Alpha-1 blocker

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Alpha-1 blockers (also called alpha-adrenergic blocking agents) constitute a variety of drugs that reduce the effect alpha-1-adrenergic receptors. They are mainly used to treat benign prostatic hyperplasia (BPH), hypertension and post-traumatic stress disorder.[1] Alpha-1 adrenergic receptors occur in vascular smooth muscle, the central nervous system, and other tissues. When alpha blockers bind to these receptors in vascular smooth muscle, they cause vasodilation.

Over the last 40 years, a variety of drugs have been developed from non-selective alpha-1 antagonists to selective alpha-1 antagonists and alpha-1 inverse agonists.[2][3] The first drug that was used was a non-selective alpha blocker, named phenoxybenzamine and was used to treat BPH.[2] Currently, several relatively selective alpha-1 antagonists are available. As of 2018, prazosin is the only alpha-1 blocker known to act as an inverse agonist at all alpha-1 adrenergic receptor subtypes;[3][4] whereas tamsulosin is a selective antagonist for all alpha-1 subtypes.[3][5] Drugs that act as selective antagonists at specific alpha-1 adrenergic receptor subtypes have also been developed.

Medical uses[edit]

Benign prostatic hyperplasia[edit]

Benign prostatic hyperplasia (BPH) is an enlarged prostate gland. Alpha-1 blockers are the most commonly used medicine to treat BPH.[6] Alpha-1 blockers are first line treatment for the symptoms of BPH in men.[1][2][7][8] Doxazosin, terazosin, alfuzosin and tamsulosin have all been well established in treatment to reduce lower urine tract symptoms (LUTS) caused by benign prostatic hyperplasia. They are all believed to be similarly effective for this purpose. First generation alpha-1 blockers, like prazosin are not recommended to treat lower urine tract symptoms because of their blood pressure lowering effect. Second and third generations are recommended though.[1][9] In some cases alpha-1 blockers have been used in combined therapy with 5-alpha reductase blockers. Dutasteride and tamsulosin are on the market as combined therapy and results have shown that they improve symptoms significantly versus monotherapy.[9][10]

Hypertension[edit]

Alpha-1 blockers are used as second line treatment for high blood pressure. They are not thought to be good as first line treatment because there are other more selective, although they can be good for treating men with hypertension and BPH.[11] Doxazosin have shown to improve symptoms of BPH in elderly and reduce blood pressure at the same time. BPH is very common in men over 60 years old and hypertension as well.[12] Terazosin is also safe and effective to use against hypertension and BPH but is of first generation while doxazosin is second generation alpha-1 blockers.

Post-traumatic stress disorder (PTSD) and nightmares[edit]

Post-traumatic stress disorder is a disabling condition that can be caused by after some kind of life-threatening trauma. It is common in veteran soldiers who have experienced some kind of trauma.[13] Prazosin which is commonly used as antihypertension but because of the alpha-1 adrenergic activity of Prazosin, that activity has been connected to fear and startle responses.[14] Prazosin has been established as an effective and safe, brain active alpha-1 adrenergic receptor antagonist. It can be used against trauma nightmares, sleep disturbance and chronic PTSD.[15]

Adverse effects[edit]

As Alpha-1-a blockers affect the symptoms of BPH more specifically than the Alpha-1 blockers, the adverse effects seem to be more linked to the reproductive system while minimizing the effect on the blood-pressure system.[16][17] Hypotension and its complications (such as weakness and dizziness) are a constant risk, however, even though a selective alpha-1a blocker is being used. It is therefore important when starting treatment with an alpha-1 blocker to monitor the blood pressure to minimize the risk for adverse effects connected to low blood pressure.

Selectivity Adverse effects Main hepatic pathway
Prazosin Alpha-1 Dizziness, headache, drowsiness, asthenia, weakness, palpitations, nausea, vomiting, diarrhea, constipation, edema, hypotension, dyspnea, vertigo, nervousness, rash, blurred vision CYP1A1[18]
Terazosin Alpha-1 Nervousness, vertigo, palpitations, tachycardia, chest pain, hypotension, dyspnoea, nausea, constipation, diarrhoea, vomiting, pruritus, rash, back pain, impotence, dizziness, asthenia, oedema, headache, pain (extreme) CYP3A1[19]
Doxazosin Alpha-1 Hypotension (dizziness and weakness), priapism (prolonged erection), floppy iris syndrome during cataract surgery, vertigo, palpitation/tachycardia, bronchitis, cough, dyspnea, pruritus, back pain, cystitis, asthenia, peripheral oedema, retrograde ejaculation CYP3A4[20]
Silodosin Alpha-1-a Problem with ejaculation/orgasm, hypotension, dizziness, nasal congestion, diarrhoea CYP3A4[21]
Alfuzosin Alpha-1-a Dizziness(due to hypotension), upper respiratory tract infection, headache, fatigue, impotence, pain in whole body, bronchitis, gastro-intestinal pain, swelling, rash, difficulty swallowing or breathing, chest pain, fainting, hoarsing CYP3A4[22]
Tamsulosin Alpha-1-a Hypotension, problem with ejaculation/orgasm, disorientation, headache, floppy iris syndrome during cataract surgery CYP3A4 & CYP2D6[23]

By reducing alpha-1-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. The risk of first dose phenomenon may be reduced or eliminated by gradual-dose titration, since the adverse effects of Prazosin are dose-related.[7] This is also the case for Tamsulosin and it may be assumed that the others alpha-1 blockers work in a similar manner, since Tamsulosin is an alpha-1-a blocker and Prazosin is an alpha-1 blocker.[24] The risk for floppy iris syndrome during cataract surgery is elevated when the patient is using an alpha-1 blocker. This is especially the case for Tamsulosin and other alpha-1-a blockers, since alpha-1-a receptors are present also in the iris dilator muscle, which allows unopposed action of the parasympathetically innervated iris constrictor muscle and loss of iris tone.[25] This however can be treated if the eye surgeon is experienced and has knowledge of the use of alpha-1 blocker.

Interactions and contraindication[edit]

Contraindication : Allergies or hypersensitivity to alpha-1 blockers or any of the active ingredient, that includes angiodema induced by the drug. Patients with a history of orthostatic hypotension or severe hepatic impairment.[26][27]

Interactions : No interactions were recorded when administered with atenolol (beta blocker), enalapril (ACE inhibitor) and theophylline.[26] Furosemide has drop effect on plasma level for tamsulosin, and a rise in plasma level with cimetidine. No dose adjustment needs to be done when the levels are in normal range. Drugs that inhibit CYP3A4 (for example, itraconazole, ketoconazole, and ritonavir) can increase drug exposure for tamsulosin, alfuzosin, doxazosin and silodosin. Grapefruit is also a powerful inhibitor of the CYP3A4 enzyme, so concurrent use is not recommended as it may increase the plasma levels of the Alpha-1 blockers which are metabolised by the CYP3A4 enzyme.[28] Some drugs; such as Fluoxetine, Paroxetine and Ritonavir are strong inhibitors of the CYP2D6 enzyme and therefore it is not recommended to use at the same time as tamsulosin, as it may increase plasma levels of tamsulosin and increase the risk of adverse effects.[29]

Warfarin and diclofenac can increase elimination rate for tamsulosin, but has not shown effect on alfuzosin hydrochloride. Co-administration of alpha-1 inhibitor can cause hypotension.[26][27]

Since alpha-1 blockers may cause orthostatic hypotension, co-administration with antihypertensives and vasodilators must be evaluated with regards to risk-benefit as the risk for low blood pressure is greatly increased.

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 α1a-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.

List of alpha-1 blockers[edit]

Alpha-1 inhibitor Structure Use Brand name Selectivity
Prazosin Hypertension, PTSD and nightmares Minipress, Vasoflex, Lentopres and Hypovase. Alpha-1
Terazosin Hypertension and BPH Hytrin, Zaysel and Terazosin Alpha-1
Doxazosin Hypertension and BPH Cardura and Carduran Alpha-1
Silodosin BPH Rapaflo, Silodyx, Rapilif, Urief, Trupas, Urorec Alpha-1a
Alfuzosin BPH Uroxatral, Xat, Xatral, Prostetrol and Alfural. Alpha-1a
Tamsulosin BPH Alna, Flomax, Omnic Alpha-1a

Silodosin shows high affinity and selectivity for alpha-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 alpha-1b receptors in the blood vessels is thought to be reflected in its low incidence of orthostatic and vasodilatory side effects.[30]

Pharmacokinetics[edit]

Absorption : Bioavailability of tamsulosin and terazosin is around 90% during oral administration in fasting state. Food can have effect on absorption for tamsulosin if it has been ingested shortly before, Tmax for fasting state is 2,9-5,6 hours compared to 5,2–7 hours in fed state. Food has no effect on absorption of terazosin but can delay plasma level concentration for 1 hour, peak plasma level are around 1–2 hours.[31][26][32] Alfuzosin bioavailability under fed state is around 49%. Tmax is 8 hours in fed state.[33][27] Tamsulosin Cmax range was 13.9-18,6 ng/mL fastest and in fed state 7,2-15,6 ng/mL, Cmax for alfuzion is 13,6 mg/mL.[34][26]

Distribution : Tamsulosin is 99% bound to plasma and distribution volume is low 0,2l/kg.[26] Alfuzosin is 90% bound to plasma and distribution volume is 2,5l/kg.[33][27] Terazosin is 90-94% bound to plasma.[32]

Elimination : Elimination half-life for alfuzosin is around 8 hours, alfuzosin is metabolised mainly via liver. 75-91% is excreted in feces and 35% in unchanged form. Distribution volume and excretion increases with renal impairment do to less protein binding, but the half-life elimination rate is unchanged. therefore no dose adjustment is needed for low to moderate renal impairment. Delay in elimination half-life, peak concentration in plasma is double and bioavailability is changed in hepatic impairment patients. Alfuzosin should not be used for patients with renal impairment.[33][27] Tamsulosin is excreted via urine and 9% of that is unchanged on its active form, elimination half-life for tamsulosin is between 9–13 hours for healthy volunteers. The elimination half-life for target patients is around 14–15 hours. No dose adjustment is needed for patients with renal impairment and moderate hepatic impairment.[31][26] 10-20% of terazosin is excreted unchanged in urine and feces during oral administration. 40% is eliminated in urine and 60% in feces. Eliminations half-life for terazosin is between 8–13 hours. No dose adjustment is needed for patients with renal impairment. Terazosin is metabolised by the liver and is excreted by the bilary tract, so patients with moderate hepatic impairment should receive titrated doses of terazosin witch caution. Patients with severe hepatic impairment should not take terazosin do to lack of clinical data.[32][35]

Mechanism of action[edit]

Alpha 1 blockers inhibit norepinephrine (NE) from entering smooth muscles and prevent contraction.
Alpha 1 blockers inhibit norepinephrine (NE) from entering smooth muscles and prevent contraction.

Alpha-1 blockers inhibit norepinephrine which inhibits the blood vessels from contraction. It happens because alpha 1 blockers inhibit the activation of post-synaptic alpha-1 receptors by releasing catecholamines and/or the circulation which are neural. Alpha-1 blockers have no effect on renin release or cardio output.[36]

Benign prostatic hyperplasia[edit]

Alpha-1 blocker, blocks alpha receptors and it relaxes the smooth muscles in the bladder. It helps the urine to flow smoothly and it can lessen the pain. from the bladder pressing on the prostate.[37][38] Selective alpha-1 blockers are better tolerated than non-selective alpha blockers in the body and therefore works better on BPH.[2] Terazosin, tamsulosin and doxazosin are prime drug for BPH because they have a long half-life and modified release formulation. Tamsulosin is primarily used because it doesn't affect the blood pressure and the side effects of vasodilation is minimum.[37][38][2]

Hypertension[edit]

Alpha-1 blocker lowers the blood pressure by blocking alpha-1 receptors so norepinephrine can't bind the receptor and it causes the blood vessels to dilate. Without the resistance in the blood vessels the blood runs more freely.[39][40] Alpha-1 blockers have a good effect on lipoproteins in plasma, insulin resistance and it causes the glucose levels in blood to lower.[39][40][41]

Structure activity relationship (SAR)[edit]

By changing the furan ring in prazosin to tetrahydrofuran ring (as in alfuzosin) the half-life is greatly increased, allowing once-a-day dosing. Silodosin is the most selective for alpha-1a receptors.[42] The affinity and selectivity for alpha-1 receptors seems to be determined by structure between the quinazoline and the furan ring. Piperazine is present in prazosin, terazosin and doxazosin which seems to contribute to the non-selective inhibition of alpha-1 receptors.[43]

Comparison between Prazosin, Terazosin and Alfuzosin. The conversion of the furan ring to a tetrahydrofuran ring (colored purple) contributed to an increase in half-life of terazosin. The conversion of piperazine to aminopropyl (colored red) increases the selectivity for Alpha-1a receptors.

Doxazosin 2,4-diamino-6,7-dimethoxyquinazoline variations for in vitro and in vivo performance. A key factor in these structures was the derivations from 2,4-diamino-6,7-dimethoxyquinazoline nucleus that was replaced for norepinephrine. And N-1 which protonated quinazoline was also a key factor.[44]

Variation of structure 2,4-diamino-6,7-dimethoxyquinazoline.
Variation of structure 2,4-diamino-6,7-dimethoxyquinazoline.

Tamsulosin is most potent alpha 1 blocker and has the most selectivity for alpha 1a blockers. It doesn't do anything for beta-adrenoceptors because it lacks beta-hydroxyl group and therefor can not block beta-receptors.[45]

History[edit]

The first effective treatment for benign prostatic hyperplasia (BPH) was a non-selective alpha blocker phenoxybenzamine which was irreversible. Dibenzyline was the first brand name marketed. Today phenoxybenzamine is not the first choice due to many side effects like lowering blood pressure.[2]

First selective alpha-1 blocker that was approved to treat hypertension was prazosin. Prazosin was synthesized in 1974 when Constantin and Hess were trying to discover a vasodilator which had a minimal effect on cardiac activity.[7] Prazosin is a much better tolerated drug than phenoxybenzamine but the problem that still remained was it lowered the blood pressure too much.[2][8]

Terazosin was the first long lasting alpha 1 blocker approved by FDA to treat BPH. Doxazosin and Tamsulosin was approved after. The first line treatment choice today to treat BPH is tamsulosin. It is not because it is better tolerated or had greater efficacy than the previous drugs. Only because of the minimal dose titration. Alfuzosin SR (sustained release) was the fourth alpha 1 selective blocker to be approved by FDA. It is used to treat BPH and the good thing about Alfuzosin SR is, it doesn´t need a dose titration. Over 30 years of improving alpha 1 blockers for BPH it has been primarily focused on tolerance and good use.[2][40]

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