Resistance to ERK1/2 pathway inhibitors; sweet spots, fitness deficits and drug addiction.

MEK1/2 inhibitors are clinically approved for the treatment of BRAF-mutant melanoma, where they are used in combination with BRAF inhibitors, and are undergoing evaluation in other malignancies. Acquired resistance to MEK1/2 inhibitors, including selumetinib (AZD6244/ARRY-142866), can arise through amplification of BRAF or KRAS to reinstate ERK1/2 signalling. We have found that BRAF amplification and selumetinib resistance are fully reversible following drug withdrawal. This is because resistant cells with BRAF amplification become addicted to selumetinib to maintain a precise level of ERK1/2 signalling (2%-3% of total ERK1/2 active), that is optimal for cell proliferation and survival. Selumetinib withdrawal drives ERK1/2 activation outside of this critical "sweet spot" (~20%-30% of ERK1/2 active) resulting in a p57-dependent G1 cell cycle arrest and senescence or expression of NOXA and cell death with features of autophagy; these terminal responses select against cells with amplified BRAF. ERK1/2-dependent p57 expression is required for loss of BRAF amplification and determines the rate of reversal of selumetinib resistance. Growth of selumetinib-resistant cells with BRAF amplification as tumour xenografts also requires the presence of selumetinib to "clamp" ERK1/2 activity within the sweet spot. Thus, BRAF amplification confers a selective disadvantage or "fitness deficit" during drug withdrawal, providing a rationale for intermittent dosing to forestall resistance. Remarkably, selumetinib resistance driven by KRAS amplification/upregulation is not reversible. In these cells ERK1/2 reactivation does not inhibit proliferation but drives a ZEB1-dependent epithelial-to-mesenchymal transition that increases cell motility and promotes resistance to traditional chemotherapy agents. Our results reveal that the emergence of drug-addicted, MEKi-resistant cells, and the opportunity this may afford for intermittent dosing schedules ("drug holidays"), may be determined by the nature of the amplified driving oncogene (BRAF . KRAS), further exemplifying the difficulties of targeting KRAS mutant tumour cells.