Alpha-1 blocker

Alpha-1 blockers (also called alpha-adrenergic blocking agents) constitute a variety of drugs that block α₁-adrenergic receptors in arteries, smooth muscles, and central nervous system tissues.

Pharmacology

The blockade or reduction of epinephrine and norepinephrine binding on alpha adrenoreceptors reduce arteriolar resistance and increase venous capacitance causes a reflex increase in heart rate. Depending on plasma concentration they may cause orthostatic hypotension. Alpha-1 blockers may decrease LDL and triglycerides and increase HDL.

Indications

These drugs may be used to treat:

• benign prostatic hyperplasia (BPH)^[1]
  • Lower urinary tract symptoms
  • Post Transurethral microwave thermotherapy (TUMT) and Transurethral needle ablation of the prostate (TUNA) procedures
• high blood pressure (hypertension). This is not typically the drug of choice unless the patient also has BPH.
• symptoms of non inflammatory chronic pelvic pain syndrome, a type of prostatitis. As a side effect they may reduce blood pressure and result in lightheadedness.
• Post Traumatic Stress Disorder

Examples of alpha blockers

Others include:

• Trimazosin

Non-selective Adrenergic blockers:

• Phenoxybenzamine
• Phentolamine (Regitine)

Silodosin shows high affinity and selectivity for α_1A adrenergic receptors found in the prostate which ensures that it works quickly and effectively to relieve the symptoms of BPH. Silodosin's low affinity for α_1B receptors in the blood vessels is thought to be reflected in its low incidence of orthostatic and vasodilatory side effects.^[3]

Tamsulosin is relatively selective for α_1a-adrenergic receptors, which are mainly present in the prostate. Hence, it may have a more selective action in BPH with minimal effects on blood pressure.

Adverse effects and interactions

By reducing α₁-adrenergic activity of the blood vessels, these drugs may cause hypotension (low blood pressure) and interrupt the baroreflex response. In doing so, they may cause dizziness, lightheadedness, or fainting when rising from a lying or sitting posture (known as orthostatic hypotension or postural hypotension). For this reason, it is generally recommended that alpha blockers should be taken at bedtime. Additionally, the risk of first dose phenomenon may be reduced by starting at a low dose and titrating upwards as needed.

Because these medications may cause orthostatic hypotension, as well as low blood pressure in general, these agents may interact with other medications that increase risk for low blood pressure, such as other antihypertensives and vasodilators.

As discussed above, tamsulosin may have less risk for low blood pressure and orthostatic hypotension due to its selectivity for α_1a-adrenergic receptors. On the other hand, the drug (a) elevates risk for floppy iris syndrome, and (b) might show adverse drug reactions (ADRs) characteristic of the sulfa related drugs.

References

1. ↑ Höfner K (December 1999). "alpha(1)-Blocker therapy in the nineties: focus on the disease". Prostate Cancer Prostatic Dis. 2 (S4): S9–S15. PMID 12496768. doi:10.1038/sj.pcan.4500368. 
2. ↑ Yilmaz E, Batislam E, Basar MM, Tuglu D, Ferhat M, Basar H (June 2005). "The comparison and efficacy of 3 different alpha1-adrenergic blockers for distal ureteral stones". J. Urol. 173 (6): 2010–2. PMID 15879806. doi:10.1097/01.ju.0000158453.60029.0a. 
3. ↑ Nomiya M, Yamaguchi O. A quantitative analysis of mRNA expression of alpha 1 and beta-adrenoceptor subtypes and their functional roles in human normal and obstructed bladders. J Urol. 2003;170(2 Pt 1):649-653.

External links

• DrugDigest - Alpha blockers
• RxList.com - Tamsulosin
• alpha-Adrenergic Blockers at the US National Library of Medicine Medical Subject Headings (MeSH)