chemistry

Phenoxybenzamine

Phenoxybenzamine is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Phenoxybenzamine rather than just read about it. In short: Phenoxybenzamine (PBZ), sold under the brand names Dibenzyline and Dibenyline, is a non-selective, irreversible alpha blocker. Uses It is used in the treatment of hypertension, and specifically that caused by pheochromocytoma.

Key takeaways

Phenoxybenzamine belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Phenoxybenzamine to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Phenoxybenzamine from memory before moving on to harder problems.

Reference excerpt

Phenoxybenzamine (PBZ), sold under the brand names Dibenzyline and Dibenyline, is a non-selective, irreversible alpha blocker.

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, 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. It is also used in complex regional pain syndrome (CRPS) type 1 due to its antiadrenergic effects. It has shown to be beneficial if used in the first 3 months of the CRPS diagnosis.

Investigational Phenoxybenzamine has long been known to block ejaculation without affecting semen quality or ability to achieve orgasm, which could make it an effective male contraceptive. This effect is completely reversible, and is believed to be the result of alpha-1 adrenoceptor blockade in the longitudinal muscles of the vas deferens. A dose of 20 mg/day results in aspermia due to reversible paralyzing effects on the vas deferens, ampulla, and ejaculatory ducts. Due to these actions, phenoxybenzamine is also useful for the treatment of premature ejaculation in men.

Pharmacology Phenoxybenzamine is used as an anti-hypertensive due to its efficacy in reducing the vasoconstriction caused by epinephrine (adrenaline) and norepinephrine. Phenoxybenzamine forms a permanent covalent bond with adrenergic receptors. Based on known information 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. Thus, it remains permanently bound to the receptor, preventing adrenaline and noradrenaline from binding. This causes vasodilation 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. A side effect of phenoxybenzamine is reflex tachycardia. As a non-selective alpha receptor antagonist, it will also affect both the postsynaptic alpha-1 and presynaptic alpha-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. Clinically, non-selective alpha antagonists block alpha receptors (but do not differentiate between alpha-1 and alpha-2). They are used as antihypertensives because they block alpha-receptor-mediated vasoconstriction. The block on alpha-2 receptors further potentiates beta-effects, increasing cardiac output. Phenoxybenzamine has a long-lasting action, binding covalently to the alpha receptors. Its only current clinical use is in preparing patients with pheochromocytoma for surgery; its irreversible antagonism and the resultant depression in the maximum of the agonist dose-response curve are desirable in a situation where surgical manipulation of the tumour may release a large bolus of pressor amine into the circulation. Typically, phenoxybenzamine is not used in the long term, as new receptors are made to upregulate alpha stimulation. The main limiting side-effects of alpha antagonists are that the baroreceptor reflex is disrupted and thus this can cause postural hypotension. Phenoxybenzamine also has irreversible antagonist/weak partial agonist properties at the serotonin 5-HT2A receptor. Due to its 5-HT2A receptor antagonism, phenoxybenzamine is useful in the treatment of carcinoid tumor, a neoplasm that secretes large amounts of serotonin and causes diarrhea, bronchoconstriction, and flushing.

Stereoisomerism Phenoxybenzamine contains a stereocenter, so there are two enantiomers, the (R)- and the (S)-forms. All commercial preparations contain the drug as racemate.

See also Ketanserin Pindobind Phentolamine Phenoxyethylamine

References

External links Media related to Phenoxybenzamine at Wikimedia Commons

Illustrations

Worked examples

Example 1 — a first encounter with Phenoxybenzamine

Start with the simplest possible case. Write down what Phenoxybenzamine claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Phenoxybenzamine before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Phenoxybenzamine ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Phenoxybenzamine

In research: Phenoxybenzamine appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Phenoxybenzamine in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Phenoxybenzamine is common in secondary-school and first-year university syllabi. It links to neighbouring topics 5-HT2A antagonists, Alkylating agents, Alpha-1 blockers, so understanding it makes those chapters shorter.
In everyday life: Look for Phenoxybenzamine outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Phenoxybenzamine in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Phenoxybenzamine means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Phenoxybenzamine out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Phenoxybenzamine in simple terms?

Phenoxybenzamine (PBZ), sold under the brand names Dibenzyline and Dibenyline, is a non-selective, irreversible alpha blocker. Uses It is used in the treatment of hypertension, and specifically that caused by pheochromocytoma.

Why does Phenoxybenzamine matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Phenoxybenzamine?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Phenoxybenzamine.