Background: Immunosuppressive therapy with the mammalian target of rapamycin (mTOR) inhibitors requires a fine balance between allograft maintenance and drug-related side effects.
Methods: In this study we examined the feasibility of monitoring TOR inhibitor-based immunosuppression by assessment of the phosphorylation status at the Thr(389) site of the p70S6 kinase in peripheral blood mononuclear cells (PBMCs). At total of 36 patients with renal transplants and 8 healthy controls were enrolled.
Results: We found that sirolimus treatment was associated with a pronounced inhibition of p70S6 kinase phosphorylation, as compared to healthy donors or otherwise immunosuppressed patients. In sirolimus-treated patients, phosphorylation of the p70S6 kinase was significantly inhibited when sirolimus trough levels were > 6 ng/mL. In contrast, for trough levels <6 ng/mL, the degree of inhibition of p70S6 kinase phosphorylation showed a high degree of interindividual variability. We recorded a total of five clinical relevant rejection episodes in this patient category. Intriguingly, rejecters uniformly maintained a high degree of phosphorylation independent of the sirolimus trough level whereas non-rejecters showed a significant inhibition of phosphorylation.
Conclusion: Therefore, the phosphorylation status of the p70S6 kinase appears to provide more relevant information on the desired effect of sirolimus in target cells as compared to trough level measurements. Moreover, this assay provides an opportunity to safely titer down sirolimus levels to avoid overimmunosuppression and, on the other hand, to identify patients with insufficient TOR inhibitor therapy that are at risk for rejection.