| Buprenorphine (BN) is used as an analgesic and also for the treatment of opioid dependence. Even though the pharmacology of BN has been studied extensively in animal and human pain models, little attention has been given to the potential role of its major metabolite, norbuprenorphine (NBN), with regard to its activity or contribution to the overall effect of BN. In addition, the pharmacokinetics/pharmacodynamics (PK/PD) of BN and NBN and elucidation of all possible metabolites of NBN were not available due to a lack of sensitive analytical methods, and due to the complex task of metabolite identification.; In this thesis, a sensitive, qualitative and quantitative method was developed to analyze BN, NBN and its metabolites. The PK/PD of both BN and NBN were explored. First, we developed and validated a LC-TIS-MS-MS method for simultaneous determination of BN and NBN in rat and human plasma. We then identified and iv elucidated the structures of seven metabolites (M1-M7) of NBN. NBN is metabolized by Phase I metabolizing enzymes and then by Phase II enzymes to form conjugated and sulfate metabolites. Biliary excretion is the major route of elimination for free NBN and renal excretion for NBN metabolites. The biliary excretion pathway also allowed enterohepatic recirculation, thus, offering the possibility of NBN remaining in the system longer.; The pharmacokinetics of BN and NBN were characterized using a two-compartment PK model. BN and NBN were absorbed very rapidly and were assumed to reach target sites rapidly, and were eliminated from the system at moderate to slow rates based on terminal half-lives of 5.5-9.7 h. BN and NBN have a large volume of distribution of 13-14 L/kg, which exceeded the total body water in rat (∼0.7 L/kg), indicating their extensive distribution into tissue compartments.; We examined the effects of NBN on gastrointestinal transit in rats and established that NBN is equipotent with BN in delaying transit at a dose of 0.3 mg/kg, s.c.; Finally, we established a concentration-effect relationship of BN and NBN based on a direct link PK-PD model. The plasma concentration and analgesic effect were concomitantly entered into a sigmoid Emax model. BN showed full efficacy at 0.5 mg/kg, s.c., where Emax = 99.3% and EC50 = 20 ng/ml. NBN showed a robust efficacy at 3.0 mg/kg, where Emax = 84.8 and EC50 = 221 ng/ml. At a higher dose for BN (3.0 mg/kg) the maximum analgesic effect was lower and the EC50 was higher than at dose of 0.5 mg/kg.; In summary, we characterized the pharmacokinetics and pharmacodynamics, and offered the complete profile of BN and NBN in this thesis. Based on its long half-life, very large volume of distribution and efficacy, we clearly verified NBN contribution to the overall pharmacodynamic effect of BN. |
