FEMARA
Clinical safety rating: caution
Comprehensive clinical and safety monograph for FEMARA (FEMARA).
Aromatase inhibitor; inhibits the conversion of androgens to estrogens by competitively binding to the aromatase enzyme, thereby reducing estrogen levels in peripheral tissues and tumors.
| Metabolism | Hepatic metabolism via CYP2A6 and CYP3A4, primarily by CYP2A6; also undergoes glucuronidation. The major metabolite is a carbinol derivative, which is inactive. |
| Excretion | Letrozole is extensively metabolized in the liver, primarily to an inactive carbinol metabolite. Approximately 90% of an oral dose is excreted in urine, with about 6% as unchanged drug and 84% as metabolites. Fecal excretion accounts for less than 10%. |
| Half-life | The terminal elimination half-life of letrozole is approximately 2 days (range 24–48 hours). Steady-state concentrations are achieved within 2–6 weeks of daily dosing. The long half-life supports once-daily dosing. |
| Protein binding | Letrozole is weakly protein bound (approximately 60%), primarily to albumin. The free fraction is about 40%. |
| Volume of Distribution | The apparent volume of distribution is approximately 1.9 L/kg (range 1.2–2.6 L/kg). This large Vd suggests extensive tissue distribution and good penetration into peripheral tissues. |
| Bioavailability | Absolute bioavailability after oral administration is nearly complete, estimated at >99.9%, indicating negligible first-pass metabolism. |
| Onset of Action | Suppression of plasma estradiol levels is observed within 24 hours after a single oral dose, with maximal suppression (≥80% reduction from baseline) achieved within 2–3 days of once-daily dosing. |
| Duration of Action | After discontinuation, plasma estradiol levels gradually return to baseline over 2–3 weeks. The duration of estrogen suppression correlates with the drug's half-life and persists for approximately 2–3 days after the last dose, with residual effects lasting up to 7 days. |
2.5 mg orally once daily.
| Dosage form | TABLET |
| Renal impairment | No dose adjustment required for GFR ≥30 mL/min. Insufficient data for GFR <30 mL/min. |
| Liver impairment | Child-Pugh A or B: 2.5 mg orally once daily. Child-Pugh C: Not recommended. |
| Pediatric use | Not approved for use in pediatric patients. |
| Geriatric use | No specific dose adjustment required; monitor for increased sensitivity. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for FEMARA (FEMARA).
| Breastfeeding | Breastfeeding is not recommended during letrozole therapy. It is unknown if letrozole is excreted in human milk; however, due to the potential for serious adverse reactions in the nursing infant, women should discontinue nursing or discontinue the drug. M/P ratio is not available. |
| Teratogenic Risk | Pregnancy Category X: FEMARA (letrozole) is contraindicated in pregnancy. Based on its mechanism of action (aromatase inhibition) and animal studies, there is a risk of fetal harm, including potential teratogenic effects (skeletal and visceral malformations) and embryotoxicity. First trimester exposure is particularly concerning; if pregnancy occurs during treatment, the patient should be apprised of the potential hazard. |
■ FDA Black Box Warning
None
| Serious Effects |
["Hypersensitivity to letrozole or any component of the formulation","Premenopausal women (due to risk of ovarian hyperstimulation and lack of efficacy)","Pregnancy and breastfeeding"]
| Precautions | ["Bone effects: Increased risk of osteoporosis and bone fractures due to estrogen suppression; monitor bone mineral density and consider use of bisphosphonates.","Cardiovascular effects: Increased incidence of ischemic cardiovascular events compared to tamoxifen; monitor for cardiovascular disease.","Hepatic effects: Elevations in liver enzymes; monitor liver function periodically.","Pregnancy: Pregnancy category D; can cause fetal harm. Advise use of effective contraception during treatment."] |
| Food/Dietary | No specific food interactions. Grapefruit and grapefruit juice do not interact with letrozole. Avoid alcohol due to potential liver effects. |
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| Fetal Monitoring | Monitor pregnancy status (negative serum hCG prior to initiation and regular intervals). If pregnancy occurs, fetal ultrasound and assessment for potential developmental abnormalities are advised. Monitor maternal bone density due to estrogen suppression; consider baseline and periodic DXA scans. No specific fetal monitoring required outside of standard prenatal care if exposure occurs. |
| Fertility Effects | FEMARA is used off-label for ovulation induction due to its antiestrogenic effects on the endometrium and cervical mucus; however, in oncology dosing (2.5 mg daily), it may impair fertility. Aromatase inhibition suppresses estrogen synthesis, potentially affecting follicular development and ovulation. In premenopausal women, ovarian function may be temporarily disrupted; advise women of childbearing potential to use effective contraception during treatment and for at least 3 weeks after discontinuation. Fertility may return after treatment cessation. |
| Clinical Pearls | FEMARA (letrozole) is a nonsteroidal aromatase inhibitor used for hormone receptor-positive breast cancer. Monitor bone mineral density due to increased fracture risk. Use caution in patients with severe hepatic impairment (Child-Pugh class C). Avoid use in premenopausal women unless concurrent ovarian suppression. Common side effects include hot flashes, arthralgia, and fatigue. |
| Patient Advice | Take exactly as prescribed, usually once daily with or without food. · Report any unusual bone pain or fractures to your healthcare provider. · Use effective non-hormonal contraception if premenopausal and on ovarian suppression. · May cause dizziness or fatigue; avoid driving if affected. · Do not take estrogen-containing supplements or therapies while on this drug. · Keep all appointments for bone density monitoring and blood tests. |