Calcitriol is a steroid hormone, vitamin D metabolite and agonist of vitamin D receptor (VDR), also known as calcitriol receptor. It regulates absorption of calcium from intestine, resorption of bone calcium and calcium excretion via kidneys. This compound also controls cell cycle, promotes cell differentiation, triggers apoptosis and acts as anti-inflammatory factor within the tumor microenvironment. It has anti-proliferative effects in malignant epithelial cells and in tumour-derived endothelial cells (TDEC). Also, it is a potent inhibitor of retinal neoangiogenesis in vitro.