A judge has ruled against the NHS after the health service refused to fund Biomarin's Kuvan for a young boy.

The seven-year-old has severe autism and phenylketonuria (PKU), which inhibits his ability to digest protein. His autism makes it difficult for him to follow the strict dietery resitrictions usually used to treat PKU, so his doctors sought funding for Kuvan (sapropterin dihydrochloride), which would allow him to get a proportion of vitamins and minerals from ordinary food.

His counsel, Ian Wise QC, said that funding had been repeatedly refused by NHS England with differing reasons given. The judge, Mrs Justice Andrews, quashed the decision and remitted it for reconsideration.

The case succeeded on the basis of one of the three grounds advanced – concerning the decision in relation to the clinical effectiveness of the drug. Andrews said the reason for turning down the initial application had been that Kuvan was not clinically effective and so overturned the decision.

"If 'clinical effectiveness' is properly interpreted, the evidence that Kuvan is clinically effective is overwhelming,” she said.

But she warned: "Whilst this judgment is bound to give rise to a degree of optimism, I must caution against raising hopes too high. The fact that this claim for judicial review has succeeded does not mean that there will necessarily be a favourable outcome to this IFR (individual funding request) application."

Jenni Richards QC, for NHS England, said there was no basis for impugning on rationality grounds the judgments reached as to clinical and cost-effectiveness or equity of funding.

The mainstay of PKU treatment was – and would remain, even with Kuvan – dietary control.

She said that the child's consultant acknowledged that his overall development outcome would mostly be affected by the severity of his autism rather than his PKU and that Kuvan would not be expected to significantly alter or improve his behaviour.​