Background: EC has high rates of PI3K pathway alteration including PTEN mutation (50%) or IHC loss (>50%), PIK3CA mutation (25-40%) and PIK3R1 (20%) mutation. MK-2206 is an allosteric inhibitor of AKT, an effector kinase of PI3K signals. We hypothesized that pts whose tumors harbored PIK3CA mutations would be more likely to benefit from MK-2206 than those without PIK3CA mutation. Methods: Pts had recurrent or advanced EC; all histologies except MMMT were eligible. Up to 2 prior chemo lines were permitted; excluding prior treatment with PI3K/MTOR inhibitors. The first 19 pts were treated with MK-2206 200mg QW; due to initial skin toxicity rates, the starting dose was amended to 135mg QW. Co-primary endpoints were objective response and 6 mo PFS. The first 37 pts were stratified retrospectively. PIK3CA MT included R88Q, K111N, E110K, E418K, C420R, E453K, E542K/V,E545K, Q546R, H701P, M1043V, H1047R/L/Y changes. Independent Simon 2-stage tests were planned within PIK3CA MT and WT stratum: for MT, n1=15 and n2=10 pts would allow discrimination of RR<5% and 6moPFS<10% versus RR>25% or 6moPFS>35%; for WT, n1=31 and n2=24 pts would discriminate RR<5% and 6moPFS<10% versus RR>20% or 6moPFS>25%. Results: 37 pts were enrolled (1 ineligible) before accrual was stopped as timely CLIA-compliant prospective mutation analysis was not feasible. By PIK3CA mutation analysis, 9 pts were MT: 1 pt had both PR and 6moPFS. 27 pts were WT: 1 pt had PR and 3 pts had 6moPFS. 2 pts with 6moPFS were treated at 200mg; 2 pts with 6moPFS were treated at 135mg. Each group had 1 PR. The most common toxicity was grade 3 rash (19%). Grade 3 and 4 toxicities occurred in 50% and 8% of pts. Exploratory analysis of histology found all pts with 6moPFS were classified as serous (4 of 8) as compared to all other histologies (0 of 28, p=.001). Targeted exome sequencing and copy number analysis of the PI3K pathway and PTEN IHC are underway. Associative studies will be reported. Conclusions: There is limited single agent activity of MK-2206 in both PIK3CA MT and PIK3CA WT EC populations. Activity was detected in pts with serous histology tumors and warrants further study. Clinical trial information: NCT01307631.