Medicines for Phenylketonuria

What it is

Phenylketonuria (PKU) is an inherited condition in which the body cannot properly break down a substance called phenylalanine — an amino acid (building block of protein) found in protein-containing foods. This is because of a missing or reduced enzyme. If phenylalanine is not broken down, it builds up in the blood and, importantly, can damage the developing brain, potentially causing serious learning disability and other problems — but, crucially, this harm can be prevented if the condition is picked up early and managed properly from the start. In the UK, PKU is one of the conditions screened for in the newborn blood spot ("heel prick") test in the first days of life, so that it can be identified and treatment started before any harm occurs. This early detection and lifelong management is why children with PKU, who are diagnosed and treated from birth, can develop normally. The key to management is controlling the amount of phenylalanine in the diet. PKU is a lifelong condition, and understanding and following the dietary management is central.

How it is treated

The mainstay of managing PKU is a carefully controlled diet, low in phenylalanine, started as early as possible after diagnosis (which, thanks to newborn screening, is usually within the first weeks of life) and continued lifelong, guided by a specialist team including dietitians. Because phenylalanine comes from protein, this means limiting high-protein foods (such as meat, fish, eggs, dairy, nuts and pulses) and avoiding the artificial sweetener aspartame (which contains phenylalanine), while taking special protein substitutes and supplements that provide the other amino acids and nutrients the body needs for growth and health without the phenylalanine. The diet is carefully balanced and monitored, with regular blood tests to check phenylalanine levels and adjust the diet, and the amount of phenylalanine that can be tolerated is individualised. Following the diet is especially critical in childhood (for brain development) and, very importantly, during pregnancy in women with PKU (to protect the developing baby), and lifelong management is now recommended. Some people may benefit from specific medicines that help in certain forms of PKU. Ongoing specialist support helps families and individuals manage what is a demanding but effective treatment. The reassuring message is that PKU, though a serious condition if untreated, is very effectively managed — picked up by newborn screening and controlled with a special diet from birth, so that children can develop normally, with lifelong management and specialist support.