MEPIN

Indications

• Hypertension
• Pregnancy-induced hypertension

Dosage

Children: Refer to the BNF for Children for appropriate dosing information.

Adults: Initially 250 mg 2 to 3 times a day, gradually increased as necessary at intervals of at least 2 days; maximum daily dose is 3 g.

Mechanism of action

Methyldopa is metabolized to alpha-methylnorepinephrine, which acts as an agonist at presynaptic alpha-2 adrenergic receptors in the brainstem. This stimulation inhibits adrenergic neuronal outflow and reduces norepinephrine release, leading to decreased sympathetic nervous system activity and lower blood pressure. Additionally, methyldopa inhibits aromatic L-amino acid decarboxylase, contributing to decreased levels of norepinephrine and other biogenic amines, although this mechanism plays a minimal role in its antihypertensive effect.

Pharmacodynamics

The antihypertensive effects of methyldopa are mainly mediated by its active metabolite, alpha-methylnorepinephrine. This metabolite decreases peripheral sympathetic tone and lowers arterial pressure. Methyldopa reduces levels of serotonin, dopamine, norepinephrine, and epinephrine, resulting in lower blood pressure without significantly affecting renal blood flow or glomerular filtration rate. It may also slow heart rate in some patients. The onset of action occurs within 12 to 24 hours, with peak effects seen in 4 to 6 hours after administration.

Pharmacokinetics

Methyldopa is administered orally and is well absorbed, with peak plasma concentrations typically occurring within 1 to 2 hours. It undergoes extensive hepatic metabolism, primarily to alpha-methylnorepinephrine. The elimination half-life is approximately 2 hours, with metabolites excreted in urine. Blood pressure effects can last for 24 to 48 hours post-discontinuation.

Contra-indications

• Severe bradyarrhythmia secondary to second- or third-degree AV block
• Acute porphyrias

Adverse effects

• Constipation
• Depression
• Breast enlargement
• Cardiac inflammation
• Dizziness
• Dry mouth
• Fatigue
• Headache
• Nausea
• Postural hypotension
• Sedation
• Cognitive impairment
• Vomiting
• Diarrhoea
• Eosinophilia
• Gynaecomastia
• Haemolytic anaemia
• Skin reactions
• Delusions
• Hallucinations
• Granulocytopenia
• Lupus-like syndrome
• Pancreatitis
• Psychiatric disorders

Interactions

• Entacapone increases exposure to methyldopa (unknown effect)

Precautions

• Cerebrovascular disease
• History of depression
• Mild to moderate heart failure
• History of hepatic impairment
• Bradyarrhythmia
• Polyneuropathy
• Raynaud’s syndrome
• Other occlusive peripheral vascular disease
• Elderly patients may experience intolerance

Pregnancy

May lower fetal heart rate. Avoid oral use unless potential benefit outweighs risk.

Breast-feeding

Use with caution; consult healthcare provider.

Storage

Store in a cool, dry place away from direct sunlight.

Formulations

• Oral tablets
• Oral suspension

Indications

• Hypertension
• Angina pectoris (prophylaxis)

Dosage

Children: Not licensed for use in children.

Adults: Initially 5 mg three times a day, which may be adjusted according to response, with a typical maintenance dose of 10-20 mg three times a day. For modified-release formulations, the initial dose is often 20 mg twice daily.

Mechanism of action

Nifedipine blocks voltage-gated L-type calcium channels in vascular smooth muscle and myocardial cells. This action prevents calcium ions from entering the cells during depolarization, reducing peripheral arterial resistance and dilating coronary arteries. This results in lowered blood pressure and increased oxygen delivery to the heart, alleviating angina symptoms. The mechanism by which nifedipine inhibits calcium influx is not completely understood, but it is believed to involve interference with ion-control gating mechanisms of the calcium channels.

Pharmacodynamics

Nifedipine is an inhibitor of L-type voltage-gated calcium channels, which reduces blood pressure and increases oxygen supply to the heart. The immediate-release formulations require dosing three times daily due to their shorter duration of action. The typical daily dose ranges from 10 to 120 mg, with careful monitoring for excessive hypotension and potential angina exacerbation.

Pharmacokinetics

Nifedipine is rapidly absorbed following oral administration, with peak plasma concentrations typically occurring within 30 minutes to 2 hours for immediate-release forms. Its bioavailability is affected by first-pass metabolism, and it is extensively metabolized in the liver. The half-life of nifedipine varies, but it generally ranges from 2 to 5 hours. The drug is primarily excreted as metabolites in the urine, with only a small fraction excreted unchanged.

Contra-indications

• Cardiogenic shock
• Aortic stenosis
• Severe maternal hypotension
• Fatal fetal hypoxia in pregnancy

Adverse effects

• Constipation
• Malaise
• Oedema
• Allergic reactions
• Angioedema
• Hypotension
• Reflex tachycardia

Interactions

• Grapefruit juice (severe increase in exposure)
• Rifampicin (severe decrease in exposure)
• Enzalutamide (moderate decrease in exposure)
• Dronedarone (moderate increase in exposure)
• Antifungal azoles (moderate increase in exposure)
• Miconazole (moderate increase in exposure)
• Cobicistat (moderate increase in exposure)
• Crizotinib (moderate increase in exposure)
• Dabrafenib (moderate decrease in exposure)
• Idelalisib (moderate increase in exposure)

Precautions

• Monitor blood pressure and heart rate regularly
• Use with caution in patients with significant left ventricular dysfunction
• Avoid excessive decrease in blood pressure
• Consider initial low doses in patients with renal impairment

Pregnancy

May inhibit labour; not to be used in multiple pregnancies unless no other acceptable alternative.

Breast-feeding

Manufacturer advises to avoid; present in breast milk.

Storage

Store at room temperature, away from moisture and heat.

Formulations

• Immediate-release capsules
• Modified-release formulations
• Oral suspension
• Solution for infusion

Mechanism of action

The exact mechanism of methyldopa is not fully elucidated; however, the main mechanisms of methyldopa involve its actions on alpha-adrenergic receptor and the aromatic L-amino acid decarboxylase enzyme, to a lesser extent. The sympathetic outflow is regulated by alpha (α)-2 adrenergic receptors and imidazoline receptors expressed on adrenergic neurons within the rostral ventrolateral medulla. Methyldopa is metabolized to α‐methylnorepinephrine via dopamine beta-hydroxylase activity and, consequently, alpha-methylepinephrine via phenylethanolamine-N-methyltransferase activity. Mediating the therapeutic effects of methyldopa, α‐methylnorepinephrine and α-methylepinephrine active metabolites are agonists at presynaptic alpha-2 adrenergic receptors in the brainstem. Stimulating alpha-2 adrenergic receptors results in the inhibition of adrenergic neuronal outflow and attenuation of norepinephrine release in the brainstem. Consequently, the output of vasoconstrictor adrenergic signals to the peripheral sympathetic nervous system is reduced, leading to a reduction in blood pressure. The L-isomer of alpha-methyldopa also reduces blood pressure by inhibiting aromatic L-amino acid decarboxylase, also known as DOPA decarboxylase, which is an enzyme responsible for the syntheses of dopamine and serotonin. Inhibiting this enzyme leads to depletion of biogenic amines such as norepinephrine. However, inhibition of aromatic L-amino acid decarboxylase plays a minimal role in the blood-pressure‐lowering effect of methyldopa. METHYLDOPA...HAS HYPOTENSIVE ACTION INDEPENDENT OF ITS ANTIADRENERGIC ACTIONS; THIS IS PROBABLY PARTLY CENTRAL DEPRESSANT ACTION @ VASOMOTOR CENTER & PARTLY PERIPHERAL ACTION OF UNKNOWN MECHANISM. ... Alpha-methylnorepinephrine acts in the brain to inhibit adrenergic neuronal outflow from the brainstem, and this central effect is principally responsible for its antihypertensive action. IN CONSCIOUS RENAL HYPERTENSIVE RATS ALPHA-METHYLDOPA PRODUCED A LONG-LASTING FALL IN BLOOD PRESSURE WHICH WAS PARTIALLY ATTENUATED BY PRETREATMENT WITH NALTREXONE (5 MG/KG SC). PRETREATMENT WITH ANTISERUM TO BETA-ENDORPHIN APPLIED LOCALLY, ALSO BLOCKED THE DEPRESSOR RESPONSE. THESE RESULTS SUGGEST THAT THE FALL IN BLOOD PRESSURE OBSERVED AFTER ALPHA-METHYLDOPA AND ITS ACTIVE METABOLITE ALPHA-METHYLNORADRENALINE INVOLVES A BETA-ENDORPHIN LIKE PEPTIDE; A POSSIBLE SITE OF ACTION IS THE NUCLEUS TRACTUS SOLITARII. A REVIEW ON THE MECHANISM OF ACTION.

Pharmacodynamics

Antihypertensive effects of methyldopa are mostly mediated by its pharmacologically active metabolite, alpha-methylnorepinephrine, which works as an agonist at central inhibitory alpha-adrenergic receptors. Stimulation of alpha-adrenergic receptors leads to decreased peripheral sympathetic tone and reduced arterial pressure. Methyldopa causes a net reduction in the tissue concentration of serotonin, dopamine, norepinephrine, and epinephrine. Overall, methyldopa lowers both standing blood pressure and especially supine blood pressure, with infrequent symptomatic postural hypotension. Methyldopa also reduces plasma renin activity but has negligible effects on glomerular filtration rate, renal blood flow, or filtration fraction. It also has no direct effect on cardiac function but in some patients, a slowed heart rate may occur. Following oral administration, blood-pressure-lowering effects are observed within 12 to 24 hours in most patients, and a maximum reduction in blood pressure occurs in 4 to 6 hours. Blood pressure returns to pre-treatment levels within 24 to 48 hours following drug discontinuation. Following intravenous administration, the blood-pressure-lowering effects of methyldopa last for about 10 to 16 hours.

Mechanism of action

Nifedipine blocks voltage gated L-type calcium channels in vascular smooth muscle and myocardial cells. This blockage prevents the entry of calcium ions into cells during depolarization, reducing peripheral arterial vascular resistance and dilating coronary arteries. These actions reduce blood pressure and increase the supply of oxygen to the heart, alleviating angina. The principal physiologic action of nifedipine is to inhibit the transmembrane influx of extracellular calcium ions across the membranes of myocardial cells and vascular smooth muscle cells, without changing serum calcium concentrations. Calcium plays important roles in the excitation-contraction coupling processes of the heart and vascular smooth muscle cells and in the electrical discharge of the specialized conduction cells of the heart. The membranes of these cells contain numerous channels that carry a slow inward current and that are selective for calcium. Activation of these slow calcium channels contributes to the plateau phase (phase 2) of the action potential of cardiac and vascular smooth muscle cells. The exact mechanism whereby nifedipine inhibits calcium ion influx across the slow calcium channels is not known, but the drug is thought to inhibit ion-control gating mechanisms of the channel, deform the slow channel, and/or interfere with release of calcium from the sarcoplasmic reticulum. By inhibiting calcium influx, nifedipine inhibits the contractile processes of cardiac and vascular smooth muscle, thereby dilating the main coronary and systemic arteries. Nifedipine is a peripheral arterial vasodilator which acts directly on vascular smooth muscle. The binding of nifedipine to voltage-dependent and possibly receptor-operated channels in vascular smooth muscle results in an inhibition of calcium influx through these channels. Stores of intracellular calcium in vascular smooth muscle are limited and thus dependent upon the influx of extracellular calcium for contraction to occur. The reduction in calcium influx by nifedipine causes arterial vasodilation and decreased peripheral vascular resistance which results in reduced arterial blood pressure.

Pharmacodynamics

Nifedipine is an inhibitor of L-type voltage gated calcium channels that reduces blood pressure and increases oxygen supply to the heart. Immediate release nifedipine's duration of action requires dosing 3 times daily. Nifedipine dosing is generally 10-120mg daily. Patients should be counselled regarding the risk of excessive hypotension, angina, and myocardial infarction.