Figure 1.

The theophylline metabolic pathway. Molar percentages of the metabolites analyzed in our preterm infant group are expressed in parentheses. CYP1A2 is a major hepatic enzyme involved in the decomposition of approximately 90–95% of theophylline in human adults. CYP1A2 mainly catalyzes the conversion of theophylline to 1‐methylxanthine and 3‐methylxanthine (N‐demethylation), while CYP2E1 and CYP3A4 catalyze the conversion of theophylline to 1,3‐dimethyluric acid (8‐hydroxylation). 1‐Methylxanthine is oxidized very rapidly to 1‐methyluric acid by xanthine oxidase