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

Targeting telomerase for its advent in cancer therapeutics

Telomerase has emerged as an important primary target in anticancer therapy. It is a distinctive reverse transcriptase enzyme, which extends the length of telomere at the 3' chromosomal end, and uses telomerase reverse transcriptase (TERT) and telomerase RNA template-containing domains. Telomerase has a vital role and is a contributing factor in human health, mainly affecting cell aging and cell proliferation. Due to its unique feature, it ensures unrestricted cell proliferation in malignancy and plays a major role in cancer disease. The development of telomerase inhibitors with increased specificity and better pharmacokinetics is being considered to design and develop newer potent anticancer agents. Use of natural and synthetic compounds for the inhibition of telomerase activity can lead to an opening of new vistas in cancer treatment. This review details about the telomerase biochemistry, use of natural and synthetic compounds; vaccines and oncolytic virus in therapy that suppress the telomerase activity. We have discussed structure-activity relationships of various natural and synthetic telomerase inhibitors to help medicinal chemists and chemical biology researchers with a ready reference and updated status of their clinical trials. Suppression of human TERT (hTERT) activity through inhibition of hTERT promoter is an important approach for telomerase inhibition.

Oncolytic virus delivery: from nano-pharmacodynamics to enhanced oncolytic effect

With the advancement of a growing number of oncolytic viruses (OVs) to clinical development, drug delivery is becoming an important barrier to overcome for optimal therapeutic benefits. Host immunity, tumor microenvironment and abnormal vascularity contribute to inefficient vector delivery. A number of novel approaches for enhanced OV delivery are under evaluation, including use of nanoparticles, immunomodulatory agents and complex viral–particle ligands along with manipulations of the tumor microenvironment. This field of OV delivery has quickly evolved to bioengineering of complex nanoparticles that could be deposited within the tumor using minimal invasive image-guided delivery. Some of the strategies include ultrasound (US)-mediated cavitation-enhanced extravasation, magnetic viral complexes delivery, image-guided infusions with focused US and targeting photodynamic virotherapy. In addition, strategies that modulate tumor microenvironment to decrease extracellular matrix deposition and increase viral propagation are being used to improve tumor penetration by OVs. Some involve modification of the viral genome to enhance their tumoral penetration potential. Here, we highlight the barriers to oncolytic viral delivery, and discuss the challenges to improving it and the perspectives of establishing new modes of active delivery to achieve enhanced oncolytic effects.

Oncolytic virotherapy of breast cancer

The use of replication competent viruses that selectively target and destroy cancer cells has rapidly evolved over the past decade and numerous innovative oncolytic viruses have been created. Many of these promising anti-cancer agents have recently entered into clinical trials (including those on breast cancer) and demonstrated encouraging safety and efficacy. Virotherapeutic strategies are thus of considerable interest to combat breast cancer in both (i) the primary disease situation in which relapse should be avoided as good as possible and (ii) in the metastatic situation which remains incurable to date. Here, we summarize data from preclinical and clinical trials using oncolytic virotherapy to treat breast cancer. This includes strategies to specifically target breast cancer cells, to arm oncolytic viruses with additional therapeutic transgenes and an outlining of future challenges when translating these promising therapeutics "from bench to bedside".

Self-inactivating helper virus for the production of high-capacity adenoviral vectors

Standard methods for producing high-capacity adenoviral vectors (HC-Ads) are based on co-infection with a helper adenovirus (HV). To avoid HV encapsidation, its packaging signal (Ψ) is flanked by recognition sequences for recombinases expressed in the producing cells. However, accumulation of HV and low yield of HC-Ad are frequently observed, due in part to insufficient recombinase expression. We describe here a novel HV (AdTetCre) in which Ψ is flanked by loxP sites that can be excised by a chimeric MerCreMer recombinase encoded in the same viral genome. Efficient modulation of cleavage was obtained by simultaneous control of MerCreMer expression using a tet-on inducible system, and translocation to the nucleus by 4-hydroxytamoxifen (TAM). Encapsidation of AdTetCre was strongly inhibited by TAM plus doxycicline. Using AdTetCre and 293Cre4 cells for the production of HC-Ads, we found that cellular and virus-encoded recombinases cooperate to minimize HV contamination. The method was highly reproducible and allowed the routine production of different HC-Ads in a medium-scale laboratory setting in adherent cells, with titers >10¹⁰ infectious units and <0.1% HV contamination. The residual HVs lacked Ψ and were highly attenuated. We conclude that self-inactivating HVs based on virally encoded recombinases are promising tools for the production of HC-Ads.

Tropism-modification strategies for targeted gene delivery using adenoviral vectors

Achieving high efficiency, targeted gene delivery with adenoviral vectors is a long-standing goal in the field of clinical gene therapy. To achieve this, platform vectors must combine efficient retargeting strategies with detargeting modifications to ablate native receptor binding (i.e. CAR/integrins/heparan sulfate proteoglycans) and “bridging” interactions. “Bridging” interactions refer to coagulation factor binding, namely coagulation factor X (FX), which bridges hepatocyte transduction in vivo through engagement with surface expressed heparan sulfate proteoglycans (HSPGs). These interactions can contribute to the off-target sequestration of Ad5 in the liver and its characteristic dose-limiting hepatotoxicity, thereby significantly limiting the in vivo targeting efficiency and clinical potential of Ad5-based therapeutics. To date, various approaches to retargeting adenoviruses (Ad) have been described. These include genetic modification strategies to incorporate peptide ligands (within fiber knob domain, fiber shaft, penton base, pIX or hexon), pseudotyping of capsid proteins to include whole fiber substitutions or fiber knob chimeras, pseudotyping with non-human Ad species or with capsid proteins derived from other viral families, hexon hypervariable region (HVR) substitutions and adapter-based conjugation/crosslinking of scFv, growth factors or monoclonal antibodies directed against surface-expressed target antigens. In order to maximize retargeting, strategies which permit detargeting from undesirable interactions between the Ad capsid and components of the circulatory system (e.g. coagulation factors, erythrocytes, pre-existing neutralizing antibodies), can be employed simultaneously. Detargeting can be achieved by genetic ablation of native receptor-binding determinants, ablation of “bridging interactions” such as those which occur between the hexon of Ad5 and coagulation factor X (FX), or alternatively, through the use of polymer-coated “stealth” vectors which avoid these interactions. Simultaneous retargeting and detargeting can be achieved by combining multiple genetic and/or chemical modifications.

Analysis of adenovirus trans-complementation-mediated gene expression controlled by melanoma-specific TETP promoter in vitro

BACKGROUND

Human adenoviruses (Ads) have substantial potential for clinical applications in cancer patients. Conditionally replicating adenoviruses (CRAds) include oncolytic adenoviruses in which expression of the immediate early viral transactivator protein E1A is controlled by a cancer cell-selective promoter. To enhance efficacy, CRAds are further armed to contain therapeutic genes. Due to size constraints of the capsid geometry, the capacity for packaging transgenes into Ads is, however, limited. To overcome this limitation, the employment of E1A-deleted replication-deficient viruses carrying therapeutic genes in combination with replication-competent CRAd vectors expressing E1A in trans has been proposed. Most trans-complementing studies involved transgene expressions from strong ubiquitous promoters, and thereby relied entirely on the cancer cell specificity of the CRAd vector.

RESULTS

Here we tested the trans-complementation of a CRAd and a replication-deficient transgene vector containing the same cancer cell-selective promoter. Hereto, we generated two new vectors expressing IL-2 and CD40L from a bicistronic expression cassette under the control of the melanoma/melanocyte-specific tyrosinase enhancer tyrosinase promoter (TETP), which we previously described for the melanoma-specific CRAd vector AdDeltaEP-TETP. These vectors gave rise to tightly controlled melanoma-specific transgene expression levels, which were only 5 to 40-fold lower than those from vectors controlled by the nonselective CMV promoter. Reporter analyses using Ad-CMV-eGFP in combination with AdDeltaEP-TETP revealed a high level of trans-complementation in melanoma cells (up to about 30-fold), but not in non-melanoma cells, unlike the AdCMV-eGFP/wtAd5 binary vector system, which was equally efficient in melanoma and non-melanoma cells. Similar findings were obtained when replacing the transgene vector AdCMV-eGFP with AdCMV-IL-2 or AdCMV-CD40L. However, the combination of the novel AdTETP-CD40L/IL-2 vector with AdDeltaEP-TETP or wtAd5 gave reproducible moderate 3-fold enhancements of IL-2 by trans-complementation only.

CONCLUSIONS

The cancer cell-selective TETP tested here did not give the expected enforceable transgene expression typically achieved in the Ad trans-complementing system. Reasons for this could include virus-mediated down regulation of limiting transcription factors, and/or competition for such factors by different promoters. Whether this finding is unique to the particular promoter system tested here, or also occurs with other promoters warrants further investigations.

Oncolytic (replication-competent) adenoviruses as anticancer agents

IMPORTANCE OF THE FIELD

Whilst therapies for neoplasies have advanced tremendously in the last few decades, there is still a need for new anti-cancer treatments. One option is genetically-engineered oncolytic adenovirus (Ad) 'vectors'. These kill cancer cells via the viral replication cycle, and amplify the anti-tumor effect by producing progeny virions able to infect neighboring tumor cells.

AREAS COVERED IN THIS REVIEW

We provide a description of basic Ad biology and summarize the literature for oncolytic Ads from 1996 to the present.

WHAT THE READER WILL GAIN

An overall view of oncolytic Ads, the merits and drawbacks of the various features of these vectors, and obstacles to further development and future directions for research.

TAKE HOME MESSAGE

Ads are attractive for gene therapy because they are relatively innocuous, easy to produce in large quantities, genetically stable, and easy to manipulate. A variety of have been constructed and tested, in pre-clinical and clinical experiments. Oncolytic Ads proved to be remarkably safe; no dose-limiting toxicity was observed in any clinical trial, and the maximum tolerated dose was not reached. At present, the major challenge for researchers is to increase the efficacy of the vectors, and to incorporate oncolytic virotherapy into existing treatment protocols.

Growth inhibition of human multiple myeloma cells by an oncolytic adenovirus carrying the CD40 ligand transgene

PURPOSE

The growth-inhibitory activity of recombinant CD40 ligand (CD40L) is well documented in human multiple myeloma (MM). We examined MM-targeted delivery of CD40L by a conditional replicative oncolytic adenovirus, AdEHCD40L.

EXPERIMENTAL DESIGN

The growth-regulatory activity of AdEHCD40L was determined in vitro and in vivo. Differential analysis with AdEHCD40L and parental virus (AdEHNull)-infected cultures allowed the identification of cellular and molecular pathways modulated by the CD40L transgene.

RESULTS

Conditional expression of viral E1A and CD40L transgene was shown in human MM lines RPMI 8226 [interleukin (IL)-6 independent] and Kas-6/1 (IL-6 dependent) under hypoxic conditions commonly found in MM in situ. AdEHCD40L inhibited MM cell growth more effectively than AdEHNull. This enhanced growth-inhibitory activity was abrogated by cotreatment with a CD40L antibody. Chemoresistant MM lines (MR20 and LR5) were similarly susceptible to AdEHCD40L treatment. AdEHCD40L induced apoptosis and S-phase cell cycle blockade while uniquely up-regulating the previously described proapoptotic elements tumor necrosis factor-related apoptosis-inducing ligand, Fas, and IL-8. Intratumoral injections of AdEHCD40L reduced the growth of severe combined immunodeficient/hu RPMI 8226 xenografts by >50% compared with 28% reduction by AdEHNull. Adenoviral hexon and CD40L were detected in AdEHCD40L-treated tumors at day 35 after infection primarily in necrotic areas, suggesting viral replicative activity.

CONCLUSIONS

These findings show that CD40L acts in concert with viral oncolysis to produce MM growth inhibition through activation of cellular apoptosis. The direct growth-inhibitory activity of AdEHCD40L, together with the well-known immune-potentiating features of CD40L, may be clinically applicable for the experimental treatment of MM or plasma cell leukemia.

Targeting cancer by transcriptional control in cancer gene therapy and viral oncolysis

Cancer-specificity is the key requirement for a drug or treatment regimen to be effective against malignant disease--and has rarely been achieved adequately to date. Therefore, targeting strategies need to be implemented for future therapies to ensure efficient activity at the site of patients' tumors or metastases without causing intolerable side-effects. Gene therapy and viral oncolysis represent treatment modalities that offer unique opportunities for tumor targeting. This is because both the transfer of genes with anti-cancer activity and viral replication-induced cell killing, respectively, facilitate the incorporation of multiple mechanisms restricting their activity to cancer. To this end, cellular mechanisms of gene regulation have been successfully exploited to direct therapeutic gene expression and viral cell lysis to cancer cells. Here, transcriptional targeting has been the role model and most widely investigated. This approach exploits cellular gene regulatory elements that mediate cell type-specific transcription to restrict the expression of therapeutic genes or essential viral genes, ideally to cancer cells. In this review, we first discuss the rationale for such promoter targeting and its limitations. We then give an overview how tissue-/tumor-specific promoters are being identified and characterized. Strategies to apply and optimize such promoters for the engineering of targeted viral gene transfer vectors and oncolytic viruses-with respect to promoter size, selectivity and activity in the context of viral genomes-are described. Finally, we discuss in more detail individual examples for transcriptionally targeted virus drugs. First highlighting oncolytic viruses targeted by prostate-specific promoters and by the telomerase promoter as representatives of tissue-targeted and pan-cancer-specific virus drugs respectively, and secondly recent developments of the last two years.

Adenoviral vector-based strategies for cancer therapy

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

A simplified in vitro ligation approach to clone an E1B55k-deleted double-targeted conditionally-replicative adenovirus

BACKGROUND

Construction of conditionally-replicative Adenovirus (CRAd) is complex and time-consuming. While homologous recombination (HR) using a two-plasmid system in bacteria is commonly used to generate CRAds, alternative methods may be required when HR fails. Previously, in vitro ligation has been suggested to facilitate construction of E1/E3-deleted, replication-incompetent Ad vectors. However, in vitro ligation has only rarely been used to generate CRAds and may be a complex procedure for molecular biologists who are not experts in the field.

METHODS AND RESULTS

A modified in vitro ligation approach was developed to construct a double-targeted, E1B55k-deleted CRAd. The method allowed the incorporation of a tumor-specific promoter, e.g. the heat-shock protein 70 (hsp70) promoter, upstream of E1a, deletion of the E1B55k gene, and HR-free cloning of the recombined E1Delta55k gene into the Ad genome. The genetic structure of the CRAd was confirmed using restriction analysis and PCR. The replication rate of the hsp70E1Delta55k CRAd was 1.5-2% of Ad without E1Delta55k deletion.

CONCLUSION

A 3-step cloning approach can generate a double-targeted, E1B55k-deleted CRAd using a straight-forward, modified in vitro ligation procedure.

Human adenovirus replicates in immunocompetent models of pancreatic cancer in Syrian hamsters

The preclinical evaluation of toxicity and antitumor effect of conditionally replicative (oncolytic) adenoviruses is hampered by the inability of human adenoviruses to replicate efficiently in murine cells. The Syrian golden hamster (Mesocricetus auratus) has been suggested as a permissive animal for adenoviral replication, and cancer cell lines derived from various hamster tumors are available. We provide evidence that wild-type adenovirus type 5 is able to infect and replicate in the pancreatic cancer cell lines HaP-T1 and H2T both in vitro and in vivo. Determination of cytopathic effect, viral spread, progeny production, and the expression of late viral proteins indicates that the complete viral cycle of adenovirus takes place, albeit less efficiently than in highly permissive human cancer cell lines A549 and HuH7. Intrahepatic inoculation of HaP-T1 and H2T cells gave rise to tumors in the liver of hamsters that resemble metastases of pancreatic cancer. The growth of HaP-T1-induced nodules was faster compared with those derived from H2T, but both caused progressive liver infiltration and peritoneal dissemination. When adenovirus was inoculated in these lesions, productive replication took place and newly formed infective virions could be recovered 4 days after administration. In conclusion, the Syrian hamster models described here offer the opportunity to evaluate the effect of oncolytic adenoviruses in an immunocompetent animal and may be a valuable tool in the preclinical evaluation of these agents.

A novel chromogranin-A promoter-driven oncolytic adenovirus for midgut carcinoid therapy

PURPOSE

The use of replication-selective oncolytic adenoviruses is an emerging therapeutic approach for cancer, which thus far has not been employed for carcinoids. We therefore constructed Ad[CgA-E1A], a novel replication-selective oncolytic adenovirus, where the chromogranin A (CgA) promoter controls expression of the adenoviral E1A gene.

EXPERIMENTAL DESIGN

The Ad[CgA-E1A] virus was evaluated for E1A protein expression, replication ability, and cytolytic activity in various cell lines. It was also evaluated for treatment of xenografted human carcinoid tumors in nude mice. To use Ad[CgA-E1A] for the treatment of carcinoid liver metastases, it is important that normal hepatocytes do not support virus replication to minimize hepatotoxicity. We therefore evaluated CgA protein expression in normal hepatocytes. We also evaluated CgA gene expression in normal hepatocytes and microdissected tumor cells from carcinoid metastases.

RESULTS

We found that Ad[CgA-E1A] replicates similarly to wild-type virus in tumor cells with neuroendocrine features, including the BON carcinoid cell line and the SH-SY-5Y neuroblastoma cell lines, whereas it is attenuated in other cell types. Thus, cells where the CgA promoter is active are selectively killed. We also found that Ad[CgA-E1A] is able to suppress fast-growing human BON carcinoid tumors in nude mice. Furthermore, CgA is highly expressed in microdissected cells from carcinoid metastases, whereas it is not expressed in normal hepatocytes.

CONCLUSION

Ad[CgA-E1A] is an interesting agent for the treatment of carcinoid liver metastases in conjunction with standard therapy for these malignancies.

A conditionally replicative, Wnt/beta-catenin pathway-based adenovirus therapy for anaplastic thyroid cancer

Thyroid cancer affects between 10,000 and 15,000 people per year in the US. Typically, this disease can be controlled with surgical resection and radioiodide treatment. However, resistance to these conventional therapies is observed in some patients, who develop intractable anaplastic thyroid cancer (ATC), for which no effective therapies exist. Recently, a sizable fraction of undifferentiated or poorly differentiated thyroid cancers were shown to contain mutations in beta-catenin, an oncogenic protein involved in the etiology of cancers of many tissues. We developed a conditionally replicative adenovirus (named 'HILMI') which, by virtue of TCF response elements drives E1A and E1B expression, replicates specifically in cells with an active Wnt/beta-catenin pathway. We show that several thyroid cancer cell lines, derived from undifferentiated or anaplastic tissues and possessing an active Wnt/beta-catenin pathway, are susceptible to cell killing by HILMI. Furthermore, viral replication in ATC cells as xenograft tumors in nude mice was observed, and prolonged survival of mice with ATC tumors was observed following administration of the HILMI therapeutic vector. The results warrant further development of this therapeutic approach for ATC patients.

Selective replication of E1B55K-deleted adenoviruses depends on enhanced E1A expression in cancer cells

E1B55K-deleted dl1520 could selectively replicate in cancer cells and has been used in clinical trials as an antitumor agent. The mechanism of virus selective replication in cancer cells, including a possible role of p53, is unclear. Studies with established cancer cell lines have demonstrated that some cancer cells are resistant to dl1520 replication, regardless of the p53 status. Hep3B cells supported the E1b-deleted adenoviruses to replicate, whereas Saos2 cells were resistant to viral replication. We applied p53-null Hep3B and Saos2 cells as models to clarify the replication ability of E1B55K-deleted adenoviruses with different expression levels of E1a. We show that lower E1A expression in Saos2 may be the reason for the poor replication in some cancer cells due to the fact that E1a promoter was less activated in Saos2 than in Hep3B. We also demonstrate that the E1B55K protein can increase E1A expression in Saos2 cells for efficient virus replication. In addition, the upstream regions of the E1a promoter have transcriptional activity in Hep3B cells but not in Saos2 cells. The viral E1B55K protein may activate cancer cellular factor(s) that targets the upstream regions of the E1a gene to increase its expression. This is the first study demonstrating that E1B55K protein affects the E1A production levels that is related to cancer selective replication. Our studies have suggested that increase of E1A expression from E1b-deleted adenoviruses may enhance killing cancer cells that otherwise are resistant to viral replication.

Efficient generation of double heterologous promoter controlled oncolytic adenovirus vectors by a single homologous recombination step in Escherichia coli

Background

Oncolytic adenoviruses are promising agents for the multimodal treatment of cancer. However, tumor-selectivity is crucial for their applicability in patients. Recent studies by several groups demonstrated that oncolytic adenoviruses with tumor-/tissue-specific expression of the E1 and E4 genes, which are pivotal for adenoviral replication, have a specificity profile that is superior to viruses that solely target the expression of E1 or E4 genes. Presently the E1 and E4 regions are modified in a time consuming sequential fashion.

Results

Based on the widely used adenoviral cloning system AdEasy we generated a novel transfer vector that allows efficient and rapid generation of conditionally replication-competent adenovirus type 5 based vectors with the viral E1 and E4 genes under the transcriptional control of heterologous promoters. For insertion of the promoters of interest our transfer vector has two unique multiple cloning sites. Additionally, our shuttle plasmid allows encoding of a transgene within the E1A transcription unit. The modifications, including E1 mutations, are introduced into the adenoviral genome by a single homologous recombination step in Escherichia coli. Subsequently infectious viruses are rescued from plasmids. As a proof-of-concept we generated two conditionally replication-competent adenoviruses Ad.Ki•COX and Ad.COX•Ki with the promoters of the Ki-67 protein and the cyclooxygenase-2 (COX-2) driving E1 and E4 and vice versa.

Conclusion

We demonstrated with our cloning system efficient generation of double heterologous promoter controlled oncolytic adenoviral vectors by a single homologous recombination step in bacteria. The generated viruses showed preferential replication in tumor cells and in a subcutaneous HT-29 colon cancer xenograft model the viruses demonstrated significant oncolytic activity comparable with dl327.

Dual cancer-specific targeting strategy cures primary and distant breast carcinomas in nude mice

Limitations of current viral-based gene therapies for malignant tumors include lack of cancer-specific targeting and insufficient tumor delivery. To ameliorate these problems and develop a truly effective adenovirus gene-based therapy for cancer, we constructed a conditionally replication competent adenovirus (CRCA) manifesting the unique properties of tumor-specific virus replication in combination with production of a cancer-selective cytotoxic cytokine, melanoma differentiation associated gene-7/interleukin-24 (mda-7/IL-24), which embodies potent bystander antitumor activity. Cancer cell selective tropism was ensured by engineering the expression of the adenoviral E1A protein, necessary for viral replication, under the control of a minimal promoter region of progression elevated gene-3 (PEG-3), which functions selectively in diverse cancer cells with minimal activity in normal cells. In the E3 region of this CRCA, we introduced the mda-7/IL-24 gene, thereby mediating robust production of this cytokine as a function of adenovirus replication. Infection of this CRCA (designated Ad.PEG-E1A-mda-7) in normal mammary epithelial cells and breast cancer cells confirmed cancer cell selective adenoviral replication, mda-7/IL-24 expression, growth inhibition, and apoptosis induction. Injecting Ad.PEG-E1A-mda-7 into human breast cancer xenografts in athymic nude mice completely eradicated not only the primary tumor but also distant tumors (established on the opposite flank of the animal) thereby implementing a cure. This dual cancer-specific targeting strategy provides an effective approach for treating breast and other human neoplasms with the potential for eradicating both primary tumors and metastatic disease. Additionally, these studies support the potential use of mda-7/IL-24 in the therapy of malignant cancers.

Tamoxifen-regulated adenoviral E1A chimeras for the control of tumor selective oncolytic adenovirus replication in vitro and in vivo

Pharmacological control is a desirable safety feature of oncolytic adenoviruses (oAdV). It has recently been shown that oAdV replication may be controlled by drug-dependent transcriptional regulation of E1A expression. Here, we present a novel concept that relies on tamoxifen-dependent regulation of E1A activity through functional linkage to the mutated hormone-binding domain of the murine estrogen receptor (Mer). Four different E1A-Mer chimeras (ME, EM, E(DeltaNLS)M, MEM) were constructed and inserted into the adenoviral genome under control of a lung-specific surfactant protein B promoter. The highest degree of regulation in vitro was seen for the corresponding oAdVs Ad.E(DeltaNLS)M and Ad.MEM, which exhibited an up to 100-fold higher oAdV replication in the presence as compared with the absence of 4-OH-tamoxifen. Moreover, destruction of nontarget cells was six- and 13-fold reduced for Ad.E(DeltaNLS)M and Ad.MEM, respectively, as compared with Ad.E. Further investigations supported tamoxifen-dependent regulation of Ad.E(DeltaNLS)M and Ad.MEM in vivo. Induction of Ad.E(DeltaNLS)M inhibited growth of H441 lung tumors as efficient as a control oAdV expressing E1A. E(DeltaNLS)M and the MEM chimeras can be easily inserted into a single vector genome, which extends their application to existing oAdVs and strongly facilitates in vivo application.

Oncolytic viral therapies

Molecular research has vastly advanced our understanding of the mechanism of cancer growth and spread. Targeted approaches utilizing molecular science have yielded provocative results in the treatment of cancer. Oncolytic viruses genetically programmed to replicate within cancer cells and directly induce toxic effect via cell lysis or apoptosis are currently being explored in the clinic. Safety has been confirmed and despite variable efficacy results several dramatic responses have been observed with some oncolytic viruses. This review summarizes results of clinical trials with oncolytic viruses in cancer.

Use of replicating oncolytic adenoviruses in combination therapy for cancer

Oncolytic virotherapy is the use of genetically engineered viruses that specifically target and destroy tumor cells via their cytolytic replication cycle. Viral-mediated tumor destruction is propagated through infection of nearby tumor cells by the newly released progeny. Each cycle should amplify the number of oncolytic viruses available for infection. Our understanding of the life cycles of cytolytic viruses has allowed manipulation of their genome to selectively kill tumor cells over normal tissue. Because the mechanism of tumor destruction is different, oncolytic virotherapy should work synergistically with current modes of treatment such as chemotherapy and radiation therapy. This article focuses on oncolytic adenoviruses that have been created and tested in preclinical and clinical trials in combination with chemotherapy, radiation therapy, and gene therapy.

Tissue-specific promoters for cancer gene therapy

Adenoviral cancer gene therapy approaches have resulted in promising recent results. Following only a decade of intense development, some of the crucial obstacles are now being overcome. Insufficient transduction has been the main limitation of earlier approaches. A new approach for increasing transduction of tumour cells is utilisation of replication-competent oncolytic agents, such as conditionally replicating adenoviruses (CRADs). The anti-tumour effect is caused by replication of the virus per se and, thus, replication must be restricted to tumour cells to protect normal tissues from damage. Tissue-specific promoters (TSPs) represent a powerful tool for decreasing the toxicity of cancer gene therapy to normal tissues and have previously been utilised for specific mutation compensation or delivery of prodrug-converting enzymes. However, TSPs can also be used for controlling crucial viral replication regulators and consequent restriction of replication to tumour cells. Initial clinical trials have demonstrated the safety and suggested efficacy for TSP-controlled CRADs as a novel approach for cancer gene therapy.

Selective replicating viral vectors : potential for use in cancer gene therapy

Treatment of cancer is limited by toxicity to normal tissue with standard approaches (chemotherapy, surgery and radiotherapy). The use of selective replicating viral vectors may enable the targeting of gene-modified viruses to malignant tissue without toxic effect. Studies of these vectors have demonstrated tumour-selective replication and minimal evidence of replication in normal tissue. The most advanced clinical results reported involve gene-modified adenoviral vectors. Several completed, histologically confirmed responses to local/regional injection have been induced, particularly in recurrent squamous cell carcinoma involving the head and neck region. Dose limiting toxicity above 10(13) viral particles per injection has been observed. Anti-tumour effect is demonstrable in animal models without evidence of significant toxicity when these vectors are used alone or in combination with chemotherapy, radiation therapy or as gene delivery vehicles. Preliminary clinical trials, particularly with E1B-deleted adenoviruses, report evidence of clinical activity in comparison with expected historical responses. Enhancement in replication selectivity to malignant tissue is also demonstrated preclinically and clinically with an E1B-deleted adenovirus utilising a prostate-specific antigen promoter. Other selective replicating viral vectors such as herpes simplex virus and vaccinia virus have also been explored clinically and suggest evidence of activity in patients with cancer. Modifications may one day enable more aggressive use of these new and exciting therapeutics as systemic gene delivery vehicles.

Systemic chemotherapy for patients with bladder cancer – current controversies and future directions

Many localised, superficial bladder cancers can be effectively controlled. However, disease which has spread to nodes outside the pelvis or to distant organs is generally incurable and systemic therapies, rather than surgery, are appropriate. Combination chemotherapy based around established cytotoxic drugs such as cisplatin has proven benefit in palliating symptoms and prolonging survival in responsive patients with advanced disease. Combination chemotherapies which include newer cytotoxic drugs such as gemcitabine provide the potential for equivalent efficacy with less toxicity than established regimens. Between the extremes of superficial and advanced disease, muscle-invasive bladder cancers have traditionally been treated, with curative intent, by radical surgery or radiotherapy. However, newly published data suggest, for the first time, genuine survival benefits from peri-operative chemotherapy. This article reviews the evidence for cisplatin-based chemotherapy in advanced disease, assesses the potential benefits of newer cytotoxic drugs, discusses the latest evidence pertaining to peri-operative chemotherapy in muscle-invasive disease, and looks forward to potential new biological agents in the systemic therapy of bladder cancer.

Analyses of melanoma-targeted oncolytic adenoviruses with tyrosinase enhancer/promoter-driven E1A, E4, or both in submerged cells and organotypic cultures

We have generated novel conditionally replicative adenoviruses (CRAds) targeted to melanoma cells. In these adenoviruses, the E4 region (AdΔ24TyrE4) or both E1 and E4 regions (Ad2xTyr) were controlled by a synthetic tyrosinase enhancer/promoter (Tyr2E/P) specific for melanocytes. The properties of these CRAds were compared with wild-type adenovirus (Adwt) and our previous CRAd with a targeted E1A CRII mutation (AdTyrΔ24) in submerged cultures of melanoma cells and nonmelanoma control cells. We showed that AdΔ24TyrE4 had a cell type selectivity similar to AdTyrΔ24 but had a distinct block in viral reproduction in nonmelanoma cells and that Ad2xTyr had an augmented selectivity for melanoma cells. These viruses were additionally tested in organotypic cultures of melanoma cell lines, primary human keratinocytes (PHKs), or mixed cell populations. Unexpectedly, the CRAds exhibited somewhat different cell type selectivity profiles in these cultures relative to those observed in submerged cultures, demonstrating the importance of multiple assay systems. Specifically, AdTyrΔ24 and Ad2xTyr were selective for melanoma cells, whereas AdΔ24TyrE4 exhibited no selectivity, similar to Adwt. AdTyrΔ24 and Ad2xTyr were strongly attenuated in their ability to lyse PHKs in organotypic cultures. Furthermore, Ad2xTyr had a superior melanoma selectivity in organotypic cultures of cocultivated melanoma cells and PHKs. The enhanced selectivity for melanoma cells exhibited by Ad2xTyr provides a window of opportunity for therapeutic application. These studies also demonstrate that organotypic cultures derived from mixtures of tumor and normal cells represent a promising new model for analysis of CRAd specificity and toxicity.

Virotherapeutics: conditionally replicative adenoviruses for viral oncolysis

Viral oncolysis, or virotherapy, is an endeavor to use viruses as therapeutic agents in an effort to exploit their highly evolved qualities of host cell killing and simultaneous multiplication and spread. This review describes the concept of oncolytic adenoviruses, also called conditionally replicative adenoviruses (CRAds), and recent developments--inspired by early clinical results--that aim at the optimization of CRAd efficacy. Molecular strategies applied for the development of oncolytic adenoviruses include (i) the genetic manipulation of the expression and/or function of key regulatory viral proteins in order to restrict viral replication and spread to tumor cells, (ii) the engineering of the adenoviral capsid for efficient and tumor-targeted infection, and (iii) the incorporation of heterologous genes to facilitate combination therapies or tracking of the virus. Initial clinical trials have provided proof-of-concept for adenoviral oncolysis in patients and a favorable safety profile for oncolytic adenoviruses has been demonstrated. In conclusion, adenoviral oncolysis, with its distinct therapeutic mechanism, shows remarkable therapeutic potential. Advanced generations of virotherapeutics are currently in development.

Endocrine aspects of cancer gene therapy

The field of cancer gene therapy is in continuous expansion, and technology is quickly moving ahead as far as gene targeting and regulation of gene expression are concerned. This review focuses on the endocrine aspects of gene therapy, including the possibility to exploit hormone and hormone receptor functions for regulating therapeutic gene expression, the use of endocrine-specific genes as new therapeutic tools, the effects of viral vector delivery and transgene expression on the endocrine system, and the endocrine response to viral vector delivery. Present ethical concerns of gene therapy and the risk of germ cell transduction are also discussed, along with potential lines of innovation to improve cell and gene targeting.

Replicating Adenoviruses in Cancer Therapy

The potential use of adenoviruses in therapy against cancer has evoked a rapidly moving field of research. Unlike conventional gene therapy vectors, oncolytic adenoviruses retain the ability to replicate. However, replication is restricted as much as possible to tumor cells, with the aim of eliminating these cells through viral cytotoxicity. The two key issues are to improve the efficiency of virus replication and cell killing while ensuring the specificity of these activities for tumor cells. Wild-type adenoviruses as such may already be usable for cancer therapy. Strategies to further improve efficiency and specificity include the partial or complete removal of viral genes. The idea is that functions carried out by the corresponding gene products are not required for replication in tumor cells, but are needed in normal cells. Accordingly, the removal of genes encoding E1B-55 kDa or E1B-19 kDa, or the mutation of E1A may improve the selective killing of tumor cells. On the other hand, the overexpression of the adenovirus death protein (ADP) may enhance viral spread and oncolytic efficiency. Other strategies to improve the specific oncolytic activity of replicating adenoviruses have been pursued. For instance, some promoters are active specifically in tumor cells, and these promoters were introduced into the viral genome, to regulate essential viral genes. Moreover, replicating viruses were engineered to express toxic proteins or drug converters. A number of these viruses have been tested successfully using tumor xenografts in nude mice as a model system. An oncolytic adenovirus lacking the E1B-55 kDa gene product, termed dl1520 or ONYX015, was injected into squamous cell carcinomas of head and neck in phase II clinical trials, and the results were encouraging when chemotherapy was applied in parallel. In the future, further progress might be achieved on the level of virus constructs, but also by refining and adjusting simultaneous conventional therapies, and by standardizing the assessment of the clinical outcome. Recent progress has been made towards the use of replicating virus constructs in cancer therapy. The goal of these developments is to remove cancerous cells from patients with the help of viruses that selectively replicate in these cells. These viruses are generally termed oncolytic viruses. Some convenient properties of adenovirus make this virus particularly useful for this purpose. It infects a large number of human cell types, especially epithelial cells, which give rise to the vast majority of human malignancies. It can be grown easily and to high titers, and the creation of virus recombinants is well established. Finally, a large body of basic research has already been carried out on this virus, facilitating its manipulation. Various approaches to use adenovirus as a cancer drug have been reviewed (Alemany et al. 1999a, 2000; Curiel 2000; Galanis et al. 2001b; Gromeier 2001; Heise and Kirn 2000; Kirn 2000a; Kirn et al. 2001; Kirn and McCormick 1996; Smith and Chiocca 2000; Sunamura 2000; Wells 2000; Wodarz 2001). The aim of this chapter is to provide an integrated overview of these strategies.

Adenovirus vectors: biology, design, and production

The use of adenovirus as a gene transfer vehicle arose from early reports of recombinant viruses carrying heterologous DNA fragments. Adenovirus vectors offer many advantages for gene delivery: they are easy to propagate to high titers, they can infect most cell types regardless of their growth state, and in their most recent embodiments they can accommodate large DNA inserts. In this chapter, the development of adenovirus vectors is reviewed, from the use of so-called first-generation, E1-deleted viruses to the latest generation high-capacity, helper-dependent vectors. Examples of their use in the clinic are described, as are the current areas in which improvements to these vectors are being explored.

A novel tetracycline-controlled transactivator-transrepressor system enables external control of oncolytic adenovirus replication

The use of restricted replication-competent adenoviruses (RRCAs) inducing tumor cell-specific lysis is a promising approach in cancer gene therapy. However, the use of RRCAs in humans carries considerable risk, since after injection into the patient, further regulation or inhibition of virus replication from the outside is impossible. Therefore, we have developed a novel system allowing external pharmacological control of RRCA replication. We show here that a tumor-selective E1B-deleted RRCA can be tightly regulated by use of doxycycline (dox)-controlled adenoviral E1A gene expression, which in turn determines vector replication. RRCA replication is switched on by addition and switched off by withdrawal of dox. The system results in efficient tumor cell killing after induction by dox, whereas cells are unaffected by the uninduced system. It was also employed for efficient external control of transgene expression from cotransfected replication-deficient adenovectors. Furthermore, the use of a liver cell-specific human alpha1-antitrypsin (hAAT)-promoter driving a tetracycline-controlled transcriptional silencer allowed specific protection of cells with hAAT-promoter activity in the absence of dox in vitro and in vivo, delineating a new principle of 'tissue protective' gene therapy. The concept of external control of RRCAs may help to improve the safety of cancer gene therapy.

Telomerase-dependent oncolytic adenovirus for cancer treatment

Conditionally replicative adenovirus (CRAD) is an attractive anticancer agent as it can selectively replicate in tumor cells. Expression of telomerase reverse transcriptase (TERT) is a unique tumor cell characteristic, being absent in normal postmitotic cells. Thus, we constructed a TERT promoter regulated CRAD for tumor-specific oncolysis by replacing the endogenous adenovirus E1A promoter with that of human TERT (Adv-TERTp-E1A). We showed that its replication was severely attenuated in TERT-negative cells, but that it replicated almost as efficiently as wild-type adenovirus in TERT-positive cells. Accordingly, Adv-TERTp-E1A conferred cytopathicity to TERT-positive, but not TERT-negative, cells. In vivo replication of Adv-TERTp-E1A after local administration into a xenograft model of human hepatocellular carcinoma in nude mice was demonstrated by an increase in adenovirus titers in tumor extracts by several orders of magnitude between 6 h and 3 days postvector injection. Furthermore, significant inhibition of tumor growth with substantial necrotic tumor areas staining positively for adenovirus was observed with Adv-TERTp-E1A, but not with a control replication-deficient adenovirus. There was also the absence of hepatotoxicity in tumor-bearing animals after intratumoral delivery of the CRAD. The results indicate that the TERT promoter-driven CRAD is capable of tumor-selective replication and oncolysis in vitro and in vivo, and can be utilized as an adjuvant treatment agent for cancer.

Replicative oncolytic adenoviruses in multimodal cancer regimens

The use of replication-competent viruses that have a cytolytic cycle has emerged as a viable strategy (oncolytic virotherapy) to specifically kill tumor cells and the field has advanced to the point of clinical trials. A theoretical advantage of replicative oncolytic viruses is that their numbers should increase via viral replication within infected tumor cells and resulting viral progeny can then infect additional cells within the tumor mass. The life cycle of a virus involves multiple interactions between viral and cellular proteins/genes, which maximize the ability of the virus to infect and replicate within cells. Understanding such interactions has led to the design of numerous genetically engineered adenovirus (Ad) vectors that selectively kill tumor cells while sparing normal cells. These viruses have also been modified to function as therapeutic gene delivery vehicles, thus augmenting their anticancer capacity. In addition, the oncolytic mode of tumor killing differs from that of standard anticancer therapies, providing the possibility for synergistic interactions with other therapies in a multimodal antitumor approach. In this review, we describe the oncolytic Ad vectors tested in preclinical and clinical models and their use in combination with chemo-, radio-, and gene therapies.

Transcriptional Targeting in Cancer Gene Therapy

Cancer gene therapy has been one of the most exciting areas of therapeutic research in the past decade. In this review, we discuss strategies to restrict transcription of transgenes to tumour cells. A range of promoters which are tissue-specific, tumour-specific, or inducible by exogenous agents are presented. Transcriptional targeting should prevent normal tissue toxicities associated with other cancer treatments, such as radiation and chemotherapy. In addition, the specificity of these strategies should provide improved targeting of metastatic tumours following systemic gene delivery. Rapid progress in the ability to specifically control transgenes will allow systemic gene delivery for cancer therapy to become a real possibility in the near future.

Heat-directed suicide gene therapy for breast cancer

Adjuvant hyperthermia can improve treatment outcome for locally recurrent breast cancer (LRBC). Previously, we demonstrated that infection of human breast cancer cells with a recombinant adenovirus expressing beta-galactosidase from the human hsp70b gene promoter (Ad.70b.betagal) results in 50- to 800-fold increases in reporter gene expression following heat treatment (30 minutes at 43 degrees C). Here, we describe a heat-directed suicide gene therapy strategy based on an adenoviral vector (Ad.70b.CDTK) in which expression of the dual prodrug-activating E. coli cytosine deaminase/herpes simplex virus thymidine kinase (CDTK) fusion gene is under the control of the hsp70b promoter. Treatment of T47D and MCF-7 breast cancer cells with mild hyperthermia (43 degrees C/30 minutes) and prodrugs (100 microg/ml 5-fluorocytosine and 10 microg/ml ganciclovir) following infection with Ad.70b.CDTK (10-100 PFU/cell) resulted in 30- to 60-fold decreases in clonogenic survival relative to control cultures treated with heat or prodrugs alone. Clonogenic survival declined even further (up to 240-fold) following heat treatment at 41.5 degrees C for 120 minutes. A decreased clonogenic survival was accompanied by tumor cell apoptosis. These results demonstrate that this combined treatment strategy can be highly effective against heat- and radiation-resistant breast tumor cells and supports the continued development of heat-directed CDTK suicide gene therapy strategies for LRBC.

Prospects for CD40-directed experimental therapy of human cancer

CD40, a member of the tumor necrosis factor receptor (TNF-R) family, is a surface receptor best known for its capacity to initiate multifaceted activation signals in normal B cells and dendritic cells (DCs). CD40-related treatment approaches have been considered for the experimental therapy of human leukemias, lymphomas, and multiple myeloma, based on findings that CD40 binding by its natural ligand (CD40L), CD154, led to growth modulation of malignant B cells. Recent studies also exploited the selective expression of the CD40 receptor on human epithelial and mesenchymal tumors but not on most normal, nonproliferating epithelial tissues. Ligation of CD40 on human breast, ovarian, cervical, bladder, non small cell lung, and squamous epithelial carcinoma cells was found to produce a direct growth-inhibitory effect through cell cycle blockage and/or apoptotic induction with no overt side effects on their normal counterparts. CD154 treatment also heightened tumor rejection immune responses through DC activation, and by increasing tumor immunogenicity through up-regulation of costimulatory molecule expression and cytokine production of epithelial cancer cells. These immunopotentiating features can produce a "bystander effect" through which the CD40-negative tumor subset is eliminated by activated tumor-reactive cytotoxic T cells. However, the potential risk of systemic inflammation and autoimmune consequences remains a concern for systemic CD154-based experimental therapy. The promise of CD154 as a tumor therapeutic agent to directly modulate tumor cell growth, and indirectly activate antitumor immune response, may depend on selective and/or restricted CD154 expression within the tumor microenvironment. This may be achieved by inoculating cancer vaccines of autologous cancer cells that have been transduced ex vivo with CD154, as documented by recently clinical trials. This review summarizes recent findings on CD154 recombinant protein- and gene therapy-based tumor treatment approaches, and examines our understanding of the multifaceted molecular mechanisms of CD154-CD40 interactions.

Oncolytic adenoviruses for treatment of brain tumours

Standard therapies are not capable of curing patients with malignant glioma; more than 90% of patients die within 2 years after diagnosis. Gene therapy appeared as a promising new approach for this disease. However, results of clinical trials with replication deficient viral vectors were disappointing. The main reasons being poor transduction efficiency of adenovirus towards glioma cells and limited spread and distribution of the vector in the tumour. With the increasing knowledge of viral genetics and its functions, an attractive alternative tool to kill malignant glioma cells has been developed: Replicating adenovirus as an oncolytic agent. This type of therapy, also referred to as virotherapy, has the potential to overcome some of the limitations connected with replication deficient adenoviral vectors. In this review the authors describe the latest developments in strategies that are being used to create a tumour- or glioma-selective replicating adenovirus. Special attention is given to the methods of viral delivery to an infiltrating tumour in the brain, regarding optimal dose and toxicity. Furthermore, the role of conventional antitumour treatments, such as irradiation and chemotherapy, in enhancing the effect of virotherapy is being emphasised.

Adenoviruses in oncology: a viable option?

The feasibility of using adenoviruses for gene therapy has been under close scrutiny recently, as it has become clear that significant toxicity can result from the strong immune response created by intravenous administration of large doses of first generation adenovirus vectors. This suggests that other vectors could be more useful for treatment of metabolic and hereditary disease, where widespread transduction is often necessary for effective gene replacement, and the viability of target cells is important. However, promising recent results in human cancer trials have confirmed that adenoviruses can be very useful in oncology. For cancer treatment, the unparalleled transduction efficacy of adenovirus in dividing and dormant cells is a major benefit. As the goal in cancer gene therapy is to kill infected tumour cells, long-term transgene expression is not necessary. In addition, the immune response generated against infected cells could be useful for eradicating uninfected tumour. Importantly, more than 670 cancer patients have been treated with adenovirus intratumorally, intra-arterially, intraperitoneally and intravenously with very manageable adverse effects and no unexpected severe or lethal toxicity. Currently, the most promising approaches are based on replication-competent agents that allow efficient tumour penetration because of their capacity for tissue-specific replication. In addition to transcriptional control, it is becoming clear that targeting is necessary for efficient tumour transduction and less infection of normal tissues. Exciting results are anticipated when the first selectively replicating targeted adenoviruses go to clinical trials. In conclusion, intense gene therapy and virological research have suggested that while other vectors could be more useful for treatment of hereditary disease, adenoviruses are highly promising and safe agents for oncology, as suggested in a number of early phase clinical trials.

New oncolytic adenoviruses with hypoxia- and estrogen receptor-regulated replication

Oncolytic adenoviruses with restricted replication can be produced if the expression of crucial transcription units of the virus is controlled by tissue- or tumor-specific promoters. Here we describe a method for the rapid incorporation of exogenous promoters into the E1A and E4 regions of the human adenovirus type 5 genome. Using this system, we have generated AdEHT2 and AdEHE2F, two conditionally replicative adenoviruses for the treatment of breast cancer. The expression of the E1A gene in both viruses is controlled by a minimal dual-specificity promoter that responds to estrogens and hypoxia. The tight regulation of E1A expression correlated with the ability of these viruses to replicate and kill human cancer cells that express estrogen receptors, or are maintained under hypoxic conditions. The telomerase reverse transcriptase (TERT) promoter and the E2F-1 promoter are preferentially activated in cancer cells. They were introduced into the E4 region of AdEHT2 and AdEHE2F, respectively. The telomerase core promoter failed to block the replication of the virus in telomerase-negative cells. In contrast, AdEHE2F was attenuated in nontransformed quiescent cells growing under normoxic conditions, suggesting that an intact pRB pathway with low levels of E2F transcription factors acts as a negative modulator for the virus. These data indicate that the simultaneous regulation of E1A and E4 viral transcription units by the appropriate combination of promoters can increase the tumor selectivity of oncolytic adenoviruses.

Transcriptional targeting of conditionally replicating adenovirus to dividing endothelial cells

Conditionally replicating adenoviruses (CRADs) are a novel strategy in cancer treatment and clinical trials using CRADs targeted to tumor cells have been reported recently. We hypothesized that it would be possible to construct CRADs targeted to dividing endothelial cells, which are present in the tumor endothelium. We utilized the regulatory elements of Flk-1 and endoglin genes, which have been shown to be highly overexpressed in angiogenic endothelial cells, to construct two CRADs: Ad.Flk-1, which has adenoviral E1A gene under the control of the Flk-1 enhancer/promoter, and Ad.Flk-Endo, which harbors the same Flk-1 enhancer/promoter as Ad.Flk-1, plus it has the adenoviral E1B gene under control of the endoglin promoter. Viral titer measurements by plaque assay showed that in human umbilical vein endothelial cells (HUVECs), both CRADs replicated at levels comparable to that of wild-type adenovirus. In Flk-1 and endoglin negative Hep3B and A549 cells, however, the replication of Ad.Flk-1 and Ad.Flk-Endo was reduced by 30-fold and 600-fold, respectively. Cytotoxicity assays demonstrated that both CRADs killed HUVECs as effectively as wild-type adenovirus and their cytotoxicity in Hep3B and A549 cells was comparable to nonreplicating control adenovirus. Furthermore, there was a striking inhibition (83-91%) of capillary network formation in an in vitro angiogenesis assay when HUVECs were infected with Ad.Flk-1 or Ad.Flk-Endo as compared with the nonreplicating control virus. These results demonstrate that CRADs can be transcriptionally targeted to dividing endothelial cells with high specificity, and that the combined use of Flk-1 and endoglin regulatory elements has a synergistic effect on targeting specificity. This principle may be incorporated into novel therapeutic agents to develop anti-angiogenic treatment for cancer.

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.

A rapid and easy method for production and selection of recombinant adenovirus genomes

Adenoviruses are used widely as vectors for gene therapy. Due to the large size of their genome there is a low frequency of unique restriction sites and many techniques have been described to construct recombinant viruses. Whatever the considered technique, the Escherichia coli strain BJ5183 is used to obtain recombinant adenovirus genomes in a plasmid, or to construct defective viral backbones which will be used to produce infectious viral particles by homologous recombination in HEK293 cells. Unfortunately BJ5183 bacteria do not produce a sufficient amount of plasmid DNA to allow for restriction analysis. Plasmids have to be transferred into another strain to detect the expected construction. It is reported now that the common E. coli strain, Top10F' can be used for the construction of recombinant adenovirus genomes. A plasmid carrying a kanamycin resistance gene and containing the two ends of the adenovirus genome was used. It permits modification by classical molecular biology techniques or homologous recombination at both ends of the genome. The remainder of the genome is introduced by homologous recombination in Top10F'. Several homologous recombination steps were successfully performed without the steps of extraction and introduction of plasmid DNA in another strain to check the plasmids obtained.

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.

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.

Cancer gene therapy: fringe or cutting edge?

Direct targeting of cancer cells with gene therapy has the potential to treat cancer on the basis of its molecular characteristics. But although laboratory results have been extremely encouraging, many practical obstacles need to be overcome before gene therapy can fulfil its goals in the clinic. These issues are not trivial, but seem less formidable than the challenge of killing cancers selectively and rationally--a challenge that has been successfully addressed.

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.

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.