Absorption

Analysis of plasma, urine, and feces following a single oral 200 mg dose of tritium-labeled flutamide to human volunteers showed that the drug is rapidly and completely absorbed. Following a single 250 mg oral dose to normal adult volunteers, the biologically active alpha-hydroxylated metabolite reaches maximum plasma concentrations in about 2 hours, indicating that it is rapidly formed from flutamide. Food has no effect on bioavailability of flutamide.

Distribution

In male rats neither flutamide nor any of its metabolites is preferentially accumulated in any tissue except the prostate after an oral 5 mg/kg dose of 14C-flutamide. Total drug levels were highest 6 hours after drug administration in all tissues. Levels declined at roughly similar rates to low levels at 18 hours. The major metabolite was present at higher concentrations than flutamide in all tissues studied. Following a single 250 mg oral dose to normal adult volunteers, low plasma levels of flutamide were detected. The plasma half-life for the alpha-hydroxylated metabolite of flutamide is about 6 hours. Flutamide, in vivo, at steady-state plasma concentrations of 24 to 78 ng/mL, is 94% to 96% bound to plasma proteins. The active metabolite of flutamide, in vivo, at steady-state plasma concentrations of 1556 to 2284 ng/mL, is 92% to 94% bound to plasma proteins.

Metabolism

The composition of plasma radioactivity, following a single 200 mg oral dose of tritium-labeled flutamide to normal adult volunteers, showed that flutamide is rapidly and extensively metabolized, with flutamide comprising only 2.5% of plasma radioactivity 1 hour after administration. At least 6 metabolites have been identified in plasma. The major plasma metabolite is a biologically active alpha-hydroxylated derivative which accounts for 23% of the plasma tritium 1 hour after drug administration. The major urinary metabolite is 2-amino-5-nitro-4-(trifluoromethyl)phenol.

Excretion

Flutamide and its metabolites are excreted mainly in the urine with only 4.2% of the dose excreted in the feces over 72 hours.

Geriatric

Following multiple oral dosing of 250 mg t.i.d. in normal geriatric volunteers, flutamide and its active metabolite approached steady-state plasma levels (based on pharmacokinetic simulations) after the fourth flutamide dose. The half-life of the active metabolite in geriatric volunteers after a single flutamide dose is about 8.1 hours and at steady state in 9.6 hours.

Race

There are no known alterations in flutamide absorption, distribution, metabolism, or excretion due to race.

Renal Impairment

Following a single 250 mg dose of flutamide administered to subjects with chronic renal insufficiency, there appeared to be no correlation between creatinine clearance and either Cmax or AUC of flutamide. Renal impairment did not have an effect on the Cmax or AUC of the biologically active alpha-hydroxylated metabolite of flutamide. In subjects with creatinine clearance of < 29 mL/min, the half-life of the active metabolite was slightly prolonged. Flutamide and its active metabolite were not well dialyzed. Dose adjustment in patients with chronic renal insufficiency is not warranted.

Hepatic Impairment

No information on the pharmacokinetics of flutamide in hepatic impairment is available (see BOXED WARNINGS, Hepatic Injury).

Women, Pediatrics

Flutamide has not been studied in women or pediatric patients.

Drug-Drug Interactions

Interactions between flutamide capsules and LHRH agonists have not occurred. Increases in prothrombin have been noted in patients receiving warfarin therapy (see PRECAUTIONS).

Stage B2-C Prostatic Carcinoma

The effects of hormonal treatment combined with radiation were studied in 466 patients (231 flutamide capsules + goserelin acetate implant + radiation, 235 radiation alone) with bulky primary tumors confined to the prostate (stage B2) or extending beyond the capsule (stage C), with or without pelvic node involvement.

In this multicentered, controlled trial, administration of flutamide capsules (250 mg t.i.d.) and goserelin acetate (3.6 mg depot) prior to and during radiation was associated with a significantly lower rate of local failure compared to radiation alone (16% vs. 33% at 4 years, P < 0.001). The combination therapy also resulted in a trend toward reduction in the incidence of distant metastases (27% vs. 36% at 4 years, P = 0.058). Median disease-free survival was significantly increased in patients who received complete hormonal therapy combined with radiation as compared to those patients who received radiation alone (4.4 vs. 2.6 years, P < 0.001). Inclusion of normal PSA level as a criterion for disease-free survival also resulted in significantly increased median disease-free survival in patients receiving the combination therapy (2.7 vs. 1.5 years, P < 0.001).

Stage D2 Prostatic Carcinoma

To study the effects of combination therapy in metastatic disease, 617 patients (311 leuprolide + flutamide, 306 leuprolide + placebo) with previously untreated advanced prostatic carcinoma were enrolled in a large multicentered, controlled clinical trial.

Three and one-half years after the study was initiated, median survival had been reached. The median actuarial survival time was 34.9 months for patients treated with leuprolide and flutamide versus 27.9 months for patients treated with leuprolide alone. This 7 month increment represents a 25% improvement in overall survival time with the flutamide therapy. Analysis of progression-free survival showed a 2.6 month improvement in patients who received leuprolide plus flutamide, a 19% increment over leuprolide and placebo.

Use in Women

Flutamide capsules are for use only in men. This product has no indication for women and should not be used in this population, particularly for nonserious or non life-threatening conditions.

Fetal Toxicity

Flutamide may cause fetal harm when administered to a pregnant woman (see Pregnancy).

Aniline Toxicity

One metabolite of flutamide is 4-nitro-3-fluoro-methylaniline. Several toxicities consistent with aniline exposure, including methemoglobinemia, hemolytic anemia and cholestatic jaundice have been observed in both animals and humans after flutamide administration. In patients susceptible to aniline toxicity (e.g., persons with glucose-6-phosphate dehydrogenase deficiency, hemoglobin M disease and smokers), monitoring of methemoglobin levels should be considered.

Teratogenic Effects

Pregnancy Category D

There was decreased 24 hour survival in the offspring of pregnant rats treated with flutamide at doses of 30, 100, or 200 mg/kg/day (approximately 3, 9, and 19 times the human dose). A slight increase in minor variations in the development of the sternebrae and vertebrae was seen in fetuses of cats treated with two higher doses. Feminization of the male cats also occurred at the two higher dose levels. There was a decreased survival rate in the offspring of rabbits receiving the highest dose (15 mg/kg/day, equal to 1.4 times the human dose).

Cardiovascular System

Hypertension in 1% of patients

Central Nervous System

CNS (drowsiness/confusion/depression/anxiety/nervousness) reactions occurred in 1% of patients

Gastrointestinal System

Anorexia 4%, and other GI disorders occurred in 6% of patients

Hematopoietic System

Anemia occurred in 6%, leukopenia in 3%, and thrombocytopenia in 1% of patients

Liver and Biliary System

Hepatitis and jaundice in less than 1% of patients

Skin

Irritation at the injection site and rash occurred in 3% of patients

Other

Edema occurred in 4%, genitourinary and neuromuscular symptoms in 2%, and pulmonary symptoms in less than 1% of patients.

In addition, the following spontaneous adverse experiences have been reported during the marketing of flutamide: hemolytic anemia, macrocytic anemia, methemoglobinemia, sulfhemoglobinemia, photosensitivity reactions (including erythema, ulceration, bullous eruptions, and epidermal necrolysis), and urine discoloration. The urine was noted to change to an amber or yellow-green appearance which can be attributed to the flutamide and/or its metabolites. Also reported were cholestatic jaundice, hepatic encephalopathy, and hepatic necrosis. The hepatic conditions were often reversible after discontinuing therapy; however, there have been reports of death following severe hepatic injury associated with use of flutamide.

Malignant breast neoplasms have occurred rarely in male patients being treated with flutamide.

Abnormal Laboratory Test Values

Laboratory abnormalities including elevated SGOT, SGPT, bilirubin values, SGGT, BUN, and serum creatinine have been reported.