Sirolimus, but not the structurally related RAD (everolimus), enhances thenegative effects of cyclosporine on mitochondrial metabolism in the rat brain

1 Clinical studies have shown enhancement of cyclosporine toxicity when co- administered with the immunosuppressant sirolimus. We evaluated the biochem ical mechanisms underlying the sirolimus/ cyclosporine interaction on rat b rain metabolism using magnetic resonance spectroscopy (MRS) and compared th e effects of sirolimus with those of the structurally related RAD. 2 Two-week-old rats (25 g) were allocated to the following treatment groups (all n=6): I. control, II. cyclosporine (10 mg kg(-1) d(-1)), III. sirolim us (3 mg kg(-1) d(-1)), IV. RAD (3 mg kg(-1) d(-1)), V. cyclosporine + siro limus and VI. cyclosporine + RAD. Drugs were administered by oral gavage fo r 6 days. Twelve hours after the last dose, metabolic changes were assessed in brain tissue extracts using multinuclear MRS. 3 Cyclosporine significantly inhibited mitochondrial glucose metabolism (gl utamate: 78 +/- 6% of control; GABA: 67 +/- 12%; NAD(+): 76 +/- 3%; P <0.05 ), but increased lactate production. Sirolimus and RAD inhibited cytosolic glucose metabolism via lactate production (sirolimus: 81 +/- 3% of control, RAD: 69 +/- 2%; P <0.02). Sirolimus enhanced cyclosporine-induced inhibiti on of mitochondrial glucose metabolism (glutamate: 60 +/- 4%; GABA: 59 +/- 8%; NAD(+): 45 +/- 5%; P <0.02 versus cyclosporine alone). Lactate producti on was significantly reduced. In contrast, RAD antagonized the effects of c yclosporine (glutamate, GABA, and NAD(+), not significantly different from controls). 4 The results can partially be explained by pharmacokinetic interactions: c o-administration increased the distribution of cyclosporine and sirolimus i nto brain tissue, while co-administration with RAD decreased cyclosporine b rain tissue concentrations. In addition RAD, but not sirolimus, distributed into brain mitochondria. 5 The combination of cyclosporine/RAD compares favourably to cyclosporinels irolimus in regards to their effects on brain high-energy metabolism and ti ssue distribution in the rat.