Binding of theophylline in human serum determined by ultrafiltration and equilibrium dialysis.

Abstract

1 Binding of theophylline (80 ,tmol/l) was determined in serum from healthy subjects by equilibrium dialysis and by ultrafiltration, using [3H]-theophylline, at 22°C and at different pH-values. pH was regulated by gassing with CO2 or by dialysing the serum against a phosphate buffer before use. 2 Binding of theophylline in serum was 34-38% determined by equilibrium dialysis and 41-45% determined by ultrafiltration at pH 7.4-7.5. The protein concentration in serum decreased by 12-16% during equilibrium dialysis and increased by 20% during ultrafiltration. The intersubject variation in binding was small. 3 Binding of theophylline in serum was pH-dependent with 25-30% bound at pH 7.0 and 58-60% bound at pH 8.1-8.3. Binding was significantly correlated to the fraction of ionized theophylline. 4 The binding of theophylline in normal human serum is about 35 40% at pH 7.4 and 22°C. The difference in binding observed between equilibrium dialysis and ultrafiltration may be explained by the opposite changes in protein concentration during the experiment. 5 Control of pH is necessary to obtain physiologically relevant data on drug binding in serum.

pH lability in serum during equilibrium dialysis.

“…According to published data, when pH of serum increased by 0.1 unit from 7.4, the free fraction of propranolol decreased by 9% , of oxprenolol by 8% , of imipramine by 6% (Kristensen & Gram, 1982), of pethidine by 7% (La Rosa et al, 1984), and of theophylline by 8% (Vallner et al, 1979) or 3-4% (Shaw etal., 1982;Br0rs et al, 1983).…”

“…The composition of Krebs Ringer phosphate buffer and of Krebs Ringer bicarbonate buffer was as described previously (Br0rs et al, 1983), except that gassing with CO2 was not performed in the present study. …”

“…For several of these drugs, 30% or more decrease in free fraction is observed if precautions to avoid a pH increase in serum are not taken , Br0rs et al, 1983. Frequently, pH during the experiments on drug serum binding is not accounted for, and procedures which in our experience lead to inappropriate pH levels are used.…”

Do saliva concentrations predict plasma unbound theophylline concentrations? A problem re‐examined.

“…The mean plasma protein binding in these patients of 58.2 + 5.09%, is similar in males (57.7 + 5.06%) and females (58.8 + 5.1%). These findings are consistent with those of other authors using ultrafiltration (Antal et al, 1981;Aslaksen et al, 1981;Yurchak & Jusko, 1976;Piafsky etal., 1980;Mangione etal., 1978) but somewhat higher than those obtained by equilibrium dialysis at 37°C (Buss et al, 1983;Brors et al, 1983) showing that theophylline binding is temperature dependent, as well as pH dependent (Vallner et al, 1979). There also appears to be a very weak correlation between age and free fraction (r = 0.411) such that theophylline plasma protein binding decreases with advancing years, as reported for other drugs (Wallace et al, 1976;Patterson et al, 1982).…”

The protein binding of timegadine determined by equilibrium dialysis.

“…Timegadine acts by inhibiting both cyclo-oxygenase and lipoxygenase activity thus preventing the formation of prostaglandins (Ahnfelt-Ronne & ArrigoniMartelli, 1980). It is generally thought that only the free fraction of a drug is pharmacologically active and subject to elimination (Koch-Weser & Sellers, 1976;Brors et al, 1983;Buss et al, 1983). It therefore follows that the elimination half-life (T½1) could be greater for highly bound drugs.…”