5 alpha reductase inhibitors

 

• 5-alpha reductase inhibitors (finasteride and dutasteride) inhibit the conversion of testosterone to dihydrotestosterone (DHT), thereby reducing DHT levels and lowering serum PSA levels by approximately 50% after 6-12 months of therapy.
• 5-alpha reductase converts testosterone \rightarrow dihydrotestosterone (DHT), the more potent androgen responsible for prostatic growth.
• Inhibition of this enzyme:
  • Decreases intraprostatic DHT levels
  • Shrinks prostate volume
  • Improves urinary flow
  • Reduces the risk of acute urinary retention and need for surgery in BPH
• Testosterone levels are usually unchanged or may show a slight compensatory increase; they are not significantly reduced.
• PSA levels typically decrease by about 50% after prolonged therapy. Therefore, in patients taking 5-alpha reductase inhibitors, the measured PSA should generally be doubled when screening for prostate cancer.
• Common indications:
  • Benign prostatic hyperplasia (BPH)
  • Androgenetic alopecia (finasteride)
• Common adverse effects:
  • Decreased libido
  • Erectile dysfunction
  • Ejaculatory dysfunction
  • Reduced semen volume
  • Gynecomastia (less common)
• Examples:
  • Finasteride: Inhibits Type II 5-alpha reductase
  • Dutasteride: Inhibits both Type I and Type II isoenzymes

Statement Breakdown

• By inhibiting this enzyme, it reduces the levels of DHT, which can help prevent hair loss and shrink the prostate in affected individuals. (Statement 1)
• Effect on Testosterone: Testosterone levels are usually unchanged or may show a slight compensatory increase; they are not significantly reduced. (Statement 2 ruled out)
• Effect on PSA: PSA levels typically decrease by about 50% after prolonged therapy (Statement 3 ruled out).
• Therefore, in patients taking 5-alpha reductase inhibitors, the measured PSA should generally be doubled when screening for prostate cancer.

Learning Objective

Property	Details
Drug class	5-alpha reductase inhibitors
MOA	Inhibits the enzyme type 2 5-alpha reductase, which converts testosterone to dihydrotestosterone (DHT)
Therapeutic uses	Treatment of benign prostatic hyperplasia (BPH) and male pattern baldness
Dosage forms	Oral tablet and topical solution
Pharmacokinetics	Absorbed well orally; undergoes extensive hepatic metabolism via CYP3A4 enzyme; eliminated via urine and feces
Contraindications	Hypersensitivity to finasteride, pregnancy, breastfeeding
Adverse effects	Impotence, decreased libido, ejaculation disorders, gynecomastia and tenderness, allergic reactions.