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The authors sought to determine the effect of concomitant peroral (PO) administration of cyclosporine (CsA) and sirolimus (SRL, rapamycin) on the tissue distributions of CsA and SRL in the rat. Groups of four adult male Wistar-Furth rats were treated for 14 days with 2.5, 5.0, or 10.0 mg CsA/kg·day. Other groups of four adult male Wistar-Furth rats were treated for 14 days with a 1-to-6.25 weight-to-weight ratio of SRL to CsA at SRL doses of 0.4, 0.8, or 1.6 mg/kg·day. Concentrations of CsA and SRL in homogenates of heart, intestinal, kidney, liver, lung, muscle, spleen, and testes were compared to those in whole blood (WB). There was a large, dose-dependent, distinctive distribution of CsA among rat tissues, as has previously been well documented. At a constant molar dose ratio, concomitant oral administration of SRL produced an approximately two-fold increase in the concentrations of CsA in rat tissues, although SRL did not change the CsA tissue-to-WB partition coefficients. Concomitant oral CsA administration produced dose-dependent increases in SRL tissue concentrations and decreases in the SRL tissue-to-WB partition coefficients. The increases in tissue and WB concentrations on coadministration of both agents may be explained either by an increase in absorption caused by competition between the two agents for binding sites on P-glycoprotein in the gut, a reduced rate of metabolism, or to an as yet unidentified elimination mechanism. The dose-independent and unchanged CsA tissue-to-WB partition coefficients suggest that SRL does not affect the equilibrium of CsA between the central and tissue compartments, namely the tissue uptake or intracellular binding. Altered values of the SRL tissue-to-WB partition coefficients suggest that, under the conditions studied, CsA disturbs the equilibrium of SRL between the central and tissue compartments.