Developmental toxicants

Reproductive toxicity is any effect of a substance that interfere with reproductive ability or capacity (UNECE, 2004). Developmental toxicity comprises the part of reproductive toxicity

1. Introduction

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dealing with the effects on the offspring, i.e. adverse effects induced during pregnancy or as a result of parental exposure, that can be manifested at any point in the life span of the organism (UNECE, 2004). Chemicals or drugs are considered development toxicants when they display developmental toxicity and they have predominant effects on the offspring (Schardein, 2000).

The implication is that a substance producing abnormal development at doses, which also induce maternal toxicity, will be regulated by the toxicity to the adult and hence will not necessarily be classified as developmental toxic.

Most developmental toxicants produce abnormalities only during certain critical periods of gestation (Carlson, 2004; Schardein, 2000). In line, some stages of development are more susceptible to toxicants than others (Fig. 1.2). Exposure of the embryo to a toxicant during cleavage and the formation of the epiblast and hypoblast are unlikely to result in congenital abnormalities because it either damages so few cells that the embryo can recover or it damages so many cells that the embryo dies. The period with maximal susceptibility to developmental harm is between the third and eighth week (the embryonic period), because most organogenesis occurs in this interval and interference with the processes may lead to gross malformations. Most organs are well established after the eighth week of gestation, making it unlikely that major malformations will be induced. Abnormalities arising during the third to the ninth month of gestation, the foetal period, tend to be functional or to be disturbances in the growth of established body parts. It should be considered that some developmentally harmful agents might cause their effects at the molecular level at an early stage of development, although the effects may not be recognized before later, perhaps postnatally. Other agents may destruct already established structures. A recent literature analysis of the standardised toxicological endpoints assessed in current in vivo developmental toxicity testing of chemicals revealed the frequency of endpoints with statistically significant findings for 202 developmental toxicants (Fig. 1.3) (Bremer et al., 2007). The data showed the most common manifestations of developmental toxicity to be post-implantation loss and death, abnormal offspring bodyweight, malformation of the skeleton, external limbs, digits, mouth, jaw and skull as well as malformations of the visceral (pertaining to the internal organs) neurological, urogenital and cardiovascular tissues.

Developmental toxicity is governed by dose-effect relations (Schardein, 2000). Because it is a multicellular phenomenon induction of abnormalities only occurs above a certain threshold of exposure and is not a stochastic phenomenon such as induction of cancer and mutations for which the risk decreases with lower doses but theoretically never disappears. The dose-response curves for developmental toxic effects are probably rather steep, lying within the doses, which will kill the embryo and those without effect.

1. Introduction

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The development is not equally susceptible or sensitive to the influence of a given substance for all species and individuals (Frankos, 1985; Hurtt et al., 2003; Schardein, 2000). An agent that has a specific teratogenic effect in some species may have little or no effect in other species, a teratogen may produce similar defects at different frequencies in various species, or a teratogen may induce certain abnormalities in some species that are entirely different from those induced in other species (Schardein, 2000). Teratogenic responses may exhibit similar inter-individual differences (Schardein, 2000). Genetic or environmental factors may influence the responsiveness of different species and individuals (Schardein, 2000). The environmental factors may be diet, season and temperature. The genetic factors may be maternal weight, foetal weight and number, size and constitution of placenta, intrauterine associations and hormone levels.

Inter- and intra-species differences in rate of metabolism or metabolic products will also add to the variations in teratogenic responses. Susceptibility to a teratogen will furthermore depend on it accessibility to the embryo/foetus: There is only a limited placental barrier per se, as only substances with a molecular weight of more than 1000 are completely excluded (Schardein, 2000). Binding to plasma proteins is probably a more important determinant of whether a substance reaches the embryo, as placental transfer is modulated by the free plasma concentration, lipid solubility and ionization of the substance (Schardein, 2000).

Analysing veterinary drugs, for which residues have been demonstrated in human food, demonstrated overall positive concordances between rat, rabbit and mice of 74% for teratogenicity and 56% for fetotoxicity (Hurtt et al., 2003). Another study on substances identified as human teratogens showed levels of positive concordance between human and mouse of 85%, rat of 80%, rabbit of 60%, hamster of 45% and monkey of 30% (Frankos, 1985).

For non-human teratogens, the positive concordance was even lower, as it was 35% for mouse, 50% for rat, 70% for rabbit, 35% for hamster and 80% for monkey (Frankos, 1985). Overall, teratogenic responses in animals are generally indicative of human teratogenicity, but the concordance is unsatisfying hampering developmental toxicity assessments (Frankos, 1985;

Hurtt et al., 2003; Schardein, 2000).

To achieve a full understanding of the mechanisms of actions of developmental toxicants requires explanation of all events from exposure to the occurrence of the developmental defect (NRC, 2000). This includes understanding of: 1) the toxicant's kinetics and means of absorption, distribution, metabolism and excretion within the mother and offspring, 2) the toxicant's (or its metabolites) molecular interactions in the offspring or with maternal or extraembryonic components supporting development, 3) the consequences of the molecular interaction on

1. Introduction

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cellular functions or processes and 4) the consequences of the altered cellular functions or processes on the developmental outcome.

Fig. 1.2 The bars indicate the time in human pre-natal development with susceptibility of the formation of different organs to developmental harm from teratogens. Black and white bars indicate respectively high and low

susceptibility. Source: Schardein (2000).

Fig. 1.3 Frequency of standardised toxicological endpoints yielding statistically significant findings for 202 substances classified as developmental toxicants. Source: Bremer et al. (2005).

Normal mammalian development is extremely complex, and is still not understood in detail. The facts, that the conceptus is in continuous development, that there might be a myriad

1. Introduction

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of points at which a toxicant may interact with an important molecular component to induce developmental changes and that one toxicant may interact with several points, further complicate the understanding of the mechanisms of actions of developmental toxicants. Advances in research on signalling pathways and genetic regulatory circuits have identified key processes in embryo developmental (Gilbert, 2006; NRC, 2000). A report from the U.S. National Research Council identified signalling pathways of importance for development in most animals (NRC, 2000). Several of these pathways seem to be active only in specific periods of normal development, i.e. either before organogenesis and cytodifferentiation (development of specialised cell structures and functions) or after organogenesis and cytodifferentiation (Table 1.2). Studies of the alternation by developmental toxicants of these processes may give new knowledge on mechanisms of actions of developmental toxicity. Present research effort has already lead to some understanding of molecular interactions and hence the mechanisms of some developmental toxicants (or their metabolites). Developmental toxic agents e.g. interact with translocation of cytosolic receptors to the nucleus regulating gene expression, bind covalently to DNA resulting in abnormal transcription, bind covalently or non-covalently to proteins causing abnormal function, oxidise proteins or lipids changing their structure and thus functionality and bind to sulfhydryl groups playing a crucial role in maintenance of protein structure and biological activity (NRC, 2000). Moreover, research has confirmed the presence of several molecular interactions of some toxicants (NRC, 2000).

Table 1.2 The National Research Council identified 17 known signalling pathways and their specific periods of activity during development. 6 of these are crucial for early development in most animals, while 4 are more used in late development. Modified from: NRC (2000).

1. Introduction

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