1. Clinical Overview
A fixed-dose combination (FDC) therapy for the management of moderate to severe symptoms of Benign Prostatic Hyperplasia (BPH). Silodosin is a highly selective alpha-1A adrenergic receptor antagonist that provides rapid relief from dynamic symptoms (e.g., urinary flow). Dutasteride is a dual 5-alpha reductase inhibitor (inhibits both Type I and Type II isoenzymes) that reduces prostate volume by inhibiting the conversion of testosterone to dihydrotestosterone (DHT), addressing the static component of BPH. This combination provides synergistic relief of lower urinary tract symptoms (LUTS) and reduces the risk of disease progression and acute urinary retention.
| Onset | Duration | Bioavailability |
|---|---|---|
| Silodosin: Symptomatic relief within 2-4 hours. Dutasteride: Reduction in prostate size and serum DHT observed within 1-2 weeks, but maximum clinical benefit may take 3-6 months. | Silodosin: Approximately 24 hours (once-daily dosing). Dutasteride: Very long; due to its long half-life, effects persist for weeks after discontinuation. | Silodosin: Approximately 32% (in fasting state). Dutasteride: About 60% (absolute bioavailability). |
2. Mechanism of Action
The combination works via two complementary mechanisms. Silodosin selectively blocks alpha-1A adrenoceptors, which are predominant in the prostate stroma, bladder neck, and prostatic urethra. This blockade inhibits the sympathetically mediated contraction of smooth muscle in these areas, reducing dynamic urethral resistance and improving urine flow. Dutasteride inhibits both Type I and Type II isoforms of the enzyme 5-alpha reductase, which is responsible for converting testosterone to the more potent androgen dihydrotestosterone (DHT) within the prostate gland. Reduced intraprostatic DHT leads to involution of the epithelial glandular tissue, decreasing prostate volume (static component) over time.
3. Indications & Uses
- Treatment of symptomatic Benign Prostatic Hyperplasia (BPH) in men with moderate to severe symptoms (IPSS score >7)
- To improve urinary flow, reduce symptoms of obstruction (hesitancy, weak stream, straining) and irritation (frequency, urgency, nocturia)
- To reduce the risk of acute urinary retention (AUR) and the need for BPH-related surgery
4. Dosage & Administration
Adult Dosage: One tablet (Silodosin 8mg + Dutasteride 0.5mg) orally once daily, preferably with the same meal each day to standardize absorption.
Administration: Swallow the tablet whole with a glass of water, preferably with or immediately after a meal (to reduce risk of orthostatic hypotension and standardize Silodosin absorption). Do not crush, break, or chew the tablet. If a dose is missed, take it as soon as remembered unless it is almost time for the next dose. Do not double the dose.
5. Side Effects
Common side effects may include:
- Retrograde ejaculation or ejaculation failure (Silodosin effect)
- Decreased libido (Dutasteride effect)
- Dizziness/Postural dizziness
- Headache
- Nasopharyngitis/Rhinitis
- Diarrhea
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Strong CYP3A4 Inhibitors (Ketoconazole, Itraconazole, Ritonavir, Clarithromycin) | Significantly increases Silodosin plasma concentration, increasing risk of adverse effects like hypotension. Contraindicated in hepatic impairment. | Major |
| Other Alpha-Adrenergic Blockers (Prazosin, Tamsulosin, Terazosin, Doxazosin) | Additive hypotensive and orthostatic effects. Generally not recommended. | Major |
| Antihypertensives (Beta-blockers, ACE inhibitors, Calcium channel blockers, Diuretics) | Potentiates hypotensive effect. Monitor blood pressure, especially during initiation. | Moderate |
| Phosphodiesterase-5 Inhibitors (Sildenafil, Tadalafil, Vardenafil) | Additive vasodilatory and hypotensive effects. Use with caution and separate administration times. | Moderate |
| Moderate CYP3A4 Inhibitors (Erythromycin, Verapamil, Diltiazem) | May increase Silodosin levels. Use with caution and monitor. | Moderate |
| Warfarin | Dutasteride is highly protein-bound; theoretical displacement interaction. Monitor INR closely, especially during initiation/discontinuation. | Moderate |
7. Patient Counselling
- DO take the tablet once daily, preferably with the same meal (e.g., breakfast or dinner).
- DO swallow the tablet whole with water. Do not crush, break, or chew.
- DO inform all your doctors and your surgeon (especially eye surgeon for cataract) that you are taking this medicine.
- DO get your PSA tested regularly as advised by your urologist, and inform them you are on this medication.
- DONT donate blood while taking this medicine and for at least 6 months after stopping it (to prevent transmission to a pregnant woman).
- DONT start any new prescription, over-the-counter medicine, or herbal supplement without consulting your doctor.
- DONT change your dose or stop taking the medicine without consulting your doctor, even if you feel better.
8. Toxicology & Storage
Overdose: Primarily an extension of Silodosin's alpha-blockade effects: severe hypotension, dizziness, tachycardia, syncope. Dutasteride overdose is unlikely to cause acute toxicity due to its wide therapeutic index but may lead to increased anti-androgenic effects.
Storage: Store below 30°C. Protect from light and moisture. Keep in the original blister pack or container. Keep out of reach and sight of children. Do not use after the expiry date printed on the pack. Dispose of unused medicine properly; do not flush.