 Temsirolimus is a structural analog of sirolimus rapamycin that has been formulated for IV or oral administration for the treatment of various malignancies. Sirolimus was shown to have potent immunosuppressive as well as antifungal and antitumor properties. Its mechanism of action results in part from binding to an intracellular cytoplasmic protein, FK506 tacrolimus binding protein FKBP-12. The complex of sirolimus bound to FKBP-12 blocks the activity of mTOR. 