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Fareston

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Fareston

CLINICAL PHARMACOLOGY

Mechanism of Action

Toremifene is a nonsteroidal triphenylethylene derivative. Toremifene binds to estrogen receptors and may exert estrogenic, antiestrogenic, or both activities, depending upon the duration of treatment, animal species, gender, target organ, or endpoint selected. In general, however, nonsteroidal triphenylethylene derivatives are predominantly antiestrogenic in rats and humans and estrogenic in mice. In rats, toremifene causes regression of established dimethylbenzanthracene (DMBA)-induced mammary tumors. The antitumor effect of toremifene in breast cancer is believed to be mainly due to its antiestrogenic effects, i.e., its ability to compete with estrogen for binding sites in the cancer, blocking the growth-stimulating effects of estrogen in the tumor.

Pharmacodynamics

Toremifene causes a decrease in the estradiol-induced vaginal cornification index in some postmenopausal women, indicative of its antiestrogenic activity. Toremifene also has estrogenic activity as shown by decreases in serum gonadotropin concentrations (FSH and LH).

Effects on Cardiac Electrophysiology

The effect of 20 mg, 80 mg, and 300 mg of toremifene on QT interval was evaluated in a double-blind, randomized study in healthy male subjects aged 18 to 45 years. The QT interval was measured at steady state of toremifene (Day 5 of dosing), including the time of peak plasma concentration (Tmax), at 13 time points (4 ECGs/time point) over 24 hours post dose in a time matched analysis. The 300 mg dose of toremifene (approximately five times the highest recommended dose 60 mg) was chosen because this dose produces exposure to toremifene that will cover the expected exposures that may result from potential drug interactions and hepatic impairment [see DRUG INTERACTIONS].

Dose and concentration-related increases in the QTc interval and T wave changes were observed (see Table 1). These effects are believed to be caused by toremifene and N-demethyltoremifene. Toremifene had no effects on heart rate, PR and QRS interval duration [see BOXED WARNING and WARNINGS AND PRECAUTIONS].

Table 1: QTc Prolongation in Healthy Male Volunteers

Treatment Arm Mean (90% CI) QTc, ms QTc > 60 ms (n, %) QTc > 500 ms (n, %)
Toremifene 20 mg (N = 47) 7 (0.9, 13.6) 0 0
Toremifene 80 mg (N = 47) 26 (21.1, 31.2) 2 (4.3%) 0
Toremifene 300 mg (N = 48) 65 (60.1, 69.2) 43 (89.6%) 5 (10.4%)

Pharmacokinetics

Absorption

Toremifene is well absorbed after oral administration and absorption is not influenced by food. Peak plasma concentrations are obtained within 3 hours. Toremifene displays linear pharmacokinetics after single oral doses of 10 to 680 mg. After multiple dosing, dose proportionality was observed for doses of 10 to 400 mg. Steady state concentrations were reached in about 4-6 weeks.

Distribution

Toremifene has an apparent volume of distribution of 580 L and binds extensively ( > 99.5%) to serum proteins, mainly albumin.

Metabolism

Toremifene is extensively metabolized, principally by CYP3A4 to N-demethyltoremifene which is also antiestrogenic but with weak in vivo antitumor potency. Serum concentrations of N-demethyltoremifene are 2 to 4 times higher than toremifene at steady state.

Following multiple dosing with toremifene in 20 healthy volunteers, plasma toremifene exposure was lower on Day 17 compared to Day 5 by approximately 14%. N-demethyltoremifene exposure was higher on Day 17 compared to Day 5 by approximately 80%. Based on these data and an in vitro induction study in human hepatocytes, auto-induction of CYP3A4 by toremifene is likely. The effect of auto-induction on efficacy was likely captured following prolonged dosing in the clinical studies.

Elimination

The plasma concentration time profile of toremifene declines biexponentially after absorption with a mean distribution half-life of about 4 hours and an elimination half-life of about 5 days. Elimination half-lives of major metabolites, N-demethyltoremifene and (Deaminohydroxy) toremifene, were 6 and 4 days, respectively. Mean total clearance of toremifene was approximately 5 L/h. Toremifene is eliminated as metabolites primarily in the feces, with about 10% excreted in the urine during a 1-week period. Elimination of toremifene is slow, in part because of enterohepatic circulation.

Renal insufficiency

The pharmacokinetics of toremifene and N-demethyltoremifene were similar in normals and patients with impaired kidney function.

Hepatic insufficiency

The mean elimination half-life of toremifene was increased by less than twofold in 10 patients with hepatic impairment (cirrhosis or fibrosis) compared to subjects with normal hepatic function. The pharmacokinetics of N-demethyltoremifene were unchanged in these patients. Ten patients on anticonvulsants (phenobarbital, clonazepam, phenytoin, and carbamazepine) showed a twofold increase in clearance and a decrease in the elimination half-life of toremifene.

Geriatric patients

The pharmacokinetics of toremifene were studied in 10 healthy young males and 10 elderly females following a single 120 mg dose under fasting conditions. Increases in the elimination half-life (4.2 versus 7.2 days) and the volume of distribution (457 versus 627 L) of toremifene were seen in the elderly females without any change in clearance or AUC. The median ages in the three controlled studies ranged from 60 to 66 years. No significant age-related differences in FARESTON effectiveness or safety were noted.

Food

The rate and extent of absorption of FARESTON are not influenced by food; thus FARESTON may be taken with or without food.

Race

The pharmacokinetics of toremifene in patients of different races has not been studied. Fourteen percent of patients in the North American Study were non-Caucasian. No significant race-related differences in FARESTON effectiveness or safety were noted.

Clinical Studies

Three prospective, randomized, controlled clinical studies (North American, Eastern European, and Nordic) were conducted to evaluate the efficacy of FARESTON for the treatment of breast cancer in postmenopausal women. The patients were randomized to parallel groups receiving FARESTON 60 mg (FAR60) or tamoxifen 20 mg (TAM20) in the North American Study or tamoxifen 40 mg (TAM40) in the Eastern European and Nordic studies. The North American and Eastern European studies also included high-dose toremifene arms of 200 and 240 mg daily, respectively. The studies included postmenopausal patients with estrogen-receptor (ER) positive or estrogen-receptor (ER) unknown metastatic breast cancer. The patients had at least one measurable or evaluable lesion. The primary efficacy variables were response rate (RR) and time to progression (TTP). Survival (S) was also determined. Ninety-five percent confidence intervals (95% CI) were calculated for the difference in RR between FAR60 and TAM groups and the hazard ratio (relative risk for an unfavorable event, such as disease progression or death) between TAM and FAR60 for TTP and S.

Two of the 3 studies showed similar results for all effectiveness endpoints. However, the Nordic Study showed a longer time to progression for tamoxifen (see table).

Clinical Studies

Study North American Eastern European Nordic
Treatment Group FAR60 TAM20 FAR60 TAM40 FAR60 TAM40
No. Patients 221 215 157 149 214 201
Responses
CR1 + PR2 14 + 33 11 + 30 7 + 25 3 + 28 19 + 48 19 + 56
RR3 (CR + PR)% 21.3 19.1 20.4 20.8 31.3 37.3
Difference in RR 2.2   -0.4   -6.0  
   95% CI4 for Difference in RR -5.8 to 10.2 -9.5 to 8.6 -15.1 to 3.1
Time to Progression (TTP)
Median TTP (mo.) 5.6 5.8 4.9 5.0 7.3 10.2
Hazard Ratio (TAM/FAR) 1.01   1.02   0.80  
   95% CI4 for Hazard Ratio (%) 0.81 to 1.26 0.79 to 1.31 0.64 to 1.00
Survival (S)
Median S (mo.) 33.6 34.0 25.4 23.4 33.0 38.7
Hazard Ratio (TAM/FAR) 0.94   0.96   0.94  
   95% CI4 for Hazard Ratio (%) 0.74 to 1.24 0.72 to 1.28 0.73 to 1.22
1CR = complete response; 2PR = partial response; 3RR = response rate; 4CI = confidence interval

The high-dose groups, toremifene 200 mg daily in the North American Study and 240 mg daily in the Eastern European Study, were not superior to the lower toremifene dose groups, with response rates of 22.6% and 28.7%, median times to progression of 5.6 and 6.1 months, and median survivals of 30.1 and 23.8 months, respectively. The median treatment duration in the three pivotal studies was 5 months (range 4.2-6.3 months).

Last reviewed on RxList: 6/3/2011
This monograph has been modified to include the generic and brand name in many instances.

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