Study to Evaluate the Safety and Antitumor Activity of CX-2009 Monotherapy and in Combination With CX-072 in Advanced Breast Cancer

Eligible patients will be enrolled to the treatment arm based on breast cancer subtype. Patients will receive study treatment on Day 1 of a Q3W cycle. Treatment with CX-2009 monotherapy (Arms A and B) or CX-2009 in combination with CX-072 (Arm C) will be given until disease progression or symptomatic deterioration, unacceptable toxicity necessitating treatment discontinuation, or if the patient meets certain study defined criteria for discontinuation. On-treatment tumor assessments, will occur every 6 weeks per RECIST v1.1 for the first 48 weeks, and every 12 weeks thereafter.