Mode of Action: Impaired Fetal Leydig Cell Function—Effects on Male Reproductive Development Produced by Certain Phthalate Esters

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Abstract
Certain phthalate esters (di-2-ethylhexyl; di-n-butyl and butyl benzyl) have profound effects on the developing male reproductive system when administered orally to pregnant experimental animals during a critical window of development. These esters produce a syndrome of adverse effects that are characteristic of a disturbance in androgen-mediated development and include a variety of reproductive tract malformations and effects on developmental phenotypic markers. A testicular dysgenesis syndrome has been proposed to explain the secular increases in a number of human male reproductive deficits, including decreased semen parameters, increased incidence of cryptorchidism and hypospadias (two of the most common human birth defects), and increased incidence of testicular (germ cell derived) cancer. The rodent phthalate data lend support to the hypothesis. This example illustrates a number of points in the use of the Human Relevance Framework. First, chemical agents may have more than one mode of action (MOA): for example, phthalate-induced peroxisome proliferation leading to hepatocarcinogensis, compared with the induction of developmental effects via effects on androgen signaling. Second, the case demonstrates the life-stage sensitivity of the response to these compounds. Third, because humans may be exposed to multiple phthalate esters producing adverse effects on reproductive development, these compounds may be useful in testing the utility of the Human Relevance Framework (HRF) approach for evaluating cumulative and aggregate risk.