Inclusion Criteria:
* A diagnosis of metastatic or unresectable WD/DD liposarcoma. DD liposarcoma must be present. Unresectable is defined as if the primary tumor a) cannot be safely removed surgically or b) would benefit from systemic therapy prior to a surgical approach
* Measurable disease by RECIST 1.1
a. Target lesions must not be chosen from a previously irradiated field unless there has been radiographically and/or pathologically documented tumor progression in that lesion prior to enrollment
* Age ≥ 18 years
* ECOG performance status 0 or 1
* Adequate organ and marrow function as defined below (ULN indicates institutional upper limit of normal):
1. Absolute neutrophil count ≥ 1.5 x 109/L
2. Hemoglobin ≥ 8.0 g/dL
3. WBC ≥ 3.0 x 109/L
4. Platelets ≥ 100 x 109/L
5. ≤ 1.5 X ULN OR Direct bilirubin ≤ ULN for subjects with total bilirubin levels \> 1.5 ULN. Except patients with Gilbert's disease (≤3x ULN)
6. AST (SGOT) /ALT (SGPT) ≤ 3 x institutional ULN
7. Creatinine Clearance \> 30 mL/min (calculated by Cockcroft-Gault method)
* Women of child-bearing potential and men must agree to use adequate contraception (hormonal or barrier method of birth control or abstinence) during the trial period through at least 120 days after the last dose of study treatment.
* Ability to understand and the willingness to sign a written informed consent document.
* Ability to swallow tablets or capsules
* Patients with brain metastasis that have been treated with definitive surgery or radiation, and have been clinically stable for 3 months are eligible
Exclusion Criteria:
* Patients who have not recovered from clinically significant adverse events of prior therapy to ≤ NCI CTCAE v5 Grade 1, except alopecia and stable neuropathy, which must have resolved to Grade ≤ 2 or baseline.
* Patients receiving any other investigational agents.
* Patients who have received prior treatment with a selective CDK4 inhibitor or an anti-PD-1/PD-L1 agent
* Uncontrolled intercurrent illness including, but not limited to, known ongoing or active infection, including uncontrolled HIV, active hepatitis B or C, symptomatic congestive heart failure, unstable angina pectoris, uncontrolled cardiac arrhythmias, psychiatric illness/social situations that would limit compliance with study requirements, clinically significant interstitial lung disease or active noninfectious pneumonitis, or active infection requiring systemic therapy
1. Patients with a CD4+ count of \> 300 and an undetectable viral load who are currently on HAART are eligible for inclusion
2. Patients with NYHA class III or IV congestive heart failure within 6 months of study treatment will be excluded
* Pregnant women and women who are breast-feeding.
* History or evidence of symptomatic autoimmune disease in past 2 years prior to enrollment.
a. Replacement therapy (e.g., thyroxine for hypothyroidism, insulin for diabetes or physiologic corticosteroid replacement therapy for adrenal or pituitary insufficiency) is not considered a form of systemic treatment for autoimmune disease
* Prolonged QTcF \> 450 ms for men and \> 470 ms for women at Screening.
* Patients who have received a live vaccine within 30 days of the start date of the planned study therapy. Note: Seasonal influenza vaccines for injection are generally inactivated flu vaccines and are allowed; however intranasal influenza vaccines are live attenuated vaccines, and are not allowed
* Radiation therapy within 2 weeks prior to study Day 1
* Prior organ transplantation including allogenic stem-cell transplantation
* Known prior severe hypersensitivity to investigational product or any component in its formulations, including known severe hypersensitivity reactions to monoclonal antibodies (NCI CTCAE v 5 Grade ≥ 3)
* Patients who require concomitant use of medications that strongly induce or inhibit CYP3A (per section 15.0)
