Reproductive Toxicity and Teratogenicity in Visual Studio .NET

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21.5.3 Reproductive Toxicity and Teratogenicity
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The aim of developmental and reproductive testing is to examine the potential for a compound to interfere with the ability of an organism to reproduce. This includes testing to assess reproductive risk to mature adults as well as the developing individual at various stages of life, from conception to sexual maturity. Traditionally animal studies have been conducted in three segments : (I) in adults, treatment during a pre-mating period and optionally continuation for the female through implantation or lactation; (II) in pregnant animals treatment during the major period of organogenesis; and (III) treatment of pregnant/lactating animals from the completion of organogenesis through lactation (peri- and postnatal study). Although guidelines addressing treatment regimens have been rather similar throughout the world, required end points measured in adults and developing organisms have varied. International harmonization of guidelines has demonstrated the need for exibility in testing for reproductive and developmental toxicity, and toxicologists are now often challenged to design unique studies to examine potential effects on all the parameters considered in the classical segment I, II, and III studies. In adults, these include development of mature egg and
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TOXICITY TESTING
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sperm, fertilization, implantation, delivery of offspring (parturition), and lactation. In the developing organism, these include early embryonic development, major organ formation, fetal development and growth, and postnatal growth including behavioral assessments and attainment of full reproductive function. These evaluations are usually best carried out in several separate studies.
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Some De nitions in Reproductive Biology. Discussion of a bit of reproductive biology may be helpful in the understanding of study designs to evaluate reproductive and developmental toxicity. Tests to assess general reproductive performance and fertility are generally conducted using rats. In the rat, multiple eggs are ovulated from mature follicles in the ovary. The follicle that an egg leaves behind develops into glandular tissue known as a corpora lutea. The corpora lutea secretes progesterone, a hormone needed to maintain pregnancy (unlike humans in which progesterone is secreted by the placenta). Corpora lutea are visible as blisterlike protuberances on the ovary. A count of the corpora lutea in the ovary allows one to determine the maximum number of potential offspring for that pregnancy. Fertilized eggs develop into zygotes that may attach to the wall of the uterus (implantation). The discrete areas of implantation may be observed and counted upon examination of the uterus at C-section. Calculation of pre- and postimplantation loss are important end points in a reproductive toxicity study. Pre-implantation loss is the death of a fertilized ova prior to implantation in the uterine wall. Postimplantation loss (i.e., resorption and/or fetal death) is the death of the conceptus after implantation in the uterine wall and prior to parturition. Postimplantation loss can be broken down into early and late resorptions and fetal death. A late resorption has discernable features such as limbs, eyes, and nose, whereas an early resorption has none of these features. Single- and Multiple-Generation Tests. Fertility and general reproductive performance can be evaluated in single and multiple generation tests. These tests are usually conducted using rats. Fertility is de ned as the ability to produce a pregnancy, while the ability to produce live offspring is known as fecundity. An abbreviated protocol for a single-generation test is shown in Figure 21.2. In typical tests 25 males per dose group are treated for 70 days prior to mating and 25 females per dose group are treated for 14 days pre-mating. The number of animals is chosen to yield at least 20 pregnant females per dose group including controls. The treatment durations are selected to coincide with critical times during which spermatogenesis and ovulation occur. It takes approximately 70 days in the rat for spermatogonial cells to become mature sperm capable of fertilization. In the female rat the estrus cycle length is 4 to 5 days and a 14-day dosing period is considered suf cient time to detect potential effects on hormonal or other systems that may effect ovulation. In some study designs both males and females are treated for 70 days pre-mating.
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