Beta-adrenergic (Ritodrine)

Mechanism

Mechanism of Action

Ritodrine is a selective agonist of β₂-adrenergic receptors located in myometrial smooth muscle cells and in the bronchial and vascular tree.

Relaxation Signaling by cAMP

1. Activation of Adenylate Cyclase (Gs pathway): The binding of ritodrine to myometrial β₂ receptors activates the Gs stimulatory subunit, which stimulates cytosolic adenylate cyclase, catalyzing the conversion of ATP to cyclic adenosine monophosphate (cAMP).
2. Phosphorylation of MLCK by PKA: Elevated levels of cAMP activate protein kinase A (PKA), which directly phosphorylates myosin light chain kinase (MLCK). Phosphorylated MLCK loses affinity for the calcium-calmodulin complex.
3. Calcium Sequestration: PKA additionally stimulates the uptake of calcium back to the sarcoplasmic reticulum mediated by the ATP-dependent calcium pump (SERCA) and activates calcium-dependent potassium channels, hyperpolarizing the myocyte cell membrane.
4. Uterine relaxation: The gravid uterus interrupts its contractile activity.

Pharmacokinetics

Pharmacokinetics

• Routes of administration: Intravenous (continuous regulated infusion with infusion pump); The oral route was previously used for maintenance, but was withdrawn due to ineffectiveness and poor safety profile.
• Distribution: Large apparent volume of distribution. It easily crosses the placental barrier, reaching rapid hemodynamic equilibrium in the plasma of the fetus.
• Metabolism: Extensive hepatic conjugation via sulfation and glucuronidation to inactive hydrophilic metabolites.
• Half-life (t_1/2): Short, approx. 2 to 2.5 hours after stopping the intravenous infusion.
• Excretion: Renal in 90% in the form of conjugates and unchanged drug.

Indicators and dose

Indications

• Acute tocolysis in uncomplicated imminent preterm delivery to allow fetal lung maturation.
• Emergency intrapartum tocolysis: Treatment of uterine tachysystole or hypertonia associated with acute fetal distress ("intrauterine fetal resuscitation").

Security

Contraindications

Critical Safety Alert: Maternal Cardiovascular Complications

Due to overlap and interaction with cardiac β₁-adrenergic receptors at therapeutic doses, ritodrine induces massive systemic adrenergic stimulation. It is strictly contraindicated in patients with:

• Underlying maternal heart disease (coronary insufficiency, valvular stenosis, cardiomyopathy, arrhythmias).
• Uncontrolled maternal hyperthyroidism (increased risk of thyroid storm and fatal arrhythmias).
• Decompensated or unstable maternal diabetes mellitus (β₂ agonists stimulate hepatic glycogenolysis and glucagon secretion, inducing acute hyperglycemic diabetic ketoacidosis).
• Premature placental abruption or active genital bleeding.
• Amniotic infection (chorioamnionitis).

Adverse Effects (ADR)

• Maternal Cardiovascular: Severe tachycardia (HR > 120-140 bpm), intense palpitations, decrease in diastolic blood pressure with increased differential pressure and acute pulmonary edema (especially if associated with fluid volume overload and concomitant infusion of corticosteroids).
• Metabolic: Pronounced hyperglycemia and severe hypokalemia (β₂ receptors stimulate the cellular Na+/K+ ATPase pump, causing the internalization of potassium ions from the plasma to the intracellular space).
• Fetal: Reflex fetal tachycardia and risk of rebound hypoglycemia in the newborn in the first hours of life after birth (due to secondary hyperinsulinemia induced in utero).

Dosing and Monitoring

Continuous intravenous infusion: Start with an infusion rate of 50 to 100 µg/min (6 to 12 mL/hour of a standard solution of 150 mg of ritodrine in 500 mL of 0.9% physiological saline). It can be increased progressively at intervals of 10-15 minutes at 50 µg/min until uterine activity stops or until a maximum safe infusion rate of 350 µg/min is reached.

Strict monitoring required: Measure maternal heart rate (suspend or reduce infusion rate if HR exceeds 120-130 bpm), lung auscultation every 4 hours (monitor for basal crackles suggestive of acute pulmonary edema), control of serum glucose and potassium levels every 6 to 12 hours, and rigorous fluid balance (limit total fluid intake to < 1500-2000 mL in 24 hours).

Pregnancy and Breastfeeding

Exclusively indicated for short-term tocolysis between 22 and 37 weeks of gestation. Not applicable for use during active breastfeeding.