An 11-year-old boy, who is now free of blood cancer after undergoing a new type of treatment, has told Sky News the drug gave him a "burst of energy".
Arthur D'Hulst had leukaemia but traditional chemotherapy failed to clear it, leaving him very weak, and the side effects were rough.
He was offered a new drug to try called blinatumomab, or blina, which is an immunotherapy that is "kinder" and more targeted and far less toxic than chemo.
Arthur was one of the first children to take part in the trial at London's Great Ormond Street Hospital (GOSH).
Blina made him feel much better. It works by targeting a protein, called CD19, on the leukaemia cells so the person's immune system can recognise them.
The immune system can then attack and kill the leukaemia cells.
Arthur told Sky News: "Chemo was very tiring and made me feel like I always wanted to sleep and I never had any energy to do anything.
"But blina was like, I had a burst of energy. I could go outside."
The drug comes in a bag of liquid that is administered through a plastic tube which goes into a vein.
GOSH said on its website: "It is usually given 24 hours a day for four weeks followed by a two-week break without the infusion. It may be given at home using a portable infusion pump."
Arthur carried the treatment with him in a rucksack which he called his "blina backpack" that pumped the drug into his body day and night.
Although Arthur had to return to GOSH every four days to have the drug topped up, the rest of the time he was at home with his family.
It meant he could do other things - like playing on swings in a park - while the treatment was happening.
The backpack stayed with him continuously including in bed and he was able to get a good night's sleep despite the pump making a noise.
His mother Sandrine Heutz said blina "feels like the future".
She said: "It did feel so effective in terms of doing the job of getting rid of the cancer but also in the way that he was able to live with it."
At the end of last April, Arthur had the final operation to remove the tubing from his arm which his mother said was a big step and he was "free".
In the UK, about 450 children each year are diagnosed with Arthur's type of cancer.
Dr Sujith Samarasinghe, from GOSH, explained that chemotherapy "is like a poison" which "hits everything, the leukaemia cells, as well as all your normal cells within the body".
"What the antibody does is it's like a homing beacon. It lights up the leukaemia cells so that the children's immune system can kill the leukaemia cells, and just the leukaemia cells."
Blina is not officially licensed for use in children like Arthur but it is being tentatively used in 20 hospitals around the country with good results.