Inclusion Criteria:

          1. Evidence of a personally signed and dated informed consent document indicating that
             the subject has been informed of all pertinent aspects of the study;

          2. Subject is willing and able to comply with scheduled visits, treatment plan,
             laboratory tests, and other study procedures;

          3. 18 years of age or older;

          4. Patient agrees not to participate in another interventional study while on treatment;

          5. Histologically diagnosed endometrioid or high-grade serous ovarian cancer, estrogen
             (ER) and/or progesterone (PR) receptor positive (defined as > 10% by
             immunohistochemistry);

          6. Patients must have completed 2 previous courses of chemotherapy:

               1. The penultimate regimen must be a platinum-based chemotherapy course prior to
                  enrolment on the study:

               2. For the last chemotherapy course prior to enrolment on the study:

                    -  There is no pre-specified regimen;

                    -  It may contain a Platinum salt or not (depending upon Platinum sensitivity),
                       at discretion of treating Physician;

                    -  Patients must have demonstrated disease progression by RECIST v1.1 to the
                       last treatment

                    -  Patients must be treated on the study within 8 weeks of completion of their
                       final dose of second line regimen;

          7. Formalin fixed, paraffin embedded tumor sample from the primary tumor must be
             available for central testing;

          8. Eastern Cooperative Oncology Group (ECOG) performance status ≤ 2;

          9. Adequate bone marrow function at screening:

               -  Absolute Neutrophil Count (ANC) ≥ 1,500/mm³ (≥ 1.5x109/L)

               -  Platelets ≥ 100,000/mm³ or ≥ 100 x 109/L

               -  Hemoglobin ≥ 9.0 g/dL;

         10. Adequate liver function at screening:

               -  Total serum bilirubin ≤ 1.5 x upper limit of normal (ULN) (≤ 3.0 x ULN if Gilbert
                  Syndrome)

               -  Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤ 3.0 x ULN
                  (≤ 5.0 x ULN if there is tumor involvement in the liver)

               -  Alkaline phosphatase ≤ 2.5 x ULN (≤ 5.0 x ULN if there is tumor involvement in
                  the liver);

         11. Adequate renal function at screening:

             - Serum creatinine ≤ 1.5 x ULN or estimated creatinine clearance ≥ 50mL/min;

         12. Evidence of non-childbearing potential:

               -  Postmenopausal (defined as at least 1 year without any menses) prior to
                  screening, or

               -  Radiation-induced oophorectomy with last menses >1 year ago, or

               -  Surgical sterilisation (bilateral oophorectomy or hysterectomy).

        Exclusion Criteria:

          1. Patients with a known hypersensitivity to Palbociclib or Letrozole or any of the
             excipients of the product;

          2. Previous treatment with CDK inhibitors or endocrine therapy;

          3. Persistent toxicities (grade 2 or greater) caused by previous cancer therapy
             (excluding alopecia);

          4. Patients with second primary cancer, except: adequately treated non-melanoma skin
             cancer, curatively treated in-situ cancer of the cervix, Ductal Carcinoma in Situ
             (DCIS), stage 1 grade 1 endometrial carcinoma curatively treated with no evidence of
             disease for 3 years;

          5. Patients receiving any chemotherapy, radiotherapy, within 3 weeks from the last dose
             prior to study entry;

          6. Patients with symptomatic uncontrolled brain metastases. A scan to confirm the absence
             of brain metastases is not required;

          7. Major surgical procedure within 3 weeks prior to study randomization, or one is
             planned during the course of the study;

          8. Patients considered poor medical risk due to a serious, uncontrolled medical disorder,
             non-malignant systemic disease or active, uncontrolled infection. Examples include,
             but are not limited to, uncontrolled ventricular arrhythmia, recent (within 6 months)
             myocardial infarction, cerebrovascular accident, gastrointestinal bleeding, or any
             psychiatric disorder that prohibits obtaining informed consent;

          9. Patients that have difficulty taking oral medication or any digestive tract
             dysfunction or inflammatory bowel disease that would interfere with the intestinal
             absorption of drugs (eg, partial bowel obstruction or malabsorption);

         10. Patients have received potent inhibitors or inducers of CYP3A4 within 7 days prior to
             randomization;

         11. Pregnant or breast feeding women;

         12. Patient has a known history of positive test for human immunodeficiency virus (HIV);

         13. Patients with known hepatic disease (ie, Hepatitis B or C);

         14. Subjects who are investigational site staff members directly involved in the conduct
             of the trial and their family members, site staff members otherwise supervised by the
             Investigator, or subject who are Pfizer employees directly involved in the conduct of
             the trial;

         15. Treatment with any investigational product during the last 28 days;

         16. QTc > 480ms, QT syndrome, Brugada syndrome, history QTc prolongation or Torsade de
             Points;

         17. Other severe acute or chronic medical or psychiatric condition or laboratory
             abnormality that may increase the risk associated with study participation or may
             interfere with the interpretation of study results and, in the judgment of the
             investigator, would make the subject inappropriate for entry into this study.