Objective: In vivo, after subcutaneous injection, insulin glargine (21(A)-Gly-31(B)-Arg-32(B)-Arg-human insulin) is enzymatically processed into 21(A)-Gly-human insulin (metabolite 1 [M1]). 21(A)-Gly-des-30(B)-Thr-human insulin (metabolite 2 [M2]) is also found. In vitro, glargine exhibits slightly higher affinity, whereas M1 and M2 exhibit lower affinity for IGF-1 receptor, as well as mitogenic properties, versus human insulin. The aim of the study was to quantitate plasma concentrations of glargine, M1, and M2 after subcutaneous injection of glargine in male type 1 diabetic subjects.
Research design and methods: Glargine, M1, and M2 were determined in blood samples obtained from 12, 11, and 11 type 1 diabetic subjects who received single subcutaneous doses of 0.3, 0.6, or 1.2 units · kg(-1) glargine in a euglycemic clamp study. Glargine, M1, and M2 were extracted using immunoaffinity columns and quantified by a specific liquid chromatography-tandem mass spectrometry assay. Lower limit of quantification was 0.2 ng · mL(-1) (33 pmol · L(-1)) per analyte.
Results: Plasma M1 concentration increased with increasing dose; geometric mean (percent coefficient of variation) M1-area under the curve between time of dosing and 30 h after dosing (AUC(0-30h)) was 1,261 (66), 2,867 (35), and 4,693 (22) pmol · h · L(-1) at doses of 0.3, 0.6, and 1.2 units · kg(-1), respectively, and correlated with metabolic effect assessed as pharmacodynamics-AUC(0-30h) of the glucose infusion rate following glargine administration (r = 0.74; P < 0.01). Glargine and M2 were detectable in only one-third of subjects and at a few time points.
Conclusions: After subcutaneous injection of glargine in male subjects with type 1 diabetes, exposure to glargine is marginal, if any, even at supratherapeutic doses. Glargine is rapidly and nearly completely processed to M1 (21(A)-Gly-human insulin), which mediates the metabolic effect of injected glargine.