Factors influencing theophylline serum protein binding.

We studied a number of influences on theophylline binding to serum proteins using equilibrium dialysis (37 degrees), a modified Krebs-Ringer bicarbonate buffer (pH 7.4), and 8-14C-theophylline with unlabeled theophylline (30 microgram/ml) added to sera from healthy subjects. Theophylline protein binding rose by 18.6% as pH rose from 7.0 to 7.8 (percent theophylline bound = 28.2 +/- 4.3 at pH 7.0 and 46.8 +/- 4.9 at pH 7.8, n = 5). Average theophylline binding to the proteins at 37 degrees in serum samples from 10 normal adults was 39.3 +/- 3.44%, which is 89.9% lower than the average of 48.2 +/- 3.74% for the same samples at 26 degrees. Theophylline binding was 6.1% higher with 0.1 mole/l phosphate buffer, pH 7.4, than with a modified Krebs-Ringer bicarbonate buffer, pH 7.4. Of the 19 drugs and metabolites tested for competition with theophylline for binding sites on serum proteins, 10 induced decreases in binding ranging from 6.8% in the case of furosemide to 18.3% for sodium salicylate. The latter was the only drug that induced a decrease in theophylline binding at concentrations that would be achieved in the therapy of same patients (i.e., patients on long-term salicylate therapy). All the other drugs that decreased theophylline binding did so at much greater concentrations than their usual therapeutic levels. The mean +/- SD of theophylline bound in 51 fresh serum samples from healthy adults was 48.6 +/- 10.2%; the pH of these specimens varied from 7.6 to 8.7. After adjusting pH to 7.4, theophylline binding was lowered to 37.6 +/- 4.5% and intersubject variability decreased. We recommend that the pH of serum specimens be adjusted to 7.4, or to the original pH of the blood specimen if it differs significantly from 7.4 (i.e., in acidotic or alkalotic patients). The wide range of reported values for theophylline binding to serum proteins in normal and asthmatic adults at least partly results from differences in the conditions used for the separation of free from bound drug.