BIOPHARMACEUTICS & DRUG DISPOSITION Biopharm. Drug Dispos. 29: 231–235 (2008) Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/bdd.606 A Disposition Kinetic Study of Tramadol in Rat Perfused Liver Mohammad-Reza Rouini a, *, Mahmoud Ghazi-Khansari b , Yalda H. Ardakani a , Zahra Dasian a and Hoda Lavasani a a Biopharmaceutics and Pharmacokinetic Division, Department of Pharmaceutics, Faculty of Pharmacy and Pharmaceutical Sciences Research Centre, Tehran University of Medical Sciences, 14155-6451, Tehran, Iran b Department of Pharmacology, School of Medicine, Tehran University of Medical Sciences, Tehran, Iran ABSTRACT: A recirculated perfusion system was used to investigate the metabolism of tramadol, an analgesic agent, in the isolated perfused rat liver. Tramadol was added to the perfusion medium at a concentration of 300 ng/ml, and the perfusate samples were collected for 180 min. The concentration of tramadol and its three main metabolites O-desmethyltramadol (M1) and N-desmethyltramadol (M2) and N,O-didesmethyltramadol (M5) were determined in perfusate samples by a rapid HPLC method. All through the study, the phase I metabolism of tramadol led to the formation of M1 metabolite from early sampling points while M5 metabolite was detectable after 50 min in 6 out of 10 perfused livers and the M2 metabolite was not detectable in any experiment. The kinetic parameters of tramadol and two detectable metabolites (M1 and M5) were then calculated in perfusate samples. The tramadol concentration decreased from 297.8 to 159.6 ng/ ml, with a mean half-life of 232.4 min and a hepatic clearance of 0.73 ml/min. After 180 min, the mean concentration of M1 reached 59.5 ng/ml, resulting in a metabolic ratio of 16%, while the formation of M5 metabolite continued to a mean concentration of 14.6 ng/ml resulting in a metabolic ratio of 2% using AUC (0À180min) . Copyright # 2008 John Wiley & Sons, Ltd. Key words: tramadol; pharmacokinetic; liver perfusion; rat Introduction Tramadol hydrochloride trans-( Æ )-2-[(dimethyla- mino) methyl]-1-(3-methoxyphenyl)-cyclohexanol hydrochloride (Figure 1), is a centrally acting analgesic with efficacy and potency ranging between weak opioids and morphine [1]. Tramadol produces analgesia by synergistically combining weak m-opioid and monoaminergic (noradrenalin and serotonin) mediated mechanisms [2]. Its therapeutic plasma concentration has been re- ported to be in the range of 100–300 ng/ml [3]. Tramadol is rapidly and almost completely absorbed after oral administration but its absolute bioavailability is only 65–70% due to first-pass metabolism [4]. This analgesic is rapidly and extensively metabolized in the liver. The principal metabolic pathways, O- and N-desmethylation, involve cytochrome P-450 isoenzymes 2D6, 2B6 and 3A4, respectively. The primary metabolites O-desmethyltramadol (M1) and N-desmethyltramadol (M2) may be further metabolized to three additional second- ary metabolites namely, N,N-didesmethyltrama- dol (M3), N,N,O-tridesmethyltramadol (M4) and N,O-didesmethyltramadol (M5). Of all these metabolites, only M1 is pharmacologically active. In phase II, O-demethylated metabolites are *Correspondence to: Department of Pharmaceutics, Faculty of Pharmacy, Tehran University of Medical Sciences, Tehran, 14155- 6451, Iran. E-mail: rouini@tums.ac.ir Received 8 October 2007 Revised 21 November 2007 Accepted 3 December 2007 Copyright # 2008 John Wiley & Sons, Ltd.