Apatinib Combined With S-1 in the Second-line Treatment of Advanced Pancreatic Cancer

1. To observe the curative efficacy of apatinib and S-1 on patients by analyzing the data of overall survival (OS), complete remission (CR) or partial remission (PR), the rate of progression free survival (PFS), levels of CA19-9 and VEGFR in serum \[Time Frame: Evaluation at 2 month intervals through study completion from the date of study entry until the date of progression, up to 1 year\]. 2. To observe any adverse events, including abnormal clinical symptoms and vital signs, abnormal laboratory examinations, and to record the clinical features, severity, occurrence time, duration, management and prognosis of all subjects during the clinical study, and to determine the correlation between these adverse events and the experimental drugs. . The safety of drugs used in advanced pancreatic cancer was evaluated by CTCAE v4.0