 ARM A: Patients receive gemcitabine hydrochloride intravenously IV over 30 minutes on days 1 and 8, and cisplatin IV over 60 minutes on day 1. Patients also receive ATR kinase inhibitor VX-970 IV over 60 minutes on days 2 and 9. Treatment repeats every 21 days for 6 courses in the absence of disease progression or unacceptable toxicity. 