Recombinantly expressed CYP450 systems (rCYPs) are often used to screen for irreversible/quasi-irreversible enzyme inhibitors during drug development. The concentration- and time-dependent inactivation of CYP2D6 by methylenedioxymethamphetamine (MDMA) was compared in three different rCYP2D6 systems (yeast microsomes, Supersomes™ and Bactosomes™) under the conditions of the most commonly used protocols in assessing mechanism-based inactivation (MBI). MDMA (2-20µM) was pre-incubated with enzyme for 0, 2.5 and 5min followed by a five-fold dilution and further incubation with dextromethorpan (DEX) (50µM). The formation of dextrorphan (DOR) from DEX was used as a specific marker of CYP2D6 activity. Concentration- and time-dependent inactivation of CYP2D6 by MDMA was observed with each rCYP system. However, the apparent kinetic parameters for MBI (kinact, the maximum inactivation rate constant and KI, the inhibitor concentration associated with half maximal rate of inactivation) were significantly greater (p<0.05) for Bactosomes™ (0.95 ± 0.33min-1), 42.9 ± 20.1µM) than those found using yeast microsomes (0.28 ± 0.04min-1, 2.86 ±.18µM) and Supersomes™ (0.38 ± 0.05min-1), 3.66 ± 0.10µM). After correction for depletion of MDMA during pre-incubation, (kinact and KI values determined using Bactosomes™ decreased significantly but remained higher than for the other rCYP systems (p<0.05). Substantial metabolism of DOR after its formation from DEX was also observed using Supersomes™ and Bactosomes™. Sub-optimal study design when investigating MBI may compromise the quantitative characterization of inhibitory characteristics using some rCYP systems.
September 1, 2007
Author(s): Linh Van, Judith Hargreaves, Martin Lennard, Geoffrey Tucker, Amin Rostami-Hodjegan
Year: September 1, 2007