Pharmacokinetics of melatonin in human sexual maturation

To determine whether melatonin pharmacokinetics change during puberty, we infused melatonin iv in 9 prepubertal, 8 pubertal, and 16 adult subjects and measured melatonin in serum and saliva, and 6-hydroxymelatonin sulfate in urine. A pilot study of 3 adult males showed dose linearity, absence of saturation kinetics, and unaltered metabolism and urinary excretion for doses of 0.1, 0.5, and 5.0 micrograms/kg. All other subjects received 0.5 microgram/kg melatonin. The results of pharmacokinetic parameters calculated from serum melatonin showed no significant gender differences in adults. However, developmental differences were significant between prepubertal children and adults for terminal elimination rate constant (1.08 +/- 0.25 vs. 0.89 +/- 0.11 h-1), elimination half-life (0.67 +/- 0.12 vs. 0.79 +/- 0.10 h), and area under the concentration-time curve (250.9 +/- 91.8 vs. 376.9 +/- 154.3 (pg/mL).h, respectively). At all time points melatonin levels were higher in serum than in saliva, and the ratio between serum and salivary melatonin varied up to 55-fold within and between individuals. Results based on salivary melatonin showed significant differences between prepubertal children and adults for the terminal elimination rate constant (1.90 +/- 0.95 vs. 1.06 +/- 0.28 h-1). The described group differences in pharmacokinetic parameters suggest that prepubertal children metabolize melatonin faster than adults. The inconsistent ratio between serum and salivary melatonin calls for caution in the use of salivary melatonin for pharmacokinetic studies or to infer pineal function. The present findings, suggestive of faster melatonin metabolism in prepubertal children, combined with the known decline of serum melatonin with age and higher excretion rate of the metabolite in prepubertal children lead us to conclude that the prepubertal pineal gland has a higher melatonin secretion rate than the adult gland.