5∝-REDUCTASE INIHIBITORS...
EXAMPLES: Dutasteride, finasteride.
MECHANISM OF ACTION: Competitively inhibit the metabolism of testosterone to
dihydrotestosterone (a more potent androgen) in peripheral tissues. Reduced circulating
dihydrotestosterone leads to reduced prostatic volume and thereby relief of voiding
symptoms.
- Benign prostatic hyperplasia.
- Should not be given in women, children or adolescents.
- Severe liver disease.
- Impotence.
- Decreased libido.
- Ejaculation disorders.
- Breast tenderness/enlargement.
- Metabolised in the liver with the majority excreted via the GI tract.
- t½ for dutasteride is 3–5 weeks (at therapeutic concentrations); t½ for finasteride is 5–6 h (longer in patients >70 years).
- No specific drug monitoring required.
- Plasma levels of dutasteride may be increased if long-term administration with protease inhibitors (ritonavir, indinavir) or antifungals (ketoconazole, itraconazole).
- Finasteride can be used in combination with doxazosin (a blocker) to treat benign prostatic hyperplasia.
- These drugs cause a reduction in serum PSA levels of ~50%. Therefore, interpretation of PSA levels should take this into account.
- Women of childbearing potential should avoid handling broken tablets/capsules.
- Patients may require several months treatment before a benefit is observed.
