title: Monitoring Cyp2b10 mRNA expression at cessation of 2-year carcinogenesis bioassay in mouse liver provides evidence for a carcinogenic mechanism devoid of human relevance: the dalcetrapib experience.
abstract: BACKGROUND
  Dalcetrapib is a cholesteryl ester transfer protein (CETP) modulator in clinical assessment for cardiovascular outcome benefits. In compliance with regulatory requirements, dalcetrapib was evaluated in rodent 2-year carcinogenesis bioassays. In the mouse bioassay, male mice demonstrated increased liver weight and statistically increased incidences of hepatocellular adenoma/carcinoma. Hepatic cytochrome p450 (Cyp) 2b10 mRNA induction and increased Cyp2b10 enzyme activity signify activation of hepatic nuclear receptor constitutive androstane receptor (CAR), a widely established promoter of rodent-specific hepatic tumors. We therefore monitored hepatic Cyp2b10 mRNA and its enzyme activity in a subset of dalcetrapib-treated male mice from the bioassay.
  
  
  METHODS
  Liver samples were obtained from ~1/3 of male mice from each dose group including vehicle-controls (mean and earliest study day of death 678 and 459 respectively). Quantitative real time PCR (qRT-PCR) was performed to determine Cyp2b10 mRNA expression and Cyp1a-, Cyp2b10- and Cyp3a-selective activities were monitored.
  
  
  RESULTS
  Cyp2b10 mRNA was strongly induced by dalcetrapib with an expected wide inter-individual variation (5-1421-fold). Group average fold-induction versus vehicle-controls showed a dose-related increase from 48-fold (250mg/kg/day) to 160-fold (750mg/kg/day), which declined slightly at 2000mg/kg/day (97-fold). Cyp enzyme activities showed approximate doubling of total Cyp P450 content per milligram protein and a 9-fold increase in Cyp2b10-selective pentoxyresorufin O-dealkylase activity (750mg/kg/day).
  
  
  CONCLUSIONS
  These data from hepatic Cyp2b10 monitoring are strongly suggestive of CAR activation by dalcetrapib, a mechanism devoid of relevance towards hepatocarcinogenesis in humans; results show feasibility of Cyp2b10 as a surrogate marker for this mechanism at cessation of a carcinogenesis bioassay.
authors.affiliation: F. Hoffmann-La Roche Ltd., Pharma Research and Early Development, Non-Clinical Safety, Grenzacherstrasse 124, 4070 Basel, Switzerland.
authors.forename: J-C
authors.initials: JC
authors.lastname: Hoflack
authors.forename: L
authors.initials: L
authors.lastname: Mueller
authors.forename: S
authors.initials: S
authors.lastname: Fowler
authors.forename: A
authors.initials: A
authors.lastname: Braendli-Baiocco
authors.forename: N
authors.initials: N
authors.lastname: Flint
authors.forename: O
authors.initials: O
authors.lastname: Kuhlmann
authors.forename: T
authors.initials: T
authors.lastname: Singer
authors.forename: A
authors.initials: A
authors.lastname: Roth
mesh_terms: D000577:Amides; D000818:Animals; D000924:Anticholesteremic Agents / Q000008:administration & dosage / Q000633:toxicity*; D001189:Aryl Hydrocarbon Hydroxylases / Q000235:genetics*; D001681:Biological Assay / Q000379:methods; D000072467:Cytochrome P450 Family 2; D004305:Dose-Response Relationship, Drug; D016903:Drug Monitoring / Q000379:methods; D004952:Esters; D005240:Feasibility Studies; D015971:Gene Expression Regulation, Enzymologic / Q000187:drug effects; D006801:Humans; D008099:Liver / Q000187:drug effects* / Q000201:enzymology / Q000378:metabolism; D008297:Male; D051379:Mice; D008813:Mice, Inbred ICR; D016133:Polymerase Chain Reaction; D012333:RNA, Messenger / Q000378:metabolism*; D013045:Species Specificity; D013250:Steroid Hydroxylases / Q000235:genetics*; D013438:Sulfhydryl Compounds / Q000008:administration & dosage / Q000633:toxicity*; D013997:Time Factors
chemical_list: D000577:Amides; D000924:Anticholesteremic Agents; D004952:Esters; D012333:RNA, Messenger; D013438:Sulfhydryl Compounds; C411602:dalcetrapib; D013250:Steroid Hydroxylases; D001189:Aryl Hydrocarbon Hydroxylases; C101679:Cyp2b10 protein, mouse; D000072467:Cytochrome P450 Family 2