Contribution of CYP2E1 and CYP3A to acetaminophen reactive metabolite formation.

BACKGROUND: CYP2E1, 1A2, and 3A4 have all been implicated in the formation of N-acetyl-p-benzoquinone imine (NAPQI), the reactive intermediate of acetaminophen (INN, paracetamol), in studies in human liver microsomes and complementary deoxyribonucleic acid-expressed enzymes. However, recent pharmacokinetic evidence in humans has shown that the involvement of CYP1A2 is negligible in vivo. The purpose of this study was to evaluate the respective roles of CYP2E1 and 3A4 in vivo. METHODS: The involvement of CYP2E1 was assessed through pretreatment of adult human volunteers with disulfiram to inhibit the enzyme and the role of CYP3A4 through its induction in a second cohort of adults with rifampin (INN, rifampicin). Each of the respective studies was an open-label, balanced-randomized crossover design. Blood samples were obtained serially for 12 hours and urine was collected for 24 hours after acetaminophen administration. Acetaminophen was assayed in plasma, and acetaminophen and metabolites were assayed in urine. RESULTS: The recovery of the thiol metabolites formed by conjugation of NAPQI with glutathione was decreased by 69%, and the formation clearance of NAPQI was decreased by 74% (both P < .01) by pretreatment with disulfiram. Rifampin pretreatment had no effect on the formation of NAPQI or the recovery of thiol metabolites formed by conjugation of NAPQI with glutathione. CONCLUSIONS: CYP2E1 accounts for the formation of NAPQI in intact humans; the contribution of other isozymes of cytochrome P450 appears to be negligible. Under some conditions, disulfiram may be useful in diminishing the formation of NAPQI after acetaminophen overdose.

Mechanism of cytochrome P450 activation by caffeine and 7,8-benzoflavone in rat liver microsomes.

Caffeine and 7,8-benzoflavone activate CYP3A2 in rat liver microsomes. Both activators appear to enhance enzyme activity by an increase in Vmax and to a lesser extent a decrease in Km. Additive effect studies demonstrated that the two activators oppose one another's effect. Electron transfer steps in the cytochrome P450 cycle are involved in the mechanism of cytochrome P450 activation, as indicated by the lack of effect of caffeine or 7,8-benzoflavone on cumene hydroperoxide-supported oxidation of acetaminophen by cytochrome P450. The involvement of cytochrome b5 in the formation of N-acetyl-p-benzoquinone imine (NAPQI) was implicated through a synergistic effect of NADH on the NADPH-supported reaction. Anti-cytochrome b5, but not anti-cytochrome P450 reductase IgG, diminished the activation effect of caffeine on NAPQI formation. Neither antibody altered the effect of 7,8-benzoflavone on NAPQI formation. The impairment of NAPQI formation by cytochrome b5 antibody suggests that cytochrome P450 activation by caffeine but not 7,8-benzoflavone is mediated in part through enhancement of the transfer of the second electron to cytochrome P450 from cytochrome b5.

Immunogenicity of recombinant hepatitis B vaccine in urban black children from Ga-Rankuwa, Bophuthatswana, South Africa.

Many areas in Southern Africa have a relatively high endemicity for hepatitis B for which the only effective medical measure is vaccination. The aim of this study was to evaluate the antibody response to a recombinant hepatitis B vaccine (Engerix B; Smith Kline-Beecham) in a black urban population, with the use of the recommended regimen and a low dose, short course. One hundred eleven children seronegative for hepatitis B virus (5 to 19 years old) were randomized to receive one of the two vaccination schedules (20 micrograms at zero, 1 and 6 months or 2 micrograms at zero, 1 and 2 months). Antibody to hepatitis B surface antigen was determined 6 to 8 weeks after the last dose by radioimmunoassay (Ausab; Abbott Laboratories). The recommended schedule gave a seroconversion rate of 100% with a geometric mean titer of 585.9 mIU/ml. The low dose, short course schedule produced a seroconversion rate of 63.8% and a geometric mean titer of 73.8 mIU/ml. In the 5- to 9-year-old individuals, however, 71.6% seroconverted (geometric mean titer 114.2 mIU/ml). For cost reasons further investigations on low dose regimens are indicated.