Finasteride

FINASTERIDE — finasteride tablet, film coated
Physicians Total Care, Inc.

Finasteride, a synthetic 4-azasteroid compound, is a specific inhibitor of steroid Type II 5α-reductase, an intracellular enzyme that converts the androgen testosterone into 5α-dihydrotestosterone (DHT).

Finasteride is 4-azaandrost-1-ene-17-carboxamide, N -(1,1-dimethylethyl)-3-oxo-,(5α,17β)-. The empirical formula of finasteride is C23 H36 N2 O2 and its molecular weight is 372.55. Its structural formula is:

image of chemical structure

Finasteride is a white crystalline powder with a melting point near 250°C. It is freely soluble in chloroform and in lower alcohol solvents, but is practically insoluble in water.

Finasteride tablets for oral administration are film-coated tablets that contain 5 mg of finasteride and the following inactive ingredients: FD&C Blue #2 Aluminum Lake, hypromellose, lactose monohydrate, lauryl macrogolglycerides, magnesium stearate, microcrystalline cellulose, polyethylene glycol, pregelatinized starch, sodium starch glycolate and titanium dioxide.

CLINICAL PHARMACOLOGY

The development and enlargement of the prostate gland is dependent on the potent androgen, 5 α -dihydrotestosterone (DHT). Type II 5 α −reductase metabolizes testosterone to DHT in the prostate gland, liver and skin. DHT induces androgenic effects by binding to androgen receptors in the cell nuclei of these organs.

Finasteride is a competitive and specific inhibitor of Type II 5α-reductase with which it slowly forms a stable enzyme complex. Turnover from this complex is extremely slow (t½ ~30 days). This has been demonstrated both in vivo and in vitro. Finasteride has no affinity for the androgen receptor. In man, the 5 α -reduced steroid metabolites in blood and urine are decreased after administration of finasteride.

In man, a single 5-mg oral dose of finasteride produces a rapid reduction in serum DHT concentration, with the maximum effect observed 8 hours after the first dose. The suppression of DHT is maintained throughout the 24-hour dosing interval and with continued treatment. Daily dosing of finasteride at 5 mg/day for up to 4 years has been shown to reduce the serum DHT concentration by approximately 70%. The median circulating level of testosterone increased by approximately 10-20% but remained within the physiologic range.

Adult males with genetically inherited Type II 5 α -reductase deficiency also have decreased levels of DHT. Except for the associated urogenital defects present at birth, no other clinical abnormalities related to Type II 5 α -reductase deficiency have been observed in these individuals. These individuals have a small prostate gland throughout life and do not develop BPH.

In patients with BPH treated with finasteride (1-100 mg/day) for 7-10 days prior to prostatectomy, an approximate 80% lower DHT content was measured in prostatic tissue removed at surgery, compared to placebo; testosterone tissue concentration was increased up to 10 times over pretreatment levels, relative to placebo. Intraprostatic content of prostate-specific antigen (PSA) was also decreased.

In healthy male volunteers treated with finasteride for 14 days, discontinuation of therapy resulted in a return of DHT levels to pretreatment levels in approximately 2 weeks. In patients treated for three months, prostate volume, which declined by approximately 20%, returned to close to baseline value after approximately three months of discontinuation of therapy.

PharmacokineticsAbsorption

In a study of 15 healthy young subjects, the mean bioavailability of finasteride 5-mg tablets was 63% (range 34-108%), based on the ratio of area under the curve (AUC) relative to an intravenous (IV) reference dose. Maximum finasteride plasma concentration averaged 37 ng/mL (range, 27-49 ng/mL) and was reached 1-2 hours postdose. Bioavailability of finasteride was not affected by food.

Mean steady-state volume of distribution was 76 liters (range, 44-96 liters). Approximately 90% of circulating finasteride is bound to plasma proteins. There is a slow accumulation phase for finasteride after multiple dosing. After dosing with 5 mg/day of finasteride for 17 days, plasma concentrations of finasteride were 47 and 54% higher than after the first dose in men 45-60 years old (n=12) and greater than or equal to 70 years old (n=12), respectively. Mean trough concentrations after 17 days of dosing were 6.2 ng/mL (range, 2.4-9.8 ng/mL) and 8.1 ng/mL (range, 1.8-19.7 ng/mL), respectively, in the two age groups. Although steady state was not reached in this study, mean trough plasma concentration in another study in patients with BPH (mean age, 65 years) receiving 5 mg/day was 9.4 ng/mL (range, 7.1-13.3 ng/mL; n=22) after over a year of dosing.

Finasteride has been shown to cross the blood brain barrier but does not appear to distribute preferentially to the CSF.

In 2 studies of healthy subjects (n=69) receiving finasteride 5 mg/day for 6-24 weeks, finasteride concentrations in semen ranged from undetectable (less than 0.1 ng/mL) to 10.54 ng/mL. In an earlier study using a less sensitive assay, finasteride concentrations in the semen of 16 subjects receiving finasteride 5 mg/day ranged from undetectable (less than 1.0 ng/mL) to 21 ng/mL. Thus, based on a 5-mL ejaculate volume, the amount of finasteride in semen was estimated to be 50- to 100-fold less than the dose of finasteride (5 µg) that had no effect on circulating DHT levels in men (see also PRECAUTIONS, Pregnancy).

Finasteride is extensively metabolized in the liver, primarily via the cytochrome P450 3A4 enzyme subfamily. Two metabolites, the t-butyl side chain monohydroxylated and monocarboxylic acid metabolites, have been identified that possess no more than 20% of the 5α-reductase inhibitory activity of finasteride.

In healthy young subjects (n=15), mean plasma clearance of finasteride was 165 mL/min (range, 70-279 mL/min) and mean elimination half-life in plasma was 6 hours (range, 3-16 hours). Following an oral dose of 14 C-finasteride in man (n=6), a mean of 39% (range, 32-46%) of the dose was excreted in the urine in the form of metabolites; 57% (range, 51-64%) was excreted in the feces.

The mean terminal half-life of finasteride in subjects greater than or equal to 70 years of age was approximately 8 hours (range, 6-15 hours; n=12), compared with 6 hours (range, 4-12 hours; n=12) in subjects 45-60 years of age. As a result, mean AUC(0-24 hr ) after 17 days of dosing was 15% higher in subjects greater than or equal to 70 years of age than in subjects 45-60 years of age (p=0.02).

Pediatric: Finasteride pharmacokinetics have not been investigated in patients less than 18 years of age.

Gender: Finasteride pharmacokinetics in women are not available.

Geriatric: No dosage adjustment is necessary in the elderly. Although the elimination rate of finasteride is decreased in the elderly, these findings are of no clinical significance. See also Pharmacokinetics, Excretion , PRECAUTIONS, Geriatric Use and DOSAGE AND ADMINISTRATION.

Race: The effect of race on finasteride pharmacokinetics has not been studied.

Renal Insufficiency: No dosage adjustment is necessary in patients with renal insufficiency. In patients with chronic renal impairment, with creatinine clearances ranging from 9.0 to 55 mL/min, AUC, maximum plasma concentration, half-life, and protein binding after a single dose of 14 C-finasteride were similar to values obtained in healthy volunteers. Urinary excretion of metabolites was decreased in patients with renal impairment. This decrease was associated with an increase in fecal excretion of metabolites. Plasma concentrations of metabolites were significantly higher in patients with renal impairment (based on a 60% increase in total radioactivity AUC). However, finasteride has been well tolerated in BPH patients with normal renal function receiving up to 80 mg/day for 12 weeks, where exposure of these patients to metabolites would presumably be much greater.

Hepatic Insufficiency: The effect of hepatic insufficiency on finasteride pharmacokinetics has not been studied. Caution should be used in the administration of finasteride in those patients with liver function abnormalities, as finasteride is metabolized extensively in the liver.

(also see PRECAUTIONS, Drug Interactions)

No drug interactions of clinical importance have been identified. Finasteride does not appear to affect the cytochrome P450-linked drug metabolism enzyme system. Compounds that have been tested in man have included antipyrine, digoxin, propranolol, theophylline, and warfarin, and no clinically meaningful interactions were found.

Mean (SD) Pharmacokinetic Parameters in Healthy Young Subjects (n=15)
Mean (± SD)
Bioavailability63% (34-108%)*
Clearance (mL/min)165 (55)
Volume of Distribution76 (14)
Half-Life (hours6.2 (2.1)
Mean (SD) Noncompartmental Pharmacokinetic Parameters After Multiple Doses of 5 mg/day in Older Men
Mean (± SD)
45 — 60 years old (n=12)≥ 70 years old (n=12)
AUC (ng•hr/mL)389 (98)463 (186)
Peak Concentration (ng/mL)46.2 (8.7)48.4 (14.7)
Time to Peak (hours)1.8 (0.7)1.8 (0.6)
Half-Life (hours )*6.0 (1.5)8.2 (2.5)

*First-dose values, all other parameters are last-dose values Clinical Studies

Finasteride 5 mg/day was initially evaluated in patients with symptoms of BPH and enlarged prostates by digital rectal examination in two 1-year, placebo-controlled, randomized, double-blind studies and their 5-year open extensions.

Finasteride was further evaluated in a long-term efficacy and safety study, a double-blind, randomized, placebo-controlled, 4-year, multicenter study. 3040 patients between the ages of 45 and 78, with moderate to severe symptoms of BPH and an enlarged prostate upon digital rectal examination, were randomized into the study (1524 to finasteride, 1516 to placebo) and 3016 patients were evaluable for efficacy. 1883 patients completed the 4-year study (1000 in the finasteride group, 883 in the placebo group).

Effect on Symptom Score

Symptoms were quantified using a score similar to the American Urological Association Symptom Score, which evaluated both obstructive symptoms (impairment of size and force of stream, sensation of incomplete bladder emptying, delayed or interrupted urination) and irritative symptoms (nocturia, daytime frequency, need to strain or push the flow of urine) by rating on a 0 to 5 scale for six symptoms and a 0 to 4 scale for one symptom, for a total possible score of 34.

Patients in a long-term efficacy and safety study had moderate to severe symptoms at baseline (mean of approximately 15 points on a 0-34 point scale). Patients randomized to finasteride who remained on therapy for 4 years had a mean (± 1 SD) decrease in symptom score of 3.3 (± 5.8) points compared with 1.3 (± 5.6) points in the placebo group. (See Figure 1.) A statistically significant improvement in symptom score was evident at 1 year in patients treated with finasteride vs placebo (−2.3 vs −1.6), and this improvement continued through Year 4.

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Figure 1 Symptom Score in a long-term efficacy and safety study

Results seen in earlier studies were comparable to those seen in a long-term efficacy and safety study. Although an early improvement in urinary symptoms was seen in some patients, a therapeutic trial of at least 6 months was generally necessary to assess whether a beneficial response in symptom relief had been achieved. The improvement in BPH symptoms was seen during the first year and maintained throughout an additional 5 years of open extension studies.

Effect on the Need for Surgery

In a long-term efficacy and safety study, efficacy was also assessed by evaluating treatment failures. Treatment failure was prospectively defined as BPH-related urological events or clinical deterioration, lack of improvement and/or the need for alternative therapy. BPH-related urological events were defined as urological surgical intervention. Complete event information was available for 92% of the patients. The following table (Table 1) summarizes the results.

Table 1 All Treatment Failures in a long-term efficacy and safety study
Patients(%)*
Event PlaceboN=1503 FinasterideN=1513 RelativeRisk ** 95 % CI PValue **
All Treatment Failures37.126.20.68(0.57 to0.79)less than 0.001
Surgical Interventions for BPH10.14.60.45(0.32 to0.63)less than 0.001
Two consecutive symptom scores greater than 209.26.7
Bladder Stone0.40.5
Incontinence2.11.7
Renal Failure0.50.6
UTI5.74.9
Discontinuation due to worsening of BPH, lack of improvement, or to receive other medical treatment21.813.3

** Hazard ration based on log rank test.
Compared with placebo, finasteride was associated with a significantly lower need for BPH-related surgery [13.2% for placebo vs 6.6% for finasteride; 51% reduction in risk, 95% CI: (34 to 63%)]. Compared with placebo, finasteride was associated with a significantly lower risk for surgery [10.1% for placebo vs 4.6% for finasteride; 55% reduction in risk, 95% CI: (37 to 68%)]; see Figure 2.
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Figure 2

Placebo Group
No. of events, cumulative3789121162
No. at risk per year1503145413741314
Finasteride Group
No. of events, cumulative18404969
No. at risk per year1513148314381410

Effect on Maximum Urinary Flow Rate

In the patients in a long-term efficacy and safety study who remained on therapy for the duration of the study and had evaluable urinary flow data, finasteride increased maximum urinary flow rate by 1.9 mL/sec compared with 0.2 mL/sec in the placebo group.

There was a clear difference between treatment groups in maximum urinary flow rate in favor of finasteride by month 4 (1.0 vs 0.3 mL/sec) which was maintained throughout the study. In the earlier 1-year studies, increase in maximum urinary flow rate was comparable to a long-term efficacy and safety study and was maintained through the first year and throughout an additional 5 years of open extension studies.

Effect on Prostate Volume

In a long-term efficacy and safety study, prostate volume was assessed yearly by magnetic resonance imaging (MRI) in a subset of patients. In patients treated with finasteride who remained on therapy, prostate volume was reduced compared with both baseline and placebo throughout the 4-year study. Finasteride decreased prostate volume by 17.9% (from 55.9 cc at baseline to 45.8 cc at 4 years) compared with an increase of 14.1% (from 51.3 cc to 58.5 cc) in the placebo group (p less than 0.001). (See Figure 3.)

Results seen in earlier studies were comparable to those seen in a long-term efficacy and safety study. Mean prostate volume at baseline ranged between 40-50 cc. The reduction in prostate volume was seen during the first year and maintained throughout an additional five years of open extension studies.

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Figure 3. Prostate Volume in a long-term efficacy and safety study

Prostate Volume as a Predictor of Therapeutic Response

A meta-analysis combining 1-year data from seven double-blind, placebo-controlled studies of similar design, including 4491 patients with symptomatic BPH, demonstrated that, in patients treated with finasteride, the magnitude of symptom response and degree of improvement in maximum urinary flow rate were greater in patients with an enlarged prostate at baseline.

Summary of Clinical Studies

The data from these studies, showing improvement in BPH-related symptoms, reduction in treatment failure (BPH-related urological events), increased maximum urinary flow rates, and decreasing prostate volume, suggest that finasteride arrests the disease process of BPH in men with an enlarged prostate.

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