Source: Medicines and Medical Devices Safety Authority (NZ) Revision Year: 2022 Publisher: Viatris Ltd, PO Box 11-183, Ellerslie, AUCKLAND www.viatris.co.nz Telephone 0800 168 169
Pharmacotherapeutic group: Endocrine therapy. Hormone antagonist and related agents: aromatase inhibitor
ATC code: L02BG04
The elimination of oestrogen-mediated growth stimulation is a prerequisite for tumour response in cases where the growth of tumour tissue depends on the presence of oestrogens and endocrine therapy is used. In postmenopausal women, oestrogens are mainly derived from the action of the aromatase enzyme, which converts adrenal androgens – primarily androstenedione and testosterone – to oestrone and oestradiol. The suppression of oestrogen biosynthesis in peripheral tissues and the cancer tissue itself can, therefore, be achieved by specifically inhibiting the aromatase enzyme.
Letrozole is a non-steroidal aromatase inhibitor. It inhibits the aromatase enzyme by competitively binding to the haem of the aromatase cytochrome P450, resulting in a reduction of oestrogen biosynthesis in all tissues where present.
In healthy postmenopausal women, single doses of 0.1 mg, 0.5 mg and 2.5 mg letrozole suppress serum oestrone and oestradiol by 75%, 78% and 78% from baseline, respectively. Maximum suppression is achieved in 48 to 78 hours.
In postmenopausal patients with advanced breast cancer, daily doses of 0.1 to 5 mg suppressed plasma concentrations of oestradiol, oestrone, and oestrone sulphate by 75 to 95% from baseline in all patients treated. With doses of 0.5 mg and higher, many values of oestrone and oestrone sulphate were below the limit of detection in the assays, indicating that higher oestrogen suppression is achieved with these doses. Oestrogen suppression was maintained throughout treatment in all these patients.
Letrozole is highly specific in inhibiting aromatase activity. Impairment of adrenal steroidogenesis has not been observed. No clinically relevant changes were found in the plasma concentrations of cortisol, aldosterone, 11-deoxycortisol, 17-hydroxy-progesterone and ACTH or in plasma renin activity among postmenopausal patients treated with a daily dose of 0.1 to 5 mg letrozole. The ACTH stimulation test performed after 6 and 12 weeks of treatment with daily doses of 0.1 mg, 0.25 mg, 0.5 mg, 1 mg, 2.5 mg and 5 mg letrozole did not indicate any attenuation of aldosterone or cortisol production. Thus, glucocorticoid and mineralocorticoid supplementation is not necessary.
No changes were noted in plasma concentrations of androgens (androstenedione and testosterone) among healthy postmenopausal women after 0.1 mg, 0.5 mg and 2.5 mg single doses of letrozole or in plasma concentrations of androstenedione among postmenopausal patients treated with daily doses of 0.1 to 5 mg, indicating that the blockade of oestrogen biosynthesis does not lead to accumulation of androgenic precursors. Plasma levels of LH and FSH were not affected by letrozole in patients, nor is thyroid function as evaluated by TSH, T4 and T3 uptake test.
BIG 1-98 was a multicentre, double-blind study in which over 8,000 postmenopausal women with hormone receptor-positive early breast cancer were randomised to one of the following treatments:
The primary endpoint was disease free survival (DFS), secondary efficacy endpoints were time to distant metastasis (TDM), overall survival (OS), distant disease-free survival (DDFS), systemic disease-free survival (SDFS), invasive contralateral breast cancer and time to breast cancer recurrence.
Data in Table 4 reflects results of the Primary Core Analysis (PCA) based on data from the monotherapy arms (A and B) and from the two switching arms (C and D) at a median treatment duration of 24 months and a median follow-up of 26 months and at a median treatment duration of 32 months and a median follow-up of 60 months.
The 5-year DFS rates were 84% for letrozole and 81.4% for tamoxifen.
Table 4. Primary core analysis: disease-free and overall survival at a median follow-up of 26 months and at median follow-up of 60 months (ITT population):
| Primary Core Analysis | ||||||
|---|---|---|---|---|---|---|
| Median follow-up 26 months | Median follow-up 60 months | |||||
| Letrozole N=4003 | Tamoxifen N=4007 | HR1 (95% CI) P | Letrozole N=4003 | Tamoxifen N=4007 | HR1 (95% CI) P | |
| Disease-free survival (primary) - events (protocol definition2) | 351 | 428 | 0.81 (0.70,0.93) 0.003 | 585 | 664 | 0.86 (0.77,0.96) 0.008 |
| Overall survival (secondary) Number of deaths | 166 | 192 | 0.86 (0.70,1.06) | 330 | 374 | 0.87 (0.75,1.01) |
HR = Hazard ratio; CI = Confidence interval
1 Log rank test, stratified by randomisation option and use of chemotherapy (yes/no).
2 DFS events: loco-regional recurrence, distant metastasis, invasive contralateral breast cancer, second (nonbreast) primary malignancy, death from any cause without a prior cancer event.
The Monotherapy Arms Analysis (MAA) long-term update of the efficacy of letrozole monotherapy compared to tamoxifen monotherapy (median duration of adjuvant treatment: 5 years) is presented in Table 5.
Table 5. Monotherapy arms analysis: disease-free and overall survival at a median follow-up of 96 months (ITT population):
| Letrozole N=2463 | Tamoxifen N=2459 | Hazard Ratio1 (95% CI) | P Value | |
|---|---|---|---|---|
| Disease-free survival events (primary)2 | 626 | 698 | 0.87 (0.78, 0.97) | 0.01 |
| Time to distant metastasis (secondary) | 301 | 342 | 0.86 (0.74, 1.01) | 0.06 |
| Overall survival (secondary) - deaths | 393 | 436 | 0.89 (0.77, 1.02) | 0.08 |
| Censored analysis of DFS3 | 626 | 649 | 0.83 (0.74, 0.92) | |
| Censored analysis of OS3 | 393 | 419 | 0.81 (0.70, 0.93) |
1 Log rank test, stratified by randomisation option and use of chemotherapy (yes/no).
2 DFS events: loco-regional recurrence, distant metastasis, invasive contralateral breast cancer, second (non-breast) primary malignancy, death from any cause without a prior cancer event.
3 Observations in the tamoxifen arm censored at the date of selectivity switching to letrozole.
The sequential treatments analysis (STA) addresses the second primary question of BIG 1-98, namely whether sequencing of tamoxifen and letrozole would be superior to monotherapy. There were no significant differences in DFS, OS, SDFS, or DDFS from switch with respect to monotherapy (Table 6).
Table 6. Sequential treatments analysis of disease-free survival with letrozole as initial endocrine agent (STA switch population):
| N | Number of events1 | Hazard ratio2 | (97.5% confidence interval) | Cox model P-value | |
|---|---|---|---|---|---|
| [Letrozole →] Tamoxifen | 1460 | 254 | 1.03 | (0.84, 1.26) | 0.72 |
| Letrozole | 1464 | 249 |
1 Protocol definition, including second non-breast primary malignancies, after switch/beyond two years
2 Adjusted by chemotherapy use
There were no significant differences in DFS, OS, SDFS or DDFS in any of the STA from randomisation pairwise comparisons (Table 7).
Table 7. Sequential treatments analyses from randomisation (STA-R) of disease-free survival (ITT STA-R population):
| Letrozole → Tamoxifen | Letrozole | |
| Number of patients | 1540 | 1546 |
| Number of patients with DFS events (protocol definition) | 330 | 319 |
| Hazard ratio1 (99% CI) | 1.04 (0.85, 1.27) | |
| Letrozole → Tamoxifen | Tamoxifen2 | |
| Number of patients | 1540 | 1548 |
| Number of patients with DFS events (protocol definition) | 330 | 353 |
| Hazard ratio1 (99% CI) | 0.92 (0.75, 1.12) | |
1 Adjusted by chemotherapy use (yes/no)
2 626 (40%) patients selectively crossed to letrozole after tamoxifen arm unblinded in 2005
Study D2407 is an open-label, randomized, multicentre post approval safety study designed to compare the effects of adjuvant treatment with letrozole and tamoxifen on bone mineral density (BMD) and serum lipid profiles. A total of 262 patients were assigned either letrozole for 5 years or tamoxifen for 2 years followed by letrozole for 3 years.
At 24 months there was a statistically significant difference in the primary end-point; the lumbar spine (L2-L4) BMD showed a median decrease of 4.1% for letrozole compared to a median increase of 0.3% for tamoxifen.
No patient with a normal BMD at baseline became osteoporotic during 2 years of treatment and only 1 patient with osteopenia at baseline (T score of -1.9) developed osteoporosis during the treatment period (assessment by central review).
The results for total hip BMD were similar to those for lumbar spine but less pronounced. There was no significant difference between treatments in the rate of fractures – 15% in the letrozole arm, 17% in the tamoxifen arm.
Median total cholesterol levels in the tamoxifen arm were decreased by 16% after 6 months compared to baseline and this decrease was maintained at subsequent visits up to 24 months. In the letrozole arm, total cholesterol levels were relatively stable over time, giving a statistically significant difference in favour of tamoxifen at each time point.
In a multicentre, double-blind, randomized, placebo-controlled study (MA-17), over 5,100 postmenopausal women with receptor-positive or unknown primary breast cancer who had completed adjuvant treatment with tamoxifen (4.5 to 6 years) were randomised to either letrozole or placebo for 5 years.
The primary endpoint was disease-free survival, defined as the interval between randomisation and the earliest occurrence of loco-regional recurrence, distant metastasis, or contralateral breast cancer.
The first planned interim analysis at a median follow-up of around 28 months (25% of the patients being followed-up for at least 38 months) showed that letrozole significantly reduced the risk of breast cancer recurrence by 42% compared with placebo (hazard ratio 0.58; 95% CI 0.45, 0.76; P=0.00003). The benefit in favour of letrozole was observed regardless of nodal status. There was no significant difference in overall survival: (letrozole 51 deaths; placebo 62; HR 0.82; 95% CI 0.56, 1.19).
Consequently, after the first interim analysis the study was unblinded and continued in an open-label fashion and patients in the placebo arm were allowed to switch to letrozole for up to 5 years. Over 60% of eligible patients (disease-free at unblinding) opted to switch to letrozole. The final analysis included 1,551 women who switched from placebo to letrozole at a median of 31 months (range 12 to 106 months) after completion of tamoxifen adjuvant therapy. Median duration for letrozole after switch was 40 months.
The final analysis conducted at a median follow-up of 62 months confirmed the significant reduction in the risk of breast cancer recurrence with letrozole.
Table 8. Disease-free and overall survival (Modified ITT population):
| Median follow-up 28 months1 | Median follow-up 62 months | |||||
|---|---|---|---|---|---|---|
| Letrozole | Placebo | HR (95% CI)2 | Letrozole | Placebo | HR (95% CI)2 | |
| N=2582 | N=2586 | P value | N=2582 | N=2586 | P value | |
| Disease-free survival3 | ||||||
| Events | 92 (3.6%) | 155 (6.0%) | 0.58 (0.45, 0.76) 0.00003 | 209 (8.1%) | 286 (11.1%) | 0.75 (0.63, 0.89) |
| 4-year DFS rate | 94.4% | 89.8% | 94.4% | 91.4% | ||
| Disease-free survival3, including deaths from any cause | ||||||
| Events | 122 (4.7%) | 193 (7.5%) | 0.62 (0.49, 0.78) | 344 (13.3%) | 402 (15.5%) | 0.89 (0.77, 1.03) |
| 5-year DFS rate | 90.5% | 80.8% | 88.8% | 86.7% | ||
| Distant metastases | ||||||
| Events | 57 (2.2%) | 93 (3.6%) | 0.61 (0.44, 0.84) | 142 (5.5%) | 169 (6.5%) | 0.88 (0.70, 1.10) |
| Overall survival | ||||||
| Deaths | 51 (2.0%) | 62 (2.4%) | 0.82 (0.56, 1.19) | 236 (9.1%) | 232 (9.0%) | 1.13 (0.95, 1.36) |
| Deaths4 | -- | -- | -- | 2365 (9.1%) | 1706 (6.6%) | 0.78 (0.64, 0.96) |
HR = Hazards ration; CI = confidence interval
1 When the study was unblinded in 2003, 1551 patients in the randomized placebo arm (60% of those eligible to switch – i.e. who were disease-free) switched to letrozole at a median 31 months after randomization. The analyses presented here ignore the selective crossover.
2 Stratified by receptor status, nodal status and prior adjuvant chemotherapy.
3 Protocol definition of disease-free survival events: loco-regional recurrence, distant metastasis or contralateral breast cancer.
4 Exploratory analysis, censoring follow-up times at the date of switch (if it occurred) in the placebo arm.
5 Median follow-up 62 months.
6 Median follow-up until switch (if it occurred) 37 months.
In the MA-17 bone substudy in which concomitant calcium and vitamin D were given, greater decreases in BMD compared to baseline occurred with letrozole compared with placebo. The only statistically significant difference occurred at 2 years and was in total hip BMD (letrozole median decrease of 3.8% vs placebo median decrease of 2.0%).
In the MA-17 lipid substudy there were no significant differences between letrozole and placebo in total cholesterol or in any lipid fraction.
In the updated quality of life substudy there were no significant differences between treatments in physical component summary score or mental component summary score, or in any domain score in the SF-36 scale. In the MENQOL scale, significantly more women in the letrozole arm than in the placebo arm were most bothered (generally in the first year of treatment) by those symptoms deriving from oestrogen deprivation – hot flushes and vaginal dryness. The symptom that bothered most patients in both treatment arms was aching muscles, with a statistically significant difference in favour of placebo.
One controlled double-blind trial was conducted comparing letrozole 2.5 mg to tamoxifen 20 mg as first-line therapy in postmenopausal women with advanced breast cancer. In 907 women, letrozole was superior to tamoxifen in time to progression (primary endpoint) and in overall objective tumour response, time to treatment failure and clinical benefit. The results are summarised in Table 9.
Table 9. Results at a median follow-up of 32 months:
| Variable | Statistic | Letrozole N=453 | Tamoxifen N=454 |
|---|---|---|---|
| Time to progression | Median | 9.4 months | 6.0 months |
| (95% CI for median) | (8.9, 11.6 months) | (5.4, 6.3 months) | |
| Hazard ratio (HR) | 0.72 | ||
| (95% CI for HR) | (0.62, 0.83) | ||
| P<0.0001 | |||
| Objective response rate (ORR) | CR+PR | 145 (32%) | 95 (21%) |
| (95% CI for rate) | (28, 36%) | (17, 25%) | |
| Odds ratio | 1.78 | ||
| (95% CI for odds ratio) | (1.32, 2.40) | ||
| P=0.0002 | |||
Time to progression was significantly longer, and response rate significantly higher for letrozole irrespective of whether adjuvant anti-oestrogen therapy had been given or not. Time to progression was significantly longer for letrozole irrespective of dominant site of disease. Median time to progression was 12.1 months for letrozole and 6.4 months for tamoxifen in patients with soft tissue disease only and median 8.3 months for letrozole and 4.6 months for tamoxifen in patients with visceral metastases.
Study design allowed patients to cross over upon progression to the other therapy or discontinue from the study. Approximately 50% of patients crossed over to the opposite treatment arm and crossover was virtually completed by 36 months. The median time to crossover was 17 months (letrozole to tamoxifen) and 13 months (tamoxifen to letrozole).
Letrozole treatment in the first-line therapy of advanced breast cancer resulted in a median overall survival of 34 months compared with 30 months for tamoxifen (logrank test P=0.53, not significant). The absence of an advantage for letrozole on overall survival could be explained by the crossover design of the study.
Two well-controlled clinical trials were conducted comparing two letrozole doses (0.5 mg and 2.5 mg) to megestrol acetate and to aminoglutethimide, respectively, in postmenopausal women with advanced breast cancer previously treated with anti-oestrogens.
Time to progression was not significantly different between letrozole 2.5 mg and megestrol acetate (P=0.07). Statistically significant differences were observed in favour of letrozole 2.5 mg compared to megestrol acetate in overall objective tumour response rate (24% vs 16%, P=0.04), and in time to treatment failure (P=0.04). Overall survival was not significantly different between the 2 arms (P=0.2).
In the second study, the response rate was not significantly different between letrozole 2.5 mg and aminoglutethimide (P=0.06). Letrozole 2.5 mg was statistically superior to aminoglutethimide for time to progression (P=0.008), time to treatment failure (P=0.003) and overall survival (P=0.002).
The safety and efficacy of letrozole has not been demonstrated in the neoadjuvant treatment of breast cancer.
Use of letrozole in men with breast cancer has not been studied.
Letrozole is rapidly and completely absorbed from the gastrointestinal tract (mean absolute bioavailability: 99.9%). Food slightly decreases the rate of absorption (median tmax 1 hour fasted versus 2 hours fed; and mean Cmax 129 ± 20.3 nmol/L fasted versus 98.7 ± 18.6 nmol/L fed), but the extent of absorption (AUC) is not changed. The minor effect on the absorption rate is not considered to be of clinical relevance, and therefore letrozole may be taken without regard to meal times.
Plasma protein binding of letrozole is approximately 60%, mainly to albumin (55%). The concentration of letrozole in erythrocytes is about 80% of that in plasma. After administration of 2.5 mg 14C-labelled letrozole, approximately 82% of the radioactivity in plasma was unchanged compound. Systemic exposure to metabolites is therefore low. Letrozole is rapidly and extensively distributed to tissues. Its apparent volume of distribution at steady state is about 1.87 ± 0.47 L/kg.
Metabolic clearance to a pharmacologically inactive carbinol metabolite is the major elimination pathway of letrozole (CLm= 2.1 L/h) but is relatively slow when compared to hepatic blood flow (about 90 L/h). The cytochrome P450 isoenzymes 3A4 and 2A6 were found to be capable of converting letrozole to this metabolite. Formation of minor unidentified metabolites, and direct renal and faecal excretion play only a minor role in the overall elimination of letrozole. Within 2 weeks after administration of 2.5 mg 14C-labelled letrozole to healthy postmenopausal volunteers, 88.2 ± 7.6% of the radioactivity was recovered in urine and 3.8 ± 0.9% in faeces. At least 75% of the radioactivity recovered in urine up to 216 hours (84.7 ± 7.8% of the dose) was attributed to the glucuronide of the carbinol metabolite, about 9% to two unidentified metabolites, and 6% to unchanged letrozole.
The apparent terminal elimination half-life in plasma is about 2 to 4 days. After daily administration of 2.5 mg, steady-state levels are reached within 2 to 6 weeks. Plasma concentrations at steady state are approximately 7 times higher than concentrations measured after a single dose of 2.5 mg, while they are 1.5 to 2 times higher than the steady-state values predicted from the concentrations measured after a single dose, indicating a slight non-linearity in the pharmacokinetics of letrozole upon daily administration of 2.5 mg. Since steady-state levels are maintained over time, it can be concluded that no continuous accumulation of letrozole occurs.
The pharmacokinetics of letrozole were dose proportional after single oral doses up to 10 mg (dose range: 0.01 to 30 mg) and after daily doses up to 1.0 mg (dose range: 0.1 to 5 mg). After a 30 mg single, oral dose there was a slightly dose over-proportional increase in AUC value. The dose over-proportionality is likely to be the result of a saturation of metabolic elimination processes. Steady levels were reached after 1 to 2 months at all dosage regimens tested (0.1 to 5.0 mg daily).
Age had no effect on the pharmacokinetics of letrozole.
In a study involving 19 volunteers with varying degrees of renal function (24-hour creatinine clearance 9 to 116 mL/min), no effect on the pharmacokinetics of letrozole was found after a single dose of 2.5 mg. In addition to the above study assessing the influence of renal impairment on letrozole, a covariate analysis was performed on the data of two pivotal studies (Study AR/BC2 and Study AR/BC3). Calculated creatinine clearance (CLcr) [Study AR/BC2 range: 19 to 187 mL/min; Study AR/BC3 range: 10 to 180 mL/min] showed no statistically significant association between letrozole plasma trough levels at steady-state (Cmin). Furthermore, data of these studies in secondline metastatic breast cancer showed no evidence of an adverse effect of letrozole on CLcr or an impairment of renal function.
Therefore, no dose adjustment is required for patients with renal impairment (CLcr ≥10 mL/min). Little information is available in patients with severe impairment of renal function (CLcr <10 mL/min).
In a similar study involving subjects with varying degrees of hepatic function, the mean AUC values of the volunteers with moderate hepatic impairment (Child-Pugh score B) was 37% higher than in normal subjects, but still within the range seen in subjects without impaired function. In a study comparing the pharmacokinetics of letrozole after a single oral dose in eight subjects with liver cirrhosis and severe hepatic impairment (Child-Pugh score C) to those in healthy volunteers (n=8), AUC and t½ increased by 95 and 187%, respectively. Thus, letrozole should be administered with caution to patients with severe hepatic impairment and after consideration of the risk/benefit in the individual patient.
In a variety of preclinical safety studies conducted in standard animal species, there was no evidence of systemic or target organ toxicity.
Letrozole showed a low degree of acute toxicity in rodents exposed up to 2000 mg/kg. In dogs letrozole caused signs of moderate toxicity at 100 mg/kg.
In repeated-dose toxicity studies in rats and dogs up to 12 months, the main findings observed can be attributed to the pharmacological action of the compound. The no-adverse-effect level was 0.3 mg/kg in both species.
Oral administration of letrozole to female rats resulted in decreases in mating and pregnancy ratios and increases in pre-implantation loss.
Both in vitro and in vivo investigations on letrozole’s mutagenic potential revealed no indications of any genotoxicity.
In a 104-week rat carcinogenicity study, no treatment-related tumours were noted in male rats. In female rats, a reduced incidence of benign and malignant mammary tumours at all the doses of letrozole was found.
In a 104-week mouse carcinogenicity study, no treatment-related tumours were noted in male mice. In female mice, a generally dose-related increase in the incidence of benign ovarian granulosa theca cell tumours was observed at all doses of letrozole tested. These tumours were considered to be related to the pharmacological inhibition of oestrogen synthesis and may be due to increased LH resulting from the decrease in circulating oestrogen.
Letrozole was embryotoxic and fetotoxic in pregnant rats and rabbits following oral administration at clinically relevant doses. In rats that had live foetuses, there was an increase in the incidence of foetal malformations including domed head and cervical/centrum vertebral fusion. An increased incidence of foetal malformations was not seen in the rabbit. It is not known whether this was an indirect consequence of the pharmacological properties (inhibition of oestrogen biosynthesis) or a direct drug effect (see sections 4.3 and 4.6).
Preclinical observations were confined to those associated with the recognised pharmacological action, which is the only safety concern for human use derived from animal studies.
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