An experimental cancer-killing virus has been administered to a human patient for the first time, with hopes the testing will ultimately reveal evidence of a new means of successfully fighting cancer tumors in people's bodies.

The drug candidate, called CF33-hNIS (aka Vaxinia), is what's called an oncolytic virus, a genetically modified virus designed to selectively infect and kill cancer cells while sparing healthy ones.

In the case of CF33-hNIS, the modified pox virus works by entering cells and duplicating itself. Eventually, the infected cell bursts, releasing thousands of new virus particles that act as antigens, stimulating the immune system to attack nearby cancer cells.

Previous research in animal models has shown the drug can harness the immune system in this way to hunt and destroy cancer cells, but up until now no testing has been done in humans.

That's just changed, with co-developers of the drug – the City of Hope cancer care and research center in Los Angeles, and Australia-based biotech company Imugene – now announcing that the first clinical trial in human patients is underway.

"Our previous research demonstrated that oncolytic viruses can stimulate the immune system to respond to and kill cancer, as well as stimulate the immune system to be more responsive to other immunotherapies," says City of Hope oncologist and principal investigator Daneng Li.

"We believe CF33-hNIS has the potential to improve outcomes for our patients."

Unlocking that potential will first depend on showing that CF33-hNIS is safe for people to take, with the first phase of the trial focusing on the safety and tolerability of the drug.

The intervention is expected to enroll 100 participants in total, each being an adult patient with metastatic or advanced solid tumors who has previously tried at least two prior lines of standard treatment.

Once enrolled in the trial, these individuals will receive low doses of the experimental treatment via direct injection or intravenously.

If early results are successful and CF33-hNIS is deemed safe and well tolerated, additional tests will investigate how the drug pairs with pembrolizumab, an existing antibody treatment already used in cancer immunotherapy.

The version of the virus now being clinically trialed produces human sodium iodide symporter (hNIS), a protein that enables researchers to image and monitor viral replication, as well as allowing an additional way to damage the cancer cells by adding radioactive iodine.

Before efficacy is determined, however, researchers will first be looking to see how well the drug is handled by patients, recording the frequency and severity of any adverse effects, and also investigating how well participants fare as low doses are escalated.

Secondary measures – including assessments of how effectively CF33-hNIS shrinks treated tumors – will be analyzed later on down the track, but with the trial expected to take two years across multiple anticipated clinical sites, it will likely be some time before we know the results in detail.

Which doesn't mean we can't get excited about the broad potential here; just that we should keep our expectations in check, because promising results in pre-clinical experiments don't necessarily guarantee similarly successful results in subsequent research involving human patients.

If the drug does turn out to be safe and well-tolerated, we could be looking at a powerful new tool for fighting tumors, described as a "game-changer because of how potent it is and because of its ability to recruit and activate immune cells," according to surgical oncologist Susanne Warner, who previously led a team studying the effects of CF33 on tumors in mice.

"Our oncolytic virus trains the immune system to target a specific cancer cell," she said in 2020.

"Meaning if a similar cancer cell ever tries to regrow, the immune system will be ready and waiting to shut it down."

Nobody knows for sure yet if CF33-hNIS will work the same wonders in people, but if it can, it stands to become only the second FDA-approved oncolytic virus therapy for cancer, following on the heels of a drug called Talimogene laherparepvec (T-VEC), a modified version of the herpes simplex virus, which is used in the treatment of melanoma.