Teratogenicity of etretinate during early pregnancy in the rat and its correlation with maternal plasma concentrations of the drug

A human induced pluripotent stem cell-based in vitro assay predicts developmental toxicity through a retinoic acid receptor-mediated pathway for a series of related retinoid analogues

The relative developmental toxicity potency of a series of retinoid analogues was evaluated using a human induced pluripotent stem (iPS) cell assay that measures changes in the biomarkers ornithine and cystine. Analogue potency was predicted, based on the assay endpoint of the ornithine/cystine (o/c) ratio, to be all-trans-retinoic acid>TTNPB>13-cis-retinoic acid≈9-cis-retinoic acid>acitretin>etretinate>retinol. These rankings correlate with in vivo data and demonstrate successful application of the assay to rank a series of related toxic and non-toxic compounds. The retinoic acid receptor α (RARα)-selective antagonist Ro 41-5253 inhibited the cystine perturbation caused by all-trans-retinoic acid, TTNPB, 13-cis-retinoic acid, 9-cis-retinoic acid, and acitretin. Ornithine was altered independent of RARα in all retinoids except acitretin. These results suggest a role for an RARα-mediated mechanism in retinoid-induced developmental toxicity through altered cystine metabolism.

Structure-based thresholds of toxicological concern (TTC): guidance for application to substances present at low levels in the diet

The threshold of toxicological concern (TTC) is a pragmatic risk assessment tool that is based on the principle of establishing a human exposure threshold value for all chemicals, below which there is a very low probability of an appreciable risk to human health. The concept that there are levels of exposure that do not cause adverse effects is inherent in setting acceptable daily intakes (ADIs) for chemicals with known toxicological profiles. The TTC principle extends this concept by proposing that a de minimis value can be identified for many chemicals, in the absence of a full toxicity database, based on their chemical structures and the known toxicity of chemicals which share similar structural characteristics. The establishment and application of widely accepted TTC values would benefit consumers, industry and regulators. By avoiding unnecessary toxicity testing and safety evaluations when human intakes are below such a threshold, application of the TTC approach would focus limited resources of time, cost, animal use and expertise on the testing and evaluation of substances with the greatest potential to pose risks to human health and thereby contribute to a reduction in the use of animals. An Expert Group of the European branch of the International Life Sciences Institute-ILSI Europe-has examined the TTC principle for its wider applicability in food safety evaluation. The Expert Group examined metabolism and accumulation, structural alerts, endocrine disrupting chemicals and specific endpoints, such as neurotoxicity, teratogenicity, developmental toxicity, allergenicity and immunotoxicity, and determined whether such properties or endpoints had to be taken into consideration specifically in a step-wise approach. The Expert Group concluded that the TTC principle can be applied for low concentrations in food of chemicals that lack toxicity data, provided that there is a sound intake estimate. The use of a decision tree to apply the TTC principle is proposed, and this paper describes the step-wise process in detail. Proteins, heavy metals and polyhalogenated-dibenzodioxins and related compounds were excluded from this approach. When assessing a chemical, a review of prior knowledge and context of use should always precede the use of the TTC decision tree. The initial step is the identification and evaluation of possible genotoxic and/or high potency carcinogens. Following this step, non-genotoxic substances are evaluated in a sequence of steps related to the concerns that would be associated with increasing intakes. For organophosphates a TTC of 18microg per person per day (0.3 microg/kg bw/day) is proposed, and when the compound is not an OP, the TTC values for the Cramer structural classes III, II and I, with their respective TTC levels (e.g. 1800, 540 and 90 microg per person per day; or 30, 9 and 1.5 microg/kg bw /day), would be applied sequentially. All other endpoints or properties were shown to have a distribution of no observed effect levels (NOELs) similar to the distribution of NOELs for general toxicity endpoints in Cramer classes I, II and III. The document was discussed with a wider audience during a workshop held in March 2003 (see list of workshop participants).

Down-regulation of regulatory proteins for differentiation and proliferation in murine fetal hypoplastic lungs: altered mesenchymal-epithelial interactions

We compared proliferation (growth) and differentiation (development) related proteins in normal and hypoplastic fetal murine lungs. The hypoplastic lungs were created in CD-1 fetal mice by nitrofen exposure (25 mg per pregnant mouse given intragastrically on gestational day 8 [Gd8]), as published earlier. The lungs were harvested at Gd14, 16, 19 and from neonates. Immunoblot analyses were carried out for transcription factors (oncogenic proteins, nuclear receptor, and transmembrane receptor proteins) in severely hypoplastic murine fetal lungs with coexistent diaphragmatic hernia, and results were compared with those derived from normal lungs of equivalent age. These proteins have proposed roles in the regulation of proliferation and differentiation processes of fetal lungs. We have shown that the product of the oncogene c-myc was reduced in hypoplastic lungs at all stages of gestation, whereas c-Fos protein levels were variable. These proteins are known to regulate transcription of various developmental proteins, such as those responsible for proliferation and differentiation. Further, the nuclear transcription factors thyroid transcription factor-1 (TITF-1) and glucocorticoid receptor (GR) were reduced, and thyroid hormone receptor (TR) and retinoic acid receptors (RARs) were inhibited in severely hypoplastic lungs compared to normal lungs of equivalent gestational stage, except in neonatal lungs, where signals for RARs were seen. TITF-1 is known to localize in bronchial epithelial cells in developing lungs. It is restricted to type II pneumocytes with gestational development in the normal lungs and regulates surfactant proteins. Earlier, we have reported that surfactant proteins are reduced in hypoplastic lungs. In the current study, reduced GR and TITF-1 proteins may play a role in reducing surfactant proteins in the hypoplastic lungs. The significant inhibition in TR and RARalpha in the severely hypoplastic lungs reflects on affected epithelial cell maturation and alveolar formation, respectively. Altered RARbeta levels correlate with affected lung growth and branching morphogenesis of nitrofen-exposed lungs. A transmembrane receptor protein EGFR was reduced in hypoplastic lungs, suggesting the involvement of altered mesenchymal-epithelial signal transduction pathways. We conclude (1) Our data suggest altered levels of various nuclear transcription factors in the murine fetal hypoplastic lungs; (2) Reduced levels TITF-1 protein in hypoplastic lungs may have caused the functional immaturity of distal lung, immature airways and thus may affect overall differentiation of lungs. These results correlated with low levels of surfactant proteins in these lungs; (3) TR and RAR inhibition indicate their roles through reduced or retarded proliferation and differentiation processes in the severely hypoplastic lungs; (4) GR down-regulation in developing fetal murine hypoplastic lungs indicate delayed development, and GR up-regulation in affected neonates may be induced by stress/stretch caused at birth due to air-breathing; (5) Down- regulation of EGFR indicate altered mesenchymal-epithelial interactions and possible influence on lung proliferation and differentiation.

Correlation of structural class with no-observed-effect levels: a proposal for establishing a threshold of concern

The relationship between chemical structure and toxicity was explored through the compilation of a large reference database consisting of over 600 chemical substances tested for a variety of endpoints resulting in over 2900 no-observed-effect levels (NOELs). Each substance in the database was classified into one of three structural classes using a decision tree approach. The resulting cumulative distributions of NOELs for each of the structural classes differed significantly from one another, supporting the contention that chemical structure defines toxicity. The database was used to derive a threshold of acceptable human exposure for each of the structural classes that could be applied in the absence of specific toxicity data on a substance within one of the three structural classes. The human exposure thresholds provide guidance on the degree of testing and evaluation required for substances that lack toxicity data.

The safety of etretinate as long-term therapy for psoriasis: results of the etretinate follow-up study

BACKGROUND

Etretinate is an aromatic retinoid given orally to treat severe psoriasis, a chronic disease that often requires long-term therapy.

OBJECTIVE

We assessed the safety of long-term therapy with etretinate for psoriasis.

METHODS

This 5-year prospective study of a cohort of 956 patients with psoriasis treated with etretinate assessed the frequency of adverse events in relation to total use and in relation to the frequency of these events in control populations.

RESULTS

Our data do not provide evidence for an increased risk of cardiovascular disease, cancer, diabetes, or inflammatory bowel disease in association with long-term etretinate use. Although some patients reported that joint problems improved with the use of etretinate, a greater number associated the use of etretinate with joint problems.

CONCLUSION

With proper patient selection and monitoring, long-term etretinate therapy (up to 4 years) does not appear to be accompanied by a substantial increased risk of major adverse effects.

Retinoic acid and retinoic acid receptors in development

The vitamin A derivative retinoic acid (RA) and related compounds (retinoids) are utilized as signaling molecules in a diverse array of developmental and physiological regulatory processes, including many important in the developing and mature nervous system. Retinoids function by interaction with high affinity receptors of the nuclear receptor family, which also mediate the effects of steroid and thyroid hormones and which act in the nucleus as transcription factors. This review summarizes current knowledge of the molecular mechanisms of retinoid action, the complex role of retinoid receptors in a variety of hormonal signaling processes, and illustrates current efforts to more fully understand the biological functions of retinoid receptors through analysis of downstream gene regulatory networks and studies of mouse gene knockout systems.

Inner ear malformations induced by isotretinoin in hamster fetuses

Inner ear malformations induced in anotic hamster fetuses following maternal treatment with 50 mg/kg isotretinoin (13-cis-retinoic acid) on gestational day 8 are described. Computer-assisted three dimensional reconstruction was used. Two general types of defective vestibulocochlear development were seen. Defects were bilateral and correlated with extent of middle ear deficiency and severity of mandibular defects. In the more severely affected fetuses the inner ear was limited to an epithelial sac with occasional small projections, no apparent innervation and a correspondingly reduced otic capsule. In most of the fetuses examined the inner ear was less severely affected and was characterized by a reduction in the number of semicircular ducts and alterations in the size and shape of the cochlear duct. These defects are similar to those seen in a child with the isotretinoin embryopathy. Pathogenesis may result from a direct effect on otic epithelium or from faulty inductive interactions with the rhombencephalon or with periotic neural crest cells.