Drug disposition in the neonatal animal, with particular reference to the foal.

The research article investigates how newborn animals, specifically foals, handle drugs differently than their adult counterparts due to changes in drug distribution, metabolism, and excretion during their early life stages. These alterations can influence drug concentrations in plasma and at receptor sites.

Drug Disposition in Neonates

• Newborn animals, or neonates, react differently to drugs compared to adults due to modifications in the processes of drug distribution, metabolism, and excretion. These changes can affect the drug concentrations in the blood plasma and at the locations where the drug behaves (receptor sites).

Characteristics of the Neonatal Period

• The neonatal period, the time just after birth, exhibits several distinguishing characteristics that influence drug disposition. These include enhanced absorption of drugs from the gastrointestinal tract, diminished binding of drugs with proteins in the blood plasma, and an increased volume of distribution of drugs in the extracellular fluid or total body water.
• The ‘blood-brain’ barrier, a filtering mechanism of the body that keeps the circulating blood away from the brain extracellular fluid, is more permeable in newborns which allows more of the drug to penetrate.
• Many drugs are eliminated more slowly in newborns due to the immaturity of drug metabolism and excretion pathways.

Metabolic and Renal Function Development

• Newborn animals have deficient hepatic microsomal oxidative reactions and glucuronide conjugation (types of drug metabolism) for some time after birth. This could affect the duration of action of certain lipid-soluble drugs.
• Also, due to the kidneys’ functional immaturity, the renal excretion of polar drugs and drug metabolites is decreased.
• In terms of overall renal function, maturity usually is reached within two weeks after birth in ruminants and pigs, whereas, in other domestic animals, it might take at least four weeks.

Continuing Maturation After Birth

• Significant physiological and biochemical development occurs in the first five days after birth with maturation continuing more slowly over the next five weeks.
• The time taken for any process to reach functional maturity depends on the process itself and varies with the species of animal.
• The absorption, disposition and response to drugs in the first 24 hours after birth may be unique to that period.