Pharmacokinetics of [3H]ohmefentanyl in rats

Ohmefentanyl (OMF) is a new potent analgesic derived from fentanyl. The pharmacokinetic characteristics of [3H]OMF in rats was studied in this paper. Time courses of concentration of [3H]OMF in plasma after bolus iv could be described as a 3-compartment open kinetic model. The major part of a single dose (1 microgram/kg) of [3H]OMF was rapidly eliminated from plasma by uptake of the active drug by tissues or organs and by biotransformation into hydrophilic metabolites. In respect to its lipophilic nature of OMF the concentrations of [3H]OMF in tissues or organs were much higher than those in plasma. At 1 min after iv, the concentrations of [3H]OMF in the lung, kidney, heart and brain stem were 40, 15, 7.5 and 6 times respectively higher than those in plasma. [3H]OMF and its hydrophilic metabolites were mainly excreted via the kidneys. The urine and feces in 72 h contained about 53% and 15% respectively of the total radioactivity administered. Approximately 65% of [3H]OMF was bound to plasma protein at pH 7.4, 4 degrees C. The bindings were the same for all concentrations of [3H]OMF between 0.02 and 1 ng/ml plasma in vitro.