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Oncolytic Virotherapy: A Contest between Apples and Oranges

2017; Elsevier BV; Volume: 25; Issue: 5 Linguagem: Inglês

10.1016/j.ymthe.2017.03.026

1525-0024

Stephen J. Russell, Kah-Whye Peng,

Herpesvirus Infections and Treatments

Viruses can be engineered or adapted for selective propagation in neoplastic tissues and further modified for therapeutic transgene expression to enhance their antitumor potency and druggability. Oncolytic viruses (OVs) can be administered locally or intravenously and spread to a variable degree at sites of tumor growth. OV-infected tumor cells die in situ, releasing viral and tumor antigens that are phagocytosed by macrophages, transported to regional lymph nodes, and presented to antigen-reactive T cells, which proliferate before dispersing to kill uninfected tumor cells at distant sites. Several OVs are showing clinical promise, and one of them, talimogene laherparepvec (T-VEC), was recently granted marketing approval for intratumoral therapy of nonresectable metastatic melanoma. T-VEC also appears to substantially enhance clinical responsiveness to checkpoint inhibitor antibody therapy. Here, we examine the T-VEC paradigm and review some of the approaches currently being pursued to develop the next generation of OVs for both local and systemic administration, as well as for use in combination with other immunomodulatory agents. Viruses can be engineered or adapted for selective propagation in neoplastic tissues and further modified for therapeutic transgene expression to enhance their antitumor potency and druggability. Oncolytic viruses (OVs) can be administered locally or intravenously and spread to a variable degree at sites of tumor growth. OV-infected tumor cells die in situ, releasing viral and tumor antigens that are phagocytosed by macrophages, transported to regional lymph nodes, and presented to antigen-reactive T cells, which proliferate before dispersing to kill uninfected tumor cells at distant sites. Several OVs are showing clinical promise, and one of them, talimogene laherparepvec (T-VEC), was recently granted marketing approval for intratumoral therapy of nonresectable metastatic melanoma. T-VEC also appears to substantially enhance clinical responsiveness to checkpoint inhibitor antibody therapy. Here, we examine the T-VEC paradigm and review some of the approaches currently being pursued to develop the next generation of OVs for both local and systemic administration, as well as for use in combination with other immunomodulatory agents. Oncolytic virotherapy uses replication-competent viruses that have been adapted to amplify and spread selectively at sites of tumor growth.1Russell S.J. Peng K.W. Bell J.C. Oncolytic virotherapy.Nat. Biotechnol. 2012; 30: 658-670Crossref PubMed Scopus (973) Google Scholar, 2Seymour L.W. Fisher K.D. Oncolytic viruses: finally delivering.Br. J. Cancer. 2016; 114: 357-361Crossref PubMed Scopus (83) Google Scholar In situ killing of the infected tumor cells, either by the infection or the host immune system, creates a local intratumoral inflammatory milieu containing all the ingredients necessary to boost systemic antitumor immunity (Figure 1). The relative contributions of these two modes of tumor cell killing (direct viral oncolysis and boosted antitumor immunity) is heavily impacted by neutralizing antibodies and virus-reactive T cells generated during previous virus exposures. Virus exposure history is, therefore, an important driver of interindividual variability in oncolytic virus (OV) responses, especially to the first dose of virus administered, and may further lead to progressive changes in the magnitude of response with each successive dose. Talimogene laherparepvec (T-VEC) is an oncolytic herpes simplex virus type 1 (HSV-1) that was recently granted regulatory marketing approval for the treatment of inoperable malignant melanoma.3Rehman H. Silk A.W. Kane M.P. Kaufman H.L. Into the clinic: talimogene laherparepvec (T-VEC), a first-in-class intratumoral oncolytic viral therapy.J. Immunother. Cancer. 2016; 4: 53Crossref PubMed Scopus (229) Google Scholar, 4Bommareddy P.K. Patel A. Hossain S. Kaufman H.L. Talimogene laherparepvec (T-VEC) and other oncolytic viruses for the treatment of melanoma.Am. J. Clin. Dermatol. 2017; 18: 1-15Crossref PubMed Scopus (148) Google Scholar Administered intratumorally into one or more accessible (usually cutaneous) lesions every 2 weeks for up to 18 months, the virus produced durable systemic responses in 16% of treated patients,5Andtbacka R.H. Kaufman H.L. Collichio F. Amatruda T. Senzer N. Chesney J. Delman K.A. Spitler L.E. Puzanov I. Agarwala S.S. et al.Talimogene laherparepvec improves durable response rate in patients with advanced melanoma.J. Clin. Oncol. 2015; 33: 2780-2788Crossref PubMed Scopus (1608) Google Scholar and has since shown extraordinarily promising activity in the same disease indication when combined with ipilumumab or pembrolizumab checkpoint antibody therapy.6Puzanov I. Milhem M.M. Minor D. Hamid O. Li A. Chen L. Chastain M. Gorski K.S. Anderson A. Chou J. et al.Talimogene laherparepvec in combination with ipilimumab in previously untreated, unresectable stage IIIB-IV melanoma.J. Clin. Oncol. 2016; 34: 2619-2626Crossref PubMed Scopus (376) Google Scholar, 7Long G.V. Dummer R. Ribas A. Puzanov I. VanderWalde A. Andtbacka R.H.I. Michielin O. Olszanski A.J. Malvehy J. Cebon J.S. et al.Efficacy analysis of MASTERKEY-265 phase 1b study of talimogene lapherparevec (T-VEC) and pembrolizuman (pembro) for unresectable stage IIIB-IV melanoma.J. Clin. Oncol. 2016; 34Google Scholar Efforts are now underway to explore the potential of T-VEC therapy across a spectrum of different cancers. Unsurprisingly, the successful emergence of T-VEC as an FDA (Food and Drug Administration)-approved drug is accelerating commercial development efforts in the oncolytic virotherapy world and is having a positive energizing effect on numerous academic groups and early-stage companies working with a broad spectrum of OVs deriving from many different virus families and engineered in various ways. In this review article, to commemorate the 20th anniversary of the journal Molecular Therapy, we will focus initially on T-VEC, giving a brief overview of its origins, design, development, and mechanism of action. We will then address three key assumptions that are driving and sustaining current efforts to develop additional OVs and platforms. The first assumption is that T-VEC is not the last word in local intratumoral virotherapy and that better OVs will come. The second assumption is that systemic virotherapy will prove superior to the local T-VEC treatment paradigm. The third assumption is that, even if viruses with superior activity are not found, a wider selection of OVs will facilitate better treatment outcomes that can be achieved with T-VEC alone; for example, through sequential use of different OVs in a given patient and by selecting OVs based on specific characteristics of a given patient and his or her tumor. Finally, we will discuss approaches to combination therapy, patient selection, and OV development in veterinary practice, all of which are likely to accelerate progress during the coming years. Our intent is not to provide a comprehensive overview of the field, and we apologize to those many investigators whose outstanding contributions are not discussed. For more comprehensive coverage of specific topics, the interested reader is referred to a number of previously published reviews addressing the early history of the field,8Kelly E. Russell S.J. History of oncolytic viruses: genesis to genetic engineering.Mol. Ther. 2007; 15: 651-659Abstract Full Text Full Text PDF PubMed Scopus (470) Google Scholar the design and construction of oncolytic viruses,9Maroun J. Muñoz-Alía M. Ammayappan A. Schulze A. Peng K.-W. Russell S. Designing and building oncolytic viruses.Future Virol. 2017; (Published online March 31, 2017)https://doi.org/10.2217/fvl-2016-0129Crossref Scopus (86) Google Scholar the various approaches to oncolytic immunotherapy,10Keller B.A. Bell J.C. Oncolytic viruses—immunotherapeutics on the rise.J. Mol. Med. (Berl.). 2016; 94: 979-991Crossref PubMed Scopus (42) Google Scholar and clinical experience to date as well as ongoing trials.11Lawler S.E. Speranza M.C. Cho C.F. Chiocca E.A. Oncolytic viruses in cancer treatment: a review.JAMA Oncol. 2016; (Published July 21, 2016)https://doi.org/10.1001/jamaoncol.2016.2064Crossref PubMed Scopus (334) Google Scholar Viruses were first shown to have definite antitumor activity in the 1950s.8Kelly E. Russell S.J. History of oncolytic viruses: genesis to genetic engineering.Mol. Ther. 2007; 15: 651-659Abstract Full Text Full Text PDF PubMed Scopus (470) Google Scholar Encouraged by occasional case reports of spontaneous tumor regressions that coincided with viral illnesses, a variety of newly identified viruses were intentionally administered to cancer patients. Not infrequently, treated tumors regressed, but it remained unknown whether this was due to direct oncolysis or amplified antitumor immunity. Whatever their cause, the responses were erratic, short lived, and sometimes associated with unacceptable normal tissue toxicities (e.g., lethal encephalitis). Interest waned but was revived after 1989, when Dr. Robert Martuza reported in Science that a genetically modified HSV-1 virus whose thymidine kinase gene had been inactivated led to tumor control without associated encephalitis when administered into an intracerebral glioma in a mouse model.12Martuza R.L. Malick A. Markert J.M. Ruffner K.L. Coen D.M. Experimental therapy of human glioma by means of a genetically engineered virus mutant.Science. 1991; 252: 854-856Crossref PubMed Scopus (783) Google Scholar This landmark study set the stage for an oncolytic virotherapy renaissance that has continued to this day, with all manner of viruses being engineered to enhance their tumor specificity, safety, druggability, immunogenicity, and oncolytic potency. HSV continued to attract considerable attention as a malleable platform from which a variety of tumor-selective variants have been generated. A favorite approach was to destroy the neurovirulence of lab-adapted strains by disrupting both copies of the γ-34.5 gene whose encoded protein normally suppresses key antiviral responses of the host cell. Without γ-34.5, HSV is unable to shut down the host cell interferon response (by suppressing TBK-1 [TANK-binding kinase 1]), block PKR (protein kinase R)-mediated shutoff of protein synthesis in the infected cell, or inhibit autophagy via beclin-1 blockade.13Wilcox D.R. Longnecker R. The herpes simplex virus neurovirulence factor γ34.5: revealing virus-host interactions.PLoS Pathog. 2016; 12: e1005449Crossref PubMed Scopus (35) Google Scholar Some of these γ-34.5-deleted HSVs were tested clinically, most notably in glioma patients, with encouraging but not definitive results.14Kaur B. Chiocca E.A. Cripe T.P. Oncolytic HSV-1 virotherapy: clinical experience and opportunities for progress.Curr. Pharm. Biotechnol. 2012; 13: 1842-1851Crossref PubMed Scopus (51) Google Scholar T-VEC (or JS1/ICP34.5−/ICP47−/GM-CSF [granulocyte macrophage colony-stimulating factor], the virus that was to become known as T-VEC) was originally described in 2003.15Liu B.L. Robinson M. Han Z.Q. Branston R.H. English C. Reay P. McGrath Y. Thomas S.K. Thornton M. Bullock P. et al.ICP34.5 deleted herpes simplex virus with enhanced oncolytic, immune stimulating, and anti-tumour properties.Gene Ther. 2003; 10: 292-303Crossref PubMed Scopus (560) Google Scholar The thinking behind the design of this virus was that its lab-adapted, γ-34.5-deleted predecessors had been over-attenuated. Thus, T-VEC was derived from a fresh pathogenic virus isolate obtained from the cold sore of a lab worker. Although it was initially attenuated by disrupting both copies of the γ-34.5 gene, the attenuation was partially reversed by engineering US11, whose product also blocks the shutoff of host cell protein synthesis, to be expressed at an earlier stage in the virus infection cycle. In addition to these de-attenuating modifications, the virus was engineered to more effectively boost the antitumor immune response. This was achieved by deleting the ICP47 gene, whose product suppresses antigen presentation by the infected cell, and by inserting two copies of the GM-CSF gene into the virus to activate and promote the differentiation of locally resident antigen-presenting cells (APCs) in the infected tumor. T-VEC was rapidly advanced to the clinic and shown to be active in malignant melanoma, shrinking injected tumors and sometimes leading to the regression of distant metastatic lesions.16Kaufman H.L. Kim D.W. DeRaffele G. Mitcham J. Coffin R.S. Kim-Schulze S. Local and distant immunity induced by intralesional vaccination with an oncolytic herpes virus encoding GM-CSF in patients with stage IIIc and IV melanoma.Ann. Surg. Oncol. 2010; 17: 718-730Crossref PubMed Scopus (403) Google Scholar The phase 3 T-VEC registration trial was launched in May 2009, 2 years before FDA approvals were granted for the anti-CTLA4 antibody ipilumumab and the B-raf inhibitor vemurafenib. Thus, the control randomization arm was subcutaneous GM-CSF, which has very little antimelanoma activity. Between May 2009 and July 2011, 436 patients with unresectable (stage III or IV) melanoma were randomly assigned to intralesional T-VEC or subcutaneous GM-CSF administered every 2 weeks. The durable response rate (responses lasting at least 6 months) was 16.3% in the T-VEC arm and 2.1% in the GM-CSF arm, and T-VEC was associated with a longer overall survival of 23.3 months versus 18.9 months with GM-CSF.5Andtbacka R.H. Kaufman H.L. Collichio F. Amatruda T. Senzer N. Chesney J. Delman K.A. Spitler L.E. Puzanov I. Agarwala S.S. et al.Talimogene laherparepvec improves durable response rate in patients with advanced melanoma.J. Clin. Oncol. 2015; 33: 2780-2788Crossref PubMed Scopus (1608) Google Scholar Based on these positive findings, a biologics license application was filed, and U.S. marketing approval was granted in October 2015, with European and Australian approvals granted shortly thereafter. Arguably, T-VEC is an ideal intratumoral cancer vaccine. It spreads locally within the injected tumor and kills tumor cells by in situ necroptosis, causing them to release tumor antigens, viral antigens, damage-associated molecular patterns (DAMPs), and GM-CSF, providing what is possibly a near-perfect environment for activated APCs to phagocytose a mixture of viral and tumor antigens for presentation to CD4+ helper and CD8+ cytotoxic T cells in the regional lymph nodes.17Kohlhapp F.J. Kaufman H.L. Molecular pathways: mechanism of action for talimogene laherparepvec, a new oncolytic virus immunotherapy.Clin. Cancer Res. 2016; 22: 1048-1054Crossref PubMed Scopus (188) Google Scholar Co-presentation of viral and tumor antigens by individual APCs that have "fed" on virus-infected tumor lysate greatly increases the probability that tumor-reactive cytotoxic T lymphocytes (CTLs) recognizing tumor-specific MHC (major histocompatibility complex)-neoantigenic peptide complexes will be stimulated in an environment that is rich in helper T cell cytokines (e.g., from virus-reactive T helper cells), increasing the probability of their amplification, release, and subsequent trafficking back to sites of tumor growth, the basis of the systemic tumor responses. Can we improve upon this? Will alternative HSV configurations prove superior to T-VEC? There are many new herpes OVs under development encoding matrix-degrading enzymes to enhance their intratumoral spread,18Hong C.S. Fellows W. Niranjan A. Alber S. Watkins S. Cohen J.B. Glorioso J.C. Grandi P. Ectopic matrix metalloproteinase-9 expression in human brain tumor cells enhances oncolytic HSV vector infection.Gene Ther. 2010; 17: 1200-1205Crossref PubMed Scopus (33) Google Scholar fusogenic membrane glycoproteins to enhance their potency,19Nakamori M. Fu X. Meng F. Jin A. Tao L. Bast Jr., R.C. Zhang X. Effective therapy of metastatic ovarian cancer with an oncolytic herpes simplex virus incorporating two membrane fusion mechanisms.Clin. Cancer Res. 2003; 9: 2727-2733PubMed Google Scholar, 20Simpson G.R. Coffin R.S. Construction and characterization of an oncolytic HSV vector containing a fusogenic glycoprotein and prodrug activation for enhanced local tumor control.Methods Mol. Biol. 2009; 542: 551-564Crossref PubMed Scopus (5) Google Scholar and/or receptor-targeted attachment proteins,21Menotti L. Cerretani A. Hengel H. Campadelli-Fiume G. Construction of a fully retargeted herpes simplex virus 1 recombinant capable of entering cells solely via human epidermal growth factor receptor 2.J. Virol. 2008; 82: 10153-10161Crossref PubMed Scopus (91) Google Scholar, 22Goins W.F. Hall B. Cohen J.B. Glorioso J.C. Retargeting of herpes simplex virus (HSV) vectors.Curr. Opin. Virol. 2016; 21: 93-101Crossref PubMed Scopus (18) Google Scholar microRNA targets,23Lee C.Y. Rennie P.S. Jia W.W. MicroRNA regulation of oncolytic herpes simplex virus-1 for selective killing of prostate cancer cells.Clin. Cancer Res. 2009; 15: 5126-5135Crossref PubMed Scopus (87) Google Scholar, 24Mazzacurati L. Marzulli M. Reinhart B. Miyagawa Y. Uchida H. Goins W.F. Li A. Kaur B. Caligiuri M. Cripe T. et al.Use of miRNA response sequences to block off-target replication and increase the safety of an unattenuated, glioblastoma-targeted oncolytic HSV.Mol. Ther. 2015; 23: 99-107Abstract Full Text Full Text PDF PubMed Scopus (57) Google Scholar and tissue-specific promoters25Kambara H. Okano H. Chiocca E.A. Saeki Y. An oncolytic HSV-1 mutant expressing ICP34.5 under control of a nestin promoter increases survival of animals even when symptomatic from a brain tumor.Cancer Res. 2005; 65: 2832-2839Crossref PubMed Scopus (179) Google Scholar, 26Yoo J.Y. Haseley A. Bratasz A. Chiocca E.A. Zhang J. Powell K. Kaur B. Antitumor efficacy of 34.5ENVE: a transcriptionally retargeted and "Vstat120"-expressing oncolytic virus.Mol. Ther. 2012; 20: 287-297Abstract Full Text Full Text PDF PubMed Scopus (45) Google Scholar to target their tropisms. With the tighter tumor specificity of these targeted HSVs, it is now considered reasonable to further de-attenuate them (e.g., by reconstituting γ-34.5) to more fully restore their antitumor potency. Will the newer oncolytic HSVs prove superior to T-VEC? Can T-VECs potency be increased simply by increasing the dose or optimizing the intratumoral injection technique? What can be gained by changing the composition of the inflammatory tumor lysate by encoding interleukins, cytokines, and T cell chemokines in the viral genome? Will tumor-reactive T cells be more effectively engaged by viruses encoding proteins that kill uninfected as well as infected tumor cells? The answers to these questions are difficult to predict or to model preclinically, but, judging by the intensity of activity in this area at the present time, answers will soon be forthcoming. So what about other viruses? HSV is by no means the only platform showing promise as an intratumoral cancer therapy, and melanoma is not the only tumor that responds to oncolytic virotherapy. In brain cancer, for example, in addition to several oncolytic HSVs, patients are being treated with viruses from other families27Foreman P.M. Friedman G.K. Cassady K.A. Markert J.M. Oncolytic virotherapy for the treatment of malignant glioma.Neurotherapeutics. 2017; (Published March 6, 2017)https://doi.org/10.1007/s13311-017-0516-0Crossref PubMed Scopus (96) Google Scholar and with encouraging results. Examples include a recombinant poliovirus incorporating a neuroattenuating rhinovirus internal ribosome entry site ([PV-RIPO] NCT: 01491893);28Goetz C. Dobrikova E. Shveygert M. Dobrikov M. Gromeier M. Oncolytic poliovirus against malignant glioma.Future Virol. 2011; 6: 1045-1058Crossref PubMed Scopus (38) Google Scholar a C-type retrovirus encoding the enzyme cytosine deaminase, which activated the prodrug 5-fluorocytosine to 5-fluorouracil (Toca-511);29Cloughesy T.F. Landolfi J. Hogan D.J. Bloomfield S. Carter B. Chen C.C. Elder J.B. Kalkanis S.N. Kesari S. Lai A. et al.Phase 1 trial of vocimagene amiretrorepvec and 5-fluorocytosine for recurrent high-grade glioma.Sci. Transl. Med. 2016; 8: 341ra75Crossref PubMed Scopus (128) Google Scholar an integrin-targeted serotype 5 adenovirus neuroattenuated by an E1A deletion (DNX-2401);30Lang F. Conrad C. Gomez-Manzano C.D. Tufaro F. Yung W. Sawaya R. Weinberg J. Prabhu S. Fuller G. Aldape K. Fueyo J. First-in-human phase I clinical trial of oncolytic delta-24-RGD (DNX-2401) with biological endpoints: Implications for viro-immunotherapy.Neuro Oncol. 2014; 16: iii39Crossref Google Scholar and a nonengineered rat parvovirus (H-1PV).31Geletneky K. Huesing J. Rommelaere J. Schlehofer J.R. Leuchs B. Dahm M. Krebs O. von Knebel Doeberitz M. Huber B. Hajda J. Phase I/IIa study of intratumoral/intracerebral or intravenous/intracerebral administration of Parvovirus H-1 (ParvOryx) in patients with progressive primary or recurrent glioblastoma multiforme: ParvOryx01 protocol.BMC Cancer. 2012; 12: 99Crossref PubMed Scopus (123) Google Scholar Also, in non-brain-cancer indications, there are many examples of promising clinical responses following intratumoral administration of non-T-VEC oncolytics. Noteworthy examples include H101, an E1B-deleted serotype 5 adenovirus that was approved by the Chinese FDA in 2005 for intratumoral therapy of head and neck cancer in combination with standard chemotherapy;32Xia Z.J. Chang J.H. Zhang L. Jiang W.Q. Guan Z.Z. Liu J.W. Zhang Y. Hu X.H. Wu G.H. Wang H.Q. et al.[Phase III randomized clinical trial of intratumoral injection of E1B gene-deleted adenovirus (H101) combined with cisplatin-based chemotherapy in treating squamous cell cancer of head and neck or esophagus].Chin. J. 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Importantly, from a drug development perspective, viruses being developed clinically for non-melanoma indications might never have to prove superiority in a head-to-head comparison with T-VEC, but this depends, to some extent, on whether and how rapidly T-VEC marketing approvals are pursued and granted in other cancer indications. This is a critical question for OV drug developers that must be carefully considered before initiating expensive randomized trials. With each new drug approval for a given cancer, the bar for newer drug approvals is set progressively higher. Newer drugs always have to prove superiority over existing drugs as determined by higher response rates, better durability of response, longer survival, lower toxicity, and/or activity in treatment refractory disease. So with the approval of T-VEC as a melanoma therapy, a high bar has been set for future OV approvals in that indication, and the race is truly on to show clinical benefit of many competing OV platforms in other cancers. However, given the diverse tissue tropisms of naturally occurring viruses, it seems highly unlikely that T-VEC could prove superior to all other intratumoral oncolytic agents across all tumor histologies. T-VEC does, however, have a theoretical edge over non-herpes OVs in that the natural behavior of HSV-1, the natural precursor of T-VEC, is to reactivate repeatedly throughout life, causing significant local tissue damage in the face of a robust adaptive immune response.35Chentoufi A.A. Benmohamed L. Mucosal herpes immunity and immunopathology to ocular and genital herpes simplex virus infections.Clin. Dev. Immunol. 2012; 2012: 149135PubMed Google Scholar Thus, as an in situ vaccine that can amplify tumor neoantigen-reactive T cells even in the face of pre-existing antiviral immunity, T-VEC seems like a potentially ideal drug to partner with immune checkpoint antibody therapies. Early clinical data addressing this question are already strongly supportive of the concept, showing greatly improved response rates in patients with stage III/IV melanoma treated with a combination of anti-CTLA4 or anti-PD1 antibody therapy plus T-VEC.6Puzanov I. Milhem M.M. Minor D. Hamid O. Li A. Chen L. Chastain M. Gorski K.S. Anderson A. Chou J. et al.Talimogene laherparepvec in combination with ipilimumab in previously untreated, unresectable stage IIIB-IV melanoma.J. Clin. Oncol. 2016; 34: 2619-2626Crossref PubMed Scopus (376) Google Scholar, 7Long G.V. Dummer R. Ribas A. Puzanov I. VanderWalde A. Andtbacka R.H.I. Michielin O. Olszanski A.J. Malvehy J. Cebon J.S. et al.Efficacy analysis of MASTERKEY-265 phase 1b study of talimogene lapherparevec (T-VEC) and pembrolizuman (pembro) for unresectable stage IIIB-IV melanoma.J. Clin. Oncol. 2016; 34Google Scholar However, there is emerging evidence for similar synergistic interaction with checkpoint antibody therapy in melanoma patients treated with intratumoral CVA21 (Viralytics, 2016, Society for the Immunotherapy of Cancer, conference), and with an unarmed oncolytic HSV-1 not encoding GM-CSF.36Andtbacka R.H.I. Ross M.I. Agarwala S.S. Taylor M.H. Vetto J.T. Neves R.I. Daud A. Khong H.T. Ungerleider R.S. Boran A.D.W. et al.Preliminary results from phase II study of combination treatment with HF10, a replication-competent HSV-1 oncolytic virus, and ipilimumab in patients with stage IIIb, IIIc, or IV unresectable or metastatic melanoma.J. Clin. Oncol. 2016; 34PubMed Google Scholar Responses to the combination of CVA21 plus pembrolizumab were even observed in T-VEC refractory patients (Viralytics, 2016, Society for the Immunotherapy of Cancer, conference). Detailed analysis of individual lesion response rates in melanoma patients receiving intratumoral T-VEC showed complete responses in 46.1% of injected lesions, 30.1% of uninjected non-visceral lesions, and only 9.4% of uninjected visceral lesions.5Andtbacka R.H. Kaufman H.L. Collichio F. Amatruda T. Senzer N. Chesney J. Delman K.A. Spitler L.E. Puzanov I. Agarwala S.S. et al.Talimogene laherparepvec improves durable response rate in patients with advanced melanoma.J. Clin. Oncol. 2015; 33: 2780-2788Crossref PubMed Scopus (1608) Google Scholar It is clear from this analysis that tumors respond better to oncolytic virotherapy when they actually get infected with the virus. The rationale for systemic virus delivery to all sites of tumor dissemination is, therefore, compelling, and it is expected that, if adequate systemic delivery can be achieved, response rates will be maximized. So is efficient systemic virus delivery feasible, and how can it be achieved? The barriers to successful intravenous therapy are well known:1Russell S.J. Peng K.W. Bell J.C. Oncolytic virotherapy.Nat. Biotechnol. 2012; 30: 658-670Crossref PubMed Scopus (973) Google Scholar, 37Miller A. Russell S.J. The use of the NIS reporter gene for optimizing oncolytic virotherapy.Expert Opin. Biol. Ther. 2016; 16: 15-32Crossref PubMed Scopus (49) Google Scholar massive dilution of the virus in the bloodstream; neutralization by antiviral antibodies and complement proteins; virus particle sequestration in liver Kuppfer cells and splenic macrophages; and the limited permeability of tumor neovessels. Considering these barriers, it is easy to understand why T-VEC is considered a poor candidate for systemic application. It is difficult to manufacture in sufficient quantities for systemic administration and is susceptible to rapid neutralization by circulating anti-HSV-1 antibodies in at least 50% of treatment-eligible patients.38Smith J.S. Robinson N.J. Age-specific prevalence of infection with herpes simplex virus types 2 and 1: a global review.J. Infect. Dis. 2002; 186: S3-S28Crossref PubMed Scopus (683) Google Scholar, 39Xu F. Schillinger J.A. Sternberg M.R. Johnson R.E. Lee F.K. Nahmias A.J. Markowitz L.E. Seroprevalence and coinfection with herpes simplex virus type 1 and type 2 in the United States, 1988-1994.J. Infect. Dis. 2002; 185: 1019-1024Crossref PubMed Scopus (214) Google Scholar Also, HSV particles are large (150–200 nm) and, hence, less likely to extravasate from tumor neovessels, even from those with abnormally increased permeability. Viremic dissemination of natural virus infections is typically sustained by the continuous release of progeny particles (e.g., poliovirus, smallpox) or infected cells (e.g., measles-infected lymphocytes and monocytes) from one or more primary sites of infection.40Virgin S. Pathogenesis of viral infection.in: Fields Virology. Volume 1. Lippincott, Williams and Wilkins, Philadelphia, PA2007: 327-388Google Scholar, 41Griffin D.E. Measles virus.in: Fields Virology. Volume 2. Lippincott, Williams and Wilkins, Philadelphia, PA2007: 1551-1586Google Scholar Also, when viremia is cell associated, dissemination may continue even after the appearance of neutralizing antiviral antibodies.42Zingher A. Mortimer P. Convalescent whole blood, plasma and serum in the prophylaxis of measles: JAMA, 12 April, 1926; 1180-1187.Rev. Med. Virol. 2005; 15: 407-418Crossref PubMed Scopus (29) Google Scholar, 43Moore A.E. 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