Pharmacokinetic variability which is due to difference in drug concentration at the site of action(as reflected from plasma drug concentration) because of inter individual differences in the drug absorption, distribution, metabolism and excretion.
The major causes of inter subject pharmacokinetic variability are :-
Comprehensive examination of the effect of age on drug elimination has been reported for ceftriaxone.
Clearance increased from 0.9 to 2.5 ml/min. when comparing patient 1 to 8 days old with patient 9 to 30 days old.
The mean clearance of ceftriaxone in children ranging in age from 1 to 12 months and from 1 to 6 years was 6.2 ml/min. and 9.1 ml/min. respectively.
The 18 to 49 years old age group had the highest clearance of ceftriaxone, 17 ml/min.
Older groups of patient had progressively lower value of clearance.
Very elderly patients 75 to 92 years of age had an average clearance of about 8 ml/min.
DRUG METABOLISM IN NEWBORN:
The most dramatic age-related differences in drug elimination often occur between the newborn and adult.
Most of the enzymatic microsomal systems required for drug metabolism are present at birth, but their concentration are usually lower than adult levels.
In general drugs subject to biotransformation are eliminated more slowly in newborn than in adult.
Chronic exposure of the fetus to antiepileptic drugs throughout gestation may lead to induction of drug-metabolizing enzyme.
Sulfate conjugation seems to be as efficient in newborns as in adults, but conjugation with glucuronic acid is considerably reduced, reaching adult level only after 3 years of age.
This deficiency is responsible for the serious adverse effects observed in newborn after administration of chloramphenicol a drug that is ordinarily conjugated with glucuroni acid.
The clearance of morphine in newborn (1 to 4 days old) was less than half life found in older infants (6 vs. 24 ml/min/kg).
The amount of unchanged theophylline in urine in premature infants decreases with postnatal age, whereas the excretion of metabolites increases with age.
PLASMA PROTEIN BINDING IN NEWBORN:
Difference in plasma protein binding and tissue binding of drugs have also been reported between newborn and adults.
In each case binding in plasma protein is level in the newborns than in the adults.
Increase in plasma protein binding is an increase in apparent volume of distribution in the newborn.
The relatively low plasma protein binding in neonates is often associated with elevated level of bilirubin, which is avidly bound to albumin and may complete with drug for binding sites.
At one time it was thought that competition between drugs bilirubin for binding sites on albumin could result in displacement of bilirubin leading to its deposition in the central nervous system and kernicterus.
decreased in plasma protein binding drugs and differences in body composition with respect to TBW and ECF.
RENAL EXCRETION IN NEWBORN:
Although the ratio of kidney weight to total body weight in the newborns is twice that in the adults anatomically and functionally immature, all aspect of renal function are reduced.
The difference process of renal excretion on mature at different rates,
Average glomerular filtration rate is 38.5 ml/min.
immature renal function affects the elimination of amino glycosides, indomethacine, digoxin, penicilline, sulfonamides and many other drugs.
The risk of ADR. In newborn is high.
The elevated digoxin level in the neonates are related to the low renal clearance of digoxin, which is half compared to children and adults with normal renal function.
DRUG METABOLISM IN CHILDREN:
In general drug metabolism is impaired in neonates compared to adults.
Older infants age (6 months to 12 years) has higher drug metabolism capacity rates compared to adults.
for example, clindamycin theophylline valproic acid Has faster elimination rate.