Cytolytic viruses as potential anti-cancer agents

A new compound, phomaherbarine A, induces cytolytic reactivation in epstein-barr virus-positive B cell lines

Epstein-Barr virus (EBV), the first virus found to induce cancer in humans, has been frequently detected in various types of B cell lymphomas. During its latent phase, EBV expresses a limited set of proteins crucial for its persistence. Induction of the lytic phase of EBV has shown promise in the treatment of EBV-associated malignancies. The present study assessed the ability of phomaherbarine A, a novel compound derived from the endophytic fungus Phoma herbarum DBE-M1, to stimulate lytic replication of EBV in B95-8 cells. Phomaherbarine A was found to efficiently initiate the expression of both early and late EBV lytic genes in B95-8 cells, with this initiation being further heightened by the addition of phorbol myristate acetate and sodium butyrate. Moreover, phomaherbarine A demonstrated notable cytotoxicity against the EBV-associated B cell lymphoma cell lines B95-8 and Raji. Mechanistically, phomaherbarine A induces apoptosis in these cells through the activation of caspase-3/7. When combined with ganciclovir, phomaherbarine A does not interfere with the reduction of viral replication by ganciclovir and sustains its apoptosis induction. In conclusion, these findings indicate that phomaherbarine A may be a promising candidate for therapeutic intervention in patients with EBV-associated B cell lymphomas.

The Interference between SARS-CoV-2 and Tyrosine Kinase Receptor Signaling in Cancer

Cancer and viruses have a long history that has evolved over many decades. Much information about the interplay between viruses and cell proliferation and metabolism has come from the history of clinical cases of patients infected with virus-induced cancer. In addition, information from viruses used to treat some types of cancer is valuable. Now, since the global coronavirus pandemic erupted almost a year ago, the scientific community has invested countless time and resources to slow down the infection rate and diminish the number of casualties produced by this highly infectious pathogen. A large percentage of cancer cases diagnosed are strongly related to dysregulations of the tyrosine kinase receptor (TKR) family and its downstream signaling pathways. As such, many therapeutic agents have been developed to strategically target these structures in order to hinder certain mechanisms pertaining to the phenotypic characteristics of cancer cells such as division, invasion or metastatic potential. Interestingly, several authors have pointed out that a correlation between coronaviruses such as the SARS-CoV-1 and -2 or MERS viruses and dysregulations of signaling pathways activated by TKRs can be established. This information may help to accelerate the repurposing of clinically developed anti-TKR cancer drugs in COVID-19 management. Because the need for treatment is critical, drug repurposing may be an advantageous choice in the search for new and efficient therapeutic compounds. This approach would be advantageous from a financial point of view as well, given that the resources used for research and development would no longer be required and can be potentially redirected towards other key projects. This review aims to provide an overview of how SARS-CoV-2 interacts with different TKRs and their respective downstream signaling pathway and how several therapeutic agents targeted against these receptors can interfere with the viral infection. Additionally, this review aims to identify if SARS-CoV-2 can be repurposed to be a potential viral vector against different cancer types.

Safety, biodistribution and viral shedding of oncolytic vaccinia virus TG6002 administered intravenously in healthy beagle dogs

Oncolytic virotherapy is an emerging strategy that uses replication-competent viruses to kill tumor cells. We have reported the oncolytic effects of TG6002, a recombinant oncolytic vaccinia virus, in preclinical human xenograft models and canine tumor explants. To assess the safety, biodistribution and shedding of TG6002 administered by the intravenous route, we conducted a study in immune-competent healthy dogs. Three dogs each received a single intravenous injection of TG6002 at 105 PFU/kg, 106 PFU/kg or 107 PFU/kg, and one dog received three intravenous injections at 107 PFU/kg. The injections were well tolerated without any clinical, hematological or biochemical adverse events. Viral genomes were only detected in blood at the earliest sampling time point of one-hour post-injection at 107 PFU/kg. Post mortem analyses at day 35 allowed detection of viral DNA in the spleen of the dog which received three injections at 107 PFU/kg. Viral genomes were not detected in the urine, saliva or feces of any dogs. Seven days after the injections, a dose-dependent antibody mediated immune response was identified. In conclusion, intravenous administration of TG6002 shows a good safety profile, supporting the initiation of clinical trials in canine cancer patients as well as further development as a human cancer therapy.

Assessing and Overcoming Resistance Phenomena against a Genetically Modified Vaccinia Virus in Selected Cancer Cell Lines

Genetically modified vaccinia viruses (VACVs) have been shown to possess profound oncolytic capabilities. However, tumor cell resistance to VACVs may endanger broad clinical success. Using cell mass assays, viral replication studies, and fluorescence microscopy, we investigated primary resistance phenomena of cell lines of the NCI-60 tumor cell panel to GLV-1h94, a derivative of the Lister strain of VACV, which encodes the enzyme super cytosine deaminase (SCD) that converts the prodrug 5-fluorocytosine (5-FC) into the chemotherapeutic compound 5-fluorouracil (5-FU). After treatment with GLV-1h94 alone, only half of the cell lines were defined as highly susceptible to GLV-1h94-induced oncolysis. When adding 5-FC, 85% of the cell lines became highly susceptible to combinatorial treatment; none of the tested tumor cell lines exhibited a "high-grade resistance" pattern. Detailed investigation of the SCD prodrug system suggested that the cytotoxic effect of converted 5-FU is directed either against the cells or against the virus particles, depending on the balance between cell line-specific susceptibility to GLV-1h94-induced oncolysis and 5-FU sensitivity. The data provided by this work underline that cellular resistance against VACV-based virotherapy can be overcome by virus-encoded prodrug systems. Phase I/II clinical trials are recommended to further elucidate the enormous potential of this combination therapy.

Current Perspectives on Therapies, Including Drug Delivery Systems, for Managing Glioblastoma Multiforme

Glioblastoma multiforme (GBM), a standout among the most dangerous class of central nervous system (CNS) cancer, is most common and is an aggressive malignant brain tumor in adults. In spite of developments in modality therapy, it remains mostly incurable. Consequently, the need for novel systems, strategies, or therapeutic approaches for enhancing the assortment of active agents meant for GBM becomes an important criterion. Currently, cancer research focuses mainly on improving the treatment of GBM via diverse novel drug delivery systems. The treatment options at diagnosis are multimodal and include radiation therapy. Moreover, significant advances in understanding the molecular pathology of GBM and associated cell signaling pathways have opened opportunities for new therapies. Innovative treatment such as immunotherapy also gives hope for enhanced survival. The objective of this work was to collect and report the recent research findings to manage GBM. The present review includes existing novel drug delivery systems and therapies intended for managing GBM. Reported novel drug delivery systems and diverse therapies seem to be precise, secure, and relatively effective, which could lead to a new track for the obliteration of GBM.

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.

Paramyxoviruses activation by host proteases in cultures of normal and cancer cells

Multiplication of paramyxovirus Sendai and Newcastle disease virus (NDV) was studied in cultures of normal and tumor cells. Production of noninfectious virus with uncleaved F0 was observed in canine kidney cell line MDCK (line H) and its derivatives carrying tetracycline-regulated expression of transmembrane protease HAT or TMPRSS2 with trypsin-like cleavage specificity. Under tetracycline induction, a cleavage F0 (65 kD)>F1 (50 kD)+F2(15 kD) and production of infectious virus were observed in these cell cultures. Under tetracycline induction, the additional subunit 38K (m.w. 38 kDa) of the F protein was detected both in infected MDCK-HAT cells and in newly synthesized Sendai virus in addition to F0, F1 and F2, indicating thereby a second HAT-sensitive proteolytic site in the F0 molecule. Highly infectious virus containing cleaved F1+F2 was produced in cultures of cancer cells Caco-2 and H1299. Virus Sendai synthesized in H1299 cells contained 38 K subunit indicating a cleavage of the F0 at a second site by H1299 host cell proteases. Levels of cleaved F1+F2 and infectious virions were higher at the late stage of infection in cancer cells, suggesting thus the induction of virus-activating proteases in Caco-2 and H1299 cells under infection with paramyxoviruses. NDV virus was found to induce more rapid death of cancer cells Caco-2 than Sendai virus. Cooperatively, the obtained data show that cancer cells in distinction to nonmalignant cells can synthesize protease(s) activating infectivity of paramyxoviruses. Thus, they are more vulnerable to paramyxovirus infection than normal cells.

Going viral: a review of replication-selective oncolytic adenoviruses

Oncolytic viruses have had a tumultuous course, from the initial anecdotal reports of patients having antineoplastic effects after natural viral infections a century ago to the development of current cutting-edge therapies in clinical trials. Adenoviruses have long been the workhorse of virotherapy, and we review both the scientific and the not-so-scientific forces that have shaped the development of these therapeutics from wild-type viral pathogens, turning an old foe into a new friend. After a brief review of the mechanics of viral replication and how it has been modified to engineer tumor selectivity, we give particular attention to ONYX-015, the forerunner of virotherapy with extensive clinical testing that pioneered the field. The findings from those as well as other oncolytic trials have shaped how we now view these viruses, which our immune system has evolved to vigorously attack, as promising immunotherapy agents.

Therapeutic potential of oncolytic Newcastle disease virus: a critical review

Newcastle disease virus (NDV) features a natural preference for replication in many tumor cells compared with normal cells. The observed antitumor effect of NDV appears to be a result of both selective killing of tumor cells and induction of immune responses. Genetic manipulations to change viral tropism and arming the virus with genes encoding for cytokines improved the oncolytic capacity of NDV. Several intracellular proteins in tumor cells, including antiapoptotic proteins (Livin) and oncogenic proteins (H-Ras), are relevant for the oncolytic activity of NDV. Defects in the interferon system, found in some tumor cells, also contribute to the oncolytic selectivity of NDV. Notwithstanding, NDV displays effective oncolytic activity in many tumor types, despite having intact interferon signaling. Taken together, several cellular systems appear to dictate the selective oncolytic activity of NDV. Some barriers, such as neutralizing antibodies elicited during NDV treatment and the extracellular matrix in tumor tissue appear to interfere with spread of NDV and reduce oncolysis. To further understand the oncolytic activity of NDV, we compared two NDV strains, ie, an attenuated virus (NDV-HUJ) and a pathogenic virus (NDV-MTH-68/H). Significant differences in amino acid sequence were noted in several viral proteins, including the fusion precursor (F0) glycoprotein, an important determinant of replication and pathogenicity. However, no difference in the oncolytic activity of the two strains was noted using human tumor tissues maintained as organ cultures or in mouse tumor models. To optimize virotherapy in clinical trials, we describe here a unique organ culture methodology, using a biopsy taken from a patient’s tumor before treatment for ex vivo infection with NDV to determine the oncolytic potential on an individual basis. In conclusion, oncolytic NDV is an excellent candidate for cancer therapy, but more knowledge is needed to ensure success in clinical trials.

Prominent IL-12 production and tumor reduction in athymic nude mice after Toxoplasma gondii lysate antigen treatment

Toxoplasma gondii is an intracellular protozoan parasite that causes a Th1 cellular immunity. Our previous study showed that T. gondii lysate antigen (TLA) treatment in S180 tumor-bearing mice resulted in tumor reduction by suppressing CD31 expression, a marker of angiogenesis. In the present study, to investigate tumor suppressive effect of TLA under the absence of T lymphocytes, athymic nude mice were compared with euthymic mice in the anti-tumorigenic effect triggered by TLA in CT26 tumors. According to the results, intratumorally injected TLA reduced tumor growth and TIMP-1 level, a metastatic marker, in both euthymic and athymic mice. TLA treatment led to a sharp increase in IL-12 expression in serum cytokine profiling of athymic mice, and increased MyD88 signals in macrophages derived from the bone marrow, implying the activation of innate immunity. The selective induction of IL-12 by TLA treatment had an anti-tumorigenic effect.

Combination of bladder cancer-specific oncolytic adenovirus gene therapy with cisplatin on bladder cancer in vitro

Bladder cancer-specific oncolytic adenovirus Ad/PSCAE/UPII/E1A, carrying E1A gene regulated by human Uroplakin II (UPII) promoter and prostate stem cell antigen enhancer (PSCAE), could kill bladder tumor cells preferentially. The aim of this study was to examine the effects of Ad/PSCAE/UPII/E1A combined with cisplatin on human bladder cancer cells and to identify the underlying mechanisms. The combined effects of Ad/PSCAE/UPII/E1A and cisplatin on EJ, 5637, and BIU-87 bladder cancer cells were evaluated by MTT cell proliferation assay. Cell apoptosis was detected by flow cytometry with fluorescein isothiocyanate-conjugated annexin V (annexin V-FITC) and propidium iodide staining. The activation of the caspase pathway and the expression of Bcl-2 family proteins were determined by western blot assay. Ad/PSCAE/UPII/E1A adenovirus vector could infect bladder cancer cell lines selectively and induce growth inhibition effectively. Of note, the combination treatment of cisplatin and Ad/PSCAE/UPII/E1A could inhibit the proliferation of bladder cancer cells significantly compared with the "alone" treatment. Furthermore, Ad/PSCAE/UPII/E1A plus cisplatin combined treatment resulted in enhanced apoptosis in bladder cancer cells. The enhanced antitumor effects in vitro elicited by Ad/PSCAE/UPII/E1A plus cisplatin were closely related to the increased Fas expression and cleavage of caspase-8 and Bid and decrease in the ratio of anti- to pro-apoptotic proteins followed by activation of caspase-9 and caspase-3, which may contribute to the activation of extrinsic and intrinsic apoptotic pathways. Our results indicate that the combination of Ad/PSCAE/UPII/E1A with cisplatin exerts a synergistic antitumor effect on human bladder cancer cells and is a potential combined treatment strategy for bladder cancer.

Experimental studies on treatment of pancreatic cancer with double-regulated duplicative adenovirus AdTPHre-hEndo carrying human endostatin gene

BACKGROUND

Gene-virus targeted therapy is a promising new method of treating pancreatic cancer. To increase the efficacy and decrease the side-effect, we constructed a conditionally replicative adenovirus (CRAd) expressing human endostatin, with a human Telomoerase Reverse Transcriptase (hTERT) promoter for the regulation of the early stage of adenovirus expression of gene E1a and a Hypoxia Response Element (HRE) promoter to regulate the gene E1b.

METHODS

A gene recombination technique was adopted to construct and generate the adenovirus AdTPHre-hEndo. Pancreatic cancer cells were studied both in vitro and in vivo. Western blotting was adopted to observe the expressions of protein E1A and E1B; duplication assay was applied to observe the selective duplication capability of recombinant cells. MTT assay was applied to measure the lethal effects of virus on pancreatic cancer cells, and ELISA was adopted to detect the human endostatin gene expression. A pancreatic cancer transplantation tumor model of nude mice was constructed to observe the antitumor effects of the virus.

RESULTS

Double-regulated duplicative adenovirus AdTPHre-hEndo genes were successfully constructed. Duplication and lethal assays proved that AdTPHre-hEndo could replicate specifically in pancreatic cancer cells and kill them. The endostatin expression in a cultured supernatant from tumor cells was significantly higher than that obtained from non-duplicative adenovirus vectors carrying that gene. The animal experiment demonstrated that AdTPHre-hEndo has a high capability to limit pancreatic cancer growth.

CONCLUSIONS

AdTPHre-hEndo has a special ability to duplicate and kill pancreatic cancer cells in in vitro and in vivo experiments, thus providing a new gene-virus-based treatment system for pancreatic cancer.

Fiber-substituted conditionally replicating adenovirus Ad5F35 induces oncolysis of human bladder cancer cells in in vitro analysis

OBJECTIVE

To assess the oncolytic effect of fiber-substituted conditionally replicating adenovirus type 5 (Ad5) F35 vector on human bladder cancer cell lines such as 253J, 5637, KK-47, T24, TCCSUP, and UMUC-3 cells.

MATERIALS AND METHODS

Ad5F35 and Ad5 conditionally replicating adenovirus vectors containing the E1 gene controlled by the human midkine promoter (Ad5F35/MKp-E1 and Ad5/MKp-E1, respectively) were constructed. Reverse transcriptase-polymerase chain reaction and cell viability assay were performed in cells transfected with Ad5F35/MKp-E1 or Ad5/MKp-E1.

RESULTS

Of the bladder cancer cells used, considerably lower expression of mRNA for Coxsackie and adenovirus receptor, an Ad5 receptor, was found with T24 and TCCSUP cells. However, the mRNA for CD46, an Ad35 receptor, was abundantly expressed in all the cell types. Ad5F35/MKp-E1 induced oncolysis in a plaque formation unit-dependent manner for all the bladder cancer cells used, with greater efficacy than Ad5/MKp-E1 for T24, TCCSUP, and 253J cells.

CONCLUSION

The results of the present study have shown that Ad5F35/MKp-E1 is more useful for the gene therapy of bladder cancer than Ad5/MKp-E1 is for some cell lines.

p53 cooperates with DNA methylation and a suicidal interferon response to maintain epigenetic silencing of repeats and noncoding RNAs

Large parts of mammalian genomes are transcriptionally inactive and enriched with various classes of interspersed and tandem repeats. Here we show that the tumor suppressor protein p53 cooperates with DNA methylation to maintain silencing of a large portion of the mouse genome. Massive transcription of major classes of short, interspersed nuclear elements (SINEs) B1 and B2, both strands of near-centromeric satellite DNAs consisting of tandem repeats, and multiple species of noncoding RNAs was observed in p53-deficient but not in p53 wild-type mouse fibroblasts treated with the DNA demethylating agent 5-aza-2'-deoxycytidine. The abundance of these transcripts exceeded the level of β-actin mRNA by more than 150-fold. Accumulation of these transcripts, which are capable of forming double-stranded RNA (dsRNA), was accompanied by a strong, endogenous, apoptosis-inducing type I IFN response. This phenomenon, which we named "TRAIN" (for "transcription of repeats activates interferon"), was observed in spontaneous tumors in two models of cancer-prone mice, presumably reflecting naturally occurring DNA hypomethylation and p53 inactivation in cancer. These observations suggest that p53 and IFN cooperate to prevent accumulation of cells with activated repeats and provide a plausible explanation for the deregulation of IFN function frequently seen in tumors. Overall, this work reveals roles for p53 and IFN that are key for genetic stability and therefore relevant to both tumorigenesis and the evolution of species.

Anti-leukemic activity of Newcastle disease virus strains AF2240 and V4-UPM in murine myelomonocytic leukemia in vivo

Newcastle disease virus (NDV) is a member of the Paramyxoviridae that has caused severe economic losses in poultry industry worldwide. Several strains of NDV were reported to induce cytolysis to cancerous cell lines. It has prompted much interest as anticancer agent because it can replicate up to 10,000 times better in human cancer cells than in most normal cells. In this study, two NDV strains, viserotropic-velogenic strain AF2240 and lentogenic strain V4-UPM, showed cytolytic activity and apoptosis induction against Mouse myelomoncytic leukemia (WEHI 3B). The cytolytic effects of NDV strains were determined using microtetrazolium (MTT) assay. The cytolytic dose - fifty percent (CD(50)) were 2 and 8HAU for AF2240 and V4-UPM strains, respectively. Cells treated with NDV strains showed apoptotic features compared to the untreated cells under fluorescence microscope. NDV induced activation of caspase-3 and DNA laddering in agarose gel electrophoresis which confirmed the apoptosis. The anti-leukemic activity of both strains was evaluated on myelomoncytic leukemia BALB/c mice. The results indicated that both NDV strains significantly decreased liver and spleen weights. It also decreased blasts cell percentage in blood, bone marrow and spleen smears of treated mice (p<0.05). Histopathological studies for spleen and liver confirmed the hematological results of blood and bone marrow. From the results obtained, the exposure to both NDV stains AF2240 and V4-UPM showed similar results for Ara-c. In conclusion NDV strains AF2240 and V4-UPM can affect WEHI 3B leukemia cells in vitro and in vivo.

The emerging role of viruses in the treatment of solid tumours

There is increasing optimism for the use of non-pathogenic viruses in the treatment of many cancers. Initial interest in oncolytic virotherapy was based on the observation of an occasional clinical resolution of a lymphoma after a systemic viral infection. In many cancers, by comparison with normal tissues, the competency of the cellular anti-viral mechanism is impaired, thus creating an exploitable difference between the tumour and normal cells, as an unimpeded viral proliferation in cancer cells is eventually cytocidal. In addition to their oncolytic capability, these particular viruses may be engineered to facilitate gene delivery to tumour cells to produce therapeutic effects such as cytokine secretion and anti -tumour immune responses prior to the eventual cytolysis. There is now promising clinical experience with these viral strategies, particularly as part of multimodal studies, and already several clinical trials are in progress. The limitations of standard cancer chemotherapies, including their lack of specificity with consequent collateral toxicity and the development of cross-resistance, do not appear to apply to viral-based therapies. Furthermore, virotherapy frequently restores chemoradiosensitivity to resistant tumours and has also demonstrated efficacy against cancers that historically have a dismal prognosis. While there is cause for optimism, through continued improvements in the efficiency and safety of systemic delivery, through the emergence of alternative viral agents and through favourable clinical experiences, clinical trials as part of multimodal protocols will be necessary to define clinical utility. Significant progress has been made and this is now a major research area with an increasing annual bibliography.

CUG2, a novel oncogene confers reoviral replication through Ras and p38 signaling pathway

As we have recently found a novel oncogene, the cancer-upregulated gene 2 (CUG2), which was elevated in a variety of tumor tissues such as the ovary, liver, lung and pancreas, we examined whether reovirus could efficiently induce cytolysis in cancer cells expressing CUG2 and thus be used as a potential cancer therapeutic agent. In this study, we describe experiments in which we use reovirus to treat NIH3T3 cells stably expressing either CUG2 (NIH-CUG2) or vector only (NIH-Vec). NIH-CUG2 cells readily support reoviral proliferation and undergo apoptosis, whereas NIH-Vec cells are highly resistant to reoviral infection and virus-induced apoptosis. This notable result may be explained by the observation that CUG2 expression inhibits PKR activation, leading to reoviral proliferation in nonpermissive NIH3T3 cells. Furthermore, reovirus infection results in almost complete regression of tumorgenic NIH-CUG2 cells in transplanted nude mice. As we found that CUG2 enhances activation of MAPK (ERK, JNK and p38), Src kinase and Ras, we examined whether CUG2 confers reoviral replication independent of the Ras or p38 MAPK signaling pathway. From these experiments we found that either inhibition of p38 MAPK or Ras blocks reoviral proliferation even in the presence of CUG2 but inhibition of ERK, JNK and Src kinase does not, indicating that activation of p38 MAPK and Ras has critical roles in reoviral replication in CUG2-expressing tumor cells. Accordingly, we propose that reovirus can be useful in the treatment of transformed cells expressing CUG2, which is commonly detected in various tumor tissues.

Nuclear accumulation of beta-catenin protein indicates activation of wnt signaling in chemically induced rat nephroblastomas

Aberrant wnt signaling caused by mutations in CTNNB1 occurs in about 15% of Wilms tumors, and these mutations appear to be dependent on the concomitant mutational inactivation of the zinc-finger protein WT1. Nuclear beta-catenin protein, a substitute marker of active wnt signaling, has been detected in an even higher proportion (>50%) of Wilms tumors, suggesting alternative genetic pathways leading to beta-catenin activation. Thus, targeting wnt signaling may become an important future therapeutic strategy in Wilms tumor patients. Currently, chemically induced rat nephroblastomas provide the only available rodent model for this tumor. To determine the contribution of active wnt signaling in this model, we investigated 24 chemically induced rat nephroblastomas for beta-catenin protein expression and for Ctnnb1 and WT1 mutations. Immunohistochemistry showed focal strong nuclear accumulation of beta-catenin protein in 18 of 24 tumors, although in a heterogenous pattern. Blastemal and mesenchymal compartments displayed nuclear-positive cells more frequently than areas of epithelial differentiation. Interestingly, we found no mutation of exon 3 of Ctnnb1 and no mutation within the zinc-finger region of WT1 in any of the 24 tumors analyzed. In conclusion, our findings suggest activation of wnt signaling in the majority (63%) of chemically induced rat nephroblastomas. Nuclear expression of beta-catenin in the absence of Ctnnb1 mutations implies, however, alternate mutational targets in rat nephroblastomas.

Oncolytic gene therapy for canine cancers: teaching old dog viruses new tricks

The use of viruses to treat cancer has been studied for decades. With the advancement of molecular biology, viruses have been modified and genetically engineered to optimize their ability to target cancer cells. Canine viruses, such as distemper virus and adenovirus, are being exploited for the treatment of canine cancer as the dog has proven to be a good comparative model for human cancer research and proof of concept investigations. In this review, we introduce the concept of oncolytic viruses and describe some of the preliminary attempts to use oncolytic viruses for the treatment of canine cancer.

Not gene therapy, but genetic surgery-the right strategy to attack cancer

In this review, I will suggest to divide all the approaches united now under common term "gene therapy" into two broad strategies of which the first one uses the methodology of targeted therapy with all its characteristics, but with genes in the role of agents targeted at a certain molecular component(s) presumably crucial for cancer maintenance. In contrast, the techniques of the other strategy are aimed at the destruction of tumors as a whole using the features shared by all cancers, for example relatively fast mitotic cell division or active angiogenesis. While the first strategy is "true" gene therapy, the second one is more like genetic surgery when a surgeon just cuts off a tumor with his scalpel and has no interest in knowing delicate mechanisms of cancer emergence and progression. I will try to substantiate the idea that the last strategy is the only right one, and its simplicity is paradoxically adequate to the super-complexity of tumors that originates from general complexity of cell regulation, strongly disturbed in tumor cells, and especially from the complexity of tumors as evolving cell populations, affecting also their ecological niche formed by neighboring normal cells and tissues. An analysis of the most widely used for such a "surgery" suicide gene/prodrug combinations will be presented in some more details.

Construction and characterization of an oncolytic HSV vector containing a fusogenic glycoprotein and prodrug activation for enhanced local tumor control

A large number of oncolytic viral vectors are currently under clinical development for cancer therapy. Herpes simplex virus type 1 (HSV-1) has demonstrated particular promise in this field, showing genetically engineered selective tumor replication and cytotoxicity in a wide variety of tumor types, without damaging healthy tissues. Enhanced activity has been observed when a range of therapeutic genes has been inserted into various oncolytic HSV genomes. Here, we discuss methods used to develop and characterize an oncolytic HSV virus that combines expression of a highly potent prodrug activating gene (yeast cytosine deaminase/uracil phosphoribosyltransferase fusion [Fcy::Fur]) and the fusogenic glycoprotein from gibbon ape leukemia virus (GALV) for enhanced local tumor control.

E2F promoter-regulated oncolytic adenovirus with p16 gene induces cell apoptosis and exerts antitumor effect on gastric cancer

Replication-competent adenovirus (RCAd) constitutes an alternative in cancer therapy. For obtaining advanced RCAd generations with high oncolytic capability and a good safety profile, we constructed an E2F promoter-regulated RCAd carrying p16 gene, AdE2F-p16, in which the E1a gene was controlled by the E2F promoter. The experimental data showed that the E2F promoter endowed AdE2F-p16 with high specificity in cancer cells. While rarely replicating in normal cells, AdE2F-p16 could replicate in p16-deficient cancer cells, with 2,937- to 160,000-fold increased replicative capability in different cancer cell lines. AdE2F-p16 expressed p16 within cancer cells and led to potent antitumor efficacy in gastric cancer xenografts in nude mice, with a tumor inhibition rate of 59.14%. Due to the combined effects of cancer cell apoptosis induced by p16 expression and oncolysis by virus replication, the E2F promoter-regulated, p16-armed RCAd provides a promising strategy for cancer gene therapy.

Coronavirus genetically redirected to the epidermal growth factor receptor exhibits effective antitumor activity against a malignant glioblastoma

Coronaviruses are positive-strand RNA viruses with features attractive for oncolytic therapy. To investigate this potential, we redirected the coronavirus murine hepatitis virus (MHV), which is normally unable to infect human cells, to human tumor cells by using a soluble receptor (soR)-based expression construct fused to an epidermal growth factor (EGF) receptor targeting moiety. Addition of this adapter protein to MHV allowed infection of otherwise nonsusceptible, EGF receptor (EGFR)-expressing cell cultures. We introduced the sequence encoding the adaptor protein soR-EGF into the MHV genome to generate a self-targeted virus capable of multiround infection. The resulting recombinant MHV was viable and had indeed acquired the ability to infect all glioblastoma cell lines tested in vitro. Infection of malignant human glioblastoma U87DeltaEGFR cells gave rise to release of progeny virus and efficient cell killing in vitro. To investigate the oncolytic capacity of the virus in vivo, we used an orthotopic U87DeltaEGFR xenograft mouse model. Treatment of mice bearing a lethal intracranial U87DeltaEGFR tumor by injection with MHVsoR-EGF significantly prolonged survival compared to phosphate-buffered saline-treated (P = 0.001) and control virus-treated (P = 0.004) animals, and no recurrent tumor load was observed. However, some adverse effects were seen in normal mouse brain tissues that were likely caused by the natural murine tropism of MHV. This is the first demonstration of oncolytic activity of a coronavirus in vivo. It suggests that nonhuman coronaviruses may be attractive new therapeutic agents against human tumors.

Expression of HBX, an oncoprotein of hepatitis B virus, blocks reoviral oncolysis of hepatocellular carcinoma cells

Although reovirus has been used in tests as a potential cancer therapeutic agent against a variety of cancer cells, its application to hepatocellular carcinoma cells, in which the hepatitis B virus (HBV) X (HBX) protein of HBV plays a primary role, has not yet been explored. Here, we describe experiments in which we use reovirus to treat Chang liver carcinoma cells expressing either a vector only (Chang-vec) or a vector encoding HBX protein (Chang-HBX). Although Chang-vec cells readily support reoviral proliferation and undergo apoptosis, Chang-HBX cells are highly resistant to reoviral infection and virus-induced apoptosis, even though HBX protein induces activation of Ras and inactivation of PKR, which are normally thought to enhance reoviral oncolysis. The resistance of Chang-HBX cells to reovirus may instead be explained by HBX-induced downregulation of death receptor 5 and activation of Stat1. Phosphorylated Stat1 activates interferon (IFN)-stimulated regulatory element (ISRE)- and IFN-gamma-activated sequence (GAS)-mediated transcription, leading to the production of IFN-beta, whereas the reduced expression of Stat1 with its siRNA results in a decrease in IFN-beta production, by which Chang-HBX cells eventually succumb to reovirus infection. This result further indicates that HBX induces the establishment of an antiviral state through Stat1 activation. Thus, it appears that active Ras does not override the antiviral effect mediated by the activation of Stat1. Accordingly, we report that HBX, an oncoprotein of HBV, can prevent reoviral oncolysis of hepatocellular carcinoma. This suggests there may be limits to the practical application of reovirus in the treatment of human cancers already expressing other oncoviral proteins.

Targeted delivery of a suicide gene to human colorectal tumors by a conditionally replicating vaccinia virus

We have generated a thymidine kinase gene-deleted vaccinia virus (VV) (Copenhagen strain) that expressed the fusion suicide gene FCU1 derived from the yeast cytosine deaminase and uracil phosphoribosyltransferase genes. Intratumoral inoculation of this thymidine kinase gene-deleted VV encoding FCU1 (VV-FCU1) in the presence of systemically administered prodrug 5-fluorocytosine (5-FC) produced statistically significant reductions in the growth of subcutaneous human colon cancer in nude mice compared with thymidine kinase gene-deleted VV treatments or with control 5-fluorouracil alone. A limitation of prodrug therapies has often been the requirement for the direct injection of the virus into relatively large, accessible tumors. Here we demonstrate vector targeting of tumors growing subcutaneously following systemic administration of VV-FCU1. More importantly we also demonstrate that the systemic injection of VV-FCU1 in nude mice bearing orthotopic liver metastasis of a human colon cancer, with concomitant administration of 5-FC, leads to substantial tumor growth retardation. In conclusion, the insertion of the fusion FCU1 suicide gene potentiates the oncolytic efficiency of the thymidine kinase gene-deleted VV and represents a potentially efficient means for gene therapy of distant metastasis from colon and other cancers.

Telomerase-specific oncolytic virotherapy for human hepatocellular carcinoma

AIM: To evaluate the therapeutic efficiency of replicative adenovirus CNHK300 targeted in telomerase-positive hepatocellular carcinoma.

METHODS: CNHK300, ONYX-015 (55 kDa protein deleted adenovirus) and wtAd5 (wild type adenovirus 5) were compared, and virus proliferation assay, cell viability assay, Western blot and fluorescence microscopy were used to evaluate the proliferation and cytolysis selectivity of CNHK300.

RESULTS: The replicative multiples in Hep3B and HepGII after 48 h of CNHK300 proliferation were 40 625 and 65 326 fold, respectively, similar to that of wtAd5.. However, CNHK300 exhibited attenuated replicative ability in normal fibroblast cell line BJ. CNHK300 could lyse hepatocellular carcinoma cells at a low multiplicity of infection (MOI), but could not affect growth of normal cells even at a high MOI.

CONCLUSION: CNHK300 is a cancer-selective replication-competent adenovirus which can cause oncolysis of liver cancer cells as well as wtAd5 (wild type adenovirus 5), but had severely attenuated replicative and cytolytic ability in normal cells. This novel strategy of cancer treatment offers a promising treatment platform.

Carboxypeptidase-G2-based gene-directed enzyme-prodrug therapy: a new weapon in the GDEPT armoury

Gene-directed enzyme-prodrug therapy (GDEPT) aims to improve the therapeutic ratio (benefit versus toxic side-effects) of cancer chemotherapy. A gene encoding a 'suicide' enzyme is introduced into the tumour to convert a subsequently administered non-toxic prodrug into an active drug selectively in the tumour, but not in normal tissues. Significant effects can now be achieved in vitro and in targeted experimental models, and GDEPT therapies are entering the clinic. Our group has developed a GDEPT system that uses the bacterial enzyme carboxypeptidase G2 to convert nitrogen mustard prodrugs into potent DNA crosslinking agents, and a clinical trial of this system is pending.

Midkine promoter-based conditionally replicative adenovirus for targeting midkine-expressing human bladder cancer model

OBJECTIVES

To develop a novel therapeutic strategy against human bladder cancer using Ad-MK-E1a-a midkine (MK) promoter-regulated, conditionally replicating, adenovirus.

METHODS

We tested several human cancer cell lines in vitro, including those of bladder cancer (KK47, 5637, and T24), lung cancer (A549), and head and neck cancer (H891). In each cell line, we examined MK mRNA expression by TaqMan real-time quantitative polymerase chain reaction, MK promoter activity, after plasmid transfection, using a luciferase assay, and the transduction efficiency by co-transfection with the cytomegalovirus-beta-gal plasmid. In these cells, we assessed the cell type-specific replication of Ad-MK-E1a virus by measuring the E1a DNA copy number by real-time polymerase chain reaction and the cell growth inhibition due to this virus using the Alamar blue assay. In animal studies, nude mice were subcutaneously inoculated with KK47 cells and later intratumorally injected with phosphate-buffered saline or Ad5-CMV-LacZ or Ad-MK-E1a.

RESULTS

The MK mRNA expression level and MK promoter-driven luciferase activity were relatively greater and markedly increased, respectively, in the 5637, A549, and KK47 cells than in the T24 and H891 cells. After Ad-MK-E1a infection, the E1a DNA copy number increased more significantly in the KK47, 5637, and A549 cells than in the T24 and H891 cells. At a multiplicity of infection of 0.01, Ad-MK-E1a significantly inhibited KK47 and 5637 cell growth. In vivo, Ad-MK-E1a injection markedly inhibited KK47 tumor growth.

CONCLUSIONS

We have demonstrated the antitumor effect of Ad-MK-E1a in a human bladder cancer model overexpressing MK mRNA.

Treatment of implanted mammary tumors with recombinant vesicular stomatitis virus targeted to Her2/neu

Vesicular stomatitis virus (VSV) is being developed for cancer therapy. We have created a recombinant replicating VSV (rrVSV) that targeted to Her2/neu expressing breast cancer cells and expresses mouse GM-CSF. We now tested the efficacy of this rrVSV in the treatment of peritoneal tumor implants of D2F2/E2 cells, a BALB/c mouse mammary tumor cell line, which was stably transfected to express Her2/neu. Mice were treated 1 day following tumor implantation with either 2 x 10(8) infectious doses rrVSV or conditioned media (CM). All control animals developed massive peritoneal tumor with a median survival of 16 days. Nine of 10 rrVSV treated mice survived long term with no evidence of tumor. rrVSV had much less efficacy in treating implants of the parent D2F2 cells that did not express Her2/neu. The median survival was 13.5 days in mice treated with CM and 21 days in those treated with rrVSV. There was one long term survivor in the rrVSV treated group. None of the rrVSV treated animals showed evidence of viral toxicity. Three of 7 long term survivors did not develop tumor when rechallenged first with D2F2/E2 and then with D2F2 cells. Both successful therapy and resistance to rechallenge were T-cell dependent. These studies demonstrate that targeted rrVSV eliminated peritoneal implants of Her2/neu expressing tumor and elicited an anti-tumor T-cell immunologic response.

Vesicular stomatitis virus infection triggers apoptosis associated with decreased DeltaNp63alpha and increased Bax levels in the immortalized HaCaT keratinocyte cell line

In view of the powerful inherent oncolytic activity exhibited by the vesicular stomatitis virus (VSV) in several tumor types, we set out to investigate the susceptibility of the immortalized HaCaT keratinocyte cell line to VSV, and analyzed the role of apoptosis in the VSV-mediated induction of cell death. Indirect immunofluorescence assays, Western blot analyses and plaque titrations demonstrated that the HaCaT cell line was permissive to VSV replication. The results of ELISA for detection of the enrichment of nucleosomes in the cytoplasm of apoptotic cells revealed that VSV infection elicits the apoptotic death of HaCaT cells. Mock-infected HaCaT cells displayed the endogenous expression of DeltaNp63alpha, p53 mutated on UV hot spots (p53(mt)), Bcl-2 and p21 Bax. The levels of DeltaNp63alpha and p53(mt) were decreased, Bcl-2 remained unaffected, while the expressions of p21Bax and p18 Bax were increased in VSV-infected HaCaT cells. Together, these data demonstrate that VSV replicates efficiently and triggers apoptosis in the immortalized HaCaT keratinocyte cell line. The VSV-mediated alterations in the expressions of DeltaNp63alpha and Bax may be implicated in the apoptotic responses of infected cells and may also sensitize to other apoptotic stimuli. These findings may stimulate further studies with the goal of developing VSV-based virotherapy into an effective modality for the treatment of epithelial-derived malignant tumors of the skin.

Oncolytic Treatment and Cure of Neuroblastoma by a Novel Attenuated Poliovirus in a Novel Poliovirus-Susceptible Animal Model

Neuroblastoma is one of the most common solid tumors in children. Treatment is of limited utility for high-risk neuroblastoma and prognosis is poor. Resistance of neuroblastoma to conventional therapies has prompted us to search for a novel therapeutic approach based on genetically modified polioviruses. Poliovirus targets motor neurons leading to irreversible paralysis. Neurovirulence can be attenuated by point mutations or by exchange of genetic elements between different picornaviruses. We have developed a novel and stable attenuated poliovirus, replicating in neuroblastoma cells, by engineering an indigenous replication element (cre), copied from a genome-internal site, into the 5'-nontranslated genomic region (mono-crePV). An additional host range mutation (A(133)G) conferred replication in mouse neuroblastoma cells (Neuro-2a(CD155)) expressing CD155, the poliovirus receptor. Crossing immunocompetent transgenic mice susceptible to poliovirus (CD155 tg mice) with A/J mice generated CD155 tgA/J mice, which we immunized against poliovirus. Neuro-2a(CD155) cells were then transplanted into these animals, leading to lethal tumors. Despite preexisting high titers of anti-poliovirus antibodies, established lethal s.c. Neuro-2a(CD155) tumors in CD155 tgA/J mice were eliminated by intratumoral administrations of A(133)Gmono-crePV. No signs of paralysis were observed. Interestingly, no tumor growth was observed in mice cured of neuroblastoma that were reinoculated s.c. with Neuro-2a(CD155). This result indicates that the destruction of neuroblastoma cells by A(133)Gmono-crePV may lead to a robust antitumor immune response. We suggest that our novel attenuated oncolytic poliovirus is a promising candidate for effective oncolytic treatment of human neuroblastoma or other cancer even in the presence of present or induced antipolio immunity.

p53-expressing conditionally replicative adenovirus CNHK500-p53 against hepatocellular carcinoma in vitro

AIM: To develop a conditionally replicative gene-viral vector system called CNHK500-p53, which contains dual promoters within the E1 region, and combines the advantages of oncolytic virus and gene therapies for hepatocellular carcinoma (HCC).

METHODS: CNHK500-p53 was constructed by using human telomerase reverse transcriptase (hTERT) promoter to drive adenovirus E1a gene and hypoxia response element (HRE) promoter to drive adenovirus E1b gene. p53 gene expressing cassette was inserted into the genome of replicative virus. Viral replication experiments, cytopathic effect (CPE) and methyl thiazolyl tetrazolium (MTT) assay were performed to test the selective replication and oncolytic efficacy of CNHK500-p53.

RESULTS: Immunohistochemistry verified that infection with CNHK500-p53 was associated with selective replication of adenovirus and production of p53 protein in telomerase-positive and hypoxia-inducible factor-dependent HCC cells. p53 protein secreted from HepG2, infected with CNHK500-p53 was significantly higher than that infected with nonreplicative adenovirus Ad-p53 in vitro (388 ± 34.6 μg/L vs 76.3 ± 13.17 μg/L). Viral replication experiments showed that replication of CNHK500-p53 and CNHK500 or WtAd5, was much stronger than that of Ad-p53 in tested HCC cell lines. CPE and MTT assay indicated that CNHK500-p53 selectively replicated in and killed HCC cells while leaving normal cells unaffected.

CONCLUSION: A more efficient gene-viral system is developed by combining selective oncolysis with exogenous expression of p53 against HCC cells.

Increased safety with preserved antitumoral efficacy on hepatocellular carcinoma with dual-regulated oncolytic adenovirus

PURPOSE

A dual-regulated adenovirus variant CNHK500, in which human telomerase reverse transcriptase promoter drove the adenovirus 5 (Ad5) E1a gene and hypoxia-response promoter controlled the E1b gene, was engineered. This virus has broad anticancer spectrum and higher specificity compared with mono-regulated adenovirus CNHK300. The objective of the current study is to show its antitumor selectivity and therapeutic potential.

EXPERIMENTAL DESIGN

The antitumor specificity of human telomerase reverse transcriptase and hypoxia response promoters was evaluated in a panel of tumor and normal cells. Under the control of these promoters, the tumor-selective expression of E1a and E1b genes was evaluated. Further in vitro antitumor specificity and potency of this virus were characterized by viral replication and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Subsequently, hepatocellular carcinoma xenografts were established to evaluate CNHK500 antitumor efficacy in vivo by different routes of virus administration and different dosages.

RESULTS

Human telomerase reverse transcriptase and hypoxia response promoters were activated in a tumor-selective manner or under hypoxia treatment in a broad panel of cells. Selective adenoviral early gene expression, efficient viral replication, and oncolysis were observed in all tested cancer cells with more attenuated replication capacity in normal cells. Significant regression of hepatocellular carcinoma xenografts and prolonged survival were observed by either i.t. or i.v. administration.

CONCLUSIONS

CNHK500 greatly reduced side effects in normal cells via dual control of adenoviral essential genes while still preserving potent antitumor efficacy on broad-spectrum cancer cells in vitro and in vivo. It can be used as a powerful therapeutic agent not only for liver cancers but also for other solid tumors.

Antitumor activity of an hTERT promoter-regulated tumor-selective oncolytic adenovirus in human hepatocellular carcinoma

AIM: To construct a tumor-selective replication-competent adenovirus (RCAd), SG300, using a modified promoter of human telomerase reverse transcriptase (hTERT).

METHODS: The antitumor efficacy of SG300 in hepatocellular carcinoma was assessed in vitro and

in vivo. In vitro cell viability by MTT assay was used to assess the tumor-selective oncolysis and safety features of SG300, and in vivo antitumor activity of SG300 was assessed in established hepatocellular carcinoma models in nude mice.

RESULTS: SG300 could lyse hepatocellular carcinoma cells at a low multiplicity of infection (MOI), but could not affect growth of normal cells even at a high MOI. Both in Hep3B and SMMC-7721 xenograft models of hepatocellular carcinoma, SG300 had an obvious antitumor effect, resulting in a decrease in tumor volume. Its selective oncolysis to tumor cells and safety to normal cells was also superior to that of ONYX-015. Pathological examination of tumor specimens showed that SG300 replicated selectively in cancer cells and resulted in apoptosis and necrosis of cancer cells.

CONCLUSION: hTERT promoter-regulated replicative adenovirus SG300 has a better cancer-selective replication-competent ability, and can specifically kill a wide range of cancer cells with positive telomerase activity, and thus has better potential for targeting therapy of hepatocellular carcinoma.

Phase I, Open-Label, Dose-Escalating Study of a Genetically Engineered Herpes Simplex Virus, NV1020, in Subjects with Metastatic Colorectal Carcinoma to the Liver

Current regimens of systemic chemotherapy result in only modest lengthening of survival in patients with advanced stage, liver-dominant, metastatic colorectal cancer who have failed first-line chemotherapy. The objective of this study was to investigate the safety and tolerability of NV1020, a replication-competent, attenuated, genetically engineered herpes simplex virus type 1 (HSV-1), in patients with hepatic colorectal metastases refractory to first-line chemotherapy. A phase I, open-label, dose-escalating study of a single 10-min hepatic arterial infusion of NV1020 in four cohorts. Three patients in each cohort received doses of 3 x 10(6), 1 x 10(7), 3 x 10(7), and 1 x 10(8) plaque-forming units. Adverse events were either mild or moderate in severity, and self-limiting. Only three serious adverse events (one transient rise in serum y-glutamyltransferase, one diarrhea, and one leukocytosis) experienced by three patients were considered to be possibly or probably related to NV1020. There were no deaths during the study, and there was no evidence of disseminated herpes infection. Viral presence was detected in only one saliva sample and two serum samples from one asymptomatic patient in the highest dose cohort. In the first week after viral administration only rare and minor increases were noted for tumor necrosis factor-alpha (six samples; three patients; peak, 40 pg/ml), interleukin (IL)-1 (two samples; two patients; peak, 28 pg/ml), and interferon-y (four samples; two subjects; peak, 54 pg/ml). No IL-2 was detected. Mild liver enzyme elevations were self-limiting and not associated with clinical symptoms. We conclude that NV1020, a genetically engineered but replication-competent HSV-1 oncolytic virus, can be safely administered into the hepatic artery without significant effects on normal liver function.

Construction of doxycyline-dependent mini-HIV-1 variants for the development of a virotherapy against leukemias

T-cell acute lymphoblastic leukemia (T-ALL) is a high-risk type of blood-cell cancer. We describe the improvement of a candidate therapeutic virus for virotherapy of leukemic cells. Virotherapy is based on the exclusive replication of a virus in leukemic cells, leading to the selective removal of these malignant cells. To improve the safety of such a virus, we constructed an HIV-1 variant that replicates exclusively in the presence of the nontoxic effector doxycycline (dox). This was achieved by replacement of the viral TAR-Tat system for transcriptional activation by the Escherichia coli-derived Tet system for inducible gene expression. This HIV-rtTA virus replicates in a strictly dox-dependent manner. In this virus, additional deletions and/or inactivating mutations were introduced in the genes for accessory proteins. These proteins are essential for virus replication in untransformed cells, but dispensable in leukemic T cells. These minimized HIV-rtTA variants contain up to 7 deletions/inactivating mutations (TAR, Tat, vif, vpR, vpU, nef and U3) and replicate efficiently in the leukemic SupT1 T cell line, but do not replicate in normal peripheral blood mononuclear cells. These virus variants are also able to efficiently remove leukemic cells from a mixed culture with untransformed cells. The therapeutic viruses use CD4 and CXCR4 for cell entry and could potentially be used against CXCR4 expressing malignancies such as T-lymphoblastic leukemia/lymphoma, NK leukemia and some myeloid leukemias.

Rapid adaptation of a recombinant vesicular stomatitis virus to a targeted cell line

Vesicular stomatitis virus (VSV) is being developed for cancer therapy. We created a recombinant replicating VSV (rrVSV) that preferentially infected Her2/neu-expressing breast cancer cells. This rrVSV did not express the native VSV-G glycoprotein (gp). Instead, it expressed a chimeric Sindbis gp which included a single-chain antibody (SCA) directed to the human Her2/neu receptor. The virus infected mouse mammary carcinoma cells (D2F2/E2) expressing Her2/neu 23-fold better than the parent cells (D2F2). However, viral growth in cultured D2F2/E2 cells was curtailed after several cycles, and viral yield was very poor at 2 x 10(4) infectious doses (ID)/ml. We performed in vitro serial passage in D2F2/E2 cells to evolve a virus with improved growth that could be used for preclinical therapy trials in mice. Fifteen passes generated an adapted virus that progressed through multiple cycles in cultured D2F2/E2 cells until all cells were infected and had a viral yield of 1 x 10(8) ID/ml. Sequencing of the entire viral genomes found only 2 mutations in the adapted virus. Both mutations occurred in the gp gene segment coding for the SCA. An additional N-glycosylation site was created by one of the mutations. The adapted virus showed higher density of gp on the viral envelope, improved infectivity, much greater stability, higher burst size, and decreased induction of cellular interferon. The specificity for cells expressing the Her2/neu receptor was unchanged. These studies demonstrate that serial passage can be used to rapidly evolve a VSV genome encoding an improved chimeric glycoprotein.