nefit from treatment. As mutations in MEK or ERK had not been described until recently, much attention has focused on mutations of RAS and RAF, long known cellular oncogenes and immediate upstream activators of MEK, as possible molecular markers of sensitivity to MEK inhibition. Mutations in RAS and RAF are common in human tumours and typically Belinostat PX105684 demonstrate mutual exclusivity, suggesting that either mutation might exert its oncogenic activity through common downstream proteins, such as the MEK/ ERK kinase module. Using small molecule MEK inhibitors in cells with RAS or BRAF mutations, Solit et al. have recently demonstrated that tumours with BRAF mutations display enhanced sensitivity to MEK inhibition when compared with wild type cells and cells harbouring various RAS mutations.
In addition, following treatment with MEK inhibitors, the growth of tumours in BRAF mutant xenografts was completely suppressed, whereas RAS mutant tumours were only partially inhibited. Extension of this study to the NCI 60 cell lines, for which a large body of data from inhibitor screening assays could be interrogated, yielded supportive Danusertib 827318-97-8 information, the top ranking compounds that scored on V600EBRAF positive lines happen to represent predominantly MEK inhibitors with similar effectiveness as CI 1040. From a molecular standpoint, recent data from Garnett et al. indicate that, even though a small fraction of BRAF mutations generates an enzyme that is impaired in its ability to activate the downstream MEK/ERK cascade, kinase impaired mutants also work through the mitogenic cascade culminating in ERK activation.
The mechanism is rescue of kinaseimpaired mutant BRAF by wild type C RAF through a process that involves 14 3 3 mediated hetero oligomerization and transactivation. Alternatively, measurement of baseline levels of doubly phosphorylated ERK, the direct target of MEK enzymatic activity, in tumour biopsies and/or archived tumour tissue could be used to identify patients/tumour types in which the MEK/ERK module is constitutively active and that would potentially benefit from MEK inhibition based therapeutic strategies. To this end, as well as for pharmacodynamic monitoring purposes, Western blot and flow cytometry based methods have been devised that allow the accurate and quantitative detection of phosphorylated ERK in tumour tissue.
Though a mild association is seen between baseline pERK levels in archived tumour samples and subsequent stable disease, pERK inhibition in either peripheral blood mononuclear cells or in tumour tissues from patients receiving MEK inhibitor therapy has not correlated with clinical benefit. Therefore, the presence of activated ERK, as well as the percentage of ERK inhibition, may not be sufficient in themselves as a guide to the anticancer effects of MEK inhibition. One possible explanation for the failure Tortora et al. Page 11 Drug Resist Updat. Author manuscript, available in PMC 2008 September 23. NIH PA Author Manuscript NIH PA Author Manuscript NIH PA Author Manuscript of pERK reduction during MEK inhibitor therapy to predict clinical outcome is that tumour pERK levels are examined at pre specified time points, and these data may reflect ERK activation at that time, but may not differentiate between short lived mitogen activation and sustained constitutive MAPK pathway activation. Kinetics consideration may also be of importance in determining the overall effect of MEK blockade in different clinical situations, as recently