Caffeine 25 mg, Chlorpheniramine Maleate 2 mg, Nimesulide 100 mg, Phenylephrine 10 mg Tablets

Caffeine is structurally related to other methylxanthines, theophylline, and theobromine. It is a bronchial smooth muscle relaxant, a CNS stimulant, a cardiac muscle stimulant, and a diuretic.

Although the mechanism of action of caffeine in apnea of prematurity is not known, several mechanisms have been hypothesized. These include: (1) stimulation of the respiratory center, (2) increased minute ventilation, (3) decreased threshold to hypercapnia, (4) increased response to hypercapnia, (5) increased skeletal muscle tone, (6) decreased diaphragmatic fatigue, (7) increased metabolic rate, and (8) increased oxygen consumption.

Most of these effects have been attributed to antagonism of adenosine receptors, both A1 and A2 subtypes, by caffeine, which has been demonstrated in receptor binding assays and observed at concentrations approximating those achieved therapeutically.

After oral administration of 10 mg caffeine base/kg to preterm neonates, the peak plasma level (Cmax) for caffeine ranged from 6 to 10 mg/L and the mean time to reach peak concentration (Tmax) ranged from 30 minutes to 2 hours. The Tmax was not affected by formula feeding. The absolute bioavailability, however, was not fully examined in preterm neonates.

Caffeine is rapidly distributed into the brain. Caffeine levels in the cerebrospinal fluid of preterm neonates approximate their plasma levels. The mean volume of distribution of caffeine in infants (0.8 to 0.9 L/kg) is slightly higher than that in adults (0.6 L/kg). Plasma protein binding data are not available for neonates or infants. In adults, the mean plasma protein binding in vitro is reported to be approximately 36%.

Hepatic cytochrome P450 1A2 (CYP1A2) is involved in caffeine biotransformation. Caffeine metabolism in preterm neonates is limited due to their immature hepatic enzyme systems.

Interconversion between caffeine and theophylline has been reported in preterm neonates; caffeine levels are approximately 25% of theophylline levels after theophylline administration and approximately 3 to 8% of caffeine administered would be expected to convert to theophylline.

In young infants, the elimination of caffeine is much slower than that in adults due to immature hepatic and/or renal function. Mean half-life (T1/2) and fraction excreted unchanged in urine (Ae) of caffeine in infants have been shown to be inversely related to gestational/postconceptual age. In neonates, the T1/2 is approximately 3 to 4 days and the Ae is approximately 86% (within 6 days). By 9 months of age, the metabolism of caffeine approximates that seen in adults (T1/2 = 5 hours and Ae = 1%).

Studies examining the pharmacokinetics of caffeine in neonates with hepatic or renal insufficiency have not been conducted. Caffeine citrate should be administered with caution in preterm neonates with impaired renal or hepatic function. Serum concentrations of caffeine should be monitored and dose administration of caffeine citrate should be adjusted to avoid toxicity in this population.

Chlorphenamine reversibly blocks histamine H1 receptors and also has anticholinergic activity. This reduces histamine-mediated effects on smooth muscle and capillary permeability.

The tablet reference describes gastrointestinal absorption, distribution into the central nervous system and metabolism mainly in the liver to demethylated products. The reported plasma half-life is approximately 12–15 hours. Drug-related material is eliminated in urine. These oral findings should not be assigned to an injectable chlorphenamine product.

Nimesulide inhibits cyclo-oxygenase-dependent prostaglandin synthesis. This archived mechanism reference is not a current prescribing recommendation; hepatic safety restrictions and local authorization must be checked separately.

The archived oral 100 mg reference reports plasma peaks at 2–3 hours, protein binding up to 97.5% and a half-life of 3.2–6 hours. Hepatic pathways include CYP2C9 and formation of an active hydroxylated metabolite. About 50% is eliminated through urine and 29% through feces after metabolism; only 1–3% is unchanged urinary drug. These findings are not controlled-release or injection data.

Phenylephrine acts directly on adrenergic receptors, predominantly through alpha-adrenergic activity, producing vasoconstriction. This mechanism alone does not establish clinical effectiveness for a particular oral product.

The oral-capsule reference reports extensive presystemic metabolism, including metabolism in intestinal cells, with approximately 40% systemic bioavailability. Peak concentrations occur around 1–2 hours and mean plasma half-life is about 2–3 hours. Further hepatic metabolism occurs after absorption; parent drug and metabolites are eliminated in urine. These are oral-reference findings, not injectable or ophthalmic pharmacokinetics.