Monthly Update: Oncologic, Endocrine & Metabolic: Replicating oncolytic viruses: An overview

Herpesviruses: hijacking the Ras signaling pathway

Cancer is the final result of the accumulation of several genetic alterations occurring in a cell. Several herpesviruses and especially gamma-herpesviruses have played an important role in Cancer Biology, contributing significantly to our comprehension of cell signaling and growth control pathways which lead to malignancy. Unlike other infectious agents, herpesviruses persist in the host by establishing a latent infection, so that they can reactivate periodically. Interestingly, some herpesviruses are able to either deliver or induce the expression of cellular oncogenes. Such alterations can result in the derailment of the normal cell cycle and ultimately shift the balance between continuous proliferation and programmed cell death. Herpesvirus infection employs key molecules of cellular signaling cascades mostly to enhance viral replication. However, most of these molecules are also involved in essential cellular functions, such as proliferation, cellular differentiation and migration, as well as in DNA repair mechanisms. Ras proteins are key molecules that regulate a wide range of cellular functions, including differentiation, proliferation and cell survival. A broad field of medical research is currently focused on elucidating the role of ras oncogenes in human tumor initiation as well as tumor progression and metastasis. Upon activation, Ras proteins employ several downstream effector molecules such as phosphatidylinositol 3-kinase (PI3-K) and Raf and Ral guanine nucleotide-dissociation stimulators (RALGDS) to regulate a cascade of events ranging from cell proliferation and survival to apoptosis and cellular death. In this review, we give an overview of the impact that herpesvirus infection has on the host-cell Ras signaling pathway, providing an outline of their interactions with the key cascade molecules with which they associate. Several of these interactions of viral proteins with member of the Ras signaling pathway may be crucial in determining herpesviruses' oncogenic potential or their oncomodulatory behavior. The questions that emerge concern the potential role of these molecules as therapeutic targets both for viral infections and cancer. Understanding the means by which viruses may cause oncogenesis would therefore provide a deeper knowledge of the overall oncogenic process.

Oncolysis of pancreatic tumour cells by a gamma34.5-deleted HSV-1 does not rely upon Ras-activation, but on the PI 3-kinase pathway

The ability of viruses to selectively target, replicate within, and destroy tumour cells without deleterious effects in normal cells (oncolysis), makes the use of viruses as an attractive tool for cancer treatment. Pancreatic adenocarcinoma, being insensitive to traditional therapy and having a rather poor prognosis, represents a suitable target to evaluate viral oncolysis as a novel therapeutic approach. Herpes simplex virus (HSV) has been reported to produce an oncolytic effect in cells overexpressing Ras. As Ras signalling is frequently aberrant in pancreatic cancer, we compared four pancreatic cell lines (which differ in the presence of mutated or wild-type ras) for their ability to support growth of gamma34.5-replication attenuated HSV-1 (R3616). Our data show that permissiveness to viral replication is neither associated with enhanced Ras signalling nor with defective PKR activity. By contrast, we provide evidence that disregulation of the PI 3-kinase signalling pathway allows conditionally replication-defective R3616 virus to overcome the cellular antiviral activity.

Oncolytic viral therapy for breast cancer with herpes simplex virus type 1 mutant HF 10

BACKGROUND AND OBJECTIVES

Many genetically engineered viruses have been evaluated for their potential as therapeutic agents in the treatment of malignant tumors. We applied a spontaneously generated, highly attenuated herpes simplex virus (HSV) type-1 clone, HF10, to the treatment of breast cancer. In this study, we investigated the ability of HF10 to infect and lyse human and murine breast cancer cells in vitro and tested its efficacy in an immuno-competent animal model of breast cancer.

METHODS

To assess the therapeutic efficacy of HF10 against subcutaneous tumors in vivo, mouse breast cancer cells were injected into the backs of mice, which were then treated with HF10. Tumor volume and survival rate were used as measures of the antitumor effect in the in vivo experiments. In vitro viral cytotoxity assays and replication assays were also performed in human breast cancer cell lines.

RESULTS

In the in vivo study, tumor growth was suppressed and long-term survival rates were prolonged. HF10 was effective in producing cytolytic effects in vitro at various multiplicities of infection (MOI) in all cell lines tested.

CONCLUSIONS

HF10 demonstrated antitumor effects in our animal model. The viral growth and oncolytic effect of HF10 in the human breast cancer cell line suggest that HF10 is potentially effective in the clinical treatment of human cancer. These results indicate that replication-competent HSV-1 mutants hold significant promise as cancer therapeutic agents.

Viral oncolysis for malignant liver tumors

Viral oncolysis represents a unique strategy to exploit the natural process of viral replication to kill tumor cells. Although this concept dates back nearly a century, recent advances in the fields of molecular biology and virology have enabled investigators to genetically engineer viruses with greater potency and tumor specificity. In this article we review the general mechanisms by which oncolytic viruses achieve their antineoplastic efficacy and specificity. We focus on the development of several classes of oncolytic viruses for the treatment of malignant liver tumors, including adenoviruses, vaccinia viruses, and herpes simplex viruses, and discuss the results of clinical trials for these viruses. We also describe results from our laboratory research program, which is focused on developing effective, liver tumor-specific Herpes simplex virus 1 mutants.

Viral oncolysis

The concept of using replicating viruses as anticancer agents is not a new one, but the ability to genetically modify these viruses into increasingly potent and tumor-specific vectors is a recent phenomenon. As more is learned about the functions of viral gene products in controlling the mammalian cell cycle and in disabling cellular defense mechanisms, specific viral functions can be augmented or eliminated to enhance antineoplastic efficacy. In this article, general mechanisms by which oncolytic viruses achieve their antitumor efficacy and specificity are reviewed. The paradoxical roles of the immune response are addressed with respect to oncolytic viral therapy, as it, on one hand, impedes the spread of viral infection, and on the other, augments tumor cell destruction through the recruitment of T cells "vaccinated" against tumor antigens. The most commonly used oncolytic viruses are each reviewed in turn, including adenoviruses, herpes simplex viruses, vaccinia viruses, reoviruses, and Newcastle disease viruses. Special attention is focused on the unique biology of each of these viruses as well as the status of several of these mutants in clinical trials.

Intra-arterial administration of a replication-selective adenovirus (dl1520) in patients with colorectal carcinoma metastatic to the liver: a phase I trial

Both replication-incompetent and replication-selective adenoviruses are being developed for the treatment of cancer and other diseases. Concerns have been raised about the safety of intra-vascular adenovirus administration following a patient death on a clinical trial with a replication-defective adenovirus. In addition, the feasibility of vascular delivery to distant tumors has been questioned. dl1520 (ONYX-015) is a replication-selective adenovirus that has previously shown safety and antitumoral activity following intratumoral injection. This is the first report of intra-vascular administration with a genetically engineered, replication-selective virus. A phase I dose-escalation trial was performed in patients with liver-predominant gastrointestinal carcinoma (n = 11 total; primarily colorectal). dl1520 was infused into the hepatic artery at doses of 2 x 10(8)-2 x 10(1)2 particles for two cycles (days 1 and 8). Subsequent cycles of dl1520 were administered in combination with intravenous 5-fluorouracil (5-FU) and leucovorin. No dose-limiting toxicity, maximally tolerated dose or treatment-emergent clinical hepatotoxicity were identified following dl1520 infusion. Mild to moderate fever, rigors and fatigue were the most common adverse events. Antibody titers increased significantly in all patients. Viral replication was detectable in patients receiving the highest two doses. An objective response was demonstrated in combination with chemotherapy in a patient who was refractory to both 5-FU and dl1520 as single agents. Therefore, hepatic artery infusion of the attenuated adenovirus dl1520 was well-tolerated at doses resulting in infection, replication and chemotherapy-associated antitumoral activity.

ONYX-015, an E1B gene-attenuated adenovirus, causes tumor-specific cytolysis and antitumoral efficacy that can be augmented by standard chemotherapeutic agents

The 55-kilodalton (kDa) protein from the E1B-region of adenovirus binds to and inactivates the p53 gene, which is mutated in half of human cancers. We have previously shown that the replication and cytopathogenicity of an E1B, 55-kDa gene-attenuated adenovirus, ONYX-015, is blocked by functional p53 in RKO and U20S carcinoma lines. We now report that normal human cells were highly resistant to ONYX-015-mediated, replication-dependent cytolysis. In contrast, a wide range of human tumor cells, including numerous carcinoma lines with either mutant or normal p53 gene sequences (exons 5-9), were efficiently destroyed. Antitumoral efficacy was documented following intratumoral or intravenous administration of ONYX-015 to nude mouse-human tumor xenografts; efficacy with ONYX-015 plus chemotherapy (cisplatin, 5-fluorouracil) was significantly greater than with either agent alone.