OBJECTIVEIn vivo, after subcutaneous injection, insulin glargine (21A-Gly-31B-Arg-32B-Arg-human insulin) is enzymatically processed into 21A-Gly-human insulin (metabolite 1 [M1]). 21A-Gly-des-30B-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 METHODSGlargine, 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.RESULTSPlasma 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 (AUC0–30h) 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-AUC0–30h 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.CONCLUSIONSAfter 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 (21A-Gly-human insulin), which mediates the metabolic effect of injected glargine.

OBJECTIVETo investigate concentration of plasma insulin glargine after its subcutaneous dosing compared with concentration of its metabolites 1 (M1) and 2 (M2) in subjects with type 2 diabetes.RESEARCH DESIGN AND METHODSNine subjects underwent a 32-h euglycemic glucose clamp study (0.4 units/kg glargine after 1 week of daily glargine administration). Glargine, M1, and M2 were measured by a specific liquid chromatography-tandem mass spectrometry assay.RESULTSGlargine was detected in only five of the nine subjects, at few time points, and at negligible concentrations. M1 was detected in all subjects and exhibited the same pattern as traditional radioimmunoassay-measured plasma insulin. M2 was not detected at all.CONCLUSIONSAfter subcutaneous injection, glargine was minimally detectable in blood, whereas its metabolite M1 accounted for most (>90%) of the plasma insulin concentration and metabolic action of the injected glargine.