Difference between revisions of "Phenoxybenzamine"

Latest revision as of 15:46, 19 September 2010

Phenoxybenzamine (marketed under the trade name Dibenzyline) is a non-specific, irreversible alpha antagonist.

Uses

It is used in the treatment of hypertension, and specifically that caused by pheochromocytoma. It has a slower onset and a longer lasting effect compared with other alpha blockers.

It was also the first alpha blocker to be used for treatment of benign prostatic hyperplasia,^[1] although it is currently seldom used for that indication due to unfavourable side effects.

It has been used in the treatment of hypoplastic left heart syndrome.^[2]

It is also used in Complex Regional Pain Syndrome Type 1 due to its anti-adrenergic affects. It has shown to be beneficial if used in the first 3 months of CRPS diagnosis.

It is currently under investigation for use as a male contraceptive. Phenoxybenzamine has been found to block ejaculation, which not only gives it the potential to be an effective contraceptive, but could also lead to much cleaner sex in that it would hinder the release of semen-borne pathogens. Studies have found that the quality of the semen is unaffected and the results are reversible by simply discontinuing the treatment.^[3]

Pharmacology

Phenoxybenzamine is used as an anti-hypertensive due to its efficacy in reducing the vasoconstriction caused by adrenaline and noradrenaline. Phenoxybenzamine forms a permanent covalent bond with adrenergic receptors. Based on what we know about the structures of these receptors, it likely involves attack by the cysteine at position 3.36 in transmembrane helix 3 to form a stable linkage.^[4] Thus, it remains permanently bound to the receptor, preventing adrenaline and noradrenaline from binding. This causes vasodilatation in blood vessels, due to its antagonistic effect at the alpha-1 adrenoceptor found in the walls of blood vessels, resulting in a drop in blood pressure.

It will also affect the postsynaptic alpha 1 and 2 receptors in the nervous system, and so reduce sympathetic activity. This results in further vasodilation, pupil constriction, an increase in GI tract motility and secretions, and glycogen synthesis.

It also has partial agonist/antagonist properties at the serotonin 5-HT2A receptor. Due to its 5-HT2A antagonism, it is useful in the treatment of carcinoid tumor, a neoplasm that secretes large amounts of serotonin and causes diarrhea, bronchoconstriction, and flushing. ^[5]

References

es:Fenoxibenzamina ja:フェノキシベンザミン

1. ↑ Caine M, Perlberg S, Meretyk S (1978). "A placebo-controlled double-blind study of the effect of phenoxybenzamine in benign prostatic obstruction". British journal of urology. 50 (7): 551–4. doi:10.1111/j.1464-410X.1978.tb06210.x. PMID 88984. CS1 maint: Multiple names: authors list (link)
2. ↑ Lua error in package.lua at line 80: module 'Module:Citation/CS1/Suggestions' not found.
3. ↑ Lua error in package.lua at line 80: module 'Module:Citation/CS1/Suggestions' not found.
4. ↑ Frang H, Cockcroft V, Karskela T, Scheinin M, Marjamäki A (2001). "Phenoxybenzamine binding reveals the helical orientation of the third transmembrane domain of adrenergic receptors". J. Biol. Chem. 276 (33): 31279–84. doi:10.1074/jbc.M104167200. PMID 11395517. CS1 maint: Multiple names: authors list (link)
5. ↑ Katzung, Trevor et al. Pharmacology Board Review. p.153. Mcgraw Hill, 2007.