LONDON • A one-year-old girl who was close to dying has become the first in the world to be treated with "designer" immune cells genetically engineered to reverse her cancer, doctors said on Thursday.

Layla Richards was suffering from leukaemia, but was saved after scientists used a new gene-editing technique to manipulate cells to fight the disease at Great Ormond Street Hospital in central London.

"As this was the first time that the treatment had been used, we didn't know if or when it would work and so we were over the moon when it did," said Professor Paul Veys, director of bone marrow transplant at the hospital. "Her leukaemia was so aggressive that such a response is almost a miracle."

The baby was diagnosed with acute lymphoblastic leukaemia - the most common form of childhood leukaemia - when she was just 14 weeks old. She was treated with chemotherapy and a bone marrow transplant, but the cancer returned and doctors told her parents to consider palliative end-of-life care.

But dad Ashleigh Richards, 30, refused to give up. He told the BBC: "I didn't want to go down that road. I'd rather that she tried something new and I took the gamble."

The family was then offered an experimental treatment under development at the hospital, in which doctors modified white blood cells - T-cells - from a healthy donor so that they seek out and kill drug-resistant leukaemia.

"She was sick and in lots of pain, so we had to do something," her father said. "Doctors explained that even if we could try the treatment, there was no guarantee that it would work but we prayed it would."

Layla was given a small infusion of the genetically-engineered cells known as UCART19 cells. A few weeks later, consultants told her parents that the treatment had worked and she has no trace of leukaemia in her body.

Doctors stressed that the experimental technique had just been used once and that the results need to be replicated, but said it was potentially very promising.

Prof Veys said Layla has been in remission for only a few months. "The only way we will find out if this is a cure is by waiting that one or two years, but even having got this far from where we were is a major, major step," he told the BBC.

AGENCE FRANCE-PRESSE