Letrozole has no progesterone, androgene or estrogen effects, and is not a mechanism based inhibitor of CYP2C9 or CYP3A4. No reports of CYP-inducing effects. However, side effects such as nausea, anorexia and vomiting may be potentially porphyrinogenic through reduction in caloric intake.

Letrozole, a benzhydryltriazole derivative, is a selective, nonsteroidal aromatase inhibitor.

Letrozole is used in the treatment of advanced or locally advanced breast cancer. It is administered orally. Common adverse reactions of letrozole that can be confused with an acute porphyric attack are nausea, vomiting, depression, diarrhea and constipation. Side effects such as nausea, anorexia and vomiting may be potentially porphyrinogenic through reduction in caloric intake.

Letrozole is metabolized in the liver by CYP3A4 and 2A6. In human liver microsomes, letrozole has been shown to strongly inhibit in vitro metabolic reactions catalyzed by CYP2A6 and to moderately inhibit reactions catalyzed by CYP2C19; thus, concomitant use of letrozole may result in decreased metabolism and increased plasma concentrations of agents metabolized by these hepatic microsomal enzymes. Negligible inhibitory effect on CYP2C9 and CYP3A (Jeong, 2009). No CYP-related interactions reported for the drug in clinical use.

Letrolzole was used uneventfully in the treatment of breast cancer in 59 year old PV woman (Thiery-Vuillemin, 2008).

Uneventful use reported in 1 patient with acute porphyria.