Methyldopa
Methyldopa, sold under the brand name Aldomet among others, is a medication used for high blood pressure. It is one of the preferred treatments for high blood pressure in pregnancy. For other types of high blood pressure including very high blood pressure resulting in symptoms other medications are typically preferred. It can be given by mouth or injection into a vein. Onset of effects is around 5 hours and they last about a day.Common side effects include sleepiness. More severe side effects include red blood cell breakdown, liver problems, and allergic reactions. Methyldopa is in the alpha-2 adrenergic receptor agonist family of medication. It works by stimulating the brain to decrease the activity of the sympathetic nervous system.Methyldopa was discovered in 1960. It is on the World Health Organizations List of Essential Medicines.

Medical uses
Methyldopa is used in the clinical treatment of the following disorders:

Hypertension (or high blood pressure)
Gestational hypertension (or pregnancy-induced hypertension) and pre-eclampsia.

Side effects
Methyldopa is capable of inducing a number of adverse side effects, which range from mild to severe. Nevertheless, they are generally mild when the dose is less than 1 gram per day. Side effects may include:

Psychological
Depression or even suicidal ideation, as well as nightmares
Apathy or anhedonia, as well as dysphoria
Anxiety, especially of the social anxiety variant
Decreased alertness, awareness, and wakefulness
Impaired attention, focus, and concentration
Decreased desire, drive, and motivation
Fatigue or lethargy or malaise or lassitude
Sedation or drowsiness or somnolence or sleepiness
Agitation or restlessness
Cognitive and memory impairment
Derealization or depersonalization, as well as mild psychosis
Sexual dysfunction including impaired libido, desire, and drive
Physiological
Dizziness, lightheadedness, or vertigo
Miosis or pupil constriction
Xerostomia or dry mouth
Gastrointestinal disturbances such as diarrhea or constipation
Headache or migraine
Myalgia or muscle aches, arthralgia or joint pain, or paresthesia ("pins and needles")
Restless legs syndrome (RLS)
Parkinsonian symptoms such as muscle tremors, rigidity, hypokinesia, or balance or postural instability
Akathisia, ataxia, dyskinesia as well as even tardive dyskinesia, or dystonia
Bells palsy or facial paralysis
Sexual dysfunction consisting of impaired erectile dysfunction or anorgasmia
Hyperprolactinemia or excess prolactin, gynecomastia/breast enlargement in males, or amenorrhoea or absence of menstrual cycles in females
Bradycardia or decreased heart rate
Hypotension or decreased blood pressure (though this may also be considered a therapeutic benefit)
Orthostatic hypotension (also known as postural hypotension)
Hepatitis, hepatotoxicity, or liver dysfunction or damage
Pancreatitis or inflammation of the pancreas
Warm autoimmune hemolytic anemia or deficiency in red blood cells (RBCs)
Myelotoxicity or bone marrow suppression, potentially leading to thrombocytopenia or blood platelet deficiency or leukopenia or white blood cell (WBC) deficiency
Hypersensitivity such as lupus erythematosus, myocarditis, or pericarditis
Lichenoid reactions such as skin lesions or rashes
Pallor

Rebound/withdrawal
Rebound hypertension via withdrawal on account of tolerance upon the abrupt discontinuation of methyldopa has been reported.

Mechanism of action
The mechanism of action of methyldopa is not fully clear. Although it is a centrally acting sympathomimetic, it does not block reuptake or transporters. It may reduce the dopaminergic and serotonergic transmission in the peripheral nervous system and it indirectly affects norepinephrine (noradrenaline) synthesis. Methyldopa acts on alpha-2 adrenergic receptors, which are found on the pre synaptic nerve terminal. This inhibits the synthesis of norepinephrine by inhibiting tyrosine hydroxylase. 
The S-enantiomer of methyldopa is a competitive inhibitor of the enzyme aromatic L-amino acid decarboxylase (LAAD), which converts L-DOPA into dopamine. L-DOPA can cross the blood brain barrier and thus methyldopa may have similar effects. LAAD converts it into alpha-methyldopamine, a false prescursor to norepinephrine, which in turn reduces synthesis of norepinephrine in the vesicles. Dopamine beta hydroxylase (DBH) converts alpha-methyldopamine into alpha-methylnorepinephrine, which is an agonist of the presynaptic α2-adrenergic receptor causing inhibition of neurotransmitter release.

Pharmacokinetics
Maximum decrease in blood pressure occurs 4-6 hours after oral dosage. The half-life of methyldopa is 105 minutes. Methyldopa exhibits variable absorption from the gastrointestinal tract. It is metabolized in the liver and intestines and is excreted in urine.

History
When methyldopa was first introduced, it was the mainstay of antihypertensive treatment, but its use has declined on account of relatively severe adverse side effects, with increased use of other safer and more tolerable agents such as alpha blockers, beta blockers, and calcium channel blockers. Additionally, it has yet to be associated with reducing adverse cardiovascular events including myocardial infarction and stroke, or overall all-cause mortality reduction in clinical trials. Nonetheless, one of methyldopas still current indications is in the management of pregnancy-induced hypertension (PIH), as it is relatively safe in pregnancy compared to many other antihypertensives which may affect the fetus.

See also
Difluoromethyldopa
D-DOPA (dextrodopa)
L-DOPA (levodopa; trade names Sinemet, Pharmacopa, Atamet, Stalevo, Madopar, Prolopa, etc.)
L-DOPS (droxidopa)
Dopamine (Intropan, Inovan, Revivan, Rivimine, Dopastat, Dynatra, etc.)
Norepinephrine (noradrenaline; Levophed, etc.)
Epinephrine (adrenaline; Adrenalin, EpiPed, Twinject, etc.)

References


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