Renal function and diuretic therapy in infants and children. Part I

Drugs in human milk. Clinical pharmacokinetic considerations

Drugs ingested by a lactating mother would be expected to appear in human milk to some extent and be ingested by a breast-feeding infant. Drugs pass from maternal plasma into milk by passive diffusion and are distributed within the aqueous, protein and lipid phases of milk. Distribution into milk will be affected by physiochemical characteristics of the drug: acid-base characteristics, relative protein binding in plasma and milk, and lipid solubility, as well as milk composition. The milk-to-plasma concentration ratio is the most commonly quoted index of drug distribution into human milk. However, calculation of the daily infant dose of drug ingested in milk, and from this the dose in milk relative to the maternal dose on a weight-adjusted basis, is a more relevant indicator of infant exposure to a drug. This is particularly true for drugs with a high volume of distribution, for which only a small proportion of the mother's dose is contained within the plasma and available for distribution into milk. A better indication of infant exposure to a drug is the steady-state plasma drug concentration in a breast-feeding infant, the major determinants of which are the dose rate (via milk) and the oral availability and clearance in the infant. Although in neonates the rate of absorption may be different from adults, there is little evidence that its extent is significantly different. Clearance, however, is impaired in very young infants, particularly if premature. The decreased clearance would result in a proportional increase in steady-state plasma concentrations in the breast-feeding infant. Consideration of the dose ingested in milk and the approximate clearance in infants of different ages allows estimation of likely steady-state plasma concentrations in breast-feeding infants. From these considerations, recommendations regarding the safety of drugs during breast-feeding can be made. Drugs which are very toxic or have dose-independent toxicity should be considered separately. Recommendations regarding 'social' drugs such as nicotine, alcohol, caffeine and theobromine are particularly difficult, as doses are uncontrolled and vary variable.

Principles of drug biodisposition in the neonate. A critical evaluation of the pharmacokinetic-pharmacodynamic interface (Part I)

Rational pharmacotherapy is dependent upon an understanding of the clinical pharmacokinetic and pharmacodynamic properties of the drugs employed. Although the available data on drug biodisposition and action in the neonate have increased considerably in the last few years, pharmacokinetic-pharmacodynamic interactions for many drugs remain poorly understood. The ontogeny of drug absorption, distribution, metabolism, and elimination are addressed in this review. Drug absorption from any site depends upon both the physicochemical properties of the drug and a variety of patient factors. Absorption of orally administered drugs may be affected by changes in gastric acidity and emptying time as well as by bile salt pool size, bacterial colonisation, and extraintestinal disease states such as congestive heart failure. Factors affecting drug absorption following intramuscular, percutaneous, and rectal administration are also discussed. Drug distribution in the neonate is influenced by a variety of important and predictable age-dependent factors. The developmental aspects of protein binding and body water compartments are described. Additionally, hepatic drug metabolism assumes an important role in understanding the pharmacokinetic and pharmacodynamic properties of many compounds. Certain biotransformation pathways, including hydroxylation by the P450 mono-oxygenase system and glucuronidation, demonstrate only limited activity at birth, while other pathways, such as sulphate or glycine conjugation, appear very efficient at birth. Elimination of drugs excreted unchanged in the urine is dramatically reduced in the newborn, compared with older infants and children, due to immaturity of both glomerular filtration and tubular secretory processes. The glomerular filtration rate remains markedly reduced prior to 34 weeks gestational age, increasing as a function of post-conceptual age until adult values are achieved by approximately 2.5 to 5 months of age. Tubular secretory capacity is also limited at birth, approaching adult values by approximately 7 months of age. Published reports describing the pharmacokinetics and pharmacodynamics of commonly used drugs in the neonatal period, as well as differences in drug biodisposition among premature infants, full term neonates, and older infants and children, are reviewed. Our recommendations for neonatal drug therapy are based upon a critical interpretation of these data, an understanding of fetal development and maturational processes, and an understanding of how disease states may affect drug biodisposition in the neonate.

Changed pharmacokinetics under the influence of age

Age significantly affects therapeutics in both general and specific ways. In newborns and in infants various physiological processes are still developing, whereas in elderly there may be decreased efficiency or capacity of physiological processes. Unexpected or 'idiosyncratic' responses to drugs in the very old or in the very young often can be explained by age-related changes in absorption, distribution, metabolism, end-organ responsiveness and excretion. Adolescence is often associated with rapid growth and changing body composition. Special problems with adolescents are poor compliance and drug abuse. Adults show a rather stable pharmacokinetic profile, although the cardiac output diminishes and the peripheral resistance increases about 1% annually. Exposure to enzyme-inducing agents (nicotine, cimetidine) influences the pharmacokinetic parameters of both adolescents and adults.

Urinary sodium excretion and the renin-aldosterone system in new-born calves

Urinary Na+ and K+ excretion, plasma aldosterone levels (PAL) and plasma renin activity (PRA) were measured in three groups of four 3-day-old calves infused with aldosterone, ethacrynic acid and hydrochlorothiazide or with vehicle. Aldosterone infusion (16.7 micrograms/kg body wt. given by rapid injection, followed by the infusion of 33.3 micrograms/kg body wt. during 6 h) decreased urinary Na+ concentration and excretion during the 6 h period of infusion. This effect disappeared during the following 18 h. Ethacrynic acid (2 mg/kg body wt. by rapid injection, followed by the infusion of 1.5 mg/kg body wt..h during 6 h) and hydrochlorothiazide (8 mg/kg body wt. by rapid injection, followed by the infusion of 6 mg/kg body wt..h during 6 h) increased urinary volume and urinary excretion of Na+, but decreased urinary K+ concentration without affecting urinary K+ excretion during the 6 h period of infusion. During the same time, the plasma Na+ concentration decreased, PRA increased rapidly and was followed by a significant rise in PAL. These results demonstrate that in very young healthy calves, the renal tubules are able to respond to aldosterone. The renin-aldosterone system is also operative in these animals since it responds to Na+ depletion induced by the combination of diuretics and inhibitors of tubular Na+ reabsorption.

Urinary cyclic AMP in spot urine of healthy children

We present reference values for the excretion of cAMP in spot urine collected between 09.00 and 12.00 hours in 143 healthy children aged 2-200 months. The excretion of cAMP was creatinine-corrected and expressed as a substance concentration ratio (UcAMP/crea)U due to a positive significant correlation between the excretion of cAMP and creatinine (r = 0.68, p less than 0.001). The mean value (95% significance limits) for (UcAMP/crea)U was 748 mumol/mol (254-2206 mumol/mol). A logarithmic transformation of the ratio was used, since preliminary analysis showed uneven distribution; when the logarithmic transformation was used, the data appeared evenly distributed. There was no significant difference between the results for girls and boys. The value of lg(UcAMP/crea)U was related to the age or body surface area with decreasing values at higher age or body surface area. (r = -0.55 and r = -0.57, p less than 0.001). Spot urine for measurement of urinary cAMP instead of a 24 h collection appears preferable due to the practicability of the test in children.

Effects of ethacrynic acid in the newborn infant

The effects of ethacrynic acid on electrolyte and water excretion were examined in 10 neonates with fluid overload states. Ethacrynic acid (1 mg/kg IV) produced a 10-fold increase in FENa+ and FECl - and a sixfold increase in urine volume and osmolar clearance. These effects peaked within 1 hour of ethacrynic acid administration, and progressively decreased to the baseline value by 5 hours after drug administration. The mean excretion of potassium increased by 280% in 1 hour, and returned to baseline value after 5 hours. The mean Ca++ excretion increased fourfold after 1 hour, and the effect lasted for 4 hours. Two infants developed mild hyponatremia. Our data emphasize the prolonged diuretic and saluretic effect of ethacrynic acid in neonates.

Determinants of the diuretic response to furosemide in infants with congestive heart failure

A wide interindividual variation in the degree of diuresis produced following treatment with furosemide has been observed in infants with congestive heart failure. A method of pharmacokinetic analysis that separates drug disposition from renal response to furosemide was used to compare the effect of this agent in infants with congestive heart failure with the effect seen in infants with fluid retention secondary to noncardiac factors. No differences in the drug elimination rate, urinary drug excretion, or renal diuretic response between the 2 groups were seen. Infants in both groups exhibited a wide range in all variables examined. The marked interindividual variation in response originates from decreased delivery of furosemide to its site of action in some patients and a decreased responsiveness to the drug in others.

Pharmacokinetics of diuretics and methylxanthines in the neonate

The elimination of diuretics and methylxanthines is considerably slower in the neonate than in the adult. Dose guidelines, especially during long term maintenance, must be adjusted to account for this slower drug elimination. Pharmacokinetic studies and the requisite pharmacologic evaluation on diuretics such as hydrochlorothiazide, spironolactone, ethacrynic acid and others should be done. Furosemide undergoes biotransformation in the newborn producing an acid metabolite and a glucuronide conjugate. Methylxanthines are effective in the treatment of neonatal apnea. Plasma elimination of theophylline is exceedingly slow, more so with caffeine. Decreased elimination is partly explained by decreased oxidative biotransformation. Caffeine is excreted in the urine of the newborn mainly unchanged (85%) in contrast to the adult where caffeine is a minor portion of urinary excretion (2%). Theophylline is methylated to caffeine and may possibly exert additive pharmacologic effects.

The occurrence of hyperaldosteronism in infants with congestive heart failure

Serum aldosterone and plasma renin were measured in 20 normal infants and 15 infants with congestive cardiac failure. Serum aldosterone was significantly increased (151 +/- 38 ng/dl mean +/- standard error of the mean) in patients before treatment when compared with aldosterone in normal infants (29 +/- 7 ng/dl). Increasing serum aldosterone was related to increasing plasma renin. The response to furosemide appeared to be inversely related to serum aldosterone concentrations. In four infants, administration of an aldosterone antagonist (spironolactone) resulted in improved diuresis and decreased serum aldosterone. Hyperaldosteronism is an important factor contributing to fluid and sodium retention in infants with heart failure.

Pharmacokinetic disposition and protein binding of furosemide in newborn infants

Using a one-compartment model, the pharmacokinetic disposition of furosemide was studied in eight premature and term neonates with fluid overload. Following a single intravenous injection of furosemide (1 to 1.5 mg/kg), multiple blood samples were obtained from a heelstick or an arterial catheter and analyzed for furosemide by gas liquid chromatography. The mean (+/- SE) apparent volume of distribution was 829.2 +/- 118.9 ml/kg; t1/2 was 7.7 +/- 1.0 hour; elimination rate constant was 0.102 +/- 0.013/hour and plasma clearance was 81.61 +/- 14.98 ml/kg/hour. Compared to the disposition of furosemide in normal adults. AVd is almost fourfold greater in the neonate with an eightfold prolongation in plasma t1/2, an eightfold decrease in Ke1, and a twofold decrease in plasma clearance. Neither gestational and postnatal age nor birth weight correlated with the pharmacokinetic variables. No significant change in reserve bilirubin-binding capacity after an intravenous dose of furosemide was noted. Slow elimination of furosemide may partly explain the prolonged diuretic and saluretic effect of furosemide in the neonate.

The pharmacokinetics and metabolism of the anilide local anaesthetics in neonates. I. Lignocaine

The pharmacokinetics and metabolism of lignocaine in premature neonates was studied after subcutaneous administration. The collection of serial urine together with a limited number of blood samples from neonates enabled simultaneous computer fitting of data to a pharmacokinetic model. The disposition kinetics of lignocaine in four neonates were compared with similar data reported for adults. Neonates had prolonged t1/2 (neonate mean: 3.16 h; adult mean: 1.80 h), and an increased total volume of distribution (neonate mean: 2.75 l/kg; adult mean: 1.11 l/kg) compared with adults. Total plasma clearance (Cltp) normalised on body weight showed no significant difference between neonates (mean: 0.610 l/h/kg) and adults (mean: 0.550 l/h/kg). The urinary excretion of lignocaine and several of its metabolites was studied in 8 neonates and 11 adults. Neonates were shown to excrete much more unchanged lignocaine (mean: 19.67%) compared with adults (mean: 4.27%) and the proportion of the dose excreted as 4-hydroxyxylidine is considerably reduced in neonates (neonate mean: 8.89%; adult mean: 63.78%). The use of the two pharmacokinetic parameters, t1/2 and Cltp, as indices of drug elimination ability are discussed.

The pharmacologic effects of furosemide therapy in the low-birth-weight infant

The pharmacologic effects of furosemide were studied in six infants (mean gestation 30.7 weeks; mean birth weight 1,490 gm) at ages 10 to 57 days. Furosemide (for clinical indication, standardized at 1 mg/kg) was given intravenously over one minute; data were collected over the ensuing 24 hours. For three hours following furosemide administration, a significant diuresis was observed. Sodium excretion, percent fractional sodium excretion, and potassium excretion were significantly increased and urinary pH significantly decreased for six hours following the administration of furosemide. Creatinine and free water clearances were slightly elevated, although not significantly. Furosemide is an effective diuretic, the onset of pharmacologic action was within one hour, the peak action was sustained for three hours, and the duration of action was six hours. The net fluid, sodium, and potassium losses following a 1 mg/kg single dose were 28 ml, and 3.6 and 0.3 mEq/kg, respectively.

A study of factors affecting blood pressure and angiotensin II in newborn infants

Blood pressure has been measured using the Doppler ultrasonic technique in 144 infants in the first 11 days of life. Venous angiotensin II levels measured by radioimmunoassay were determined in 75 of these babies. Highly significant correlations were found between systolic blood pressure and gestational age and body weight. AII was inversely related to blood pressure and significantly related to birth weight. Babies with respiratory distress were found to be significantly hypotensive and to have markedly raised AII levels.