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Determination of the Serum Protein Binding of Oxycodone and Morphine Using Ultrafiltration

Protein binding of oxycodone and morphine in human serum was determined in vitro using ultrafiltration. Binding studies were also performed using both purified human serum albumin and human α1-acid glycoprotein (AAG). Albumin was found to be the major binding protein for both oxycodone and morphine. The serum protein binding of both oxycodone and morphine was independent of drug concentration in the therapeutic range (5–100 ng/ml), but was dependent on protein concentration. In addition, bound fractions of oxycodone and morphine increased with increasing concentrations of both albumin and A AG. At physiological pH and temperature, the mean (± SD) serum protein binding of oxycodone was 45.1% (± 0.4%) and that of morphine was 35.3% (± 0.2%) A decrease in temperature from 37 to 23°C significantly increased the serum protein binding of oxycodone and morphine by 8–9% (p < 0.0001) and 7–10% (p < 0.0001), respectively, indicating the importance of maintaining the temperature at 37° C during protein binding experiments. A reduction in pH from 7.75–8.85 to 7.4 significantly reduced serum protein binding of both oxycodone and morphine by 4–5% (p < 0.0001) and 4–7% (p < 0.0001), respectively. Serum samples, to which known concentrations of oxycodone had been added and which were stored at −20° C, showing a gradual but significant decline (p < 0.0001) in serum protein binding of oxycodone from –45 to 39% during the 4-week storage period. Although serum protein binding of oxycodone and morphine is dependent on protein concentration and pH – factors that may vary in different disease states–any changes in the binding of oxycodone or morphine are unlikely to alter the pharmacological effects of these drugs because of their normally low extent of binding (45 and 35%, respectively).