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

Antiviral activity of cidofovir against telomerase-specific replication-selective oncolytic adenovirus, OBP-301 (Telomelysin)

We constructed a replication-competent oncolytic adenovirus, OBP-301 (Telomelysin), in which human telomerase reverse transcriptase (hTERT) promoter drives E1 genes. OBP-301 is currently being used in a phase-I clinical trial for various types of tumors. Under such conditions, anti-adenoviral agents should be available for safety use against OBP-301 since any adenoviral viremia could cause severe adverse effects. Cidofovir (CDV) is an acyclic nucleoside phosphonate that has a broad antiviral activity against DNA viruses. Here, we examined the antiviral effects of CDV against OBP-301. The in vitro cytopathic effects of OBP-301 were suppressed by CDV. Moreover, CDV decreased the adenoviral E1A gene copy number after OBP-301 infection. These results suggest that CDV is a potentially useful antiviral agent for OBP-301.

Biological approaches to therapy of pancreatic cancer

Pancreatic cancer is a lethal disease and notoriously difficult to treat. Only a small proportion is curative by surgical resection, whilst standard chemotherapy for patients with advanced disease has only modest effect with substantial toxicity. Clearly there is a need for the continual development of novel therapeutic agents to improve the current situation. Improvement of our understanding of the disease has generated a large number of studies on biological approaches targeting the molecular abnormalities of pancreatic cancer, including gene therapy and signal transduction inhibition, antiangiogenic and matrix metalloproteinase inhibition, oncolytic viral therapy and immunotherapy. This article provides a review of these approaches, both investigated in the laboratories and in subsequent clinical trials.

Cancer cell death enhances the penetration and efficacy of oncolytic herpes simplex virus in tumors

The success of tumor oncolytic virotherapy is limited by the poor penetration of virus in tumors. Interstitial collagen fibers and the narrow spacing between cancer cells are major barriers hindering the movement of large viral particles. To bypass the cellular barrier, we tested the hypothesis that the void space produced by cancer cell apoptosis enhances the initial spread and efficacy of oncolytic herpes simplex virus (HSV). In mice with mammary tumors, apoptosis was induced by doxycycline-regulated expression/activation of CD8/caspase-8, paclitaxel, or paclitaxel plus tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). In both collagen-poor and collagen-rich tumors, apoptosis or necrosis increased the initial intratumoral spread of HSV. Compared with the isolated pattern of HSV infection generally located in the center of control tumors, apoptosis induction and a single i.t. injection of virus produced an interconnected and diffuse pattern of infection, which extended from the tumor center to the periphery. This interconnected pattern of viral infection correlated with the formation of void spaces and channel-like structures in apoptosis-rich tumor areas. We also show that the i.t. injection of HSV after caspase-8 activation or paclitaxel-TRAIL pretreatment retards tumor growth, whereas HSV administration before tumor cell death induction did not improve therapeutic efficacy. Hence, our findings show that the induction of cancer cell death before the injection of oncolytic HSV enhances intratumoral virus delivery/penetration and antitumor efficacy.

Biodistribution and kinetics of the novel selective oncolytic adenovirus M1 after systemic administration

Oncolytic adenoviruses represent a promising novel therapeutic option for the treatment of cancer. Despite their demonstrated safety in human clinical trials, the fundamental properties of oncolytic adenovirus biodistribution, spread, viral persistence, and replication in vivo have not been well characterized. The aim of this study was to evaluate the kinetics of viral distribution, spread, replication, and antitumoral efficacy after i.v. administration of a novel oncolytic mutant M1. This mutant consists of the E1A CR2-deleted Adv5 with a fragment of antisense polo-like kinase 1 (plk1) cDNA inserted into the deleted 6.7K/gp19K region, which combines oncolytic properties with efficient plk1 silencing, as described in our previous reports. In the present study, we established a new human orthotopic gastric carcinoma with a high frequency metastasis mouse model and showed that M1 spread not only in local primary tumors but also in disseminated metastases. M1 could effectively replicate in tumor cells leading to "oncolysis" and was able to eliminate expression of the targeted gene plk1 in human orthotopic gastric carcinoma model mice. Therefore, i.v. administration of M1 could prolong the survival time of tumor-bearing mice.

Myxoma virus and oncolytic virotherapy: a new biologic weapon in the war against cancer

Oncolytic virotherapy is an innovative alternative to more conventional cancer therapies. The ability of some viruses to specifically target and kill malignant cancerous cells while leaving normal tissue unscathed has opened a large repertoire of new and selective cancer killing therapeutic candidates. Poxviruses, such as vaccinia virus, have a long history of use in humans as live vaccines and have more recently been studied as potential platforms for delivery of immunotherapeutics and attenuated variants of vaccinia have been explored as oncolytic candidates. In contrast, the poxvirus myxoma virus is a novel oncolytic candidate that has no history of use in humans directly, as it has a distinct and absolute host species tropism to lagomorphs (rabbits). Myxoma virus has been recently shown to be able to also selectively infect and kill human tumor cells, a unique tropism that is linked to dysregulated intracellular signalling pathways found in the majority of human cancers. This review outlines the existing knowledge on the tropism of myxoma virus for human cancer cells, as well as preclinical data exhibiting its ability to infect and clear tumors in animal models of cancer. This is an exciting new therapeutic option for treating cancer, and myxoma virus joins a growing group of oncolytic virus candidates that are being developed as a new class of cancer therapies in man.

Induction of Efficient Antitumor Immunity Using Dendritic Cells Activated by Recombinant Sendai Virus and Its Modulation by Exogenous IFN-β Gene

Dendritic cell (DC)-based cancer immunotherapy has been paid much attention as a new and cancer cell-specific therapeutic in the last decade; however, little clinical outcome has been reported. Current limitations of DC-based cancer immunotherapy include sparse information about which DC phenotype should be administered. We here report a unique, representative, and powerful method to activate DCs, namely recombinant Sendai virus-modified DCs (SeV/DC), for cancer immunotherapy. In vitro treatment of SeV without any bioactive gene solely led DCs to a mature phenotype. Even though the expression of surface markers for DC activation ex vivo did not always reach the level attained by an optimized amount of LPS, superior antitumor effects to B16F1 melanoma, namely tumor elimination and survival, were obtained with use of SeV-GFP/DC as compared with those seen with LPS/DC in vivo, and the effect was enhanced by SeV/DC-expressing IFN-beta (SeV-murine IFN-beta (mIFN-beta)/DC). In case of the treatment of an established tumor of B16F10 (7-9 mm in diameter), a highly malignant subline of B16 melanoma, SeV-modified DCs (both SeV-GFP/DC and SeV-mIFN-beta/DC), but not immature DC and LPS/DC, dramatically improved the survival of animals. Furthermore, SeV-mIFN-beta/DC but not other DCs could lead B16F10 tumor to the dormancy, associated with strongly enhanced CD8+ CTL responses. These results indicate that rSeV is a new and powerful tool as an immune booster for DC-based cancer immunotherapy that can be significantly modified by IFN-beta, and SeV/DC, therefore, warrants further investigation as a promising alternative for cancer immunotherapy.

Paclitaxel and vincristine potentiate adenoviral oncolysis that is associated with cell cycle and apoptosis modulation, whereas they differentially affect the viral life cycle in non-small-cell lung cancer cells

Chemotherapy, including microtubule (MT)-interacting agents, can enhance the tumor-eradicating activity of replication-competent adenoviruses. The purpose of this study was to obtain more insight into the mechanism underlying this enhancement that may be exploited for the development of improved therapy. Two MT-interacting agents with opposite activity, paclitaxel (PTX) that stabilizes and vincristine (VCR) that destabilizes MTs, were found to synergistically enhance adenoviral oncolysis in non-small-cell lung cancer (NSCLC) cells. To explore the possibility that these drugs affect the viral life cycle by modulating adenoviral gene expression, we used a quantitative reverse transcription-polymerase chain reaction assay and found that PTX, but not VCR, increased the expression of E1A13S, ADP and Penton genes, which correlated with an increase in viral particle assembly and release. Next, the effect of combined treatment on cell-cycle progression was studied. Both drugs suppressed adenovirus-induced S-phase arrest and instead caused G2/M arrest, which was accompanied by an increase in apoptotic cells. Taken together, the enhancement of oncolysis by MT-interacting drugs appears not to require specific MT transport or scaffold functions. Our findings suggest that MT-interacting drug-induced cellular signals that modulate cell-cycle arrest and apoptosis are primarily on the basis of their oncolysis-enhancing activity.

Gene therapy in gynecological cancer

Gynecological malignancies remain a major source of morbidity and mortality worldwide. In the USA alone, more than 77,000 women are diagnosed annually and over 28,000 die of some form of a gynecological malignancy. Many of these women will fail conventional therapy, leaving few remaining treatment options. Gene therapy presents one possible alternative treatment modality although, unfortunately, it is currently more theoretical than practical. Here, some of the basic science behind gene therapy is reviewed, different delivery systems used to transport the therapeutic gene are discussed, different methods of achieving a therapeutic effect are examined, some of the key trials in ovarian, endometrial, cervical and vulvar cancer research are highlighted and the future of gene therapy is explored.

Markedly enhanced cytolysis by E1B-19kD-deleted oncolytic adenovirus in combination with cisplatin

Oncolytic adenoviruses are currently being developed as novel antitumor therapeutics. To enhance their therapeutic potential, adenoviruses are being administered in combination with standard chemotherapy. Adenoviral vectors used in these clinical trials, however, can be destructive as they encode intact E1B 19-kDa protein, which can block the apoptotic pathway induced by a variety of chemotherapeutic agents. Previously, we have shown that oncolytic adenovirus Ad-DeltaE1B19/55, deleted for sequence encoding E1B 19-kDa and E1B 55-kDa proteins, exhibits marked enhancement in cytolytic and apoptotic activity [Kim, J., Cho, J.Y., Kim, J.H., Jung, K.C., and Yun, C.O. (2002). Cancer Gene Ther. 9, 725-736]. In the current study, we assess the therapeutic value of Ad- DeltaE1B55 and Ad-DeltaE1B19/55 in combination with cisplatin. A marked increase in cytotoxicity was observed for both Ad-DeltaE1B55 and Ad-DeltaE1B19/55 when combined with cisplatin. Relative to each other in all cell lines examined, the combination of the double-deleted adenovirus, Ad-DeltaE1B19/55, plus cisplatin exhibited a greater cell-killing effect than did the single-deleted adenovirus, Ad-DeltaE1B55, plus cisplatin. Propidium iodide staining and TUNEL analysis also revealed that the combination of cisplatin with Ad-DeltaE1B19/55 caused greater induction of apoptosis than that with Ad-DeltaE1B55. Similarly, in vivo, the combination of Ad-DeltaE1B55 or Ad-DeltaE1B19/55 with cisplatin also induced greater antitumor effect in a human cervical xenograft model. TUNEL staining showed that the apoptotic level was significantly higher in tumor tissue treated with Ad-DeltaE1B19/55 plus cisplatin than with any other treatment. In addition, viral presence was confirmed by immunohistological staining, with increased numbers of adenoviral particles detected in wider areas of tumors treated with Ad-DeltaE1B19/55 oncolytic adenovirus plus cisplatin. Taken together, these findings demonstrate that cisplatin in combination with E1B- 19kD-deleted oncolytic adenovirus may enhance therapeutic efficacy (via active induction of apoptosis), eliciting a greater efficacy profile than that with E1B-19kD-expressing oncolytic adenovirus.

A Phase I Trial of Intravenous CG7870, a Replication-Selective, Prostate-Specific Antigen–Targeted Oncolytic Adenovirus, for the Treatment of Hormone-Refractory, Metastatic Prostate Cancer

CG7870 is a replication-selective oncolytic adenovirus genetically engineered to replicate preferentially in prostate tissue. In a previous phase I/II clinical trial of intraprostatic delivery of CG7870 for locally recurrent prostate cancer this virus was well tolerated. In this phase I study CG7870 was administered as a single intravenous infusion in a group-sequential dose escalation design (1 x 10(10) to 6 x 10(12) viral particles (vp)) to 23 patients with hormone-refractory metastatic prostate cancer. Flulike symptoms (fever, fatigue, rigors, nausea, and/or vomiting) were the most common adverse events. Three therapy-related grade 3 adverse events were reported, one of which (fatigue) was serious. At doses greater than 10(12) vp all five patients experienced asymptomatic grade 1 to 2 transaminitis and/or isolated d-dimer elevations starting on day 2 through 8; dose escalation was therefore halted at 6 x 10(12) vp. All tested patients had CG7870 genomes present in the peripheral blood for at least 90 minutes after infusion; patients in the highest dose group had persistence of genomes through 29 days. A "secondary" or "delayed" peak in plasma CG7870 genome copies (defined as a >10-fold increase in CG7870 genomes from nadir concentration) suggestive of active viral replication and shedding into the bloodstream was detected in 16/23 (70%) patients. CG7870 was detected in the saliva of 3 patients, whereas all urine samples tested negative. All patients developed antibodies to CG7870. Dose-related increases in interleukins 6 and 10 (IL-6, IL-10) blood levels were detected. The peak IL-6 concentration after CG7870 treatment was associated with a transient, asymptomatic decrease in blood pressure. No partial or complete prostate-specific antigen (PSA) responses were observed; however, 5 patients had a decrease in serum PSA of 25% to 49% following a single treatment, including 3 of 8 patients at the highest dose levels.

Use of synthetic vectors for neutralising antibody resistant delivery of replicating adenovirus DNA

Use of synthetic vectors to deliver genomes of conditionally replicating lytic viruses combines the strengths of viral and non-viral approaches by enabling neutralising antibody resistant deployment of cancer virotherapy. Adenovirus is particularly suitable for this application since all proteins essential for replication can be expressed from the input DNA, although the presence of terminal protein (TP) covalently linked to the 5' termini of the input virus genomes both improves expression of transgenes encoded in the input DNA and also enhances replication. These roles of TP were distinguished in experiments where E1-deleted Ad(GFP)DNA bearing TP (Ad(GFP)DNA-TP), delivered with DOTAP, gave a two-fold greater frequency of transduction than Ad(GFP)DNA(without TP) in non-complementing A549 cells, while in 293 cells (which support replication of E1-deleted viruses) the presence of TP mediated a much greater differential transgene expression, commensurate with its ability to promote replication. Subsequent studies using AdDNA for virotherapy, therefore, included covalently linked TP. AdDNA-TP delivered to A549 cells using a synthetic polyplex vector was shown to be resistant to levels of neutralising antisera that completely ablated infection by wild-type adenovirus, enabling polyplex/Ad(wild type)DNA-TP to mediate a powerful cytopathic effect. Similarly in vivo, direct injection of a polyplex/Ad(wild type)DNA-TP into A549 tumours was neutralising antibody-resistant and enabled virus replication, whereas intact virus was neutralised by the antibody and failed to infect. The delivery of adenovirus genomes-TP using synthetic vectors should provide a strategy to bypass neutralising antibodies and facilitate clinical application of replicating adenovirus for cancer virotherapy.

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.

RETRACTED: Selective targeting of checkpoint kinase 1 in tumor cells with a novel potent oncolytic adenovirus

This article has been retracted: please see Elsevier Policy on Article Withdrawal (http://www.elsevier.com/locate/withdrawalpolicy). This article has been retracted at the request of the editor-in-chief. Similarities were reported between images in this article and an article in Clinical Cancer Research (Zhou et al., 2005, Clin. Cancer Res. 11, 8431-8440, https://10.1158/1078-0432.CCR-05-1085). Q.J., J.Z., X.H., G.C., Y.L., K.L., L.Z., and D.M. were all authors of the Clinical Cancer Research paper as well. These concerns were initially reported in a Pubpeer thread (https://pubpeer.com/publications/FF881782FF5AFD316D42E0C0F00766). Image analysis performed by the editorial office confirmed findings of image recycling in Figures 2A and 3B of the Molecular Therapy article. This reuse (and in part misrepresentation) of data without appropriate attribution represents a severe abuse of the scientific publishing system.

Targeted molecular therapy of GBM

Major advances in molecular biology, cellular biology and genomics have substantially improved our understanding of cancer. Now, these advances are being translated into therapy. Targeted therapy directed at specific molecular alterations is already creating a shift in the treatment of cancer patients. Glioblastoma (GBM), the most common brain cancer of adults, is highly suited for this new approach. GBMs commonly overexpress the oncogenes EGFR and PDGFR, and contain mutations and deletions of tumor suppressor genes PTEN and TP53. Some of these alterations lead to activation of the P13K/Akt and Ras/MAPK pathways, which provide targets for therapy. In this paper, we review the ways in which molecular therapies are being applied to GBM patients, and describe the tools of these approaches: pathway inhibitors, monoclonal antibodies and oncolytic viruses. We describe strategies to: i) target EGFR, its ligand-independent variant EGFRvIII, and PDGFR on the cell surface, ii) inhibit constitutively activate RAS/MAPK and PI3K/Akt signaling pathways, iii) target TP53 mutant tumors, and iv) block GBM angiogenesis and invasion. These new approaches are likely to revolutionize the treatment of GBM patients. They will also present new challenges and opportunities for neuropathology.

Gene therapy developments for pancreatic cancer

Treatment options for pancreatic cancer have limited success and it is therefore an appropriate target for the development of new strategies, including gene therapy. Gene therapy approaches include inhibition of activated oncogenes (KRAS, LSM1) with antisense and RNA interference strategies, replacement of inactivated tumour suppressor genes (TP53, CDKN2A, CDKN1A), targeting of cell signalling pathways, gene-directed prodrug-activation therapies and the use of replication-competent oncolytic viruses. Angiogenesis and apoptosis have also been targeted for gene therapy. Clinical trials of gene therapy have shown only moderate anti-tumour effects. As there are many genetic abnormalities in pancreatic cancer, strategies combining different targets or indeed different modalities of treatment, may be more successful. Identification of new targets and improvements in delivery and targeting may further improve the efficacy of gene therapy in pancreatic cancer.

The chemical nature of the 2'-substituent in the pentose-sugar dictates the pseudoaromatic character of the nucleobase (pKa) in DNA/RNA

We here show that the pKa (error limit: 0.01 to 0.03 pKa unit) of a nucleobase in a nucleotide can be modulated by the chemical nature of the 2'-substituent at the sugar moiety. This has been evidenced by the measurement of nucleobase pKa in 47 different model nucleoside 3',5'-bis- and 3'-mono-ethylphosphates. The fact that the electronic character of each of the 2'-substituents (Fig. 1) alters the chemical shift of the H2' sugar proton, and also alters the pKa of the nucleobase in the nucleotides has been evidenced by a correlation plot of pKa of N3 of pyrimidine (T/C/U) or pKa of N7 of 9-guaninyl with the corresponding deltaH2' chemical shifts at the neutral pH, which shows linear correlation with high Pearson's correlation coefficients (R = 0.85-0.97). That this modulation of the pKa of the nucleobase by a 2'-substituent is a through-bond as well as through-space effect has been proven by ab initio determined pKa estimation. Interestingly, experimental pKas of nucleobases from NMR titration and the calculated pKas (by ab initio calculations utilizing closed shell HF 6-31G** basis set) are linearly correlated with R = 0.98. It has also been observed that the difference of ground and protonated/de-protonated HOMO orbital energies (DeltaHOMO, a.u.) for the nucleobases (A/G/C/T/U) are well correlated with their pK(a)s in different 2'-substituted 3',5'-bis-ethylphosphate analogs suggesting that only the orbital energy of HOMO can be successfully used to predict the modulation of the chemical reactivity of the nucleobase by the 2'-substituent. It has also been demonstrated that pKa values of nucleobases in 3',5'-bis-ethylphosphates (Table 1) are well correlated with the change in dipole moment for the respective nucleobases after protonation or de-protonation. This work thus unambiguously shows that alteration of the thermodynamic stability (Tm) of the donor-acceptor complexes [ref. 20], as found with various 2'-modified duplexes in the antisense, siRNA or in triplexes by many workers in the field, is a result of alteration of the pseudoaromatic character of the nucleobases engineered by alteration of the chemical nature of the 2'-substitution.

The effect of hypoxia on the uptake, replication and lytic potential of group B adenovirus type 3 (Ad3) and type 11p (Ad11p)

Replicating, tumor selective viruses are being tested as potential treatments for human cancers. Hypoxia is a pathophysiological cancer condition that alters the lytic potential of the replication-competent adenovirus serotype 5 (Ad5) virus by a mechanism independent of receptor levels or internalization rates. We extend these initial studies to examine the potential effects of hypoxia on the group B adenoviruses (Ads), adenovirus type 3 (Ad3) (group B1) and adenovirus type 11p (Ad11p) (group B2). Receptor expression (CD46) is not altered by hypoxia. However, the lytic potential is compromised in a cell-dependent fashion. Consequently, our study suggests that group B replicating Ad-based treatments, like the group C Ad-5-based viruses, will need to be modified in order to effectively treat hypoxic components of human tumors.

Oncolytic virotherapy: approaches to tumor targeting and enhancing antitumor effects

The application of replicating viruses for the treatment of cancers represents a novel therapy that is distinct from traditional treatment modalities. It is apparent that the genetic changes that a virus produces within an infected cell in order to create an environment conducive to viral replication are often similar to the processes involved in cellular transformation. These include uncontrolled cellular proliferation, prevention of apoptosis, and resistance to host organism immune effector mechanisms. Deletions of viral genes involved in these processes have been exploited to produce viral mutants whose replication is selective for transformed cells. The use of tissue-specific transcriptional response or RNA stability elements to control the expression of critical viral genes has also resulted in targeted viruses. Work also is being undertaken to restrict or alter the tropism of viruses by altering their ability to infect certain cell types. Finally, the addition of exogenous genes can be used to increase the virus's lytic potential and/or bystander killing; to further induce the host's immune response against cancer cells; and/or to permit the controlled downregulation of viral replication if necessary. The combination of different tumor-targeting mutations in parallel with the expression of foreign genes has resulted in the evolution of second- and third-generation viruses that continue to become further distinct from their native parental strains. The movement of these viruses into the clinic has begun to demonstrate the potential of this approach in the treatment of cancers.

In vivo Imaging of Adenovirus Transduction and Enhanced Therapeutic Efficacy of Combination Therapy with Conditionally Replicating Adenovirus and Adenovirus-p27

Gene therapy is hampered by poor gene transfer to the tumor mass. We previously proposed a combination adenoviral gene therapy containing a conditionally replicating adenovirus (CRAD) expressing mutant E1 (delta24RGD) and a replication-defective E1-deleted adenovirus to enhance the efficiency of gene transfer. Mutant E1 expressed by delta24RGD enables the replication of replication-defective adenoviruses in tumors when cancer cells are co-infected with both viruses. In this study, gene transfer rates in xenografts tumors were monitored by bioluminescence in cells infected with the replication-defective adenovirus-luciferase (ad-luc). Tumor masses treated with CRAD + ad-luc showed dramatically stronger and more prolonged luciferase expression than ad-luc-treated tumors and this expression spread through the entire tumor mass without significant systemic spread. Transduction with CRAD + replication-defective adenovirus-p27 increased the expression of p27 by 24-fold versus transduction with ad-p27 alone. Treatment of a lung cancer cell line and of established lung cancer xenografts with CRAD + adenovirus-p27 also induced stronger growth suppression than treatment with either virus alone. These findings confirm the selective replication of E1-deleted adenovirus containing a therapeutic gene due to the presence of mutant E1 produced by delta24RGD in tumors. Moreover, this replication increased the therapeutic gene transfer rate and enhanced its antitumor effects.

Development of transcriptionally regulated oncolytic adenoviruses

Changes initiated at the cellular and systemic levels as a result of viral infection or neoplastic transformation share significant overlap. Therefore, the use of replicating viruses to treat tumors has long been postulated as a promising avenue for oncolytic therapy. Over the last 10 years, transcriptionally regulated adenoviruses have become a popular platform for the development of such oncolytic viruses. Placement of heterologous promoters in front of key adenoviral transcription units to achieve tumor- or tissue-specific viral replication is well documented. Various derivatives of this general strategy have led to considerable insight into its limitations, pitfalls, and potential. Although a general process can be described by which to develop transcriptionally regulated adenoviruses, it is apparent that few set rules can yet be defined as to what constitutes a safe, stable, and therapeutically effective vector. Clinical experiences to date suggest the short-term potential for this class of therapeutics lies in combination therapy regimens. Such lessons from the clinic suggest the next generation of transcriptionally regulated oncolytic adenoviruses take advantage of the ability of the platform to carry transgenes in order to deliver a multimodal therapy from a single agent. Beyond this 'arming' of the vectors lies the detargeting, retargeting, and coating of adenoviruses to improve the delivery of the agent to the treatment site(s). As a therapeutic platform, transcriptionally regulated adenoviruses are at an early stage of development with considerable opportunities for advancement.

Inactivating intracellular antiviral responses during adenovirus infection

DNA viruses promote cell cycle progression, stimulate unscheduled DNA synthesis, and present the cell with an extraordinary amount of exogenous DNA. These insults elicit vigorous responses mediated by cellular factors that govern cellular homeostasis. To ensure productive infection, adenovirus has developed means to inactivate these intracellular antiviral responses. Among the challenges to the host cell is the viral DNA genome, which is viewed as DNA damage and elicits a cellular response to inhibit replication. Adenovirus therefore encodes proteins that dismantle the cellular DNA damage machinery. Studying virus-host interactions has yielded insights into the molecular functioning of fundamental cellular mechanisms. In addition, it has suggested ways that viral cytotoxicity can be exploited to offer a selective means of restricted growth in tumor cells as a therapy against cancer. In this review, we discuss aspects of the intracellular response that are unique to adenovirus infection and how adenoviral proteins produced from the early region E4 act to neutralize antiviral defenses, with a particular focus on DNA damage signaling.

Adenoviral Strategies for the Gene Therapy of Cancer

Despite slow clinical progress, efforts to develop specific nontoxic cancer gene therapies are increasing exponentially. Adenoviral vectors are one of the most popular vehicles for gene transfer currently being used in worldwide clinical trials for cancer. Over the past decade our knowledge of the adenoviral life cycle together with the discovery of novel tumor antigens has permitted the targeting of adenoviral vectors to specific tumors. Targeting adenoviral vectors to tumors is crucial for their use in clinical applications in order to allow for systemic administration and the use of reduced vector doses. In addition, novel approaches to tumor killing have also been explored, which will have greater potency and selectivity than currently available treatments such as chemotherapy or radiation. This review discusses the basic concepts behind the use of adenoviral vectors for cancer gene therapy and their potential for clinical application, as well as ongoing and completed clinical trials.

Novel oncolytic adenovirus selectively targets tumor-associated polo-like kinase 1 and tumor cell viability

PURPOSE

Polo-like kinase 1 (plk1) is a serine/threonine protein kinase essential for multiple mitotic processes. Previous observations have validated plk1 as a promising therapeutic target. Despite being conceptually attractive, the potency and specificity of current plk1-based therapies remain limited. We sought to develop a novel plk1-targeting strategy by constructing an oncolytic adenovirus to selectively silence plk1 in tumor cells.

EXPERIMENTAL DESIGN

Two artificial features were engineered into one wild-type adenovirus type 5 (wt-Adv5) genome to generate a new oncolytic adenovirus (M1). First, M1 contains a 27-bp deletion in E1A region, which confers potent, oncolytic efficacy. Second, M1 is armed with a fragment of antisense plk1 cDNA that substitutes the E3 region encoding 6.7K and gp19K. In this design, tumor-selective replication of M1 would activate the native adenovirus E3 promoters to express the antisense plk1 cDNA preferentially in tumor cells and silence tumor-associated plk1 protein.

RESULTS

By virtue of combining oncolysis with plk1 targeting, M1 exhibited potent antitumoral efficacy in vitro and in vivo. Systemic administration of M1 plus cisplatin induced complete tumor regression in 80% of orthotopic hepatic carcinoma model mice that were otherwise resistant to cisplatin and disseminated metastases.

CONCLUSIONS

Coupling plk1 targeting with oncolysis had shown superior antitumor efficacy. Present findings would benefit the development of novel oncolytic adenoviruses generally applicable to a wide range of molecule-based therapeutics.

Adenovirus infection and cytotoxicity of primary mantle cell lymphoma cells

Mantle cell lymphoma (MCL) is a distinct form of non-Hodgkin's lymphoma (NHL) derived from CD5+ B cells. MCL cells overexpress cyclin D1 as a consequence of translocation of the gene into the immunoglobulin heavy-chain gene locus. MCL is an aggressive form of NHL with frequent relapses after standard-dose chemotherapy. In this context, a variety of novel therapies for patients with MCL have been investigated. In this study, we use an expanded panel of attenuated adenoviruses to study adenovirus-mediated cytotoxicity of MCL cells. Our results demonstrate: 1) adenovirus infection of MCL cells despite the absence of receptor/coreceptor molecules known to be important for adenovirus infection of other cells types; 2) cytotoxicity of MCL cells after infection with specific adenovirus mutants; 3) a high degree of cytotoxicity after infection of some patient samples with viruses lacking the E1B 19k "antiapoptotic" gene; and 4) cytotoxicity after infection with viruses containing mutations in E1A pRb or p300 binding. The extent of cytotoxicity with the panel of viruses demonstrated interpatient variability, but 100% cytotoxicity, as determined by molecular analysis, was detected in some samples. These studies provide the foundation for: 1) the development of adenoviruses as cytotoxic agents for MCL and 2) analyses of key regulatory pathways operative in MCL cells.

Genetic incorporation of HSV-1 thymidine kinase into the adenovirus protein IX for functional display on the virion

Adenoviral vectors have been exploited for a wide range of gene therapy applications. Direct genetic modification of the adenovirus capsid proteins has been employed to achieve alteration of vector tropism. We have defined the carboxy-terminus of the minor capsid protein pIX as a locus capable of presenting incorporated ligands on the virus capsid surface. Thus, we sought to exploit the possibility of incorporating functional proteins at pIX. In our current study, we incorporated the herpes simplex virus type 1 (HSV-1) thymidine kinase (TK) within pIX to determine if a larger protein of this type could retain functionality in this context. Our study herein clearly demonstrates our ability to rescue viable adenoviral particles that display functional HSV-1 TK as a component of their capsid surface. DNA packaging and cytopathic effect were not affected by this genetic modification to the virus, while CAR-dependent binding was only marginally affected. Using an in vitro [3H]-thymidine phosphorylation assay, we demonstrated that the kinase activity of the protein IX-TK fusion protein incorporated into adenoviral virions is functional. Analysis of cell killing after adenovirus infection showed that the protein IX-TK fusion protein could also serve as a therapeutic gene by rendering transduced cells sensitive to gancyclovir. Using 9-[4-[18F]-fluoro-3-(hydroxymethyl)butyl]guanine ([18F]-FHBG; a positron-emitting TK substrate), we demonstrated that we could detect specific cell binding and uptake of adenoviral virions containing the protein IX-TK fusion protein at 1 h post-infection. Our study herein clearly demonstrates our ability to rescue viable adenoviral particles that display functional HSV-1 TK as a component of their capsid surface. The alternative display of HSV-1 TK on the capsid may offer advantages with respect to direct functional applications of this gene product. In addition, the determination of an expanded upper limit of incorporable proteins on pIX highlights its unique utility as a locus for placement of functional vector constructs.

Effect of hypoxia on Ad5 infection, transgene expression and replication

Oxygen deprivation (hypoxia) is a common feature of various human maladies, including cardiovascular diseases and cancer; however, the effect of hypoxia on Ad-based gene therapies has not been described. In this study, we evaluated how hypoxia (1% pO(2)) affects different aspects of Ad-based therapies, including attachment and uptake, transgene expression, and replication, in a series of cancer cell lines and primary normal cells. We found that hypoxia had no significant effect on the expression or function of the Ad5 attachment (Coxsackievirus and Adenovirus Receptor) and internalization (alpha(v) integrins) proteins, nor on the human cytomegalovirus-driven expression of an exogenous gene carried by a replication-incompetent Ad. Viral replication, however, was compromised by hypoxic conditions. Our studies revealed hypoxia-induced reductions in E1A levels that were mediated at the post-transcriptional level. E1A drives cells into the viral replication optimal S phase of the cell cycle; consequently, the combination of reduced E1A protein and hypoxia-induced G1 arrest of cells may be responsible for the lack of efficient viral replication under hypoxic conditions. Consequently, while traditional replication-incompetent Ad-based vectors appear to be viable delivery systems for hypoxia-associated disease indications, our studies suggest that Oncolytic Ads may need additional factors to efficiently treat hypoxic regions of human tumors.

Hypoxia reduces adenoviral replication in cancer cells by downregulation of viral protein expression

Successful cancer therapy using replicating viral vectors relies on the spread of virus from infected to uninfected cells. To date, there has been limited clinical success in the use of replicating adenoviruses. In animal models, established xenograft tumors are rarely eliminated despite the persistence of high viral titers within the tumor. Hypoxia is a prevalent characteristic of solid tumors, whereas adenovirus naturally infects tissues exposed to ambient oxygen concentrations. Here, we report that hypoxia (1% oxygen) reduces adenoviral replication in H1299 and A549 lung cancer cells, BxPC-3 pancreatic cancer cells, LNCaP prostate cancer cells and HCT116 colon cancer cells. However, hypoxia does not reduce cell viability or restrict S-phase entry. Importantly, the production of E1a and fiber proteins under hypoxic conditions was substantially decreased at 24 and 48 h compared to room air controls. In contrast, Northern analysis showed similar levels of E1a mRNA in room air and hypoxic conditions. In conclusion, a level of hypoxia similar to that found within solid tumors reduces the replication of adenoviral vectors by reduction of viral protein expression without a reduction in mRNA levels. To further improve oncolytic therapy using a replicating adenovirus, it is important to understand the mechanism through which hypoxia and the virus interact to control expression of viral and cellular proteins, and consequently to develop means to overcome decreased viral production in hypoxic conditions.

Viruses with deletions in antiapoptotic genes as potential oncolytic agents

Replication-selective oncolytic viruses have emerged as a new treatment platform for cancers. However, selectivity and potency need to be improved before virotherapy can become a standard treatment modality. In addition, mechanisms that can be incorporated to enable targeting a broad range of cancer types are highly desirable. Cancer cells are well known to have multiple blocks in apoptosis pathways. On the other hand, viruses have evolved to express numerous antiapoptotic genes to antagonize apoptosis induced upon infection. Viruses with deletions in antiapoptotic genes can therefore be complemented by antiapoptotic genetic changes in cancer cells for efficient replication and oncolysis. In this review, we summarize the recent development of this concept, the potential obstacles, and future directions for optimization.

Virus-associated RNA I-deleted adenovirus, a potential oncolytic agent targeting EBV-associated tumors

Given the growing number of tumor types recognizably associated with EBV infection, it is critically important that therapeutic strategies are developed to treat such tumors. Replication-selective oncolytic adenoviruses represent a promising new platform for anticancer therapy. Virus-associated I (VAI) RNAs of adenoviruses are required for efficient translation of viral mRNAs. When the VAI gene is deleted, adenovirus replication is impeded in most cells (including HEK 293 cells). EBV-encoded small RNA1 is uniformly expressed in most EBV-associated human tumors and can functionally substitute for the VAI RNAs of adenovirus. It enables replication to proceed through complementation of VAI-deletion mutants. We hypothesized that VAI-deleted adenovirus would selectively replicate in EBV-positive tumor cells due to the presence of EBV-encoded small RNA1 with no (or poor) replication in normal or EBV-negative tumor cells. In this report, we show that high levels of replication occurred in the VAI-deleted mutant in the EBV-positive tumor cells compared with low (or negligible) levels in EBV-negative and normal human primary cells. Correspondingly, high toxicity levels were observed in EBV-positive tumor cells but not in EBV-negative tumor or normal human primary cells. In vivo, VAI-deleted adenovirus showed superior antitumoral efficacy to wild-type adenovirus in EBV-positive tumor xenografts, with lower hepatotoxicity than wild-type adenovirus. Our data suggest that VAI-deleted adenovirus is a promising replication-selective oncolytic virus with targeting specificity for EBV-associated tumors.

ADP-overexpressing adenovirus elicits enhanced cytopathic effect by induction of apoptosis

Replication-competent adenoviruses (Ad's) are emerging as a promising new modality for treatment of cancer. Selective replication of viral agents in tumor may lead to improved efficacy over nonreplicating Ad's due to their inherent ability to multiply, lyse, and spread to surrounding cells. We have previously shown that an E1B 55 kDa-deleted adenovirus (YKL-1) exhibits tumor-specific replication and cell lysis, but its cytolytic effects were reduced in comparison to the wild-type adenovirus. To increase the oncolytic potency of YKL-1, we have reintroduced the Ad death protein (ADP) gene under the control of either a CMV or an MLP promoter at the E3 region of YKL-1, generating YKL-cADP and YKL-mADP Ad's, respectively. ADP is an 11.6 kDa protein encoded by the E3 transcription unit, and is required to kill adenovirus-infected cells efficiently. However, to date, the mechanism by which ADP mediates cell death has not been clearly defined. In this study, we report that ADP-overexpressing Ad markedly enhanced cytolytic effect (up to 100-fold) against all tumor cell lines tested, but did not increase cytopathic effect in normal skin fibroblast, BJ. Moreover, plaque size formed by YKL-cADP was substantially larger than that of YKL-1, indicating an enhancement in cell lysis. TUNEL (terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling) assay and Annexin-V/PI double staining indicate that ADP-mediated cytotoxicity was largely driven by apoptosis. Finally, YKL-cADP adenovirus also showed superior antitumor effect than YKL-1 and YKL-mADP in C33A cervical and Hep3B hepatoma xenograft tumor models. Taken together, these lines of evidence demonstrate that the newly generated adenovirus expressing ADP under the CMV promoter induces efficient but tumor-selective cell lysis, which is critical for adding therapeutic value to replicating adenovirus for its use in cancer gene therapy.

Productive replication of human adenovirus type 5 in canine cells

Development of immunocompetent patient-like models that allow direct analysis of human adenovirus-based conditionally replicative adenoviruses (CRAds) would be beneficial for the advancement of these oncolytic agents. To this end, we explored the possibility of cross-species replication of human adenovirus type 5 (Ad5) in canine cells. With a panel of canine tumor cell lines of both epithelial and mesenchymal derivations, we demonstrate that human Ad5 can productively infect canine cells. Since the biological behavior and clinical presentation of certain dog tumors closely resemble those of their human counterparts, our results raise the possibility of exploiting canine models for preclinical analysis of candidate CRAd agents designed for human virotherapy.

Effective Gene Therapy of Biliary Tract Cancers by a Conditionally Replicative Adenovirus Expressing Uracil Phosphoribosyltransferase: Significance of Timing of 5-Fluorouracil Administration

In order to enhance the efficacy of conditionally replicating adenoviruses (CRAd) in the treatment of cancers of the biliary tract, we studied the efficacy in vitro and in vivo of AxE1CAUP, a CRAd vector that carries a gene for uracil phosphoribosyltransferase (UPRT), which converts 5-fluorouracil (5-FU) directly to 5-fluorouridine monophosphate and greatly enhances the cytotoxicity of 5-FU. AxE1CAUP replicated and induced an increased UPRT expression in biliary cancer cells more efficiently than AxCAUP, a nonreplicative adenovirus carrying the UPRT gene. Whereas AxCAUP and AxE1AdB, a CRAd without the UPRT gene, modestly increased the sensitivity of BC cells to 5-FU, AxE1CAUP markedly increased the sensitivity, especially when the timing of 5-FU administration was appropriately chosen. AxE1CAUP replicated much less efficiently in normal WI-38 fibroblasts without any change in the sensitivity to 5-FU. In nude mice with s.c. biliary cancer xenografts, i.t. AxE1CAUP/5-FU therapy inhibited tumor growth significantly more strongly than AxCAUP/5-FU or AxE1AdB/5-FU therapy. Furthermore, in mice with peritoneally disseminated biliary cancer, i.p. AxE1CAUP efficiently proliferated in the tumors, decreased the tumor burden, and prolonged the survival of the mice when 5-FU was started 10 or 15 days after the vector inoculation, whereas earlier initiation of 5-FU resulted in early eradication of the vector and no survival benefit. The present study shows that the CRAd expressing UPRT was a more potent sensitizer of biliary cancer to 5-FU, than was a nonreplicative UPRT-encoding vector or a CRAd without UPRT gene, even at a lower dose of the vector, and that timing of 5-FU administration was a key factor to maximize the efficacy. This gene therapy with appropriately timed administration of 5-FU should be useful in overcoming the resistance of biliary cancers to 5-FU.

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.

Overexpression of adenovirus E3-11.6K protein induces cell killing by both caspase-dependent and caspase-independent mechanisms

Recent studies have shown enhanced antitumor efficacy with adenoviruses that either lack E1B-19K or overexpress E3-11.6K (also known as adenoviral death protein). E1B-19K is a well-characterized anti-apoptotic protein, and viruses with E1B-19K deletions show increased cytopathicity. However, the mechanism of cell killing by E3-11.6K, which plays an important role in killing infected cells and virion release, is not well characterized. To understand the mechanism of cell killing following E3-11.6K overexpression, we constructed a recombinant adenovirus, Ad-ME, by introducing viral major late promoter upstream of the E3-11.6K sequence. Similar to the E1B-19K-deleted virus, E1B/19K-, Ad-ME induced cell death to a greater extent than the wild-type virus. Cell shrinkage, membrane blebbing, activation of caspases 3 and 9, cleavage of poly(ADP-ribose)polymerase (PARP), DNA degradation, and ratio of ADP to ATP in Ad-ME-infected cells indicated that apoptosis contributes to cell death following E3-11.6K overexpression. However, the levels of activation of caspases 3 and 9 were lower in cells infected with Ad-ME compared to those infected with E1B/19K-. Furthermore, cell killing by Ad-ME was not effectively inhibited by Z-VAD-FMK, a general caspase inhibitor. Taken together, our results suggest both caspase-dependent and caspase-independent mechanisms of cell killing due to overexpression of E3-11.6K.

Combination Therapy with Conditionally Replicating Adenovirus and Replication Defective Adenovirus

Low gene transfer rate is the most substantial hurdle in the practical application of gene therapy. One strategy to improve transfer efficiency is the use of a conditionally replicating adenovirus (CRAD) that can selectively replicate in tumor cells. We hypothesized that conventional E1-deleted adenoviruses (ad) can become replication-competent when cotransduced with a CRAD to selectively supply E1 in trans in tumors. The resulting selective production of large numbers of the E1-deleted ad within the tumor mass will increase the transduction efficiency. We used a CRAD (Delta24RGD) that produces a mutant E1 without the ability to bind retinoblastoma but retaining viral replication competence in cancer cells with a defective pRb/p16. Ad-lacZ, adenovirus-luciferase (ad-luc), and adenovirus insulin-like growth factor-1R/dominant-negative (ad-IGF-1R/dn; 482, 950) are E1-deleted replication-defective adenoviruses. The combination of CRAD and ad-lacZ increased the transduction efficiency of lacZ to 100% from 15% observed with ad-lacZ alone. Transfer of media of CRAD and ad-lacZ cotransduced cells induced the transfer of lacZ (media transferable bystander effect). Combination of CRAD and ad-IGF-1R/dn increased the production of truncated IGF-1R or soluble IGF-1R > 10 times compared with transduction with ad-IGF-1R/dn alone. Combined intratumoral injection of CRAD and ad-luc increased the luciferase expression about 70 times compared with ad-luc alone without substantial systemic spread. Combined intratumoral injection of CRAD and ad-IGF-1R/482 induced stronger growth suppression of established lung cancer xenografts than single injections. The combination of CRAD and E1-deleted ad induced tumor-specific replication of CRAD and E1-deleted ad and increased the transduction rate and therapeutic efficacy of these viruses in model tumors.

Infectivity-enhanced cyclooxygenase-2-based conditionally replicative adenoviruses for esophageal adenocarcinoma treatment

The employment of conditionally replicative adenoviruses (CRAd) constitutes a promising alternative for cancer treatment; however, in the case of esophageal adenocarcinoma (EAC) the lack of an appropriate tumor-specific promoter and relative resistance to adenovirus infection have hampered the construction of CRAds with clinically applicable specificity and efficacy. By combining transcriptional targeting with infectivity enhancement for CRAds, we generated novel cyclooxygenase-2 (Cox-2) promoter-controlled replicative viral agents for the treatment of EAC. We used infectivity enhancement based on incorporation of an RGD-4C motif into the HI loop of the adenoviral (Ad) fiber knob domain as well as replacement of the Ad5 knob with the Ad3 knob. The Cox-2 promoter was highly active in EAC, whereas showing no significant activity in Cox-2-negative cell lines and primary cells isolated from normal mouse esophagus and stomach. Evaluation of infectivity-enhanced vectors revealed that the transduction and virus-cell binding ability of Ad5/Ad3-chimera were significantly more efficient than that of unmodified and Arg-Gly-Asp (RGD)-modified vectors. All of the Cox-2 CRAds demonstrated replication and subsequent oncolysis in EAC cells but not in Cox-2-negative cells in vitro, thus confirming the dependence of their replication on the Cox-2 promoter activity. Ad5/Ad3 CRAds exhibited significantly improved oncolysis and progeny production compared with unmodified and RGD-modified vectors without sacrificing tumor selectivity. Whereas unmodified and RGD-modified CRAds showed insignificant therapeutic effect in vivo, Ad5/Ad3 CRAds remarkably suppressed tumor growth of established xenografts in mice. Thus, our studies have demonstrated that Ad5/Ad3-chimeric Cox-2 promoter-driven CRAds are selective and potent agents for the treatment of EAC.

Tumor-specific gene therapy for uterine cervical cancer using MN/CA9-directed replication-competent adenovirus

Although gene therapies using tissue-specific promoters have been reported to be a promising tool for treating cancers, few studies have explored this possibility for uterine cervical cancer. MN/CA9 is a transmembrane glycoprotein that was first identified in the human cervical carcinoma cell line, HeLa. Since MN/CA9 protein is highly expressed in uterine cervical cancer tissues, but not in normal cervix, we constructed a tumor-specific replication-competent adenoviral vector utilizing MN/CA9 promoter (Ad-MN/CA9-E1a), which can replicate only in MN/CA9-expressing cells. Infection of Ad-MN/CA9-E1a to MN/CA9-positive uterine cervical cancer cells (HeLa, C-33 A and SiHa) resulted in much stronger Ad5 E1a protein expressions compared with MN/CA9-negative cells (SK-RC-29), suggesting a tissue-specific replication of this recombinant adenovirus. In vitro cytotoxicity assay revealed that the growth of MN/CA9-positive cells was significantly inhibited with 0.01-1 MOI of Ad-MN/CA9-E1a, but the growth of MN/CA9-negative cells (SK-RC-29) could only be inhibited by as many as 100 MOI. Intratumoral injection of Ad-MN/CA9-E1a effectively induced growth delay of HeLa tumors in nude mice. These results suggest that a novel replication-competent adenoviral vector mediated by MN/CA9 promoter, Ad-MN/CA9-E1a, can selectively replicate in MN/CA9-expressing tumors with cytotoxic effects and may be utilized for the treatment of uterine cervical cancer.

A Novel Conditionally Replicative Adenovirus Vector Targeting Telomerase-Positive Tumor Cells

PURPOSE

To develop a novel conditionally replicative adenovirus vector that targets telomerase-positive cancer cells.

EXPERIMENTAL DESIGN

A telomerase gene-derived promoter was used to control the expression of the E1a gene so that the E1a gene is only expressed in telomerase-positive tumor cells. In addition, a reporter gene was also engineered into the vector so that its infection and replication can be monitored easily.

RESULTS

A novel recombinant adenovirus vector that could selectively replicate in telomerase-positive cancer cells was made successfully. This vector showed active replication in a panel of cancer cells and minimal replication in normal human fibroblast or epithelial cells. The recombinant vector could effectively lyse various cultured tumor cells even at very low multiplicity of infection. The replication efficiency in tumor cells is over 10(3)-fold more than normal fibroblast and epithelial cells. In s.c. tumor models, the newly developed telomerase-selective adenovirus vectors exhibited significantly more virus replication and reporter gene expression.

CONCLUSIONS

The telomerase-targeted adenovirus vector has significant potential as an oncolytic virus as well as a tumor-specific therapeutic gene delivery vehicle.

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.

E3 gene manipulations affect oncolytic adenovirus activity in immunocompetent tumor models

Oncolytic replication-selective adenoviruses constitute a rapidly growing therapeutic platform for cancer. However, the role of the host immune response and the E3 immunoregulatory genes of the human adenovirus were unknown until now. We identified four mouse carcinoma lines of variable permissivity for adenoviral gene expression, cytopathic effects and/or burst size. To determine E3 gene effects in immunocompetent tumor-bearing hosts, we injected tumors with one of three adenoviruses: Ad5 (E3 wild type), dl309 (del. E3 10.4/14.5, 14.7 kDa) or dl704 (del. E3 gp19 kDa). Compared with Ad5 and dl704, dl309 was cleared much more rapidly and/or its activity was lower in all four models. Intratumoral injection with dl309 resulted in markedly greater macrophage infiltration and expression of both tumor necrosis factor and interferon-gamma. Adenovirus replication, CD8(+) lymphocyte infiltration and efficacy were similar upon intratumoral injection with either dl704 or Ad5. E3-dependent differences were not evident in athymic mice. These findings have important implications for the design of oncolytic adenoviruses and may explain the rapid clearance of E3-10.4/14.5,14.7-deleted adenoviruses in patients.

Novel immunocompetent murine tumor models for the assessment of replication-competent oncolytic adenovirus efficacy

Oncolytic replication-selective adenoviruses constitute a rapidly expanding experimental approach to the treatment of cancer. However, due to the lack of an immunocompetent and replication-competent efficacy model, the role of the host immune response and viral E3 immunoregulatory genes remained unknown. We screened nine murine carcinoma lines for adenovirus (Ad5) uptake, gene expression, replication, and cytopathic effects. In seven of these murine cell lines the infectability and cytopathic effects were similar to those seen with human carcinoma lines. Surprisingly, productive viral replication was demonstrated in several lines; replication varied from levels similar to those for some human carcinoma lines (e.g., CMT-64) to very low levels. Seven of these lines were grown as subcutaneous xenografts in immunocompetent mice and were subsequently injected directly with Ad5, saline, or a replication-deficient control adenovirus particle to assess intratumoral viral gene expression, replication, and antitumoral effects. E1A, coat protein expression, and cytopathic effects were documented in five xenografts; Ad5 replication was demonstrated in CMT-64 and JC xenografts. Ad5 demonstrated significant efficacy compared to saline and nonreplicating control Ad particles in both replication-permissive xenografts (CMT-64, JC) and poorly permissive tumors (CMT-93); efficacy against CMT-93 tumors was significantly greater in immunocompetent mice compared to athymic mice. These murine tumor xenograft models have potential for elucidating viral and host immune mechanisms involved in oncolytic adenovirus antitumoral effects.

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.

Regulation of mRNA production by the adenoviral E1B 55-kDa and E4 Orf6 proteins

The E1B 55-kDa and E4 Orf6 proteins of human subgroup C adenoviruses both counter host cell defenses mediated by the cellular p53 protein and regulate viral late gene expression. A complex containing the two proteins has been implicated in induction of selective export of viral late mRNAs from the nucleus to the cytoplasm, with concomitant inhibition of export of the majority of newly synthesized cellular mRNAs. The molecular mechanisms by which these viral proteins subvert cellular pathways of nuclear export are not yet clear. Here, we review recent efforts to identify molecular and biochemical functions of the E1B 55-kDa and E4 Orf6 proteins required for regulation of mRNA export, the several difficulties and discrepancies that have been encountered in studies of these viral proteins, and evidence indicating that the reorganization of the infected cell nucleus and production of viral late mRNA at specific intra-nuclear sites are important determinants of selective mRNA export in infected cells. In our view, it is not yet possible to propose a coherent molecular model for regulation of mRNA export by the E1B 55-kDa and E4 Orf6 proteins. However, it should now be possible to address specific questions about the roles of potentially relevant properties of these viral proteins.

Progress and problems with the use of viral vectors for gene therapy

Gene therapy has a history of controversy. Encouraging results are starting to emerge from the clinic, but questions are still being asked about the safety of this new molecular medicine. With the development of a leukaemia-like syndrome in two of the small number of patients that have been cured of a disease by gene therapy, it is timely to contemplate how far this technology has come, and how far it still has to go.

Dr. Gary J. Becker Young Investigator Award: intraarterial adenovirus for metastatic gastrointestinal cancer: activity, radiographic response, and survival

PURPOSE

To determine the antitumoral activity and radiographic response pattern of intraarterial administration of a selective replication-competent adenovirus in patients with hepatic metastases from gastrointestinal carcinomas.

MATERIALS AND METHODS

Thirty-five patients were treated, seven in the dose-escalation phase and 28 at high doses. Inclusion criteria allowed mild laboratory value and performance status abnormalities and as much as 50% replacement of hepatic volume by tumor. An attenuated adenovirus that selectively replicates in p53-deficient cells (Onyx-015) was administered by hepatic arterial infusion at doses as high as 2 x 10(12) particles for two cycles. Subsequent cycles (maximum of eight total) were administered in combination with intravenous 5-fluorouracil (5-FU) and leucovorin.

RESULTS

Tumor responses were demonstrated in combination with chemotherapy, even in 5-FU-resistant patients. The 15 patients who responded radiographically showed a pattern of acute tumor enlargement despite normalization of laboratory and clinical parameters, followed by very slow regression of tumor size. Radiographic response did not correlate with p53 status. Median survival of radiographic responders (475 days) was significantly longer than that of nonresponders (143 days).

CONCLUSIONS

Hepatic arterial infusion of the replication-selective adenovirus Onyx-015 in combination with chemotherapy resulted in tumor regressions in select patients, including some in whom previous chemotherapy had failed. A biphasic radiographic response pattern was demonstrated. The mechanism of action appears to be more complex than that seen in vitro.