Introduction In Argentina 18.000 new cases of breast cancer (BC) are diagnosed each year and it is the commonest cause of cancer death in women reaching 5600 deaths per year.

In postmenopausal women with advanced or metastatic estrogen receptor-(ER) positive, Her2-negative BC, the combination of Palbociclib (P) + Letrozol (L) or Fulvestrant (F) is a good option of treatment.

The objective was to assess clinical benefit, evolution and safety with P + L or F in the context of daily clinical practice.

Methodology We performed an observational study.

Patients (pts) who started CDK4/6 inhibitors P treatment between April 2016 and June 2020 were included.

Results 54 pts with median age 61 years (r:31–85) were analyzed. 11 premenopausal women.

29 pts (53,7%) performed P + L and 25 (46,3%) P + F.

5 pts presented with the novo metastatic disease. The main localization of metastases was bone in 24 pts, lymphatic in 14, liver in 10 and lung in 6.

Clinical benefit:

8 pts (14,8%) stable disease, 18 pts (33,4%) partial response and 3 pts (5,5%) complete response.

Dose reduction to 100 mg P occurred in 7 pts.

27 pts (50%) had cycle delays (7–14 days), mainly due to Grado 3 neutropenia (60%).

Most common nonhematologic adverse events:

asthenia (40%) and fatigue (35%).

26 pts (48,2%) still under treatment.

28 pts (51,8%) discontinued treatment, owing to disease progression in 25 pts and toxicities in 3.

Conclusion a clinical benefit was observed in 48,2% of our pts with an adequate tolerance and the adherence to treatment was maintained with acceptable toxicity profile.