Pabinafusp alfa is a fusion protein consisting of a humanized anti-hTfR antibody and hIDS. The humanized anti-hTfR antibody is considered to bind to TfRs expressed in the luminal cell membrane of brain microvascular endothelial cells composing the blood-brain barrier, being transported together with the TfR, passing through brain microvascular endothelial cells via transcytosis (which involves the mechanisms of uptake into the cell via endocytosis and release outside the cell via exocytosis) and further penetrating the basal lamina into neuronal cells in the brain parenchyma. In addition, after binding of pabinafusp alfa to M6PR on the cell membrane and cellular uptake via endocytosis, newly formed endosomes are thought to fuse with lysosomes and move into them. In patients with MPS, the type of accumulating GAG differs depending on the MPS phenotype, and CNS symptoms develops in patients with MPS phenotypes that are associated with the accumulation of HS. HS concentrations in CSF are high in patients with MPS-II who have intellectual disability, compared with those who do not have intellectual disability. These findings and other factors have suggested that HS plays a vital role in the development of CNS symptoms. The hIDS of pabinafusp alfa is considered to promote the degradation of HS in the brain parenchyma, thereby suppressing the development or progression of CNS symptoms. It has been demonstrated that pabinafusp alfa has binding affinity for M6PR and hTfR, and is taken up by human fibroblasts via the receptors.