CLINICAL PHARMACOLOGY
Mechanism Of Action
Breast cancer cell growth may
be estrogen-dependent. Aromatase is the principal enzyme that converts
androgens to estrogens both in pre-and postmenopausal women. While the main
source of estrogen (primarily estradiol) is the ovary in premenopausal women,
the principal source of circulating estrogens in postmenopausal women is from
conversion of adrenal and ovarian androgens (androstenedione and testosterone)
to estrogens (estrone and estradiol) by the aromatase enzyme in peripheral
tissues.
Exemestane is an irreversible,
steroidal aromatase inactivator, structurally related to the natural substrate
androstenedione. It acts as a false substrate for the aromatase enzyme, and is
processed to an intermediate that binds irreversibly to the active site of the
enzyme, causing its inactivation, an effect also known as “suicide inhibition.”
Exemestane significantly lowers circulating estrogen concentrations in
postmenopausal women, but has no detectable effect on adrenal biosynthesis of
corticosteroids or aldosterone. Exemestane has no effect on other enzymes
involved in the steroidogenic pathway up to a concentration at least 600 times
higher than that inhibiting the aromatase enzyme.
Pharmacodynamics
Effect On Estrogens
Multiple doses of exemestane
ranging from 0.5 to 600 mg/day were administered to postmenopausal women with
advanced breast cancer. Plasma estrogen (estradiol, estrone, and estrone
sulfate) suppression was seen starting at a 5-mg daily dose of
exemestane, with a maximum suppression of at least 85% to 95% achieved at a
25-mg dose. Exemestane 25 mg daily reduced whole body aromatization (as
measured by injecting radiolabeled androstenedione) by 98% in postmenopausal
women with breast cancer. After a single dose of exemestane 25 mg, the maximal
suppression of circulating estrogens occurred 2 to 3 days after dosing and
persisted for 4 to 5 days.
Effect On Corticosteroids
In multiple-dose trials of doses up to 200 mg daily,
exemestane selectivity was assessed by examining its effect on adrenal steroids.
Exemestane did not affect cortisol or aldosterone secretion at baseline or in
response to ACTH at any dose. Thus, no glucocorticoid or mineralocorticoid
replacement therapy is necessary with exemestane treatment.
Other Endocrine Effects
Exemestane does not bind significantly to steroidal
receptors, except for a slight affinity for the androgen receptor (0.28%
relative to dihydrotestosterone). The binding affinity of its
17dihydrometabolite for the androgen receptor, however, is 100 times that of
the parent compound. Daily doses of exemestane up to 25 mg had no significant
effect on circulating levels of androstenedione, dehydroepiandrosterone
sulfate, or 17-hydroxyprogesterone, and were associated with small decreases in
circulating levels of testosterone. Increases in testosterone and
androstenedione levels have been observed at daily doses of 200 mg or more. A
dose-dependent decrease in sex hormone binding globulin (SHBG) has been
observed with daily exemestane doses of 2.5 mg or higher. Slight, nondose-dependent
increases in serum luteinizing hormone (LH) and follicle-stimulating hormone
(FSH) levels have been observed even at low doses as a consequence of feedback
at the pituitary level. Exemestane 25 mg daily had no significant effect on
thyroid function [free triiodothyronine (FT3), free thyroxine (FT4), and
thyroid stimulating hormone (TSH)].
Coagulation And Lipid Effects
In study 027 of postmenopausal women with early breast
cancer treated with exemestane (N=73) or placebo (N=73), there was no change in
the coagulation parameters activated partial thromboplastin time [APTT],
prothrombin time [PT], and fibrinogen. Plasma HDL cholesterol was decreased
6–9% in exemestane treated patients; total cholesterol, LDL cholesterol,
triglycerides, apolipoprotein-A1, apolipoprotein-B, and lipoprotein-a were
unchanged. An 18% increase in homocysteine levels was also observed in
exemestane treated patients compared with a 12% increase seen with placebo.
Pharmacokinetics
Following oral administration to healthy postmenopausal
women, plasma concentrations of exemestane decline polyexponentially with a
mean terminal half-life of about 24 hours. The pharmacokinetics of exemestane
are dose proportional after single (10 to 200 mg) or repeated oral doses (0.5 to
50 mg). Following repeated daily doses of exemestane 25 mg, plasma
concentrations of unchanged drug are similar to levels measured after a single
dose. Pharmacokinetic parameters in postmenopausal women with advanced breast
cancer following single or repeated doses have been compared with those in
healthy, postmenopausal women. After repeated dosing, the average oral
clearance in women with advanced breast cancer was 45% lower than the oral
clearance in healthy postmenopausal women, with corresponding higher systemic
exposure. Mean AUC values following repeated doses in women with breast cancer
(75.4 ng•h/mL) were about twice those in healthy women (41.4
ng•h/mL).
Absorption
Following oral administration, exemestane appeared to be
absorbed more rapidly in women with breast cancer than in the healthy women,
with a mean tmax of 1.2 hours in the women with breast cancer and 2.9 hours in
healthy women. Approximately 42% of radiolabeled exemestane was absorbed from
the gastrointestinal tract. A high-fat breakfast increased AUC and Cmax of
exemestane by 59% and 39%, respectively, compared to fasted state.
Distribution
Exemestane is distributed extensively into tissues.
Exemestane is 90% bound to plasma proteins and the fraction bound is
independent of the total concentration. Albumin and α11-acid glycoprotein
both contribute to the binding. The distribution of exemestane and its
metabolites into blood cells is negligible.
Metabolism
Exemestane is extensively metabolized, with levels of the
unchanged drug in plasma accounting for less than 10% of the total
radioactivity. The initial steps in the metabolism of exemestane are oxidation
of the methylene group in position 6 and reduction of the 17-keto group with
subsequent formation of many secondary metabolites. Each metabolite accounts
only for a limited amount of drug-related material. The metabolites are
inactive or inhibit aromatase with decreased potency compared with the parent
drug. One metabolite may have androgenic activity [see CLINICAL PHARMACOLOGY].
Studies using human liver preparations indicate that cytochrome P 450 3A4 (CYP
3A4) is the principal isoenzyme involved in the oxidation of exemestane.
Exemestane is metabolized also by aldoketoreductases.
Elimination
Following administration of radiolabeled exemestane to
healthy postmenopausal women, the cumulative amounts of radioactivity excreted
in urine and feces were similar (42 ± 3% in urine and 42 ± 6% in feces over a
1-week collection period). The amount of drug excreted unchanged in urine was
less than 1% of the dose.
Specific Populations
Geriatric
Healthy postmenopausal women aged 43 to 68 years were
studied in the pharmacokinetic trials. Age-related alterations in exemestane
pharmacokinetics were not seen over this age range.
Gender
The pharmacokinetics of exemestane following
administration of a single, 25-mg tablet to fasted healthy males (mean age 32
years) were similar to the pharmacokinetics of exemestane in fasted healthy
postmenopausal women (mean age 55 years).
Race
The influence of race on exemestane pharmacokinetics has
not been evaluated.
Hepatic Impairment
The pharmacokinetics of exemestane have been investigated
in subjects with moderate or severe hepatic impairment (Childs-Pugh B or C).
Following a single 25-mg oral dose, the AUC of exemestane was approximately 3
times higher than that observed in healthy volunteers.
Renal Impairment
The AUC of exemestane after a single 25-mg dose was
approximately 3 times higher in subjects with moderate or severe renal
insufficiency (creatinine clearance <35 mL/min/1.73 m²) compared with the
AUC in healthy volunteers.
Pediatric
The pharmacokinetics of exemestane have not been studied
in pediatric patients.
Drug Interactions
Exemestane does not inhibit any of the major CYP
isoenzymes, including CYP 1A2, 2C9, 2D6, 2E1, and 3A4.
In a pharmacokinetic interaction study of 10 healthy
postmenopausal volunteers pretreated with potent CYP 3A4 inducer rifampicin 600
mg daily for 14 days followed by a single dose of exemestane 25 mg, the mean
plasma Cmax and AUC 0–∞ of exemestane were decreased by 41% and 54%,
respectively [see DOSAGE AND ADMINISTRATION and DRUG INTERACTIONS].
In a clinical pharmacokinetic study, coadministration of
ketoconazole, a potent inhibitor of CYP 3A4, has no significant effect on
exemestane pharmacokinetics. Although no other formal drug-drug interaction
studies with inhibitors have been conducted, significant effects on exemestane
clearance by CYP isoenzyme inhibitors appear unlikely.
Clinical Studies
Adjuvant Treatment In Early Breast Cancer
The Intergroup Exemestane Study 031 (IES) was a
randomized, double-blind, multicenter, multinational study comparing exemestane
(25 mg/day) vs. tamoxifen (20 or 30 mg/day) in postmenopausal women with early
breast cancer. Patients who remained disease-free after receiving adjuvant
tamoxifen therapy for 2 to 3 years were randomized to receive an additional 3
or 2 years of AROMASIN or tamoxifen to complete a total of 5 years of hormonal
therapy.
The primary objective of the study was to determine
whether, in terms of disease-free survival, it was more effective to switch to
AROMASIN rather than continuing tamoxifen therapy for the remainder of five
years. Disease-free survival was defined as the time from randomization to time
of local or distant recurrence of breast cancer, contralateral invasive breast
cancer, or death from any cause.
The secondary objectives were to compare the two regimens
in terms of overall survival and long-term tolerability. Time to contralateral
invasive breast cancer and distant recurrence-free survival were also
evaluated.
A total of 4724 patients in the intent-to-treat (ITT)
analysis were randomized to AROMASIN (exemestane tablets) 25 mg once daily (N =
2352) or to continue to receive tamoxifen once daily at the same dose received
before randomization (N = 2372). Demographics and baseline tumor
characteristics are presented in Table 5. Prior breast cancer therapy is
summarized in Table 6.
Table 5: Demographic and Baseline Tumor
Characteristics from the IES Study of Postmenopausal Women with Early Breast
Cancer (ITT Population)
Parameter |
Exemestane
(N = 2352) |
Tamoxifen
(N = 2372) |
Age (years): Median age (range) |
63.0 (38.0 - 96.0) |
63.0 (31.0 - 90.0) |
Race, n (%): |
Caucasian |
2315 (98.4) |
2333 (98.4) |
Hispanic |
13 (0.6) |
13 (0.5) |
Asian |
10 (0.4) |
9 (0.4) |
Black |
7 (0.3) |
10 (0.4) |
Other/not reported |
7 (0.3) |
7 (0.3) |
Nodal status, n (%): |
Negative |
1217 (51.7) |
1228 (51.8) |
Positive |
1051 (44.7) |
1044 (44.0) |
1-3 Positive nodes |
721 (30.7) |
708 (29.8) |
4-9 Positive nodes |
239 (10.2) |
244 (10.3) |
>9 Positive nodes |
88 (3.7) |
86 (3.6) |
Not reported |
3 (0.1) |
6 (0.3) |
Unknown or missing |
84 (3.6) |
100 (4.2) |
Histologic type, n (%): |
Infiltrating ductal |
1777 (75.6) |
1830 (77.2) |
Infiltrating lobular |
341 (14.5) |
321 (13.5) |
Other |
231 (9.8) |
213 (9.0) |
Unknown or missing |
3 (0.1) |
8 (0.3) |
Receptor status*, n (%): |
ER and PgR Positive |
1331 (56.6) |
1319 (55.6) |
ER Positive and PgR Negative/Unknown |
677 (28.8) |
692 (29.2) |
ER Unknown and PgR Positive**/Unknown |
288 (12.2) |
291 (12.3) |
ER Negative and PgR Positive |
6 (0.3) |
7 (0.3) |
ER Negative and PgR Negative/Unknown (none positive) |
48 (2.0) |
58 (2.4) |
Missing |
2 (0.1) |
5 (0.2) |
Tumor Size, n (%): |
≤ 0.5 cm |
58 (2.5) |
46 (1.9) |
> 0.5 - 1.0 cm |
315 (13.4) |
302 (12.7) |
> 1.0 - 2 cm |
1031 (43.8) |
1033 (43.5) |
> 2.0 - 5.0 cm |
833 (35.4) |
883 (37.2) |
> 5.0 cm |
62 (2.6) |
59 (2.5) |
Not reported |
53 (2.3) |
49 (2.1) |
Tumor Grade, n (%): |
G1 |
397 (16.9) |
393 (16.6) |
G2 |
977 (41.5) |
1007 (42.5) |
G3 |
454 (19.3) |
428 (18.0) |
G4 |
23 (1.0) |
19 (0.8) |
Unknown/Not Assessed/Not reported |
501 (21.3) |
525 (22.1) |
* Results for receptor status include the results of the
post-randomization testing of specimens from subjects for whom receptor status
was unknown at randomization.
** Only one subject in the exemestane group had unknown ER status and positive
PgR status. |
Table 6: Prior Breast Cancer Therapy of Patients in
the IES Study of Postmenopausal Women with Early  Breast Cancer (ITT
Population)
Parameter |
Exemestane
(N = 2352) |
Tamoxifen
(N = 2372) |
Type of surgery, n (%): |
Mastectomy |
1232 (52.4) |
1242 (52.4) |
Breast-conserving |
1116 (47.4) |
1123 (47.3) |
Unknown or missing |
4 (0.2) |
7 (0.3) |
Radiotherapy to the breast, n (%): |
Yes |
1524 (64.8) |
1523 (64.2) |
No |
824 (35.5) |
843 (35.5) |
Not reported |
4 (0.2) |
6 (0.3) |
Prior therapy, n (%): |
Chemotherapy |
774 (32.9) |
769 (32.4) |
Hormone replacement therapy |
567 (24.1) |
561 (23.7) |
Bisphosphonates |
43 (1.8) |
34 (1.4) |
Duration of tamoxifen therapy at randomization (months): |
Median (range) |
28.5 (15.8 - 52.2) |
28.4 (15.6 - 63.0) |
Tamoxifen dose, n (%): |
20 mg |
2270 (96.5) |
2287 (96.4) |
30 mg* |
78 (3.3) |
75 (3.2) |
Not reported |
4 (0.2) |
10 (0.4) |
*The 30 mg dose was used only in Denmark, where this dose
was the standard of care. |
After a median duration of therapy of 27 months and with
a median follow-up of 34.5 months, 520 events were reported, 213 in the
AROMASIN group and 307 in the tamoxifen group (Table 7).
Table 7: Primary Endpoint Events (ITT Population)
Event |
First Events
N (%) |
Exemestane
(N = 2352) |
Tamoxifen
(N = 2372) |
Loco-regional recurrence |
34 (1.45) |
45 (1.90) |
Distant recurrence |
126 (5.36) |
183 (7.72) |
Second primary -contralateral breast cancer |
7 (0.30) |
25 (1.05) |
Death - breast cancer |
1 (0.04) |
6 (0.25) |
Death - other reason |
41 (1.74) |
43 (1.81) |
Death - missing/unknown |
3 (0.13) |
5 (0.21) |
Ipsilateral breast cancer |
1 (0.04) |
0 |
Total number of events |
213 (9.06) |
307 (12.94) |
Disease-free survival in the
intent-to-treat population was statistically significantly improved [Hazard
Ratio (HR) = 0.69, 95% CI: 0.58, 0.82, P = 0.00003, Table 8, Figure 1] in the
AROMASIN arm compared to the tamoxifen arm. In the hormone receptor-positive
subpopulation representing about 85% of the trial patients, disease-free
survival was also statistically significantly improved (HR = 0.65, 95% CI:
0.53, 0.79, P = 0.00001) in the AROMASIN arm compared to the tamoxifen arm.
Consistent results were observed in the subgroups of patients with node
negative or positive disease, and patients who had or had not received prior
chemotherapy.
An overall survival update at
119 months median follow-up showed no significant difference between the two
groups, with 467 deaths (19.9%) occurring in the AROMASIN group and 510 deaths
(21.5%) in the tamoxifen group.
Table 8: Efficacy Results from the IES Study in
Postmenopausal Women with Early Breast Cancer
|
Hazard Ratio (95% CI) |
p-value (log-rank test) |
ITT Population |
Disease-free survival |
0.69 (0.58-0.82) |
0.00003 |
Time to contralateral breast cancer |
0.32 (0.15-0.72) |
0.00340 |
Distant recurrence-free survival |
0.74 (0.62-0.90) |
0.00207 |
Overall survival |
0.91 (0.81-1.04) |
0.16* |
ER and/or PgR positive |
Disease-free survival |
0.65 (0.53-0.79) |
0.00001 |
Time to contralateral breast cancer |
0.22 (0.08-0.57) |
0.00069 |
Distant recurrence-free survival |
0.73 (0.59-0.90) |
0.00367 |
Overall survival |
0.89 (0.78-1.02) |
0.09065* |
*Not adjusted for multiple
testing. |
Figure 1: Disease-Free
Survival in the IES Study of Postmenopausal Women with Early Breast Cancer (ITT
Population)
Treatment Of Advanced Breast
Cancer
Exemestane 25 mg administered
once daily was evaluated in a randomized double-blind, multicenter,
multinational comparative study and in two multicenter single-arm studies of
postmenopausal women with advanced breast cancer who had disease progression
after treatment with tamoxifen for metastatic disease or as adjuvant therapy.
Some patients also have received prior cytotoxic therapy, either as adjuvant
treatment or for metastatic disease.
The primary purpose of the
three studies was evaluation of objective response rate (complete response [CR]
and partial response [PR]). Time to tumor progression and overall survival were
also assessed in the comparative trial. Response rates were assessed based on
World Health Organization (WHO) criteria, and in the comparative study, were
submitted to an external review committee that was blinded to patient
treatment. In the comparative study, 769 patients were randomized to receive
AROMASIN (exemestane tablets) 25 mg once daily (N = 366) or megestrol acetate
40 mg four times daily (N = 403). Demographics and baseline characteristics are
presented in Table 9.
Table 9: Demographics and
Baseline Characteristics from the Comparative Study of Postmenopausal Women
with Advanced Breast Cancer Whose Disease Had Progressed after Tamoxifen
Therapy
Parameter |
AROMASIN
(N = 366) |
Megestrol Acetate
(N = 403) |
Median Age (range) |
65 (35-89) |
65 (30-91) |
ECOG Performance Status |
0 |
167 (46%) |
187 (46%) |
1 |
162 (44%) |
172 (43%) |
2 |
34 (9%) |
42 (10%) |
Receptor Status |
ER and/or PgR + |
246 (67%) |
274 (68%) |
|
|
|
ER and PgR unknown |
116 (32%) |
128 (32%) |
Responders to prior tamoxifen |
68 (19%) |
85 (21%) |
NE for response to prior tamoxifen |
46 (13%) |
41 (10%) |
Site of Metastasis |
Visceral ± other sites |
207 (57%) |
239 (59%) |
Bone only |
61 (17%) |
73 (18%) |
Soft tissue only |
54 (15%) |
51 (13%) |
Bone & soft tissue |
43 (12%) |
38 (9%) |
Measurable Disease |
287 (78%) |
314 (78%) |
Prior Tamoxifen Therapy |
Adjuvant or Neoadjuvant |
145 (40%) |
152 (38%) |
Advanced Disease, Outcome |
CR, PR, or SD ≥ 6 months |
179 (49%) |
210 (52%) |
SD < 6 months, PD or NE |
42 (12%) |
41 (10%) |
Prior Chemotherapy |
For advanced disease ± adjuvant |
58 (16%) |
67 (17%) |
Adjuvant only |
104 (28%) |
108 (27%) |
No chemotherapy |
203 (56%) |
226 (56%) |
The efficacy results from the
comparative study are shown in Table 10. The objective response rates observed
in the two treatment arms showed that AROMASIN was not different from megestrol
acetate. Response rates for AROMASIN from the two single-arm trials were 23.4%
and 28.1%.
Table 10: Efficacy Results from the Comparative Study
of Postmenopausal Women with Advanced Breast Cancer Whose Disease Had
Progressed after Tamoxifen Therapy
Response Characteristics |
AROMASIN
(N=366) |
Megestrol Acetate
(N=403) |
Objective Response Rate = CR + PR (%) |
15.0 |
12.4 |
Difference in Response Rate (AR-MA) |
2.6 |
95% C.I. |
7.5, -2.3 |
CR (%) |
2.2 |
1.2 |
PR (%) |
12.8 |
11.2 |
SD ≥ 24 Weeks (%) |
21.3 |
21.1 |
Median Duration of Response (weeks) |
76.1 |
71.0 |
Median TTP (weeks) |
20.3 |
16.6 |
Hazard Ratio (AR-MA) |
0.84 |
Abbreviations: CR = complete response, PR = partial response, SD = stable disease (no change), TTP = time to tumor progression, C.I. = confidence interval, MA = megestrol acetate, AR = AROMASIN |
There were too few deaths
occurring across treatment groups to draw conclusions on overall survival
differences. The Kaplan-Meier curve for time to tumor progression in the
comparative study is shown in Figure 2.
Figure 2: Time to Tumor Progression in the Comparative
Study of Postmenopausal Women With Advanced Breast Cancer Whose Disease Had
Progressed After Tamoxifen Therapy