Pharmacokinetics and Prodrug Principle
When administered orally, the drug exhibits relatively low bioavailability, with no more than 50% of the initial dose reaching the systemic circulation unchanged. However, the therapeutic value of methyldopa lies not in its peripheral action, but in its ability to readily cross the blood-brain barrier (BBB) and enter the central nervous system.
The drug's mechanism of action strictly follows the prodrug principle. Once inside the brain, the parent molecule exerts no direct effect, but instead undergoes a sequential two-step biochemical transformation:
- In the first step, the substance is converted into the intermediate metabolite — $\alpha$-methyldopamine.
- In the second step, final transformation into the active compound occurs — $\alpha$-methylnorepinephrine.
It is the resulting $\alpha$-methylnorepinephrine that acts as the target agent: it selectively stimulates $\alpha_2$-adrenergic receptors located on the membranes of neurons in the nucleus tractus solitarius (NTS).
Hemodynamic Effects and Time of Action
Stimulation of central $\alpha_2$-adrenergic receptors in the nucleus tractus solitarius triggers a cascade of hemodynamic changes that together provide a pronounced antihypertensive effect. The process unfolds as follows:
- First, there is a significant decrease in vasomotor center tone.
- Consequently, total peripheral resistance (TPR) decreases as blood vessels relax.
- An additional factor is the reduction in cardiac output.
- Summary of effects: Reliable and stable reduction of blood pressure (BP).
It is important to consider the drug's time profile. Because methyldopa requires time to cross the blood-brain barrier and undergo complex metabolic transformation within the CNS, it cannot be used to treat acute hypertensive emergencies. The onset of the therapeutic effect is delayed, appearing only 3–5 hours after oral administration. Meanwhile, the achieved effect is long-lasting, persisting for up to 24 hours.
Extensive Side Effect Profile
The widespread use of methyldopa in routine practice is severely limited by a broad spectrum of adverse effects. Adverse reactions can affect various organ systems:
- Central nervous system: The most frequent complaints include pronounced sedation, sluggishness, and constant drowsiness.
- Autonomic manifestations: Patients often experience dry mouth, gastrointestinal motility disorders (constipation), and orthostatic hypotension (a sharp drop in blood pressure when moving to an upright position).
- Fluid and electrolyte balance: The drug actively promotes the retention of sodium ($Na^+$) and water, clinically manifesting as peripheral edema.
- Specific and severe reactions: Long-term use carries a risk of hepatotoxicity (severe liver dysfunction). Cases of drug-induced parkinsonism, severe hematologic abnormalities (including leukopenia and hemolytic anemia), and pronounced sexual dysfunction have also been reported.
Place in Modern Clinical Practice
In modern pharmacological classification, methyldopa belongs to the first-generation centrally acting antihypertensive agents. Its class counterparts include well-known drugs such as clonidine and guanfacine.
Today, the use of methyldopa in general cardiology practice is strictly limited. Physicians avoid prescribing it to the general patient population due to the high frequency of adverse effects, objective difficulties in selecting the optimal dosage, and the high risk of a dangerous withdrawal syndrome upon abrupt cessation of therapy.
Nevertheless, the drug holds a uniquely important clinical niche. Methyldopa is officially recognized as the drug of choice for the treatment of arterial hypertension in pregnancy. In this specific clinical scenario, its proven safety for the fetus and reliable blood pressure control completely outweigh the potential risks to the mother.