High-dose enzyme replacement therapy in murine Hurler syndrome

Mucopolysaccharidosis type I (MPS I) is an autosomal recessive disease that is systemic, including progressive neurodegeneration, mental retardation and death before the age of 10 years. MPS I results from deficiency of α-L-iduronidase (IDUA) in lysosomes and subsequent accumulation of glycosaminoglycans (GAG). Clinical enzyme replacement therapy (ERT) with intravenous laronidase reverses some aspects of MPS I disease (e.g., hepatomegaly, splenomegaly, glycosaminoglycanuria) and ameliorates others (e.g., pulmonary function, cardiac disease, arthropathy, exercise tolerance). However, neurologic benefits are thought to be negligible because the blood-brain barrier (BBB) blocks enzyme from reaching the central nervous system (CNS). We considered the possibility that a very high dose of intravenous laronidase might be able to traverse the BBB in small quantities, and provide some metabolic correction in the brain. To address this question, high-dose laronidase was administered (11.6 mg/kg, once per week, 4 weeks) to adult MPS I mice. IDUA enzyme activity in the cortex of treated mice increased to 97% of that in wild type mice (p<0.01). GAG levels in cortex were reduced by 63% of that from untreated MPS I mice (p<0.05). Further, immunohistochemical analysis showed that treatment reduced secondary GM3-ganglioside accumulation in treated MPS I mice. Water T-maze tests showed that the learning abnormality in MPS I mice was reduced (p<0.0001). In summary, repeated, high-dose ERT facilitated laronidase transit across the BBB, reduced GAG accumulation within the CNS, and rescued cognitive impairment.