Treatment with insulin glargine does not suppress serum IGF‐1

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Abstract
Aims A 6–8‐fold higher insulin‐like growth factor 1 (IGF‐1) receptor binding affinity in vitro is reported for the insulin analogue glargine compared with human insulin. This study evaluates the in vivo significance by exploring the growth hormone (GH)–IGF‐1 axis. Assuming a higher binding affinity of insulin glargine to pituitary IGF‐1 receptors, serum IGF‐1 concentrations should decrease via negative feedback. Methods In a crossover study, insulin glargine or NPH insulin, respectively, were used in identical doses as basal insulins in treatment periods of 3 weeks. Results Overall glycaemic control was not different between the treatment regimens. In contrast to the hypothesis, serum IGF‐1 concentrations were higher during insulin glargine treatment compared with NPH insulin in patients with Type 1 diabetes (177 ± 18 vs. 159 ± 18 µg/l, P < 0.02, n = 17, age 28 ± 2 years). The effect on IGF‐1 was most pronounced in male patients with Type 1 diabetes (174 ± 11 vs. 146 ± 10 µg/l, P < 0.02, n = 10), but was not significant in patients with Type 2 diabetes (92 ± 9 vs. 86 ± 8 µg/l, NS, n = 25, age 66 ± 2 years). Conclusions In contrast to our hypothesis, serum IGF‐1 did not decrease, but rose during insulin glargine treatment, suggesting an absence of relevant IGF‐1‐like activity of glargine at the level of the pituitary. Improved plasma glucose at dawn during glargine treatment may intensify growth hormone surges and increase IGF‐1 synthesis. Significant increases were seen in younger patients, compatible with the higher activity of the GH–IGF‐1 axis in this age group.