Aromatase is the major enzyme metabolizing buprenorphine in human placenta

Buprenorphine (BUP) is a partial opiate agonist used for treatment of the adult and the pregnant addicted to this class of narcotics. The kinetic parameters for transplacental transfer and the metabolism of BUP during its perfusion in a placental lobule were the subject of an earlier report from our laboratory. The aim of this investigation is to identify and characterize the enzyme catalyzing the metabolism of BUP in term human placenta. Norbuprenorphine (norBUP) is the only metabolite formed as determined by high performance liquid chromatography and mass spectrometry. The activity of the enzyme responsible for BUP metabolism is highest in the microsomal fraction and lowest in the cytosolic, with the mitochondrial in between. Compounds with selective affinity to the enzyme aromatase (CYP 19), namely 4-hydroxyandrostenedione and aminoglutethimide, caused >70% inhibition of norBUP formation. Monoclonal antibodies raised against CYP 19 were the most potent inhibitors of BUP dealkylation. A comparison between the data obtained from the saturation isotherm for BUP dealkylation by placental microsomes and a commercially available system of cDNA-expressed CYP 19 indicated similar kinetic parameters, with apparent K_m values of 12 ± 4.0 and 14 ± 8.0 μM, respectively. Therefore, aromatase is the major enzyme catalyzing the biotransformation of BUP to norBUP in term human placentas obtained from healthy pregnancies. The minor involvement of other cytochrome P450 isoforms or enzyme(s) in the metabolism of BUP in placental tissue cannot be ruled out.