Mutations of the RAS family of genes are frequent events in acute myeloid leukaemia (AML), occurring in 10% of children and 10-20% of adults. NRAS mutations promote proliferation through activation of the Ras/Raf/MEK/ERK signalling pathway. Several MEK inhibitors have shown promising pre-clinical activity in AML, with a number of compounds currently in adult phase I/II clinical trials, including AZD6244, GSK1120212 and AS703026. Paediatric data and models of resistance to MEK inhibition in AML however, are limited. We therefore undertook a comprehensive preclinical evaluation of MEK inhibitors in paediatric AML and developed a clinically relevant model of resistance.

The in vitro efficacy of 7 MEK inhibitors was determined using a diverse panel of 6 paediatric and 5 adult AML cell lines. Efficacy varied across the cell lines however, all paediatric lines were significantly more sensitive to at least one MEK inhibitor compared to Ara-C with the exception of the Down Syndrome associated AML line, CMK. Although basal MEK activation varied, phospho-MEK was reduced following MEK inhibitor treatment.  Cell cycle analyses demonstrated sensitive cell lines develop sub-G1 accumulation, with apoptosis confirmed by Annexin V positivity. In addition, a matched panel of peripheral blood blasts retrieved at diagnosis and relapse from a child with MLL-rearranged AML revealed that the relapse sample exhibited greater in vitro sensitivity to MEK inhibition compared to diagnosis.

Long-term persistent culture of THP-1 cells (MLL-rearranged and NRAS mutated) with AZD6244 and AS703026, resulted in high-level resistance (> 20x IC50). Importantly, cells displayed cross-resistance not only to these two compounds but also to GSK1120212. Next-generation sequencing approaches are currently in progress to determine the mechanism of resistance and identify potential therapeutic strategies which may overcome MEK inhibitor resistance. Taken together, these data will help inform the optimal therapeutic use of MEK inhibitors in AML.