This is an open-label Phase 1b dose-escalation study to assess the safety, tolerability, and PK of vantictumab when combined with docetaxel. Vantictumab will be administered IV on Day 1 of each 21-day cycle. Docetaxel (75 mg/m2) will be administered IV on Day 1 of each cycle. The planned dose levels of vantictumab are 5 and 10 mg/kg. Depending on safety in this study, additional lower or intermediate dose levels may be evaluated. Depending on emerging safety data from the Phase 1a study 18R5-001 with continuing dose escalation, additional higher dose levels of vantictumab may be evaluated in this study. Alternative dosing schedules of vantictumab may be explored based on emerging nonclinical and clinical data for safety, PD, PK and efficacy. The starting dose for a new dosing schedule will be chosen to result in an AUC equivalent to the highest dose level that cleared on thepreviously studied dosing schedule. No dose escalation of vantictumab will be allowed within a dose cohort.