Background: The RAF/MEK/ERK and PI3K/AKT signaling pathways are commonly deregulated in CRC. Each can serve as a compensatory mechanism mediating resistance to single pathway blockade. Combined inhibition with MK-2206 and selumetinib (AZD6244) could enhance antitumor activity. Tolerable doses for the combination in solid tumors (ASCO 2011, abstr 3004) are less than single agent MTDs. We conducted a biomarker driven trial of MK-2206 and selumetinib at the established combination doses to determine extent of pERK and pAKT inhibition in tumor biopsies (bx), using new, clinically validated immunoassays for pERK and pAKT. Methods: Patients (pts) with advanced CRC, refractory to standard therapy, ECOG ≤ 2, adequate organ function, and disease amenable to bx were treated with oral MK-2206, 90 mg QW and selumetinib, 75 mg daily, stratified for KRAS mutation status (+ or WT). For each strata, pAKT and pERK levels from paired tumor bx, baseline and 3-6 hrs post-dose in 3 pts each on C1D1 or C1D22, were measured using a quantitative, chemiluminescence immunoassay (dilution linearity R² = 0.985 pERK, 0.995 pAKT; % CV at 0 < 6). 70% inhibition was considered biologically significant based on levels observed in preclinical models that demonstrated antitumor activity. Results: 11 pts enrolled to date (KRAS +, 6 pts; WT, 5 pts). 2/9 pts had SD after 2 cycles. Gr 1/ 2 toxicities: rash, reversible retinal detachment, liver abnormalities, hypoalbuminemia, lymphopenia. Target protein inhibition data are available for 8 paired bx samples (Table). Conclusions: Although 4/8 pts demonstrated biologically significant inhibition in one marker, at the MTD for this combination no pt had ≥ 70% inhibition of both targets. If repeated dosing does not produce the desired inhibition of pERK and pAKT, we will conclude that the dose reductions for each agent necessitated by the toxicity of the agents used in combination preclude the possibility of providing adequate dual pathway inhibition. Loss of PTEN and DNA mutation analyses (KRAS, BRAF, PIK3Ca, AKT) are planned.

	% Inhibition of phosphorylation
	pt	pERK	pAKT
C1D1
KRAS +	1	46	15
	2	>77	4
	3	61	23
C1D1
KRAS WT	4	53	19
	5	>70	34
	6	48	78
C1D22
KRAS +	7	77	20
	8	0	60