PHARMACOKINETICS AND DISPOSITION Diosmin pretreatment affects bioavailability of metronidazole Received: 15 April 2002 / Accepted: 6 November 2002 / Published online: 1 March 2003 Ó Springer-Verlag 2003 Abstract Objective: To screen for inhibitory effects of diosmin on cytochrome P 450 -mediated metabolism of metronidazole in healthy volunteers. Design: Before/ after non-blinded investigation conducted in healthy male volunteers. Methods: After an overnight fast, metronidazole (two 400-mg tablets) was administered to 12 volunteers, either alone or after a 9-day pretreatment period with a once- daily dose of diosmin 500-mg tablets under direct observation. Serum concentrations of metronidazole up to 48 h postdose and urinary concentrations of metronidazole and its two major metabolites up to 24 h postdose were measured using reversed-phase high- performance liquid chromatography. Results: Metronicazole plasma AUC (0–¥) and C max were significantly higher after diosmin pretreatment by (mean) 27% and 24%, respectively. However, time to reach peak concentration (t max ) was not affected signif- icantly. Urinary excretion of acid and hydroxy metab- olites in urine was decreased significantly, while excretion of unchanged metronidazole was increased. Conclusion: Diosmin pretreatment significantly altered the metabolism of metronidazole, as demonstrated by changes in plasma pharmacokinetics as well as by urinary recovery of both parent drug and its major metabolites. This may be caused by the inhibition of cytochrome P 450 enzymes. Keywords Diosmin Æ Metronidazole Æ CYP3A4 Introduction Diosmin (3’, 5, 7-trihydroxy-4’-methoxyflavone 7-ruti- noside) is a flavone (Fig. 1) [1] used for the treatment of haemorrhoids and of chronic venous insufficiency of the lower limbs [2]. A pure synthetic diosmin (Venex) was used in the study. Diosmin and diosmetin are natural dietary agonists of the aryl hydrocarbon receptor (AhR), causing a potent increase in CYP1A1 tran- scription and its activity; however, only diosmetin is capable of inhibiting CYP1A1 enzyme activity [3]. In pharmacokinetic studies on diosmin and diometin, both flavonoids were rapidly metabolised, diosmetin was partly excreted in bile as the glucuronide and sulphate, and diosmin was partly excreted in bile as such and as the glucuronide conjugate. Both the parent compounds were completely absent in urine. Diosmetin had a long plasma elimination half-life ranging from 26 h to 43 h [4, 5]. Flavones and flavonols are in general more potent enzyme inhibitors specifically for CYP1A1 and CYP1A2 than flavanones, isoflavones and chalcones [6]. The in- hibitory effects of tangeretin, green tea flavonoids and other flavonoids on CYP1A, CYP2B, CYP2E1 and CYP3A enzymes were examined in rat and human liver microsomes [7]. Specifically, flavones or flavonoids that contain C5, C7 and C4’ hydroxyl groups were potent inhibitors of CYP1A1 and 1A2 activity [8]. Further- more, naturally occurring plant flavonoids may acutely upregulate the apparent activity of P-glycoprotein (P- gp), while flavonoids like quercetin, rutin, galangin, kaempferol, genistein and daidzein inhibited the P-gp pump efflux activity [9]. Thus, in general, flavones have the potential to alter the pharmacokinetics of drugs by at least two mechanisms. Metronidazole, a nitroimidazole derivative (1-hy- droxy-ethyl-2-methy-5-nitroimidazole), is widely used for the treatment of protozoan and anaerobic bacterial infections. Metronidazole is metabolised primarily in the liver by CYP 3A4 and CYP 2C9, the two major Eur J Clin Pharmacol (2003) 58: 803–807 DOI 10.1007/s00228-002-0543-5 K. Rajnarayana Æ Mada S. Reddy Devarakonda R. Krishna K. Rajnarayana Æ M.S. Reddy Æ D.R. Krishna (&) Drug Metabolism and Clinical Pharmacokinetics Division, University College of Pharmaceutical Sciences, Kakatiya University, AP-506009, Warangal, India E-mail: dr_krishna@hotmail.com Tel.: +91–8712–572567 Fax: +91–8712–438844