Hormone Receptor Positive Metastatic Breast Cancer in Patient With Multiple Sclerosis

72-year-old female with severe multiple sclerosis (MS) involving bilateral lower extremities. She was initially diagnosed with stage IIB (T2, N1) invasive ductal carcinoma of the left breast 4years ago. Tumor size 2.7 cm, grade 2, ER/PR positive, HER-2 negative. She underwent left mastectomy with 1/1 sentinel lymph node involved. Oncotype recurrence score was 33 and she received adjuvant TC chemotherapy x 4 followed by adjuvant Tamoxifen. She had right breast recurrence in 2 years (while on tamoxifen); T2, N1(mi), ER positive, PR negative, and HER-2 negative. She had right mastectomy and 1/1 sentinel lymph node had microscopic disease. Adjuvant endocrine therapy with aromatase inhibitor was started but she did not tolerate and was held after 1 year. She had recurrent/metastatic breast cancer in bilateral axillary lymph nodes 8 months ago; biopsy of lymph node showed ER/PR positive and HER-2 negative disease. PET scan showed disease in bilateral axilla only. She was started on capecitabine and fulvestrant but developed Grade 3 GI and dermal toxicity from Xeloda although follow up CT scan shows interval response to therapy.

Question. What further therapy would you recommend with underlying history of MS?