The recombinant fusion protein SHP631 consists of a chimeric monoclonal antibody binding to human insulin receptor and iduronate-2-sulfatase (I2S). This product is being developed as an enzyme replacement therapy to treat cognitive symptoms of Hunter’s syndrome. Because the current therapy (idursulfase, brand name Elaprase from Shire) cannot cross the blood–brain barrier (BBB), SHP631 is being developed to do so, enabling the presence of I2S in the brain. The enzymatic activity of this molecule is measured using the substrate 4-methyl umbelliferyl-α-L-idopyranosiduronic…