Ribosomal S6 kinase and AKT phosphorylation as pharmacodynamic biomarkers in patients with myelodysplastic syndrome treated with RAD001.

BACKGROUND: Myelodysplastic syndrome (MDS) continues to cause major morbidity and mortality; thus, novel treatments are needed. The mammalian target of rapamycin (mTOR) inhibitor RAD001 (everolimus) inhibits cellular pathways important to MYC protein stability and cell growth. Pharmacodynamic biomarkers could be useful in distinguishing between (1) disease resistance that occurs even though mTOR is successfully inhibited (suggesting a need for a different treatment strategy) and (2) resistance that might respond to changes in drug dosage or schedule.

PATIENTS AND METHODS: This was a small phase II trial of RAD001 in patients with low- and intermediate-1-risk MDS (n = 7). Protein S6K1 (S6) is downstream of mTOR, whereas protein kinase B (AKT) is upstream of mTOR. Therefore, to evaluate the pharmacodynamic effects of RAD001, S6 and AKT phosphorylation (pS6, pAKT) were measured by peripheral blood flow cytometry.

RESULTS: Sequential weeks of RAD001 produced a decrease in pS6, whereas pAKT was maintained or increased. There were no clinical responses despite the biomarker evidence of intended pharmacodynamic effect.

CONCLUSION: The pS6:pAKT ratio could be useful as a biomarker of target inhibition by RAD001 (clinicaltrials.gov identifier: NCT00809185).