Glycemic control and chronic dosing of rhesus monkeys with a fusion protein of iduronidase and a monoclonal antibody against the human insulin receptor

Abstract

Hurler's syndrome, or mucopolysaccharidosis type I, is a lysosomal storage disorder caused by mutations in the gene encoding the lysosomal enzyme iduronidase (IDUA). The disease affects both peripheral tissues and the central nervous system (CNS). Recombinant IDUA treatment does not affect the CNS, because IDUA does not cross the blood-brain barrier (BBB). To enable BBB penetration, human IDUA was re-engineered as an IgG-IDUA fusion protein, where the IgG domain is a genetically engineered monoclonal antibody (MAb) against the human insulin receptor (HIR). The HIRMAb penetrates the brain from the blood via transport on the endogenous BBB insulin receptor and acts as a molecular Trojan horse to deliver the fused IDUA to the brain. Before human testing, the HIRMAb-IDUA fusion protein was evaluated in a 6-month weekly dosing toxicology study at doses of 0, 3, 9, and 30 mg/kg/week of the fusion protein administered to 40 rhesus monkeys. The focus of the present study is the effect of chronic high dose administration of this fusion protein on plasma glucose and long-term glycemic control. The results show that the HIRMAb has weak insulin agonist activity and causes hypoglycemia at the high dose, 30 mg/kg, after intravenous infusion in normal saline. When dextrose is added to the saline infusion solution, no hypoglycemia is observed at any dose. An intravenous glucose tolerance test performed at the end of the 6 months of chronic treatment showed no change in glucose tolerance at any dose of the HIRMAb-IDUA fusion protein.

Fig. 1.

Plasma glucose is plotted versus time after the start of a 30 min infusion of the HIRMAb-IDUA fusion protein at a dose of either 0 mg/kg (left) or 30 mg/kg (right). Data are shown for individual monkeys (6 males and 6 females in each treatment group). The HIRMAb-IDUA fusion protein is infused in 50 ml of normal saline with no glucose supplement at the start (week 1) of the study. The horizontal bar defines a plasma glucose of 40 mg%, which is a minimum value for all but one of the saline-infused monkeys.

Fig. 2.

Plasma glucose is plotted versus time after the start of a 30-min infusion of the HIRMAb-IDUA fusion protein at a dose of 0, 3, 9, or 30 mg/kg. The HIRMAb-IDUA fusion protein is infused in 50 ml of 10% dextrose/normal saline at the end (week 25) of the study. The 23-h plasma glucose was 80 ± 4, 80 ± 5, 78 ± 7, and 80 ± 4 mg% for the 0, 3, 9, and 30 mg/kg treatment groups, respectively. Data are mean ± S.E. (n = 6–12 combined sexes in each group).

Fig. 3.

CSF glucose at 0, 3, and 23 h after HIRMAb-IDUA fusion protein infusion is shown for 0, 3, 9, and 30 mg/kg doses of the HIRMAb-IDUA fusion protein. Data are mean ± S.E. (n = 8–12 combined sexes in each group). The HIRMAb-IDUA fusion protein is infused in 50 ml of normal saline at the start (week 1) of the study. *, P < 0.05 difference from control (0 mg/kg) as determined by analysis of variance.

Fig. 4.

CSF glucose is plotted versus the corresponding plasma glucose at 3 h after the intravenous infusion of the HIRMAb-IDUA fusion protein for all four treatment groups (0, 3, 9, and 30 mg/kg). Data for individual monkeys is shown. The slope was determined by linear regression analysis. CSF and plasma glucose were determined after HIRMAb-IDUA infusion during the first week of the study.