Protein binding of fentanyl and its metabolite nor-fentanyl in human plasma, albumin and α-1 acid glycoprotein

1. Fentanyl is a highly lipophilic opioid commonly used to treat cancer pain. Plasma protein binding (PPB) of fentanyl in human plasma is reported as 80-85%, however it is unclear whether fentanyl binds primarily to albumin (ALB) or alpha-1 acid glycoprotein (AAG) and no studies have been conducted on the metabolite, nor-fentanyl. Fentanyl is also known to bind to plasticware and ultrafiltration (UF) devices which impacts adversely on binding experiments. 2. PPB of fentanyl and nor-fentanyl to ALB and AAG in isotonic phosphate buffer solution and seeded human plasma was quantified. PPB was also performed in plasma samples obtained from cancer patients receiving transdermal fentanyl. The adsorption of fentanyl and norfentanyl to UF devices and plasticware commonly used in PPB studies was also assessed. 3. Fentanyl was shown to bind primarily to ALB as opposed to AAG, with nor-fentanyl exhibiting negligible binding to plasma proteins. Total PPB of fentanyl was 86-89% in seeded human plasma. PPB in 56 cancer patient samples was 95.1 +/- 3.52% for fentanyl and 32.4 +/- 21.9% for nor-fentanyl. 4. UF was shown to be a reliable and convenient method for PPB studies, thereby removing the need for complex testing for adsorption of the drug to plasticware during UF.