Please cite this article in press as: Abass, K., et al., Human variation and CYP enzyme contribution in benfuracarb metabolism in human in vitro hepatic models. Toxicol. Lett. (2013), http://dx.doi.org/10.1016/j.toxlet.2013.08.023 ARTICLE IN PRESS G Model TOXLET-8467; No. of Pages 10 Toxicology Letters xxx (2013) xxx–xxx Contents lists available at ScienceDirect Toxicology Letters jou rn al hom epage: www.elsevier.com/locate/toxlet Human variation and CYP enzyme contribution in benfuracarb metabolism in human in vitro hepatic models Khaled Abass a,b,∗,1 , Petri Reponen b,c , Sampo Mattila c , Arja Rautio a , Olavi Pelkonen b a Center for Arctic Medicine, Thule Institute, University of Oulu, Finland b Pharmacology and Toxicology Unit, Institute of Biomedicine, P.O. Box 5000, FI-90014 University of Oulu, Oulu, Finland c Department of Chemistry, P.O. Box 3000, FI-90014 University of Oulu, Finland h i g h l i g h t s • Human interindividual variability in the metabolism of benfuracarb was 5.4-fold (CL int ). • CYP3A4 was the major enzyme of carbofuran formation and CYP2C9 of sulfoxide formation. • The carbofuran pathway repre- sents 80–98% of the total hepatic metabolism of benfuracarb. • CYP3A4-catalyzed metabolism is the main source of human interindivid- ual differences. g r a p h i c a l a b s t r a c t a r t i c l e i n f o Article history: Received 7 May 2013 Received in revised form 26 August 2013 Accepted 28 August 2013 Available online xxx Keywords: Benfuracarb Human variability Cytochrome P450s CYP assignment Metabolism Risk assessment a b s t r a c t Human responses to the toxicological effects of chemicals are often complicated by a substantial interindi- vidual variability in toxicokinetics, of which metabolism is often the most important factor. Therefore, we investigated human variation and the contributions of human-CYP isoforms to in vitro metabolism of benfuracarb. The primary metabolic pathways were the initial sulfur oxidation to benfuracarb-sulfoxide and the nitrogen-sulfur bond cleavage to carbofuran (activation). The K m , V max , and CL int values of carbo- furan production in ten individual hepatic samples varied 7.3-, 3.4-, and 5.4-fold, respectively. CYP2C9 and CYP2C19 catalyzed benfuracarb sulphur oxidation. Carbofuran formation, representing from 79% to 98% of the total metabolism, was catalyzed predominantly by CYP3A4. The calculated relative contribu- tion of CYP3A4 to carbofuran formation was 93%, while it was 4.4% for CYP2C9. The major contribution of CYP3A4 in benfuracarb metabolism was further substantiated by showing a strong correlation with CYP3A4-selective markers midazolam-1 ′ -hydroxylation and omeprazole-sulfoxidation (r = 0.885 and 0.772, respectively). Carbofuran formation was highly inhibited by the CYP3A inhibitor ketoconazole. Moreover, CYP3A4 marker activities were relatively inhibited by benfuracarb. These results confirm that human CYP3A4 is the major enzyme involved in the in vitro activation of benfuracarb and that CYP3A4-catalyzed metabolism is the primary source of interindividual differences. © 2013 Elsevier Ireland Ltd. All rights reserved. ∗ Corresponding author at: Center for Arctic Medicine, Thule Institute, P.O. Box 7300, FI-90014 University of Oulu, Oulu, Finland. Tel.: +358 294 483563. E-mail addresses: khaled.megahed@oulu.fi, khaled.m.abass@gmail.com (K. Abass). 1 Permanent address: Department of Pesticides, Menoufia University, P.O. Box 32511, Egypt. 1. Introduction Protecting public health is the primary reason why risk assess- ment of pesticides is very important. Humans are inevitably exposed to pesticides in various ways, at different dose levels, and for varying periods of time (Boobis et al., 2008; Ellenhorns et al., 1997). The characterization of the metabolic features of a pesticide in humans is an essential piece of scientific information needed for 0378-4274/$ – see front matter © 2013 Elsevier Ireland Ltd. All rights reserved. http://dx.doi.org/10.1016/j.toxlet.2013.08.023