In 2005, Merck bought the rights to Kuvan and Peg-Pal, another PKU treatment, outside the US and Japan.
BioMarin will pay Merck €340m for Kuvan and about €185m in additional milestones for both products. The deal will take effect in January next year.
Kuvan is indicated to treat hyperphenylalaninemia (HPA) due to PKU in patients of all ages who have shown to be responsive to the product, or because of tetrahydrobiopterin (BH4) deficiency.
Clinical data suggested that Kuvan produces reduces blood phenylalanine concentration in a large subset of patients.
Kuvan is approved in 51 countries globally, including member states of the European Union and the US.
Merck member of the executive board and CEO of Healthcare Belén Garijo said: "Returning the rights of Kuvan and Peg-Pal to BioMarin will allow Merck to fully focus on its core businesses, as well as further align R&D investment behind key strategic areas."
BioMarin chairman and CEO Jean-Jacques Bienaimé said: "We will leverage our established worldwide infrastructure and strong relationships within the PKU community to ensure that patients globally have access to Kuvan, and potentially pegvaliase upon approval.
"We look forward to expanding our PKU franchise beyond the US and Canadian markets and into our existing commercial footprint of about 60 countries where Kuvan is currently sold."
Image: Pützerturm, a landmark of the headquarters of Merck KGaA, Darmstadt, Germany. Photo: courtesy of Merck KgaA.