PIK3CA and breast cancer: Interestingly, metformin has been shown to decrease leptin levels in patients with either breast or endometrial cancer.100,101 In patients with endometrial cancer, metformin reduces cancer cell proliferation (as measured by Ki-67 staining) and has inhibitory effects on the PI3K–mTOR signalling pathways in the presurgical window.71,72 In a trial of 200 non-diabetic patients with breast cancer, metformin did not significantly decrease breast cancer cell proliferation.