Colorectal cancer is the **third most common cancer in the world**, responsible for roughly 900,000 deaths every year. And within that category, there is a particularly hard subset: patients whose tumours carry a **KRAS mutation** — a fault in one of the most common cancer-driving genes — and whose disease is classified as **microsatellite stable (MSS)**.
For these patients, the last decade's revolution in cancer immunotherapy has largely passed them by.
Immune checkpoint inhibitors — the drugs that have transformed outcomes in melanoma, lung cancer, and certain forms of colorectal cancer — generally work by lifting molecular brakes that prevent the immune system from attacking tumours. In **microsatellite-unstable (MSI-H)** colorectal cancer, this approach has been transformative. But MSS tumours — which account for approximately **85% of all colorectal cancers** — have a different molecular character. They don't respond the same way. They are what oncologists call **immunologically cold**: the immune system doesn't recognise them as threats, doesn't mount a meaningful attack, and checkpoint inhibitors can't change that.
For second-line KRAS-mutant MSS metastatic colorectal cancer, standard chemotherapy currently achieves an objective response rate of roughly **6–11%** and a median overall survival of around **11 months**.
Then there is the **REO-022 study data**.
**A Virus From Nature**
Pelareorep is not a synthetic molecule or an engineered antibody. It is a **reovirus** — a naturally occurring, non-pathogenic virus (meaning it does not cause disease in healthy humans) that has a remarkable property: it preferentially infects and replicates inside cells with activated KRAS signalling.
KRAS-mutant cancer cells, it turns out, create a cellular environment that reoviruses find particularly hospitable. The virus enters, replicates, and eventually causes the cancer cell to die. But the killing is only part of the story.
As the virus destroys cancer cells, it does something immunologically important: it generates **immunogenic cell death** — a form of tumour cell destruction that sends alarm signals to the immune system, recruiting T-cells and other immune cells to the tumour site. In other words, it transforms a cold, immunologically invisible tumour into something **hot** — something the immune system can recognise, attack, and remember.
This is the mechanism researchers at **Oncolytics Biotech** have spent years developing and validating. And the completed REO-022 study is their most compelling data to date.
**The REO-022 Results**
The REO-022 study evaluated pelareorep in combination with **bevacizumab** (an anti-angiogenic agent that cuts off tumour blood supply) and **FOLFIRI** (a standard chemotherapy backbone) in patients with second-line RAS-mutated, MSS metastatic colorectal cancer — the exact population where immunotherapy has consistently failed.
The results:
📊 **Objective response rate (ORR): 33%** — versus approximately 6–11% with standard-of-care alone ⏱️ **Median progression-free survival (PFS): 16.6 months** — versus 5.7 months with standard treatment 🗓️ **Median overall survival (OS): 27 months** — versus 11.2 months with standard treatment
That is a tripling of survival time in a patient population that immunotherapy was not supposed to work in.
The mechanism makes scientific sense. KRAS-mutant cells are precisely what the reovirus is evolved to exploit. Pelareorep finds them, infects them, kills them, and in doing so, lights up the tumour for immune attack — the kind of immune attack that checkpoint inhibitors can then amplify.
Translational analyses from the study have confirmed the biology: pelareorep treatment **increases KRAS-mutant-specific T-cell populations** in the bloodstream — direct evidence that the immune system is being activated against the cancer.
**FDA Fast Track and the REO-033 Trial**
The REO-022 data were compelling enough to prompt the **FDA to grant Fast Track Designation** to pelareorep in combination with bevacizumab and FOLFIRI for second-line KRAS-mutant MSS metastatic colorectal cancer in **February 2026**. Fast Track status is reserved for therapies that address serious conditions and demonstrate substantial potential to fill an unmet medical need — it expedites the review process and allows more frequent communication with the FDA.
In **March 2026**, Oncolytics Biotech launched **REO-033** — a new randomised Phase 2 clinical trial designed to confirm the REO-022 findings in a larger, controlled study. The first site activation is anticipated later in March, with preliminary data expected before the end of 2026.
REO-033 is the pivotal step from promising signal to confirmed treatment. If the randomised data holds, it will form the basis for a potential marketing authorisation application.
**Why This Matters Beyond the Numbers**
The KRAS gene was first identified as an oncogene in the 1980s. For three decades, it was considered **"undruggable"** — a mutation so embedded in cancer biology that no drug could target it reliably. The development of KRAS inhibitors in recent years represented the first crack in that wall. Pelareorep represents something different: not targeting KRAS directly, but exploiting it.
The reovirus doesn't need to block KRAS. It *uses* it — entering cells precisely because KRAS is overactive, turning the cancer's own mutation against it.
And it does it while making the tumour visible to the immune system — solving, in a single mechanism, both the problem of the cancer's molecular resistance and its immunological invisibility.
For patients with KRAS-mutant MSS colorectal cancer, there are currently no approved immunotherapy options and limited second-line chemotherapy choices. Median survival is measured in months.
Twenty-seven months is not a cure. But it is more than double what is currently possible. It is extra years. It is time.
The REO-033 results are expected later this year.
For a disease that has resisted every immune approach since checkpoint inhibitors arrived, a naturally occurring virus is doing what engineered antibodies could not. 🦠🎯
*Sources: Oncolytics Biotech (oncolyticsbiotech.com) · FDA Fast Track Designation announcement (February 2026) · REO-033 trial initiation (March 2026) · Targeted Oncology · Cancer Network · Cure Today · OncDaily · BusinessWire*
