5α-reductase inhibitors

Key examples

• Finasteride

 

Common indications

• In benign prostatic hyperplasia, 5α-reductase inhibitors are a second-line medical treatment after α-blockers. They improve lower urinary tract symptoms, such as difficulty passing urine, urinary retention and poor urinary flow, and reduce the need for prostate-related surgery. However, it can take several months for this effect to become evident clinically. For this reason, an α-blocker is usually preferred for initial therapy, with a 5α-reductase inhibitor added if the response is poor or if the prostate is particularly bulky.
• Male-pattern baldness

 

Physiology

• The intracellular enzyme 5α-reductase converts testosterone to its more active metabolite dihydrotestosterone. 

 

Mechanisms of action

• 5α-reductase inhibitors reduce the size of the prostate gland. 
• As dihydrotestosterone stimulates prostatic growth, inhibition of its production by 5α-reductase inhibitors reduces prostatic enlargement and improves urinary flow. 

 

Important adverse effects

• The most common adverse effects of 5α-reductase inhibitors relate to their anti-androgen action. These include impotence and reduced libido, which are usually transient, and breast tenderness and enlargement (gynaecomastia), which can affect patient adherence to treatment. 
• An additional effect of androgen inhibition is hair growth, which can be exploited to advantage in treatment of male-pattern baldness.  
• Breast cancer has been reported in men taking finasteride.

 

Warnings

• Exposure of a male foetus to 5α-reductase inhibitors may cause abnormal development of the external genitalia.
• It is therefore important that pregnant women do not take these drugs and are not exposed to them.