Background: MM-121 is a fully human monoclonal antibody targeting the epidermal growth factor receptor family member ErbB3. ErbB3 has been implicated in driving cancer growth and in the development of resistance to conventional chemotherapies across multiple malignancies. Here we present results of an open-label, Phase 1, multicenter, non-randomized, dose-escalation trial which recently completed enrollment evaluating MM-121 in combination with one of the following chemotherapies: Gemcitabine (Arm A, n=11), carboplatin (Arm B, n=11), pemetrexed (Arm C, n=10), or cabazitaxel (Arm D, n=11). Methods: Patients were treated in a dose escalation “3+3” design to assess the safety, tolerability and pharmacokinetics (PK) of MM-121 administered weekly in combination with anticancer therapies in subjects with advanced cancer. Doses were escalated until the maximum tolerated dose (MTD) was identified or the combination was shown to be tolerable at the highest planned doses. Secondary objectives included: Determining the objective response rate, clinical benefit rate, PK and immunogenicity of MM-121. Data summarized are as of 1/17/2013 from a live database. Results: Overall, 43 patients, [22 (51%) female and 21 (49%) male] have been treated with a median treatment duration of 57 days (range 1-302). The median age was 59 years (range 42-84) and patients had received a median of four prior lines of therapy (range 0-13). Common (>20%) adverse events of any grade and causality across all arms included diarrhea (74%), nausea (54%), fatigue (51%), anemia (44%), vomiting (33%), hypokalemia (30%), decreased appetite (26%), thrombocytopenia (26%), peripheral edema (23%), neutropenia (21%), and constipation (21%). Four DLTs were observed: Two in combination with carboplatin (G4 thrombocytopenia and G3 rash), one with gemcitabine (G4 thrombocytopenia), and one with pemetrexed (G4 hyperuricemia). Overall 38 (88%) patients were evaluable for response and the overall clinical benefit rate (PR or SD >18 weeks), is 32% (12/38). Conclusions: MM-121 can be combined at its recommended single agent dose with standard doses of gemcitabine, pemetrexed, and cabazitaxel and adapted doses of carboplatin. Clinical trial information: NCT01447225.