Everolimus Is Comparably Effective to Rapamycin Acting Synergistically with Imatinib in the Treatment of Constitutively Active Mutations of the Juxtamembrane Domain of c-Kit.

Kit cITD is a complex internal tandem duplication involving the juxtamembrane (JMD) and kinase domain of stem cell factor receptor c-kit which was identified recently in 7% of childhood AML. kit cITD induces constitutive proliferation and apoptosis resistance that can be synergistically blocked with imatinib (im) and rapamycin (ra).

Due to a long half-life ra has disadvantageous pharmacological characteristics making drug handling especially difficult in children with serum concentrations quickly raising to toxic levels. We therefore compared ra with its ester analogue everolimus (RAD001, ev) which has a better bioavailability and a shorter half-life in order to assess equivalence and potential functional superiority. Ba/F3 cells, stably transfected with c-kit WT and kit cITD, were treated with im, ra and ev alone and in combination. In proliferation assays of WT and cITD cell lines ev was a significantly more potent inhibitor than ra. When ra or ev were combined with low im concentrations proliferation was synergistically inhibited with both drugs but again significantly better with ev. In cITD expressing cells ra and ev alone were comparably poor inducers of apoptosis. In assays combining im with ra or ev both combinations showed a significant and comparable synergism with an additional dose effect of the mTOR inhibitors between 3nM and 0.3 nM. In a NOD/SCID animal model all animals injected with kit cITD succumbed due to multi-organ infiltration of c-kit positive cells within 20 days. Survival was prolonged in animals treated with im/ra and ev. We therefore conclude that ev is at least equivalent to ra and can replace ra exploring a molecular based therapeutic attempt in susceptible children with identified mutations of the JMD of c-kit.