Late expression of p53 from a replicating adenovirus improves tumor cell killing and is more tumor cell specific than expression of the adenoviral death protein

Oncolytic adenovirus as pancreatic cancer-targeted therapy: Where do we go from here?

Pancreatic cancer remains one of the deadliest cancers with extremely high mortality rate, and the number of cases is expected to steadily increase with time. Pancreatic cancer is refractory to conventional cancer treatment options, like chemotherapy and radiotherapy, and commercialized immunotherapeutics, owing to its immunosuppressive and desmoplastic phenotype. Due to these reasons, development of an innovative treatment option that can overcome these challenges posed by the pancreatic tumor microenvironment (TME) is in an urgent need. The present review aims to summarize the evolution of oncolytic adenovirus (oAd) engineering and usage as therapeutics (either monotherapy or combination therapy) over the last decade to overcome these hurdles to instigate a potent antitumor effect against desmoplastic and immunosuppressive pancreatic cancer.

Evaluation of a Novel Oncolytic Adenovirus Silencing SYVN1

Oncolytic adenoviruses are promising new anticancer agents. To realize their full anticancer potential, they are being engineered to express therapeutic payloads. Tumor suppressor p53 function contributes to oncolytic adenovirus activity. Many cancer cells carry an intact TP53 gene but express p53 inhibitors that compromise p53 function. Therefore, we hypothesized that oncolytic adenoviruses could be made more effective by suppressing p53 inhibitors in selected cancer cells. To investigate this concept, we attenuated the expression of the established p53 inhibitor synoviolin (SYVN1) in A549 lung cancer cells by RNA interference. Silencing SYVN1 inhibited p53 degradation, thereby increasing p53 activity, and promoted adenovirus-induced A549 cell death. Based on these observations, we constructed a new oncolytic adenovirus that expresses a short hairpin RNA against SYVN1. This virus killed A549 cells more effectively in vitro and inhibited A549 xenograft tumor growth in vivo. Surprisingly, increased susceptibility to adenovirus-mediated cell killing by SYVN1 silencing was also observed in A549 TP53 knockout cells. Hence, while the mechanism of SYVN1-mediated inhibition of adenovirus replication is not fully understood, our results clearly show that RNA interference technology can be exploited to design more potent oncolytic adenoviruses.

Concepts in Oncolytic Adenovirus Therapy

Oncolytic adenovirus therapy is gaining importance as a novel treatment option for the management of various cancers. Different concepts of modification within the adenovirus vector have been identified that define the mode of action against and the interaction with the tumour. Adenoviral vectors allow for genetic manipulations that restrict tumour specificity and also the expression of specific transgenes in order to support the anti-tumour effect. Additionally, replication of the virus and reinfection of neighbouring tumour cells amplify the therapeutic effect. Another important aspect in oncolytic adenovirus therapy is the virus induced cell death which is a process that activates the immune system against the tumour. This review describes which elements in adenovirus vectors have been identified for modification not only to utilize oncolytic adenovirus vectors into conditionally replicating adenoviruses (CRAds) that allow replication specifically in tumour cells but also to confer specific characteristics to these viruses. These advances in development resulted in clinical trials that are summarized based on the conceptual design.

Effect of Transgene Location, Transcriptional Control Elements and Transgene Features in Armed Oncolytic Adenoviruses

Clinical results with oncolytic adenoviruses (OAds) used as antitumor monotherapies show limited efficacy. To increase OAd potency, transgenes have been inserted into their genome, a strategy known as "arming OAds". Here, we review different parameters that affect the outcome of armed OAds. Recombinant adenovirus used in gene therapy and vaccination have been the basis for the design of armed OAds. Hence, early region 1 (E1) and early region 3 (E3) have been the most commonly used transgene insertion sites, along with partially or complete E3 deletions. Besides transgene location and orientation, transcriptional control elements, transgene function, either virocentric or immunocentric, and even the codons encoding it, greatly impact on transgene levels and virus fitness.

Unleashing the Full Potential of Oncolytic Adenoviruses against Cancer by Applying RNA Interference: The Force Awakens

Oncolytic virus therapy of cancer is an actively pursued field of research. Viruses that were once considered as pathogens threatening the wellbeing of humans and animals alike are with every passing decade more prominently regarded as vehicles for genetic and oncolytic therapies. Oncolytic viruses kill cancer cells, sparing healthy tissues, and provoke an anticancer immune response. Among these viruses, recombinant adenoviruses are particularly attractive agents for oncolytic immunotherapy of cancer. Different approaches are currently examined to maximize their therapeutic effect. Here, knowledge of virus–host interactions may lead the way. In this regard, viral and host microRNAs are of particular interest. In addition, cellular factors inhibiting viral replication or dampening immune responses are being discovered. Therefore, applying RNA interference is an attractive approach to strengthen the anticancer efficacy of oncolytic viruses gaining attention in recent years. RNA interference can be used to fortify the virus’ cancer cell-killing and immune-stimulating properties and to suppress cellular pathways to cripple the tumor. In this review, we discuss different ways of how RNA interference may be utilized to increase the efficacy of oncolytic adenoviruses, to reveal their full potential.

Combining Oncolytic Virotherapy with p53 Tumor Suppressor Gene Therapy

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

Functional Screening Identifies Human miRNAs that Modulate Adenovirus Propagation in Prostate Cancer Cells

Oncolytic adenoviruses represent a novel class of anticancer agents. Their efficacy in killing cancer cells is variable, suggesting that there is room for improvement. Host miRNAs have been shown to play important roles in susceptibility of cells to replication of different viruses. This study investigated if adenovirus replication in human prostate cancer cells is influenced by host cell miRNA expression. To this end, human miRNA expression in response to adenovirus infection was analyzed, and functional screens for lytic adenovirus replication were performed using synthetic miRNA mimic and inhibitor libraries. Adenovirus infection generally reduced miRNA expression. On top of this nonspecific interference with miRNA biogenesis, a set of miRNAs, including in particular miR-222, was found specifically reduced. Another set of miRNAs was found to promote adenovirus-induced death of prostate cancer cells. In most cases, this did not stimulate adenovirus propagation. The exception was miR-26b. Overexpression of miR-26b inhibited adenovirus-induced NF-κB activation, augmented adenovirus-mediated cell death, increased adenovirus progeny release, and promoted adenovirus propagation and spread in several human prostate cancer cell lines. This suggests that miR-26b is particularly useful to be combined with oncolytic adenovirus for more effective treatment of prostate cancer.

Adeno-associated virus enhances wild-type and oncolytic adenovirus spread

The contamination of adenovirus (Ad) stocks with adeno-associated viruses (AAV) is usually unnoticed, and it has been associated with lower Ad yields upon large-scale production. During Ad propagation, AAV contamination needs to be detected routinely by polymerase chain reaction without symptomatic suspicion. In this study, we describe that the coinfection of either Ad wild type 5 or oncolytic Ad with AAV results in a large-plaque phenotype associated with an accelerated release of Ad from coinfected cells. This accelerated release was accompanied with the expected decrease in Ad yields in two out of three cell lines tested. Despite this lower Ad yield, coinfection with AAV accelerated cell death and enhanced the cytotoxicity mediated by Ad propagation. Intratumoral coinjection of Ad and AAV in two xenograft tumor models improved antitumor activity and mouse survival. Therefore, we conclude that accidental or intentional AAV coinfection has important implications for Ad-mediated virotherapy.

Adenoviral protein V promotes a process of viral assembly through nucleophosmin 1

Adenoviral infection induces nucleoplasmic redistribution of a nucleolar nucleophosmin 1/NPM1/B23.1. NPM1 is preferentially localized in the nucleoli of normal cells, whereas it is also present at the nuclear matrix in cancer cells. However, the biological roles of NPM1 during infection are unknown. Here, by analyzing a pV-deletion mutant, Ad5-dV/TSB, we demonstrate that pV promotes the NPM1 translocation from the nucleoli to the nucleoplasm in normal cells, and the NPM1 translocation is correlated with adenoviral replication. Lack of pV causes a dramatic reduction of adenoviral replication in normal cells, but not cancer cells, and Ad5-dV/TSB was defective in viral assembly in normal cells. NPM1 knockdown inhibits adenoviral replication, suggesting an involvement of NPM1 in adenoviral biology. Further, we show that NPM1 interacts with empty adenovirus particles which are an intermediate during virion maturation by immunoelectron microscopy. Collectively, these data implicate that pV participates in a process of viral assembly through NPM1.

Selectivity and efficiency of late transgene expression by transcriptionally targeted oncolytic adenoviruses are dependent on the transgene insertion strategy

Key challenges facing cancer therapy are the development of tumor-specific drugs and potent multimodal regimens. Oncolytic adenoviruses possess the potential to realize both aims by restricting virus replication to tumors and inserting therapeutic genes into the virus genome, respectively. A major effort in this regard is to express transgenes in a tumor-specific manner without affecting virus replication. Using both luciferase as a sensitive reporter and genetic prodrug activation, we show that promoter control of E1A facilitates highly selective expression of transgenes inserted into the late transcription unit. This, however, required multistep optimization of late transgene expression. Transgene insertion via internal ribosome entry site (IRES), splice acceptor (SA), or viral 2A sequences resulted in replication-dependent expression. Unexpectedly, analyses in appropriate substrates and with matching control viruses revealed that IRES and SA, but not 2A, facilitated indirect transgene targeting via tyrosinase promoter control of E1A. Transgene expression via SA was more selective (up to 1,500-fold) but less effective than via IRES. Notably, we also revealed transgene-dependent interference with splicing. Hence, the prodrug convertase FCU1 (a cytosine deaminase-uracil phosphoribosyltransferase fusion protein) was expressed only after optimizing the sequence surrounding the SA site and mutating a cryptic splice site within the transgene. The resulting tyrosinase promoter-regulated and FCU1-encoding adenovirus combined effective oncolysis with targeted prodrug activation therapy of melanoma. Thus, prodrug activation showed potent bystander killing and increased cytotoxicity of the virus up to 10-fold. We conclude that armed oncolytic viruses can be improved substantially by comparing and optimizing strategies for targeted transgene expression, thereby implementing selective and multimodal cancer therapies.

Directed adenovirus evolution using engineered mutator viral polymerases

Adenoviruses (Ads) are the most frequently used viruses for oncolytic and gene therapy purposes. Most Ad-based vectors have been generated through rational design. Although this led to significant vector improvements, it is often hampered by an insufficient understanding of Ad’s intricate functions and interactions. Here, to evade this issue, we adopted a novel, mutator Ad polymerase-based, ‘accelerated-evolution’ approach that can serve as general method to generate or optimize adenoviral vectors. First, we site specifically substituted Ad polymerase residues located in either the nucleotide binding pocket or the exonuclease domain. This yielded several polymerase mutants that, while fully supportive of viral replication, increased Ad’s intrinsic mutation rate. Mutator activities of these mutants were revealed by performing deep sequencing on pools of replicated viruses. The strongest identified mutators carried replacements of residues implicated in ssDNA binding at the exonuclease active site. Next, we exploited these mutators to generate the genetic diversity required for directed Ad evolution. Using this new forward genetics approach, we isolated viral mutants with improved cytolytic activity. These mutants revealed a common mutation in a splice acceptor site preceding the gene for the adenovirus death protein (ADP). Accordingly, the isolated viruses showed high and untimely expression of ADP, correlating with a severe deregulation of E3 transcript splicing.

Oncolytic-adenovirus-expressed RNA interference for cancer therapy

IMPORTANCE OF THE FIELD

RNA interference (RNAi) has generated considerable excitement for its potential cancer therapeutic applications. Because of the difficulties in delivering a large amount of siRNA to cancer cells and the short half-life of siRNA, it is important to choose an efficient delivery system for transduction of siRNA into target cells. Oncolytic adenovirus offers a better platform by virtue of its high transfection efficiency and selective replication in cancer cells.

AREAS COVERED IN THIS REVIEW

This review focuses on the synergism between oncolytic adenovirus and siRNA antitumor responses, and discusses recent progresses in oncolytic-adenovirus-expressed siRNA.

WHAT THE READER WILL GAIN

siRNA-expressing oncolytic adenovirus can generate a significantly enhanced antitumor effect through gene knockdown and viral oncolysis.

TAKE HOME MESSAGE

Due to its potency and target specificity, using siRNA delivery by oncolytic adenovirus has generated much excitement and will open new avenues for treatment of human cancer.

Comparison of oncolytic adenoviruses for selective eradication of oral cancer and pre-cancerous lesions

Oncolytic adenoviruses are being investigated as potential anti-cancer agents. Selective lytic replication in cancer cells is essential for an effective and safe treatment. In this study, we compared 11 oncolytic adenoviruses in relevant cell cultures to assess their use for treating oral cancer and pre-cancerous lesions. We determined the cytotoxicity of oncolytic adenovirus infection and calculated selectivity indices for cytotoxicity to cancer cells compared with normal oral keratinocytes and fibroblasts. Keratinocytes were very sensitive to wild-type adenovirus serotype 5 (Ad5); 1- to 3-log more than head and neck squamous cell carcinoma (HNSCC) cells. The potencies of oncolytic adenoviruses to kill HNSCC cells within 7 days after infection ranged from approximately 10 times less potent to approximately 10 times more potent than Ad5. The selectivity indices determined on fibroblasts and keratinocytes differed markedly. Two oncolytic adenoviruses were more selective than Ad5 for HNSCC cells compared with fibroblasts; and five viruses showed selective replication on HNSCC cells compared with keratinocytes. Overall, CRAd-S.RGD with E1A driven by the survivin promoter and an infectivity-enhancing capsid modification showed the most favourable cytotoxicity pattern; being very potent in killing HNSCC cells, only slightly less effective than Ad5 in killing pre-neoplastic keratinocytes and the least toxic to normal keratinocytes.

Transcriptionally regulated, prostate-targeted gene therapy for prostate cancer

Prostate cancer is the most frequently diagnosed cancer and the second leading cause of cancer deaths in American males today. Novel and effective treatment such as gene therapy is greatly desired. The early viral based gene therapy uses tissue-nonspecific promoters, which causes unintended toxicity to other normal tissues. In this chapter, we will review the transcriptionally regulated gene therapy strategy for prostate cancer treatment. We will describe the development of transcriptionally regulated prostate cancer gene therapy in the following areas: (1) Comparison of different routes for best viral delivery to the prostate; (2) Study of transcriptionally regulated, prostate-targeted viral vectors: specificity and activity of the transgene under several different prostate-specific promoters were compared in vitro and in vivo; (3) Selection of therapeutic transgenes and strategies for prostate cancer gene therapy (4) Oncolytic virotherapy for prostate cancer. In addition, the current challenges and future directions in this field are also discussed.

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.

Armed replicating adenoviruses for cancer virotherapy

Conditionally replicating adenoviruses (CRAds) have many advantages as agents for cancer virotherapy and have been safely used in human clinical trials. However, replicating adenoviruses have been limited in their ability to eliminate tumors by oncolysis. Thus, the efficacy of these agents must be improved. To this end, CRAds have been engineered to express therapeutic transgenes that exert antitumor effects independent of direct viral oncolysis. These transgenes can be expressed under native gene control elements, in which case placement within the genome determines the expression profile, or they can be controlled by exogenous promoters. The therapeutic transgenes used to arm replicating adenoviruses can be broadly classified into three groups. There are those that mediate killing of the infected cell, those that modulate the tumor microenvironment and those with immunomodulatory functions. Overall, the studies to date in animal models have shown that arming a CRAd with a rationally chosen therapeutic transgene can improve its antitumor efficacy over that of an unarmed CRAd. However, a number of obstacles must be overcome before the full potential of armed CRAds can be realized in the human clinical context. Hence, strategies are being developed to permit intravenous delivery to disseminated cancer cells, overcome the immune response and enable in vivo monitoring of the biodistribution and activity of armed CRAds.

Directed evolution generates a novel oncolytic virus for the treatment of colon cancer

Background

Viral-mediated oncolysis is a novel cancer therapeutic approach with the potential to be more effective and less toxic than current therapies due to the agents selective growth and amplification in tumor cells. To date, these agents have been highly safe in patients but have generally fallen short of their expected therapeutic value as monotherapies. Consequently, new approaches to generating highly potent oncolytic viruses are needed. To address this need, we developed a new method that we term “Directed Evolution” for creating highly potent oncolytic viruses.

Methodology/Principal Findings

Taking the “Directed Evolution” approach, viral diversity was increased by pooling an array of serotypes, then passaging the pools under conditions that invite recombination between serotypes. These highly diverse viral pools were then placed under stringent directed selection to generate and identify highly potent agents. ColoAd1, a complex Ad3/Ad11p chimeric virus, was the initial oncolytic virus derived by this novel methodology. ColoAd1, the first described non-Ad5-based oncolytic Ad, is 2–3 logs more potent and selective than the parent serotypes or the most clinically advanced oncolytic Ad, ONYX-015, in vitro. ColoAd1's efficacy was further tested in vivo in a colon cancer liver metastasis xenograft model following intravenous injection and its ex vivo selectivity was demonstrated on surgically-derived human colorectal tumor tissues. Lastly, we demonstrated the ability to arm ColoAd1 with an exogenous gene establishing the potential to impact the treatment of cancer on multiple levels from a single agent.

Conclusions/Significance

Using the “Directed Evolution” methodology, we have generated ColoAd1, a novel chimeric oncolytic virus. In vitro, this virus demonstrated a >2 log increase in both potency and selectivity when compared to ONYX-015 on colon cancer cells. These results were further supported by in vivo and ex vivo studies. Furthermore, these results have validated this methodology as a new general approach for deriving clinically-relevant, highly potent anti-cancer virotherapies.

Syncytia formation affects the yield and cytotoxicity of an adenovirus expressing a fusogenic glycoprotein at a late stage of replication

Fusogenic membrane glycoproteins (FMGs) may enhance the cytotoxicity of conditionally replicative adenoviruses. However, expression at early stages of infection impairs virus replication. We have inserted the hyperfusogenic form of the gibbon ape leukemia virus (GALV) envelope glycoprotein as a new splice unit of the major late promoter (MLP) to generate a replication-competent adenovirus expressing this protein. At high multiplicity of infection (MOI), this virus replicated efficiently forming clumps of fused cells and showing a faster release. In contrast, at low MOI, infected cells formed syncytia where only one nucleus contained virus DNA, decreasing total virus production but increasing cytotoxicity.

Cellular genetic tools to control oncolytic adenoviruses for virotherapy of cancer

Key challenges facing cancer therapy are the development of tumor-specific drugs and the implementation of potent multimodal treatment regimens. Oncolytic adenoviruses, featuring cancer-selective viral cell lysis and spread, constitute a particularly interesting drug platform towards both goals. First, as complex biological agents, adenoviruses allow for rational drug development by genetic incorporation of targeting mechanisms that exert their function at different stages of the viral replication cycle. Secondly, therapeutic genes implementing diverse cancer cell-killing activities can be inserted into the oncolytic adenovirus genome without loss of replication potential, thus deriving a "one-agent combination therapy". This article reviews an intriguing approach to derive oncolytic adenoviruses, which is to insert cellular genetic regulatory elements into adenovirus genomes for control of virus replication and therapeutic gene expression. This approach has been thoroughly investigated and optimized during the last decade for transcriptional targeting of adenovirus replication and gene expression to a wide panel of tumor types. More recently, further cellular regulatory mechanisms, such as mRNA stability and translation regulation, have been reported as tools for virus control. Consequently, oncolytic adenoviruses with a remarkable specificity profile for prostate cancer, gastrointestinal cancers, liver cancer, breast cancer, lung cancer, melanoma, and other cancers were derived. Such specificity profiles allow for the engineering of new generations of oncolytic adenoviruses with improved potency by enhancing viral cell binding and entry or by expressing therapeutic genes. Clearly, genetic engineering of viruses has great potential for the development of innovative antitumor drugs--towards targeted and multimodal cancer therapy.

Enhanced tumor cell kill by combined treatment with a small-molecule antagonist of mouse double minute 2 and adenoviruses encoding p53

Strategies to treat cancer by restoring p53 tumor suppressor functions are being actively investigated. These approaches range from expressing an exogenous p53 gene in p53 mutant cancers to antagonizing a p53 inhibitor in p53 wild-type (WT) cancer cells. In addition, exogenous p53 is used to strengthen the anticancer efficacy of oncolytic adenoviruses. Many cancers express high levels of the major negative regulator of p53, mouse double minute 2 (MDM2) protein. Recently, a novel class of highly potent and specific MDM2 antagonists, the Nutlins, was identified. We envisioned that Nutlins could protect both endogenous and exogenous p53 from MDM2-mediated inactivation. We therefore investigated treating human cancer cells with a combination of adenovirus-mediated p53 gene therapy and Nutlin. Combination treatment resulted in broadly effective cell kill of p53 WT and p53-negative cancer cells. Cytotoxicity was associated with profound cell cycle checkpoint activation and apoptosis induction. We also tested Nutlin in combination with oncolytic adenoviruses. Nutlin treatment accelerated viral progeny burst from oncolytic adenovirus-infected cancer cells and caused an estimated 10- to 1,000-fold augmented eradication of p53 WT cancer cells. These findings suggest that Nutlins are promising compounds to be combined with p53 gene therapy and oncolytic virotherapy for cancer.

Cancer selective adenoviruses

Ten years ago Frank McCormick proposed dl1520 as an oncolytic adenovirus. Although great as an inspiration for better oncolytic viruses it was far from a good product. As Onyx-015, it underwent a wish-fulfilling clinical development program seizing the opportunity left by its p53-targeted non-replicative counterpart Ad-p53. Now, facing a skeptical environment, more selective and potent oncolytic adenoviruses await their clinical opportunity. However, advance in key issues remains elusive, such as, selectivity or retargeting at the level of cell receptors to improve pharmacokinetics. Preclinical models and a few clinical data on biodistribution show that only a minimal proportion of the injected dose reaches the tumors after systemic administration. Once in the tumor, the virus must overcome barriers to efficient spread imposed by stroma and immune responses. Arming the oncolytic virus with transgenes is a natural combination of virotherapy and gene therapy strategies. Transgenes that increase virus production or cellular spread may help to overcome these barriers. Cytotoxic transgenes can help to eliminate tumor cells but need to be compatible with efficient virus replication. These challenges require a careful approach to clinical development and a great deal of collaboration to launch clinical tests with a virus backbone that contains intellectual property from multiple sources.

Melanoma cultures show different susceptibility towards E1A-, E1B-19 kDa- and fiber-modified replication-competent adenoviruses

Replicating adenovirus (Ad) vectors with tumour tissue specificity hold great promise for treatment of cancer. We have recently constructed a conditionally replicating Ad5 AdDeltaEP-TETP inducing tumour regression in a xenograft mouse model. For further improvement of this vector, we introduced four genetic modifications and analysed the viral cytotoxicity in a large panel of melanoma cell lines and patient-derived melanoma cells. (1) The antiapoptotic gene E1B-19 kDa (Delta19 mutant) was deleted increasing the cytolytic activity in 18 of 21 melanoma cells. (2) Introduction of the E1A 122-129 deletion (Delta24 mutant), suggested to attenuate viral replication in cell cycle-arrested cells, did not abrogate this activity and increased the cytolytic activity in two of 21 melanoma cells. (3) We inserted an RGD sequence into the fiber to extend viral tropism to alphav integrin-expressing cells, and (4) swapped the fiber with the Ad35 fiber (F35) enhancing the tropism to malignant melanoma cells expressing CD46. The RGD-fiber modification strongly increased cytolysis in all of the 11 CAR-low melanoma cells. The F35 fiber-chimeric vector boosted the cytotoxicity in nine of 11 cells. Our results show that rational engineering additively enhances the cytolytic potential of Ad vectors, a prerequisite for the development of patient-customized viral therapies.

Adenovirus-mediated p53 tumor suppressor gene therapy of osteosarcoma

The clinical outcome for osteosarcoma (OS) remains discouraging despite efforts to optimize treatment using conventional modalities including surgery, radiotherapy and chemotherapy. Novel therapeutic approaches based on our expanding understanding of the mechanisms of tumor cell killing have the potential to alter this situation. Tumor suppressor gene therapy aims to restore the function of a tumor suppressor gene lost or functionally inactivated in cancer cells. One such molecule, the p53 tumor suppressor gene plays a critical role in safeguarding the integrity of the genome and preventing tumorigenesis. Introduction of wild-type (wt) p53 into transformed cells has been shown to be lethal for most cancer cells in vitro, but clinical trials of p53 gene replacement have had limited success. Analysis of these clinical trials highlighted the insufficient efficacy of current vectors and low proapoptotic activity of wt p53 as a single agent in vivo. In this review, a contemporary summarization of the current status of adenovirus-mediated p53 gene therapy of OS is presented. Advancement in our understanding of p53 tumor suppressor activity, the molecular biology of chemoresistant OS, and recent advances in tumor targeting with adenoviral vectors are also addressed. Based on these parameters, prospects for future investigations are proposed.

Development of a transposon-based approach for identifying novel transgene insertion sites within the replicating adenovirus

Therapeutic gene delivery from an oncolytic adenovirus (Ad) is one approach to enhancing the potency of Ad-based virotherapies for cancer. To identify therapeutic transgene insertion sites compatible with the replicating virus, a methodology that broadly scans the viral genome is needed. To address this we modified a transposon (Tn7)-based in vitro transposition system to take advantage of its nonprejudiced scanning ability to identify insertion sites compatible with viral replication. Using this system with a plasmid containing an E3-deleted Ad5, we identified several unique sites for promoter-based expression cassette insertions within the Ad genome. The transposon-based expression cassette is bounded by PmeI restriction endonuclease sites unique to the transposon, making expression cassette substitutions easy to perform. Additional expression cassettes containing different promoters and reporter genes were substituted into two of the newly identified transgene insertion sites. The results suggest that the ease and orientation of expression cassette substitution depend on both the insertion site location and the promoter and gene of the replacement expression cassette. These studies establish the transposon-based system as an efficient approach to scanning the Ad genome and identifying insertion sites compatible with viral replication and represents a powerful tool for the development of armed therapeutic viruses for cancer.

p53 promotes adenoviral replication and increases late viral gene expression

The tumor suppressor protein, p53, plays a critical role in viro-oncology. However, the role of p53 in adenoviral replication is still poorly understood. In this paper, we have explored further the effect of p53 on adenoviral replicative lysis. Using well-characterized cells expressing a functional p53 (A549, K1neo, RKO) and isogenic derivatives that do not (K1scx, RKOp53.13), we show that virus replication, late virus protein expression and both wtAd5 and ONYX-015 virus-induced cell death are impaired in cells deficient in functional p53. Conversely, by transfecting p53 into these and other cells (IIICF/c, HeLa), we increase late virus protein expression and virus yield. We also show, using reporter assays in IIICF/c, HeLa and K1scx cells, that p53 can cooperate with E1a to enhance transcription from the major late promoter of the virus. Late viral protein production is enhanced by exogenous p53. Taken together, our data suggest that functional p53 can promote the adenovirus (Ad) lytic cycle. These results have implications for the use of Ad mutants that are defective in p53 degradation, such as ONYX-015, as agents for the treatment of cancers.

Late expression of nitroreductase in an oncolytic adenovirus sensitizes colon cancer cells to the prodrug CB1954

We have constructed an oncolytic adenovirus expressing the Escherichia coli nitroreductase gene nfsB from an internal ribosome entry site (IRES) in the adenovirus L5 major late transcript. The virus (Tcf-NTR) has Tcf transcription factor-binding sites in the E1A, E1B, and E4 promoters, which restrict viral replication to cells that have activation of the Wnt signaling pathway. This virus was compared with an E1B-55K-deleted virus expressing nitroreductase (NTR) from a cytomegalovirus (CMV) promoter in the E1B-55K region [CRAd-NTR(PS1217H6)]. Both viruses express NTR in colorectal cancer cell lines and show increased cytopathic effect in the presence of the prodrug CB1954. Unlike the Tcf-NTR virus, the CMV-NTR virus expresses NTR in human lung fibroblasts and sensitizes these normal cells to CB1954. The in vivo activity of the viruses was tested in SW620 xenografts in nude mice by intravenous injection of 1,011 particles of virus followed 1 week later by intraperitoneal injections of CB1954. The CMV-NTR virus produced minimal effects in this model. The median time to form 1,000-mm(3) tumors in mice treated with the Tcf-NTR virus plus CB1954 was increased from 14 to 26 days (p=0.003), but this was due mainly to the direct oncolytic effect of the virus. Combination therapy with 3 x 10(11) particles of Tcf-NTR virus (given intravenously) and the mammalian target of rapamycin (mTOR) inhibitor RAD001 (everolimus) (given orally) significantly improved survival (median, >50 days), and addition of CB1954 to this regimen further delayed tumor growth. These results show that the Tcf-NTR virus is more tumor selective and active than the CMV-NTR virus. At the level of transduction that can be achieved currently with oncolytic viruses given intravenously, drugs such as RAD001, which do not require activation by the virus, produce greater increases in efficacy than prodrugs such as CB1954.

Oncolysis and suppression of tumor growth by a GFP-expressing oncolytic adenovirus controlled by an hTERT and CMV hybrid promoter

One of the challenges of oncolytic virotherapy is the inability to easily track or monitor virus activity during treatment. Here we describe the construction and functional characterization of Ad/hTC-GFP-E1, an oncolytic virus whose transgenes GFP and E1A are both under the control of a synthetic promoter (hTC). This promoter consists of sequences from the human telomorase reverse transcriptase promoter and a minimal cytomegalovirus (CMV) early promoter. The tumor-specific expression of E1A and GFP was demonstrated by Western blot and fluorescent microscope analyses, and the tumor-specific cytotoxicity by crystal-violet staining and cell viability assays. Viral replication and tumor cell lysis occurred at multiplicities of infection (MOI) as low as 100 viral particles per cell in sensitive cell lines. No overt cytotoxic effect was observed in normal human fibroblasts, even at MOIs over 2000 vp. The presence of oncolytic vector was easily visualized and quantitated in vitro and in vivo, in correlation with viral replication. Intralesional administration of the virus into subcutaneous H1299 (NSCLC) tumor xenografts significantly suppressed tumor growth and provided a survival benefit. Together, these results demonstrate that an hTERT-specific oncolytic adenovirus expressing an hTERT-specific transgene is applicable for cancer therapy.

Modification of the p53 transgene of a replication-competent adenovirus prevents mdm2- and E1b-55kD-mediated degradation of p53

Clinical efficacy of adenovirus-mediated cancer gene therapy has been limited thus far. To improve its oncolytic effect, a replication-competent adenoviral vector was previously constructed to express high levels of p53 at a late time point in the viral life cycle. p53 expression from this vector improved tumor cell killing and viral spread in vitro. However, p53 function is antagonized by cellular mdm2 and adenoviral E1b-55kD, both of which are known to bind to and inactivate p53. Therefore, a new vector (Adp53W23S) that expresses a modified p53 transgene, which does not bind to E1b-55kd and mdm2, was constructed. The modified p53 protein was demonstrated to have a substantially prolonged half-life, and its localization was predominantly nuclear. Viral replication was unaffected by expression of the modified p53 and cancer cell killing was improved in vitro. However, in a xenograft model, efficacy was not significantly different from control virus. In conclusion, expression of a degradation-resistant p53 transgene late in the life cycle of a replication-competent adenovirus improves p53 stability and cancer cell killing in vitro. However, other factors, such as the adenoviral E1b-19kD and E1a proteins, which oppose p53 function, and limitations to viral spread need to be addressed to further improve in vivo efficacy.

Oncolytic virotherapy for cancer treatment: challenges and solutions

Advances in gene modification and viral therapy have led to the development of a variety of vectors in several viral families that are capable of replication specifically in tumor cells. Because of the nature of viral delivery, infection, and replication, this technology, oncolytic virotherapy, may prove valuable for treating cancer patients, especially those with inoperable tumors. Current limitations exist, however, for oncolytic virotherapy. They include the body's B and T cell responses, innate inflammatory reactions, host range, safety risks involved in using modified viruses as treatments, and the requirement that most currently available oncolytic viruses require local administration. Another important constraint is that genetically enhanced vectors may or may not adhere to their replication restrictions in long-term applications. Several solutions and strategies already exist, however, to minimize or circumvent many of these limitations, supporting viral oncolytic therapy as a viable option and powerful tool in the fight against cancer.

Oncolytic adenoviruses - selective retargeting to tumor cells

Virotherapy is an approach for the treatment of cancer, in which the replicating virus itself is the anticancer agent. Virotherapy exploits the lytic property of virus replication to kill tumor cells. As this approach relies on viral replication, the virus can self-amplify and spread in the tumor from an initial infection of only a few cells. The success of this approach is fundamentally based on the ability to deliver the replication-competent viral genome to target cells with a requisite level of efficiency. With virotherapy, while a number of transcriptional retargeting strategies have been utilized to restrict viral replication to tumor cells, this review will focus primarily on transductional retargeting strategies, whereby oncolytic viruses can be designed to selectively infect tumor cells. Using the adenoviral vector paradigm, there are three broad strategies useful for viral retargeting. One strategy uses heterologous retargeting ligands that are bispecific in that they bind both to the viral vector as well as to a cell surface target. A second strategy uses genetically modified viral vectors in which a cellular retargeting ligand is incorporated. A third strategy involves the construction of chimeric recombinant vectors, in which a capsid protein from one virus is exchanged for that of another. These transductional retargeting strategies have the potential for reducing deleterious side effects, and increasing the therapeutic index of virotherapeutic agents.

Conditionally replicative adenovirus expressing degradation-resistant p53 for enhanced oncolysis of human cancer cells overexpressing murine double minute 2

Conditionally replicative adenoviruses (CRAd) are under investigation as anticancer agents. Previously, we found that the CRAd AdDelta24-p53, expressing the p53 tumor suppressor protein from its genome, more effectively killed most human cancer cells than did its parent AdDelta24. However, a minority of cancer cell lines poorly responded to the oncolysis-enhancing effect of p53. Here we show that refractory cell lines expressed high levels of the major negative p53 regulator murine double minute 2 (MDM2). To obviate MDM2-mediated inactivation of CRAd-encoded p53, we constructed the new CRAd AdDelta24-p53(14/19) encoding a p53 variant incapable of binding to MDM2. AdDelta24-p53(14/19) was approximately 10 times more effective than AdDelta24-p53 in killing cancer cell lines with high levels of human MDM2, but not cells with low MDM2. This finding supports the notion that exogenous expression of functional p53 augments the anticancer efficacy of CRAds. In addition, it confirms that high MDM2 expression is a molecular determinant of resistance against oncolysis enhancement by exogenous wild-type p53. Moreover, it shows that efficacy enhancement by restoration of functional p53 can also be accomplished in cancer cells expressing a p53 inhibitor. This further expands the utility of CRAds expressing functional p53 variants for effective virotherapy of cancer and thus their possible contribution to the advancement of individualized molecular medicine.

Identification of novel insertion sites in the Ad5 genome that utilize the Ad splicing machinery for therapeutic gene expression

Therapeutic transgene expression from oncolytic viruses represents one approach to increasing the effectiveness of these agents as cancer therapeutics. In the case of the oncolytic adenovirus (Ad), however, the genomic packaging capacity is constrained. To address this, we explored whether a transposon-based system could identify sites in the viral genome where endogenous Ad promoters could drive transgene expression via splicing and still maintain the replication capacity of the virus. Using GFP as a reporter gene and an E3-deleted Ad genome as a target, we tested three splicing signals. RACE analysis confirmed that gene expression from the GFP-expressing Ads occurs via splicing and traced expression to the Ad major late promoter (MLP). Replacement of the GFP transposon by an equivalent splice acceptor-luciferase expression cassette in the same orientation confirmed that substitute transgenes are also expressed via splicing from the MLP. Interestingly, insertion of the substitute transgene in the opposite orientation also resulted in expression that, in some cases, originated from within the ITR region of the viral genome. In summary, splice acceptor sequences can be used to control transgene expression from endogenous Ad promoters and this represents a genomically economical approach to arming oncolytic Ads.

Conditionally replicative adenovirus for gastrointestinal cancers

The clinical outcome of advanced gastrointestinal (GI) cancers (especially pancreatic and oesophageal cancers) is dismal, despite the advance of conventional therapeutic strategies. Cancer gene therapy is a category of new therapeutics, among which conditionally replicative adenovirus (CRAd) is one promising strategy to overcome existing obstacles of cancer gene therapy. Various CRAds have been developed for GI cancer treatment by taking advantage of the replication biology of adenovirus. Some CRAds have already been tested in clinical trials, but have fallen short of initial expectations. Concerns for clinical applicability include therapeutic potency, replication selectivity and interval end points in clinical trials. In addition, improvement of experimental animal models is needed for a deeper understanding of CRAd biology. Despite these obstacles, CRAds continue to be an exciting area of investigation with great potential for clinical utility. Further virological and oncological research will eventually lead to full realisation of the therapeutic potential of CRAds in the field of GI cancers.

AdEasy-based cloning system to generate tropism expanded replicating adenoviruses expressing transgenes late in the viral life cycle

Replicating adenoviral vectors (RAds) hold great promise for the treatment of cancer. Significant therapeutic effects of these vectors do not only rely on tumor targeting but also on efficient release of viral progeny from host cells. Cytotoxic genes expressed late in the adenoviral life cycle can significantly enhance viral release and spreading. Therefore, an adenoviral cloning system that allows easy integration of established tumor targeting techniques together with late expression of transgenes can be a valuable tool for the development of RAds. We expanded the features of the widely used AdEasy adenoviral cloning system toward the production of tropism modified replicating adenoviral vectors that express transgenes late in the viral life cycle. Three vectors (pIRES, pFIBER and pAdEasy-Sce) that facilitate easy manipulation of the adenoviral fiber region were established. Unique BstBI and I-Sce-1 restriction sites facilitate the introduction of retargeting peptides in the fiber HI-loop and of genes of interest in the fiber transcription unit. We validated the system by constructing an E1-positive adenovirus with an RGD motif in the fiber HI-loop and green fluorescent protein (GFP) expressed from the fiber transcription unit (AdDelta24Fiber-rgd-GFP). Additionally, assessment of E1-negative replication-deficient vectors confirmed strict dependence upon E1 expression for the expression of transgenes inserted into the fiber transcription unit. This flexible cloning system allows for straightforward construction of tropism expanded replicating adenoviral vectors that express transgenes late in the adenoviral life cycle.

Viable adenovirus vaccine prototypes: high-level production of a papillomavirus capsid antigen from the major late transcriptional unit

Safe, effective, orally delivered, live adenovirus vaccines have been in use for three decades. Recombinant derivatives of the live adenovirus vaccines may prove an economical alternative to current vaccines for a variety of diseases. To explore that possibility, we constructed a series of recombinants that express the major capsid protein (L1) of canine oral papillomavirus (COPV), a model for mucosal human papillomavirus (HPV) infection. Vaccination with virus-like particles (VLPs) composed of recombinant HPV L1 completely prevents persistent HPV infection [Koutsky, L. A., Ault, K. A., Wheeler, C. M., Brown, D. R., Barr, E., Alvarez, F. B., Chiacchierini, L. M. & Jansen, K. U. (2002) N. Engl. J. Med. 347, 1645-1651], suggesting that L1 expressed from recombinant adenoviruses might provide protective immunity. In our recombinants, COPV L1 is incorporated into adenovirus late region 5 (Ad L5) and is expressed as a member of the adenoviral major late transcriptional unit (MLTU). COPV L1 production by the most prolific recombinant is comparable to that of the most abundant adenoviral protein, hexon. COPV L1 production by recombinants is influenced by Ad L5 gene order, the specific mRNA processing signals associated with COPV L1, and the state of a putative splicing inhibitor in the COPV L1 gene. Recombinant COPV L1 protein assembles into VLPs that react with an antibody specific for conformational epitopes on native COPV L1 protein that correlate with protection in vivo. The designs of these recombinants can be applied directly to the production of recombinants appropriate for assessing immunogenicity and protective efficacy in animal models and in human trials.

Replication-dependent transgene expression from a conditionally replicating adenovirus via alternative splicing to a heterologous splice-acceptor site

BACKGROUND

Oncolytic viruses are promising anticancer agents because they selectively kill cancer cells and multiply within a tumor. Their oncolytic potency might be improved by expressing a therapeutic gene from the virus genome. In this regard, proper kinetics and level of transgene expression are important. In addition, expression of cytotoxic transgene products should be confined to cancer cells. Here, we developed oncolytic adenoviruses that provide transgene expression dependent on viral replication.

METHODS

We constructed an oncolytic adenovirus that expresses luciferase under regulation of the endogenous major late promoter (MLP) via alternative splicing to an inserted splice-acceptor site analogous to that of the adenovirus serotype 40 long fiber gene. Splicing of the luciferase transcript was studied by RT-PCR analysis. Expression was measured in the presence and absence of the flavonoid apigenin, an inhibitor of viral replication.

RESULTS

The inserted splice-acceptor site was properly recognized by the adenoviral splicing machinery. Luciferase expression levels were markedly higher than levels obtained with the cytomegalovirus (CMV) promoter, especially at late stages of infection. Inhibiting adenovirus replication reduced luciferase expression levels dramatically by 4 to 5 logs, whereas expression levels with the CMV-luciferase adenovirus were only moderately affected (2 logs).

CONCLUSIONS

Transgene delivery using the endogenous late gene expression machinery resulted in an expression pattern distinct from expression driven by the conventional CMV promoter. The high expression levels and strict coupling of expression to viral replication should be useful for adequate monitoring of replication and might provide a platform for the design of armed conditionally replicating adenoviruses (CRAds) with enhanced oncolytic potency.

Conditionally replicating adenoviruses kill tumor cells via a basic apoptotic machinery-independent mechanism that resembles necrosis-like programmed cell death

Conditionally replicating adenoviruses (CRAds) represent a promising class of novel anticancer agents that are used for virotherapy. The E1ADelta24 mutation-based viruses, Ad5-Delta24 [CRAd(E3-); E3 region deleted] and infectivity-enhanced Ad5-Delta24RGD [CRAd(E3+)] have been shown to potently eradicate tumor cells. The presence of the E3 region in the latter virus is known to improve cell killing that can be attributed to the presence of the oncolysis-enhancing Ad death protein. The more precise mechanism by which CRAds kill tumor cells is unclear, and the role of the host cell apoptotic machinery in this process has been addressed only in a limited way. Here, we examine the role of several major apoptotic pathways in the CRAd-induced killing of non-small-cell lung cancer H460 cells. As expected, CRAd(E3+) was more potent than CRAd(E3-). No evidence for the involvement of the p53-Bax apoptotic pathway was found. Western blot analyses demonstrated strong suppression of p53 expression and unchanged Bax levels during viral replication, and stable overexpression of human papillomavirus type 16-E6 in H460 cells did not affect killing by both CRAds. CRAd activity was also not hampered by stable overexpression of anti-apoptotic Bcl2 or BclXL, and endogenous Bcl2/BclXL protein levels remained constant during the oncolytic cycle. Some evidence for caspase processing was obtained at late time points after infection; however, the inhibition of caspases by the X-linked inhibitor of apoptosis protein overexpression or cotreatment with zVAD-fmk did not inhibit CRAd-dependent cell death. Analyses of several apoptotic features revealed no evidence for nuclear fragmentation or DNA laddering, although phosphatidylserine externalization was detected. We conclude that despite the known apoptosis-modulating abilities of individual Ad proteins, Ad5-Delta24-based CRAds trigger necrosis-like cell death. In addition, we propose that deregulated apoptosis in cancer cells, a possible drug resistance mechanism, provides no barrier for CRAd efficacy.

Conditionally Replicating Adenoviruses Expressing Short Hairpin RNAs Silence the Expression of a Target Gene in Cancer Cells

RNA interference (RNAi) is a posttranscriptional silencing mechanism triggered by double-stranded RNA that was recently shown to function in mammalian cells. Expression of cancer-associated genes was knocked down by expressing short hairpin RNAs (shRNAs) in cancer cells. By virtue of its excellent target specificity, RNAi may be used as a new therapeutic modality for cancer. The success of this approach will largely depend on efficient delivery of shRNAs to tumor cells. Tumor-selective replication competent viruses are especially suited to efficiently deliver anticancer genes to tumors. In addition, their intrinsic capacity to kill cancer cells makes these viruses promising anticancer agents per se. In this study, conditionally replicating adenoviruses were constructed encoding shRNAs targeted against firefly luciferase. These replicating viruses were shown to specifically silence the expression of the target gene in human cancer cells down to 30% relative to control virus. This finding offers the promise of using RNAi in the context of cancer gene therapy with oncolytic viruses.

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.

5-Fluorocytosine increases the toxicity of Wnt-targeting replicating adenoviruses that express cytosine deaminase as a late gene

Clinical studies with oncolytic adenoviruses have shown that existing viruses are safe but lack efficacy. To selectively increase the toxicity of oncolytic adenoviruses targeting colon tumours, we have inserted the yeast cytosine deaminase gene (yCD) after the fibre gene in the major late transcript. yCD was expressed using either an internal ribosome entry site (IRES) or by alternative splicing of a new exon analogous to the Ad41 long fibre exon. The IRES-CD virus gave higher yCD expression on Western blots. Both approaches result in yCD expression restricted to the period after viral DNA replication. Viral burst size was reduced by less than approximately 10-fold by 5-fluorocytosine (5-FC), showing that expression of yCD as a late gene is compatible with virus replication. Cytopathic effect assays in colon cancer cell lines showed that both yCD viruses have approximately 10-fold increased toxicity in the presence of the prodrug 5-FC, which is converted to 5-fluorouracil (5-FU) by yCD. Toxicity was higher following addition of 5-FC immediately after infection. The largest gain in toxicity was seen in HT29 colon cancer cells, which are the least permissive colon cancer cells for the parental virus, indicating that the new 5-FC/yCD viruses may have broader applications for colon cancer therapy than their predecessors.

Mode of transgene expression after fusion to early or late viral genes of a conditionally replicating adenovirus via an optimized internal ribosome entry site in vitro and in vivo

The expression of therapeutic genes by oncolytic viruses is a promising strategy to improve viral oncolysis, to augment gene transfer compared with a nonreplicating adenoviral vector, or to combine virotherapy and gene therapy. Both the mode of transgene expression and the locale of transgene insertion into the virus genome critically determine the efficacy of this approach. We report here on the properties of oncolytic adenoviruses which contain the luciferase cDNA fused via an optimized internal ribosome entry site (IRES) to the immediate early adenoviral gene E1A (AdDeltaE1AIL), the early gene E2B (AdDeltaE2BIL), or the late fiber gene (AdDeltafiberIL). These viruses showed distinct kinetics of transgene expression and luciferase activity. Early after infection, luciferase activities were lower for these viruses, especially for AdDeltaE2BIL, compared with nonreplicating AdTL, which contained the luciferase gene expressed from the strong CMV promoter. However, 6 days after infection, luciferase activities were approximately four (AdDeltaE1AIL) to six (AdDeltafiberIL) orders of magnitude higher than for AdTL, reflecting virus replication and efficient transgene expression. Similar results were obtained in vivo after intratumoral injection of AdDeltaE2BIL, AdDeltafiberIL, and AdTL. AdDeltafiberIL and the parental virus, Ad5-Delta24, resulted in similar cytotoxicity, but AdDeltaE2BIL and AdDeltaE1AIL were slightly attenuated. Disruption of the expression of neighboring viral genes by insertion of the transgene was minimal for AdDeltaE2BIL and AdDeltafiberIL, but substantial for AdDeltaE1AIL. Our observations suggest that insertion of IRES-transgene cassettes into viral transcription units is an attractive strategy for the development of armed oncolytic adenoviruses with defined kinetics and strength of transgene expression.

Armed therapeutic viruses: strategies and challenges to arming oncolytic viruses with therapeutic genes

Oncolytic viruses are attractive therapeutics for cancer because they selectively amplify, through replication and spread, the input dose of virus in the target tumor. To date, clinical trials have demonstrated marked safety but have not realized their theoretical efficacy potential. In this review, we consider the potential of armed therapeutic viruses, whose lytic potential is enhanced by genetically engineered therapeutic transgene expression from the virus, as potential vehicles to increase the potency of these agents. Several classes of therapeutic genes are outlined, and potential synergies and hurdles to their delivery from replicating viruses are discussed.