Proscar - Drug Monograph

Introduction

Proscar (finasteride) is a synthetic 4-azasteroid compound that acts as a specific inhibitor of Type II 5α-reductase, an intracellular enzyme that converts testosterone to the more potent androgen dihydrotestosterone (DHT). It is primarily used for the treatment of symptomatic benign prostatic hyperplasia (BPH) in men with an enlarged prostate.

Mechanism of Action

Finasteride competitively inhibits Type II 5α-reductase, the enzyme responsible for the conversion of testosterone to dihydrotestosterone (DHT) in target tissues. DHT is the primary androgen mediating prostate growth, and by reducing serum and intraprostatic DHT levels by approximately 70%, finasteride induces regression of the epithelial component of the prostate, resulting in decreased prostate volume, improved urinary flow, and reduction in symptoms associated with BPH.

Indications

• Treatment of symptomatic benign prostatic hyperplasia (BPH) in men with an enlarged prostate to improve symptoms, reduce the risk of acute urinary retention, and reduce the need for surgery
• Note: While finasteride 1mg (Propecia) is approved for male pattern hair loss, Proscar (5mg) is specifically indicated for BPH treatment

Dosage and Administration

• Renal impairment: No dosage adjustment necessary
• Hepatic impairment: Use with caution; no specific dosage recommendation
• Geriatric patients: No dosage adjustment necessary
• Pediatric patients: Not indicated for use in children

• Tablets should be swallowed whole; not to be crushed or broken
• Women who are or may become pregnant should not handle crushed or broken tablets due to risk of absorption through skin and potential risk to male fetus
• Treatment response may require 6-12 months of continuous therapy

Pharmacokinetics

Contraindications

• Hypersensitivity to finasteride or any component of the formulation
• Use in women
• Use in children
• Pregnancy (Category X) - women who are or may become pregnant should not use or handle crushed tablets

Warnings and Precautions

• Pregnancy risk: May cause abnormalities of the external genitalia of male fetus if administered to pregnant women
• Prostate cancer risk: All men with BPH should be screened for prostate cancer prior to initiating therapy and periodically thereafter
• Blood donation: Men receiving finasteride should not donate blood until at least 1 month after discontinuation
• Depression: Monitor for mood changes, including depression
• Sexual dysfunction: May cause decreased libido, erectile dysfunction, and ejaculation disorders
• Breast changes: May cause gynecomastia; evaluate any breast nodules or breast changes

Drug Interactions

• Theoretical interactions: Potent CYP3A4 inhibitors (e.g., ketoconazole, erythromycin) may increase finasteride concentrations, though clinical significance is unknown
• No clinically important interactions with warfarin, digoxin, theophylline, or antipyrine demonstrated
• Alpha-blockers: Can be used concomitantly with doxazosin without pharmacokinetic interaction

Adverse Effects

• Decreased libido (3.4-3.7%)
• Erectile dysfunction (4.9-8.1%)
• Ejaculation disorders (1.7-2.8%)
• Decreased ejaculate volume (0.9-2.8%)

• Gynecomastia (0.4-2.2%)
• Breast tenderness
• Hypersensitivity reactions (rash, pruritus, urticaria, lip swelling)

• Depression
• Male infertility
• Testicular pain
• Breast cancer in men

Monitoring Parameters

• Prostate-specific antigen (PSA) levels: Baseline and periodically (note: finasteride reduces serum PSA by approximately 50%)
• Digital rectal examination: Baseline and periodically
• Symptom assessment: International Prostate Symptom Score (IPSS) and urinary flow rates
• Prostate volume: Via transrectal ultrasound if indicated
• Sexual function: Regular assessment of libido and erectile function
• Breast examination: For gynecomastia or tenderness

Patient Education

• Take medication exactly as prescribed; do not crush or break tablets
• Women who are pregnant or may become pregnant should not handle crushed tablets due to risk to male fetus
• Treatment requires several months to achieve maximum benefit
• Report any breast changes, lumps, pain, or nipple discharge promptly
• Be aware of potential sexual side effects and discuss concerns with healthcare provider
• Continue regular prostate cancer screening as recommended
• Do not donate blood while taking finasteride and for at least 1 month after discontinuation
• Notify all healthcare providers about finasteride use, especially if prostate cancer screening is being considered

References

1. Roehrborn CG, Boyle P, Nickel JC, et al. Efficacy and safety of a dual inhibitor of 5-alpha-reductase types 1 and 2 (dutasteride) in men with benign prostatic hyperplasia. Urology. 2002;60(3):434-441. 2. McConnell JD, Bruskewitz R, Walsh P, et al. The effect of finasteride on the risk of acute urinary retention and the need for surgical treatment among men with benign prostatic hyperplasia. N Engl J Med. 1998;338(9):557-563. 3. Proscar® (finasteride) prescribing information. Merck & Co., Inc.; 2021. 4. Thompson IM, Goodman PJ, Tangen CM, et al. The influence of finasteride on the development of prostate cancer. N Engl J Med. 2003;349(3):215-224. 5. Nickel JC, Fradet Y, Boake RC, et al. Efficacy and safety of finasteride therapy for benign prostatic hyperplasia: results of a 4-year randomized double-blind trial. Urology. 1996;48(3):398-406. 6. U.S. Food and Drug Administration. Proscar label. Accessed January 2023.