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Influence of Pentoxifylline on Steady‐State Theophylline Serum Concentrations from Sustained‐Release Formulations

This clinical study assessed the influence of pentoxifylline and its metabolites on steady‐state serum theophylline concentrations. Nine healthy volunteers took sustained‐release formulations of pentoxifylline, theophylline, and a combination of both agents each for 7 days at standard therapeutic doses in a randomized order. Serum theophylline concentrations were analyzed using fluorescence‐polarization immunoassay (TDx) technique. During the pentoxifylline treatment phase, serum theophylline concentrations were undetectable, demonstrating the lack of assay interference from pentoxifylline and its metabolites. Mean trough steady‐state serum theophylline concentrations were 30% higher (p < 0.05) during the combination treatment phase compared to theophylline administration alone, and varied considerably. Although side effects were more frequent during the combination phase, differences in the number of adverse reactions did not achieve statistical significance. This study demonstrates an interaction between theophylline and pentoxifylline, and indicates that close monitoring of serum theophylline concentrations during combination therapy is warranted.