G207 (NCT03911388)

Most childhood brain tumors start in the cerebellum, low at the back of the head, where balance and coordination live. That is where medulloblastoma begins, the most common malignant brain tumor of childhood.

Treatment is a hard road: surgery, radiation to the whole brain and spine, then months of chemotherapy. Many children are cured, but it is tough on a growing brain. And when the tumor comes back after all of that, there are very few options left. That is the problem this trial is trying to solve.

Here is the idea. What if the treatment were a virus?

Almost everyone has met herpes simplex virus type one, the cold-sore virus. It is naturally good at getting into nerve cells. Scientists rebuilt it so it can grow inside cancer cells, but not healthy brain. The rebuilt virus is called G207. The strategy: let it do what viruses do, only inside the tumor, and let the immune system notice.

How does it work? Two edits to the virus's genes.

The first edit takes away the virus's key. Every healthy cell has an alarm: when it senses a virus, an enzyme called PKR shuts down the cell's protein factory, and the infection stalls. Natural herpes carries a gene that switches that alarm off. Scientists deleted it. Now healthy cells stop G207. But many cancer cells disabled that alarm on their way to becoming cancer. Inside them, G207 multiplies.

The second edit takes away a tool. To copy itself, the virus needs an enzyme that builds DNA. Scientists broke the virus's own copy, so G207 must borrow it from the cell it is in, and only dividing cells have it. Tumor cells divide constantly. Most mature brain cells do not.

Together, the two edits mean the virus can only thrive in the tumor. It multiplies inside cancer cells until they burst, spreads to the cancer cells next door, and all that debris draws in the immune system.

That last part may matter most. In the first trial of G207 in children, biopsies before treatment showed almost no immune cells in the tumor, what doctors call a cold tumor. Months later, the same tumors were crowded with T cells. Twelve children were treated. No serious side effects were blamed on the virus, and median survival was twelve point two months, against a historical five point six. A small first study, not proof, published in the New England Journal of Medicine in twenty twenty-one.

One more piece. In the lab, a small dose of radiation makes tumor cells produce more of the very enzyme the virus lost, which helps it multiply. So most children in this trial get one low dose of radiation the day after the virus.

That first trial treated the upper brain. This one takes G207 to the cerebellum for the first time, after mouse studies showed it was safe there. Children and young adults from three to twenty-one whose cerebellar tumor has returned receive a single infusion of G207 through fine catheters placed in the tumor. The first question is safety; the second is dose. It is led by Gregory Friedman at M D Anderson, with sites in Houston, Birmingham, and St. Louis.

Everything in this film comes from the published record, and it is not medical advice.