Rapid disappearance of naloxone was observed in a group of very low birthweight infants examined for naloxone kinetics. Five infants (BW 1.20±0.25kg, GA 29±1wk) received 0.04 mg/per kg of naloxone intravenously, four within the first week of life and one on day 26. Serial serum samples were obtained at specific time intervals and frozen for subsequent analysis. Serum naloxone concentrations were measured by the radioimmunoassay method of Berkowitz (1975).

Serial naloxone concentration at 5 min. was 51.5±13.4pmole/ml, at 15 min. 36.7±4.0pmole/ml, at 30 min. 28.9±5.1 pmole/ml, at 60 min. 20.4±5.9pmole/ml, at 120 min. 7.3±2.4pmole/ml, and 240 min. 1.5±0.4pmole/ml. No naloxone was detected at the next sample time (12hrs). The elimination rate constant (Ke) calculated from the decay portion of the elimination curve was 0.823±0.130/hr. The calculated half life (t½) was 51.8±9.2min. No correlations were found between Ke and t½ and initial serum level, birthweight, gestational age, and postnatal age.

This group of infants demonstrated rapid elimination of intravenous naloxone, consistent with results obtained in adults.

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Stile, I., Fort, M., Marotta, F. et al. PHARMACOKINETICS OF NALOXONE IN PREMATURE INFANTS. Pediatr Res 18, 161 (1984).

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