In vitro metabolism of benzo[a]pyrene and dibenzo[def,p]chrysene in rodent and human hepatic microsomes

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous and often carcinogenic contaminants released into the environment during natural and anthropogenic combustion processes. Benzo[a]pyrene (B[a]P) is the prototypical carcinogenic PAH, and dibenzo[def,p]chrysene (DBC) is a less prevalent, but highly potent transplacental carcinogenic PAH. Both are metabolically activated by isoforms of the cytochrome P450 enzyme superfamily to form reactive carcinogenic and cytotoxic metabolites. Metabolism of B[a]P and DBC was studied in hepatic microsomes of male Sprague-Dawley rats, naïve and pregnant female B6129SF1/J mice, and female humans, corresponding to available pharmacokinetic data. Michaelis–Menten saturation kinetic parameters including maximum rates of metabolism (VMAX, nmol/min/mg microsomal protein), affinity constants (KM, μM), and rates of intrinsic clearance (CLINT, ml/min/kg body weight) were calculated from substrate depletion data. CLINT was also estimated from substrate depletion data using the alternative in vitro half-life method. VMAX and CLINT were higher for B[a]P than DBC, regardless of species. Clearance for both B[a]P and DBC was highest in naïve female mice and lowest in female humans. Clearance rates of B[a]P and DBC in male rat were more similar to female human than to female mice. Clearance of DBC in liver microsomes from pregnant mice was reduced compared to naïve mice, consistent with reduced active P450 protein levels and elevated tissue concentrations and residence times for DBC observed in previous in vivo pharmacokinetic studies. These findings suggest that rats are a more appropriate model organism for human PAH metabolism, and that pregnancy's effects on metabolism should be further explored.

Human in Vivo Pharmacokinetics of [14C]Dibenzo[def,p]chrysene by Accelerator Mass Spectrometry Following Oral Microdosing

“…Overall, across a number of concentrations, 55 ± 7% (at the low end of previously reported recoveries of 61–101% in human studies with pharaceuticals, 18 perhaps due to the much lower mass of the dose utilized in this study) was the average DBC recovery when reconstituted in methanol/water, and this value was used for efficiency corrections in the PK parameters reported below. Recent work by Crowell et al 35 had determined this to be the optimal extraction method for DBC, as well as the diol, and tetraol metabolites.…”

Human Microdosing with Carcinogenic Polycyclic Aromatic Hydrocarbons: In Vivo Pharmacokinetics of Dibenzo[def,p]chrysene and Metabolites by UPLC Accelerator Mass Spectrometry

“…14,15 More recently, pregnancy was shown to significantly impact PAH metabolism and pharmacokinetics in mice. 32–34 Crowell et al, observed a 2- to 10-fold reduction in P450 activities during pregnancy in B6129F1 mice that significantly altered the pharmacokinetics of DBC and its major metabolites compared to non-pregnant mice. 33 …”

Arsenite Interacts with Dibenzo[def,p]chrysene (DBC) at Low Levels to Suppress Bone Marrow Lymphoid Progenitors in Mice

“…DBC toxicity is likely caused by the products of its biotransformation [15]. DBC-dihydrodiol epoxide (DBCDE) is formed through the action of two enzymes, CYP1B1 and epoxide hydrolase [16]. DBC radical cation is formed through the action of peroxidases [17], and DBC-dione obtained through the breakdown of DBC-11, 12-dihydrodiol by aldoketoreductases (AKR), [18].…”

“…However, these studies were conducted at higher concentrations of As +3 (micromolar ranges) than used in our present work. As with DMBA [7], DBC is mostly metabolized by CYP1B1 in mouse lymphoid tissues [16]. Therefore, if high doses of As +3 interfere with the induction or activity of CYP1B1, this may explain the lack of interaction between As +3 and DBC at the higher concentration of As +3 .…”