Oncolytic Viruses Primed for Growth

Robert Andtbacka, MD

Amid the rapid expansion of immunotherapy for a wide range of tumor types, oncolytic virus therapies are generating growing attention from researchers and pharmaceutical developers, raising the potential for a new class of immune-enhancing drugs.

More than 16 genetically modified virus therapies are being explored in clinical trials in the United States, either as monotherapy or in combination with other therapies, according to an industry report¹ and a search of the ClinicalTrials.gov database. Although that number is outpaced by interest in the leading form of anticancer immunotherapy, the checkpoint blockade agents, and the emerging chimeric antigen receptor T-cell therapies, recent industry developments show heightened expectations for oncolytic viruses.

In February 2018, Merck agreed to pay $394 million to acquire Viralytics, an Australian company that is developing Cavatak (CVA21), a formulation of coxsackievirus type A21.² The drug is being studied in combination with pembrolizumab (Keytruda), a PD-1 inhibitor that Merck developed, in melanoma, prostate, lung, and bladder cancers.

In another partnership that is moving forward, PsiOxus Therapeutics, based in the United Kingdom, announced that its oncolytic virus candidate, NC-348, has been approved for use in human trials, qualifying the company for a $15 million milestone payment from BristolMyers Squibb.³ PsiOxus previously received $50 million from Bristol-Myers to develop NG-348, which the company describes as an enadenotucirev virus, a genetically modified adenovirus, that is “armed” with 2 immunomodulatory membrane-integrated T-cell—engaging proteins.

As it stands now, the only FDA-approved oncolytic virus is talimogene laherparepvec (T-VEC; Imlygic), a form of herpes simplex virus type 1 (HSV-1) that has been genetically modified to express granulocyte-macrophage colony-stimulating factor. T-VEC was approved in 2015 for the locoregional treatment of recurrent, unresectable melanoma; it is injected into cutaneous, subcutaneous, and nodal lesions.⁴

T-VEC has proved to be a durable, well-tolerated therapy that has been effective as monotherapy in patients with no visceral to minimally visceral disease, Robert H.I. Andtbacka, MD, CM, a leading investigator of the drug, said during a presentation at the 14th Annual International Symposium on Melanoma and Other Cutaneous Malignancies^® that Physicians’ Education Resource^® hosted in New York City in February. For patients with visceral disease, T-VEC combination therapies would be more effective, he said.

Oncolytic viral therapy exerts a direct cytotoxic effect on the tumor and appears to enhance the efficacy of other immunotherapies, said Andtbacka, who is a surgeon and investigator at the Huntsman Cancer Institute at the University of Utah in Salt Lake City, where he also is an associate professor in the Division of Surgical Oncology.

“We know with these therapies that we can also use them to change the tumor microenvironment,” he said in an interview with OncologyLive^®. “In patients who don’t respond to PD-1 inhibition, we can use these intralesional therapies to change the tumor microenvironment and make some of these nonresponders into responders.”

Andtbacka pointed out 2 trials that, if successful, would lead to an expanded role for T-VEC in melanoma: a phase II study of neoadjuvant T-VEC with surgery versus surgery alone in patients with surgically resectable stage IIIb/c melanoma (NCT02211131) and the phase III MASTERKEY-265/KEYNOTE-034 study of pembrolizumab with or without T-VEC in patients with treatment-naïve, unresectable stage III or IV melanoma (NCT02263508).

Although T-VEC and other oncolytic virus therapies have been primarily studied in melanoma thus far, Andtbacka sees the potential for their utility in other tumor types, including liver metastases. “We really are expanding this into other cancers,” he said.

Mechanisms of Action

Indeed, the approval of T-VEC has served as a proof of concept for the modality. “It’s an exciting time for oncolytic viruses, and now that there’s been a virus approved by the FDA, there’s more interest in the field,” said Samuel D. Rabkin, PhD, an associate professor of surgery at Harvard Medical School and an associate virologist at Massachusetts General Hospital, both in Boston.The concept of using viruses as anticancer agents dates back more than 100 years, when doctors first observed temporary clinical remission of cancer after naturally acquired systemic infections. These observations led to multiple clinical trials treating patients with cancer with wild-type viruses. Natural virulence could not be controlled, however, and anticancer viral therapy fell out of favor with researchers and clinicians. Throughout the 1950s and 1960s, numerous attempts were made to utilize new animal models and methods for virus propagation, but these efforts were abandoned following limited success.⁵

Over the past 2 decades, however, the advent of genetic engineering and improved knowledge of molecular and cancer biology have refined oncolytic virus therapy design and allowed it to reemerge as a potentially powerful therapeutic option for advanced cancers. While success has been observed in clinical trials assessing oncolytic virus monotherapies, preliminary results from early-stage clinical trials suggest that combinations with other forms of therapy may enhance the clinical efficacy of oncolytic viruses even further.

Cancer cells in general serve as excellent hosts for oncolytic virus replication; they often have impairments in protection mechanisms against viral infections (eg, the interferon-beta signaling pathway) that allow for increased viral replication.⁶ In addition, other hallmarks of cancer—including resistance to cell death, evasion of growth suppressors, genome instability, DNA damage stress, and avoidance of immune destruction—provide advantages for increased oncolytic virus efficacy. Oncolytic virus therapies have unique mechanisms of action that allow them to target multiple immune processes (FIGURE).⁷ They are designed to allow selective viral replication in cancer cells without damaging normal cells. This can be accomplished by selecting for nonpathogenic viruses that replicate preferentially in cancer cells (eg, pelareorep [Reolysin]), or by directly engineering the virus genome (eg, T-VEC and G47Δ, both derived from HSV-1).

Figure. Immunotherapy Mechanisms⁷

Genetic engineering of virus particles has been an effective strategy for controlling viral replication within cancer cells and has become the standard for therapeutic development. Viruses with deletion of the HSV-1 γ-34.5 gene, such as T-VEC and G47Δ, are unable to replicate in normal cells because these viruses are unable to inhibit the host’s interferon-induced shutoff of protein synthesis after infection.⁸ Viruses with this deletion, however, can replicate in cancer cells. Deletion of the α-47 gene, which antagonizes the host cell transporter associated with antigen presentation, is also common (such as in T-VEC and G47Δ) and is thought to enhance the antitumor immune response.⁹

Toca 511 (vocimagene amiretrorepvec) employs a different strategy. It is a replicating nonlytic retrovirus that encodes the cytosine deaminase (CD) protein and can turn 5-FC into 5-fluouracil (5-FU), which activates the immune system against cancer, explained Clark C. Chen, MD, PhD, the Lyle French Chair in Neurosurgery and head of the Departmanet of Neurosurgery at the University of Minnesota in Minneapolis. The virus is injected into the site of highest tumor concentration in the brain by a surgeon. Later, Toca FC (extended-release 5-fluorocytosine), a novel formulation of an antifungal drug, 5-FC, is given to the patient orally, and the CD gene converts the 5-FC into 5-FU.¹⁰

Although most oncolytic viruses in development are DNA viruses, RNA viruses, such as CVA21, may have advantages for targeting some cancers, according to Howard L. Kaufman, MD, chief medical officer of Replimune. Kaufman presented preclinical data at the Society for Immunotherapy of Cancer 2017 Annual Meeting to demonstrate that variation in intratumoral antiviral machinery may make some tumors more susceptible to infection with an RNA virus than with a DNA virus. Although this has yet to be investigated in the clinical setting, he suggested that prospective testing in future clinical trials could help assess whether the tumor’s antiviral machinery is associated with response to DNA or RNA viruses.

The anticancer effect of oncolytic viruses was initially attributed to apoptosis, autophagy, or both (ie, direct oncolysis) of the tumor. Masahiro Toda, MD, PhD, and colleagues, however, demonstrated that intratumoral injection of an oncolytic HSV-1 shrank both local and distant tumors in proximity to the injection site, indicating induction of systemic antitumor immunity as well as direct oncolysis.¹¹ This immunity could play a critical role in obtaining durable responses in patients with metastatic or aggressive disease. Kaufman described the vaccine-injected tumor as an in situ vaccine source from which released antigens trigger a systemic immune response to the tumor.

Glioblastoma is Prime Target

Kaufman and Rabkin agreed that, because of new techniques in interventional radiology and minor surgery, direct injection of the virus is possible for most tumors and would be more effective than intravenous administration, which tends to result in high titers of neutralizing antiviral antibodies. “With modern neurosurgical techniques, almost all areas [of the brain] are accessible to biopsy,” said Rabkin. “If you can perform a biopsy, you can deliver the virus.”Although T-VEC is the only oncolytic virus currently approved by the FDA for treatment of patients with unresectable melanoma, multiple clinical trials of oncolytic virus therapies are assessing efficacy against numerous cancer types (TABLE). These trials will help clarify whether effectiveness varies among viruses and tumor types.

Table. Selected Clinical Trials Investigating Oncolytic Virus Therapies

According to Rabkin, glioblastoma has been a primary target for oncolytic virus therapy due to an urgent need for durable therapies. “You have a disease for which the life span is rather limited, and that provides opportunities and dire needs for new strategies to treat the disease,” he said.

G47Δ

Phase I and IIa studies in Japan have shown tolerability of intratumoral injection with G47Δ in patients with recurrent glioblastoma, and a phase II study (UMIN000015995) was initiated in 2015 for patients with residual or recurrent glioblastoma. In 2016, Japan’s Ministry of Health, Labour, and Welfare designated G47Δ as a Sakigake breakthrough therapy, providing early assessment and priority reviews to fast-track the virus’ approval.12

Rabkin noted that the effectiveness of oncolytic viruses throughout a range of genetic alterations and phenotypes makes them particularly attractive for treating glioblastoma, a heterogeneous type of cancer with distinct genetic subtypes. Furthermore, in vitro studies have shown that oncolytic viruses are effective against cancer stem cells in glioblastoma, which extend beyond the tumor margins and are notoriously resistant to current therapies. “To really be effective in treating the disease, one has to be able to target these cells that have migrated away from the tumor,” Rabkin said. “That is where immunotherapy looks very promising.”

Toca 511 and Toca FC

Toca 511 and Toca FC induced durable responses in patients with glioblastoma multiforme (GBM) and anaplastic astrocytoma (AA), with a median duration of response of 35.1+ months, said Chen, in a presentation during the 2017 AACRNCIEORTC International Conference on Molecular Targets and Cancer Therapeutics, held in Philadelphia in October 2017. In a pooled analysis of 3 phase I studies, the virus also proved to be well tolerated, with fewer any-grade adverse events than with standard chemotherapy.¹³

Toca 511 and Toca FC received a breakthrough therapy designation from the FDA in February 2017 for treatment of patients with recurrent high-grade glioma.¹⁴ The FDA also granted a fast track designation to the virus in July 2015.¹⁵

Currently ongoing is the Toca 5 trial, a multicenter, randomized, open-label phase II/III trial that is looking at Toca 511 and Toca 5C compared with investigator’s choice of lomustine, temozolomide (Temodar), or bevacizumab (Avastin) in patients with GBM or AA who are undergoing resection following their first or second recurrence (NCT02414165). The trial recently began enrolling patients.

DNX-2401

DNX-2401 (tasadenoturev) is an adenovirus-based oncolytic virus therapy currently being investigated in clinical trials. Data from a phase I study showed that a single intratumoral injection of DNX-2401 resulted in 20% of a cohort of patients with recurrent malignant glioma surviving more than 3 years from the time of treatment.¹⁶

In the dose-escalation study, patients were enrolled into the treatment-only group A (n = 25) or the treat-resect-treat group B (n = 12). Group A underwent stereotactic biopsy to document recurrence, followed by a single intratumoral injection of DNX-2401 at the assigned dose through the biopsy needle into the contrast-enhancing tumor mass. In group B, stereotactic biopsy and intratumoral injection of DNX-2401 through an implanted catheter was followed 2 weeks later by craniotomy with en bloc tumor resection, and then administration of a second DNX-2401 dose into several locations in the wall of the resection cavity.

Five (20%) of 25 patients in group A survived more than 3 years, including 3 patients who had complete responses (CR, defined as ≥95% reduction in the size of the enhancing tumor) and 2 patients with stable disease. All 3 patients who achieved a CR had progression-free periods of ≥3 years. Two patients (17%) in group B survived for 2 years. Across the overall population, the median survival time was 13.0 months.

The CAPTIVE trial (NCT02798406) is currently investigating the efficacy of intratumoral injection DNX-2401 followed by intravenous pembrolizumab (Keytruda) at 3-week intervals for patients with recurrent GBM.

PVS-RIPO

PVS-RIPO consists of a genetically modified poliovirus Sabin type 1 in which the internal ribosomal entry site (IRES) on the poliovirus is replaced with the IRES from human rhinovirus type 2 (HRV2). Once administered, the therapy begins duplicating within cells that express CD155/Necl5, an onco-fetal cell adhesion molecule that is common across solid tumors.

In May 2016, the FDA granted a breakthrough therapy designation for PVS-RIPO as a potential therapy for patients with recurrent GBM. The designation was supported by evidence from an ongoing phase I study that is exploring PVS-RIPO in patients with grade IV malignant glioma. According to findings presented at the 2015 American Society of Clinical Oncology Annual Meeting, the 24-month overall survival rate was 24% among 24 patients. In updated findings, 3 patients remained alive 36 months following treatment, according to investigators at Duke University who have pioneered its development.¹⁷

Combination Therapies

The oncolytic virus is currently being evaluated in a phase II trial in which patients with malignant glioma are being randomized to PVS-RIPO as monotherapy or in combination with lomustine (NCT02986178). Participants will receive a single dose of PVS-RIPO intratumorally through an intracerebral catheter placed within the enhancing portion of the tumor; those in the lomustine arm will then take a single oral dose.The randomized open-label phase III trial that led to the approval of T-VEC in patients with unresectable melanoma showed an overall response rate of 26.4% and a complete response rate of 10.8%.¹⁸ While displaying efficacy, response is likely limited due to multiple factors, including antiviral mechanisms in the host that limit viral dissemination, tumor resistance to oncogenic signaling pathways exploited by the oncolytic viruses, and immunosuppressive regulatory factors within the tumor.

Therefore, researchers have been investigating clinical approaches to enhance efficacy, including combination approaches with added immunotherapy, chemotherapy, or targeted agents. “The issue of combinations is something that is going to be very promising with oncolytic viruses, because they tend to kill cells through mechanisms that are distinct from other forms of therapy,” Rabkin said. “You can generate a much better effect with the combination.”

Some combinations of interest that are currently in development include the following:

Immune Checkpoint Inhibitors

Antiviral immune response contributes to the clinical efficacy of oncolytic viruses, though it may inhibit the cytotoxic activity of the T-cell tumor infiltrates through upregulation of CTLA-4 and/or PD-1/PD-L1 interactions.

A phase Ib clinical trial demonstrated the clinical efficacy of oncolytic viral therapy and CTLA-4 blockade. Treatment with T-VEC plus the CTLA-4 inhibitor ipilimumab (Yervoy) led to an objective response rate of 50% and a durable response rate (≥6 months) of 44% in patients with advanced unresectable melanoma.¹⁹

Furthermore, interim results from the ongoing phase Ib CAPRA trial demonstrated the clinical efficacy of combination therapy with CVA21 plus the PD-1 inhibitor pembrolizumab in patients with advanced melanoma. Of the first 15 patients evaluated, 87% achieved disease control, and objective tumor response was observed in 60% of patients.²⁰

“This trial is a good example of what we hope to see, which is that we’re getting much better responses with both agents than what you would expect from either one alone,” said Kaufman, a principal investigator for CAPRA.

Kaufman also noted that the coxsackievirus may enhance response to PD-1/PD-L1 checkpoint inhibitors by upregulating PD-L1 expression in the tumor, which may enhance responsiveness in cancers where checkpoint inhibitors alone have minimal effectiveness, such as sarcoma, breast cancer, and PD-L1—negative lung cancer. He also emphasized the importance of prospective clinical trials to assess the effectiveness of oncolytic virus combinations in different types of tumors, particularly those with a high population incidence (such as basal cell and squamous cell carcinomas) or for those with an aggressive phenotype and few viable treatment options (such as Merkel cell carcinoma).

Cyclophosphamide

Clinical Considerations

Cyclophosohamide (CPA) causes nonspecific DNA alkylation and cell death through apoptosis and modulates the immune system by killing proliferating natural killer, T, and B cells. CPA has also been shown to enhance the antitumor activity of cytotoxic T cells by suppressing local innate immune cells and depleting regulatory T cells. Adding CPA improved the antitumor effects of oncolytic viruses in preclinical trials,^21,22 and preconditioning with CPA increased intratumoral levels of the oncolytic virus in a murine model of melanoma.²² Early-phase clinical trials are also investigating the addition of CPA to an adenovirus (NCT00634231; NCT02879669) and an oncolytic vaccinia virus (NCT02630368), although some experts suggest that the shift away from nonspecific chemotherapy may limit the widespread adoption of this combination.Early trials have shown that oncolytic virus therapy is generally well tolerated and does not seem to trigger many of the immune-related toxicities associated with immune checkpoint blockade. The results of studies investigating combinations of the 2 therapies demonstrate toxicities consistent with checkpoint blockade alone. “The effect of the virus tends to be very local,” Rabkin said. “Even though it may have systemic effects, I think they’re probably less likely to lead to severe adverse events than checkpoint blockade.”

Rabkin and Kaufman noted ongoing concerns about systemic spread or transmission to household contacts, although neither has been reported in clinical trials. Kaufman stated that some patients have reported mild, self-limiting symptoms consistent with the common cold, such as a runny nose, cough, and fever from CVA21, but that the overall safety profile is relatively good. Rabkin also pointed out that the modified forms of HSV-1 used in oncolytic therapy are less likely to cause infection than is the wild-type virus, to which most of the population has been exposed. Also, infections could easily be treated with antiviral agents in the rare event of infection spread.

On the other hand, an overly strong immune response to the oncolytic virus may lead to the production of neutralizing antibodies that attenuate the antitumor effect. In a study evaluating the antitumor efficacy of using different oncolytic adenovirus therapies with autophagy-inducing immunogenic treatment in chemotherapy-refractory patients, disease control (stable or better) was achieved with 67% of the combinations. Importantly, viral replication was not limited, despite the development of capsid-specific neutralizing antibodies to the adenovirus.²³

The Future of Oncolytic Viruses

Antiviral immunity may also reduce the efficacy of multiple-dose treatment and be particularly problematic with oncolytic viruses derived from viruses against which the general population is vaccinated, such as measles and vaccinia. Strategies such as complement inhibition and the use of cell carriers as Trojan horses are under investigation to minimize the development of antiviral antibodies and improve delivery of the oncolytic virus to the tumor.Oncolytic viruses represent a new strategy to augment treatment of multiple types of cancer, particularly those with limited options. According to Kaufman, a major strength of this platform is the diversity of viruses, and further research into optimizing genetic manipulation for various types of cancer will be useful.

From the clinical perspective, Kaufman and Rabkin stated that additional trials must be conducted to identify the optimal setting and dosing. Both experts agree that treatment in the frontline setting would be most desirable, and Kaufman noted that patients might avoid more toxic agents with effective frontline treatment, including oncolytic viruses.

Furthermore, he noted, the frequency and duration of oncolytic virus administration has been largely based on empirical evidence, and patients may be able to achieve long-term results with just a few doses. “We may not need to give it as frequently as we have been,” Kaufman said. “We need to do more work to understand whether we’re overtreating some patients.”

Although clinical trials are in their early stages, Kaufman and Rabkin are optimistic about the possibilities for oncolytic virus therapy. “I think it’s a strategy that has a lot of potential and opportunity,” Rabkin said. “There’s a lot of diversity in the agents being looked at.”

References

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