Replication-competent Herpes Simplex Virus Type Research Articles

212. Phase I Trial of Intratumoral (IT) Administration of HF10, a Replication-Competent Herpes Simplex Virus Type 1, in Patients with Refractory Superficial Cancer

212. Phase I Trial of Intratumoral (IT) Administration of HF10, a Replication-Competent Herpes Simplex Virus Type 1, in Patients with Refractory Superficial Cancer

Immunization with a highly attenuated replication-competent herpes simplex virus type 1 mutant, HF10, protects mice from genital disease caused by herpes simplex virus type 2

Genital herpes is an intractable disease caused mainly by herpes simplex virus (HSV) type 2 (HSV-2), and is a major concern in public health. A previous infection with HSV type 1 (HSV-1) enhances protection against primary HSV-2 infection to some extent. In this study, we evaluated the ability of HF10, a naturally occurring replication-competent HSV-1 mutant, to protect against genital infection in mice caused by HSV-2. Subcutaneous inoculation of HF10-immunized mice against lethal infection by HSV-2, and attenuated the development of genital ulcer diseases. Immunization with HF10 inhibited HSV-2 replication in the mouse vagina, reduced local inflammation, controlled emergence of neurological dysfunctions of HSV-2 infection, and increased survival. In HF10-immunized mice, we observed rapid and increased production of interferon-γ in the vagina in response to HSV-2 infection, and numerous CD4+ and a few CD8+ T cells localized to the infective focus. CD4+ T cells invaded the mucosal subepithelial lamina propria. Thus, the protective effect of HF10 was related to induction of cellular immunity, mediated primarily by Th1 CD4+ cells. These data indicate that the live attenuated HSV-1 mutant strain HF10 is a promising candidate antigen for a vaccine against genital herpes caused by HSV-2.

Strategies for the Rapid Construction of Conditionally-Replicating HSV-1 Vectors Expressing Foreign Genes as Anticancer Therapeutic Agents

Conditionally replication-competent Herpes Simplex Virus Type 1 (HSV-1) vectors expressing foreign genes have been developed as experimental therapeutic agents. Traditional methods of virus construction, including growth selection based on thymidine kinase gene expression, and color selection based on a reporter gene expression are often time-consuming and relatively inefficient. This review summarizes the various strategies developed in recent years for the rapid and efficient construction of novel conditionally replication-competent mutant HSV expressing multiple foreign genes. Additionally, two new modifications of existing strategies, which have not been previously reported, are discussed.

Carrier cell-based delivery of replication-competent HSV-1 mutants enhances antitumor effect for ovarian cancer

Oncolytic viruses capable of tumor-selective replication and cytolysis have shown early promise as cancer therapeutics. We have developed replication-competent attenuated herpes simplex virus type 1 (HSV-1) mutants, named HF10 and Hh101, which have been evaluated for their oncolytic activities. However, the host immune system remains a significant obstacle to effective intraperitoneal administration of these viruses in the clinical setting. In this study, we investigated the use of these HSV-1 mutants as oncolytic agents against ovarian cancer and the use of human peritoneal mesothelial cells (MCs) as carrier cells for intraperitoneal therapy. MCs were efficiently infected with HSV-1 mutants, and MCs loaded with HSV-1 mutants caused cell killing adequately when cocultured with cancer cells in the presence or absence of HSV antibodies. In a mouse xenograft model of ovarian cancer, the injection of infected carrier cells led to a significant reduction of tumor volume and prolonged survival in comparison with the injection of virus alone. Our results indicate that replication-competent attenuated HSV-1 exerts a potent oncolytic effect on ovarian cancer, which may be further enhanced by the utilization of a carrier cell delivery system, based on amplification of viral load and possibly on avoidance of neutralizing antibodies.

A limited innate immune response is induced by a replication-defective herpes simplex virus vector following delivery to the murine central nervous system

Herpes simplex virus type 1 (HSV-1)-based vectors readily transduce neurons and have a large payload capacity, making them particularly amenable to gene therapy applications within the central nervous system (CNS). Because aspects of the host responses to HSV-1 vectors in the CNS are largely unknown, we compared the host response of a nonreplicating HSV-1 vector to that of a replication-competent HSV-1 virus using microarray analysis. In parallel, HSV-1 gene expression was tracked using HSV-specific oligonucleotide-based arrays in order to correlate viral gene expression with observed changes in host response. Microarray analysis was performed following stereotactic injection into the right hippocampal formation of mice with either a replication-competent HSV-1 or a nonreplicating recombinant of HSV-1, lacking the ICP4 gene (ICP4-). Genes that demonstrated a significant change (P < .001) in expression in response to the replicating HSV-1 outnumbered those that changed in response to mock or nonreplicating vector by approximately 3-fold. Pathway analysis revealed that both the replicating and nonreplicating vectors induced robust antigen presentation but only mild interferon, chemokine, and cytokine signaling responses. The ICP4- vector was restricted in several of the Toll-like receptor-signaling pathways, indicating reduced stimulation of the innate immune response. These array analyses suggest that although the nonreplicating vector induces detectable activation of immune response pathways, the number and magnitude of the induced response is dramatically restricted compared to the replicating vector, and with the exception of antigen presentation, host gene expression induced by the nonreplicating vector largely resembles mock infection.

Oncolytic virotherapy with an HSV amplicon vector expressing granulocyte–macrophage colony-stimulating factor using the replication-competent HSV type 1 mutant HF10 as a helper virus

Direct viral infection of solid tumors can cause tumor cell death, but these techniques offer the opportunity to express exogenous factors to enhance the antitumor response. We investigated the antitumor effects of a herpes simplex virus (HSV) amplicon expressing mouse granulocyte-macrophage colony-stimulating factor (mGM-CSF) using the replication-competent HSV type 1 mutant HF10 as a helper virus. HF10-packaged mGM-CSF-expressing amplicon (mGM-CSF amplicon) was used to infect subcutaneously inoculated murine colorectal tumor cells (CT26 cells) and the antitumor effects were compared to tumors treated with only HF10. The mGM-CSF amplicon efficiently replicated in CT26 cells with similar oncolytic activity to HF10 in vitro. However, when mice subcutaneously inoculated with CT26 cells were intratumorally injected with HF10 or mGM-CSF amplicon, greater tumor regression was seen in mGM-CSF amplicon-treated animals. Furthermore, mGM-CSF amplicon treatment prolonged mouse survival. Immunohistochemical analysis revealed increased inflammatory cell infiltration in the solid tumor in the mGM-CSF amplicon-treated animals. These results suggest that expression of GM-CSF enhances the antitumor effects of HF10, and HF10-packaged GM-CSF-expressing amplicon is a promising agent for the treatment of subcutaneous tumors.

Effects of G207, a conditionally replication-competent oncolytic herpes simplex virus, on the developing mammalian brain

Viral oncolytic therapy for malignant brain tumors involves local intratumoral delivery of a genetically engineered virus with tumor cell-specific lytic activity. Promising preliminary results have been achieved in preclinical models with G207, a replication-competent herpes simplex virus type 1 constructed with multiple directed mutations. Although the safety of G207 has been demonstrated in adults, application of viral oncolytic therapy to children with brain tumors has been delayed because of previous lack of data concerning the impact of a replication-competent oncolytic virus on the developing mammalian brain. In this study there was no significant difference in long-term physical development, cognitive performance, or exploratory behaviors between mice that received intracerebral inoculation of G207 or control saline at 4 days of age. However, histological examination and magnetic resonance imaging revealed frequent unilateral ventriculomegaly ipsilateral to the site of injection in only the G207 group. These results suggest that although it is unlikely that G207 will have significant adverse effects on neurodevelopmental outcomes of pediatric patients with brain tumors, an initial study of G207 in children should exclude those patients with tumors in or near the ventricular system as well as patients less than 2 years of age. Furthermore, patients in such a study will need to be closely monitored for the development of hydrocephalus.

Phase I, Open-Label, Dose-Escalating Study of a Genetically Engineered Herpes Simplex Virus, NV1020, in Subjects with Metastatic Colorectal Carcinoma to the Liver

Current regimens of systemic chemotherapy result in only modest lengthening of survival in patients with advanced stage, liver-dominant, metastatic colorectal cancer who have failed first-line chemotherapy. The objective of this study was to investigate the safety and tolerability of NV1020, a replication-competent, attenuated, genetically engineered herpes simplex virus type 1 (HSV-1), in patients with hepatic colorectal metastases refractory to first-line chemotherapy. A phase I, open-label, dose-escalating study of a single 10-min hepatic arterial infusion of NV1020 in four cohorts. Three patients in each cohort received doses of 3 x 10(6), 1 x 10(7), 3 x 10(7), and 1 x 10(8) plaque-forming units. Adverse events were either mild or moderate in severity, and self-limiting. Only three serious adverse events (one transient rise in serum y-glutamyltransferase, one diarrhea, and one leukocytosis) experienced by three patients were considered to be possibly or probably related to NV1020. There were no deaths during the study, and there was no evidence of disseminated herpes infection. Viral presence was detected in only one saliva sample and two serum samples from one asymptomatic patient in the highest dose cohort. In the first week after viral administration only rare and minor increases were noted for tumor necrosis factor-alpha (six samples; three patients; peak, 40 pg/ml), interleukin (IL)-1 (two samples; two patients; peak, 28 pg/ml), and interferon-y (four samples; two subjects; peak, 54 pg/ml). No IL-2 was detected. Mild liver enzyme elevations were self-limiting and not associated with clinical symptoms. We conclude that NV1020, a genetically engineered but replication-competent HSV-1 oncolytic virus, can be safely administered into the hepatic artery without significant effects on normal liver function.

Establishment of a novel foreign gene delivery system combining an HSV amplicon with an attenuated replication-competent virus, HSV-1 HF10

Herpes simplex virus type 1 (HSV-1)-based amplicon vectors have been used widely in genetic engineering with many advantages for gene delivery, being easily constructed. An attenuated and replication-competent HSV-1 HF10 clone demonstrating an oncolytic effect on cancer cells in vitro and in vivo has been applied recently for clinical virotherapy of breast cancers and the present studies were conducted to test its efficacy in combination with an HSV-1 amplicon. For this purpose, a new system was developed to produce high titers of the HSV-1 amplicon vector and the results showed that its package efficiency and the titer ratio to HF10 were improved by passage through two cell lines. A high ratio of amplicon/helper virus HF10 (A/H) (>1) was required to express the foreign gene efficiently. Furthermore, in order to express the foreign gene conditionally, an HSV-1 ICP8 promoter was introduced in place of the human cytomegalovirus MIE promoter, this driving expression of the transgene when replication of HF10 progressed. The methodology for simple preparation of mixtures of viruses containing the amplicon with the oncolytic virus is documented. This system should find application for studies of cancer therapy.

Oncolytic viral therapy for human ovarian cancer using a novel replication-competent herpes simplex virus type I mutant in a mouse model

Objectives Attenuated mutant strains of herpes simplex virus (HSV) have been effectively used for treatment of malignant brain tumors. As HSV-1 can infect and lyse a variety of cell types, other malignancies may also benefit from such treatment. We sought to test the feasibility of HSV-1 mutant-mediated gene therapy treatment of ovarian cancer. Methods We prepared two attenuated mutant HSV-1 strains. An HSV-1 mutant, hrR3, has replaced the gene encoding ribonucleotide reductase (RR) with the lacZ reporter gene. We also developed a new replication-competent HSV-1 mutant, HR522; this virus, expressing the lacZ reporter gene, induces syncytium formation in infected cells. We compared the efficacy of HR522 with, paclitaxel (Taxol) and hrR3 in the treatment of nude mice harboring human ovarian cancer cells. We also examined the effect of the prodrug ganciclovir (GCV) on the treatment mediated by these HSVs. Survival was evaluated by Kaplan–Meier method and log-rank test. Results The survival of mice treated with a high-titer hrR3 (5 × 10 7 plaque-forming units [PFU]) was significantly prolonged as compared with the group given paclitaxel ( P < 0.0001, log-rank test). Although the survival of mice treated with high-titer HR522 (5 × 10 7 PFU) was not significantly prolonged compared with paclitaxel-treated group ( P = 0.212, log-rank test), GCV markedly enhanced the efficacy of HR522 administration ( P < 0.005, vs paclitaxel, log-rank test). The lacZ gene product, visualized using 5-bromo-4-chloro-3-indolyl-β- d-galactopyranoside (X-gal) histochemistry, was detected in HR522-treated tumors in areas also exhibiting apoptotic changes. Conclusions These findings indicate that the combination of HR522 and GCV possesses significant therapeutic potential for treatment of ovarian cancer. Such viral therapy offers a novel approach to reductions in the dissemination of ovarian cancer.

Oncolytic herpes simplex virus vectors for cancer virotherapy.

Oncolytic herpes simplex virus type 1 (HSV-1) vectors are emerging as an effective and powerful therapeutic approach for cancer. Replication-competent HSV-1 vectors with mutations in genes that affect viral replication, neuropathogenicity, and immune evasiveness have been developed and tested for their safety and efficacy in a variety of mouse models. Evidence to-date following administration into the brain attests to their safety, an important observation in light of the neuropathogenicity of the virus. Phase I clinical traits of three vectors, G207, 1716, and NV1020, are either ongoing or completed, with no adverse events attributed to the virus. These and other HSV-1 vectors are effective against a myriad of solid tumors in mice, including glioma, melanoma, breast, prostate, colon, ovarian, and pancreatic cancer. Enhancement of activity was observed when HSV-1 vectors were used in combination with traditional therapies such as radiotherapy and chemotherapy, providing an attractive strategy to pursue in the clinic. Oncolytic HSV-1 vectors expressing "suicide" genes (thymidine kinase, cytosine deaminase, rat cytochrome P450) or immunostimulatory genes (IL-12, GM-CSF, etc.) have been constructed to maximize tumor destruction through multimodal therapeutic mechanisms. Further advances in virus delivery and tumor specificity should improve the likelihood for successful translation to the clinic.

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.

Expression of hepatitis C virus envelope glycoproteins by herpes simplex virus type 1-based amplicon vectors.

Herpes simplex virus type 1 (HSV-1)-based amplicon vectors expressing hepatitis C virus (HCV) E1 and E2 glycoproteins were investigated. HSV-1 amplicon vectors carrying the E1E2p7- or E2p7-coding sequences of HCV type 1a under the control of the HSV-1 IE4 (alpha22/alpha47) promoter were constructed. Studies of infected HepG2, WRL 68 or Vero cells indicated that HSV-1-based amplicon vectors express high levels of HCV glycoproteins that are processed correctly. Immunofluorescence microscopy combined with immunoprecipitation and endoglycosidase treatment of cells infected with the HSV-1-based vectors expressing E1 and E2 showed that the two glycoproteins were retained in the endoplasmic reticulum and had the expected glycosylation patterns. Furthermore, although most of the E1 and E2 proteins formed disulfide-linked aggregates, significant amounts of monomeric forms of the two proteins were detected by SDS-PAGE under non-reducing conditions, suggesting the presence of non-covalently associated E1 and E2. Similar results were produced by a replication-competent recombinant HSV-1 vector expressing HCV E1 and E2. These results indicated that HSV-1-based amplicon vectors represent a useful expression system for the study of HCV glycoproteins.

Preclinical safety evaluation of G207, a replication-competent herpes simplex virus type 1, inoculated intraprostatically in mice and nonhuman primates.

G207, a replication-competent herpes simplex virus type 1 (HSV-1) virus, has been previously shown to be effective against human prostate cancer xenografts in mice. This study assesses its safety in the prostate of two animal models known for their sensitivity to HSV-1. BALB/c mice were injected intraprostatically with either HSV-1 G207 or strain F and observed for 5 months. None of the G207-injected animals exhibited any clinical signs of disease or died. However, 50% of strain F-injected mice displayed sluggish, hunched behavior and died by day 13. Histopathologically, the G207-injected prostates were normal whereas strain F-injected prostates showed epithelial flattening, sloughing, and stromal edema. Four Aotus nancymae monkeys were also injected with G207 intraprostatically and observed short term (up to 21 days) and long term (56 days). Safety was assessed on the basis of clinical observations, viral biodistribution, virus shedding, and histopathology. None of the injected monkeys displayed evidence of clinical disease, shedding of infectious virus, or spread of the virus into other organs. Except for minor histological changes unrelated to the study, no significant abnormalities were observed. These results demonstrate that G207 can be safely inoculated into the prostate and should be considered for human trials for the treatment of prostate cancer.

Therapeutic Efficacy of G207, a Conditionally Replicating Herpes Simplex Virus Type 1 Mutant, for Gallbladder Carcinoma in Immunocompetent Hamsters

Gallbladder cancer is an extremely difficult disease to cure once metastases occur. In this paper, we explored the potential of G207, an oncolytic, replication-competent herpes simplex virus type 1 mutant, as a new therapeutic means for gallbladder cancer. Gallbladder carcinoma cell lines (four human and one hamster) showed nearly total cell killing within 72 h of G207 infection at a m.o.i. of 0.25 to 2.5 in vitro. The susceptibility to G207 cytopathic activity correlated with the infection efficiency demonstrated by lacZ expression. Intraneoplastic inoculation of G207 (1 × 107 pfu) in immunocompetent hamsters bearing established subcutaneous KIGB-5 tumors caused a significant inhibition of tumor growth and prolongation of survival. Repeated inoculations (three times with 4-day intervals) were significantly more efficacious than a single inoculation. In hamsters with bilateral subcutaneous KIGB-5 tumors, inoculation of one tumor alone with G207 caused regression or growth reduction of uninoculated tumors as well as inoculated tumors. In athymic mice, however, the anti-tumor effect was largely reduced in inoculated tumors and completely abolished in remote tumors, suggesting large contribution of T-cell-mediated immune responses to both local and systemic anti-tumor effect of G207. These results indicate that G207 may be useful as a new strategy for gallbladder cancer treatment.

Replication-Competent Herpes simplex Virus Type 1 Mutant Expressing an Autofluorescent Glycoprotein H Fusion Protein

The heterocomplex of glycoproteins H (gH) and L (gL) of herpes simplex virus type 1 (HSV-1) is essential for viral infectivity and is involved in viral penetration, cell-to-cell spread, and syncytium formation. We constructed an HSV-1 mutant expressing a gH-EGFP (enhanced green fluorescent protein) fusion protein under the control of the gH true late promoter. The EGFP coding sequence was cloned after the gH signal peptide into the HSV-1 genome. Superinfection of transfected, gH-nontranscomplementing cells with gH-negative HSV-1 resulted in a replication-competent recombinant virus. Cells infected with the recombinant virus exhibited strong and stable EGFP-specific fluorescence late in infection, and autofluorescence was detected in purified virions. The recombinant genotype of the mutant was confirmed by PCR. The 140-kD gH-EGFP fusion protein showed an N-linked glycosylation pattern similar to gH-1, was recognized by the conformation-dependent gH-specific monoclonal antibodies 52S and LP11 and formed a heterocomplex with gL which was transported to the cell surface and integrated into the viral envelope. Infectivity of the gH-EGFP mutant was neutralized by antibodies 52S and LP11. To our knowledge, this is the first replication-competent HSV-1 mutant expressing an autofluorescent essential glycoprotein which will be a versatile tool for studies of penetration, late gene expression, transport and tissue spread.

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.

Antitumor efficacy of regional oncolytic viral therapy for peritoneally disseminated cancer.

Oncolytic viral therapy is a promising new method of cancer treatment. Peritoneal dissemination of cancer is a common and fatal clinical condition seen in many malignancies, with few effective therapies available. G207, a multimutated replication-competent herpes simplex virus type-1, effectively treats disseminated peritoneal cancer. This study evaluates viral proliferation and subsequent tumoricidal effects in vitro and in vivo after regional viral delivery. In vitro studies demonstrate that G207 efficiently kills five human gastric cancer cell lines, and that permissiveness to viral replication is correlated with cytotoxicity. In a murine xenograft model of human gastric carcinomatosis, peritoneal delivery of G207 effectively kills tumor and prolongs survival. Data from quantitative PCR characterizes peritoneal clearance of virus after intraperitoneal injection, and identifies G207 replication within tumor cells in vivo, similar to in vitro proliferation. Further analysis of various organs confirms that G207 does not replicate within normal tissue after peritoneal delivery. Wild-type KOS viral replication was also demonstrated in vivo, with significant toxicity secondary to dissemination and encephalitis. In vivo viral proliferation of G207 is restricted to tumor cells, is correlated with in vitro assays, and is an important mechanism of anticancer efficacy.

Attenuated, replication-competent herpes simplex virus type 1 mutant G207: safety evaluation of intracerebral injection in nonhuman primates.

This study examined the safety of intracerebral inoculation of G207, an attenuated, replication-competent herpes simplex virus type 1 (HSV-1) recombinant, in nonhuman primates. Sixteen New World owl monkeys (Aotus nancymae [karyotype 1, formerly believed to be A. trivirgatus]), known for their exquisite susceptibility to HSV-1 infection, were evaluated. Thirteen underwent intracerebral inoculation with G207 at doses of 10(7) or 10(9) PFU, two were vehicle inoculated, and one served as an infected wild-type control and received 10(3) PFU of HSV-1 strain F. HSV-1 strain F caused rapid mortality and symptoms consistent with HSV encephalitis, including fever, hemiparesis, meningitis, and hemorrhage in the basal ganglia. One year after G207 inoculation, seven of the animals were alive and exhibited no evidence of clinical complications. Three deaths resulted from nonneurologic causes unrelated to HSV infection, and three animals were sacrificed for histopathologic examination. Two animals were reinoculated with G207 (10(7) PFU) at the same stereotactic coordinates 1 year after the initial G207 inoculation. These animals were alive and healthy 2 years after the second inoculation. Cerebral magnetic resonance imaging studies performed both before and after G207 inoculation failed to reveal radiographic evidence of HSV-related sequelae. Despite the lack of outwardly observable HSV pathology, measurable increases in serum anti-HSV titers were detected. Histopathological examination of multiple organ tissues found no evidence of HSV-induced histopathology or dissemination. We conclude that intracerebral inoculation of up to 10(9) PFU of G207, well above the efficacious dose in mouse tumor studies, is safe and therefore appropriate for human clinical trials.

Herpes simplex virus type 1 DNA amplified as bacterial artificial chromosome in Escherichia coli: rescue of replication-competent virus progeny and packaging of amplicon vectors.

Herpes simplex virus type 1 (HSV-1)-based amplicon vectors contain only approximately 1% of the 152-kb HSV-1 genome, and consequently, replication and packaging into virions depends on helper functions. These helper functions have been provided conventionally by a helper virus, usually a replication-defective mutant of HSV-1, or more recently, by a set of five cosmids that overlap and represent the genome of HSV-1 deleted for DNA cleavage/packaging signals (pac). In the absence of pac signals, potential HSV-1 genomes that are reconstituted from the cosmids via homologous recombination are not packageable. The resulting amplicon stocks are, therefore, virtually free of contaminating helper virus. To simplify this packing system, the HSV-1 genome was cloned and maintained stably as a single-copy, F plasmid-based bacterial artificial chromosome in E. coli. Such a plasmid containing the HSV-1 genome deleted for the pac signals (fHSV delta pac) did not generate replication-competent progeny virus on transfection into mammalian cells, but rather, it was able to support the packaging of cotransfected amplicon DNA that contained a functional pac signal. The resulting amplicon vector stocks had titers of up to 10(7) transducing units per milliliter of culture medium and efficiently transduced neural cells in the rat brain, as well as hepatocytes in the rat. The capacity of generating infectious and replication-competent HSV-1 progeny following transfection into mammalian cells was restored after insertion of a pac signal into fHSV delta pac.