Evaluation of individual and combined neurotoxicity of the immunosuppressants cyclosporine and sirolimus by in vitro multinuclear NMR spectroscopy

Neurotoxicity, a crucial side effect of immunosuppressive therapy with cycl osporine, also has been demonstrated in vitro for sirolimus, a novel macrol ide immunosuppressant, which is under clinical investigation in combination with cyclosporine. NMR spectroscopy was used to study the separate and com bined effects of cyclosporine and sirolimus on cerebral metabolism, both in brain cells and in perfused rat brain slices. The high-energy phosphate me tabolism was already affected significantly at cyclosporine concentrations as low as 100 mu g/liter: phosphocreatine was reduced by 10 +/- 2% [half-ma ximal inhibition concentration (IC50) = 1850 +/- 600 mu g/liter], and nucle oside triphosphate was reduced by 11 +/- 5% (IC50 = 1110 +/- 420 mu g/liter ; n = 4, P < .05). At 500 mu g/liter cyclosporine, N-acetylaspartate and gl utamate were de-creased by 13 +/- 7% (IC,, = 1100 +/- 330 mu g/liter) and 2 2 +/- 9% (IC50 = 360 +/- 220 mu g/liter; n = 4, P < .05), respectively. As evaluated using an algorithm based on Loewe isobolograms, combination of cy closporine and sirolimus resulted in a synergetic reduction of high-energy phosphate metabolites. Addition of sirolimus to the perfusion medium increa sed brain slice concentrations of cyclosporine, It is concluded that cyclos porine significantly reduced high-energy phosphate metabolism in brain tiss ue at in vivo relevant concentrations. Combination with sirolimus resulted in synergism, which, in part, is explained by a greater distribution of cyc losporine into the brain tissue in the presence of sirolimus.