Intermediate endpoints of primary systemic therapy in breast cancer patients

Primary systemic therapy (PST) in breast cancer offers the opportunity to explore interactions between tumor biology and administered treatment. Changes in clinical, tissue-based, or imaging markers can provide information on the mechanisms of PST activity (activity endpoints) or predict treatment efficacy (surrogate endpoints). The most frequently used intermediate endpoint for PST is pathological complete response, but its role as a surrogate parameter of efficacy has not yet been demonstrated. Changes in tumor biology after PST may occur already a few days after treatment start; this implies that new potential surrogates occurring much earlier than pathological complete response (ie, the proliferation marker Ki67) can be identified and that short-term preoperative trials (window-of-opportunity trials) can be designed using a biological parameter as a primary endpoint. From these small trials, crucial information can be gleaned about the activity of new drugs for the design of large clinical trials.