Pharmacokinetics of Intranasal versus Subcutaneous Insulin in the Mouse

Abstract

Insulin delivery to the brain has emerged as an important therapeutic target for cognitive disorders associated with abnormal brain energy metabolism. Although insulin is transported across the blood-brain barrier, peripheral routes of administration are problematic due to systemic effects of insulin on blood glucose. Intranasal (IN) administration is being investigated as an alternative route. We conducted a head-to-head comparison of subcutaneous (SC) and IN insulin, assessing plasma and brain pharmacokinetics and blood glucose levels in the mouse. SC insulin (2.4 IU) achieved therapeutically relevant concentrations in the brain (AUCbrain = 2537 h·μIU/mL) but dramatically increased plasma insulin (AUCplasma = 520 351 h·*μIU/mL), resulting in severe hypoglycemia and in some cases death. IN administration of the same dose resulted in similar insulin levels in the brain (AUCbrain = 3442 h·μIU/mL) but substantially lower plasma concentrations (AUCplasma = 354 h·μIU/mL), amounting to a ∼ 2000-fold increase in the AUCbrain:plasma ratio relative to SC. IN dosing also had no significant effect on blood glucose. When administered daily for 9 days, IN insulin increased brain glucose and energy metabolite concentrations (e.g., adenosine triphosphate and phosphocreatine) without causing overt toxicity, suggesting that IN insulin may be a safe therapeutic option for cognitively impaired patients.

Keywords: Insulin; energy metabolism; glucose; intranasal; neurocognitive impairment; pharmacokinetics.

Conflict of interest statement

Notes

The authors declare no competing financial interest.

Figures

Figure 1

IN insulin achieves similar brain concentrations with reduced plasma concentrations relative to SC insulin. Brain and plasma were harvested 1 h postadministration of vehicle or two doses of subcutaneous (SC) insulin (0.24 and 2.4 IU) or intranasal (IN) insulin (2.4 IU) in fasted mice. (Left) SC administration resulted in dose-dependent increases in insulin concentrations in the brain that reached similar levels compared with IN insulin at the 2.4 IU dose. (Right) SC insulin at both doses yielded substantially larger increases in plasma insulin concentrations compared with IN dosing (which did increase plasma insulin levels above baseline). Values depict the mean + SEM. Comparison between treatment groups by pairwise t-test; *p < 0.05, **p < 0.01, ***p < 0.001, n.s. not significant; n = 3–6/dose.

Figure 2

IN insulin achieves similar brain exposure with reduced plasma exposure relative to SC insulin. Plasma and brain were harvested at multiple time points postadministration of insulin (2.4 IU) delivered via the subcutaneous (SC) or intranasal (IN) route. SC administration resulted in (right) substantially higher insulin exposure in plasma but (left) comparable brain exposure relative to IN administration. Dotted line indicates lower limit of detection (LLOD). Values depict the mean + SEM; open points indicate a mean value of zero; n = 3–8/group.

Figure 3

IN insulin substantially improves the brain:plasma ratio relative to SC insulin. Brain to plasma ratios of (left) Cmax and (right) AUC values obtained after subcutaneous (SC) and intranasal (IN) insulin (2.4 IU) administration were calculated. IN administration resulted in a greater than 200- and 2000-fold increases in the brain:plasma insulin Cmax and AUC ratios, respectively. Values depict the mean + SEM or mean; n = 3–8/group.

Figure 4

SC insulin induces severe hypoglycemia. Blood glucose was measured at multiple time points postadministration of insulin (2.4 IU) via the subcutaneous (SC) or intranasal (IN) route. SC administration resulted in significant and prolonged reductions in blood glucose and death in about 10% of mice. IN administration of the same dose had no significant effect on blood glucose. Values depict the mean + SEM. Blood glucose changes are compared by two-way ANOVA with Bonferroni posthoc test; **p < 0.01 vs basal for SC; n = 3/group.

Figure 5

Repeated IN insulin administration increased brain concentrations of glucose and phosphorylated energy metabolites. Tissue was harvested from mice after 9 days of daily intranasal (IN) administration of vehicle or insulin (2.4 IU). Brain energy metabolites were analyzed by NMR. IN insulin administration produced an increase in (top left) glucose and (top middle) ATP as well as in (top right) the energy storage molecule phosphocreatine. There was no change in (bottom left) ADP, but there was a corresponding reduction in (bottom right) creatine. Values are depicted individually as well as the mean ± SEM. Changes due to treatment are compared by t-test; *p < 0.05, **p < 0.01; n = 7–13/group.