35 JPP 2004, 56: 35–41 ß 2004 The Authors Received May 21, 2003 Accepted September 23, 2003 DOI 10.1211/0022357022502 ISSN 0022-3573 Faculty of Pharmacy, University of Sydney, NSW 2006, Australia Ying Hong, Iqbal Ramzan, Andrew J. McLachlan Correspondence: A. J. McLachlan, Faculty of Pharmacy, University of Sydney, NSW 2006, Australia. E-mail: andrewm@pharm. usyd.edu.au Acknowledgement and funding: Ms Y. Hong is supported by the International Postgraduate Research Scholarship. We thank Dr Romina Nand for her technical assistance. Disposition of amphotericin B in the isolated perfused rat liver Ying Hong, Iqbal Ramzan and Andrew J. McLachlan Abstract The hepatic disposition and biliary excretion of amphotericin B were investigated in the isolated perfused rat liver (IPRL). Bolus dose of 50 ·g, 99 ·g and 198 ·g amphotericin B in lipoprotein-free perfusate and 198 ·g amphotericin B in perfusate with 1 ·M high-density lipoprotein (HDL) or 1 ·M low-density lipoprotein (LDL) were examined in the IPRL. Amphotericin B concentration in perfusate was measured using a validated HPLC assay. Amphotericin B was eliminated from the perfusate in a biexponential manner. The hepatic clearance (CL H ) increased in proportion to the dose administered (0.27 Å § 0.05 mL min ¡1 at low dose, 0.54 Å § 0.23 mL min ¡1 at medium dose and 1.06 Å § 0.24 mL min ¡1 at high dose), indicating non-linear hepatic disposition of amphotericin B. The hepatic extraction ratio of amphotericin B was very low (0.066 Å § 0.015). Tissue-to-perfusion partition coefficient, calculated at 120 min, increased 1.5 fold from 9.8 Å § 1.7 at low dose to 15.9 Å § 6.4 at high dose, suggesting the significant uptake and extensive retention of amphotericin B in the liver. Biliary excretion made only minor contribution to amphotericin B elimination in the IPRL, representing around 1–3% of the dose administered. No metabolites were detected in perfusate, bile and liver samples. The hepatic disposition of amphotericin B was not affected by the presence of HDL and LDL in the perfusate. In conclusion, the hepatic disposition of amphotericin B demonstrates restrictive elimination and is concentration-dependent, consistent with carrier-mediated uptake, and lipoproteins do not influ- ence amphotericin B hepatobiliary disposition. Introduction Amphotericin B is a macrocyclic polyene antifungal drug derived from Streptomyces nodosus. Its conventional formulation, amphotericin B deoxycholate (Fungizone), remains the therapeutic cornerstone for many fungal infections. Amphotericin B phar- macokinetics is characterised by extensive tissue distribution, high protein binding (91± 95%) and a long terminal elimination half-life in man of more than 15 days (Janknegt etal 1992; Groll etal 1998). In man the elimination of amphotericin B has not been fully elucidated, but biliary excretion would be expected to contribute to its elimination since the drug has a high molecular weight of >924 g mol ¡1 and is polar on one side of the lactone ring with seven hydroxyl groups. The metabolic fate of amphotericin B is unknown and no metabolites have been identified (Coukell & Brogden 1998). Furthermore, a recent study of 2 mg kg ¡ 1 liposomal amphotericin B (AmBisome) in healthy subjects attempted to identify metabolites using LC/MS/MS assay, but detected no peaks attributable to possible amphotericin B metabolites (Bekersky et al 2002). Amphotericin B is known to associate with plasma proteins, including lipoproteins that commonly transport lipids (Wasan etal 1994a). These lipoproteins also bind and subsequently transport a number of water-insoluble compounds, including amphoter- icin B (Kwong & Wasan 2002). Recent reports suggest that increased nephrotoxicity associated with amphotericin B is related to increased uptake of amphotericin B by renal LLC PK1 cells via high-affinity LDL receptors (K d ˆ 0.054 ng mL ¡ 1 ; 96 000 sites per cell) (Wasan & Lopez-Berestein 1994; Wasan et al 1994b; Wasan 1996). In contrast, HDL and amphotericin B complex is less toxic to LLC PK1 renal cells due to low- affinity HDL receptors (K d ˆ 71.43 ng mL ¡1 ; 2 sites per cell). Thus, the possibility of amphotericin B and lipoprotein complex, especially HDL and LDL, may enhance drug