The Use of a Deuterated Calibrator for In Vivo Recovery Estimations in Microdialysis Studies JO ¨ RGEN BENGTSSON, EMMA BOSTRO ¨ M, MARGARETA HAMMARLUND-UDENAES Division of Pharmacokinetics and Drug Therapy, Department of Pharmaceutical Biosciences, Uppsala University, Box 591, SE-751 24 Uppsala, Sweden Received 11 July 2007; revised 31 August 2007; accepted 6 September 2007 Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/jps.21217 ABSTRACT: One of the crucial issues in quantitative microdialysis is the reliability of recovery estimates to correctly estimate unbound drug tissue concentrations. If a deuterated calibrator is used for retrodialysis, the calibrator has the same properties as the study drug. However, recovery of the calibrator may be affected by the presence of the drug in the tissues. The aim of this study was to investigate the recovery of deuterated morphine with time in the absence and presence of morphine in rat tissues. Microdialysis probes were placed in the brain and blood of eight rats. Ringer’s solution containing D3-morphine was perfused throughout the study and recovery was estimated. After a stabilization period of 3 h, an exponential infusion of morphine was administered over 4 h. The presence of morphine did not affect the recovery of D3- morphine from brain or blood. The average recovery values (SD) were 0.145 (0.039) and 0.131 (0.048) during the stabilization and infusion periods, respectively, for the brain probe and 0.792 (0.055) and 0.790 (0.084), respectively, for the blood probe. The recovery of deuterated morphine was stable over time in the brain and in blood, and was not affected by the presence of pharmacologically concentrations of morphine. ß 2007 Wiley- Liss, Inc. and the American Pharmacists Association J Pharm Sci 97:3433–3441, 2008 Keywords: microdialysis; active transport; blood–brain barrier; efflux pumps; ABC transporters INTRODUCTION One of the crucial issues in quantitative micro- dialysis is the reliability of the recovery estimates of the drug to correctly estimate unbound drug tissue concentrations. Drug recovery can be estimated by various methods, such as retro- dialysis by drug, the use of a calibrator or the no- net-flux method. 1–4 Each of these methods has its own advantages and disadvantages. To estimate recovery using retrodialysis by drug or by calibrator, it is assumed that when the drug is administered systemically, the gain of drug from the surround- ing tissue is the same as the loss of drug from the perfusion fluid to the tissues. 5 This can be studied in vitro before an in vivo experiment. The no-net- flux or the dynamic no-net-flux methods make no assumptions of gain and loss, but the no-net-flux method requires steady-state concentrations of the drug in the tissue and the dynamic no-net-flux method requires a large number of animals, as recovery is estimated in parallel study groups with different concentrations of drug in the perfusate. 6 Estimation of recovery using retrodialysis by drug is time consuming because a wash-out period is needed after the first perfusion of the probe with the drug before the drug can be administered systemically. There is also an assumption that Correspondence to: Margareta Hammarlund-Udenaes (Telephone: þ46-18-471-4300; Fax: þ46-18-471-4003; E-mail: mhu@farmbio.uu.se) Journal of Pharmaceutical Sciences, Vol. 97, 3433–3441 (2008) ß 2007 Wiley-Liss, Inc. and the American Pharmacists Association JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 97, NO. 8, AUGUST 2008 3433