Heinrich Bürgin

Literature review on the genotoxicity, reproductive toxicity, and carcinogenicity of ethyl methanesulfonate.

In order to assess the risk of patients being exposed to an anti-AIDS medication contaminated with EMS we have performed in depth genotoxicity, general toxicity and DMPK investigations. The results of these studies are reported in the accompanying papers of this issue. Prior to starting our investigations we searched the literature for toxicity data on this well established mutagen with specific attention to dose-response relations in in vivo genotoxicity studies, since, obviously, in vivo data are pivotal for risk assessment. There are numerous published in vivo genotoxicity studies on EMS, with generally 50mg/kg - or higher - being the minimal dose used. The dose of 50mg/kg induced effects in some, but not all studies, while the dose of 100mg/kg was clearly positive in most studies, except for heritable mutations where a single dose of 100mg/kg was not observed to induce measurable effects in post-meiotic stages and even the maximal dose of 250 mg/kg was negative in pre-meiotic stages of male germ cell development. For somatic cells, NOEL values could not be derived for any of the endpoints studied. Although a large number of genotoxicity studies are available, none of the studies was sufficiently detailed to allow unambiguous conclusions about the presence of a (practical) threshold. But in most cases the dose-responses show a sublinear relationship (i.e. the slope increases with dose) which indicates that the data would not be incompatible with a threshold dose-response relationship. This stands in contrast to data on ethylnitrosourea (ENU) which has been studied concommittantly with EMS in several in vitro and in vivo genotoxicity investigations. ENU generally appeared to induce genotoxic effects with linear dose relationships. We also review the more limited data reported on teratogenicity and carcinogenicity of EMS. Induction of fetal malformations in mice appeared to have a NOEL of 100mg/kg. Classical life-time carcinogenicity studies have not been performed with EMS. Induction of mammary, lung, kidney, brain, and liver tumors has been observed after various short term treatment regimes. In none of the published studies a no effect level was reported and no exposure data are available. Overall, the experimental data do not fully characterize the carcinogenic potential of EMS and are insufficient for a risk extrapolation to humans. Although the data on teratogenicity and carcinogenicity are insufficient for assessing dose-response relations it is generally accepted that the genotoxic property of EMS is at the base of the teratogenic and carcinogenic effects.

The high sensitivity of the rabbit to the teratogenic effects of 13-cis-retinoic acid (isotretinoin) is a consequence of prolonged exposure of the embryo to 13-cis-retinoic acid and 13-cis-4-oxo-retinoic acid, and not of isomerization to all-trans-retinoic acid

Previous studies suggested that the rabbit is much more susceptible to the teratogenic action of 13-cis-retinoic acid (13-cis-RA) than the mouse or the rat, while the teratogenicity of all-trans-RA was comparable in these species. In the present study we investigated if pharmacokinetics can explain these species- and structure-related differences. The embryotoxic and teratogenic potential of all-trans-retinoid acid (all-trans-RA) and 13-cis-RA were evaluated in the Swiss hare rabbit after oral administration of daily doses of the two drugs throughout organogenesis, from gestation day (GD) 6 to 18 (plug day=GD 0). All-trans-RA was given at dose levels of 0.7, 2 or 6 mg/kg body weight per day and 13-cis-RA at 3, 7.5 or 10 mg/kg per day. The doses needed to elicit a minimum teratogenic response were found to be 6 mg/kg per day for all-trans-RA and 10 mg/kg per day for 13-cis-RA. Using these doses, transplacental pharmacokinetics of all-trans- and 13-cis-RA were performed. Pregnant rabbits were treated once daily from GD 7 to 12 and plasma and embryo samples were collected for HPLC analysis at various time intervals after the final dose. The main plasma metabolites of all-trans-and 13-cis-RA were all-trans-β-glucuronide (all-trans-RAG) and 13-cis-4-oxo-RA, respectively. The elimination of 13-cis-RA and its metabolites from maternal plasma were much slower than of all-trans-RA resulting in accumulation of the 13-cis-isomers in plasma. Marked differences in the placental transfer of the two drugs and their metabolites were observed. All-trans-RA and all-trans-4-oxo-RA were efficiently transferred to the rabbit embryo, reaching concentrations similar to the plasma levels. On the contrary, the 13-cis-isomers reached the embryo to a lesser extent. Despite its limited placental transfer, a considerable embryonic exposure to 13-cis-RA and 13-cis-4-oxo-RA was noticed after treatment with isotretinoin, as indicated by their area-under-the-concentration-time-curve (AUC) values in the embryo, which were in the same range as the corresponding AUC value of all-trans-RA after treatment with the all-trans-isomer. Our results suggest that the high sensitivity of the rabbit to the teratogenic effects of 13-cis-RA can be attributed mainly to the 13-cis-isomers and not to isomerization to all-trans-RA. The significant exposure of the rabbit embryo to 13-cis-RA and its 4-oxo metabolite is a result of their very slow excretion rates from the maternal organism. Furthermore, this study supports the view that embryonic AUC values should be considered as the most suitable pharmacokinetic correlate to retinoid induced teratogenesis.

Cardiovascular alterations in rat fetuses exposed to calcium channel blockers

Preclinical toxicologic investigation suggested that a new calcium channel blocker, Ro 40-5967, induced cardiovascular alterations in rat fetuses exposed to this agent during organogenesis. The present study was designed to investigate the hypothesis that calcium channel blockers in general induce cardiovascular malformations indicating a pharmacologic class effect. We studied three calcium channel blockers of different structure, nifedipine, diltiazem, and verapamil, along with the new agent. Pregnant rats were administered one of these calcium channel blockers during the period of cardiac morphogenesis and the offspring examined on day 20 of gestation for cardiovascular malformations. A low incidence of cardiovascular malformations was observed after exposure to each of the four calcium channel blockers, but this incidence was statistically significant only for verapamil and nifedipine. All four agents were associated with aortic arch branching variants, although significantly increased only for Ro 40-5967 and verapamil.

Embryolethality of new herbicides is not detected by the micromass teratogen tests

New herbicidal compounds (11 pyrimidine-diones, 3 benzoates and 1 sulfonamide) were found to be embryolethal but not teratogenic in rats. The range of the embryolethal dose varied from 0.2 to >200 mg/kg. This broad range enabled us to validate whether proposed in vitro teratogen tests can detect the embryolethality of these herbicides. The IC50 values (inhibition concentration 50%) for both differentiation and proliferation of midbrain and limb bud cells of rat embryos were determined and found to be above 50 μg/ml in all cases, confirming that the herbicides were not teratogenic. No correlation, however, was observed between the embryolethality in vivo and the activities in these cells. In order to test whether the potential to cause embryolethality could be predicted and detected as a general cytotoxic effect, the inhibition of colony forming ability in V79 cells was determined. The results indicated that cytotoxicity in V79 cells may be useful for preliminary testing of the embryolethal effect of herbicides.

Embryofetal Development Study of Vismodegib, a Hedgehog Pathway Inhibitor, in Rats

Vismodegib (Erivedge) is a first-in-class small-molecule hedgehog pathway inhibitor for the treatment of adults with advanced basal-cell carcinoma. Because this pathway is known to play key roles in patterning and growth during vertebrate development, vismodegib was anticipated to be embryotoxic. To support marketing applications, an embryofetal development study was completed in which a limited number of pregnant rats (n = 6/group) was administered vismodegib by oral gavage on gestation days 6 to 17. When vismodegib was administered at ≥60 mg/kg/day, doses associated with evidence of pharmacologic activity in previous rat toxicity studies, all conceptuses were resorbed at an early embryonic stage in the absence of significant maternal toxicity. When administered at 10 mg/kg/day, corresponding to an exposure (AUC0-24h ) approximately 15% of the median in patients at steady state, a variety of malformations were observed, including absent/fused digits in the hindlimb of multiple fetuses, multiple craniofacial abnormalities in one fetus, and an anorectal defect in one fetus. In addition, the incidence of variations, including dilated renal pelvis or ureter and incompletely or unossified skeletal elements, was significantly greater when compared with the controls. These results confirmed that vismodegib is likely to be embryotoxic at clinically relevant maternal exposures, and doses ≥60 mg/kg/day resulted in a 100% incidence of embryolethality that likely resulted from severe defects in early embryonic development. In contrast, craniofacial defects typically associated with hedgehog pathway inhibition were only observed in one fetus at the low dose of 10 mg/kg/day, which likely reflected minimal or intermittent pathway inhibition at low exposures.

The Effects of Interleukin-6 Signal Blockade on Fertility, Embryo-fetal Development, and Immunization In vivo

Possible effects of interleukin-6 (IL-6) on reproductive performance, embryonal development, parturition, and postnatal development have been suggested based on protein/mRNA expression level of IL-6 in related organs, but less is known about functions of IL-6 signals in these areas. Following two different approaches have been employed to investigate the role of IL-6 signals in fertility and pre-/postnatal development: administration of a rat anti-mouse IL-6 receptor antibody, MR16-1, to mice as a neutralizing antibody system, and B6.129S2-Il6(tm1Kopf)/J (IL-6 knockout [KO]) mice as a KO system. By intravenously dosing 50 mg/kg of MR16-1 every 3 days, animals in male and female fertility studies and dams in a pre-/postnatal development study exhibited plasma MR16-1 concentrations much higher than the effective plasma concentration, indicating that MR16-1 exposure was sufficient to completely block IL-6 signals. The concentration of MR16-1 in the plasma of fetuses exceeded that in the plasma of pregnant animals, and MR16-1 concentration in milk was about one-fourth of that in plasma. Both the transient IL-6 signal blockade by MR16-1, and the constitutive IL-6 signal inhibition using IL-6 KO mice in a combined fertility and pre-/postnatal development study, revealed no biologically important effects on fertility, early embryonic development to implantation, or pre-/postnatal development, including IgG/IgM production by keyhole limpet hemocyanin sensitization. These results indicate that IL-6 signals have no unique, noncompensable roles in reproduction and development in the whole body system, although contributions of IL-6 in the signaling network appear to exist, as suggested by previously published investigations.

The Effects of Interleukin-6 Signal Blockade on Immune System, Reproductive and Skeletal Development in Juvenile Mice

Interleukin-6 (IL-6) is involved in the pathogenesis of multiple disorders, including juvenile autoimmune diseases. IL-6 participates in a broad spectrum of physiological events, and the IL-6 receptor (IL-6R) is widely distributed across multiple organs. The interrelationship of development phases in juveniles together with organs involved in IL-6 signaling called for evaluations of anti-IL-6R antibody induced effects in a juvenile mouse model to assess the safety of such an approach in human juvenile arthritis. Here we show that naive mice in which IL-6 signals have been transiently blocked during the juvenile period develop normally. The fatal immunogenic reactions recorded earlier by repeated administration of the chosen rat anti-mouse IL-6R antibody, MR16-1, to mice were avoided successfully by application of a high loading dose followed by lower maintenance doses, with the support of modeling data. The high loading-dose regimen enabled us to conduct assessments without any major interference due to immunogenicity. Transient and complete inhibition of IL-6 signals from postnatal days 22 to 79 in mice exhibited no biologically important changes in sexual maturation or development of immune and skeletal systems. Although tendencies toward reductions of peripheral blood T-cell counts were observed, normal levels of antigen-specific IgG/IgM antibody productions indicating sufficient immunological functions were confirmed. Our results demonstrate that blockage of IL-6R by the neutralizing antibody does not affect juvenile development. This may be in part due to the generation or existence of compensatory pathways in the whole body system.

Micromass Culture of Midbrain Cells and its Relevance to in Vitro Mechanistic Studies

Abstract The relationship between ethylenethiourea (ETU)‐induced malformations of cultured rat whole embryos and the alterations of midbrain (MB) cells was investigated and species‐specific ETU‐induced alterations between rat and mouse MB cells were determined.