Letrozole

Letrozole (Femara®), a nonsteroidal, third-generation aromatase inhibitor administered orally once daily, has shown efficacy in the treatment of postmenopausal women with early-stage or advanced, hormone-sensitive breast cancer.

In early-stage disease, extending adjuvant endocrine therapy with letrozole (beyond the standard 5-year period of tamoxifen) improved disease-free survival; compared with placebo there was a 43% relative reduction in disease recurrences or new contralateral breast tumours at a median follow-up of 2.4 years. The results of 4 months’ neoadjuvant treatment with letrozole or tamoxifen in postmenopausal women with untreated primary disease favour letrozole.

In advanced breast cancer, letrozole was superior to tamoxifen as first-line treatment; time to disease progression was significantly longer (9.4 vs 6.0 months, p < 0.0001) and objective response rate was significantly greater with letrozole, but median overall survival was similar between groups. For second-line therapy of advanced breast cancer that had progressed on antiestrogen therapy, letrozole showed efficacy equivalent to that of anastrozole and similar to or better than that of megestrol acetate.

Letrozole is generally well tolerated and has a similar tolerability profile to tamoxifen; the most common treatment-related adverse events were hot flushes, nausea and hair thinning. In patients with tumours that had progressed on antiestrogen therapy, letrozole was tolerated as least as well as, or better than, anastrozole or megestrol acetate. In the trial of extended adjuvant therapy, adverse events reported more frequently with letrozole than placebo were hot flushes, arthralgia, myalgia and arthritis. The long-term effects of letrozole on bone mineral density or lipid profile have not been determined and these parameters may require monitoring.

In several pharmacoeconomic modelling studies from various public healthcare system perspectives, letrozole was considered a cost effective choice for first-line (vs tamoxifen) or second-line (vs megestrol acetate) treatment for advanced breast cancer in postmenopausal women.

In conclusion, letrozole 2.5 mg/day is effective in the treatment of postmenopausal women with early-stage or advanced breast cancer. The efficacy, cost effectiveness and favourable tolerability profile of letrozole are reflected in current treatment guidelines recommending the drug as first-line therapy for advanced breast cancer. Letrozole is superior to tamoxifen for first-line treatment and is at least as effective as standard second-line treatments in disease that has progressed on antiestrogen therapy. For early-stage disease, letrozole is superior to tamoxifen in the neoadjuvant setting, and prolongs disease-free survival when administered after the standard 5-year period of adjuvant tamoxifen therapy.

Letrozole is a highly specific, nonsteroidal aromatase inhibitor and a potent inhibitor of estrogen synthesis. Plasma levels of estrone, estradiol and estrone sulfate were significantly reduced by letrozole in postmenopausal women with breast cancer. Letrozole had anti-tumour effects in ovariectomised mouse models of postmenopausal estrogen-dependent breast cancer, and it reduced cellular markers of proliferation more than tamoxifen in estrogen-dependent tumours that overexpressed human epidermal growth factor receptor (HER)1 and/or HER2. Levels of bone resorption markers were increased with letrozole in healthy postmenopausal women and those with a history of breast disease.

In postmenopausal women with breast cancer, letrozole is rapidly and almost completely absorbed from the gastrointestinal tract. Food has no clinically significant effect on the absorption. The mean maximum plasma concentration following a single oral 2.5mg dose was 107 nmol/L and was reached after a median of ≈2 hours. Steady-state plasma concentrations were attained after approximately 40 days.

Letrozole is about 60% plasma-protein bound and has a large volume of distribution (1.87 L/kg). The drug is metabolised via the cytochrome P450 enzyme system and is excreted mainly in the urine. The mean elimination half-life was 82 hours.

Concurrent administration of tamoxifen and letrozole reduced the plasma levels of letrozole compared with those of letrozole administered alone.

Letrozole 2.5 mg/day (administered to postmenopausal women participating in randomised, double-blind studies) has efficacy as first- and second-line hormonal therapy in advanced breast cancer and as adjuvant and neoadjuvant therapy in early-stage breast cancer.

Extended adjuvant therapy (randomised and double-blind) with letrozole following the standard 5 years of adjuvant tamoxifen or antiestrogen therapy improved disease-free survival compared with placebo in postmenopausal women with early-stage primary breast cancer. Early beneficial effects of letrozole prompted the unblinding of this trial at a median of 2.4 years, although patients receiving letrozole will still be monitored for the planned 5-year study period.

The results of 4 months’ randomised and double-blind neoadjuvant treatment with letrozole or tamoxifen in postmenopausal women with untreated early-stage primary disease favour letrozole; the letrozole group had a greater objective response rate and more letrozole recipients underwent breast-conserving surgery compared with tamoxifen recipients.

In advanced breast cancer, letrozole was superior to tamoxifen as first-line treatment in a large, well controlled clinical study in which approximately half of the patients in each group crossed over to the other therapy on disease progression. Time to disease progression (primary endpoint) was longer with letrozole than tamoxifen (9.4 vs 6.0 months, p < 0.0001) and the objective response rate was also greater. Median overall survival was similar between groups.

Large randomised trials of second-line therapy (following antiestrogen therapy) in postmenopausal women with advanced breast cancer indicated that letrozole was at least as effective as anastrozole or megestrol acetate for primary endpoints and was superior for some secondary endpoints.

Pharmacoeconomic modelling studies conducted from the perspective of North American or European public healthcare systems indicate that letrozole was cost effective for first-line (compared with tamoxifen) or second-line (compared with megestrol acetate) treatment for advanced breast cancer in postmenopausal women. These studies considered direct costs and used data inputs from clinical trials, the published literature and expert opinion. Incremental costs per life-year or quality-adjusted life-year (QALY) gained with letrozole were well below generally accepted thresholds for cost effectiveness; incremental costs per QALY gained for letrozole as first-line treatment (relative to tamoxifen) were $Canl2 500, £2927–£3969 or £8514 and per life-year gained for the drug as second-line treatment (relative to megestrol acetate) were $Can9000, $Can5051 or £3588.

In large trials in postmenopausal women with early-stage or advanced breast cancer, letrozole was at least as well tolerated as tamoxifen, anastrozole and megestrol acetate and had a similar adverse events profile to tamoxifen and anastrozole. Data from several studies indicated that the most common treatment-related adverse events with letrozole were hot flushes 5–20%, nausea 5–11% and headache 7–11%. The following adverse events were reported in only one study: fatigue 5%, peripheral oedema 6%, dyspnoea 10% and back pain 15%.

Adverse events occurring significantly more frequently with letrozole than placebo (when administered as extended adjuvant therapy following tamoxifen therapy) were hot flushes, arthralgia, myalgia and arthritis. Vaginal bleeding occurred less frequently with letrozole than with placebo.

The most commonly occurring treatment-related adverse events with letrozole or tamoxifen during first-line treatment for advanced disease or as neoadjuvant therapy in primary untreated disease were hot flushes, nausea and hair thinning (incidence similar in both groups). The incidence of thromboembolic events was similar between groups.

Letrozole was tolerated as well as or better than anastrozole or megestrol acetate in trials of postmenopausal women with breast cancer that had been treated with an antiestrogen.

There are no long-term data on the effects of letrozole therapy on bone mineral density and blood lipid levels; monitoring of these parameters in women on long-term therapy is prudent.