This article explores the intersection of virology and oncology, examining how scientists harness viruses' natural abilities to combat cancer. Unibest is pleased to present an oncolytic virus asset, UB084, currently in Phase II trials, that is open for global licensing and co-development opportunities.

Oncolytic viruses are a promising new approach to cancer treatment that harnesses the power of viruses to selectively target and destroy cancer cells. These viruses, which can be either naturally occurring or genetically engineered, are designed to replicate specifically in cancer cells while leaving normal tissues unharmed. The ability of oncolytic viruses to infect cancer cells is based on the fact that many cancer cells have impaired defense mechanisms against viral infection, such as the interferon-beta signaling pathway. This makes cancer cells more susceptible to viral replication compared to normal cells.

The use of viruses in cancer treatment offers several advantages. They can be used to infect cancer cells and present tumor-associated antigens, which helps the immune system recognize and attack the cancer. They can also activate "danger signals" that create a less immune-tolerant tumor microenvironment, making it easier for the immune system to fight the cancer. Additionally, viruses can serve as vehicles for delivering inflammatory and immunomodulatory cytokines directly to the tumor site.

Current Approvals

Oncolytic viruses have been gaining attention as a novel cancer treatment, and several have already been approved for clinical use in various countries.

The first oncolytic virus to receive approval was Rigvir, which was approved in Latvia in 2004 for the treatment of melanoma. Rigvir is an unmodified ECHO-7 virus that has been selected for its ability to target melanoma cells. It acts as a virus-based immunomodulator, inducing an anti-tumoral response and is used for the local treatment of skin and subcutaneous metastases of melanoma.

In 2005, China approved Oncorine (H101), an E1B-deleted adenovirus, for the treatment of head and neck cancer and esophageal cancer. Oncorine has the same construct as ONYX-015, but its use and clinical data are currently limited to China.

A major milestone in the field of oncolytic viruses was the approval of T-Vec (talimogene laherparepvec, IMLYGIC, formerly OncoVEXGM-CSF) by the FDA in the USA in October 2015. T-Vec was subsequently approved in Europe in January 2016 and in Australia in May 2016. This modified form of herpes simplex type-1 virus (HSV1) encodes a human granulocyte macrophage colony-stimulating factor (GM-CSF) gene, which enhances the immune response against the cancer cells. T-Vec is currently used for the treatment of melanoma.

In Japan, Delytact, a modified HSV, was approved for the treatment of malignant glioma in 2021. Delytact is a third-generation (triple-mutated) recombinant oncolytic herpes simplex virus type 1 developed by Daiichi Sankyo Co., Ltd. It is specifically designed to target and treat malignant glioma, a type of brain cancer.

As more oncolytic viruses demonstrate their efficacy and safety in clinical trials, it is expected that additional approvals will follow, offering new treatment options for patients with various types of cancer.

General Mechanism

The general mechanism of action of OVs involves several key steps:

Selective infection of abnormal cells: OVs are engineered to target specific markers that are overexpressed in cancer cells, such as nuclear transcription factors (e.g., human telomerase reverse transcriptase, prostate-specific antigen, cyclooxygenase-2, osteocalcin) and surface markers (e.g., prostate-specific membrane antigen, folate receptor, CD20, endothelial growth factor receptor, and Her2/neu). This selective targeting allows OVs to preferentially infect and replicate within cancer cells.

Lysis and death of tumor cells: OVs directly cause the lysis and death of infected cancer cells through various mechanisms. They can disrupt the function of organelles, such as the endoplasmic reticulum, mitochondria, or lysosomes, compromising normal cellular function. Additionally, OVs can induce oxidative stress through the production of reactive nitrogen species and endoplasmic reticulum stress, which is associated with increased intracellular calcium levels. These processes contribute to the stabilization and reduction of the tumor.

Induction of immune responses: The presence of viruses in the body triggers an immune response involving both innate and adaptive immunity. Viral components, such as proteins, RNA, DNA, and capsid, are recognized as pathogen-associated molecular patterns (PAMPs) by pattern recognition receptors. This recognition leads to the production of proinflammatory cytokines (e.g., TNF-alpha and IL-2) and the recruitment of immune cells, such as neutrophils and macrophages, to the tumor site. The release of tumor-associated antigens and neoantigens (TAAs and TANs) during OV-mediated oncolysis of cancer cells further stimulates the adaptive immune response, particularly tumor-specific T-cell responses.

The combination of selective infection, direct lysis of cancer cells and immune stimulation by OVs leads to a potent anti-tumor effect. As research in this field progresses, OVs are being engineered to enhance their specificity, potency, and ability to stimulate anti-tumor immunity, making them a promising approach for the treatment of various types of cancer.

Mechanisms of oncolytic virus (OV) action. src: Tian, Y., Xie, D. & Yang, L. Engineering strategies to enhance oncolytic viruses in cancer immunotherapy. Sig Transduct Target Ther 7, 1–21 (2022).

Type of OVs

Adenovirus

Adenoviruses (AdVs) are a popular choice for the development of oncolytic viruses due to their well-characterized biology, ease of genetic manipulation, and ability to infect a wide range of cell types. AdVs have several key features that make them suitable for use as oncolytic agents:

• Structure: AdVs are nonenveloped viruses with double-stranded linear DNA genomes (~30–40 kb) and an icosahedral capsid. The capsid proteins, namely hexon, penton-base, and fiber proteins, determine the virus's tropism (the ability to infect specific cell types).

• Serotypes: Human AdVs are classified into seven different species (A–G) that contain 104 candidate serotypes as of April 2021. Serotype 5 adenovirus (Ad5) is the most commonly used viral vector in clinical studies. Ad5 enters the targeted cells via the interaction of its fiber knob with coxsackievirus and adenovirus receptors (CARs).

Adenovirus illustration. src: Adenovirus - Infectious Dis. - Medbullets Step 2/3. https://step2.medbullets.com/infectious-dis/121820/adenovirus.

To enhance the cancer selectivity of AdVs, three main strategies have been employed:

• Deletion of E1A and E1B 55K genes: The E1A and E1B 55K genes are essential for viral replication in normal cells. By deleting these genes, the modified AdVs can only replicate in cells with mutations in the retinoblastoma (pRb) and p53 tumor suppressor pathways, which are common in many types of cancer. This strategy allows the AdVs to selectively replicate and lyse cancer cells while sparing normal cells.

• Partial deletion in the E3 region: The E3 region of the AdV genome encodes proteins that help the virus evade the host immune response. By partially deleting this region, the AdVs can accommodate the insertion of immunostimulatory transgenes, such as cytokines or costimulatory molecules, which can enhance the antitumor immune response.

• Insertion of the Arg-Gly-Asp (RGD) motif: The RGD motif is a peptide sequence that binds to integrins, which are often overexpressed on the surface of cancer cells. By inserting the RGD motif into the HI loop of the AdV fiber protein, the virus's ability to infect cancer cells is improved, as it can now enter cells through both CARs and integrins.

Vaccinia virus

Vaccinia virus (VV) is another promising candidate for the development of oncolytic viruses. VV is an enveloped virus with a large, double-stranded DNA genome (approximately 190 kb in length). The large genome size allows for the insertion and high-level expression of large foreign genes, such as therapeutic transgenes or imaging reporters, without significantly compromising viral replication.

Vaccinia virus illustration. src: Harrison, S. C. et al. Discovery of antivirals against smallpox. Proceedings of the National Academy of Sciences 101, 11178–11192 (2004).

To enhance the tumor selectivity and oncolytic potency of VV, several genetic modifications have been explored:

• Deletion of thymidine kinase (TK): TK is a viral enzyme required for the synthesis of deoxyribonucleotides in non-dividing cells. By deleting the TK gene, VV becomes dependent on the cellular TK expressed at high levels in rapidly dividing cancer cells. This modification restricts VV replication to tumor cells while sparing normal, quiescent cells.

• Deletion of vaccinia type I IFN-binding protein (B18R): B18R is a viral protein that neutralizes type I interferons (IFNs), which are key mediators of the antiviral immune response. Deleting B18R increases the sensitivity of VV to the antiviral effects of IFNs, thereby limiting its replication in normal cells with an intact IFN response. In contrast, many cancer cells have defects in the IFN pathway, allowing the B18R-deleted VV to replicate selectively in these cells.

• Deletion of vaccinia growth factor (VGF): VGF is a viral protein that promotes cell proliferation and enhances viral replication. By deleting VGF, VV becomes more dependent on the growth-promoting signals present in cancer cells, leading to preferential replication in tumor cells over normal cells.

Reovirus

Reovirus (RV) is a promising oncolytic virus that has been extensively studied for its potential in cancer therapy. As a naturally occurring virus, RV has been found to preferentially replicate in and kill cancer cells while sparing normal cells. The key features of RV that make it an attractive oncolytic agent include:

• Structure: RV is a nonenveloped virus with a double-stranded RNA genome (~23.5 kb) divided into 10 segments: large (L1–3), medium (M1–3), and small (S1–4). This segmented genome allows for the potential exchange of genetic material between different RV strains, leading to the generation of novel, more potent oncolytic viruses.

• Tumor selectivity: The selectivity of RV for cancer cells is largely dependent on the Ras signaling pathway. Many human tumors have activating mutations in the Ras pathway, leading to uncontrolled cell growth and survival. RV exploits this by requiring an activated Ras pathway for its replication and release of viral progeny. In normal cells with a regulated Ras pathway, RV replication is inhibited, making it a tumor-selective oncolytic agent.

• Mechanism of cell death: In addition to its selective replication, RV induces apoptosis in cancer cells through the Ras/RalGEF/p38 pathway. This mechanism of cell death enhances the oncolytic activity of RV and contributes to its overall antitumor efficacy.

Reovirus illustration. src: Reovirus - Microbiology - Medbullets Step 1. https://step1.medbullets.com/microbiology/121551/reovirus.

Herpes simplex virus type 1

Herpes simplex virus type 1 (HSV-1) is a widely studied oncolytic virus that has shown promising results in the treatment of various types of cancer. The genetically modified HSV-1 virus, talimogene laherparepvec (T-VEC), is the first oncolytic virus approved by the FDA for the treatment of advanced melanoma. HSV-1 is a double-stranded DNA virus with a large genome of 152 kb, encoding over 74 distinct genes. The virus is approximately 200 nm in diameter. The large genome allows for the deletion of nonessential genes for replication and the insertion of engineered transgenes without affecting the packaging efficiency of the virus.

Herpes simplex virus type 1 illustration. src: Zhu, S. & and Viejo-Borbolla, A. Pathogenesis and virulence of herpes simplex virus. Virulence 12, 2670–2702 (2021).

T-VEC is an example of a genetically modified HSV-1 virus designed to enhance its oncolytic activity and safety profile. The modifications include:

• Deletion of ICP34.5: This gene encodes a neurovirulence factor. Its deletion prevents the virus from replicating in neurons, thereby reducing the risk of neurological complications. However, the virus can still replicate in tumor cells.

• Insertion of GM-CSF: T-VEC contains two copies of the gene encoding granulocyte-macrophage colony-stimulating factor (GM-CSF) in place of ICP34.5. GM-CSF promotes the maturation of dendritic cells, which can enhance the antitumor immune response.

• Deletion of ICP47: ICP47 encodes an inhibitor of antigen presentation that blocks MHC class I antigen presentation to CD8+ T cells. By deleting this gene, T-VEC can promote immune responses against tumor cells.

Other oncolytic viruses

Apart from the previously mentioned viruses, several other viruses, such as newcastle disease virus, measles virus,  seneca valley virus, poliovirus, vesicular stomatitis virus65 and parvovirus have been developed into oncolytic viruses.

Limitations of Oncolytic Viruses

While oncolytic virus has shown promising results in the treatment of cancer, there are certain limitations that should be considered:

Route of administration: Oncolytic HSV-1 is typically administered through intralesional injections directly into the tumor. This approach may not be well suited for systemic delivery via intravenous administration, which could limit its effectiveness against metastatic or inaccessible tumors. However, the phase III study of T-VEC in advanced melanoma patients has demonstrated that local intralesional injections can induce a systemic antitumor immune response, leading to the regression of remote lesions and prolonged survival.

Antiviral immunity: One major concern in oncolytic virus therapy is the potential impact of preexisting or induced antiviral immunity on the efficacy of the treatment. As oncolytic viruses are based on common human viruses, patients may have circulating antibodies against the virus due to prior exposure or vaccination. These antibodies could neutralize the oncolytic virus and reduce its ability to infect and lyse tumor cells. However, recent studies have suggested that the antiviral immune response may not always be detrimental to the success of oncolytic viruses. In fact, the initial antiviral immune response triggered by the oncolytic virus can play a crucial role in priming the antitumor immunity. The viral infection and subsequent oncolysis lead to the release of tumor-associated antigens and danger signals, which can stimulate the innate and adaptive immune system to mount an antitumor response.

Unibest's recombinant L-IFN adenovirus injection, UB084

UB084 is currently in a phase IIa clinical trial in China, and has been planned for clinical trials in the United States. UB084 possesses several significant advantages, including:

• Dual regulatory mechanism

• High safety profile

• Broad-spectrum anti-cancer efficacy

Preclinical studies have shown remarkable tumor inhibition rates of 90-100% in at least seven types of solid tumors. Additionally, UB084 demonstrates a clear distant anti-cancer effect, suggesting its potential to target metastatic tumors.

The current clinical results for UB084 have shown high safety and significant clinical benefits. Notably, its single-agent clinical efficacy is reported to be superior to that of PD-1 monoclonal antibodies and similar competing products. This highlights the potential of UB084 as a standalone therapy or in combination with other cancer treatments.

A further innovations in the delivery of oncolytic viruses has been made by attempting to treat advanced pulmonary tumors with oncolytic viruses via nebulization, which is a first-of-its-kind approach worldwide. This novel delivery method could potentially expand the application of oncolytic viruses to target lung cancers more effectively.

First attempt to treat advanced pulmonary tumors with oncolytic virus via nebulization worldwide

Unibest invites interested parties to contact them at bd@unibestcn.com to learn more about the clinical development and data summary of UB084. The asset is open to exploring potential licensing and co-development opportunities, which could accelerate the global development and commercialization of this promising therapy.

References