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Selected parameters of renal function were studied in premature infants with a significant patent ductus arteriosus who were treated with intravenous indomethacin according to a specific protocol. Urine volume, glomerular filtration rate, urine sodium, and the fractional excretion of sodium were analyzed in 17; osmolar, sodium, and free water clearances in 8; and indomethacin pharmacokinetics in 7 premature infants. All renal function parameters analyzed decreased during indomethacin therapy: urine volume and glomerular filtration rate returned to normal, while urine sodium, fractional excretion of sodium, and the osmolar, sodium, and free water clearances remained low 24 hours after cessation of therapy. The water retention, shown by the reduced free water clearance, had no apparent deleterious effects, probably because of the infants' low fluid intake. Indomethacin pharmacokinetic parameters (clearance, area under the curve) may account in part for the variability of the fractional excretion of sodium, glomerular filtration, and urine flow rate.

Metabolism of indomethacin was examined in freshly isolated hepatocytes prepared by liver collagenase perfusion of fetal (28) and neonatal rabbits of 3, 5, 10, 12, and 25 days of age. Initial cell viability was more than 90% and linear rates of metabolism were observed for up to 2 hr of incubation. Deacylation of indomethacin to desbenzoyl indomethacin showed a rapid increase early in postnatal development while microsomal O-demethylation to desmethyl indomethacin was increased significantly only in the hepatocytes from 25-day-old rabbits. Glucuronide conjugates of indomethacin and indomethacin metabolites accounted for less than 8% of the total metabolites in hepatocytes from 25-day-old rabbits. The maturational development of indomethacin in metabolism may account for previously reported gestational dependent half-life of the drug in the premature infant (9). However, the factors that regulate development of indomethacin metabolism appear to be under more that one control system.

Aqueous humor and serum indomethacin levels were determined following administration of the drug orally, as a topical 1% aqueous suspension and as a topical 1% oil suspension. Patients receiving indomethacin orally had a mean aqueous humor level below the lower limit of sensitivity of the assay, which is also below the level known to inhibit prostaglandin synthesis in vitro and a mean serum level of 642 ng/mL. Patients receiving the topical 1% aqueous suspension had a mean aqueous level of 198 ng/mL and no detectable serum level. Patients receiving the topical 1% oil suspension had a mean aqueous level of 429 ng/mL, which was significantly higher than that of the aqueous suspension. Both topical suspensions yield levels that are able to inhibit prostaglandin synthesis. Higher aqueous levels with no detectable blood levels (and thus negligible potential for systemic toxic effects) make the topical route of administration preferable to the oral route.

The disposition of oral indomethacin was studied in 9 premature infants with significant patent ductus arteriosus (sPDA) with birth weights ranging from 800 to 1,960 gm and gestational ages of 28 to 36 wk. Peak plasma concentrations in most of the subjects were not observed until 3 to 4 hr after administration and ranged from 0.027 to 0.31 microgram/ml. Plasma half-life (t1/2) of indomethacin ranged from 11 to 20 hr and correlated with gestational age. Plasma protein binding studies, in samples from the premature infant using radiolabeled indomethacin, showed no difference in binding from values obtained with adult plasma (greater than 98%). Absorption of orally administered indomethacin appears to be incomplete and plasma clearance much longer than in the adult.

A rapid and sensitive high-performance liquid chromatography-fluorescence method is described for the quantitative analysis of indomethacin and its metabolites in urine. A modified version is also shown for the detection of indomethacin in plasma. The method consists of a single extraction of an acid-buffered plasma sample or single extractions of two buffered aliquots in parallel of urine using ethyl acetate, followed by evaporation of the organic phase. Indomethacin (I) and the metabolite desmethylindomethacin (DMI) were deacylated to their fluorescent products, deschlorobenzoylindomethacin (DBI) and desmethyldeschlorobenzoylindomethacin (DMBI), respectively, prior to chromatography. The chromatographic phase utilized a reversed-phase C18-bonded column with a solvent system comprised of either 22.5% or 26% acetonitrile in 0.25% acetic acid. The elution times for indomethacin metabolites ranged from 12-26 min. The total DBI (including deacylated I) and DMBI (including deacylated DMI) in the extract were each determined using fluorometric detection, with excitation at 288 nm and emission at 390 nm (370 nm cutoff filter). An internal standard of indole-3-propionic acid was used for quantitation. The lower limit of sensitivity for I in plasma was 25 ng/ml.

Postnatal maturation for indomethacin O-demethylation in freshly isolated hepatocytes and 10,000 g liver homogenates of rabbit was found to be significantly different from that observed in rabbit hepatic microsomes. Addition of hepatic cytosol (100,000 g supernatant) from day 10 postnatal rabbits significantly inhibited indomethacin O-demethylation by hepatic microsomes from adult rabbits. Kinetic studies demonstrated that the inhibition was competitive and consistent with the concentration of cytosol. The pattern of postnatal maturation for hepatic microsomal drug metabolism may in part be related to changes in the concentration of endogenous cytosolic inhibitors.

A GLC method utilizing an electron-capture detector is described for analysis of indomethacin in blood plasma. Indomethacin is extracted with ethyl acetate from plasma buffered to pH 5.0. The ethyl acetate is evaporated to dryness, and indomethacin is derivatized to a pentafluoropropionyl ester prior to chromatography. A GLC standard is used for peak height quantitation of indomethacin. The extraction efficiency from plasma is 92 +/- 3%, and as little as 1 ng of indomethacin can be quantitatively determined.