OBJECTIVES: To evaluate the pharmacokinetics and pharmacodynamics of lansoprazole in children between 13 and 24 months of age with gastroesophageal reflux disease (GERD). METHODS: From the population of 66 children with symptomatic GERD, erosive esophagitis (> or = grade 2) or esophageal pH < 4 for > 4.2% of the 24-h period who participated in a phase I/II, open-label, multicenter (11 sites) US study, a subanalysis of 8 toddlers between 13 and 24 months of age was performed. All children were treated, based on body weight, with lansoprazole 15 mg once daily for 8 to 12 weeks. If a child were still symptomatic after 2 weeks of treatment, then the dose of lansoprazole could be increased to twice daily at the discretion of the investigator. Pharmacokinetic parameters were assessed at day 5. Twenty-four-hour median intragastric pH and the percentage of time intragastric pH > 3 or > 4 were assessed at baseline and at day 5 of treatment. Symptom response was assessed by investigator interview and daily diary. Safety was monitored by physical examinations including vital signs, adverse event assessments and laboratory evaluations. RESULTS: Pharmacokinetic analysis of 5 children found a mean time to reach maximum concentration of 1.4 h, maximal plasma concentrations of 894 ng/mL, area under the concentration time curve of 1906 ng * h/mL and a half-life of 0.66 h. Significant (P < or = 0.027) increases from baseline to day 5 were observed in mean 24-h intragastric pH (2.76-3.52) and the percentages of time pH were > 3 (29.46%-55.36%) and pH was > 4 (16.96%-40.77%). Six of the 8 children had improvement in their overall GERD symptom severity on the basis of investigator assessment, and a reduction was seen in the percentage of days with moderate, severe or very severe GERD symptoms compared with baseline. The dosage of lansoprazole was increased in 3 of the 8 children. Median fasting serum gastrin level increased from 65.0 pg/mL at baseline to 136.5 pg/mL at the final visit. Treatment-related events were mild constipation (1 subject) and mild diarrhea (1 subject). CONCLUSIONS: Although larger studies are needed to confirm these results, lansoprazole displays pharmacokinetic and pharmacodynamic parameters in children between 13 and 24 months of age that are similar to those results observed in older children as well as adults.