I am on letrozole and iBrance for mbc as well. I also was diagnosed with mbc with right breast and lining of my lung. I have been on this drug therapy since mid October. My first set of scans after a month show a reduction by half of my breast tumour and the fluid in my pleural has reduced by half. There are some side effects like fatigue and joint aches but I am happy with thereabouts so far. I take calcium and vitamin D supplements . I also had a problem with and count and my oncologist lowered my dosage of iBrance to 100