ORIGINAL ARTICLE Quantification of Basal Insulin Peglispro and Human Insulin in Adipose Tissue Interstitial Fluid by Open-Flow Microperfusion Katrin Tiffner, MSc, 1 Beate Boulgaropoulos, PhD, 1,2 Christian Ho ¨ fferer, MSc, 1 Thomas Birngruber, PhD, 1 Niels Porksen, MD, PhD, 3 Helle Linnebjerg, PhD, 3 Parag Garhyan, PhD, 3 Eric Chen Quin Lam, PhD, 4 Mary Pat Knadler, PhD, 3 Thomas R. Pieber, Univ. Prof., MD, 1,2 and Frank Sinner, PhD 1,2 Abstract Background: Restoration of the physiologic hepatic-to-peripheral insulin gradient may be achieved by either portal vein administration or altering insulin structure to increase hepatic specificity or restrict peripheral access. Basal insulin peglispro (BIL) is a novel, PEGylated basal insulin with a flat pharmacokinetic and glucodynamic profile and altered hepatic-to-peripheral action gradient. We hypothesized reduced BIL exposure in peripheral tissues explains the latter, and in this study assessed the adipose tissue interstitial fluid (ISF) concentrations of BIL compared with human insulin (HI). Methods: A euglycemic glucose clamp was performed in patients with type 1 diabetes during continuous intravenous (IV) infusion of BIL or HI, while the adipose ISF insulin concentrations were determined using open-flow mi- croperfusion (OFM). The ratio of adipose ISF-to-serum concentrations and the absolute steady-state adipose ISF concentrations were assessed using a dynamic no-net-flux technique with subsequent regression analysis. Results: Steady-state BIL concentrations in adipose tissue ISF were achieved by *16 h after IV infusion. Median time to reach steady-state glucose infusion rate across doses ranged between 8 and 22 h. The average serum concentrations (coefficient of variation %) of BIL and HI were 11,200 pmol/L (23%) and 425 pmol/L (15%), respectively. The ISF-to-serum concentration ratios were 10.2% for BIL and 22.9% for HI. Conclusions: This study indicates feasibility of OFM to measure BIL in ISF. The observed low ISF-to-serum concentration ratio of BIL is consistent with its previously demonstrated reduced peripheral action. Keywords: Insulin analogues, Peripheral tissue concentration, Open-flow microperfusion, Dynamic no-net-flux technique, Euglycemic glucose clamp. Introduction B asal insulin peglispro (BIL) is a novel, PEGylated basal insulin with a large hydrodynamic size. BIL is generated by covalent addition of 20 kDa monomethoxy poly(ethylene glycol) to lysine 28 of the B-chain of insulin lispro. 1 The prolonged duration of action (half-life of 2– 3 days) is believed to be mediated by the increased hy- drodynamic size through slower absorption and reduced clearance. 1,2 It has recently been shown that, when compared to standard of care insulin glargine, BIL had a similar hepatic activity (endogenous glucose production suppression), but a reduced peripheral activity (less stimulation of glucose up- take and less suppression of lipolysis) in healthy subjects and in patients with type 1 diabetes mellitus (T1DM). 3 BIL therefore restores a portal-peripheral insulin action gradient for the basal insulin component, which is absent with cur- rently available insulin replacement therapies. Furthermore, 1 HEALTH—Institute for Biomedicine and Health Sciences, Joanneum Research Forschungsgesellschaft mbH, Graz, Austria. 2 Division of Endocrinology and Diabetology, Department of Internal Medicine, Medical University of Graz, Graz, Austria. 3 Eli Lilly and Company, Indianapolis, Indiana. 4 Lilly-NUS Centre for Clinical Pharmacology, Singapore, Singapore. DIABETES TECHNOLOGY & THERAPEUTICS Volume 19, Number 5, 2017 ª Mary Ann Liebert, Inc. DOI: 10.1089/dia.2016.0384 1