Background and Objective: Previously, we developed a pharmacokinetic-pharmacodynamic model of allopurinol, oxypurinol, and biomarkers, hypoxanthine, xanthine, and uric acid, in neonates with hypoxic-ischemic encephalopathy, in which high initial biomarker levels were observed suggesting an impact of hypoxia. However, the full pharmacodynamics could not be elucidated in our previous study. The current study included additional data from the ALBINO study (NCT03162653) placebo group, aiming to characterize the dynamics of hypoxanthine, xanthine, and uric acid in neonates with hypoxic-ischemic encephalopathy. Methods: Neonates from the ALBINO study who received allopurinol or placebo mannitol were included. An extended population pharmacokinetic-pharmacodynamic model was developed based on the mechanism of purine metabolism, where synthesis, salvage, and degradation via xanthine oxidoreductase pathways were described. The initial level of the biomarkers was a combination of endogenous turnover and high disease-related amounts. Model development was accomplished by nonlinear mixed-effects modeling (NONMEM®, version 7.5). Results: In total, 20 neonates treated with allopurinol and 17 neonates treated with mannitol were included in this analysis. Endogenous synthesis of the biomarkers reduced with 0.43% per hour because of precursor exhaustion. Hypoxanthine was readily salvaged or degraded to xanthine with rate constants of 0.5 1/h (95% confidence interval 0.33–0.77) and 0.2 1/h (95% confidence interval 0.09–0.31), respectively. A greater salvage was found in the allopurinol treatment group consistent with its mechanism of action. High hypoxia-induced initial levels of biomarkers were quantified, and were 1.2-fold to 2.9-fold higher in neonates with moderate-to-severe hypoxic-ischemic encephalopathy compared with those with mild hypoxic-ischemic encephalopathy. Half-maximal xanthine oxidoreductase inhibition was achieved with a combined allopurinol and oxypurinol concentration of 0.68 mg/L (95% confidence interval 0.48–0.92), suggesting full xanthine oxidoreductase inhibition during the period studied. Conclusions: This extended pharmacokinetic-pharmacodynamic model provided an adequate description of the complex hypoxanthine, xanthine, and uric acid metabolism in neonates with hypoxic-ischemic encephalopathy, suggesting a positive allopurinol effect on these biomarkers. The impact of hypoxia on their dynamics was characterized, underlining higher hypoxia-related initial exposure with a more severe hypoxic-ischemic encephalopathy status.