tumor Suppressor Gene Therapy

Oncolytic virus (OV) therapy utilizes replication-competent viruses to kill cancer cells, leaving non-malignant cells unharmed. With the first U.S. Food and Drug Administration-approved OV, dozens of clinical trials ongoing, and an abundance of translational research in the field, OV therapy is poised to be one of the leading treatments for cancer. A number of recombinant OVs expressing a transgene for p53 (TP53) or another p53 family member (TP63 or TP73) were engineered with the goal of generating more potent OVs that function synergistically with host immunity and/or other therapies to reduce or eliminate tumor burden. Such transgenes have proven effective at improving OV therapies, and basic research has shown mechanisms of p53-mediated enhancement of OV therapy, provided optimized p53 transgenes, explored drug-OV combinational treatments, and challenged canonical roles for p53 in virus-host interactions and tumor suppression. This review summarizes studies combining p53 gene therapy with replication-competent OV therapy, reviews preclinical and clinical studies with replication-deficient gene therapy vectors expressing p53 transgene, examines how wild-type p53 and p53 modifications affect OV replication and anti-tumor effects of OV therapy, and explores future directions for rational design of OV therapy combined with p53 gene therapy.

A Recombinant Chimeric Ad5/3 Vector Expressing a Multistage Plasmodium Antigen Induces Protective Immunity in Mice Using Heterologous Prime-Boost Immunization Regimens

An ideal malaria vaccine should target several stages of the parasite life cycle and induce antiparasite and antidisease immunity. We have reported a Plasmodium yoelii chimeric multistage recombinant protein (P. yoelii linear peptide chimera/recombinant modular chimera), engineered to express several autologous T cell epitopes and sequences derived from the circumsporozoite protein and the merozoite surface protein 1. This chimeric protein elicits protective immunity, mediated by CD4(+) T cells and neutralizing Abs. However, experimental evidence, from pre-erythrocytic vaccine candidates and irradiated sporozoites, has shown that CD8(+) T cells play a significant role in protection. Recombinant viral vectors have been used as a vaccine platform to elicit effective CD8(+) T cell responses. The human adenovirus (Ad) serotype 5 has been tested in malaria vaccine clinical trials with excellent safety profile. Nevertheless, a major concern for the use of Ad5 is the high prevalence of anti-vector neutralizing Abs in humans, hampering its immunogenicity. To minimize the impact of anti-vector pre-existing immunity, we developed a chimeric Ad5/3 vector in which the knob region of Ad5 was replaced with that of Ad3, conferring partial resistance to anti-Ad5 neutralizing Abs. Furthermore, we implemented heterologous Ad/protein immunization regimens that include a single immunization with recombinant Ad vectors. Our data show that immunization with the recombinant Ad5/3 vector induces protective efficacy indistinguishable from that elicited by Ad5. Our study also demonstrates that the dose of the Ad vectors has an impact on the memory profile and protective efficacy. The results support further studies with Ad5/3 for malaria vaccine development.

Unlocking the promise of oncolytic virotherapy in glioma: combination with chemotherapy to enhance efficacy

Malignant glioma is a relentless burden to both patients and clinicians, and calls for innovation to overcome the limitations in current management. Glioma therapy using viruses has been investigated to accentuate the nature of a virus, killing a host tumor cell during its replication. As virus mediated approaches progress with promising therapeutic advantages, combination therapy with chemotherapy and oncolytic viruses has emerged as a more synergistic and possibly efficacious therapy. Here, we will review malignant glioma as well as prior experience with oncolytic viruses, chemotherapy and combination of the two, examining how the combination can be optimized in the future.

A Multi Targeting Conditionally Replicating Adenovirus Displays Enhanced Oncolysis while Maintaining Expression of Immunotherapeutic Agents

Studies have demonstrated that oncolytic adenoviruses based on a 24 base pair deletion in the viral E1A gene (D24) may be promising therapeutics for treating a number of cancer types. In order to increase the therapeutic potential of these oncolytic viruses, a novel conditionally replicating adenovirus targeting multiple receptors upregulated on tumors was generated by incorporating an Ad5/3 fiber with a carboxyl terminus RGD ligand. The virus displayed full cytopathic effect in all tumor lines assayed at low titers with improved cytotoxicity over Ad5-RGD D24, Ad5/3 D24 and an HSV oncolytic virus. The virus was then engineered to deliver immunotherapeutic agents such as GM-CSF while maintaining enhanced heterogenic oncolysis.

Oncolytic adenoviruses: A thorny path to glioma cure

Glioblastoma Multiforme (GBM) is a rapidly progressing brain tumor. Despite the relatively low percentage of cancer patients with glioma diagnoses, recent statistics indicate that the number of glioma patients may have increased over the past decade. Current therapeutic options for glioma patients include tumor resection, chemotherapy, and concomitant radiation therapy with an average survival of approximately 16 months. The rapid progression of gliomas has spurred the development of novel treatment options, such as cancer gene therapy and oncolytic virotherapy. Preclinical testing of oncolytic adenoviruses using glioma models revealed both positive and negative sides of the virotherapy approach. Here we present a detailed overview of the glioma virotherapy field and discuss auxiliary therapeutic strategies with the potential for augmenting clinical efficacy of GBM virotherapy treatment.

Depletion of myeloid-derived suppressor cells during interleukin-12 immunogene therapy does not confer a survival advantage in experimental malignant glioma

Myeloid-derived suppressor cells (MDSCs) accumulate in the glioma microenvironment during tumor progression and promote immunosuppression. Interleukin-12 (IL-12) immunogene therapy can alter MDSCs toward an antigen-presenting cell phenotype and these mature cells can have a central role in antigen presentation. It remains unclear, however, how MDSC depletion can affect glioma immunotherapy. In this study, we generated a replication-deficient adenoviral vector, Ad.5/3.cRGD-mIL12p70, that transduces the GL261-based murine glioma cell line, resulting in the induction of biologically active, murine IL12p70 expression. Ex vivo, IL-12 expressed by GL261 cells induced interferon-γ synthesis in CD8(+) T cells (P<0.001), CD4(+) T cells (P=0.009) and natural killer cells (P=0.036). When injected 1 week after tumor implantation, Ad.5/3.cRGD-mIL12p70 successfully prolonged the survival of glioma-bearing mice. Sixty percent of animals treated with IL-12 immunotherapy were long-term survivors over 175 days, whereas all the control group animals expired by 40 days after tumor implantation (P=0.026). Mice receiving Ad.5/3.cRGD-mIL12p70 also accumulated 50% less MDSCs in the brain than the control group (P=0.007). Moreover, in the IL-12 group, MDSCs significantly overexpressed CD80 and major histocompatibility complex class II molecules (P=0.041). Depletion of MDSCs with Gr1(+) antibody had no survival benefit induced by IL-12-mediated immunotherapy. Of note, IL-12 therapy increased the presence of myeloid dendritic cells (mDCs) in the glioma microenvironment (P=0.0069). Ultimately, the data show that in the context of IL-12 immunogene therapy, MDSCs are dispensable and mDCs may provide the majority of antigen presentation in the brain.

Adenovirus-Based Vectors for the Development of Prophylactic and Therapeutic Vaccines

Emerging and reemerging infectious diseases as well as cancer pose great global health impacts on the society. Vaccines have emerged as effective treatments to prevent or reduce the burdens of already developed diseases. This is achieved by means of activating various components of the immune system to generate systemic inflammatory reactions targeting infectious agents or diseased cells for control/elimination. DNA virus-based genetic vaccines gained significant attention in the past decades owing to the development of DNA manipulation technologies, which allowed engineering of recombinant viral vectors encoding sequences for foreign antigens or their immunogenic epitopes as well as various immunomodulatory molecules. Despite tremendous progress in the past 50 years, many hurdles still remain for achieving the full clinical potential of viral-vectored vaccines. This chapter will present the evolution of vaccines from “live” or “attenuated” first-generation agents to recombinant DNA and viral-vectored vaccines. Particular emphasis will be given to human adenovirus (Ad) for the development of prophylactic and therapeutic vaccines. Ad biological properties related to vaccine development will be highlighted along with their advantages and potential hurdles to be overcome. In particular, we will discuss (1) genetic modifications in the Ad capsid protein to reduce the intrinsic viral immunogenicity, (2) antigen capsid incorporation for effective presentation of foreign antigens to the immune system, (3) modification of the hexon and fiber capsid proteins for Ad liver de-targeting and selective retargeting to cancer cells, (4) Ad-based vaccines carrying “arming” transgenes with immunostimulatory functions as immune adjuvants, and (5) oncolytic Ad vectors as a new therapeutic approach against cancer. Finally, the combination of adenoviral vectors with other non-adenoviral vector systems, the prime/boost strategy of immunization, clinical trials involving Ad-based vaccines, and the perspectives for the field development will be discussed.

A novel adenoviral vector labeled with superparamagnetic iron oxide nanoparticles for real-time tracking of viral delivery

In vivo tracking of gene therapy vectors challenges the investigation and improvement of biodistribution of these agents in the brain, a key feature for their targeting of infiltrative malignant gliomas. The glioma-targeting Ad5/3-cRGD gene therapy vector was covalently bound to super-paramagnetic iron oxide (Fe(3)O(4)) nanoparticles (SPION) to monitor its distribution by MRI. Transduction of labeled and unlabeled vectors was assessed on the U87 glioma cell line and normal human astrocytes (NHA), and was higher in U87 compared to NHA, but was similar between labeled and unlabeled virus. An in vivo study was performed by intracranial subcortical injection of labeled-Ad5/3-cRGD particles into a pig brain. The labeled vector appeared in vivo as a T2-weighted hyperintensity and a T2-gradient echo signal at the injection site, persisting up to 72 hours post-injection. We describe a glioma-targeting vector that is labeled with SPION, thereby allowing for MRI detection with no change in transduction capability.

Enhanced transduction and replication of RGD-fiber modified adenovirus in primary T cells

Background

Adenoviruses are often used as vehicles to mediate gene delivery for therapeutic purposes, but their research scope in hematological cells remains limited due to a narrow choice of host cells that express the adenoviral receptor (CAR). T cells, which are attractive targets for gene therapy of numerous diseases, remain resistant to adenoviral infection because of the absence of CAR expression. Here, we demonstrate that this resistance can be overcome when murine or human T cells are transduced with an adenovirus incorporating the RGD-fiber modification (Ad-RGD).

Methodology/Principal Finding

A luciferase-expressing replication-deficient Ad-RGD infected 3-fold higher number of activated primary T cells than an adenovirus lacking the RGD-fiber modification in vitro. Infection with replication-competent Ad-RGD virus also caused increased cell cycling, higher E1A copy number and enriched hexon antigen expression in both human and murine T cells. Transduction with oncolytic Ad-RGD also resulted in higher titers of progeny virus and enhanced the killing of T cells. In vivo, 35–45% of splenic T cells were transduced by Ad-RGD.

Conclusions

Collectively, our results prove that a fiber modified Ad-RGD successfully transduces and replicates in primary T cells of both murine and human origin.

Unity and diversity in the human adenoviruses: exploiting alternative entry pathways for gene therapy

Human Ads (adenoviruses) have been extensively utilized for the development of vectors for gene transfer, as they infect many cell types and do not integrate their genome into host-cell chromosomes. In addition, they have been widely studied as cytolytic viruses, termed oncolytic adenoviruses in cancer therapy. Ads are non-enveloped viruses with a linear double-stranded DNA genome of 30–38 kb which encodes 30–40 genes. At least 52 human Ad serotypes have been identified and classified into seven species, A–G. The Ad capsid has icosahedral symmetry and is composed of 252 capsomers, of which 240 are located on the facets of the capsid and consist of a trimeric hexon protein and the remaining 12 capsomers, the pentons, are at the vertices and comprise the penton base and projecting fibre protein. The entry of Ads into human cells is a two-step process. In the first step, the fibre protein mediates a primary interaction with the cell, effectively tethering the virus particle to the cell surface via a cellular attachment protein. The penton base then interacts with cell-surface integrins, leading to virus internalization. This interaction of the fibre protein with a number of cell-surface molecules appears to be important in determining the tropism of adenoviruses. Ads from all species, except species B and certain serotypes of species D, utilize CAR (coxsackie and adenovirus receptor) as their primary cellular-attachment protein, whereas most species B Ads use CD46, a complement regulatory protein. Such species-specific differences, as well as adaptations or modifications of Ads required for applications in gene therapy, form the major focus of the present review.

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.

Intravascularly administered RGD-displaying measles viruses bind to and infect neovessel endothelial cells in vivo

Systemically administered vectors must cross the endothelial lining of tumor blood vessels to access cancer cells. Vectors that interact with markers on the lumenal surface of these endothelial cells might have enhanced tumor localization. Here, we generated oncolytic measles viruses (MVs) displaying alpha(v)beta(3) integrin-binding peptides, cyclic arginine-glycine-aspartate (RGD) or echistatin, on the measles hemagglutinin protein. Both viruses had expanded tropisms, and efficiently entered target cells via binding to integrins, but also retained their native tropisms for CD46 and signaling lymphocyte activation molecule (SLAM). When fluorescently labeled and injected intravascularly into chick chorioallantoic membranes (CAMs), in contrast to unmodified viruses, the integrin-binding viral particles bound to the lumenal surface of the developing chick neovessels and infected the CAM vascular endothelial cells. In a mouse model of VEGF-induced angiogenesis in the ear pinna, the integrin-binding viruses, but not the parental virus, infected cells at sites of new blood vessel formation. When given intravenously to mice bearing tumor xenografts, the integrin-binding virus infected endothelial cells of tumor neovessels in addition to tumor parenchyma. To our knowledge, this is the first report demonstrating that oncolytic MVs can be engineered to target the lumenal endothelial surface of newly formed blood vessels when administered intravenously in living animals.

Usage of integrin and heparan sulfate as receptors for mouse adenovirus type 1

Adenovirus fiber knobs are the capsid components that interact with binding receptors on cells, while an Arg-Gly-Asp (RGD) sequence usually found in the penton base protein is important for the interaction of most adenoviruses with integrin entry receptors. Mouse adenovirus type 1 (MAV-1) lacks an RGD sequence in the virion penton base protein. We tested whether an RGD sequence found in the MAV-1 fiber knob plays a role in infection. Treatment of cells with a competitor RGD peptide or a purified recombinant RGD-containing fiber knob prior to infection resulted in reduced virus yields compared to those of controls, indicating the importance of the RGD sequence for infection. An investigation of the role of integrins as possible receptors showed that MAV-1 yields were reduced in the presence of EDTA, an inhibitor of integrin binding, and in the presence of anti-alpha(v) integrin antibody. Moreover, mouse embryo fibroblasts that were genetically deficient in alpha(v) integrin yielded less virus, supporting the hypothesis that alpha(v) integrin is a likely receptor for MAV-1. We also investigated whether glycosaminoglycans play a role in MAV-1 infection. Preincubation of MAV-1 with heparin, a heparan sulfate glycosaminoglycan analog, resulted in a decrease in MAV-1 virus yields. Reduced MAV-1 infectivity was also found with cells that genetically lack heparan sulfate or cells that were treated with heparinase I. Cumulatively, our data demonstrate that the RGD sequence in the MAV-1 fiber knob plays a role in infection by MAV-1, alpha(v) integrin acts as a receptor for the virus, and cell surface heparin sulfate glycosaminoglycans are important in MAV-1 infection.

Neural stem cells target intracranial glioma to deliver an oncolytic adenovirus in vivo

Adenoviral oncolytic virotherapy represents an attractive treatment modality for central nervous system (CNS) neoplasms. However, successful application of virotherapy in clinical trials has been hampered by inadequate distribution of oncolytic vectors. Neural stem cells (NSCs) have been shown as suitable vehicles for gene delivery because they track tumor foci. In this study, we evaluated the capability of NSCs to deliver a conditionally replicating adenovirus (CRAd) to glioma. We examined NSC specificity with respect to viral transduction, migration and capacity to deliver a CRAd to tumor cells. Fluorescence-activated cell sorter (FACS) analysis of NSC shows that these cells express a variety of surface receptors that make them amenable to entry by recombinant adenoviruses. Luciferase assays with replication-deficient vectors possessing a variety of transductional modifications targeted to these receptors confirm these results. Real-time PCR analysis of the replication profiles of different CRAds in NSCs and a representative glioma cell line, U87MG, identified the CRAd-Survivin (S)-pk7 virus as optimal vector for further delivery studies. Using in vitro and in vivo migration studies, we show that NSCs infected with CRAd-S-pk7 virus migrate and preferentially deliver CRAd to U87MG glioma. These results suggest that NSCs mediate an enhanced intratumoral distribution of an oncolytic vector in malignant glioma when compared with virus injection alone.

Biodistribution of an oncolytic adenovirus after intracranial injection in permissive animals: a comparative study of Syrian hamsters and cotton rats

Conditionally replicative adenoviruses (CRAds) are often evaluated in mice; however, normal and cancerous mouse tissues are poorly permissive for human CRAds. As the cotton rat (CR) is a semipermissive animal and the Syrian hamster (SH) is a fully permissive model for adenoviral replication, we compared them in a single study following intracranial (i.c.) injection of a novel glioma-targeting CRAd. Viral genomic copies were quantified by real-time PCR in brain, blood, liver and lung. The studies were corroborated by immunohistochemical, serological and immunological assays. CR had a multiple log higher susceptibility for adenoviral infection than SH. A similar amount of genomic copies of CRAd-Survivin-pk7 and human adenovirus serotype 5 (AdWT) was found in the brain of CR and in all organs from SH. In blood and lung of CR, AdWT had more genomic copies than CRAd-Survivin-pk7 in some of the time points studied. Viral antigens were confirmed in brain slices, an elevation of serum transaminases was observed in both models, and an increase in anti-adenoviral antibodies was detected in SH sera. In conclusion, CR represents a sensitive model for studying biodistribution of CRAds after i.c. delivery, allowing for the detection of differences in the replication of CRAd-Survivin-pk7 and AdWT that were not evident in SH.

Vector therapies for malignant glioma: shifting the clinical paradigm

BACKGROUND

Malignant glioma represents one of the most aggressive and devastating forms of human cancer. At present, there exists no successful treatment for this disease. Gene therapy, or vector therapy, has emerged as a viable experimental treatment method for intracranial malignancies.

OBJECTIVE

Vector therapy paradigms that have entered the clinical arena have shown adequate safety; however, the majority of the studies failed to observe significant clinical benefits. As such, researchers have refocused their efforts on developing novel vectors as well as new delivery methods to enhance the therapeutic effect of a particular vector. In this review, we discuss common vector therapy approaches used in clinical trials, their drawbacks and potential ways of overcoming these challenges.

METHODS

We focus on the experimental evaluation of cell-based vector therapies and adenoviral and herpes simplex virus type 1 vectors in the treatment of malignant glioma.

CONCLUSION

Vector therapy remains a promising treatment strategy for malignant glioma. Although significant questions remain to be answered, early clinical data suggest safety of this approach and future studies will likely address the efficacy of the proposed therapy.

Mesenchymal stem cells effectively deliver an oncolytic adenovirus to intracranial glioma

Gene therapy represents a promising treatment alternative for patients with malignant gliomas. Nevertheless, in the setting of these highly infiltrative tumors, transgene delivery remains a challenge. Indeed, viral vehicles tested in clinical trials often target only those tumor cells that are adjacent to the injection site. In this study, we examined the feasibility of using human mesenchymal stem cells (hMSC) to deliver a replication-competent oncolytic adenovirus (CRAd) in a model of intracranial malignant glioma. To do so, CRAds with a chimeric 5/3 fiber or RGD backbone with or without CXCR4 promoter driving E1A were examined with respect to replication and toxicity in hMSC, human astrocytes, and the human glioma cell line U87MG by quantitative polymerase chain reaction and membrane integrity assay. CRAd delivery by virus-loaded hMSC was then evaluated in vitro and in an in vivo model of mice bearing intracranial U87MG xenografts. Our results show that hMSC are effectively infected by CRAds that use the CXCR4 promoter. CRAd-CXCR4-RGD had the highest replication, followed by CRAd-CXCR4-5/3, in hMSC, with comparable levels of toxicity. In U87MG tumor cells, CRAd-CXCR4-5/3 showed the highest replication and toxicity. Virus-loaded hMSC effectively migrated in vitro and released CRAds that infected U87MG glioma cells. When injected away from the tumor site in vivo, hMSC migrated to the tumor and delivered 46-fold more viral copies than injection of CRAd-CXCR4-5/3 alone. Taken together, these results indicate that hMSC migrate and deliver CRAd to distant glioma cells. This delivery strategy should be explored further, as it could improve the outcome of oncolytic virotherapy for glioma.

Survivin-driven and fiber-modified oncolytic adenovirus exhibits potent antitumor activity in established intracranial glioma

The poor prognosis of patients with malignant gliomas necessitates the development of novel therapies. Virotherapy, using genetically engineered adenovectors that selectively replicate in and kill neoplastic cells, represents one such strategy. In this study, we examined several oncolytic vectors with modified transcriptional and transductional control of viral replication. First, we incorporated the survivin promoter (S) to drive E1A gene expression. We then modified the adenovirus serotype 5 (Ad5) fiber protein via genetic knob switching or incorporation of peptide ligands to target the following glioma-associated receptors: the Ad3 attachment protein, or CD46, alpha(v) beta(3)/alpha(v)beta(5) integrins, or heparan sulfate proteoglycans. The three conditionally replicative adenoviruses, CRAd-S-5/3, CRAd-S-RGD, and CRAd-S-pk7, were then examined in vitro with respect to transduction efficiency and tissue specificity. The most promising virus was then tested in vivo for evidence of tumor growth inhibition. CRAd-S-pk7 provided the highest level of viral replication and tumor oncolysis in glioma cell lines. At the same time, we observed minimal viral replication and toxicity in normal human brain. Injection of CRAd-S-pk7 inhibited xenograft tumor growth by more than 300% (p < 0.001). Sixty-seven percent of treated mice with intracranial tumors were long-term survivors (>110 days; p < 0.005). Analysis of tumor tissue indicated increased adenoviral infectivity, decreased mitotic activity, and enhanced tumor apoptosis. These findings demonstrate the effectiveness of CRAd-S-pk7 and provide the rationale for further development of this novel oncolytic virus for glioma gene therapy.

Lysis of Dysplastic but not Normal Oral Keratinocytes and Tissue-Engineered Epithelia with Conditionally Replicating Adenoviruses

There is no effective medical treatment for oral precancer, and surgery to remove these lesions is imprecise because abnormal mucosa extends beyond the visible lesion. Development of vectors for tumor-selective viral replication has been a significant advance, and viral lysis is well suited to destruction of oral precancerous mucosa. To facilitate evaluation of new treatments, we engineered dysplastic oral epithelium using keratinocytes isolated from dysplastic lesions. We show that these model systems recapitulate the key characteristics of the clinical lesions closely, and that topical delivery of the conditionally replicating adenovirus (CRAd) dl922-947 can lyse tissue-engineered epithelia that show mild, moderate, or severe dysplasia, but normal oral epithelia are very resistant to this treatment. The lytic effect is determined by various factors, including the grade and proliferation index of the dysplastic epithelia. The presence of suprabasal cycling cells, expression of the coxsackie adenovirus receptor (CAR), the transcription cofactor p300, and other aberrations that affect the regulation of the cell cycle or apoptosis and promote viral replication may also be important. The ability of dl922-947 to destroy engineered oral dysplasia was significantly greater than that observed using wild-type adenovirus, d/1520, or viruses modified to bypass cell entry dependent on the presence of CAR. Evidence of infection in clinical dysplastic lesions was also shown ex vivo using tissue explants. We conclude that dl922-947 may provide an efficient molecular cytotoxic to dissolve oral dysplastic lesions.

Novel recombinant adenoviral vector that targets the interleukin-13 receptor alpha2 chain permits effective gene transfer to malignant glioma

Transduction of malignant glioma with adenovirus serotype 5 (Ad5) vectors is limited by the low levels of coxsackievirus and adenovirus receptor (CAR) on tumor cells. However, malignant brain tumors have been found to overexpress a glioma-associated receptor, interleukin-13 receptor alpha2 chain (IL-13Ralpha2), a marker of both glial transformation and tumor grade. To selectively target Ad5 to IL-13Ralpha2, we constructed a replication-deficient adenoviral vector that possesses an IL-13 ligand presented by a T4 phage fibritin shaft, and designated the new virus LU-13. Western blot and sequence analyses confirmed proper trimerization and ligand presentation by the T4 fibritin shaft. Confocal microscopy analysis of primary glioma suspensions incubated with viral recombinants showed that LU-13 colocalized with IL-13Ralpha2. Luciferase transduction assays conducted in both primary and passaged glioma cell cultures exhibited at least 10-fold enhanced gene transduction. Moreover, the virus preferentially bound to glioma cells, as documented by increased adenoviral E4 DNA copy number. In vitro competition assays performed with anti-human IL-13 monoclonal antibody confirmed significant attenuation of LU-13 transduction. These results were further confirmed in vivo, where LU-13 showed a 300-fold increase in transgene expression. In summary, we describe here the development of a novel and targeted adenoviral vector that binds IL-13Ralpha2. Our findings confirm the ability of LU-13 to bind IL-13Ralpha2 and increase transgene expression, making it an attractive gene therapy vector for the treatment of malignant glioma in a clinical setting.

Fiber-knob modifications enhance adenoviral tropism and gene transfer in malignant glioma

BACKGROUND

Malignant gliomas remain refractory to Ad5-mediated gene therapy due to deficiency of the coxsackie adenovirus receptor on tumor cells. The purpose of this study was to evaluate whether changes in adenoviral tropism can enhance gene transfer in the context of malignant glioma.

METHODS

We have identified several receptors that are over-expressed on tumor cells and created a series of pseudotyped Ad5 vectors that recognize these receptors: Ad5-RGD which binds alpha(v)beta3/alpha(v)beta5 integrins; Ad5/3 which contains adenovirus serotype 3 knob and binds to CD46; Ad5-Sigma which incorporates the reovirus sigma knob and binds to junctional adhesion molecule-1; and Ad5-pk7 which contains the polylysine motif and binds heparan sulfate proteoglycans. We also investigated the Ad5-CAV1 vector, which contains the knob of canine adenovirus type 1, a virus previously shown to infect glioma via an unknown mechanism. In this study, we compared these modified vectors for their ability to promote the expression of luciferase transgene both in vitro and in vivo.

RESULTS

Our results indicate that all five modified vectors attained higher mean luciferase activity vs. control. Among them, Ad5-CAV1 and Ad5-pk7 attained the highest transduction efficiency independent of different tumor lines or infection time. Ad5-Sigma and Ad5-pk7 also demonstrated the least nonspecific infection in normal human astrocytes. Most importantly, Ad5-pk7 achieved 1000-fold increased transgene expression in human glioma xenografts in vivo.

CONCLUSIONS

These results indicate that modifications of adenoviral tropism can enhance gene transfer in tumors that are poorly susceptible to adenoviral vectors and warrant further development of Ad5-pk7 for glioma gene therapy.

Retrograde infection of precerebellar nuclei neurons by injection of a recombinant adenovirus into the cerebellar cortex of normal and reeler mice

The reeler mouse is an autosomal recessive mutant mouse caused by mutation of the reelin gene and characterized by cerebellar ataxia. To determine whether the distribution pattern of precerebellar nuclei neurons in the brainstem of the reeler mouse changes, we injected a small volume of a replication-defective recombinant adenovirus carrying E. coli beta-galactosidase (lacZ) into the cerebellar cortex of normal and reeler mice. Five days later, the mice were transcardially perfused by a fixative solution. X-gal staining of coronal or sagittal sections of the brainstem revealed that many origins for reticulocerebellar, cuneocerebellar, trigeminocerebellar, and pontocerebellar projections were retrogradely labeled, but only a few olivocerebellar neurons were labeled. Retrogradely labeled neurons in the lateral reticular nucleus tended to locate more laterally and be more condensed into a small compartment in the reeler compared with their normal counterparts. Retrogradely labeled neurons in the external cuneate nucleus were more dorsally shifted in the reeler mice compared with their normal counterparts. We could not find any differences between the normal and reeler mice in the distribution patterns of their trigeminocerebellar projection neurons. Retrogradely labeled pontocerebellar neurons in the basilar pons of the reeler mouse were reduced in number compared with their normal counterparts in addition to being more ventrally and laterally shifted. These findings strongly suggest that the migration of some precerebellar nuclei neurons from the rhombic lip to their final loci may be obstructed in the reeler mice.