Absorption of glucagon-like peptide-1 can be protracted by zinc or protamine

Pharmacokinetics of several different preparations for subcutaneous administration of GLP-1(7-36)amide (GLP-1) were studied. The difference between soluble GLP-1 at pH 4.0 and a suspension of GLP-1 crystals at pH 6.9, and how addition of zinc chloride or protamine sulphate to the GLP-1 suspension affected the absorption kinetics after s.c. administration, were investigated. Four Beagle dogs received 50 mu g/kg of GLP-1 s.c. in different vehicle compositions. The vehicle contained 1 mg/ml of GLP-1, 10 mmol/l disodium phosphate, 16 mg/ml glycerol, and in some preparations either zinc chloride or protamine sulphate. The preparations were: (1) pH 4.0, (2) pH 6.9, (3) 1 equiv. (relative to GLP-1) zinc chloride pH 6.9, (4) 0.5 equiv. zinc chloride pH 6.9, (5) 3 equiv. zinc chloride pH 6.9, (6) 0.5 mg/ml protamine sulphate pH 4.0, and (7) 0.5 mg/ml protamine sulphate pH 6.9. Plasma samples were analyzed employing a sandwich enzyme-linked immunosorbant assay. Addition of zinc chloride or protamine sulphate to the GLP-1 suspension decreased the absorption rate. The plasma concentration of GLP-1 was increased to a pharmacological level for at least 15 h when administered s.c. as a zinc or protamine suspension. Furthermore, the peak plasma concentration was flattened out, when the peptide was administered as a suspension containing zinc chloride or protamine sulphate, compared to a solution.