A type of experimental gene therapy has shown potential for treating some brain tumour patients in a small clinical trial.

The treatment was found to be safe to give to patients. And around a quarter of one group of patients on the trial survived for more than 3 years after treatment.

In total, more than 4 in 10 of these patients saw their tumours shrink or stop growing after receiving the treatment, according to unpublished results presented at the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics.

The patients in the trial all had a type of brain tumour called a glioma, which had returned after previous treatment. All of the tumours were classified as ‘high grade’, meaning fast growing.

In the Phase I clinical trial, which involves 2 different steps, 56 patients were first given the gene therapy Toca 511, a modified virus that only infects growing tumour cells. Once inside the cells it delivers a gene that contains the instructions for a molecule called cytosine deaminase, which the cells then start producing.

Patients were then given a second treatment, Toca 5C, which is an inactive version of an anti-cancer drug called 5-flurouracil. This drug can’t cross the brain’s protective barrier and therefore would be unable to reach the tumour, whereas Toca 5C can.

Once inside the brain, Toca 5C is turned into 5-fluorouracil, potentially delivering a targeted attack directly to the tumour cells.

This turns the brain tumour into an anti-cancer drug “factory,” said Professor Clark Chen from the University of Minnesota in the US who led the study. He added that the treatment also activates the immune system to further attack the cancer.

Out of the 56 patients, 23 were eligible to enter the next phase of the clinical trial. Of these, 5 patients had their tumours disappear and in a further 5 the treatment prevented their disease from worsening.

The average survival within this group of patients was 14.4 months.

Although there was no comparison group (control) in this trial, the researchers claim that this is significantly longer than would be expected for patients receiving standard treatment (8 months, according to historical data).

Because of the lack of a control group, Dr Adel Samson, a Cancer Research UK-funded expert in virus treatments from the University of Leeds, said that it’s not possible to determine whether the experimental treatment boosts survival.

An ongoing Phase II/III clinical trial of this treatment—including the 23 patients from this trial and a control group—should answer this.

“More work is needed to confirm that this treatment works in patients in the way that it is intended, and to assess whether Toca 511 additionally stimulates immune cells to attack the cancer,” said Samson.

“Further clinical trials are also needed to determine whether Toca 511 improves patient outcomes.”

Toca511 & Toca FC is not approved for brain tumour patients in the UK, but earlier this year the US Food and Drug Administration gave the treatment breakthrough therapy status for high grade glioma patients whose disease has returned.