Doxorubicin encapsulated in sterically stabilized liposomes exhibits renal and biliary clearance properties that are independent of valspodar (PSC 833) under conditions that significantly inhibit nonencapsulated drug excretion.

Coadministration of anticancer drugs and multidrug resistance modulators directed against P-glycoprotein over-expressed in tumors also results in nonspecific blockade of this drug efflux pump in excretory tissues such as the liver and kidneys. These interactions often result in impaired renal and biliary clearance for anticancer agents such as doxorubicin (DOX). In the present investigation, we characterized the excretory processes associated with liposomal DOX administration to elucidate how liposome encapsulation may bypass adverse pharmacokinetic interactions between DOX and (3'-keto-Bmt1)-(Val2)-cyclosporin (Valspodar). Renal and biliary clearance properties of liposome-encapsulated DOX were compared with those for nonencapsulated DOX in the presence and absence of Valspodar using an instrumented rat model with implanted jugular vein and bile duct catheters for continuous sampling. Two types of liposomal DOX formulations were used, a drug-permeable egg phosphatidyl choline/cholesterol system and a sterically stabilized polyethylene glycol/1,2 distearoyl-sn-glycero-3-phosphocholine/cholesterol system to establish the relative roles of liposome-encapsulated and released drug on the pharmacokinetic and excretion alterations induced by Valspodar. DOX and its primary metabolites were quantitated using high-performance liquid chromatography. When Valspodar was coadministered with nonencapsulated DOX, 3.5- and 37.5-fold reductions in renal clearance (CLr) and biliary clearance (CLb), respectively, were observed, which resulted in increased plasma DOX concentrations and total exposure. However, Valspodar-induced alterations in CLr and CLb were less profound with egg phosphatidyl choline/cholesterol DOX (1.7- and 2.0-fold reductions, respectively) and negligible with the long-circulating polyethylene glycol-containing liposomal formulation. These results indicate that liposomes may circumvent Valspodar-induced DOX pharmacokinetic changes by reducing the rate of drug excretion in liver and kidney tissue to a level that is within the renal and biliary excretion capacity in the presence of P-glycoprotein blockade.