Exemestane- 13 C 3 is the 13 C-labeled Exemestane. Exemestane (FCE 24304) is a selective, irreversible and orally active steroidal aromatase inhibitor with IC50s of 30 nM and 40 nM for human placental and rat ovarian aromatase, respectively. Exemestane can be used for hormone-dependent breast cancer research.