PNNL

Abstract

Humans are routinely exposed to complex polycyclic aromatic hydrocarbons (PAHs) mixtures rather than to single PAH compounds as often assessed for toxicity hazard. Cytochrome P450 enzymes (CYPs) demonstrate substrate promiscuity, and multiple PAHs can compete as substrates for individual CYPs (e.g. CYP1A1, CYP1B1, etc.). We hypothesize that absorbed PAHs from mixtures will compete for enzymes, attenuate/inhibit PAH metabolism, and affect PAH clearance, detoxification, bioactivation, and toxicity, particularly at high exposures. The Relative Potency Factor approach used for risk assessment of complex mixtures assumes that common human exposures will not cause interactions among mixture components. The objective of this study was to assess competitive inhibition of metabolism of PAH mixtures in humans and evaluate this key assumption of the Relative Potency Factor approach. As such, we co-incubated binary mixtures of benzo[a]pyrene (BaP) and dibenzo[def,p]chrysene (DBC) in human hepatic microsomes and measured rates of enzymatic BaP and DBC disappearance. We observed competitive inhibition of BaP and DBC metabolism and measured inhibition coefficients (Ki), observing that BaP inhibited DBC metabolism more potently than DBC inhibited BaP metabolism (0.061 vs. 0.44 µM Ki, respectively). We developed an interaction physiologically based pharmacokinetic (PBPK) model by integrating PBPK models of DBC and BaP and incorporating measured metabolism inhibition coefficients. We used the interaction PBPK model to simulate different PAH exposures to identify what levels of exposures would cause changes in internal dosimetry of DBC or BaP in humans. The PBPK model predicts significant increases in BaP and DBC concentrations in blood AUCs following high oral PAH doses (= 100 mg), 5 orders of magnitude higher than typical human exposures. We also measured inhibition coefficients of Supermix-10, a mixture of the most abundant PAHs measured at the Portland Harbor Superfund Site, on BaP and DBC metabolism. We observed similar potencies of inhibition coefficients of Supermix-10 as with BaP and DBC. Overall, results of this study demonstrate that these PAHs compete for the same enzymes and, at high doses, inhibit metabolism and alter internal dosimetry of each other. This approach predicts that BaP and DBC exposures required to observe metabolic interaction are much higher than typical human exposures, consistent with assumptions used when applying the Relative Potency Factor approach for mixture risk assessment.