Current Pharmaceutical Design - Volume 13, Issue 29, 2007

The papers in this special issue of Current Pharmaceutical Design cover a wide spectrum of relevant and fascinating topics in the area of developmental and reproductive toxicology. There is increasing evidence that small gaseous molecules, such as dioxygen, carbon monoxide, nitric oxide and hydrogen sulphide, serve important roles in modulating a variety of physiological responses, including those involved in development. The article from Fukuto and Collins [1], besides providing an overview of the chemistry and biology of these soluble gases, summarizes the information regarding the teratogenicity of either excess or deficiency of gaseous signalling molecules. Drugs blocking the potassium current IKr, are potential human teratogens. The review from Karlsson et al. [2] supports the contention that the conventional methodology used for assessing drug teratogenicity, (based on repeated daily treatment during the gestational period covering organogenesis) is not optimal to characterize the teratogenic potential of agents having IKr blocking properties. In this context, the potential role of complementary teratology studies, including single gestation day dosing and studies using embryo culture, is discussed. The manuscript authored by Cecconi et al. [3], concentrates on the adverse effects of ethylenedithiocabamate, and with special reference to fungicide mancozeb, on mammalian ovary. The ability of mancozeb to induce spindle anomalies in meiotic cells and ultrastructural changes in ovarian somatic cells raises the possibility that mancozeb may also initiate cancerogenesis. The article presented by Younglay et al. [4] focuses on the potential negative impact of man-made environmental toxicants, including polychlorinated biphenyls and dioxins, organochlorines, phthalates, benzo-a-pyrene, synthetic polymers and the fungicide vinclozolin, on human developmental and reproductive function. Retinoic Acid Metabolic Blocking Agents (RAMBAs) represent a class of agent that have been designed to inhibit the CYP26 members of the cytochrome P450 family which regulates the local levels of all-trans retinoic acid. The article from McCaffery and Simons [5], besides describing the mechanism of action of the RAMBAs, also discusses the potential teratogenic potential of this class of agents. Kozyraki and Gofflot [6] summarize recent data on the biological function of the receptors cubilin, megalin and amnionless, and focus on their implication in embryonic nutrition and central nervous system malformations. The last article from Bremer and co-workers [7], besides providing a short overview on the current status of reproductive toxicity testing, introduce new concepts for assessing reproductive toxicity applicable to large scale toxicological programmes. I would like to express my sincere appreciation to the contributors. References [1] Fukuto JM, Collins MD. Interactive Endogenous Small Molecule (Gaseous) Signaling: Implications for Teratogenesis. Curr Pharm Des 2007; 13(29): 2952-2978. [2] Karlsson M, Danielsson BR, Nilsson M, Danielsson C, Webster WS. New proposals for testing drugs with-IKr blocking activity to determine their teratogenic potential. Curr Pharm Des 2007; 13(29): 2979-2988. [3] Cecconi S, Paro R, Rossi G, Macchiarelli G. The effects of the endocrine disruptors dithiocarbamates on the mammalian ovary with particular regard to mancozeb. Curr Pharm Des 2007; 13(29): 2989-3004. [4] Younglai EV, Wu YJ, Foster WG Reproductive Toxicology of Environmental Toxicants: Emerging Issues and Concerns. Curr Pharm Des 2007; 13(29): 3005-3019. [5] McCaffery P, Simons C. Prospective teratology of retinoid acid metabolic agents (RAMBAS) and loss of CYP26 activity. Curr Pharm Des 2007; 13(29): 3020-3037. [6] Kozyraki R, Gofflot F Multiligand endocytosis and congenital defects: roles of cubilin, megalin and amnionless. Curr Pharm Des 2007; 13(29): 3038-3046. [7] Bremer S, Pellizzer C, Hoffmann S, Seidle T, Hartung T. The development of new concepts for assessing reproductive toxicity applicable to large scale toxicological programmes. Curr Pharm Des 2007; 13(29): 3047-3058.

Dioxygen (O2) is an exogenously supplied gas with a number of properties that make it valuable as a biological source of energy and as a result much of life has become dependent on this molecule. Nitric oxide (NO), carbon dioxide (CO) and hydrogen sulfide (H2S) are small molecules that are sometimes in a gaseous state and that can be either exogenously or endogenously supplied. The chemistry of these four molecules allows them to share some common biological targets and signal transduction pathways as well as providing for unique aspects to the biochemistry of each one. Dioxygen can be teratogenic either in excess (hyperoxia) or in deficiency (hypoxia). Although there is a great deal known about the chemistry and physiology of dioxygen, the mechanisms by which it induces toxic endpoints, such as teratogenesis, are unknown. This review examines some fundamental concepts of these four signaling molecules and considers some of the molecular targets and pathways by which they interact. The information regarding the teratogenicity of either excess or deficiency of the four gases is summarized. Interaction information is generally unavailable for teratogenicity endpoints with the four gases and also a mechanistic understanding of the toxicodynamics of the compounds is lacking. Although it could be theoretically predicted that certain interactions would be additive, for example carbon monoxide and hypoxia, based on the physiological role of these molecules, the data is unavailable. Consequently, these small (gaseous) signaling molecules have been demonstrated to interact with respect to signaling pathways, but whether this indicates a similar result for teratogenesis remains unevaluated.

Drugs blocking the potassium current IKr, either as an intended pharmacologic effect (eg antiarrhythmics dofetilide and almokalant) or as an unwanted side-effect (eg antihistamine astemizole, propulsive drug cisapride, antidepressive drugs and macrolide antibiotics) are potential human teratogens. It is the contention of this paper that the existing repeat dose regimen used in teratology studies to fulfil regulatory requirements, does not properly identify the teratogenic risk of these drugs. Results from conventional studies for dofetilide and almokalant showed high rates of postimplantation embryonic death with few malformed fetuses. For astemizole and cisapride only embryonic death was seen. These latter results were not considered important because they occurred either in the presence of maternal toxicity and/or at high doses. Subsequent studies have shown that IKr-blockers are highly teratogenic when administered on single gestational days (GD) during a sensitive period of rat pregnancy (GD 10-14) when they induce a high incidence of stage-specific malformations. This teratogenic activity of astemizole and cisapride was missed in the original teratology studies. Mechanistically IKrblockers cause bradycardia and arrhythmia of the embryonic heart and while an embryo may be able to survive a single day exposure to a teratogenic dose, repeat dosing often leads to death of the embryo. With this review we suggest that new drugs identified at the preclinical stage of development as having IKr-blocking properties, should undergo more comprehensive teratology testing including single GD dosing and studies using embryo culture. This would further help identify and characterise their teratogenic potential.

Many human-made chemicals are called endocrine disruptors (EDs) because they have the potential to disrupt endocrine functions in exposed organisms. Many EDs can disrupt hormonal homeostasis by interfering with hormone receptor recognition, binding and activation, while others act by still unknown mechanisms. Among the EDs specifically affecting the female reproductive system, those with steroidogenic/antisteroidogenic effects have been extensively studied and the mechanisms of toxicity clarified also at molecular level. For many others, information is restricted to few epidemiological data and in vivo/in vitro experiments with animal models. This is the case of the dithiocarbamates, and in particular of the fungicide mancozeb, an ethylenedithiocarbamate widely used to protect fruit and vegetables, ginseng included, because of its low acute toxicity in humans. Although the mechanism(s) by which mancozeb may specifically act on female reproductive organs are largely unknown, data on experimental animals in vivo have demonstrated that the fungicide can induce several disturbances on estrus cycle. When used in vitro at concentrations considered too low to cause human health injuries, the fungicide impairs mouse embryo development and meiotic spindle assembly. The possibility that the female germ cell (the oocyte) could be a specific target of mancozeb suggests a role for this fungicide as probable inductor of infertility also in exposed human populations.

Environmental toxicants comprise a number of man-made organic chemicals which may resist metabolism or their metabolites may persist in the environment and accumulate in the food chain. Some of these persistent chemicals are carried over long distances via the atmospheric transport and can have biological effects in fish, wildlife and humans. In this review the relationship between structure of these chemicals, their mode of action and their possible roles in adverse developmental and reproductive processes in humans will be discussed. The focus will be on model polychlorinated biphenyls and dioxins, organochlorines, phthalates, a constituent of cigarette smoke (benzo-a-pyrene), synthetic polymers (polybrominated diphenyl ethers and polyfluorinated compounds), and a fungicide (vinclozolin).

All-trans retinoic acid (atRA) is the transcriptionally active product of vitamin A and induces gene expression via specific receptors at nM concentrations. Essential enzymes that regulate the local levels of atRA are the CYP26 members of the cytochrome P450 family, which catabolize atRA. Compounds that have been designed to inhibit these enzymes are known as Retinoic Acid Metabolic Blocking Agents (RAMBAs). Treatment with these compounds will raise endogenous atRA levels and may be therapeutic for the treatment of diseases that respond to high atRA concentrations, including several types of cancer as well as skin conditions such as psoriasis and acne. This review describes the mechanism of action of the RAMBAs and discusses the potential side effects of these compounds. atRA is highly teratogenic and the potential teratogenicity of the RAMBAs is described by comparison with the abnormalities resulting from null mutation of individual CYP26 genes. The possible effects of RAMBAs on the adult brain are also described that have the potential for harm but, in the right circumstances, may also be beneficial.

Cubilin and megalin are multiligand receptors that mediate uptake of extracellular ligands. Their function has extensively been studied in the kidney where they play a key role in vitamin B12 and vitamin D homeostasis. Amnionless is a plasma membrane protein that binds to cubilin in various epithelia; the interaction cubilin-amnionless in the gut is crucial for dietary vitamin B12 uptake. Studies in patients with gene defects in these receptors, and animal models with inactivated cubilin, megalin or amnionless suggest an important role in embryonic development and normal growth. In this review we will summarize recent data on the biological function of these receptors and focus on their implication in embryonic nutrition and central nervous system malformations.

Large scale toxicological testing programmes which are currently ongoing such as the new European chemical legislation REACH require the development of new integrated testing strategies rather than applying traditional testing schemes to thousands of chemicals. The current practice of requiring in vivo testing for every possible adverse effect endanger the success of these programmes due (i) to limited testing facilities and sufficient capacity of scientific/technical knowledge for reproductive toxicity; (ii) an unacceptable number of laboratory animals involved (iii) an intolerable number of chemicals classified as false positive. A key aspect of the implementation of new testing strategies is the determination of prevalence of reproductive toxicity in the universe of industrial chemicals. Prevalences are relevant in order to be aware on the expected rate of false classification during the toxicological testing and to implement appropriate measures for their avoidance. Furthermore, a detailed understanding on the subendpoints affected by reproductive toxicants and the underlying mechanisms will lead to more science based testing strategies integrating alternative methods without compromising the protection of consumers.