Els Van Peer

Ontogeny of CYP3A and P‐Glycoprotein in the Liver and the Small Intestine of the Göttingen Minipig: An Immunohistochemical Evaluation

Despite the increasing use of the minipig as a non‐rodent species in general and juvenile toxicity studies, knowledge on their biotransformation processes and their ontogeny is scarce. Such data are prerequisite for the correct interpretation of non‐clinical studies in this species. Therefore, the aim of our investigation was to immunohistochemically document the presence of the drug transporter P‐glycoprotein (Pgp) and the metabolizing cytochrome P450 (CYP) 3A subfamily in the livers (n = 115) and the small intestines (n = 74) of foetal, neonatal, juvenile and adult Göttingen minipigs. Pgp was expressed in the liver in all age groups, whereas its presence in the jejunum was detected from 86 days of gestation onwards. Low expression of CYP3A was detected in the jejunums and livers from foetal and neonatal piglets. During postnatal development, the immunoreactivity for CYP3A increased in both organs. A centrilobular pattern, with a more intense staining for CYP3A of the hepatocytes surrounding the central vein, was noticed in the postnatal livers. In conclusion, the presented data suggest that the intestinal and hepatic ontogeny of P‐glycoprotein and CYP3A in minipigs corresponds to that in man, in which a similar spatio‐temporal expression has been reported.

In Vitro Biotransformation of Two Human CYP3A Probe Substrates and Their Inhibition during Early Zebrafish Development

At present, the zebrafish embryo is increasingly used as an alternative animal model to screen for developmental toxicity after exposure to xenobiotics. Since zebrafish embryos depend on their own drug-metabolizing capacity, knowledge of their intrinsic biotransformation is pivotal in order to correctly interpret the outcome of teratogenicity assays. Therefore, the aim of this in vitro study was to assess the activity of cytochrome P450 (CYP)—a group of drug-metabolizing enzymes—in microsomes from whole zebrafish embryos (ZEM) of 5, 24, 48, 72, 96 and 120 h post-fertilization (hpf) by means of a mammalian CYP substrate, i.e., benzyloxy-methyl-resorufin (BOMR). The same CYP activity assays were performed in adult zebrafish liver microsomes (ZLM) to serve as a reference for the embryos. In addition, activity assays with the human CYP3A4-specific Luciferin isopropyl acetal (Luciferin-IPA) as well as inhibition studies with ketoconazole and CYP3cide were carried out to identify CYP activity in ZLM. In the present study, biotransformation of BOMR was detected at 72 and 96 hpf; however, metabolite formation was low compared with ZLM. Furthermore, Luciferin-IPA was not metabolized by the zebrafish. In conclusion, the capacity of intrinsic biotransformation in zebrafish embryos appears to be lacking during a major part of organogenesis.

Age‐related Differences in CYP3A Abundance and Activity in the Liver of the Göttingen Minipig

In view of paediatric drug development, regulatory authorities often request safety studies in juvenile animals, including minipigs. Unfortunately, knowledge on the ontogeny of the biotransformation processes in animal models remains scarce and impedes a correct interpretation of the toxicity findings. CYP3A4 is one of the most important drug‐metabolizing enzymes in human beings and shows important similarities with CYP3A in the minipig. Therefore, the aim of this study was to assess the abundance and activity of CYP3A in liver microsomes from foetal, juvenile (days 1, 3, 7 and 28) and adult male and female Göttingen minipigs. CYP3A abundance was studied by an indirect enzyme‐linked immunosorbent assay (ELISA), whereas CYP3A activity was assessed by a biotransformation assay with Luciferin‐IPA. CYP3A abundance could not be detected until day 3. From day 7 onwards, a gradual increase in expression was noted, leading to the highest abundance in adult animals. CYP3A activity was not detectable in foetuses and 1‐day‐old animals. The CYP3A activity was detectable, but below the LLOQ in day 3 animals and increased gradually with age to reach the highest level in adults. The CYP3cide and ketoconazole inhibition, and testosterone and midazolam reduction of Luciferin‐IPA metabolism in minipig liver microsomes substantiate that Luciferin‐IPA is metabolized by CYP3A in minipigs. A positive correlation was found between CYP3A abundance and biotransformation of Luciferin‐IPA (Pearson r = 0.863; p < 0.0001). In conclusion, both abundance and activity of CYP3A increased gradually in juvenile minipigs, but remained below the levels observed in adult animals.

In vitro phase I- and phase II-drug metabolism in the liver of juvenile and adult Göttingen minipigs

PurposeIn view of pediatric drug development, juvenile animal studies are gaining importance. However, data on drug metabolizing capacities of juvenile animals are scarce, especially in non-rodent species. Therefore, we aimed to characterize the in vitro biotransformation of four human CYP450 substrates and one UGT substrate in the livers of developing Göttingen minipigs.MethodsLiver microsomes from late fetal, Day 1, Day 3, Day 7, Day 28, and adult male and female Göttingen minipigs were incubated with a cocktail of CYP450 substrates, including phenacetin, tolbutamide, dextromethorphan, and midazolam. The latter are probe substrates for human CYP1A2, CYP2C9, CYP2D6, and CYP3A4, respectively. In addition, the UGT multienzyme substrate (from the UGT-GloTM assay), which is glucuronidated by several human UGT1A and UGT2B enzymes, was also incubated with the porcine liver microsomes.ResultsFor all tested substrates, drug metabolism significantly rose postnatally. At one month of age, 60.5 and 75.4% of adult activities were observed for acetaminophen and dextrorphan formations, respectively, while 35.4 and 43.2% of adult activities were present for 4-OH-tolbutamide and 1’-OH-midazolam formations. Biotransformation of phenacetin was significantly higher in 28-day-old and adult females compared with males.ConclusionsMaturation of metabolizing capacities occurred postnatally, as described in man.