Effect of prior exposure to herpes simplex virus 1 on viral vector-mediated tumor therapy in immunocompetent mice

Prodrug bioactivation and oncolysis of diffuse liver metastases by a herpes simplex virus 1 mutant that expresses the CYP2B1 transgene

BACKGROUND

Herpes simplex virus 1 (HSV-1) infection of cancer cells results in viral replication with cell destruction and liberation of progeny virion that infect adjacent tumor cells. rRp450 is a novel replication-conditional HSV-1 mutant that expresses both the endogenous herpes viral thymidine kinase gene and the rat p450 CYP2B1 transgene; p450 bioactivates such cancer prodrugs as cyclophosphamide.

METHODS

Viral cytotoxicity and replication assays were performed in colon carcinoma cells as well as primary human hepatocytes. For in vivo studies, diffuse liver metastases were generated by inoculating MC26 cells into the portal system of BALB/c mice. Mice were treated with control media, rRp450, or rRp450 plus cyclophosphamide.

RESULTS

Cytopathic effects induced by rRp450 replication in colon carcinoma cells were equivalent to those induced by wild type HSV-1 in vitro. Assays developed to separate cytotoxicity mediated by viral replication from cytotoxicity mediated by chemotherapy confirmed that HSV-1 thymidine kinase bioactivates ganciclovir and CYP2B1 bioactivates cyclophosphamide in rRp450-infected cells. rRp450 mediated cytotoxicity in the presence of cyclophosphamide was increased by 21% to 30% above that achieved by viral replication alone. Cyclophosphamide bioactivation produced bystander killing of colon carcinoma cells but not hepatocytes. In contrast to these effects of cyclophosphamide, rRp450 mediated cytotoxicity was reduced in the presence of ganciclovir. These findings are explained by further experiments showing that bioactivation of cyclophosphamide only minimally affected HSV-1 replication in colon carcinoma cells, whereas bioactivation of ganciclovir markedly attenuated HSV-1 replication. In vivo studies revealed a substantial decrease in hepatic tumor burden in all rRp450-treated animals compared to controls. The addition of cyclophosphamide augmented rRp450's in vivo anti-neoplastic effect.

CONCLUSIONS

The rRp450 mutant HSV-1 is highly oncolytic against colon carcinoma cells both in vitro and in vivo. rRp450 displays preferential replication in colon carcinoma cells compared to normal hepatocytes. Activation of cyclophosphamide by the p450 transgene augmented the anti-neoplastic effects of rRp450 without simultaneously decreasing viral replication. Oncolysis induced by HSV-1 replication combined with cyclophosphamide prodrug activation warrants further investigation as a potential therapy for colon carcinoma liver metastases.

Oncolytic virus therapy using genetically engineered herpes simplex viruses

An increasing number of oncolytic virus vectors has been developed lately for cancer therapy. Herpes simplex virus type 1 (HSV-1) vectors are particularly useful, because they can be genetically engineered to replicate and spread highly selectively in tumor cells and can also express multiple foreign transgenes. These vectors can manifest cytopathic effect in a wide variety of tumor types without damaging normal tissues, provide amplified gene delivery within the tumor, and induce specific antitumor immunity. Multiple recombinant HSV-1 vectors have been tested in patients with brain tumors and other cancers, which showed the feasibility of administering replication-competent HSV-1 vectors safely in human organs including the brain. Different approaches are currently undertaken to improve the efficacy of oncolytic HSV-1 therapy which include development of new generation vectors via further genetic engineering of existing safe vectors, combination with immune gene therapy, and combination with conventional therapies. Oncolytic virus therapy is a promising therapeutic modality that awaits establishing as an important treatment option for cancer patients in the near future.

Oncolytic herpesvirus effectively treats murine squamous cell carcinoma and spreads by natural lymphatics to treat sites of lymphatic metastases

Oncolytic herpesviruses have significant antitumoral effects in animal models when delivered directly to established tumors. Lymphatic metastases are a common occurrence for many tumor types. This study investigates the potential of an attenuated, replication-competent, oncolytic herpes simplex virus (NV1023) both to treat a primary tumor by direct injection and to travel through the lymphatic system to treat metastatic tumor within the lymph nodes draining lymph from the site of primary cancer. Isosulfan blue dye was injected into murine auricles to determine normal lymphatic drainage patterns and demonstrated consistent blue staining of a group of ipsilateral cervical lymph nodes. Auricular injections of NV1023 resulted in viral transit to these lymph nodes as measured by 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside histochemistry and viral plaque assay. An oncolytic herpesvirus (NV1066) expressing green fluorescent protein also demonstrated viral transit from the auricle to the cervical lymph nodes on fluorescence microscopy. Using the SCC VII cell line, a novel murine model of auricular squamous cell carcinoma was developed with an approximately 20% incidence of cervical lymph node metastases. Delivery of NV1023 or NV1066 to the surgical beds after excision of auricular SCC VII tumors resulted in successful viral infection of metastatic SCC VII cells within the cervical lymph nodes. After a 7-week follow-up, significantly enhanced locoregional control (p < 0.05, Fisher exact test) and disease-free survival (p < 0.05, log rank test) were evident with NV1023 treatment. This study demonstrates that the delivery of an oncolytic herpesvirus to a primary tumor site after surgical excision may have a significant impact on reducing both primary site recurrence and regional nodal metastases.

Expression of human immunodeficiency virus type 1 gp120 from herpes simplex virus type 1-derived amplicons results in potent, specific, and durable cellular and humoral immune responses

Herpes simplex virus type 1 (HSV-1) infects a wide range of cells, including dendritic cells. Consequently, HSV-1 vectors may be capable of eliciting strong immune responses to vectored antigens. To test this hypothesis, an HSV-1 amplicon plasmid encoding human immunodeficiency virus type 1 gp120 was constructed, and murine immune responses to helper virus-free amplicon preparations derived from this construct were evaluated. Initial studies revealed that a single intramuscular (i.m.) injection of 10(6) infectious units (i.u.) of HSV:gp120 amplicon particles (HSV:gp120) elicited Env-specific cellular and humoral immune responses. A potent, CD8(+)-T-cell-mediated response to an H-2D(d)-restricted peptide from gp120 (RGPGRAFVTI) was measured by a gamma interferon ELISPOT and was confirmed by standard cytotoxic-T-lymphocyte assays. Immunoglobulin G enzyme-linked immunosorbent assay analysis showed the induction of a strong, Env-specific antibody response. An i.m. or an intradermal administration of HSV:gp120 at the tail base elicited a more potent cellular immune response than did an intraperitoneal (i.p.) inoculation, although an i.p. introduction generated a stronger humoral response. The immune response to HSV:gp120 was durable, with robust cellular and humoral responses persisting at 171 days after a single 10(6)-i.u. inoculation. The immune response to HSV:gp120 was also found to be dose dependent: as few as 10(4) i.u. elicited a strong T-cell response. Finally, HSV:gp120 elicited significant Env-specific cellular immune responses even in animals that had been previously infected with wild-type HSV-1. Taken together, these data strongly support the use of helper-free HSV-1 amplicon particles as vaccine delivery vectors.

Herpes simplex virus vectors elicit durable immune responses in the presence of preexisting host immunity

Herpes simplex virus (HSV) recombinants are being developed as vaccine vectors for the expression of heterologous antigens. There is concern, however, that preexisting HSV immunity may decrease their effectiveness. We have addressed this issue in an animal model. Immunized mice were inoculated with a replication-defective HSV-1 vector that expressed the Escherichia coli beta-galactosidase protein as a model antigen. We assessed vector efficacy by analyzing the immunoglobulin G (IgG) antibody response and cellular proliferative response directed against beta-galactosidase. We report that the ability of the vector to induce antibody or proliferative responses was not diminished by preexisting immunity to HSV. Of further note, the anti-HSV and anti-beta-galactosidase IgG responses following vector administration were extremely durable in both immunized and naive mice. These results indicate that the ability of a replication-defective HSV-derived vaccine vector to elicit long-lived immune responses in mice is not impaired by prior HSV exposure.

The complement response against an oncolytic virus is species-specific in its activation pathways

A variety of oncolytic viruses (OVs) are being tested in clinical trials for different human cancers. Although the innate immune response is critical as the first line of defense in thwarting viral infection of mammalian cells, little is known of this response in the context of OV therapy of tumors. Investigations of activities against a herpes simplex OV demonstrated that HSV-seronegative sera from rats, mice, and humans efficiently neutralize this OV in vitro. Although this neutralization is due to complement, activation of this innate host defense differs in its pathways among species routinely used in preclinical tumor trials. In rats, both natural immunoglobulins and mannan-binding lectin (MBL) activate complement against the OV, while in mice only MBL is relevant to this activation. However, in humans only natural immunoglobulins play a role in complement activity. Quantitative analyses confirm that in vivo complement depletion facilitates the initial infection of tumors by systemic OVs. Therefore, complement activation against oncolytic HSV vectors proceeds through different pathways in different species. These findings are relevant to preclinical rodent studies of OV therapy and their application to human clinical trials.

Delivery of herpes simplex virus vectors through liposome formulation

Viral vectors have been widely used as gene delivery vehicles for both experimental and clinical investigations. Although these vectors are capable of achieving high gene transduction efficiency in vitro, one of the major limitations facing the therapeutic viral vectors is that the preexisting host anti-vector immunity can substantially reduce their transduction efficiency in vivo. This is especially of concern when the therapeutic remedy requires repeated systemic administration. Here we report the delivery of herpes simplex virus (HSV) derived vectors through liposome formulation. In these studies, we have prepared HSV vectors in three different forms for liposome formulation: purified viral DNA (obtained from a bacterial artificial chromosome containing an infectious HSV genome), HSV capsids, and intact viral particles. All three forms of HSV were readily transfected into cultured cells and infectious virus was efficiently generated. Furthermore, introduction of HSV vectors as DNA/liposome complexes improved in vivo transduction efficiency, by effectively evading the host anti-HSV immunity during systemic administration. We conclude that viral vectors such as HSV can be systemically delivered through liposome formulation for safe and repeated administration for gene transduction or oncolytic purposes.

Oncolytic viruses as therapeutic agents

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

Therapeutic efficacy of G207 in a novel peripheral nerve sheath tumor model

Nerve involvement poses a significant obstacle for the management of peripheral nervous system tumors, and nerve injury provides a frequent source of postoperative morbidity. The lack of suitable animal models for peripheral nerve tumors has impeded the development of alternative nerve-sparing therapies. To evaluate the effect of a multimutated replication-competent herpes simplex virus (G207) on the growth of peripheral nerve tumors and on nerve function, we developed a novel peripheral nerve sheath tumor model. Human neuroblastoma-derived cells injected into murine sciatic nerve consistently caused tumor development within the nerve sheath after 2 weeks followed by increasingly severe impairment of nerve function. Tumor treatment by a single intratumoral injection of G207 resulted in significant reduction of functional impairment, inhibition of tumor growth and prolonged survival. Direct injection of G207 viral particles into the healthy nerve sheath caused no obvious neurologic sequelae, whereas injections of wild-type virus resulted in uniform lethality. The results indicate that viral therapy might be considered as a safe alternative to surgical removal of tumors with peripheral nerve involvement.

Combination suicide/cytokine gene therapy as adjuvants to a defective herpes simplex virus-based cancer vaccine

We have used syngeneic, established bilateral subcutaneous tumor models to examine the antitumor activity of herpes simplex virus (HSV) vectors, including the induction of an immune response against non-inoculated distant tumors. In such a model with CT26 murine colon adenocarcinoma, unilateral intratumoral inoculation of replication-deficient HSV-1 tsK inhibited the growth of both the inoculated and noninoculated established tumors. To enhance this limited antitumor immune response, we generated a defective HSV vector, dvIL12-tk encoding both interleukin-12 (IL-12) and HSV thymidine kinase (TK), with tsK as the helper virus. In a 'suicide gene' strategy, ganciclovir (GCV) treatment after intratumoral inoculation of dvlacZ-tk/tsK, encoding E. coli lacZ instead of IL-12, resulted in enhanced antitumor activity. Antitumor activity was also enhanced by local expression of IL-12 from dvIL12-tk/tsK. The combination of IL-12 cytokine therapy with GCV treatment was the most efficacious approach, with significantly greater inhibition of tumor growth than IL-12 or TK + GCV alone. These results illustrate the power of combining different cancer therapy approaches; 'suicide gene' therapy, cytokine therapy, and HSV vector infection. HSV vectors are particularly well suited to this because they can accommodate the insertion of large and multiple gene sequences.

Effects of preexisting immunity on the response to herpes simplex-based oncolytic viral therapy

Herpes simplex viruses (HSV) type 1 are the basis of a number of anticancer strategies that have proven efficacious in animal models. They are natural human pathogens and the majority of adults have anti-HSV immunity. The current study examined the effect of preexisting immunity on the response to herpes-based oncolytic viral treatment of hepatic metastatic cancer in a murine model designed to simulate a clinical approach likely to be utilized for nonneurological tumors. Specifically, the anticancer effects of NV1020 or G207, two multimutated HSV-1 oncolytic viruses, were tested in immunocompetent mice previously immunized with a wild-type herpes simplex type 1 virus. Mice were documented to have humoral as well as cell-mediated immunity to HSV-1. Tumor response to oncolytic therapy was not measurably abrogated by immunity to HSV at the doses tested. The influence of route of viral administration was also tested in models of regional hepatic arterial and intravenous therapy. Route of viral administration influenced efficacy, as virus delivered intravenously produced some detectable attenuation while hepatic arterial therapy remained unaffected. These results demonstrate that when given at appropriate doses and in reasonable proximity to tumor targets, HSV-based oncolytic therapy can still be expected to be effective treatment for patients with hepatic malignancies.

Oncolytic viral therapy for human prostate cancer by conditionally replicating herpes simplex virus 1 vector G207

Over the last few years, a conditionally replicating herpes simplex virus 1 (HSV-1) vector, G207 has been used for the treatment of several malignant tumors. In this article we evaluate the anti-tumoral effect of G207 against prostate cancer in vitro and in vivo. The susceptibility of the human prostate cancer cell lines, DU145 and PC3 to G207 at a multiplicity of infection (MOI) of 0.1 was examined. In addition, the growth characteristics of G207 were assessed. Athymic mice with s.c. tumors were inoculated in vivo intraneoplastically with 1 x 10(7) plaque-forming units (PFU) of G207. For the pathological analyses, s.c. tumors were stained with X-gal. DU145 and PC3 were efficiently destroyed by G207 within 7 days. The viral yields of G207 increased time-dependently. In vivo, the intraneoplastic inoculation of G207 induced a significant inhibition of the tumor growth. The mean tumor growth ratio was significantly inhibited in the G207-treated tumors (DU145, P < 0.0001; PC3, P < 0.001 versus controls). In a pathological study, many lacZ-positive cells were diffusely present in the G207-treated tumors. G207 showed a significant antitumoral effect against human prostate cancer cell lines, and thus may be considered a useful agent for the treatment of prostate cancer.

Effect of preexisting anti-herpes immunity on the efficacy of herpes simplex viral therapy in a murine intraperitoneal tumor model

HSV-1716, a replicating nonneurovirulent herpes simplex virus type 1, has shown efficacy in treating multiple types of human tumors in immunodeficient mice. Since the majority of the human population has been previously exposed to herpes simplex virus, the efficacy of HSV-based oncolytic therapy was investigated in an immunocompetent animal tumor model. EJ-6-2-Bam-6a, a tumor cell line derived from h-ras-transformed murine fibroblast, exhibit a diffuse growth pattern in the peritoneal cavity of BALB/c mice and replicate HSV-1716 to titers observed in human tumors. An established intraperitoneal (ip) tumor model of EJ-6-2-Bam-6a in naive and HSV-immunized mice was used to evaluate the efficacy of single or multiple ip administrations of HSV-1716 (4 x 10(6) pfu/treatment) or of carrier cells, which are irradiated, ex vivo virally infected EJ-6-2-Bam-6a cells that can amplify the viral load in situ. All treated groups significantly prolonged survival versus media control with an approximately 40% long-term survival rate (cure) in the multiply treated, HSV-naive animals. Prior immunization of the mice with HSV did not significantly decrease the median survival of the single or multiply treated HSV-1716 or the carrier cell-treated groups. These studies support the development of replication-selective herpes virus mutants for use in localized intraperitoneal malignancies.

Intravesical and intravenous therapy of human bladder cancer by the herpes vector G207

G207, a conditionally replicating herpes vector, efficiently kills human bladder cancer cells in vitro. To evaluate the therapeutic potential of G207, we have established three in vivo models similar to the clinical situation. In vivo, G207 was intraneoplastically, intravesically, or intravenously inoculated in nude mice. Intraneoplastic inoculation into subcutaneous tumor caused significant tumor growth inhibition. Intravesical inoculation of G207 also caused decreased tumor growth in an orthotopic human bladder cancer model. Furthermore, multiple intravenous inoculation markedly inhibited subcutaneous tumor growth. These results suggest that intravesical therapy with G207 is effective for localized bladder tumor, especially for carcinoma in situ (CIS), and intravenous therapy with G207 is promising for invasive or metastasized bladder tumor.

Replicating herpes simplex virus vectors for cancer gene therapy

Attenuated viral vectors based on herpes simplex virus (HSV) are capable of killing cancer cells directly while sparing normal tissue in animal models of disease. This selective ability is likely due to the evolutionary constraints on the virus to establish lifelong infection in its host without causing destruction of normal tissues. However, extensive experimental animal data show that cancer cells are able to sustain a productive viral infection, which ultimately leads to cell death and tumour regression. Moreover, preliminary results generated in two Phase I clinical studies of modified replicating HSV for the treatment of brain tumours (e.g., glioblastoma multiforme) have been encouraging and suggest that the safety data generated in animals are predictive of human safety. Although much progress has been made in developing oncolytic HSV vectors for clinical use, there is still a long way to go to determine which combinations of virus, surgery, radiation and chemotherapy will provide improved therapy for the control and eradication of a variety of human cancers.

Oncolytic viruses as novel anticancer agents: turning one scourge against another

Although the use of viruses as oncolytic agents is an historic concept, the use of genetically modified viruses to selectively target tumour cells is relatively novel and recent. The ability of viruses to efficiently infect and lyse cells, combined with the potential augmentation of this effect by progeny viruses throughout the tumour provide justification for exploitation of these agents in cancer therapy. Before application to humans, though, issues related to tumour cell selectivity, lack of toxicity to normal tissues and the effect of the antiviral immune response, will have to be clarified. The more commonly used oncolytic viruses are based on mutant strains of herpes simplex virus, adenovirus and reovirus. The tumour selectivity of each of these strains is discussed, particularly the complementation of the viral defect by cellular pathways involved in tumourigenesis. The combination of oncolytic viruses with radiation, chemotherapy and gene therapy is also reviewed. Further study of the interaction of viral proteins with cellular pathways involved in cell cycle control will provide the rationale for viral mutants with increased selectivity for tumour cells.

Systemic antitumor immunity in experimental brain tumor therapy using a multimutated, replication-competent herpes simplex virus

Replication-competent, attenuated herpes simplex virus (HSV) vectors have been developed for viral oncolytic therapy of primary and metastatic malignant brain tumors. However, the role of the host immune responses in the brain has not been elucidated. N18 neuroblastoma cells were used as a tumor model in syngeneic A/J mice to test the therapeutic efficacy of G207, a conditionally replicating HSV vector, in an immunocompetent condition. G207 inoculated intraneoplastically exhibited a prominent oncolytic antitumor effect in mice harboring N18 tumors in the brain or subcutaneously, and, in addition, elicited a systemic antitumor immune response. Subcutaneous tumor therapy with G207 caused regression of a remote, established tumor in the brain or in the periphery, which was potentially mediated by the systemic antitumor immune response, and provided persistent tumor-specific protection against N18 tumor rechallenge in the brain as well as in the periphery. Antitumor immunity was associated with an elevation of specific CTL activity against N18 tumor cells that persisted for at least 13 months. The results suggest that the oncolytic antitumor action of replication-competent HSV may be augmented by induction of specific and systemic antitumor immunity effective both in the periphery and in the brain.

Corticosteroid administration does not affect viral oncolytic activity, but inhibits antitumor immunity in replication-competent herpes simplex virus tumor therapy

A multimutated, conditionally replicating herpes simplex virus type 1, G207, has been developed as an effective means of treating human malignant brain tumors. We have shown that intraneoplastic inoculation of G207 induces a specific and systemic antitumor immune response that plays an important role in the antitumor activity, in addition to the direct oncolytic action of G207. Since a large number of malignant brain tumor patients are treated with corticosteroids, it is important to evaluate whether the therapeutic efficacy of G207 is affected by corticosteroid-induced immunosuppression. For a tumor model, we used G207-permissive N18 murine neuroblastoma cells implanted subcutaneously in syngeneic A/J mice. Intraneoplastic inoculation of G207 (10(7) PFU) induced significant suppression of tumor growth whether or not dexamethasone (5 mg/kg) was given. When dexamethasone was given for an extensive time (16 days starting on day -2), all G207-treated mice showed tumor growth despite initial shrinkage, whereas in the saline group, four of eight of the G207-treated mice were cured. Dexamethasone administration significantly reduced serum neutralizing antibodies against G207 at 14 and 21 days after intraneoplastic G207 inoculation. However, there was no difference between the dexamethasone and saline groups in terms of the amount of infectious G207 isolated from tumors. Dexamethasone administration completely abolished G207-induced cytotoxic T lymphocyte activity against N18 cells. These results indicate that the oncolytic activity of G207 is retained under corticosteroid administration. However, intensive immunosuppression may diminish the long-term efficacy of G207 owing to suppression of tumor-specific cytotoxic T lymphocyte induction.