Letrozole
letrozole — tablets (Femara)
Indications
| Indication | Approved Population | Therapy Type | Status |
|---|---|---|---|
| Adjuvant treatment of HR-positive early breast cancer | Postmenopausal women | Adjuvant endocrine therapy | FDA Approved |
| Extended adjuvant treatment of early breast cancer (after 5 years of tamoxifen) | Postmenopausal women | Extended adjuvant endocrine therapy | FDA Approved |
| First-line advanced/metastatic breast cancer (HR+ or unknown) | Postmenopausal women | Systemic endocrine therapy | FDA Approved |
| Second-line advanced breast cancer (disease progression after antiestrogen therapy) | Postmenopausal women | Systemic endocrine therapy | FDA Approved |
Letrozole is a potent, non-steroidal third-generation aromatase inhibitor that suppresses plasma oestrogen levels by >98% in postmenopausal women. In the BIG 1-98 trial (N=8,010, median follow-up 96 months), letrozole demonstrated superior disease-free survival compared to tamoxifen monotherapy (HR 0.87) in postmenopausal HR-positive early breast cancer. In the MA-17 trial, extended adjuvant letrozole after 5 years of tamoxifen significantly improved disease-free survival compared to placebo. Letrozole is effective only in postmenopausal women or premenopausal women who have undergone ovarian function suppression, as it does not suppress ovarian oestrogen production.
Ovulation induction in anovulatory infertility — 2.5–7.5 mg daily on cycle days 3–7. Increasingly used as first-line for PCOS-related anovulation, supported by the AMIGOS trial (N=750). Evidence quality: High (RCT)
Controlled ovarian stimulation in IVF — Used as adjunctive to gonadotrophins, particularly in women with hormone-sensitive conditions (breast cancer survivors). Evidence quality: Moderate
Endometriosis — 2.5 mg daily combined with norethindrone acetate or GnRH agonist for refractory disease. Evidence quality: Low
Dosing
| Clinical Scenario | Starting Dose | Maintenance Dose | Maximum Dose | Notes |
|---|---|---|---|---|
| Adjuvant — early HR+ breast cancer (postmenopausal) | 2.5 mg once daily | 2.5 mg once daily for 5 years | 2.5 mg/day | May be used as upfront monotherapy or sequentially after 2–3 years of tamoxifen. Discontinue at relapse. BIG 1-98: median treatment 60 months |
| Extended adjuvant — after 5 years of tamoxifen | 2.5 mg once daily | 2.5 mg once daily for up to 5 years | 2.5 mg/day | Optimal duration unknown; planned 5 years in MA-17 trial. Discontinue at relapse. MA-17: median treatment 60 months; 58% completed ≥4.5 years |
| Advanced / metastatic breast cancer — first-line or second-line | 2.5 mg once daily | 2.5 mg once daily until progression | 2.5 mg/day | Continue until tumour progression. Can be given regardless of prior antiestrogen therapy in second-line setting. Take with or without food |
| Severe hepatic impairment (cirrhosis) | 2.5 mg every other day | 2.5 mg every other day | 2.5 mg every other day | Cirrhosis/severe hepatic impairment: ~2x exposure. 50% dose reduction. Mild-to-moderate: no adjustment needed (AUC 37% higher but within normal range). FDA PI Section 2.5 |
Unlike aminoglutethimide (first-generation aromatase inhibitor), letrozole is a highly selective inhibitor of aromatase and does not affect adrenal steroidogenesis. Patients treated with letrozole do not require glucocorticoid or mineralocorticoid replacement therapy (FDA PI). No dose adjustment is required in elderly patients or in renal impairment (CrCl ≥10 mL/min).
Pharmacology
Mechanism of Action
Letrozole is a non-steroidal competitive inhibitor of the aromatase enzyme (CYP19A1), which catalyses the final step in oestrogen biosynthesis — the conversion of androgens (androstenedione and testosterone) to oestrogens (oestrone and oestradiol) in peripheral tissues. In postmenopausal women, the adrenal glands are the principal source of oestrogen precursors, and peripheral aromatisation (in adipose tissue, muscle, and breast tumour itself) is the sole source of circulating oestrogen. Letrozole suppresses plasma oestradiol, oestrone, and oestrone sulphate by more than 98% from baseline, achieving near-complete oestrogen deprivation. This profound oestrogen suppression slows the growth of oestrogen-dependent breast cancers. Letrozole binds competitively and reversibly to the haem group of aromatase; it is highly selective for CYP19A1 and does not affect adrenal corticosteroid or aldosterone synthesis at therapeutic doses.
ADME Profile
| Parameter | Value | Clinical Implication |
|---|---|---|
| Absorption | Rapidly and completely absorbed; bioavailability ~100%; food does not affect extent of absorption | Can be taken with or without food at any time of day |
| Distribution | Weakly protein-bound (~60%); Vd ~1.9 L/kg; Css 1.5–2x single-dose predictions; steady state in 2–6 weeks | Large Vd indicates extensive tissue distribution. Slight non-linearity at daily dosing but no continuous accumulation. |
| Metabolism | CYP3A4 and CYP2A6 to pharmacologically-inactive carbinol metabolite (4,4′-methanol-bisbenzonitrile) | No active metabolites. CYP3A4 involvement is relevant for drug interactions, though no clinically significant interactions identified with cimetidine or warfarin. |
| Elimination | t½ ~2 days (48 h); urinary excretion: ≥75% as glucuronide conjugate of carbinol metabolite, ~6% unchanged drug | Long t½ supports once-daily dosing. No dose adjustment for renal impairment (CrCl ≥10 mL/min). Severe hepatic impairment doubles exposure — dose every other day. |
Side Effects
Adverse effect data are from the BIG 1-98 adjuvant trial (N=2,448 letrozole vs N=2,447 tamoxifen, median follow-up 96 months, median treatment 60 months) and the MA-17 extended adjuvant trial. The side-effect profile reflects the consequences of profound oestrogen deprivation: accelerated bone loss, musculoskeletal symptoms, and metabolic effects (hypercholesterolaemia).
| Adverse Effect | Incidence (letrozole) | Clinical Note |
|---|---|---|
| Hypercholesterolaemia | 52.3% (vs 28.6% tamoxifen) | Grade 3–4 in 0.4%. Lipid-lowering meds required in 29% vs 20% tamoxifen. Monitor lipids. |
| Hot flashes | 33.5% (vs 38.0% tamoxifen) | Lower than tamoxifen. Usually manageable with lifestyle measures. |
| Arthralgia / arthritis | 25.4% (vs 20.6% tamoxifen; Grade 3–4: 3.4%) | Most characteristic AI side effect. Can be severe enough to warrant switching therapy. Exercise may help. |
| Bone fractures (any time) | 14.7% (vs 11.4% tamoxifen) | During treatment only: 10.2% vs 7.2%. Reflects accelerated bone mineral density loss from oestrogen deprivation. |
| Night sweats | 14.5% (vs 17.4% tamoxifen) | Often coexists with hot flashes; lower rate than tamoxifen. |
| Weight increase | 12.9% (vs 15.4% tamoxifen) | Multifactorial; encourage physical activity and dietary counselling. |
| Nausea | 11.6% (vs 11.3% tamoxifen) | Similar to tamoxifen; generally mild. |
| Adverse Effect | Incidence | Clinical Note |
|---|---|---|
| Fatigue / asthenia / malaise | 9.6% | Caution when driving or using machinery (FDA PI Section 5.4). |
| Myalgia | 9.0% | Part of the musculoskeletal symptom complex characteristic of AI therapy. |
| Oedema | 6.7% | Usually peripheral; assess for cardiac causes if significant. |
| Weight decrease | 5.7% | Less common than weight gain; monitor nutritional status. |
| Vaginal bleeding | 5.3% (vs 13.1% tamoxifen) | Much less than tamoxifen. Still warrants evaluation in postmenopausal women. |
| Osteoporosis | 5.1% (vs 2.7% tamoxifen) | Nearly double the rate of tamoxifen. DEXA monitoring recommended. |
| Back pain / bone pain | 5.0–5.1% | Distinguish from disease recurrence; image if new onset or progressive. |
| Depression | 4.9% | Screen at follow-up visits; similar rate to tamoxifen (4.7%). |
| Headache | 4.3% | Usually mild; assess if persistent. |
| Dizziness / somnolence | 3.4% | Caution driving; reported in FDA PI warnings. |
| Alopecia | 3.4% | Hair thinning; usually not complete loss. |
| Adverse Effect | Estimated Frequency | Typical Onset | Required Action |
|---|---|---|---|
| Bone fractures | 14.7% vs 11.4% tamoxifen | Throughout treatment | DEXA at baseline and periodically. Vitamin D/calcium supplementation. Consider bisphosphonate if T-score <-2.0. Weight-bearing exercise. |
| Myocardial infarction | 1.7% vs 1.1% tamoxifen (at 96 mo); 1.0% vs 0.5% (during treatment) | Variable | Assess cardiovascular risk factors at baseline. Optimise lipid management. Letrozole has higher MI rate than tamoxifen. |
| Cerebrovascular accident / TIA | 3.0% vs 2.8% tamoxifen (at 96 mo) | Variable | Manage cardiovascular risk factors. Similar rate to tamoxifen. |
| Thromboembolic events | 3.2% vs 4.6% tamoxifen (at 96 mo) | Variable | LOWER than tamoxifen. Standard VTE management if occurs. |
| Cardiac failure | 1.6% vs 1.4% tamoxifen | Variable | Monitor for heart failure symptoms, especially in patients with cardiovascular comorbidities. |
Arthralgia and myalgia are the most common reasons for early discontinuation of aromatase inhibitor therapy. Up to 25% of patients report arthralgia in clinical trials, with approximately 5% experiencing symptoms severe enough to consider switching therapy. Strategies to improve tolerability include regular weight-bearing and aerobic exercise (evidence-based), adequate vitamin D supplementation (target 25-OH vitamin D ≥30 ng/mL), analgesics (NSAIDs or paracetamol), and switching to a different AI (letrozole to exemestane or vice versa). If symptoms are intolerable despite interventions, a switch to tamoxifen may be considered.
Drug Interactions
Letrozole is metabolised primarily by CYP3A4 and CYP2A6. Despite this, the FDA PI reports no clinically significant pharmacokinetic interactions with cimetidine or warfarin. The key interaction is with tamoxifen, which reduces letrozole plasma levels by 38% when given concurrently. There are comparatively few drug interactions compared to tamoxifen.
Monitoring
- Bone Mineral Density (DEXA)Baseline; periodically (q1–2 yr)
RoutineLumbar spine BMD decreased by median 4.1% at 24 months (vs +0.3% tamoxifen). Fractures 14.7% vs 11.4% tamoxifen. Start vitamin D + calcium supplementation. Consider bisphosphonate if T-score <-2.0 or fragility fracture (FDA PI Section 5.1). - Serum CholesterolBaseline; periodically
RoutineHypercholesterolaemia in 52.3% (vs 28.6% tamoxifen). Grade 3–4 in 0.4%. Lipid-lowering therapy needed in 29% of letrozole patients. Monitor in those with preexisting hyperlipidaemia (FDA PI Section 5.2). - Musculoskeletal SymptomsEvery visit
RoutineArthralgia/arthritis 25.4%. Major reason for non-adherence. Assess severity, offer management strategies (exercise, vitamin D, analgesics), and consider therapy switch if intolerable. - Liver Function TestsBaseline; if hepatic impairment
Trigger-basedModerate hepatic impairment increases AUC by 37%. Severe (cirrhosis) doubles exposure — dose every other day. Monitor LFTs if new hepatic symptoms develop. - Cardiovascular RiskBaseline; periodically
RoutineMI rate 1.0% during treatment (vs 0.5% tamoxifen). Assess and optimise cardiovascular risk factors (blood pressure, lipids, smoking, diabetes). - Pregnancy StatusBefore initiation
Trigger-basedVerify pregnancy status before starting in perimenopausal or recently postmenopausal women. Use effective contraception during and for ≥3 weeks after last dose (FDA PI Section 5.6).
Contraindications & Cautions
Absolute Contraindications
- Pregnancy — Letrozole is teratogenic. Reports of spontaneous abortions and congenital birth defects in post-marketing data. Contraindicated in pregnant women (FDA PI Section 4).
- Known hypersensitivity to letrozole or any excipient — angioedema reported in post-marketing experience.
Relative Contraindications (Specialist Input Recommended)
- Premenopausal status without ovarian suppression — Letrozole alone cannot suppress ovarian oestrogen production. In premenopausal women, use only in combination with a GnRH agonist or after confirmed ovarian suppression.
- Severe osteoporosis (T-score <-2.5 with fragility fracture) — Risk-benefit discussion and concurrent bisphosphonate therapy essential before initiating.
Use with Caution
- Hepatic impairment — Mild-to-moderate: no adjustment (AUC +37%). Severe (cirrhosis): 2.5 mg every other day (FDA PI Section 2.5).
- Pre-existing cardiovascular disease — MI rate higher than tamoxifen (1.0% vs 0.5% during treatment). Optimise modifiable risk factors.
- Pre-existing hyperlipidaemia — Hypercholesterolaemia in 52.3%. Proactive lipid monitoring and management.
Letrozole causes clinically significant decreases in bone mineral density. In the BIG 1-98 adjuvant trial, lumbar spine BMD decreased by a median of 4.1% at 24 months (vs an increase of 0.3% with tamoxifen). The incidence of bone fractures was 14.7% for letrozole vs 11.4% for tamoxifen at median 96-month follow-up. The incidence of osteoporosis was 5.1% vs 2.7%. Consider BMD monitoring and bone-protective strategies (vitamin D, calcium, bisphosphonates when indicated) for all patients on letrozole therapy.
Patient Counselling
Purpose of Therapy
Letrozole works by blocking the body’s ability to make oestrogen, which in turn starves oestrogen-sensitive breast cancer cells. It is taken as one small tablet daily, usually for 5 years (sometimes up to 10 years). Taking the tablet consistently every day for the full duration is important — clinical trials show that women who complete the prescribed course have significantly better long-term outcomes.
How to Take
Take one 2.5 mg tablet by mouth once a day, at approximately the same time each day. It can be taken with or without food. If a dose is missed, take it as soon as remembered unless it is nearly time for the next dose — do not take two tablets at once.
Sources
- Femara (letrozole) tablets — Full Prescribing Information. Novartis Pharmaceuticals Corporation. Revised 12/2024. NDA 020726/S-043. FDA Label (2024)Primary source for all dosing, indications, adverse reactions (BIG 1-98 and FACE trial data), drug interactions, PK data, and monitoring recommendations used in this monograph.
- Femara (letrozole) tablets — DailyMed Prescribing Information. DailyMedSearchable online version of the FDA-approved letrozole label with detailed PK, clinical pharmacology, and clinical studies sections.
- BIG 1-98 Collaborative Group. Letrozole therapy alone or in sequence with tamoxifen in women with breast cancer. N Engl J Med. 2009;361(8):766–776. doi:10.1056/NEJMoa0810818BIG 1-98 trial (N=8,010). Demonstrated letrozole monotherapy superior to tamoxifen monotherapy for DFS (HR 0.87) in postmenopausal HR+ early breast cancer. Source for all BIG 1-98 adverse reaction data.
- Goss PE, Ingle JN, Martino S, et al. Randomized trial of letrozole following tamoxifen as extended adjuvant therapy in receptor-positive breast cancer: updated findings from NCIC CTG MA.17. J Natl Cancer Inst. 2005;97(17):1262–1271. doi:10.1093/jnci/dji250MA-17 trial. Extended adjuvant letrozole after 5 years of tamoxifen significantly improved DFS vs placebo. Established the extended adjuvant indication.
- Regan MM, Neven P, Giobbie-Hurder A, et al. Assessment of letrozole and tamoxifen alone and in sequence for postmenopausal women with steroid hormone receptor-positive breast cancer: the BIG 1-98 randomised clinical trial at 8.1 years median follow-up. Lancet Oncol. 2011;12(12):1101–1108. doi:10.1016/S1470-2045(11)70270-4Long-term BIG 1-98 analysis confirming sustained DFS benefit with letrozole at 8.1-year median follow-up, supporting 5-year adjuvant use.
- Legro RS, Brzyski RG, Diamond MP, et al. Letrozole versus clomiphene for infertility in the polycystic ovary syndrome (AMIGOS). N Engl J Med. 2014;371(2):119–129. doi:10.1056/NEJMoa1313517AMIGOS trial (N=750). Demonstrated letrozole superiority to clomiphene citrate for ovulation induction in PCOS (live birth rate 27.5% vs 19.1%). Supports off-label fertility indication.
- Burstein HJ, Lacchetti C, Griggs JJ, et al. Adjuvant Endocrine Therapy for Women With Hormone Receptor–Positive Breast Cancer: ASCO Clinical Practice Guideline Focused Update. J Clin Oncol. 2019;37(5):423–436. doi:10.1200/JCO.18.01160ASCO guideline recommending an aromatase inhibitor as part of adjuvant endocrine therapy for postmenopausal women with HR+ breast cancer, with duration and sequencing guidance.
- Hadji P, Aapro MS, Body JJ, et al. Management of Aromatase Inhibitor-Associated Bone Loss (AIBL) in postmenopausal women with hormone sensitive breast cancer: joint position statement of the IOF, CABS, ECTS, IEG, ESCEO, IMS, and SIOG. J Bone Oncol. 2017;7:1–12. doi:10.1016/j.jbo.2017.03.001Multi-society consensus on managing AI-associated bone loss: recommends DEXA monitoring, vitamin D/calcium, exercise, and bisphosphonate thresholds.
- Bhatnagar AS. The discovery and mechanism of action of letrozole. Breast Cancer Res Treat. 2007;105 Suppl 1:7–17. doi:10.1007/s10549-007-9696-3Original discovery paper from the Novartis group describing the development, selectivity, and mechanism of action of letrozole as a competitive aromatase inhibitor.
- Geisler J, Haynes B, Anker G, Dowsett M, Lonning PE. Influence of letrozole and anastrozole on total body aromatisation and plasma estrogen levels in postmenopausal breast cancer patients evaluated in a randomised, cross-over study. J Clin Oncol. 2002;20(3):751–757. doi:10.1200/JCO.2002.20.3.751Head-to-head crossover study demonstrating letrozole achieves >99% aromatase inhibition, more potent than anastrozole (>97%), with correspondingly lower residual oestrogen levels.
- Pfister CU, Martoni A, Zamagni C, et al. Effect of age and single versus multiple dose pharmacokinetics of letrozole (Femara) in breast cancer patients. Biopharm Drug Dispos. 2001;22(5):191–197. doi:10.1002/bdd.273PK study confirming no effect of age (35–80+ years) on letrozole pharmacokinetics, supporting no dose adjustment in elderly patients.
- Desta Z, Kreutz Y, Nguyen AT, et al. Plasma letrozole concentrations in postmenopausal women with breast cancer are associated with CYP2A6 genetic variants, body mass index, and age. Clin Pharmacol Ther. 2011;90(5):693–700. doi:10.1038/clpt.2011.174Identifies CYP2A6 polymorphisms and BMI as determinants of letrozole plasma concentrations, with potential implications for dose optimisation.