Turning tumors into "in situ vaccines"! Replimune (REPL.US) oncolytic virus passes FDA, bringing a new generation of immune reactivation strategies for PD-1 resistant melanoma

According to Zhihui Finance APP, the U.S. drug regulatory authority has officially approved a melanoma treatment drug developed by the emerging oncology immunotherapy biotech company Replimune Group Inc. (REPL.US) for patients who have not benefited from immunotherapy—a major victory for the innovative pharmaceutical company, after two previous rejections of this drug by the regulator. The FDA stated that the drug was approved based on a single-arm trial involving 91 patients with advanced melanoma, in which 24% of patients experienced tumor reduction.

The fate of this drug has been a focal point of debate surrounding the FDA’s review standards, which have been shifting, especially after the resignation of agency director Marty Makary in May. During Makary’s tenure, several high-profile drugs were denied approval or delayed, sparking controversy among biotech companies, U.S. Congress, and patient groups: for patients with severe conditions and few alternative treatments, is the FDA taking too stringent a stance towards potential therapies?

The emerging oncology immunotherapy biotech company Replimune Group was established in 2015 and is headquartered in Woburn, Massachusetts, USA. Its core technology is its proprietary RPx-engineered HSV-1 oncolytic immunotherapy platform, aiming to directly lyse tumors locally while inducing a systemic anti-tumor immune response. Its first approved product, Tudriqev, is a key commercial validation of this platform. Replimune is not like comprehensive pharma giants such as Pfizer or Merck, but rather an innovation-driven biotech company highly focused on oncolytic virus and cancer immunotherapy. It has long operated under high R&D, clinical, and regulatory risk; with Tudriqev’s approval by the FDA in August 2026, the company has transitioned from a purely clinical-stage R&D firm to launching its first commercial product.

Approved after two FDA rejections! Replimune’s melanoma therapy reverses fate, accelerated approval reignites value elasticity of innovative drugs

The drug will be marketed under the trade name Tudriqev and is administered by direct injection into the melanoma tumor. It contains a genetically engineered virus intended to both kill the injected tumor and broadly activate the immune system to attack tumors elsewhere in the body. The FDA stated the approval is based on a single-arm study with 91 patients with advanced melanoma, of whom 24% had tumor shrinkage.

Replimune’s CEO, Dr. Sushil Patel, stated: “This is a transformational moment for Replimune, marking the fruition of years of pioneering research and bringing Tudriqev to patients with urgent unmet needs for new treatment options.”

After the FDA announced the news, Replimune’s share price surged, closing up 8.7% on Thursday. The stock is up 32% year-to-date.

The company stated that a typical course of treatment will cost $450,000. The FDA reported that side effects include fatigue, fever, infection, and chills.

The therapy received accelerated approval and has not yet demonstrated survival benefit through controlled trials. The company is conducting a confirmatory trial with 400 patients to prove this benefit.

The approval concludes a 13-month ordeal. Initially, the drug seemed likely to be approved, but the FDA rejected the application in July 2025, citing problems with trial design and patient population.

After resubmitting in autumn last year, the FDA rejected Replimune’s application again this April, stating the data provided was “insufficient to conclude substantial evidence of efficacy.” However, just weeks after Makary’s resignation, the FDA agreed in late May to re-review the application. This paved the way for an advisory committee meeting on July 30, where FDA staff detailed their many concerns regarding the company’s single-arm trial data.

FDA staff noted in briefing documents for the July 30 meeting that since the supporting trial lacked a control group and Replimune’s therapy was given in combination with another approved melanoma drug—Bristol Myers Squibb’s Opdivo—the results were difficult to interpret in isolation.

Nevertheless, the FDA’s external advisory panel ultimately voted in favor—10 to 3—that, despite these limitations, the evidence supporting the drug was assessable and clinically meaningful.

Tudriqev validates the oncolytic virus platform’s value, opening commercial possibilities for Replimune in PD-1 resistant tumors

Tudriqev (vusolimogene oderparepvec, formerly RP1) works by directly injecting genetically engineered HSV-1 herpes viruses into melanoma lesions, causing the virus to selectively replicate within the tumor and lyse cancer cells. This process converts “cold” tumors into “hot” tumors that the immune system can recognize.

RP1 also carries GALV-GP R- fusion protein and GM-CSF: the former enhances tumor cell fusion and immunogenic cell death, while the latter promotes antigen presentation and immune cell recruitment. As tumors lyse, a large amount of patient-derived tumor antigens are released, effectively creating an “in situ personalized cancer vaccine.” When combined with the PD-1 antibody nivolumab (Opdivo), the blockade that tumors have re-established in T cells is lifted, in theory enabling not only local tumor attack but also eliciting systemic immune responses against distant, uninjected metastases. The FDA has thus defined it as a novel engineered oncolytic virus immunotherapy.

From an innovation standpoint, it is among the most cutting-edge products globally for melanoma treatment, especially meeting the urgent needs of patients with unresectable advanced cutaneous melanoma post PD-1 therapy failure. However, it cannot yet be rigorously called the “world’s most effective or top-tier melanoma drug.”

The FDA previously rejected it twice, precisely due to patient heterogeneity, lack of a control group, and inability to fully differentiate the contributions of RP1 versus co-administered Opdivo. The company still needs an approximately 400-patient Phase III confirmatory trial to demonstrate true clinical benefit. By comparison, first-line therapies for advanced melanoma such as nivolumab + ipilimumab, nivolumab + relatlimab already possess randomized phase III evidence; patients with BRAF V600 mutations can opt for BRAF/MEK targeted therapies; after PD-1 failure there’s Amtagvi (lifileucel) TIL-cell therapy, which has an FDA study ORR of about 31.5%, but requires tumor excision, lymphodepleting chemotherapy, and high-dose IL-2, which are substantially more complex and toxic. Cross-trial efficacy comparisons are inappropriate, so Tudriqev’s main competitive advantage is not the “highest ORR,” but its convenience and unique mechanism—systemic immune reactivation by local injection, with no need for individualized cell manufacture or lymphodepletion.

From a biotech investment perspective, its most accurate positioning is: “one of the world’s leading next-generation oncolytic immunotherapies,” rather than an established new melanoma therapy leader. The most significant aspect of this approval is the validation of Replimune’s RPx platform—that engineered HSV-1 can serve as an important option for unresectable advanced cutaneous melanoma after PD-1 failure.

Three key variables for investors to watch are—the true uptake of Tudriqev after its launch, whether the 400-patient confirmatory trial demonstrates hard endpoints such as PFS/OS, and whether this virus platform can be replicated in other skin cancers and solid tumors. Especially if the latter two materialize, the significance will grow from a “PD-1 resistant melanoma drug” to a scalable tumor immunoengineering platform. In addition, FDA’s accelerated approval system itself requires subsequent trials to confirm clinical benefits, or else there is a regulatory risk of the indication being withdrawn.