Drugs for neurological diseases have to cross the blood‐brain barrier (BBB) to induce their therapeutic effect. In vivo drug quantification in the brain is challenging, because invasive methods damage the BBB and measurement results may be confounded by drug leakage from the blood into the brain through the disrupted BBB. Cerebral open flow microperfusion (cOFM) is an in vivo sampling technique that allows BBB healing and re‐establishment after probe implantation and before sampling is performed. It therefore provides the opportunity to sample compounds in cerebral interstitial fluid with an intact BBB. This article comprehensively describes the experimental setup and procedures, perfusate requirements, critical parameters, common problems that may occur, and their causes and solutions. Typical results from a cOFM sampling experiment are presented and discussed. This protocol provides a tool for performing pharmacokinetic and pharmacodynamic studies in mouse or rat brain with an intact BBB. © 2019 by John Wiley & Sons, Inc.

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ObjectiveThe inability of leptin to suppress food intake in diet-induced obesity, sometimes referred to as leptin resistance, is associated with several distinct pathological hallmarks. One prevailing theory is that impaired transport of leptin across the blood–brain barrier (BBB) represents a molecular mechanism that triggers this phenomenon. Recent evidence, however, has challenged this notion, suggesting that leptin BBB transport is acquired during leptin resistance.MethodsTo resolve this debate, we utilized a novel cerebral Open Flow Microperfusion (cOFM) method to examine leptin BBB transport in male C57BL/6J mice, fed a chow diet or high fat diet (HFD) for 20 days.ResultsBasal plasma leptin levels were 3.8-fold higher in HFD-fed mice (p < 0.05). Leptin administration (2.5 mg/kg) elicited similar pharmacokinetic profiles of circulating leptin. However, while leptin reduced food intake by 20% over 22 h in chow-fed mice, it did not affect food intake in HFD-fed mice. In spite of this striking functional difference, hypothalamic leptin levels, as measured by cOFM, did not differ between chow-fed mice and HFD-fed mice following leptin administration.ConclusionsThese data suggest that leptin transport across the BBB is not impaired in non-obese leptin resistant mice and thus unlikely to play a direct role in the progression of pharmacological leptin resistance.

In vivo investigation of brain pharmacokinetics and pharmacodynamics (PK/PD) is an integral part of neurological drug development. However, drugs intended to act in the brain may reach it at very low concentrations due to the protective effect of the blood-brain barrier (BBB). Consequently, very sensitive measurement methods are required to investigate PK/PD of drugs in the brain. Also, these methods must be capable of continuously assessing cerebral drug concentrations with verifiable intact BBB, as disrupted BBB may lead to compound efflux from blood into brain and to biased results. To date, only a few techniques are available that can sensitively measure drug concentrations in the brain over time; one of which is cerebral open flow microperfusion (cOFM). cOFM's key features are that it enables measurement of cerebral compound concentrations with intact BBB, induces only minor tissue reactions, and that no scar formation occurs around the probe. The membrane-free cOFM probes collect diluted cerebral interstitial fluid (ISF) samples that are containing the whole molecule spectrum of the ISF. Further, combining cOFM with an in vivo calibration protocol (e.g. Zero Flow Rate) enables absolute quantification of compounds in cerebral ISF. In general, three critical aspects have to be considered when measuring cerebral drug concentrations and recording PK/ PD profiles with cOFM: (a) the BBB integrity during sampling, (b) the status of the brain tissue next to the cOFM probe during sampling, and (c) the strategy to absolutely quantify drugs in cerebral ISF. This work aims to review recent applications of cOFM for PK/PD assessment with a special focus on these critical aspects. K E Y W O R D S blood-brain barrier, cerebral open flow microperfusion (cOFM), neurological drug development, pharmacodynamics, pharmacokinetics Thomas Altendorfer-Kroath and Joanna Hummer authors contributed equally to this study.This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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