Afinitor is an mTOR (mammalian target of rapamycin) inhibitor. mTOR is a kinase, a type of protein in the body. Kinases help all cells -- both healthy and cancer cells – get the energy they need. When kinases don’t act normally or are overactive, they help certain breast cancers grow. mTOR inhibitors work by interfering with the mTOR kinase and stopping cancer cells from getting the energy they need to grow.