Background: Several clinical trials are underway to investigate if variable forms of vitamin D (D2 vs. D3) prescribed at different doses (10,000-50,000 IUs/week) can improve the side-effects associated with treatment for estrogen receptor positive (ER+) breast cancer, specifically aromatase inhibitors (AIs.) Presumably for generalizability and potential safety purposes, these trials predominantly exclude women with metastatic breast cancer (MBC); a rapidly expanding sector of the cancer survivor population who experience significant treatment-related side-effects. Evaluation of the safety of vitamin D3 supplementation is crucial since supplementation can lead to high calcium and importantly, in lab studies have shown that vitamin D3 affects a gene that increases estrogen production. To assure that vitamin D3 does not affect the clinical effects of anti-estrogen therapies, the effect of vitamin D3 supplements on estrogen production requires an evaluation that further explores and defines its potential role in symptom management for this population.

Objectives: This pilot study will evaluate the feasibility of vitamin D3 supplementation in women with MBC, providing much needed data on the preliminary safety and efficacy of this treatment in this patient population. This study will determine: 1) if weekly supplementation of high dose vitamin D3 increases serum vitamin D levels without adverse effects related to such therapy (primary aim); 2) the effects of vitamin D3 supplementation on symptom management (secondary aim); and 3) if vitamin D3 supplementation is associated with improved inflammation (exploratory aim.)

Methods: This is an 8 week "proof of concept" study to monitor laboratory parameters and to assess potential effects on short-term outcomes. Adult, female patients (\>=18 years) with ER+ MBC (Stage IV) of any race/ethnicity and a history of vitamin D \< 30 mg/dl will be recruited from within and around LUMC. Following current clinical practice guidelines, eligible participants will receive 50,000 IUs of vitamin D3 weekly for 8 weeks. Laboratory values, muscle function and inflammation will be examined pre- and post-supplementation, while symptoms will be assessed at baseline, 4 and 8 weeks post-supplementation. We will assess if increases in vitamin D are associated with clinically significant improvements in symptoms and QOL, and decreased inflammation.