A disease assessment will be performed at study week 8 (± 3 days) by the investigator. The assessment will be based both on changes in clinical symptoms, and radiographic images. Subjects without evidence of disease progression may continue to receive treatment based on their original treatment assignment until disease progression or intolerability. Disease assessments will be performed every 8 weeks during the extended period. A safety follow-up visit will occur 4 weeks after the last dose infusion of AGS-1C4D4 and/or gemcitabine.