Digestive Diseases and Sciences, Vol. 49, No. 1 (January 2004), pp. 30–37 ( C  2004) Everolimus/Cyclosporine Interactions on Bile Flow and Biliary Excretion of Bile Salts and Cholesterol in Rats MICHAEL DETERS,* GABRIELE KIRCHNER,† THERESE KOAL,* KLAUS RESCH,* and VOLKHARD KAEVER* As a possible explanation for everolimus/cyclosporine-induced hypercholesterolemia seen in trans- plant recipients, we investigated the interactions of the immunosuppressants everolimus and cy- closporine on bile flow and biliary excretion of bile salts and cholesterol in a subchronic bile fistula model in rats because biliary excretion is a main elimination route of cholesterol. After 2 weeks of daily treatment, everolimus (1 mg/kg i.p.) and cyclosporine (5 mg/kg i.p) decreased bile flow (−45 and −36%) and biliary excretion of bile salts (−34 and −54%) and cholesterol (−25 and −39%) and increased serum concentrations of cholesterol (+40 and +17%) and triglycerides (+220 and +110%). Bile salt serum concentration was elevated only by cyclosporine (+100%), and not by everolimus. Everolimus/cyclosporine slightly enforced the cyclosporine-induced hyperlipidemia but not reduction of bile parameters, while the cyclosporine-induced increase in bile salts in serum was totally prevented. From these results we conclude that bile salt synthesis could be impaired by everolimus, which could be one reason for everolimus-induced hypercholesterolemia. KEY WORDS: bile flow; cholestasis; cyclosporine; everolimus; hypercholesterolemia; rat. The macrolide immunosuppressant everolimus (RAD) is the 40- O -(2-hydroxyethyl) derivative of sirolimus (SRL). Whereas cyclosporine (CyA), one base therapy agent cur- rently used in organ transplantation, achieves its effects principally by blocking calcineurin and thereby inhibiting interleukin-2 production, RAD, like SRL, has no effect on calcineurin. RAD reduces T-lymphocyte activation by inhibiting the interleukin-2-mediated signal transduction pathway (1–3) and interacts synergistically with CyA (4). As the significant nephrotoxicity, neurotoxicity, and hy- pertension associated with CyA can be partly attributed to calcineurin blockade (5–7), RAD would be expected to have a different toxicity profile. Favorable outcomes have Manuscript received July 9, 2003; accepted September 30, 2003. From the *Institute of Pharmacology and †Department of Gastroen- terology and Hepatology, Medical School of Hannover, 30623 Hannover, Germany. Address for reprint requests: Dr. Michael Deters, Institut f¨ ur Phar- makologie, Medizinische Hochschule Hannover, Carl-Neuberg-Str. 1, D-30625 Hannover, Germany; Deters.Michael@MH-Hannover.de. been reported with the combination of RAD and CyA in rat and nonhuman primate models of kidney, heart, or lung transplantation 1 (4, 8–15). These models have demonstrated CyA-sparing effects without loss of im- munosuppressive activity, prevention and reversal of acute rejection, and prevention of the manifestation of chronic rejection by RAD. Moreover, RAD has been shown to enhance the immunosuppressive effects of CyA on lym- phocyte proliferation in human renal transplant recipi- ents (16), underscoring the potential of RAD to reduce the incidence of acute rejection, to minimize calcineurin inhibitor-induced nephrotoxicity, and to favor long-term graft survival in humans. On the other hand, RAD in com- bination with CyA increased serum concentrations of both 1 AM1, in position of amino acid 1 hydroxylated cyclosporine metabo- lite; AM9, in position of amino acid 9 hydroxylated cyclosporine metabolite; AM1c, in position of amino acid 1 hydroxylated and cy- clizted cyclosporine; metabolite; AM4N, in position of amino acid 4 N-demethylated cyclosporine metabolite. 30 Digestive Diseases and Sciences, Vol. 49, No. 1 (January 2004) 0163-2116/04/0100-0030/0 C  2004 Plenum Publishing Corporation