Inhibition of isotretinoin teratogenicity by acetylsalicylic acid pretreatment in mice

Teratogenicity is more likely a function of primary and secondary pharmacology than caused by chemically reactive metabolites: a critical evaluation of 40 years of scientific research

The number of therapeutic drugs known to be human teratogens is actually relatively small. This may reflect the rigorous animal testing and well defined labelling. Some of these drugs were identified to have reactive metabolites and this has been postulated, historically, to be their teratogenic mechanism. These drugs include thalidomide, various anticonvulsants and retinoic acid derivatives.Many of these experiments were conducted in a period where chemically reactive metabolites were being intensely investigated and associated with all forms of toxicity. The legacy of this is that these examples are routinely cited as well established mechanisms.Examination of mechanism leads to the conclusion that the teratogenicity in humans of these compounds is likely due to the primary and secondary pharmacology of the parent drug and stable circulating metabolites and that association of reactive metabolites to this toxicity is unwarranted.

Isotretinoin: Still the cause of anxiety for teratogenicity

Isotretinoin is known as a potent teratogenic agent. Pregnancy should be ruled out before isotretinoin is prescribed. In this study, we evaluated the fetal outcomes of pregnant women who had inadvertently been exposed to isotretinoin during or before pregnancy. We collected data of pregnant women who had admitted to the Teratology Information Service due to isotretinoin exposure. Data regarding medications and comorbidities were documented. Outcomes were grouped as: live births, stillbirths, live births with congenital malformations, induced abortions (due to fetal abnormalities), spontaneous abortions, and elective abortions. We found that three women had gone to elective abortions. Three babies were live born without birth defects, of which, maternal exposure periods were between 0 and 3 weeks, 9 and 10 weeks before last menstrual period. One woman, exposed to isotretinoin up to the fourth week of gestation, decided ongoing her pregnancy with no current drug-related complications. Although many risk management programs worldwide have been used to prevent isotretinoin-related teratogenicity, the results of the present study showed that women became pregnant during isotretinoin intake and opted for pregnancy termination. Attention should be paid to the risk management programs mandating effective contraceptions, in order to lessen the drug-related terminations.

Safety issues in isotretinoin therapy

Isotretinoin (Accutane, Roche Laboratories Inc, Nutley, NJ) is an important drug, not only for the treatment of severe acne, but also for other diagnoses and in chemoprevention settings. Because the use of isotretinoin is increasing, it is important for physicians to be aware of the adverse events, toxicities, and management issues related to its use. The most important issue is that of congenital defects, which has resulted in new pregnancy prevention policies and programs implemented by the manufacturer. A relatively new concern is that of depression associated with isotretinoin use, also resulting in new policies placed by the manufacturer and the FDA. The most common adverse effects observed during treatment are mucocutaneous and ocular in nature, but laboratory abnormalities and effects in the nervous, musculoskeletal, gastrointestinal, pulmonary, hematologic, and other systems are also described. Additionally, potential drug interactions, follow-up, and toxicity prevention measures are discussed.

Zinc pretreatment inhibits isotretinoin teratogenicity and induces embryonic metallothionein in CD-1 mice

Isotretinoin (ITR), a teratogen in many species, is associated with increased oxidative stress. Metallothionein (MT) is an important tissue antioxidant whose concentrations are induced by zinc. To study the role of supplemental Zn as an inducer of embryonic MT, we injected pregnant CD-1 mice subcutaneously with saline vehicle, or 20 or 40 mg/kg Zn on gestational day (GD) 6.5. After 48 h, embryonic MT concentrations increased in a dose-related manner (r = 0.64, P < 0.05) with Zn treatment. The possible protective role of Zn pretreatment against ITR teratogenicity was investigated in vivo and in vitro. CD-1 mice were pretreated with saline or Zn (20 and 40 mg/kg) on GD 8.5 and 9.5. ITR was administered to both groups of mice via three intragastric intubations of 100 mg ITR/kg at 4 h intervals on GD 10.5. On GD 18.5, Zn pre-treated mice demonstrated decreased ITR-mediated growth retardation, cleft palates and postpartum mortality. A reduction in embryonic MT concentrations was observed in mice exposed to ITR. Mouse embryos cultured on GD 8.5 with an addition of 15 micromol/L Zn for 48 h had a sixfold greater MT concentration (688 microg/g protein) than controls. The Zn pretreatment of cultured embryos prevented malformations and lessened growth retardation caused by 24 h exposure to 17 micromol/L ITR. These results suggest that Zn-mediated induction of MT in mouse embryos could protect against ITR teratogenicity.

Receptor-selective retinoid agonists and teratogenic activity

Retinoid mechanism of action is dependent upon interaction with the retinoid nuclear RAR and RXR subfamilies of receptors. Study of ligands selective for the different receptors or which modify that interaction may provide insight into which receptors play roles in or contribute to retinoid teratogenesis. The retinoids considered here include in the RAR alpha-selective arylcarboxamidobenzoic acid CD 336 (Am580), the RAR beta/gamma-selective naphthalenecarboxylic acid CD 135 and the adamantyl-phenylcarboxamidobenzoic acid CD 394, the RAR-selective tetrahydrotetra-methylanthracenylbenzoic acid (TTAB) SRI 3961, the carboxyphenylretinamide SRI 7167-67, and the RXR-selective diarylisopropylidene SR 11217. CD 135 has a 3-fold higher affinity for RAR beta when compared with RAR gamma, whereas CD 394 has a 3-fold higher affinity for RAR gamma when compared with RAR beta. A separate investigation into potential amelioration of retinoid teratogenesis by concomitant administration of the cyclohexanetrione (Ro 31-0521) was also conducted. When pregnant hamsters were given an oral bolus of CD 336 or CD 135 during the early primitive streak stage of gestation, these retinoids proved 60-100 times more potent teratogens than all-trans-retinoic acid. Intubation of CD 394 resulted in production of terata similar to that seen after an equivalent dose of all-trans-retinoic acid. Administration of SRI 3961 found this compound 8000 times more potent than all-trans-retinoic acid, while SRI 7167-67 failed to show any evidence for developmental toxicity even after exposure to 105 mg/kg. Studies with the RXR-selective SR 11217 found it to be far less potent than all-trans-retinoic acid. These data point to the conclusion that those retinoids which have no affinity for retinoid nuclear receptors also have little potential for induction of developmental toxicity at doses which do not also provoke maternal intoxication. Comparing in vitro transcriptional activation of wild-type human RAR for the supertoxic TTNBP (Ro 13-7410) and TTAB (SRI 3961) with their relative teratogenic potency in hamster found that the more toxic congener also had the lower in vitro EC50 transactivation value (at ratios approximating their differential toxicities measured as administered dose). The RAR beta/gamma-selective CD 135 (TTNN) was not as efficient as TTNBP (Ro 13-7410) or TTAB (SRI 3961) in hRAR transactivation and CD 135 was less toxic than either Ro 13-7410 or SRI 3961. Although the RXR-selective SR 11217 failed to elicit terata after moderate doses, malformations consistent with those induced by high doses of retinoic acid could be produced following a single large bolus of SR 11217. Under the conditions here, simultaneous administration of Ro 31-0521 with all-trans-retinoic acid appeared to reduce the total percentage of abnormal fetuses seen after exposure to retinoic acid alone, but fetal body weights remained depressed and the numbers of dead embryos remained elevated, suggesting only limited influence of the cyclohexanetrione on retinoid developmental toxicity.

Biochemical toxicology of chemical teratogenesis

Although exposure during pregnancy to many drugs and environmental chemicals is known to cause in utero death of the embryo of fetus, or initiate birth defects (teratogenesis) in the surviving offspring, surprisingly, little is known about the underlying biochemical and molecular mechanisms, or the determinants of teratological susceptibility, particularly in humans. In vitro and in vivo studies based primarily on rodent models suggest that many potential embryotoxic xenobiotics are actually proteratogens that must be bioactivated by enzymes such as the cytochromes P450 and peroxidases such as prostaglandin H synthase to teratogenic reactive intermediary metabolites. These reactive intermediates generally are electrophiles or free radicals that bind covalently (irreversibly) to, or directly of indirectly oxidize, embryonic cellular macromolecules such as DNA, protein, and lipid, irreversibly altering cellular function. Target oxidation, known as oxidase stress, often appears to be mediated by reactive oxygen species (ROS) such as hydroxyl radicals. The precise nature of the teratologically relevant molecular targets remains to be established, as do the relative conditions of the various types of macromolecular lesions. Teratological suseptibility appears to be determined in part by a balance among pathways of maternal xenobiotic elimination, embryonic xenobiotic bioactivation and detoxification of the xenobiotic reactive intermediate, direct and indirect pathways for the detoxification of ROS (cytoprotection), and repair of macromolecular lesions. Due largely to immature or otherwise compromised embryonic pathways for detoxification, Cytoprotection, and repair, the embryo is relatively susceptible to reactive intermediates, and teratogenesis via this mechanism can occur from exposure to therapeutic concentrations of drugs, or supposedly safe concentrations of environmental chemicals. Greater insight into the mechanisms involved in human reactive intermediate-mediated teratogenicity, and the determinants of individual teratological susceptibility, will be necessary to reduce the unwarranted embryonic attrition from xenobiotic exposure.

Interactions in developmental toxicology: a literature review and terminology proposal

Developmental toxicologists have investigated the interactive effects from concurrent exposures to a variety of chemical and physical agents, including therapeutic drugs, industrial agents, and some biological organisms or their toxins. Of approximately 160 reports of concurrent exposures reviewed in this paper, about one third report no interactive effects (including additive effects--usually referring to response--as opposed to dose-additivity); another one third report antagonistic effects, and the final third report potentiative or synergistic effects. The quality of the studies is highly variable. Frequently, only small numbers of animals were included, and very few dose levels were evaluated. Maternal toxicity was rarely discussed. Time-effect relationships were examined infrequently. In addition, these studies are also inconsistent in the use of terms to describe interactive effects, and more than 90% of the terms were not in harmony with currently accepted definitions in toxicology. Because interaction studies will continue to be important in the future, this paper proposes uniform usage of terms for additivity and interactions in developmental toxicology: additivity (the combined effect of two or more developmental toxicants approximates the sum of the effects of the agents administered separately); antagonism (the combined effect of two or more agents, one or more of which are present at doses that would be developmentally toxic if given individually, is significantly less than the sum of the effects of the agents administered separately); potentiation (the increased effect of a developmental toxicant by concurrent action of another agent at a dose that is not developmentally toxic); synergism (the combined effect of two or more developmental toxicants is significantly greater than the sum of the effects of each agent administered alone); and, interaction if more precise terminology does not apply.

Chemical teratogenesis

This review has briefly summarized what is currently known concerning the mechanisms whereby several groups of chemicals regarded as "recognized" human teratogens elicit their respective teratogenic effects. It is evident that the extent of our understanding of mechanisms for individual chemicals varies dramatically from that of a reasonably good understanding for methotrexate and other folic acid antagonists to that of virtually no understanding for the most widely recognized human teratogen, thalidomide. Even with methotrexate, however, much remains to be learned pertaining to mechanisms--i.e., the critical links in the chain of events between dihydrofolate reductase inhibition and the manifestation of specific abnormalities. Nevertheless, we can take some comfort in being able to say that we understand the primary causative mechanism. For thalidomide, as well as several others the chemical represents both a shame and a challenge--a challenge that should be addressed with our most serious efforts.