This is a multi-center, 2-part study of AMG 655, AMG 479 or AMG 655-placebo plus gemcitabine as first-line treatment of subjects with metastatic pancreatic cancer. Part 1 is an open-label, dose-escalation phase 1b segment to determine the safety, tolerability and maximum tolerated dose of AMG 655 in combination with gemcitabine. Enrollment into part 1 of the study has been completed. Part 2 is a randomized, placebo-controlled phase 2 segment to estimate the efficacy as assessed by 6 month survival of AMG 655, AMG 479, or AMG 655-placebo in combination with gemcitabine. The phase 2 segment that will commence after dose selection in part 1. In part 2, subjects will be randomized 1:1:1 to AMG 655, AMG 479, or placebo in combination with gemcitabine.