Invasive disease-free survival (iDFS) for the combination of at least 5 years endocrine therapy and 2-year palbociclib treatment versus at least 5 years endocrine therapy alone in patients with histologically confirmed HR+/HER2- invasive early breast cancer (EBC) at 4 years. iDFS is defined as the time from randomization to the date of the first event: local/regional invasive ipsilateral recurrence, contralateral invasive breast cancer, distant recurrence, second primary invasive cancer of non-breast origin or death from any cause. Direct comparison between arms used time to iDFS events and Kaplan-Meier Log-rank analysis. Due to the medians not yet achieved, the percentage of patients considered iDFS at 4 years is reported.

Percentage of patients with a Ki67 less than 2.7%

The change in Ki67 from baseline to 14 weeks.

Clinical Response is assessed by ultrasound at the end of the treatment (week 14) according to ECOG response criteria defined in Appendix A1 of the protocol. Number of participants with clinical complete response.

An AE was any untoward medical occurrence in a participant who received study drug without regard to possibility of causal relationship. An SAE was an AE resulting in any of the following outcomes or deemed significant for any other reason: death; initial or prolonged inpatient hospitalization; life-threatening experience (immediate risk of dying); persistent or significant disability/incapacity; congenital anomaly. Treatment-emergent are events between first dose of study drug and up to 28 days after last dose that were absent before treatment or that worsened relative to pretreatment state.

An adverse event (AE) was any untoward medical occurrence in a participant who received study drug without regard to possibility of causal relationship. Serious adverse event (SAE) was an AE resulting in any of the following outcomes or deemed significant for any other reason: death; initial or prolonged inpatient hospitalization; life-threatening experience (immediate risk of dying); persistent or significant disability/incapacity; congenital anomaly.

An AE was any untoward medical occurrence attributed to study drug in a participant who received study drug. An SAE was an AE resulting in any of the following outcomes or deemed significant for any other reason: death; initial or prolonged inpatient hospitalization; life-threatening experience (immediate risk of dying); persistent or significant disability/incapacity; congenital anomaly.

Dose limiting toxicity was defined as any of the following TEAEs occurring during the second cycle of treatment and possibly attributable to the combination of letrozole plus Palbociclib: 1. Grade 4 hematologic toxicity (including platelets \<25,000/μL, ANC \<500/μL). 2. Grade 3 neutropenia associated with a documented infection or fever ≥38.5°C. 3. Grade ≥3 non-hematologic toxicities, except those that have not been maximally treated (eg, nausea, vomiting, diarrhea, hypertension). 4. Delay by ≥1 week in receiving the next scheduled dose of either study treatment due to persisting treatment-related toxicities (platelet count \<50,000/μL; ANC \<1,000/μL; nonhematologic toxicities of Grade ≥3 severity). 5. Inability to deliver at least 80% of the planned Palbociclib or letrozole doses during Cycle 2 due to toxicity possibly attributable to the study treatment.

PFS was defined as the time from randomization (or the first dose of study treatment for non-randomized studies) to the first documentation of objective tumor progression or to death due to any cause, whichever occurred first. PFS calculated as (Weeks or Months) = (first event date minus randomization or the first dose date plus 1) divided by 7 (or 30.44 if in months). PFS is usually characterized by the median, 25% percentile,75% percentile and their 95% Confidence Intervals (CIs).