The purpose of this Phase 1b/2 study is to evaluate the safety and tolerability of PF-04136309 in combination with nab-paclitaxel and gemcitabine, characterize the dose-limiting toxicities (DLTs) and overall safety profile of escalated doses of PF-04136309 and the associated schedule, determine the maximum tolerated dose (MTD), and to assess the enhancement of efficacy of PF-04136309 in combination with nab-paclitaxel and gemcitabine versus nab-paclitaxel + gemcitabine + placebo in terms of Progression Free Survival.