To determine whether MM-121 + exemestane was more effective than placebo + exemestane in prolonging progression-free survival. PFS was a time to event measure, and progression of disease is defined using Response Evaluation Criteria In Solid Tumors Criteria (RECIST v1.1), "as a 20% increase in the sum of the longest diameter of target lesions, or a measurable increase in a non-target lesion, or the appearance of new lesions". Progression free survival was defined as the number of months from the date of randomization to the date of death or progression. If neither death nor progression was observed during the study, PFS data was censored at the last non-progressive disease valid tumor assessment unless the patient was discontinued due to symptomatic deterioration. If this occurred, the patient was counted as having progressive disease (PD).

Fresh tumor samples were obtained from patients prior to enrollment and formalin-fixed for analysis. Samples were analyzed using RT-PCR for the expression of the biomarker, heregulin. Progression-free survival was assessed using RECIST v 1.1 to determine whether patients whose tumors express HRG have a lower PFS than those whose tumors do not express HRG, and to assess whether the addition of MM-121 to exemestane can increase PFS in HRG-high patients.

Patients with available tissue for RT-PCR analysis

To determine whether MM-121 + exemestane is more effective than placebo + exemestane in prolonging overall survival. This was a time-to-event analysis of time from first dose to date of death.