Plasma protein binding and distribution of fentanyl, sufentanil, alfentanil and lofentanil in blood.

The in vitro plasma protein binding and distribution in blood of fentanyl and three analogues were studied in rats, dogs and healthy volunteers. In human plasma, 84.4% of fentanyl was bound, 92.5% of sufentanil, 92.1% of alfentanil and 93.6% of lofentanil. Plasma protein binding of the four analgesics was independent of their concentration over the whole therapeutic range. Plasma protein binding of alfentanil was much less pH dependent than that of the three other analgesics. Attention was drawn to the possible contribution of the "acute phase' protein alpha 1-acid glycoprotein (alpha 1-AGP), of lipoproteins and of blood cells to the binding of fentanyl and its analogues in blood.