Silodosin is a new medication indicated for the relief of lower urinary tract symptoms associated with benign prostatic hyperplasia (BPH) in men. BPH is a common condition that involves the gradual enlargement of the prostate gland. While it may not produce troublesome symptoms in all men, an enlarged prostate can compress the urethra and obstruct urine outflow. This can result in urinary urgency, nocturia, urinary retention, and frequent urinary tract infections.

Silodosin is a selective antagonist of α1A-adrenoceptors that are predominantly found in the prostate gland, bladder, and urethra. Blocking these receptors causes relaxation of smooth muscle in these tissues, thereby reducing bladder outlet resistance. While α1-adrenoceptor antagonists are frequently used to treat BPH, silodosin may be better tolerated due to its greater selectivity. Receptor binding studies demonstrate that silodosin is 162 times more selective for α1A-adrenoceptors compared to α1B-adrenoceptors. In comparison, tamsulosin has an α1A to α1B binding affinity of 9.55. The greater selectivity of silodosin is likely to translate into fewer cardiovascular adverse effects.

The adverse effects most frequently reported during clinical trials were ejaculatory disorders, with a frequency of 23%. The incidence of orthostatic effects is very low, although rare cases of syncope have been reported. Silodosin may be co-administered with phosphodiesterase-5 inhibitors as studies using maximal doses of sildenafil and tadalafil demonstrate no clinically significant effects on mean systolic or diastolic blood pressure. Co-administration of silodosin with potent inhibitors of cytochrome P450 or P-glycoprotein is not recommended.

References:

  1. Urorec® (silodosin) Australian approved product information. Salisbury South: Mayne Pharma International Pty Ltd. Approved May 2017.
  2. Pande S, Hazra A, Kundu AK. Evaluation of silodosin in comparison to tamsulosin in benign prostatic hyperplasia: a randomized controlled trial. Indian J Pharmacol. 2014; 46(6): 601-7.
  3. Yoshida M, Kudoh J, Homma Y, Kawabe K. Safety and efficacy of silodosin for the treatment of benign prostatic hyperplasia. Clin Interv Aging. 2011; 6: 161-72.

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