Preclinical studies of a brain penetrating IgG Trojan horse-arylsulfatase fusion protein in the metachromatic leukodystrophy mouse

Metachromatic Leukodystrophy (MLD) is a neurodegenerative disease caused by mutations in the lysosomal enzyme, arylsulfatase A (ASA). ASA deficiency leads to sulfatide glycolipid accumulation in brain. Treatment of MLD with intravenous recombinant ASA is not possible because this molecule does not cross the blood-brain barrier (BBB). BBB penetration by ASA was enabled by re-engineering ASA as an IgG-ASA fusion protein, where the IgG domain is a chimeric monoclonal antibody (MAb) against the mouse transferrin receptor (TfR), designated cTfRMAb. The IgG domain acts as a molecular Trojan horse to deliver the ASA into brain via transport on the endogenous BBB TfR. The cTfRMAb-ASA fusion protein bound to the mouse TfR with high affinity and retained high ASA enzyme activity comparable to recombinant human ASA. In an intial dose ranging study, 14-week old MLD mice were treated for 5 weeks with thrice-weekly intraperitoneal or subcutaneous injections of 5 mg/kg of the cTfRMAb-ASA fusion protein. This short-term chronic administration of the cTfRMAb-ASA fusion protein was well tolerated, and no clinical injection related reactions (IRR) were observed. In contrast, the short-term administration of recombinant human ASA alone to MLD mice causes severe IRRs, which prevents long-term treatment (Hum Mol Genet 2005). Based on these findings, a long-term efficacy study was initiated, and MLD mice were treated twice-weekly with saline, the cTfRMAb isotype control alone, or the cTfRMAb-ASA fusion protein intraperitoneally at 5mg/kg for up to 12 months. Preliminary analysis completed at 6 months of treatment with u003e500 doses confirmed the safety of the cTfRMAb-ASA dosing with no clinical IRRs, normal body weight and normal hematocrit and reticulocytes. The study demonstrates for the first time that ASA, following re-engineering as an IgG fusion protein has an excellent safety profile allowing for long-term treatment of MLD mice.