Pharmacokinetics: Atenolol is incompletely absorbed from the gastrointestinal tract; following oral administration, about 50% is absorbed. Peak plasma concentrations of up to 2 mcg/mL are reached in 2-4 hrs. Atenolol has low lipid solubility. It crosses the placenta and is excreted in breast milk where concentrations higher than those in maternal plasma have been achieved. Only small amounts are reported to cross the blood-brain barrier, and plasma protein-binding is minimal. The plasma half-life is about 6-7 hrs. Atenolol undergoes little or no hepatic metabolism and is excreted mainly in the urine.
Pregnancy and the Neonate: In 6 women who had taken atenolol for at least 6 days up to the time of delivery, concentrations of atenolol in maternal and umbilical serum were approximately equal. Furthermore, in a woman who had discontinued treatment 1 day before delivery, atenolol was not found in maternal or umbilical serum. The half-life of atenolol in neonates born to mothers who had been receiving atenolol ranged from 10.5-34.6 hrs (mean 16.1 hrs) in a study of 35 term infants. Atenolol concentrations were determined in cord blood and in neonatal blood 24 hrs after delivery. The range of elimination rate was 4 times slower than in adults, a difference expected based on renal excretion of atenolol.
Atenolol diffuses into breast milk and is present in milk in concentrations similar to or higher than those in maternal blood. Bradycardia associated with ingestion of atenolol in breast milk has been reported in a 5-day old term infant. The baby improved when breastfeeding was discontinued.