Ultrafiltration using the Amicon MPS-1 for assessing methadone plasma protein binding

The percent of protein-free and protein-bound methadone were separated in methadone-spiked bank and artificial plasma, and in plasma samples taken from methadone-maintained patients using the Amicon MPS-1 ultrafiltration device. Following the separation procedure, protein-bound and protein-free methadone were extracted from the protein-bound and free fractions, and their respective concentrations were determined by gas chromatography and nitrogen-phosphorus detection. Eighty-five patient samples from 38 men and 10 women receiving methadone maintenance were collected and subjected to the ultrafiltration methodology. Two independent procedures demonstrated that, following the ultrafiltration process, no proteins were measurable in the filtrate. In addition, the ultrafiltration process was found to function independently of the concentration of methadone and the volume of sample, assuming the amount filtered never exceeded 40% of the original volume. In the patient samples, the %-free methadone varied sixfold across all patients. Female patients were found to have a mean +/- SD %-free methadone of 11.9 +/- 3.8% vs. 10.1 +/- 3.4% for men. Pearson correlation values suggest that steady-state protein-free methadone levels (r = 0.521) and total methadone levels (r = 0.491) rise as methadone dose is increased. Corresponding to these results, free methadone levels are highly correlated with total methadone levels (Pearson r = 0.85). The Amicon MPS-1 ultrafiltration device appears to be a reliable and relatively easy system to use for separating protein-free from protein-bound methadone, though further study is required to clarify the clinical applications of free methadone levels.