Herpes Simplex Virus Oncolytic Therapy for Pediatric Malignancies
Authors: Gregory K Friedman, Joseph G Pressey, Alyssa T Reddy, James M Markert, G Yancey Gillespie
Journal: Molecular Therapy (2009)

Despite improving survival rates for children with cancer, a subset of patients exist with disease resistant to traditional therapies such as surgery, chemotherapy, and radiation. These patients require newer, targeted treatments used alone or in combination with more traditional approaches. Oncolytic herpes simplex virus (HSV) is one of these newer therapies that offer promise for several difficult to treat pediatric malignancies. The potential benefit of HSV therapy in pediatric solid tumors including brain tumors, neuroblastomas, and sarcomas is reviewed along with the many challenges that need to be addressed prior to moving oncolytic HSV therapy from the laboratory to the beside in the pediatric population. Despite improving survival rates for children with cancer, a subset of patients exist with disease resistant to traditional therapies such as surgery, chemotherapy, and radiation. These patients require newer, targeted treatments used alone or in combination with more traditional approaches. Oncolytic herpes simplex virus (HSV) is one of these newer therapies that offer promise for several difficult to treat pediatric malignancies. The potential benefit of HSV therapy in pediatric solid tumors including brain tumors, neuroblastomas, and sarcomas is reviewed along with the many challenges that need to be addressed prior to moving oncolytic HSV therapy from the laboratory to the beside in the pediatric population. IntroductionAlthough pediatric cancer survival rates have improved greatly over the past 30 years, there remains a significant subset of children, ∼25%, who succumb to their disease.1Gatta G Capocaccia R Coleman MP Ries LA Berrino F Childhood cancer survival in Europe and the United States.Cancer. 2002; 95: 1767-1772Crossref PubMed Scopus (0) Google Scholar,2Ries LAG Melbert D Krapcho M Mariotto A Miller BA Feuer EJ et al.SEER Cancer Statistics Review, 1975-2004, National Cancer Institute. 2007Google Scholar Deaths occur when tumors progress in the face of optimal therapies and acquire resistance to current treatment modalities like chemotherapy or radiotherapy, or the toxicities from such therapies become too great. Augmenting the dose of current therapies is likely to increase toxicity without improving survival significantly. Therefore, novel, targeted treatment strategies that evince effective oncolysis are desperately needed to decrease toxicity and, thereby, improve quality of life and survival rates for children with malignancies. This review will focus on the potential benefit of adjunctive oncolytic virotherapy that utilizes genetically engineered herpes simplex viruses, type-1 (HSVs-1).HSV-1 is a unique virus that offers promise in treating several pediatric cancers including brain tumors, neuroblastomas, and sarcomas. The virus can be employed as oncolytic viral therapy, a direct, targeted attack or via gene therapy in which foreign genes are expressed therapeutically in cancer cells.3Markert JM Parker JN Gillespie GY Whitley RJ Genetically engineered human herpes simplex virus in the treatment of brain tumours.Herpes. 2001; 8: 17-22PubMed Google Scholar Oncolytic viral therapy relies on virus replication in infected tumor cells that die and release infectious virus. In gene therapy, expression of therapeutic foreign gene products either directly or indirectly leads to cell death. Oncolytic viruses that utilize gene therapy are perceived to be more effective at killing tumors.Genetically engineered HSV-1 has a number of advantages in the treatment of a subset of pediatric malignancies. HSV-1 is a large (152 kb, 89 genes, multiple open-reading frames), double-stranded DNA, enveloped virus that does not integrate into host DNA. HSV is a neurotropic virus thus making cancers of neural origin, including brain tumors and neuroblastomas, ideal targets.4Shah AC Benos D Gillespie GY Markert JM Oncolytic viruses: clinical applications as vectors for the treatment of malignant gliomas.J Neurooncol. 2003; 65: 203-226Crossref PubMed Scopus (96) Google Scholar However, it is equally able to infect and kill cells from a variety of cancers including sarcomas, melanomas, colon, breast, lung, prostate, and hepatic tumors.5Bharatan NS Currier MA Cripe TP Differential susceptibility of pediatric sarcoma cells to oncolysis by conditionally replication-competent herpes simplex viruses.J Pediatr Hematol Oncol. 2002; 24: 447-453Crossref PubMed Scopus (25) Google Scholar,6Kemeny N Brown K Covey A Kim T Bhargava A Brody L et al.Phase I, open-label, dose-escalating study of a genetically engineered herpes simplex virus, NV1020, in subjects with metastatic colorectal carcinoma to the liver.Hum Gene Ther. 2006; 17: 1214-1224Crossref PubMed Scopus (106) Google Scholar,7Liu R Martuza RL Rabkin SD Intracarotid delivery of oncolytic HSV vector G47Delta to metastatic breast cancer in the brain.Gene Ther. 2005; 12: 647-654Crossref PubMed Scopus (36) Google Scholar,8Yu Z Chan MK O-charoenrat P Eisenberg DP Shah JP Singh B et al.Enhanced nectin-1 expression and herpes oncolytic sensitivity in highly migratory and invasive carcinoma.Clin Cancer Res. 2005; 11: 4889-4897Crossref PubMed Scopus (26) Google Scholar,9Varghese S Rabkin SD Nielsen GP MacGarvey U Liu R Martuza RL Systemic therapy of spontaneous prostate cancer in transgenic mice with oncolytic herpes simplex viruses.Cancer Res. 2007; 67: 9371-9379Crossref PubMed Scopus (27) Google Scholar,10Watanabe D Goshima F Mori I Tamada Y Matsumoto Y Nishiyama Y Oncolytic virotherapy for malignant melanoma with herpes simplex virus type 1 mutant HF10.J Dermatol Sci. 2008; 50: 185-196Abstract Full Text Full Text PDF PubMed Scopus (23) Google Scholar Normal cells are spared, whereas tumor cells are targeted by deleting genes critical for viral replication in normal cells but not necessary in tumor cells such as the "neurovirulence" gene, γ134.5.11Chou J Kern ER Whitley RJ Roizman B Mapping of herpes simplex virus-1 neurovirulence to gamma 134.5, a gene nonessential for growth in culture.Science. 1990; 250: 1262-1266Crossref PubMed Google Scholar,12Mineta T Rabkin SD Yazaki T Hunter WD Martuza RL Attenuated multi-mutated herpes simplex virus-1 for the treatment of malignant gliomas.Nat Med. 1995; 1: 938-943Crossref PubMed Google Scholar It has been estimated that up to 30 kb of the HSV genome is nonessential for replication in tumor cells and, therefore, can be replaced with foreign DNA for gene therapy without affecting the virus' ability to infect tumor cells and replicate. From a safety standpoint, antiviral agents in clinical use are readily available in the unlikely event that the mutant HSV produces toxicity to normal tissues.Currently, four different γ134.5-deleted viruses have been used in adult phase I trials (G207, 1716, NV1020, OncoVexGM-CSF). G207 and HSV1716 have been used to treat patients with recurrent glioblastoma multiforme (GBM) (Table 1).12Mineta T Rabkin SD Yazaki T Hunter WD Martuza RL Attenuated multi-mutated herpes simplex virus-1 for the treatment of malignant gliomas.Nat Med. 1995; 1: 938-943Crossref PubMed Google Scholar,13MacLean AR ul-Fareed M Robertson L Harland J Brown SM Herpes simplex virus type 1 deletion variants 1714 and 1716 pinpoint neurovirulence-related sequences in Glasgow strain 17+ between immediate early gene 1 and the 'a' sequence.J Gen Virol. 1991; 72: 631-639Crossref PubMed Google Scholar G207 was derived from R3616 that had been created by deleting both copies of γ134.5 gene from the wild-type isolate, HSV-1(F) strain. G207 was created by inserting the lacZ gene encoding β-galactosidase into the UL39 locus, thus disabling the expression of ICP6, the heavy chain for ribonucleotide reductase (Figure 1). HSV1716 was created by deleting both copies of the γ134.5 gene (also known as RL) from the wild-type isolate, strain 17 (Figure 2). HSV-1(F) is a temperature-sensitive isolate, whereas strain 17 is not. These two HSV-1 parents likely have different degrees of virulence, as these γ134.5-deleted variants have different LD50 values for mice.14Chambers R Gillespie GY Soroceanu L Andreansky S Chatterjee S Chou J et al.Comparison of genetically engineered herpes simplex viruses for the treatment of brain tumors in a scid mouse model of human malignant glioma.Proc Natl Acad Sci USA. 1995; 92: 1411-1415Crossref PubMed Scopus (165) Google Scholar,15Bolovan CA Sawtell NM Thompson RL ICP34.5 mutants of herpes simplex virus type 1 strain 17syn+ are attenuated for neurovirulence in mice and for replication in confluent primary mouse embryo cell cultures.J Virol. 1994; 68: 48-55Crossref PubMed Google ScholarTable 1Summary of engineered HSV discussed in the textVirusParent virusDeletions/mutationsForeign gene insertions/promoterCitation numberR3616HSV-1(F)1 kb deletion in γ134.5 gene (both copies)None113616UBR36161 kb deletion in γ134.5 gene (both copies) and an interruption of the uracil DNA glycosylase geneNone73d12.CALPD120ICP4 deletionCalponin promoter130G207R36161 kb deletion in γ134.5 gene (both copies) and disabling lacZ insertion in UL39None12G47ΔG207Same as G207 with 312 bp deletion in ICP47 locus and promoter region of US11None7HF10F3.9 kb deletion in the right end of the UL and UL/IRL junction, and it expression of UL56 and latency-associated transcriptsNone10HLHF10 and L1BR1Lacks both UL56 and US3None129HSV171617759 bp deletion in γ134.5 gene (both copies)None13MtHSVFDisablinglacZ insertion into both copies of γ134.5None128NV1020R7020700 bp deletion in thymidine kinase (tk) locus and a 15 kb deletion across the joining region of the long and short components of the HSV-1 genomeHSV-1 DNA fragment encoding the tk gene fused to the α gene promoter6NV1042R7020Same as NV1020Murine IL-12, hybrid α4-tk promoter81NV1066FSame at NV1020Enhanced green fluorescent protein, CMV promoter100OncoVexGM-CSFJS-1Complete deletions of the genes encoding ICP34.5 and ICP47GM-CSF, CMV promoter21rRp450hrR3UL39 deletionRat CYP2B1 gene encodes enzyme to activate oxazophosphorines30TIMP3rHSVQ1Deletions in ICP6 and γ134.5Tissue inhibitor of metalloproteinases-3, HSV-1 immediate early 4/5 promoter103 Open table in a new tab Figure 2Schematic representation of the derivation of 1716 HSV. In clinical isolate strain 17+ HSV, a 759 bp deletion was produced that extended from the DR1/Ub boundary in the "a" sequence to remove 105 bp on the 5′ end of the RL open reading frame.13MacLean AR ul-Fareed M Robertson L Harland J Brown SM Herpes simplex virus type 1 deletion variants 1714 and 1716 pinpoint neurovirulence-related sequences in Glasgow strain 17+ between immediate early gene 1 and the 'a' sequence.J Gen Virol. 1991; 72: 631-639Crossref PubMed Google Scholar This deletion was produced in both long-repeat regions (terminal repeat-long, inverted repeat-long) of the HSV genome, but only the deletion for the TRL is shown for simplicity. The strain 17+ HSV had a LD50 of <10 PFU, whereas the 1716 mutant was significantly neuroattenuated with a LD50 of 7 × 106 PFU.View Large Image Figure ViewerDownload Hi-res image Download (PPT)In the first trial of G207, a maximum tolerated dose was not reached and no definitive dose-limiting toxicities occurred with stereotactic injection of up to 3 × 109 plaque-forming units (PFUs) of virus directly in up to 5 loci within the enhancing portions of recurrent tumors.16Markert JM Medlock MD Rabkin SD Gillespie GY Todo T Hunter WD et al.Conditionally replicating herpes simplex virus mutant, G207 for the treatment of malignant glioma: results of a phase I trial.Gene Ther. 2000; 7: 867-874Crossref PubMed Google Scholar A phase Ib trial of stereotactic catheter inoculation of G207 followed by resection and reinoculation into the tumor bed of recurrent GBMs likewise confirmed safety.17Markert JM Parker JN Buchsbaum DJ Grizzle WE Gillespie GY Whitley RJ Oncolytic HSV-1 for the treatment of brain tumours.Herpes. 2006; 13: 66-71PubMed Google Scholar Although the trials were only designed to determine safety, response was seen in some patients, and two patients were long-term survivors (>5 years). A third trial utilizing stereotactic injection of G207 into recurrent tumors in five loci followed within 24 hours by a single fraction of irradiation (5 Gy) has finished accrual and will be reported shortly.Similar results were seen in trials using HSV1716, although the highest dose achieved was 1 × 105 PFU, due to its more aggressive behavior in mice. At this dose, the virus was safe with direct injection into tumors or tumor beds after tumor resection, and resulted in several long-term survivors.18Rampling R Cruickshank G Papanastassiou V Nicoll J Hadley D Brennan D et al.Toxicity evaluation of replication-competent herpes simplex virus (ICP 34.5 null mutant 1716) in patients with recurrent malignant glioma.Gene Ther. 2000; 7: 859-866Crossref PubMed Google Scholar,19Papanastassiou V Rampling R Fraser M Petty R Hadley D Nicoll J et al.The potential for efficacy of the modified (ICP 34.5(-)) herpes simplex virus HSV1716 following intratumoural injection into human malignant glioma: a proof of principle study.Gene Ther. 2002; 9: 398-406Crossref PubMed Google Scholar,20Harrow S Papanastassiou V Harland J Mabbs R Petty R Fraser M et al.HSV1716 injection into the brain adjacent to tumour following surgical resection of high-grade glioma: safety data and long-term survival.Gene Ther. 2004; 11: 1648-1658Crossref PubMed Scopus (198) Google ScholarNV1020 is a rederived virus based on the construction of R7020 that was developed for HSV immunization. NV1020 has only one of the γ134.5 loci deleted and contains an insertion of HSV-2 sequences (Figure 3). It is a replication-competent, attenuated-engineered virus, but due to its remaining γ134.5 sequence is believed to have greater potential for neurotoxicity. Nevertheless, NV1020 was infused into the hepatic artery of patients with metastatic colorectal carcinoma to the liver, safely with minimal and self-limited serious adverse events.6Kemeny N Brown K Covey A Kim T Bhargava A Brody L et al.Phase I, open-label, dose-escalating study of a genetically engineered herpes simplex virus, NV1020, in subjects with metastatic colorectal carcinoma to the liver.Hum Gene Ther. 2006; 17: 1214-1224Crossref PubMed Scopus (106) Google Scholar A multi-institutional phase II trial is ongoing. Similarly, a second generation oncolytic HSV expressing granulocyte macrophage colony–stimulation factor (GM-CSF), OncoVexGM-CSF, was safe and well tolerated when injected intratumorally in patients with recurrent cutaneous or subcutaneous deposits of breast, head and neck, gastrointestinal cancers or malignant melanoma.21Hu JC Coffin RS Davis CJ Graham NJ Groves N Guest PJ et al.A phase I study of OncoVEXGM-CSF, a second-generation oncolytic herpes simplex virus expressing granulocyte macrophage colony-stimulating factor.Clin Cancer Res. 2006; 12: 6737-6747Crossref PubMed Scopus (225) Google Scholar Several patients had stable disease without progression.Figure 3Schematic representation of the derivation of R7020 (aka NV1020). This virus was constructed for the purpose of creating a vaccine for HSV-1 and HSV-2 that would be administered by intramuscular injection of living virus.138Meignier B Longnecker R Roizman B In vivo behavior of genetically engineered herpes simplex viruses R7017 and R7020: construction and evaluation in rodents.J Infect Dis. 1988; 158: 602-614Crossref PubMed Google Scholar HSV-1(F) was initially deleted for thymidine kinase (UL23) and UL24. The internal set of the inverted repeats, containing α0, α4, γ134.5, ORF O, and ORF P, was replaced with a novel construct that contained UL23 fused to the α4 promoter-regulatory region and 3′ to a series of HSV-2 genes (US2, HSV protein kinase, US5, glycoproteins G, D, and I, as well as a portion of E). The terminal inverted repeat remained intact. Thus, this chimeric virus, containing both HSV-1 and HSV-2 sequences, has been extensively tested in both rodent and primate species for safety (LD50 = 2.7 × 106 PFU versus 3.8 × 102 PFU for HSV-1(F)) and genetic stability.138Meignier B Longnecker R Roizman B In vivo behavior of genetically engineered herpes simplex viruses R7017 and R7020: construction and evaluation in rodents.J Infect Dis. 1988; 158: 602-614Crossref PubMed Google Scholar,139Meignier B Martin B Whitley RJ Roizman B In vivo behavior of genetically engineered herpes simplex viruses R7017 and R7020. II. Studies in immunocompetent and immunosuppressed owl monkeys (Aotus trivirgatus).J Infect Dis. 1990; 162: 313-321Crossref PubMed Google ScholarView Large Image Figure ViewerDownload Hi-res image Download (PPT)Potential causes for the failure of genetically engineered HSVs to be uniformly curative are legion, but can be broadly grouped into general categories that include, but are not limited to: (i) host antiviral immune responses, (ii) tumor genotype heterogeneity, and (iii) extracellular matrix/tumor microenvironment. Seropositivity to HSV may pose a barrier to repeated administration, however, preclinical and clinical studies suggest that it does not have a deleterious impact on the initial intratumoral administration of HSVs.16Markert JM Medlock MD Rabkin SD Gillespie GY Todo T Hunter WD et al.Conditionally replicating herpes simplex virus mutant, G207 for the treatment of malignant glioma: results of a phase I trial.Gene Ther. 2000; 7: 867-874Crossref PubMed Google Scholar,22Herrlinger U Kramm CM Aboody-Guterman KS Silver JS Ikeda K Johnston KM et al.Pre-existing herpes simplex virus 1 (HSV-1) immunity decreases, but does not abolish, gene transfer to experimental brain tumors by a HSV-1 vector.Gene Ther. 1998; 5: 809-819Crossref PubMed Google Scholar,23Chahlavi A Rabkin S Todo T Sundaresan P Martuza R Effect of prior exposure to herpes simplex virus 1 on viral vector-mediated tumor therapy in immunocompetent mice.Gene Ther. 1999; 6: 1751-1758Crossref PubMed Scopus (73) Google Scholar,24Delman KA Bennett JJ Zager JS Burt BM McAuliffe PF Petrowsky H et al.Effects of preexisting immunity on the response to herpes simplex-based oncolytic viral therapy.Hum Gene Ther. 2000; 11: 2465-2472Crossref PubMed Scopus (77) Google Scholar,25Lambright ES Kang EH Force S Lanuti M Caparrelli D Kaiser LR et al.Effect of preexisting anti-herpes immunity on the efficacy of herpes simplex viral therapy in a murine intraperitoneal tumor model.Mol Ther. 2000; 2: 387-393Abstract Full Text Full Text PDF PubMed Scopus (31) Google Scholar,26Markert JM Liechty PG Wang W Gaston S Braz E Karrasch M et al.Phase Ib trial of mutant herpes simplex virus G207 inoculated pre-and post-tumor resection for recurrent GBM.Mol Ther. 2009; 17: 199-207Abstract Full Text Full Text PDF PubMed Scopus (169) Google Scholar More concerning is the observation that infiltrating mononuclear inflammatory cells may destroy virally infected cells prematurely, limiting virus production and eventually the scope of the anti-tumor effect.27Parker JN Gillespie GY Love CE Randall S Whitley RJ Markert JM Engineered herpes simplex virus expressing IL-12 in the treatment of experimental murine brain tumors.Proc Natl Acad Sci USA. 2000; 97: 2208-2213Crossref PubMed Scopus (197) Google Scholar,28Fulci G Dmitrieva N Gianni D Fontana EJ Pan X Lu Y et al.Depletion of peripheral macrophages and brain microglia increases brain tumor titers of oncolytic viruses.Cancer Res. 2007; 67: 9398-9406Crossref PubMed Scopus (94) Google Scholar Strategies that utilize agents (e.g., cyclophosphamide) to block bone marrow–derived generation of inflammatory cells seem to benefit the oncolytic effect.29Fulci G Breymann L Gianni D Kurozomi K Rhee SS Yu J et al.Cyclophosphamide enhances glioma virotherapy by inhibiting innate immune responses.Proc Natl Acad Sci USA. 2006; 103: 12873-12878Crossref PubMed Scopus (195) Google Scholar,30Currier MA Gillespie RA Sawtell NM Mahller YY Stroup G Collins MH et al.Efficacy and safety of the oncolytic herpes simplex virus rRp450 alone and combined with cyclophosphamide.Mol Ther. 2008; 16: 879-885Abstract Full Text Full Text PDF PubMed Scopus (38) Google Scholar Likewise, use of agents either exogenously administered (cilengitide, bevacizumab) or re-engineered into the virus (vasculostatin) to inhibit vascular permeability or formation of neovasculature in the tumor could significantly limit the extravasation of inflammatory cells into the tumor bed and permit greater virus production in the tumor cells.31Kurozumi K Hardcastle J Thakur R Shroll J Nowicki M Otsuki A et al.Oncolytic HSV-1 infection of tumors induces angiogenesis and upregulates CYR61.Mol Ther. 2008; 16: 1382-1391Abstract Full Text Full Text PDF PubMed Scopus (43) Google Scholar,32Kaur B Brat DJ Devi NS Van Meir EG Vasculostatin, a proteolytic fragment of brain angiogenesis inhibitor 1, is an antiangiogenic and antitumorigenic factor.Oncogene. 2005; 24: 3632-3642Crossref PubMed Scopus (94) Google ScholarTumor genotype heterogeneity poses an inherent problem for an oncolytic virus that has been seriously attenuated to make it safe and aneurovirulent. These differences include (i) reduced or complete lack of expression of the key HSV entry molecule, nectin-1 (CD111), or its family members and (ii) a nonpermissive environment for late gene expression and virus replication events. Solutions include (i) re-engineering the virus to express ligands that bind receptors expressed exclusively on tumor cells or (ii) modulating the cellular replication machinery with irradiation or DNA-damaging agents to engage the DNA repair response, which supports virus replication.33Zhou G Roizman B Construction and properties of a herpes simplex virus 1 designed to enter cells solely via the IL-13alpha2 receptor.Proc Natl Acad Sci USA. 2006; 103: 5508-5513Crossref PubMed Scopus (46) Google Scholar,34Kamiyama H Zhou G Roizman B Herpes simplex virus 1 recombinant virions exhibiting the amino terminal fragment of urokinase-type plasminogen activator can enter cells via the cognate receptor.Gene Ther. 2006; 13: 621-629Crossref PubMed Scopus (33) Google Scholar,35Zhou G Roizman B Characterization of a recombinant herpes simplex virus 1 designed to enter cells via the IL13Ralpha2 receptor of malignant glioma cells.J Virol. 2005; 79: 5272-5277Crossref PubMed Scopus (0) Google Scholar,36Mezhir JJ Advani SJ Smith KD Darga TE Poon AP Schmidt H et al.Ionizing radiation activates late herpes simplex virus 1 promoters via the p38 pathway in tumors treated with oncolytic viruses.Cancer Res. 2005; 65: 9479-9484Crossref PubMed Scopus (33) Google Scholar,37Advani SJ Mezhir JJ Roizman B Weichselbaum RR ReVOLT: radiation-enhanced viral oncolytic therapy.Int J Radiat Oncol Biol Phys. 2006; 66: 637-646Abstract Full Text Full Text PDF PubMed Scopus (39) Google Scholar,38Aghi M Rabkin S Martuza RL Effect of chemotherapy-induced DNA repair on oncolytic herpes simplex viral replication.J Natl Cancer Inst. 2006; 98: 38-50Crossref PubMed Scopus (102) Google Scholar In some instances, it may be possible to re-engineer the virus to express mutant proteins that activate signaling pathways that would provide a more hospitable virus replication environment.39Smith KD Mezhir JJ Bickenbach K Veerapong J Charron J Posner MC et al.Activated MEK suppresses activation of PKR and enables efficient replication and in vivo oncolysis by Deltagamma(1)34.5 mutants of herpes simplex virus 1.J Virol. 2006; 80: 1110-1120Crossref PubMed Scopus (53) Google Scholar Production of a chimeric HSV that contains a gene from human cytomegalovirus, a related β-herpes virus also enhances late gene expression without compromising safety of the virus.40Shah AC Parker JN Gillespie GY Lakeman FD Meleth S Markert JM et al.Enhanced antiglioma activity of chimeric HCMV/HSV-1 oncolytic viruses.Gene Ther. 2007; 14: 1045-1054Crossref PubMed Scopus (0) Google ScholarFinally, to be effective, infectious HSV must disseminate widely throughout the tumor. Extracellular matrix generated by the tumor provides a critical barrier. A potential solution is to re-engineer the virus to express ECM-proteolyzing enzymes to enhance percolation of infectious virus particles through the interstitial spaces of the tumor.41Ganesh S Gonzalez-Edick M Gibbons D Van Roey M Jooss K Intratumoral coadministration of hyaluronidase enzyme and oncolytic adenoviruses enhances virus potency in metastatic tumor models.Clin Cancer Res. 2008; 14: 3933-3941Crossref PubMed Scopus (43) Google Scholar Many of these immune, genetic and physical barriers to effective virus oncolysis are being resolved almost as rapidly as they are identified. However, it is the lag between developing the next-generation virus and its application in a phase I clinical trial that remains the most serious impediment in the quest for more effective therapies for children with cancer. To date, no pediatric trials employing engineered HSV have been conducted. However, several preclinical studies have shown promise. The potential benefit of HSV therapy in pediatric solid tumors including brain tumors, neuroblastomas, and sarcomas is reviewed in the following section.Brain TumorsCentral nervous system tumors account for ∼25% of all childhood malignancies, and brain tumors are the leading cause of cancer-related morbidity and mortality in children (Table 2). Although survival rates for patients with low-grade, localized tumors have improved to >80%, there is a poor outcome subset of children with disseminated medulloblastoma, high-grade gliomas, intrinsic pontine gliomas, atypical teratoid-rhabdoid tumors (AT/RT), and incompletely resected or metastatic ependymomas. Medulloblastomas are the most common malignant pediatric brain tumor (∼20% of cases) and an estimated one-third of these are disseminated at diagnosis.42Packer RJ Rood BR MacDonald TJ Medulloblastoma: present concepts of stratification into risk groups.Pediatr Neurosurg. 2003; 39: 60-67Crossref PubMed Scopus (0) Google Scholar Progression-free survival in patients with high risk, metastatic medulloblastomas only approaches 50%, despite surgery, radiation, and chemotherapy.43Crawford JR MacDonald TJ Packer RJ Medulloblastoma in childhood: new biological advances.Lancet Neurol. 2007; 6: 1073-1085Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar Similarly, outcomes for children with high-grade gliomas are poor with 5-year event-free survival rates ∼20% despite multimodality therapy.44Chastagner P Kalifa C Doz F Bouffet E Gentet JC Ruchoux MM et al.Outcome of children treated with preradiation chemotherapy for a high-grade glioma: results of a French Society of Pediatric Oncology (SFOP) Pilot Study.Pediatr Blood Cancer. 2007; 49: 803-807Crossref PubMed Scopus (0) Google Scholar,45MacDonald TJ Arenson EB Ater J Sposto R Bevan HE Bruner J et al.Phase II study of high-dose chemotherapy before radiation in children with newly diagnosed high-grade astrocytoma: final analysis of Children's Cancer Group Study 9933.Cancer. 2005; 104: 2862-2871Crossref PubMed Scopus (0) Google Scholar With current chemotherapy and radiotherapy, intrinsic pontine gliomas are still uniformly fatal with a median survival rate of ∼9 months.46Korones DN Fisher PG Kretschmar C Zhou T Chen Z Kepner J et al.Treatment of children with diffuse intrinsic brain stem glioma with radiotherapy, vincristine and oral VP-16: a Children's Oncology Group phase II study.Pediatr Blood Cancer. 2008; 50: 227-230Crossref PubMed Scopus (0) Google Scholar,47Dunkel IJ Garvin JH Jr Goldman S Ettinger LJ Kaplan AM Cairo M et al.High dose chemotherapy with autologous bone marrow rescue for children with diffuse pontine brain stem tumors. Children's Cancer Group.J Neurooncol. 1998; 37: 67-73Crossref PubMed Scopus (0) Google Scholar,48Hargrave D Bartels U Bouffet E Diffuse brainstem glioma in children: critical review of clinical trials.Lancet Oncol. 2006; 7: 241-248Abstract Full Text Full Text PDF PubMed Scopus (228) Google Scholar Patients with AT/RT, a recently recognized distinct tumor that tends to occur in children <3 years, have dismal median survival rates of <1 year.49Gidwani P Levy A Goodrich J Weidenheim K Kolb EA Successful outcome with tandem myeloablative chemotherapy and autologous peripheral blood stem cell transplants in a patient with atypical teratoid/rhabdoid tumor of the central nervous system.J Neurooncol. 2008; 88: 211-215Crossref PubMed Scopus (0) Google Scholar Ependymomas most often arise in the posterior fossa from the fourth ventricle with brainstem involvement occurring in up to 50% of cases making gross total resection difficult.50Nazar GB Hoffman HJ Becker LE Jenkin D Humphreys RP Hendrick EB Infratentorial ependymomas in childhood: prognostic factors and treatment.J Neurosurg. 1990; 72: 408-417Crossref PubMed Google Scholar,51Duffner PK Horowitz ME Krischer JP Friedman HS Burger PC Cohen ME et al.Postoperative chemotherapy and delayed radiation in children less than three years of age with malignant brain tumors.N Engl J Med. 1993; 328: 1725-1731Crossref PubMed Scopus (0) Google Scholar Over a quarter of patients are <2 years of age and experience significant morbidity if potentially curative radiation doses are used.52Pierre-Kahn A Hirsch JF Roux FX Renier D Sainte-Rose C Intracranial ependymomas in childhood. Survival and functional results of 47 cases.Childs Brain. 1983; 10: 145-156PubMed Google Scholar Outcomes for children with ependymomas, who have a subtotal resection, who are <3 years of age, or who have disseminated disease to the cord are grim with survival rates of <20%.53Shu HK Sa