Palbociclib Plus Fulvestrant in Women With Hormone Receptor Positive and Human Epidermal Growth Factor Receptor Type 2 Negative Locally Advanced or Metastatic Breast Cancer Previously Treated With a CDK4/6 Inhibitor in Combination With Hormonal Therapy

Inclusion Criteria:

1. Adult (≥ 18 years of age) pre or post-menopausal women with LABC or MBC not amenableto curative treatment by surgery or radiotherapy, progressing to a CDK4/6 inhibitor incombination with aromatase inhibitor or tamoxifen in the adjuvant or metastaticsetting. To be enrolled in the present trial patients must have relapsed at leastafter 12 months from the last CDK4/6 dosage in the adjuvant setting or at progressionfrom a first line combined hormonal treatment for metastatic disease with a CDK4/6inhibitor plus AI or Tam with duration of, at least, 6 months. For metastatic disease,patients must have achieved, at least a stable disease while the first line hormonaltreatment with a CDK4/6 inhibitor plus AI or Tam to be enrolled in the trial.

2. Patients receiving up to one line of chemotherapy before the first line hormonaltreatment with a CDK4/6 inhibitor for metastatic disease may be enrolled in the study.

3. Histological confirmation of ER and/or PgR ≥ 1% and HER2 negative breast cancer (IHCstatus 0, 1+, 2+ and FISH not amplified).

4. Premenopausal women: in order to be eligible must have achieved surgical menopausewith bilateral oophorectomy or ovarian radiation or medical menopause by treatmentwith a luteinizing hormone-releasing hormone (LHRH) agonist (LHRHa) for induction ofovarian suppression.

5. Radiological or objective evidence of recurrence or progression on or after the lastsystemic therapy prior to enrollment.

6. Patients who received ≤ 28 days of fulvestrant for second line advanced breast cancertreatment prior to study enrollment are eligible.

7. Patients must have:

• At least one lesion that can be accurately measured in at least one dimension ≥ 20 mm with conventional imaging techniques or ≥ 10 mm with spiral CT or MRI

• Bone lesions: lytic or mixed (lytic + sclerotic) in the absence of measurabledisease as defined above.

8. Adequate bone marrow and coagulation and adequate organ function defined as follows:

• ANC > 1,000/mm3 (1.0 x 109/L);

• Platelets > 75,000/mm3 (75 x 109/L);

• Hemoglobin≥ 9 g/dL (90 g/L);

• Serum creatinine ≤ 1.5 x ULN or estimated creatinine clearance ≥ 60 ml/min ascalculated using the method standard for the institution;

• Total serum bilirubin≤1.5 x ULN (<2.5 ULN if Gilbert's disease);

• AST and/or ALT≤ 3 x ULN (≤ 5 x ULN if liver metastases present);

• Alkaline phosphatase ≤2.5 x ULN (≤ 5 x ULN if bone or liver metastases present);

• ECOG Performance Status≤ 2;

• Resolution of all acute toxic effects of prior therapy or surgical procedures toNational Cancer Institute (NCI) CTCAE Grade ≤1 (except alopecia).

9. Estimated life expectancy of >12 weeks.

10. Patients must perform liquid biopsy at study entry and at disease progression. Tissuebiopsy of the most accessible metastatic site at study entry and at diseaseprogression are required but not mandatory.

11. Written informed consent obtained before any screening procedure and according tolocal guidelines.

Exclusion Criteria:

1. HER2-overexpressing patients by local laboratory testing (IHC3+ staining or in situhybridization positive).

2. Patients who received > 1 line of chemotherapy as treatment for MBC.

3. Patients who received > 1 line of a CDK4/6 inhibitor in combination with hormonaltreatment for LABC or MBC or who have relapsed at less than 12 months from the end ofadjuvant treatment with a CDK4/6 inhibitor. For metastatic disease, patients with aprogressive disease within the first 6 months of treatment while on first line therapywith a CDK4/6 inhibitor, will be excluded.

4. Patients receiving chemotherapy or any type of hormonal therapy after treatment with aCDK4/6 inhibitor for metastatic disease.

5. Patients interrupting the previous treatment with CDK4/6 inhibitor for cardiac and/orhepatic toxicity and not for disease progression.

6. Pregnant, lactating women.

7. Known hypersensitivity to CDK4/6 inhibitors, fulvestrant, or to any of the excipients.

8. Radiotherapy within four weeks prior to enrollment (baseline/treatment start) exceptin case of localized radiotherapy for analgesic purpose or for lytic lesions at riskof fracture which can then be completed within two weeks prior to enrollment (baseline/treatment start). Patients must have recovered from radiotherapy toxicitiesprior to enrollment.

9. Currently receiving hormone replacement therapy, unless discontinued prior toenrollment.

10. Patients receiving concomitant immunosuppressive agents or chronic corticosteroidsuse, at the time of study entry except in cases outlined below:

11. short duration (<2 weeks) of systemic corticosteroids is allowed (e.g., chronicobstructive pulmonary disease, anti-emetic);

12. low doses of corticosteroids for brain metastasis treatment is allowed.

13. Patients with symptomatic visceral disease in need of urgent disease control (e.g.,significant dyspnea related to pulmonary lymphangitic carcinomatosis and lungmetastases or clinically meaningful symptomatic liver metastasis at the judgement oftreating investigator).

14. Symptomatic brain metastases.

15. Patients with a known history of HIV seropositivity.

16. Active, bleeding diathesis, or on oral anti-vitamin K medication (except low dosewarfarin, low-molecular-weight heparin (LMWH) and acetylsalicylic acid or equivalent,as long as the INR is ≤ 2.0).

17. Any severe and/or uncontrolled medical conditions such as: unstable angina pectoris,symptomatic congestive heart failure, myocardial infarction ≤6 months prior toenrollment, serious uncontrolled cardiac arrhythmia.

18. Acute and chronic, active infectious disorders.

19. Impairment of gastrointestinal function or gastrointestinal disease that maysignificantly alter the absorption of the study treatments (e.g., ulcerative disease,uncontrolled nausea, vomiting, diarrhea, malabsorption syndrome).

20. Inability to swallow oral medications.

21. Significant symptomatic deterioration of lung function.

22. Patients being treated with drugs recognized as being strong inhibitors or inducers ofthe isoenzyme CYP3A (Rifabutin, Rifampicin, Clarithromycin, Ketoconazole,Itroconazole, Voriconazole, Ritinavir, Telithromycin) within the last 5 days prior toenrollment.

23. History of non-compliance to medical regimens.

24. Patients refusing to perform liquid biopsy at study entry and disease progression.

25. Patients unwilling to or unable to comply with the protocol.