A second baby with aggressive leukemia has been treated in London with "designer immune cells" developed by Cellectis and, six months after treatment, she remains in remission, the French biotech firm said on Friday.

The first human use of the company's cell therapy made headlines worldwide in November when one-year-old Layla was cleared of previously incurable leukemia at Britain's Great Ormond Street Hospital (GOSH).

Details of the latest compassionate care case, also treated at GOSH, were presented at the American Society of Gene & Cell Therapy annual meeting in Washington.

The so-called UCART19 injection works by adding new genes to healthy donated immune cells known as T-cells, which arm them against leukemia.

More time is needed to see whether the therapy has cured the disease, or simply slowed its progression. But the fact that Layla is still doing well 11 months after her injection and the second case has so far been successful is encouraging.

"This is not yet statistical proof but we do now have two cases," said Cellectis Chief Executive Andre Choulika. "It shows that the therapy has some potential."

The second baby was diagnosed with leukemia at four weeks old and was treated with UCART19 in December aged 16 months, after failing to respond to other treatments.

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Other drugmakers including Novartis, Juno Therapeutics and Kite Pharma have tested genetically modified T-cells extracted from an individual patient.

The Cellectis approach, however, uses cells from a healthy donor in a process that could lead to a ready "off-the-shelf" supply for use in multiple patients.

The biotech company, working with the U.S. giant Pfizer and French drugmaker Servier, believes its method is faster and cheaper than creating single patient-specific gene therapies.

Clinical trials of UCART19 are due to start this year.