A novel tumor-specific replication-restricted adenoviral vector for gene therapy of hepatocellular carcinoma

Selectively replicating adenoviruses targeting deregulated E2F activity are potent, systemic antitumor agents

We have engineered a human adenovirus, ONYX-411, that selectively replicates in human tumor cells, but not normal cells, depending upon the status of their retinoblastoma tumor suppressor protein (pRB) pathway. Early and late viral gene expression as well as DNA replication were significantly reduced in a functional pRB-pathway-dependent manner, resulting in a restricted replication profile similar to that of nonreplicating adenoviruses in normal cells both in vitro and in vivo. In contrast, the viral life cycle and tumor cell killing activity of ONYX-411 was comparable to that of wild-type adenovirus following infection of human tumor cells in vitro as well as after systemic administration in tumor-bearing animals.

E1B-55K-deleted adenovirus expressing E1A-13S by AFP-enhancer/promoter is capable of highly specific replication in AFP-producing hepatocellular carcinoma and eradication of established tumor

Here, we constructed a recombinant replication-competent adenovirus (rRCAd; AdAFPep/Rep) that expresses both E1A-13S driven by the alpha-fetoprotein (AFP) enhancer/promoter (AFPep) lacking any silencers in the 5'-flanking region of the AFP gene, and 55K-deleted E1B driven by the cytomegalovirus (CMV) promoter. We then examined the feasibility of gene therapy utilizing this virus for AFP-producing hepatocellular carcinoma (HCC). AdAFPep/Rep lysed all the AFP-producing HCC cell lines (HuH7, HepG2, PLC/PRF/5 (P5)) examined at a multiplicity of infection (MOI) as low as 0.1 and did not lyse primary human hepatocytes (Hc) at a MOI as high as 100, indicating that the rRCAd virus can lyse AFP-producing HCC cells with a higher specificity and potency than previously reported. Furthermore, this virus was capable of complete eradication of a preestablished HuH7-cell tumor by a single intratumoral injection of 10(8) plaque-forming units (pfu) of AdAFPep/Rep. Thus, AdAFPep/Rep may be applicable for clinical use.

New tools for the construction of replication-competent adenoviral vectors with altered E1A regulation

We have designed new vectors for the construction of recombinant adenoviruses containing the early region 1A (E1A) gene under the transcriptional control of heterologous promoters. The normal E1A regulatory elements have been replaced by a convenient set of unique restriction enzyme sites, allowing for introduction of gene regulatory cassettes with relative ease. Subsequent rescue generates recombinant conditionally replicating adenovirus in which the transcription of E1A is under alternative control. This allows potentially cell-type specific expression of E1A, restricting efficient virus replication to chosen target cells. It is shown that in several viruses rescued using these constructs, E1A expression is regulated by the heterologous promoters in the expected manner. Specifically, a virus with E1A under the control of the human Cytomegalovirus Immediate Early promoter produced constitutively high levels of E1A. A second virus, with E1A expression regulated by the glucocorticoid-responsive Mouse Mammary Tumor Virus promoter produced E1A expression in a dose-dependent manner upon dexamethasone treatment. Efficient growth of this second virus also required the presence of dexamethasone.

Cytolytic viruses as potential anti-cancer agents

The resistance of cancers to conventional therapies has inspired the search for novel strategies. One such approach, namely gene therapy, is based upon the introduction of genes such as those encoding suicide proteins, tumour suppressor proteins or cytokines into tumour cells by means of a genetic vector. The efficiency with which viruses transfer their genes from one host cell to another has led to the widespread use of viruses as genetic vectors. For safety reasons, such virus vectors are generally replication-defective but, unfortunately, this has limited the efficacy of treatment by restricting the number of cells to which the therapeutic gene is delivered. For this reason, the use of replication-competent viruses has been proposed, since virus replication would be expected to lead to amplification and spread of the therapeutic genes in vivo. The replication of many viruses results in lysis of the host cells. This inherent cytotoxicity, together with the efficiency with which viruses can spread from one cell to another, has inspired the notion that replication-competent viruses could be exploited for cancer treatment. Some viruses have been shown to replicate more efficiently in transformed cells but it is unlikely that such examples will exhibit a high enough degree of tumour selectivity, and hence safety, for the treatment of patients. Our increasing knowledge of the pathogenesis of virus disease and the ability to manipulate specific regions of viral genomes have allowed the construction of viruses that are attenuated in normal cells but retain their ability to lyse tumour cells. Such manipulations have included modifying the ability of viruses to bind to, or replicate in, particular cell types, while others have involved the construction of replication-competent viruses encoding suicide proteins or cytokines. Naturally occurring or genetically engineered oncolytic viruses based upon adenovirus, herpes simplex virus, Newcastle disease virus, poliovirus, vesicular stomatitis virus, weasles virus and reovirus have been described. The results of animal studies are encouraging and a number of viruses are now being evaluated in clinical trials.

Gene therapy for malignant liver disease

For most patients with advanced or multifocal hepatocellular carcinoma (HCC) or with metastatic malignant liver disease treatment options are limited, resulting in a poor prognosis. Novel therapeutic strategies such as gene therapy are therefore urgently required. Gene therapeutic approaches use gene delivery systems (vectors) to introduce DNA constructs as therapeutic agents into living cells. Antitumour strategies include the reintroduction of tumour suppressor genes into tumour cells, the expression of foreign enzymes to render tumours susceptible to treatment with chemotherapeutic agents and the enhancement of tumour immunogenicity by expressing immunomodulatory genes or by genetic vaccination with tumour antigens. Furthermore, gene therapy may be also used for anti-angiogenesis to reduce tumour growth and metastatic potential. Other novel approaches aim at the development of genetically altered replication competent viruses, which selectively replicate in tumour cells inducing cell lysis. Although most clinical trials of antitumour gene therapy so far have failed to induce strong therapeutic effects, further improvement of antitumour gene therapy may finally result in potent clinical treatment options for patients with malignant liver tumours.

Immunotherapy of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the third leading cause of cancer death, with over a million new cases annually. It is generally advanced upon detection due to underlying liver disease, which further complicates treatment. Most of the therapeutic strategies in current use (surgery, transplantation, irradiation or chemotherapy) are either palliative or only of benefit to a small percentage of patients. This article reviews the biology of HCC, including many of the molecular changes and mechanisms leading to HCC development. This article discusses the recent innovative strategies to interfere with the progression of HCC, including novel gene therapy strategies. The most recent data supporting the use of immunotherapy for hepatocellular cancer is reviewed in detail.

Adenoviruses with Tcf binding sites in multiple early promoters show enhanced selectivity for tumour cells with constitutive activation of the wnt signalling pathway

Mutation of the adenomatous polyposis coli and beta-catenin genes in colon cancer leads to constitutive activation of transcription from promoters containing binding sites for Tcf/LEF transcription factors. We have constructed adenoviruses with Tcf binding sites in the early promoters, in order to target viral replication to colon tumours. Tcf regulation of the E1A promoter confers a 100-fold selectivity for cells with activated wnt signalling in viral burst and cytopathic effect assays. p300 is a coactivator for beta-catenin, and E1A inhibits Tcf-dependent transcription through sequestration of p300, but mutation of the p300 binding site in E1A leads to a 10-fold reduction in cytopathic effect of all of the Tcf-regulated viruses. When Tcf sites are inserted in the E1A, E1B, E2 and E4 promoters the viruses show up to 100 000-fold selectivity for cells with activated wnt signalling.

A system for small-molecule control of conditionally replication-competent adenoviral vectors

Replication-competent adenoviral vectors are potentially far more efficient than replication-defective vectors. However, for reasons of safety, there is a need to restrict viral replication both spatially, by limiting replication to certain cell types, and temporally. To control replication temporally, we have developed a system, based on the small-molecule dimerizer rapamycin, for regulating the replication of adenoviral vectors. In this system, one adenoviral vector, AdC4, expresses transcription factors whose activity is regulated by the non-immunosuppressive rapamycin analog AP21967. A second vector, Ad(Z12-I-E1aE1b19k), contains E1 genes placed downstream of binding sites for the regulated transcription factor. Co-infection of several cell lines by the vector pair leads to dimerizer-dependent E1 expression and an increase in viral replication, as shown by Southern blots and replication assays. Furthermore, expression of a reporter gene from a replication-defective vector, Ad-GM-CSF, can be augmented by up to 18-fold by co-infection with the pair of conditionally replicating vectors in the presence of dimerizer. Similar results are obtained when the vectors are directly injected into subcutaneous HT1080 xenograft tumors in nude mice. We believe that vectors based on this principle will be a useful additional tool to enhance efficiency and safety of gene delivery for anti-cancer therapy.

Head and neck cancer: gene therapy approaches. Part 1: adenoviral vectors

Treatment options for recurrent or refractory head and neck cancer are limited. The goal of gene therapy is to introduce new genetic material into cancer cells without affecting toxicity to surrounding malignant cells. The most common vehicles for delivery of genes are adenoviruses. Adenoviruses gain access to malignant and normal cell cytoplasm via viral ligand binding to a unique cell surface receptor (the coxsackie adenovirus receptor [CAR]). However, this receptor is not cancer specific. Genetic modification of adenoviral DNA can create cancer specific targeting. Adenoviruses can be modified to express cancer specific ligands thereby focusing binding to malignant tissue. Furthermore, adenoviral delivered genes can be put under cancer specific promoter control to further limit gene expression in malignant tissue. Increased antitumour activity from such modifications has been demonstrated preclinically and several clinical trials have been completed demonstrating safety and clinical activity of non-replicating and conditional replicating adenoviral vector thereby opening the door for gene delivery and cancer specific targeting.

Oncolytic biotherapy: a novel therapeutic plafform

There is a clear need for new, selective, cancer treatments that do not cause the cross-resistance which occurs with currently available chemotherapeutic agents. Gene therapy is a promising approach, but to date, it has shown limited effectiveness in clinical trials because of insufficient gene transduction. Many investigators are now revisiting the 'old' idea of using tumour-specific, replication-selective viruses or bacteria to treat cancer. These agents can be directly oncolytic, but can also be used to simultaneously express therapeutic genes in target cells or induce tumour-specific, cell-mediated immunity. We discuss the promise of this rapidly evolving field and examine the potential barriers to its success.

Intravascular adenoviral agents in cancer patients: lessons from clinical trials

A large number of adenoviral agents are being developed for the treatment of cancer. However, the treatment-related death of a patient with ornithine transcarbamylase deficiency following adenovirus administration by hepatic artery has led to serious concerns regarding the safety of intravascular adenovirus. Both replication-incompetent (rAd.p53, e.g., SCH58500) and replication-selective (dl1520, aka Onyx-015; CG7870) oncolytic adenoviruses, by intravascular administration, are in clinical trials. We review Phases I and I/II results from these clinical trials. dl1520 and rAd.p53 were well-tolerated following hepatic artery infusion at doses of up to 2x10(12) and 2.5x10(13) particles, respectively. At a dose of 7.5x10(13) particles, rAd.p53 was associated with dose-limiting cardiac output suppression; dl1520 dose escalation did not proceed higher than 2x10(12). Intravenous (i.v.) infusions of dl1520 and CG7870 have been well tolerated by i.v. infusion at doses of 2x10(13) and 6x10(12), respectively, without identification of a maximally tolerated dose to date. Mild/moderate transaminitis was demonstrated in some patients on both the hepatic arterial and i.v. trials at doses >or=10(12) particles. Interleukin (IL)-6 and IL-10 were induced in a dose-dependent manner in most patients, but significant interpatient and intrapatient (on repeat doses) variabilities were demonstrated. Evidence of p53 gene expression (Ad.p53) or viral replication (dl1520) was demonstrated in the majority of patients receiving >or=10(12) particles. Over 100 cancer patients have been treated with intravascular adenovirus constructs to date with an acceptable toxicity profile; further clinical trial testing appears appropriate in cancer patients.

Conditional gene targeting for cancer gene therapy

Current treatment of solid tumors is limited by severe adverse effects, resulting in a narrow therapeutic index. Therefore, cancer gene therapy has emerged as a targeted approach that would significantly reduce undesired side effects in normal tissues. This approach requires a clear understanding of the molecular biology of both the malignant clone and the biological vectors that serve as vehicles to target cancer cells. In this review we discuss novel approaches for conditional gene expression in cancer cells. Targeting transgene expression to malignant tissues requires the use of specific regulatory elements including promoters based on tumor biology, tissue-specific promoters and inducible regulatory elements. We also discuss the regulation of both replication and transgene expression by conditionally-replicative viruses. These approaches have the potential to restrict the expression of transgenes exclusively to tissues of interest and thereby to increase the therapeutic index of future vectors for cancer gene therapy.

Human immunodeficiency virus type 1-mediated syncytium formation is compatible with adenovirus replication and facilitates efficient dispersion of viral gene products and de novo-synthesized virus particles

Conditionally replicative adenovirus (CRAd) vectors are designed for specific oncolytic replication in tumor tissues with concomitant sparing of normal cells. As such, CRAds offer an unprecedented level of anticancer potential for malignancies that have been refractory to previous cancer gene therapy interventions. CRAd efficacy may, however, be compromised by inefficient dispersion of the replicating vector within the tumor tissue. To address this issue, we evaluated the utility of a fusogenic membrane glycoprotein (FMG), which induces the fusion of neighboring cellular membranes to form multinucleated syncytia. We hypothesized that the FMG-mediated syncytia would facilitate dispersion of the adenovirus (Ad) gene products and viral progeny. To test this, human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins, which induce syncytia in the presence of CD4+ target cells, were expressed by an Ad (Ad5HIVenv) in permissive (CD4-positive) and nonpermissive (CD4-negative) cell lines. After validating this Ad-FMG model, the efficiency of Ad replication in the presence or absence of syncytia was evaluated. The results demonstrated that syncytium formation was compatible with Ad replication and dramatically increased the dispersion of virus gene products within the cytoplasm of the syncytia as well as viral particles in the nuclei of the syncytial mass. Moreover, progeny virions were released more efficiently from syncytia compared with nonsyncytial cells. These data demonstrate the utility of FMGs as a dispersion agent and suggest that FMGs can improve the efficacy of CRAd gene therapy.

Re-engineering adenovirus regulatory pathways to enhance oncolytic specificity and efficacy

Replicating adenoviruses may prove to be effective anticancer agents if they can be engineered to selectively destroy tumor cells. We have constructed a virus (01/PEME) containing a novel regulatory circuit in which p53-dependent expression of an antagonist of the E2F transcription factor inhibits viral replication in normal cells. In tumor cells, however, the combination of p53 pathway defects and deregulated E2F allows replication of 01/PEME at near wild-type levels. The re-engineered virus also showed significantly enhanced efficacy compared with extensively studied E1b-deleted viruses such as dl1520 in human xenograft tumor models.

Exploiting genetic alterations to design novel therapies for cancer

In the 3 decades since the signing of the National Cancer Act, there has been tremendous progress in the elucidation of the molecular underpinnings of cancer. Molecular genetic studies have been particularly insightful, revealing genetic rearrangements, such as chromosomal translocations, which may be the seminal event leading to deregulated cell growth for many childhood and adult cancers. These findings have led to new diagnostic and prognostic tools but have been slow to be translated into new therapeutic modalities. This article reviews a variety of methods now under development to exploit genetic changes in cancer to develop specific anticancer agents using gene therapy, viral therapy, and immunotherapy approaches. As many of these strategies inevitably enter the clinic, it will be imperative for health care professionals to be familiar with these novel approaches as they help patients navigate the likely broad array of treatment options.

Wild-type adenovirus decreases tumor xenograft growth, but despite viral persistence complete tumor responses are rarely achieved--deletion of the viral E1b-19-kD gene increases the viral oncolytic effect

Strategies to target viral replication to tumor cells hold great promise for the treatment of cancer, but even with replicating adenoviruses complete tumor responses are rarely achieved. To evaluate replicating adenoviral vectors, we have used A549 human lung cancer nude mouse xenografts as a model system. Intratumoral injection of wild-type adenovirus (Ad309) significantly reduced tumor growth from day 14 (p = 0.04) onward; however, tumor volumes reached a plateau at day 50. At 100 days, high levels of titratable virus were present within persistent viable tumors. In contrast to viral injection into established tumors, when tumor cells were infected in vitro with wild-type virus and then mixed with uninfected tumor cells, 1% of infected cells was sufficient to prevent tumor establishment. An E1b-19kD-deleted viral mutant (Ad337) was more efficient than Ad309 in this cell-mixing model. Just 1 cell in 1000 infected with Ad337 prevented tumor growth. However, although better than wild-type virus, Ad337 was unable to eradicate established flank tumors. These data suggest that although replicating adenoviruses exhibit significant oncolytic activity, barriers within the established tumor, such as connective tissue and tumor matrix, may limit the spread of virus. Strategies to enhance viral spread through established tumors are therefore likely to greatly improve the therapeutic efficacy of replicating adenoviruses.

Replication-selective virotherapy for cancer: Biological principles, risk management and future directions

In the search for novel cancer therapies that can be used in conjunction with existing treatments, one promising area of research is the use of viral vectors and whole viruses. This review describes the underlying biological principles and current status of the field, outlines approaches for improving clinical effectiveness and discusses the unique safety and regulatory issues surrounding viral therapies.

Oncolytic viruses as therapeutic agents

The concept of using viruses as oncolytic agents has a long history. However, relatively new developments are the use of these viruses as gene delivery vehicles and the restriction of viral replication and lysis to tumour cells. The latter is attempted by the use of tumour-specific promoters, which transcriptionally target viral genes involved in replication, or by deletion of viral functions dispensable for replication in tumour cells but essential for productive infection of normal cells. In addition, retargeting of the viral tropism towards tumours by capsid modifications has been examined. Although much progress has been made in developing oncolytic vectors for clinical use, there is still a long way to go to determine which combinations of virus, gene therapy, surgery, radiation, and/or chemotherapy will provide improved therapy for the control and eradication of a variety of human cancers. First controlled clinical trials with an oncolytic adenovirus in combination with chemotherapy have shown encouraging antineoplastic activity. For future vector developments it will be crucial to achieve maximum vector distribution and transgene expression within tumours, to trigger a specific systemic immune effector response against treated and untreated lesions, and to modulate the immune system to avoid immune-mediated inactivation or destruction of the virus. In the context of replication-competent vectors, suicide genes might be used as fail-safe mechanism in the case of a runaway infection.

Delivery systems intended for in vivo gene therapy of cancer: targeting and replication competent viral vectors

Cancer gene therapy represents one of the most rapidly evolving areas in pre-clinical and clinical cancer research. Application of gene transfer techniques in clinical trials has made increasingly obvious that several issues will need to be addressed prior to meaningful incorporation of gene therapy in the care of cancer patients. Two of the most important problems to overcome are lack of selectivity of the existing vectors and low efficiency of gene transfer. This review focuses on use of targeting and replication competent vectors in order to overcome these obstacles. Targeted gene therapy of malignancies can be achieved through vector targeting or transcriptional targeting and can improve the therapeutic index of gene transfer by preventing damage of normal tissues, an important requirement if systemic gene delivery is contemplated. Replication competent viral vectors can improve the efficiency of gene transfer. Provisionally replicating viruses can also improve the therapeutic index by targeting toxicity to tumor cells. A variety of provisionally replicating viruses, such as the attenuated adenovirus ONYX-015, the adenovirus CN706 that selectively replicates in prostate cancer cells, the double mutant herpes simplex virus G207, the human reovirus, and the Newcastle disease virus are currently in clinical trials. Early clinical results and limitations in the application of these vectors are discussed.

Inclusion of the herpes simplex thymidine kinase gene in a replicating adenovirus does not augment antitumor efficacy

Replication-incompetent adenoviruses (Ad) carrying the herpes simplex thymidine kinase (HSVtk) gene have been used in a number of human cancer gene therapy trials, however transduction has generally been limited to a small minority of tumor cells. To solve this problem, replication-competent adenoviral vectors carrying transgenes such as HSVtk have been developed. However, contradictory evidence exists regarding the efficacy of these new vectors. Accordingly, we constructed and tested a replication-competent E3-deleted adenoviral vector containing the HSVtk suicide gene driven by the endogenous E3 promoter (Ad.wt.tk). This virus showed high level production of the HSVtk transgene and was more efficacious than a non-replicating virus in vitro, after injection into flank tumors, and against established intraperitoneal tumors. However, addition of ganciclovir (GCV) therapy to cells or tumor-bearing animals treated with the replicating vector containing the HSVtk suicide gene did not result in increased cell killing. Our results indicate that addition of HSVtk to a replicating Ad virus will not likely be useful in augmenting antitumor effects.

Tissue-specific, tumor-selective, replication-competent adenovirus vector for cancer gene therapy

We have previously described two replication-competent adenovirus vectors, named KD1 and KD3, for potential use in cancer gene therapy. KD1 and KD3 have two small deletions in the E1A gene that restrict efficient replication of these vectors to human cancer cell lines. These vectors also have increased capacity to lyse cells and spread from cell to cell because they overexpress the adenovirus death protein, an adenovirus protein required for efficient cell lysis and release of adenovirus from the cell. We now describe a new vector, named KD1-SPB, which is the KD1 vector with the E4 promoter replaced by the promoter for surfactant protein B (SPB). SPB promoter activity is restricted in the adult to type II alveolar epithelial cells and bronchial epithelial cells. Because KD1-SPB has the E1A mutations, it should replicate within and destroy only alveolar and bronchial cancer cells. We show that KD1-SPB replicates, lyses cells, and spreads from cell to cell as well as does KD1 in H441 cells, a human cancer cell line where the SPB promoter is active. KD1-SPB replicates, lyses cells, and spreads only poorly in Hep3B liver cancer cells. Replication was determined by expression of the E4ORF3 protein, viral DNA accumulation, fiber synthesis, and virus yield. Cell lysis and vector spread were measured by lactate dehydrogenase release and a "vector spread" assay. In addition to Hep3B cells, KD1-SPB also did not express E4ORF3 in HT29.14S (colon), HeLa (cervix), KB (nasopharynx), or LNCaP (prostate) cancer cell lines, in which the SPB promoter is not expected to be active. Following injection into H441 or Hep3B tumors growing in nude mice, KD1-SPB caused a three- to fourfold suppression of growth of H441 tumors, similar to that seen with KD1. KD1-SPB had only a minimal effect on the growth of Hep3B tumors, whereas KD1 again caused a three- to fourfold suppression. These results establish that the adenovirus E4 promoter can be replaced by a tissue-specific promoter in a replication-competent vector. The vector has three engineered safety features: the tissue-specific promoter, the mutations in E1A that preclude efficient replication in nondividing cells, and a deletion of the E3 genes which shield the virus from attack by the immune system. KD1-SPB may have use in treating human lung cancers in which the SPB promoter is active.

Evaluation of a new dual-specificity promoter for selective induction of apoptosis in breast cancer cells

The conditional expression of lethal genes in tumor cells is a promising gene therapy approach for the treatment of cancer. The identification of promoters that are preferentially active in cancer cells is the starting point for this strategy. The combination of tissue-specific and tumor-specific elements offers the possibility to artificially develop such promoters. We describe the construction and characterization of a hybrid promoter for transcriptional targeting of breast cancer. In many cases, breast cancer cells retain the expression of estrogen receptors, and most solid tumors suffer from hypoxia as a consequence of their aberrant vascularization. Estrogen response elements and hypoxia-responsive elements were combined to activate transcription in cells that present at least one of these characteristics. When a promoter containing these elements is used to control the expression of the pro-apoptotic gene harakiri, the induction of cell death can be activated by estrogens and hypoxia, and inhibited by antiestrogens such as tamoxifen. Finally, we show evidence that these properties are maintained in the context of an adenoviral vector (AdEHhrk). Therefore, infection with this virus preferentially kills estrogen receptor-positive breast cancer cells, or cells growing under hypoxic conditions. We propose the use of this promoter for transcriptional targeting of breast cancer.

Target gene therapy for alpha-fetoprotein-producing hepatocellular carcinoma by E1B55k-attenuated adenovirus

Gene therapy using replication-competent adenovirus that selectively propagates in tumor cells may be an effective treatment for cancer. We developed an adenovirus that would be replication specific for hepatocellular carcinoma (HCC). Based on our finding that the E1B55k-deficient adenovirus was able to replicate in human primary hepatocytes, we therefore designed an adenovirus carrying E1A and attenuated E1B gene driven by the alpha-fetoprotein promoter (Adv-AFP-E1AdB), thus restricting the replication specificity in AFP-producing HCC. Replication of Adv-AFP-E1AdB in primary hepatocytes was practically negligible 4 days after infection. Although Adv-AFP-E1AdB replicated slowly in AFP-producing HCC, it efficiently destroyed HCC cells independent of their p53 status. Experiments were conducted in vivo using systemic administration of Adv-AFP-E1AdB and we observed tumor size reduction in nude mice having liver cancer. The use of replication-competent adenovirus deficient of the E1B gene coupled to an AFP-targeting strategy may be a safe and efficacious treatment for HCC.

Replicating adenoviruses that target tumors with constitutive activation of the wnt signaling pathway

Despite important advances in understanding the molecular basis of cancer, few treatments have been devised which rationally target known causal oncogenic defects. Selectively replicating viruses have a major advantage over nonreplicating viruses to target these defects because the therapeutic effect of the injected virus is augmented by virus produced within the tumor. To permit rational targeting of colon tumors, we have developed replicating adenoviruses that express the viral E1B and E2 genes from promoters controlled by the Tcf4 transcription factor. Tcf4 is constitutively activated by mutations in the adenomatous polyposis coli and beta-catenin genes in virtually all colon tumors and is constitutively repressed by Groucho and CtBP in normal tissue. The Tcf-E2 and Tcf-E1B promoters are active in many, but not all, cell lines with activation of the wnt pathway. Viruses with Tcf regulation of E2 expression replicate normally in SW480 colon cancer cells but show a 50- to 100-fold decrease in replication in H1299 lung cancer cells and WI38 normal fibroblasts. Activation of wnt signaling by transduction of a stable beta-catenin mutant into normal fibroblasts renders the cells permissive for virus replication. Insertion of Tcf4 sites in the E1B promoter has only small effects on replication in vitro but significantly reduces the inflammatory response in a rodent lung model in vivo. Replicating adenoviruses with Tcf regulation of both E1B and E2 transcription are potentially useful for the treatment of liver metastases from colorectal tumors, but additional changes will be required to produce a virus that can be used to treat all colon tumors.

Tumor-specific gene expression in hepatic metastases by a replication-activated adenovirus vector

Clinical applications of tumor gene therapy require tumor-specific delivery or expression of therapeutic genes in order to maximize the oncolytic index and minimize side effects. This study demonstrates activation of transgene expression exclusively in hepatic metastases after systemic application of a modified first-generation (E1A/E1B-deleted) adenovirus vector (AdE1-) in mouse tumor models. The discrimination between tumors and normal liver tissue is based on selective DNA replication of AdE1- vectors in tumor cells. This new AdE1- based vector system uses homologous recombination between inverted repeats to mediate precise rearrangements within the viral genome. As a result of these rearrangements, a promoter is brought into conjunction with a reporter gene creating a functional expression cassette. Genomic rearrangements are dependent upon viral DNA replication, which in turn occurs specifically in tumor cells. In a mouse tumor model with liver metastases derived from human tumor cells, a single systemic administration of replication activated AdE1- vectors achieved transgene expression in every metastasis, whereas no extra-tumoral transgene induction was observed. Here we provide a new concept for tumor-specific gene expression that is also applicable for other conditionally replicating adenovirus vectors.

Selective transduction of murine myelomonocytic leukemia cells (WEHI-3B) with regular and RGD-adenoviral vectors

On the basis of the susceptibility of normal myelomonocytic cells to adenoviral vectors, we have studied the possibility of selectively transducing myelomonocytic murine leukemic cells (WEHI-3B) with regular (Reg-Ad) and genetically modified (RGD-Ad) adenoviral vectors. An 8-h incubation of WEHI-3B cells with 100 pfu of Reg-Ad vectors/cell resulted in the whole population becoming positive for transgene expression. Under identical conditions of infection, 20-30% of mouse bone marrow (BM) cells were positive for the transgene. When RGD-Ad vectors were used, a brief exposure (10 min) of WEHI-3B cells to 150 pfu of the virus/cell was enough for 100% of the leukemia cells to become positive for the marker transgene (EGFP). Under these conditions, only 15-20% of BM cells and of primitive hematopoietic progenitors (Lin(-)Sca-1(+) cells) became EGFP(+), indicating an improved selectivity of the vectors for the leukemic cells. The incubation of WEHI-3B but not normal BM cells with soluble fiber protein (FP) inhibited the infection with Reg-Ad. The use of the RGD-Ad bypassed the FP-CAR interaction required for the transduction of WEHI-3B cells with Reg-Ad, suggesting that the abrogation of this requirement accounts for the improved infectivity of these leukemic cells and for the selectivity of RGD-Ad in targeting WEHI-3B leukemia cells.

Replication-selective oncolytic adenoviruses: virotherapy aimed at genetic targets in cancer

Replication-selective oncolytic adenoviruses represent a novel cancer treatment platform. Clinical studies have demonstrated the safety and feasibility of the approach, including the delivery of adenovirus to tumors through the bloodstream (Heise et al., 1999b; Reid et al., 1999; Nemunaitis et al., 1999). The inherent ability of replication-competent adenoviruses to sensitize tumor cells to chemotherapy was a novel discovery that has led to chemosensitization strategies. These data will support the further development of adenoviral agents, including second-generation constructs containing exogenous therapeuitc genes to enhance both local and systemic antitumoral activity (Heise and Kirn, 2000; Hermiston, 2000; Agha-Mohammadi and Lotze, 2000). In addition to adenovirus, other viral species are being developed including herpesvirus, vaccinia, reovirus and measles virus (Kirn, 2000a; Martuza, 2000; Norman and Lee, 2000; Mastrangelo et al., 2000; Coffey et al., 1998; Martuza et al., 1991; Kirn, 2000b; Lattime et al., 1996). Since intratumoral spread also appears to be a substantial hurdle for viral agents, inherently motile agents such as bacteria may hold great promise for this field (Low et al., 1999; Sznol et al., 2000). Given the unknown predictive value of in vitro cell-based assays and murine tumor model systems for the efficacy and therapeutic index of replication-selective oncolytic adenoviruses in patients, we believe that encouraging adenoviral agents must be tested in well-designed clinical trials as soon as possible. Only then can the true therapeutic potential of these agents be realized.

Cancer gene therapy: developments to 2000

Cancer, at the molecular level, continues to be more thoroughly understood. With this understanding comes the opportunity for innovative therapeutic intervention. Gene therapy remains an attractive concept to treat cancer. However, a number of gene therapy clinical trials have now been reported and it is clear that barriers remain before gene therapy gains widespread clinical application. This article outlines current directions and novel developments in the field of cancer gene therapy, which attempt to overcome these obstacles.

Evaluation of E1-mutant adenoviruses as conditionally replicating agents for cancer therapy

The oncolytic effect of adenoviruses may provide an efficient means to destroy tumor tissue if viruses could be developed with sufficient selectivity and efficacy. In this report we have characterized several adenoviruses, each with different mutations in the E1 region, for selective cytopathic effect in tumor cells in vitro and for their ability to inhibit tumor growth in vivo. Of the E1 mutants tested, we have identified one, E1Adl01/07, which preferentially induces cytopathic effects in a range of tumor cells versus primary cells. In addition, E1Adl01/07 significantly inhibited tumor growth and increased survival of mice in several models of human cancer. These results suggest that E1Adl01/07 might serve as an effective cancer therapeutic, combining both selectivity and efficacy.

A novel, conditionally replicative adenovirus for the treatment of breast cancer that allows controlled replication of E1a-deleted adenoviral vectors

The efficiency of gene therapy strategies against cancer is limited by the poor distribution of the vectors in the malignant tissues. To solve this problem, a new generation of tumor-specific, conditionally replicative adenoviruses is being developed. To direct the replication of the virus to breast cancer, we have considered one characteristic present in a great proportion of these cancers, which is the expression of estrogen receptors (ERs). On the basis of the wild-type adenovirus type 5, we have constructed a conditionally replicative adenovirus (Ad5ERE2) in which the E1a and E4 promoters have been replaced by a portion of the pS2 promoter containing two estrogen-responsive elements (EREs). This promoter induces transcriptional activation of the E1a and E4 units in response to estrogens in cells that express the ERs. Ad5ERE2 is able to kill ER(+) human breast cancer cell lines as efficiently as the wild-type virus, but has decreased capacity to affect ER(-) cells. By complementation of the E1a protein in trans, Ad5ERE2 allows restricted replication of a conventional E1a-deleted adenoviral vector. When a virus expressing the proapoptotic gene Bc1-xs (Clarke et al., Proc. Natl. Acad. Sci. U.S.A. 1995;92:11024-11028) is used in combination with Ad5ERE2, the ability of both viruses to induce cell death is dramatically increased, and the effect can be modulated by addition of the antiestrogen tamoxifen.

Replicative adenoviruses for cancer therapy

Rapid advances are being made in the engineering of replication-competent viruses to treat cancer. Adenovirus is a mildly pathogenic human virus that propagates prolifically in epithelial cells, the origin of most human cancers. While virologists have revealed many details about its molecular interactions with the cell, applied scientists have developed powerful technologies to genetically modify or regulate every viral protein. In tandem, the limited success of nonreplicative adenoviral vectors in cancer gene therapy has brought the old concept of adenovirus oncolysis back into the spotlight. Major efforts have been directed toward achieving selective replication by the deletion of viral functions dispensable in tumor cells or by the regulation of viral genes with tumor-specific promoters. However, the predicted replication selectivity has not been realized because of incomplete knowledge of the complex virus-cell interactions and the leakiness of cellular promoters in the viral genome. Capsid modifications are being developed to achieve tumor targeting and enhance infectivity. Cellular and viral functions that confer greater oncolytic potency are also being elucidated. Ultimately, the interplay of the virus with the immune system will likely dictate the success of this approach as a cancer therapy.

Tumor-specific, replication-competent adenovirus vectors overexpressing the adenovirus death protein

We have constructed two novel adenovirus (Ad) replication-competent vectors, named KD1 and KD3, that may have use in anticancer therapy. The vectors have two key features. First, they markedly overexpress the Ad death protein (ADP), an Ad nuclear membrane glycoprotein required at late stages of infection for efficient cell lysis and release of Ad from cells. Overexpression of ADP was achieved by deleting the E3 region and reinserting the adp gene. Because ADP is overexpressed, KD1 and KD3 are expected to spread more rapidly and effectively through tumors. Second, KD1 and KD3 have two E1A mutations (from the mutant dl1101/1107) that prevent efficient replication in nondividing cells but allow replication in dividing cancer cells. These E1A mutations preclude binding of E1A proteins to p300 and pRB. As a result, the virus should not be able to drive cells from G(0) to S phase and therefore should not be able to replicate in normal tissues. We show that KD1 and KD3 do not replicate well in quiescent HEL-299 cells or in primary human bronchial epithelial cells, small airway epithelial cells, or endothelial cells; however, they replicate well in proliferating HEL-299 cells and human A549 lung carcinoma cells. In cultured A549 cells, KD1 and KD3 lyse cells and spread from cell to cell more rapidly than their control virus, dl1101/1107, or wild-type Ad. They are also more efficient than dl1101/1107 or wild-type Ad in complementing the spread from cell to cell of an E1(-) E3(-) replication-defective vector expressing beta-galactosidase. A549 cells form rapidly growing solid tumors when injected into the hind flanks of immunodeficient nude mice; however, when A549 cells were infected with 10(-4) PFU of KD3/cell prior to injection into mice, tumor formation was nearly completely suppressed. When established A549 tumors in nude mice were examined, tumors injected with buffer grew 13.3-fold over 5 weeks, tumors injected with dl1101/1107 grew 8-fold, and tumors injected with KD1 or KD3 grew 2.6-fold. Hep 3B tumors injected with buffer grew 12-fold over 3.5 weeks, whereas tumors injected with KD1 or KD3 grew 4-fold. We conclude that KD1 and KD3 show promise as anticancer therapeutics.

Gene therapy: designer promoters for tumour targeting

One of the biggest challenges facing cancer therapy is to generate tumour-specific treatment strategies. Gene therapy hopes to achieve this by directing the activity of therapeutic genes specifically to the sites of disease. Of paramount importance for the success of this approach is the availability of tumour-specific delivery systems: both the transductional targeting of the vector vehicle and the restriction of transgene expression to the tumour are promising strategies towards this goal. This review will focus on the recent achievements in the field of transcriptional targeting and the different strategies to improve or design promoters with the desired specificities.