Background: Activating MAPK pathway mutations (predominantly RAS) occur with a high incidence in metastatic pancreatic adenocarcinoma (mPaCa). Pimasertib is a MEK1/2 inhibitor with potent activity in cell lines and xenografts with an activated MAPK pathway. This two-part trial in patients (pts) with mPaCa comprises a dose-escalation safety run-in and a randomized phase II part (EudraCT 2009-011992-61). We defined the maximum tolerated dose (MTD), safety, pharmacokinetics (PK) and antitumor activity of two pimasertib dosing schedules (S), and the recommended phase II dose (RP2D). Methods: Dose-escalation (3+3 design) in two dosing S of oral pimasertib: once-daily (qd) - 5 days on, 2 days off (S1); and twice-daily (bid) - continuous (S2) combined with the standard dose of gemcitabine (gem). Results: 53 pts (median age 61 years and ECOG performance status 0-1) have been treated at six dose levels in S1 (15 to 120 mg qd) and at 60 and 75 mg bid in S2. MTDs were defined as 120 mg qd and 75 mg bid. Two pts had a dose-limiting toxicity (DLT) in the DLT observation period: a grade (G) 3 confusion with ataxia and disorientation at 60 mg bid and a G4 suicidal ideation at 75 mg bid. G3-4 adverse events (AEs) in >5% of pts were: neutropenia (32%), thrombocytopenia (25%), asthenia (19%), dyspnea (9%), transaminitis (9%), anemia (8%), and diarrhea, pulmonary embolism, pulmonary sepsis (6% each). Most common AEs were asthenia (70%), ocular AEs (68%), skin rash (62%), nausea (58%), diarrhea (58%), peripheral edema (51%), thrombocytopenia (49%), vomiting (45%), mucositis (43%), neutropenia (38%), decreased appetite (36%) and anemia (34%). The main ocular AE was serous retinal detachment (58%); manageable retinal vein occlusion occurred in five pts. PK data were comparable to pimasertib monotherapy and published gem data. Partial responses were noted in 10 pts and stabilisation ≥3 months in 13 pts. Hot spot mutations in genes activating the MAPK and PI3K/AKT pathway and correlation with clinical outcome are being investigated. Conclusions: Pimasertib MTDs were reached. The RP2D was defined as 60 mg bid. PK was dose proportional and associated with target inhibition. Sustained responses were seen in both dosing schedules. Clinical trial information: 2009-011992-61.