Propranolol

Propranolol (INN) is a sympatholytic non-selective beta blocker. Sympatholytics are used to treat hypertension, anxiety and panic. It was the first successful beta blocker developed. Propranolol is available in generic form as propranolol hydrochloride, as well as an AstraZeneca and Wyeth product under the brand names Inderal, Inderal LA, Avlocardyl (also available in prolonged absorption form named "Avlocardyl Retard"), Deralin, Dociton, Inderalici, InnoPran XL, Sumial, Anaprilinum (depending on marketplace and release rate), Bedranol SR (Sandoz).

Propranolol is one of the banned substances in the Olympics, presumably for its use in controlling social anxiety (stage fright) and tremors.

History and development

Scottish scientist James W. Black successfully developed propranolol in the 1960s. In 1988, he was awarded the Nobel Prize in Medicine for this discovery. Propranolol was derived from the early β-adrenergic antagonists dichloroisoprenaline and pronethalol. The key structural modification, which was carried through to essentially all subsequent beta blockers, was the insertion of a aryloxy bridge into the arylethanolamine structure of pronethalol thus greatly increasing the potency of the compound. This also apparently eliminated the carcinogenicity found with pronethalol in animal models.

Newer, more selective beta-blockers (such as nebivolol, carvedilol, or metoprolol) are now used in the treatment of hypertension.

Indications

Propranolol is indicated for the management of various conditions including:

• Hypertension
• Angina pectoris
• Tachyarrhythmias
• Myocardial infarction
• Control of tachycardia/tremor associated with anxiety, hyperthyroidism or lithium therapy.
• Essential tremor
• Migraine prophylaxis ^{[1] [2]}
• Cluster headaches prophylaxis
• Tension headache (Off the label use)
• Shaky hands

• There has been some experimentation in psychiatric areas:^[3]
  • Treating the excessive drinking of fluids in psychogenic polydipsia,^[4][5]
  • Antipsychotic-induced akathisia,^[6]
  • Aggressive behavior of patients with brain injuries^[7]
  • Post-traumatic stress disorder
• Glaucoma
• Primary exertional headache^[8]

While once first-line treatment for hypertension, the role for beta-blockers was downgraded in June 2006 in the United Kingdom to fourth-line as they perform less well than other drugs, particularly in the elderly, and evidence is increasing that the most frequently used beta-blockers at usual doses carry an unacceptable risk of provoking type 2 diabetes.^[9]

Propranolol is also used to lower portal vein pressure in portal hypertension and prevent esophageal variceal bleeding.

Off-label and investigational use

Propranolol is often used by musicians and other performers to prevent stage fright. It has been taken by surgeons to reduce their own innate hand tremors during surgery.^[10]

Propranolol 80mg daily should be used post discharge in STEMI patients.

Propranolol is currently being investigated as a potential treatment for post-traumatic stress disorder.^{[11][12][13][14]} Propranolol works to inhibit the actions of norepinephrine, a neurotransmitter that enhances memory consolidation. Studies have shown that individuals given propranolol immediately after a traumatic experience show less severe symptoms of PTSD compared to their respective control groups that did not receive the drug (Vaiva et al., 2003). Propranolol reduces the effects of nightmare-related cardiac activity by keeping sinus rhythm low during nightmares, as a higher pulse and increased adrenaline are associated with severe nightmares. However, results remain inconclusive as to the success of propranolol in treatment of PTSD, including nightmares experienced by those with PTSD. There are also many ethical and legal questions surrounding the use of Propranolol-based medications for use as a "memory dampener," including: altering (memory-recalled) evidence during an investigation, modifying behavioral response to past (albeit traumatic) experiences, the regulation of these drugs, and others.^[15]

Propranolol in combination with etodolac is currently being investigated in a Phase 3 trial of 400 colorectal cancer patients as a potential treatment for prevention of colorectal cancer recurrence.^[16] The aim of this study is to assess the use of perioperative medical intervention using a combination of a propranolol and etodolac in order to attenuate the surgically induced immunosuppression and other physiological perturbations, aiming to reduce the rate of tumor recurrence and distant metastatic disease.

Evidence from June 2008 suggests that propranolol can be used to treat severe infantile hemangiomas (IHs).^[17] This treatment has proven superior to corticosteroids, as propranolol has fewer side effects and is more effective when treating IHs.

Propranolol was investigated for possible effects on resting energy expenditure and muscle catabolism in patients with severe burns.^[18] In children with burns, treatment with propranolol during hospitalization attenuated hypermetabolism and reversed muscle wasting.

Propranolol along with a number of other membrane-acting drugs have been investigated for possible effects on Plasmodium falciparum and so the treatment of malaria. In vitro positive effects until recently had not been matched by useful in vivo anti-parasite activity against P. vinckei,^[19] or P. yoelii nigeriensis.^[20] However, a single study from 2006 has suggested that propranolol may reduce the dosages required for existing drugs to be effective against P. falciparum by 5- to 10-fold, suggesting a role for combination therapies.^[21]

Precautions and contraindications

Propranolol should be used with caution in patients with:^[22]

• Diabetes mellitus or hyperthyroidism, since signs and symptoms of hypoglycaemia may be masked.
• Peripheral vascular disease and Raynaud's syndrome, which may be exacerbated
• Phaeochromocytoma, as hypertension may be aggravated without prior alpha blocker therapy
• Myasthenia gravis, may be worsened
• Other drugs with bradycardic effects

Propranolol is contraindicated in patients with:^[22]

• Reversible airways disease, particularly asthma or chronic obstructive pulmonary disease (COPD)
• Bradycardia (<60 beats/minute)
• Sick sinus syndrome
• Atrioventricular block (second or third degree)
• Shock
• Severe hypotension
• Cocaine toxicity [per American Heart Association guidelines, 2005]

Adverse effects

Adverse drug reactions (ADRs) associated with propranolol therapy are similar to other lipophilic beta blockers (see beta blocker).

Pregnancy and lactation

Propranolol, like other beta blockers, is classified as Pregnancy category C in the United States and ADEC Category C in Australia. Beta-blocking agents in general reduce perfusion of the placenta which may lead to adverse outcomes for the neonate, including pulmonary or cardiac complications, or premature birth. The newborn may experience additional adverse effects such as hypoglycemia and bradycardia.

Most beta-blocking agents appear in the milk of lactating women. This is especially the case for a lipophilic drug like propranolol. Breastfeeding is not recommended in patients receiving propranolol therapy.

Pharmacokinetics

Propranolol is rapidly and completely absorbed, with peak plasma levels achieved approximately 1–3 hours after ingestion. Co-administration with food appears to enhance bioavailability. Despite complete absorption, propranolol has a variable bioavailability due to extensive first-pass metabolism. Hepatic impairment will therefore increase its bioavailability. The main metabolite 4-hydroxypropranolol, with a longer half-life (5.2–7.5 hours) than the parent compound (3–4 hours), is also pharmacologically active.

Propranolol is a highly lipophilic drug achieving high concentrations in the brain. The duration of action of a single oral dose is longer than the half-life and may be up to 12 hours, if the single dose is high enough (e.g., 80 mg). Effective plasma concentrations are between 10–100 ng/mL.

Toxic levels are associated with plasma concentrations above 2000 ng/ml.

Mechanism of action

Propranolol is a non-selective beta blocker, that is, it blocks the action of epinephrine and norepinephrine on both β₁- and β₂-adrenergic receptors. It has little intrinsic sympathomimetic activity (ISA) but has strong membrane stabilizing activity (only at high blood concentrations, e.g. overdosage). Research has also shown that propranolol has inhibitory effects on the norepinephrine transporter and/or stimulates norepinephrine release (present experiments have shown that the concentration of norepinephrine is increased in the synapse but do not have the ability to discern which effect is taking place).^[23] Since propranolol blocks β-adrenoceptors, the increase in synaptic norepinephrine only results in α-adrenergic activation, with the α1-adrenoceptor being particularly important for effects observed in animal models. Therefore, some have suggested that it be looked upon as an indirect α1 agonist as well as a β antagonist. Probably owing to the effect at the α1-adrenoceptor, the racemate and the individual enantiomers of propranolol have been shown to substitute for cocaine in rats, with the most potent enantiomer being S-(–)-propranolol. In addition, some evidence suggests that propranolol may function as a partial agonist at one or more serotonin receptors (possibly 5-HT_1B).

Both enantiomers of the drug have a local anesthetic (topical) effect, which is normally mediated by blockade of voltage-gated sodium channels. Few studies have demonstrated propranolol's ability to block cardiac, neuronal, and skeletal voltage-gated sodium channels, accounting for its known “membrane stabilizing effect” and anti-arrhythmic and other central nervous system effects.^{[24] [25] [26]}

Interactions

Beta blockers, including propranolol, have an additive effect with other drugs which decrease blood pressure, or which decrease cardiac contractility or conductivity. Clinically-significant interactions particularly occur with:^[22]

• verapamil
• epinephrine
• β₂-adrenergic receptor agonists
• clonidine
• ergot alkaloids
• isoprenaline
• non-steroidal anti-inflammatory drugs
• quinidine
• cimetidine
• lidocaine
• phenobarbital
• rifampicin
• Fluvoxamine slows down the metabolism of propranolol significantly leading to increased blood levels of propranolol.^[27]

Dosage

The usual maintenance dose ranges for oral propranolol therapy vary by indication:

• Hypertension, angina, essential tremor
  • 120-320 mg daily in divided doses
  • Sustained-release formulations are available in some markets.

• Migraine Prophylaxis
  • The initial dose is 80 mg Inderal daily in divided doses. The usual effective dose range is 160 mg to 240 mg per day.
    • The dosage may be increased gradually to achieve optimum migraine prophylaxis. If a satisfactory response is not obtained within four to six weeks after reaching the maximum dose, Inderal therapy should be discontinued.

• Tachyarrhythmia, anxiety (GAD), hyperthyroidism
  • 10-40 mg 3-4 times daily

• Performance anxiety
  • 5-10 mg 30min or 1.5hrs before and after performance, optionally 5-10 mg night before. Up to 40 mg if necessary, but side-effects may present.^[28]

Intravenous (IV) propranolol may be used in acute arrhythmia or thyrotoxic crisis.^[29]

Chemistry

Propranolol, 1-(iso-propylamino)-3-(1-naphthyloxy)-2-propanol, is synthesized in two ways from the same initial substance. The first way consists of reacting 1-naphthol with epichlorohydrin. Opening of the epoxide ring gives 1-chloro-3-(1-naphthyloxy)-2-propanol, which is reacted further with iso-propylamine, giving propranolol. The second method uses the same reagents in the presence of a base and consists of initially making 3-(1-naphthyloxy)propylenoxide, the subsequent reaction with isopropylamine which results in epoxide ring opening leading to the formation of propranolol.

• A.F. Crowther, L.H. Smith, U.S. Patent 3,337,628 (1967).
• A.F. Crowther, L.H. Smith, U.S. Patent 3,520,919 (1970).
• A.F. Crowther, L.H. Smith, GB 994918  (1963).
• A.F. Crowther, L.H. Smith, DE 1493897  (1963).
• A.F. Crowther, L.H. Smith, BE 640312  (1964).
• A.F. Crowther, L.H. Smith, BE 640313  (1964).

References

External links

• Stapleton MP (1997). "Sir James Black and propranolol. The role of the basic sciences in the history of cardiovascular pharmacology". Texas Heart Institute Journal 24 (4): 336–42. PMC 325477. PMID 9456487. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=325477. 
• Scientific American Interview with James McGaugh
• CBS NEWS 60 Minutes: A Pill To Forget
• U.S. National Library of Medicine: Drug Information Portal - Propranolol