Fighting Cancer with Viruses: Oncolytic Virus Therapy in China

Abstract

As part of oncolytic virotherapy to treat cancer, oncolytic viruses (OVs) can selectively infect tumor cells to promote oncolysis of cancer cells, local immunological reactions, and systemic antitumor immunity with minimal toxicity to normal tissues. The immunostimulatory properties of OVs provide enormous benefits for the treatment of cancer. A variety of OVs, including genetically engineered and natural viruses, have shown promise in preclinical models and clinical studies. In 2005, the China Food and Drug Administration approved its first OV drug, Oncorine (H101), for treatment of advanced head and neck cancer. To explore new treatment strategies, >200 recombinant or natural OVs are undergoing in-depth investigation in China, and >250 oncolytic virotherapy-related reports from the OV community in China have been published in the past 5 years. These studies investigated a variety of exogenous genes and combination therapeutic strategies to enhance the treatment effects of OVs. To date, five clinical trials covering four OV agents (Oncorine, OrienX010, KH901, and H103) are ongoing, and additional OV agents are awaiting approval for clinical trials in China. Overall, this research emphasizes that combination therapy, especially tumor immunotherapy coupled with effective system administration strategies, can promote the development of oncolytic virotherapies. This article focuses on studies that were carried out in China in order to give an overview of the past, present, and future of oncolytic virotherapy in China.

Introduction

Oncolytic viruses (OV) compose a group of natural or recombinant viruses that can selectively induce lysis of tumor cells while sparing normal cells. Oncolytic virotherapy is regarded as a type of gene therapy that uses OVs as viral vectors for the efficient delivery of therapeutic genes to tumor cells. The antitumor activity of OVs arises from two distinct mechanisms: selective replication in tumor cells, resulting in a direct lytic effect on tumor cells, and induction of systemic antitumor immunity. OVs kill tumor cells and promote release of tumor-associated antigens, viral pathogen-associated molecular pattern signals (PAMPs), cellular danger-associated molecular pattern signals (DAMPs), and cytokines. These molecules induce maturation of antigen-presenting cells (APCs) and active effector T cells, which mediate antitumor immunity upon antigen recognition.¹

Although a study reporting the use of a virus as an antitumor agent was published in 1904, more in-depth research on OVs was executed in the 1950s with the development of cell culture techniques. OV research rapidly progressed in the 1990s with the advent of improved molecular biology techniques. Currently, nearly 400 OV-related reports are published annually, and >40 OV-related clinical trials have been initiated worldwide. Three OV agents, RIGVIR, Oncorine, and Imlygic, have been approved for cancer therapy. RIGVIR is a wild-type nonpathogenic enteric cytopathic human orphan virus, and was the first OV agent approved for marketing worldwide its registration for melanoma treatment in Latvia in 2004. In October 2015, the U.S. Food and Drug Administration (FDA) approved its first OV drug, Imlygic (generic name Talimogene laherparepvec [T-Vec]), a recombinant herpes simplex virus (HSV) expressing human granulocyte-macrophage colony stimulating factor (GM-CSF). Imlygic was developed by Amgen for the treatment of melanoma in the United States, Europe, and Australia.

In 1973, a review published in a Chinese journal outlined a brief history, mechanisms, and clinical trials of OVs.² The authors believed that enhanced antitumor immunity, rather than tumor cell lysis, was a principal outcome of OV therapy, as mentioned by Jules E. Harris in 1970.³ In 1985, two research teams from the Harbin Medical University published the first report of oncolytic virotherapy in China showing that vaccinia virus, enteric cytopathogenic cervine orphan virus, and enteric cytopathogenic swine orphan virus could be used as active antitumor agents for hepatocellular carcinoma (HCC) and lung adenocarcinoma therapy.^4,5 From then until the 1990s, several reports concerning OVs were published in China annually, and all used natural OVs as antitumor agents. With the development of genetic engineering, treatments involving recombinant OVs, including oncolytic adenovirus (Adv) and oncolytic HSV, have entered the mainstream. In November 2005, the China Food and Drug Administration (CFDA) approved its first OV agent, Oncorine (H101), to treat head and neck cancer. Oncorine is a recombinant Adv in which the E1b-55kD region is deleted, allowing the virus to replicate in and destroy cancer cells with preference.⁶ The approval of Oncorine accelerated the study of oncolytic virotherapy in China, as evidenced by the rapid increase in the number of OV-related studies originating in China since 2005; this rate peaked in 2012 (Fig. 1A). More than 200 OVs have been generated and are under investigation for the treatment of various cancers, and five OV-related clinical trials have been initiated in China (Table 1). This article focuses on studies carried out by the Chinese OV community in order to show the past, present, and future of oncolytic virotherapy in China.

Figure 1.

Overview of oncolytic virus (OV)-related reports from the Chinese OV community. (A) The proportion of the reports from the Chinese OV community in global OV-related reports each year. (B) Stacked area chart for the constitution of OV in China. The different colors indicate the proportion for each viral species. These results are based on data collected from nearly 400 OV-related published articles in China from 1998 to 2016. Adv, adenovirus; HSV, herpes simplex virus; NDV, Newcastle disease virus; VSV, vesicular stomatitis virus; MV, measles virus; BTV, bluetongue virus; VV, vaccinia virus.

Table 1.

Summary of oncolytic viruses starting clinical trials in China

Clinical candidate	Virus	Sponsor	Status	Primary indication	Trial Phase	Combination	Data source
Oncorine	Adv	Xinqiao Hosptial of Chongqing	Not recruiting	Malignant hydrothorax; non-small-cell lung cancer	III	Systemic chemotherapy	clinicaltrials.gov
Oncorine	Adv	Zhejiang Cancer Hospital	Prospective registration	Advanced cervical cancer	IV	Radiotherapy and chemotherapy	www.chictr.org.cn
KH901	Adv	Chengdu Kanghong Biotechnology Co. Ltd.	Active	Head and neck cancer	II	Systemic chemotherapy	www.chinadrugtrials.org.cn
H103	Adv	Shanghai Sunway Biotech Co. Ltd.	Complete	Shanghai Sunway Biotech Co. Ltd.	I	n/a	Ref. 17
OrienX010	HSV	OrienGene Biotechnology Ltd.	Recruiting	Melanoma	Ib	n/a	clinicaltrials.gov

Adv, adenovirus; HSV, herpes simplex virus; n/a, non-applicable.

Ov Species

Natural OVs, such as the vaccina virus, cervine orphan virus, and Newcastle disease virus (NDV), were the main options during the initial phase of oncolytic virotherapy research. Since the 1990s, sequencing and viral genome recombination, together with advances in genetic engineering, has resulted in the generation of numerous recombinant OVs that carry various tumor-specific promoters and exogenous genes for virotherapy. The oncolytic Adv and HSV were the most two common types of OVs used for virotherapy in China, and appeared in 66% and 10% of studies, respectively. In recent years, however, the proportion of recombinant Adv has decreased significantly, as the diversity of OV species has increased. In 2016, recombinant Adv (40.5%), recombinant HSV (18.9%), NDV (10.8%), and alphavirus M1 (10.8%) were the most four common species of OVs used for tumor therapy (Fig. 1B).

Adv is widely used in gene therapy due to its favorable safety profile, ease of purification, and adequate capacity to carry exogenous genes. For selective replication in tumor cells, one or more early gene regions (E1b, E1a, and E3) of Adv are typically deleted or driven by a tumor-specific promoter such as α-fetoprotein (AFP) or human telomerase reverse transcriptase (hTERT) promoters.^7
–9 As a non-enveloped virus, oncolytic Adv can be easily encapsulated by cationic liposomes to facilitate systemic delivery.

HSV can be categorized into two types, HSV-1 and HSV-2, which infect humans to cause herpes labialis and genital herpes, respectively. Oncolytic HSV usually has a deletion of the virulence gene ICP34.5 to confer tumor-selective replication. As a therapeutic agent, oncolytic HSV offers several advantages, including large transgene capacity, broad host range, and excellent safety in humans through control by anti-herpetic agents.¹⁰

NDV is a natural OV that kills tumor cells selectively by capitalizing on the defective antiviral activity of tumor cells. As an avian virus, NDV is safe for human use and causes only mild transient flu-like symptoms.¹¹ Genetically modified NDV can also act as a viral vector for expression of exogenous genes in tumor cells to enhance antitumor efficiency.

Alphavirus M1, isolated from culicine mosquitoes in China, is another natural OV that can specifically induce cancer cell apoptosis.^12,13 Lin et al. revealed that tumor-selective replication of alphavirus M1 was due to deficiency of the zinc-finger antiviral protein (ZAP) that exists in multiple human cancers. ZAP inhibits viral replication by inducing viral RNA degradation and translational inhibition.¹⁴ Alphavirus M1 was shown not to be pathogenic in cynomolgus macaques after multiple rounds of intravenous injection.¹⁵ As a natural OV that is totally “made in China,” alphavirus M1 has progressed rapidly and aroused broad public concern about use of OV in China.

Oncolytic Virotherapy Applications for Cancer

Based on data from China's Health and Family Planning Statistical Yearbook 2016, the three most common cancers in China are lung carcinoma, HCC, and gastric carcinoma. Coincidentally, the top three cancers involved in basic research of oncolytic virotherapy in China are HCC, lung carcinoma, and colon carcinoma, which is similar to their frequency among the Chinese population (Fig. 2). Meanwhile, in clinical trials, the situation is entirely different. Clinical trials using OV agents to treat head and neck cancer and melanoma are the most common in China and around the world.^8,16,17 Finding agents that can successfully treat cancer, especially the most common cancers, is a goal of OV study. However, issues involving the delivery method of OV agents have limited the application for oncolytic virotherapy of cancer. Tumors located near the surface of the body have typically been chosen for clinical oncolytic virotherapy study due to the ease of intratumoral injection. Thus, development of more effective delivery strategies is needed to expand the application of OV agents in the future.

Figure 2.

Pie chart for the indication of OV-related studies in China. The analysis was conducted based on the same data mentioned in Fig. 1.

Treatment Strategies Involving Ov

OVs can promote tumor cell death directly via multiple mechanisms. Yet, they cannot destroy tumor tissue completely. As such, combination of OVs with other exogenous genes or therapeutic strategies is needed to enhance therapeutic success. In China, >60 exogenous genes have been used to arm OVs. These genes can be divided into several groups: (1) cell death–related molecules that induce tumor cell death directly, such as tumor necrosis factor–related apoptosis-inducing ligand (TRAIL),¹⁸ P53,¹⁹ apoptin, interleukin (IL)-24,²⁰ and HSV type 1-thymidine kinase (HSV1-TK); (2) anti-angiogenic molecules such as endostatin, vascular endothelial cell growth inhibitor (VEGI), and fifth kringle domain of plasminogen (K5) that could inhibit tumor tissue angiogenesis²¹; (3) immunomodulatory molecules, including immune-related cytokines (IL-2, IL-18,²² IL-12,²³ GM-CSF,²⁴ interferon), chemokines (CCL5, CCL20, CCL21), and other molecules that trigger antitumor immune activity, including virus membrane proteins (rabies virus glycoprotein, NDV HN) and HSP70; and (4) small RNA molecules such as miRNA, siRNA, shRNA, and lncRNA to silence tumor-related genes (Fig. 3).

Figure 3.

Categorization of exogenous genes carried by OVs in China. The analysis was conducted based on the same data mentioned in Fig. 1.

Combination of OVs with other therapeutic methods is another convenient therapeutic strategy. OVs combined with chemotherapy,²⁵ radiotherapy,²⁶ and vascular interventional therapies are the most common combination approaches. Some novel therapeutic methods, including molecularly targeted therapy, gene therapy, and cellular immunotherapy, have also been investigated as potential combination therapies involving OVs in China.²⁷

Delivery Methods of Ov

Although OVs can destroy tumor cells selectively, infection with OV is non-specific due to the wide distribution of OV membrane receptors. Furthermore, neutralizing antibodies against specific OVs, including Adv and HSV,^28,29 also exist in human serum that can direct efficient removal of viral particles. Another challenge is how to ensure effective infiltration of OV particles to tumor tissues. To avoid nonspecific binding and enhance OV concentration in tumor tissues, intratumoral injection is often used to administer OVs, but this approach significantly limits the therapeutic effects of OVs on metastasis. To enhance Adv transfer efficiency, Zhang et al. used the proteasome inhibitor MG-132 to improve coxsackie-Adv receptor (CAR) expression in colon cancer cells via a post-translational mechanism. The increased CAR expression levels facilitated Adv infection, in turn enhancing expression of exogenous genes and the antitumor activity of oncolytic Adv.³⁰

Encapsulating OV is another strategy to reduce nonspecific binding and avoid neutralizing antibody in serum. Cationic liposomes and tumor cell–derived microparticles (T-MPs) have been used to encapsulate Adv to increase antitumor activity and to prolong circulation time.^31,32 These features will likely facilitate systemic administration of OVs and greatly improve their therapeutic efficacy.

Dendritic cells (DCs) are professional antigen-presenting cells that are also used as an efficient carrier for OVs. Li et al. constructed a recombinant Adv expressing a prostate-specific antigen (PSA)-CD40L fusion protein that was loaded with mature DCs. Mice bearing prostate carcinoma treated with DC-loaded Adv administered via intravenous injection showed significant antitumor activity in vivo.³³ This result provides a potential strategy to combine cellular immunotherapy with oncolytic virotherapy for future tumor treatments.

OVs DEVELOPED IN CHINA

Oncolytic Adv

Adv is a non-enveloped virus with a non-segmented double-stranded DNA genome. Adv infection is a common cause of respiratory system illnesses but is also a popular viral vector for gene therapy, given its large carrying capacity for exogenous genes (>30 kb) and its ability to affect both replicating and non-replicating cells. Recombinant oncolytic Advs were the first OVs to enter clinical trials for the treatment of cancers. To date, >135 recombinant OVs have been constructed in China that are based on Adv.

H101, H102, and H103

H101 (Oncorine), the first approved OV agent by the CFDA, is a recombinant type 5 Adv that has deletion of the E1b-55kD gene and carries no exogenous gene. Two other recombinant Advs, H102 and H103, were developed for further modification. H102 introduces the tumor-specific promoter (AFP promoter) to drive expression of the Ad E1a gene. H103 carries the heat shock protein 70 (HSP70) gene that can stimulate an antitumor immune response.⁶ A Phase I clinical trial of H103 involving 27 patients with advanced-stage solid tumors showed 11.1% objective response (complete response [CR] + partial response [PR]) and 48.1% clinical benefit rate (CR, PR, mild response, and stable disease) among H103-injected tumors.¹⁶

ZD55

ZD55 is a recombinant OV derived from human Adv type 5 that carries no exogenous gene. In this OV, the E1b-55kD region is replaced by an expression box derived from the human CMV promoter and SV40 polyA element.⁹ This modification confers selective lysis of tumor cells by ZD55, which can be easily modified to carry an exogenous gene, termed “Cancer Targeting Gene-Viro-Therapy” (CTGVT). Based on ZD55, an OV family was developed that includes insertion of 20 different exogenous genes,³⁴ coding cell death–related molecules (TRAIL, Smac,¹⁸ IL-24,²⁰ MnSOD, XAF1,³⁴ CYLD, IFN-β, dNK), anti-angiogenesis molecules (mK5,²¹ soluble sflt-1), immunomodulatory molecules (ST13, IL-18),^22,35 shRNA,^36
–38 and monoclonal antibody against tumor-associated antigen CD147 (HAb18)³⁹ to explore treatment strategies for various cancers. Several tissue-specific promoters were also used to enhance tumor specificity of these OVs, such as the G250 promoter (targeting renal cell carcinoma),⁴⁰ DD3 promoter (targeting prostate cancer),⁴¹ and AFP promoter (targeting HCC).⁴²

Using the ZD55 series, many virotherapy studies focusing on diverse tumors, including HCC, colorectal carcinoma, and melanoma, have been carried out. ZD55 can be combined with other agents or methods, such as chemotherapy and radiotherapy,^25,26 to enhance therapeutic effects. ZD55 has also been combined with another ZD55 derivative to induce synergistic effects, termed “cancer-targeting dual-gene virotherapy” (CTVGT-DG). To facilitate the administration, several ZD55 sets were constructed to express a chimeric exogenous gene that combines two or more exogenous genes using a linker.^39,43 Indeed, ZD55-TRAIL-IETD-Smac, carrying TRAIL and Smac linked with IETD, shows stronger antitumor activity compared to ZD55-TRAIL or ZD55-Smac alone in various tumor cell lines.⁴³ These results indicate that “dual-gene virotherapy” can be used to enhance the therapeutic effect of OV therapy. Moreover, the IETD linker, which can be cleaved by caspase-8 and is composed of only four amino acids, could see broad use as a linker of two genes in recombinant OVs.

SG600-P53

The SG series is a recombinant oncolytic Adv family that has >10 members. To increase the capacity of gene delivery and tumor-specificity of the SG series member SG600, the E1a gene CR2 region was partly deleted, and the hTERT promoter and hormone response element (HRE) were inserted to control expression of the E1a and E1b genes, respectively.¹⁹ Based on this triple-regulated oncolytic Adv, three OVs agents were generated carrying p53,¹⁹ IL-24,⁴⁴ and programmed cell death protein 5 (Pdcd5) genes that can be used to treat HCC.

SG600-P53 carries the p53 gene under control of the CMV promoter. In a tumor xenograft mouse model, SG600-P53 achieved excellent antitumor efficacy compared to SG600, ONYX-015, and Ad-P53.¹⁹ To improve this antitumor effect further, Ad5 fiber in SG600-P53 was replaced with a chimeric Ad5/35 fiber to generate a new recombinant Adv SG635-P53, which showed a markedly enhanced antitumor effect in HCC cells.⁴⁵ The safety and potential adverse effects of SG600-derived Adv were investigated by safety pharmacology test, acute toxicity test, repeat-dose toxicity test, and systemic anaphylaxis test carried out in mice, rats, cats, guinea pigs, and cynomolgus monkeys. Results from these studies suggested that SG600-P53 is a safe antitumor therapeutic agent. To date, preclinical study of SG635-P53 is complete and will soon enter clinical trial.

M7 and M8

MXs (M1–M8) are a family of recombinant oncolytic Advs that carry different exogenous genes for cancer treatment, and were developed by the Cancer Biology Research Center at Tongji Hospital (Wuhan, Hubei, China). Use of a replication-deficient Adv vector carrying HSV-TK (ADV-TK), a suicide gene used in cancer gene therapy that can render cells susceptible to nucleoside prodrugs such as ganciclovir and acyclovir, is considered to be one of the best approaches to cancer gene therapy. HSV-TK has been used in clinical trials for human cancers since the 1990s. The ADV-TK agent is currently being developed by Shenzhen Tiandakang Gene Engineering (Shenzhen, Guangdong, China), and a Phase II study for gene therapy of late-stage head and neck cancer is ongoing. To improve the therapeutic effects of ADV-TK further, two oncolytic Adv vectors, M7 and M8, which share a 27 bp deletion in the E1a region and use HSV-TK to replace 6.7K/gp19K and the Adv death protein gene of the E3 region, respectively, were generated.⁴⁶ Evaluation and comparison of the antitumor activity and safety of M7, M8, and ADV-TK showed that both M7 and M8 displayed superior antitumor activity to ADV-TK while having similar safety profiles both in vitro and in vivo.⁴⁷ Two other groups in China also constructed oncolytic Adv carrying HSV-TK for HCC and non-small-cell lung cancer treatments.⁴⁸ Their studies indicate that the combination of oncolytic Adv and HSV-TK is a potent and safe antitumor strategy.

Oncolytic HSV OrienX010

HSV is an enveloped, double-stranded linear DNA virus that causes a variety of skin diseases in humans. The complete genomic DNA sequence of HSV-1 was first published in 1988. Since then, the HSV genome has been intensively studied. Several HSV recombinant strategies were established to generate engineered HSV for use in gene therapy and virotherapy.¹⁰ To date, more than one-tenth of published OV-related reports in China are based on recombinant HSV, which ranks second behind recombinant Adv.

OrienX010 is an oncolytic HSV expressing GM-CSF that was developed by OrienGene Biotechnology Ltd. (Beijing, China). OrienX010 shares a similar strategy with T-Vec but uses the HSV-1 CL1 strain that was isolated in China. The ICP34.5 and ICP47 genes are deleted in OrienX010, and genes encoding human GM-CSF and deactivated ICP6 gene, respectively, are inserted.²⁴ Following CFDA approval of OrienX010 in 2010, a Phase I dose escalation clinical trial of this OV to treat melanoma carried out at the Capital Medical University Cancer Center (Beijing, China) was completed. The results showed that seven of nine patients with advanced malignancy experienced disease stabilization, and the median progression-free survival was 16.6 months for all patients.²⁴

NDV

The avian virus NDV is a natural OV that selectively infects human tumor cells. NDV has both lytic and nonlytic strains. The lytic strain can kill infected cells and produce infectious progeny virus particles in cancer cells, whereas the nonlytic strain cannot. NDV has been used for clinical applications since the 1950s, and three strains (MTH-68/H, NDV-HUJ, and PV701) were used in Phase I/II clinical trials for oncolytic tumor therapy in the United States and Israel. These therapies used different routes of delivery (intravenous, peritumoral, and intratumoral) and demonstrated the safety of NDV at high viral doses. More than a dozen clinical trials reported beneficial effects of NDV-based anticancer therapies.⁴⁹ In China, the first published report using NDV as an anticancer agent appeared in 1998. In this study, 45 patients suffering from multiple kinds of tumors, including nasopharyngeal carcinoma, breast carcinoma, and melanoma, were treated. In the ensuing 20 years, several strains of NDV and its proteins (HN, F) have been studied as OV agents in China.

NDV Italien strain

NDV Italien strain is a lytic strain that can induce tumor regression in a preclinical mouse model and a preliminary clinical study. The entire genome of this strain was sequenced to facilitate construction of recombinant NDVs carrying different exogenous genes.^50,51 Using these rNDVs, some key characteristics of the NDV Italien stain were investigated, including replication rate, biodistribution, antitumor activity in vivo, preference for sialic acid linkage (α2,6-) in viral attachment, and oncolytic effect.⁵² Furthermore, a recombinant NDV (rNDV-18HL) expressing intact human-mouse chimeric HAb18 antibody was generated that combines the NDV oncolytic properties with antibody-mediated effects.⁵³ rNDV-18HL reduced intrahepatic metastasis coupled with tumor lysis and prolonged survival of HCC-bearing mice. CD147, the target of HAb18 antibody, is overexpressed in HCC cells and is involved in tumor development. Injection of iodine [¹³¹I]-conjugated HAb18 F(ab′)₂ antibody (metuximab, trade name Licartin) has been approved in China for treatment of HCC since 2005. In 2015, the CFDA approved an affinity-optimized and non-fucosylated anti-CD147 chimeric HAb18 antibody (metuzumab) for a Phase I clinical trial of non-small-cell lung cancer treatment.

NDV Anhinga strain and Clone30 strain

NDV Anhinga is a mesogenic strain that is classified as a lytic strain, whereas Clone30 is lentogenic and is classified as a nonlytic strain. Based on these two strains, several recombinant NDVs were generated to express cytosine deaminase (rClone30-CD, combined with 5-FC), IL-2 (rClone30-IL2, NDV/Anh-IL-2),^23,54 IL-12 (rClone30-IL12, rClone30-IL12-IL2),²³ and TRAIL (NDV/Anh-TRAIL).⁵⁵ These recombinant NDVs were used for HCC therapy and showed a stronger antitumor activity than wild-type NDV. In contrast to TRAIL, which causes cell apoptosis via cell death receptors, IL-2 and IL-12 regulate T-cell activity to influence antitumor immunity significantly.

The Challenge and Future of Ov in China

Although OVs have been used for cancer treatment for nearly one century, controlled scientific study and clinical trials were carried out only in the past 30 years. In China, since 2005, >30 reports annually of natural or recombinant OV agents for tumor therapy have been published. Although each study shows sufficient antitumor activity both in vitro or in tumor-bearing mice, very few of these OVs are approved for clinical trials. Compared to the United States and the European Union, which have 41 and 9 OV-related registered clinical trials (data from clinicaltrials.gov and www.clinicaltrialsregister.eu), respectively, three novel OV agents (OrienX010, KH901, and H101) for cancer treatment have been approved for Phase I/II clinical trials in China (Table 1). However, more and more OV agents have completed preclinical evaluation and are awaiting approval for clinical trials in China.

Combined therapy is a prominent trait of ZD55 variants that usually combine two or three different treatment strategies and can enhance antitumor activity significantly. These combinations are similar to the cocktail method used for acquired immune deficiency syndrome treatment and can boost antitumor activity by >100-fold compared to use of a single agent.^20,43 Another study using oncolytic Adv drew similar conclusions that oncolytic virotherapy combined with an antitumor strategy (such as HSV-TK/GCV suicide gene therapy) is an effective, safe, and convenient approach to enhance antitumor therapeutic effects.⁴⁷

There are two main mechanisms by which OVs can override tumors: selectively killing tumor cells, and activating antitumor immunity.⁵⁶ Although OVs can induce tumor cell lysis directly and efficiently in both cultured cells and human tumor xenografts, this effect is greatly attenuated in the human body because of nonspecific binding, antivirus immune response, and extracellular environment barriers.⁵⁷ It is presumed that OV replication and the expression of exogenous genes in tumor tissues is not permanent but rather is limited both spatially and temporally. To eliminate cancer, OVs must initiate or activate an antitumor immune response. Although virus infection and lysis of tumor cells could induce and augment antitumor immunity, expression of immunomodulatory genes that can act as a genetic immunotherapy will likely be a more efficient and controllable method. In China, since 2016, more than half of OV-related studies are using this strategy, and >10 kinds of immunomodulatory genes are involved in these studies (Fig. 3). Cancer immunotherapy, which harnesses the body's immune system to treat cancer, has made great progress in the past few years. Several immunotherapy drugs, from monoclonal antibody drugs (PD-1 and PD-L1) to chimeric antigen receptor T-cell therapy, have recently been approved by the FDA for cancer treatment. Combining OVs with these novel cancer immunotherapy strategies will provide new opportunities to improve the therapeutic effects of OVs and may yield unexpected positive effects.

Intravascular administration is an optimal method for tumor therapy, especially for treating metastasis. All three approved OVs are subject to administration by intratumoral injection to facilitate virus infiltration into tumor tissues. Innate immunogenicity against OVs and nonspecific binding decreases gene delivery efficiency and increases the likelihood of failure to respond to repeated treatment, whereas the extracellular environment in tumor tissue also blocks OV infiltration. Development of an efficient and safe OV delivery strategy is thus necessary to overcome these obstacles and promote the potential of OV agents. Bioengineering technology, such as encapsulation by nanomaterials, 3D cell culture, and 3D bio-printing may soon be used in the development of OV agents, including formulation, evaluation, and administration.⁵⁸ Some of the newest genome editing techniques, such as clustered regularly interspaced short palindromic repeat (CRISPR) technique and orthogonal translation system,⁵⁹ could be used for the efficient construction of novel recombinant OVs.⁶⁰

Oncolytic virotherapy has already become a new field for cancer treatment in China, although a significant gap persists between bench and bedside. With more optimized combined therapeutic strategies, especially combining OVs with novel tumor immunotherapy approaches and more efficient administration methods, the progress of OV clinical use will be accelerated in China.

Footnotes

Acknowledgments

The authors gratefully acknowledge support from the National Natural Science Foundation of China (31571434, 81201776) and the National Science and Technology Major Project (2015CB553701).

Author Disclosure

No competing financial interests exist.

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