In this study positron emission tomography (PET/CT) imaging will be used to evaluate proliferative activity in sites of metastatic disease using the investigational radiotracer [18F]fluorothymidine (FLT).

This protocol is designed to develop relationships between parameters determined from imaging biomarker studies and ones used for determination of therapeutic response to combined CDK4 Inhibitor and chemotherapy regimens. Imaging will occur prior to starting therapy on the separate therapeutic protocol UPCC 06115. Patients may undergo a second FLT PET/CT scan following a 3-day run of ribociclib (LEE011) (day -6 to -4) and a third FLT PET/CT scan on cycle 1 day 12 following two treatments of weekly Paclitaxel to compare changes in FLT uptake measures.

Inclusion Criteria:

1. At least 18 years of age
2. History of histologically or cytologically confirmed breast cancer, any ER, PR or HER2 status is allowed as long as it expresses the Rb protein.
3. At least one site of metastatic breast cancer that is outside of the liver and bone that is identified by standard imaging (e.g. CT, MRI, FDG PET/CT, ultrasound, x-ray)
4. Participants must be informed of the investigational nature of this study and provide written informed consent in accordance with institutional and federal guidelines prior to study-specific procedures.
5. Patients must be candidates to receive treatment on the therapeutic trial UPCC06115, a Phase I study of ribociclib (LEE011) and weekly paclitaxel.

Exclusion Criteria:

1. Females who are pregnant at the time of screening will not be eligible for this study, urine pregnancy test will be performed at screening in women of child-bearing potential.
2. Inability to tolerate imaging procedures in the opinion of an investigator or treating physician
3. Serious or unstable medical or psychological conditions that, in the opinion of the investigator, would compromise the subject's safety or successful participation in the study.
4. Ineligible for the therapeutic trial UPCC 06115