They monitor safety in all three clinical trial phases and beyond. Note for example the outcome measures for the Provenge PIII:
Primary Outcome Measures: To assess the safety and efficacy of Sipuleucel-T in prolonging survival of men with Metastatic Androgen Independent Prostate Cancer [ Time Frame: Overall Survival (time to death) ] [ Designated as safety issue: Yes ]
Secondary Outcome Measures: To assess the safety and efficacy of Sipuleucel-T in delaying time to objective disease progression in men with Metastatic Androgen Independent Prostate Cancer [ Time Frame: Time to objective disease progression ] [ Designated as safety issue: Yes ]
Are you referring to these "signal seeking" trials? You can find them on the clinicaltrials.gov web site. Just do a search for "bavituximab".
Official Title: A Phase II Safety and Efficacy Study of Bavituximab Plus Docetaxel in Patients With Locally Advanced or Metastatic Breast Cancer
Official Title: A Phase II Safety and Efficacy Study of Bavituximab Plus Paclitaxel and Carboplatin in Patients With Locally Advanced or Metastatic Breast Cancer
Official Title: A Phase II Safety and Efficacy Study of Bavituximab Plus Paclitaxel and Carboplatin in Patients With Previously Untreated Locally Advanced or Metastatic Non-Small Cell Lung Cancer