AOP ID and Title:

Short Title: AR antagonism leading to short AGD

Authors

Sofie Christiansen; National Food Institute, Technical University of Denmark, Kongens Lyngby, 2800 Denmark

Monica Kam Draskau; National Food Institute, Technical University of Denmark, Kongens Lyngby, 2800 Denmark

Terje Svingen; National Food Institute, Technical University of Denmark, Kongens Lyngby, 2800 Denmark

Status

Abstract

This AOP links Androgen receptor antagonism during fetal life with short anogenital distance (AGD) in male offspring. A short AGD around birth is a marker for feminization of male fetuses and is associated with male reproductive disorders, including reduced fertility in adulthood. Although a short AGD is not necessarily ‘adverse’ from a human health perspective, it is considered an ‘adverse outcome’ in OECD test guidelines; AGD measurements are mandatory in specific tests for developmental and reproductive toxicity in chemical risk assessment (TG 443, TG 421/422, TG 414).

The AR is a nuclear receptor involved in the transcriptional regulation of various target genes during development and adulthood across species. Its main ligand is testosterone and dihydrotestosterone (DHT). Under normal physiological conditions, testosterone produced mainly by the testicles, is converted in peripheral tissues by 5α-reductase into DHT, which in turn binds AR and activates downstream target genes. AR signaling is necessary for normal masculinization of the developing fetus, including differentiation of the levator ani/bulbocavernosus (LABC) muscle complex in male fetuses. The LABC complex does not develop in the absence, or low levels of, androgen signaling, as in female fetuses.

The key events in this pathway is antagonism of the AR in target cells of the primitive perineal region, which leads to inactivation of the AR and failure to properly masculinize the perineum/LABC complex. In this instance, the local levels of testosterone or DHT may be normal, but prevented from binding the AR.

Summary of the AOP

Events

Molecular Initiating Events (MIE), Key Events (KE), Adverse Outcomes (AO)

Key Event Relationships

Stressors

Finasteride

Intrauterine exposure in rats can result in shorter male AGD in male offspring as reported in:

Bowman et al (2003), Toxicol Sci 74:393-406; doi: 10.1093/toxsci/kfg128

Christiansen et al (2009), Environ Health Perspect 117:1839-1846; doi: 10.1289/ehp.0900689

Schwartz et al (2019), Toxicol Sci 169:303-311; doi: 10.1093/toxsci/kfz046

 

 

Flutamide

Finasteride is a selective androgen receptor (AR) antagonist (Simard et al 1986) that has been shown to induce shorter male AGD in rats after in utero exposure (Foster & Harris 2005; Hass et al 2007; Kita et al 2016; McIntyre et al 2001; Mylchreest et al 1999; Scott et al 2007; Welsh et al 2007).

 

References:

Foster & Harris (2005), Toxicol Sci 85:1024-1032; doi: 10.1093/toxsci/kfi159

Hass et al (2007), Environ Health Perspect 115(suppl 1):122-128; doi: 10.1289/ehp.0360

Kita et al (2016), Toxicology 368-369:152-161; doi: 10.1016/j.tox.2016.08.021

McIntyre et al (2001), Toxicol Sci 62:236-249; doi: 10.1093/toxsci/62.2.236

Mylchreest et al (1999), Toxicol Appl Pharmacol 156:81-95; doi: 10.1006/taap.1999.8643

Scott et al (2007), Endocrinology 148:2027-2036; doi: 10.1210/en.2006-1622

Simard et al (1986), Mol Cell Endocrinol 44:261-270; doi: 10.1016/0303-7207(86)90132-2

Overall Assessment of the AOP

Domain of Applicability

Life Stage Applicability Taxonomic Applicability Sex Applicability

References

1. Schwartz CL, Christiansen S, Vinggaard AM, Axelstad M, Hass U and Svingen T (2019), Anogenital distance as a toxicological or clinical marker for fetal androgen action and risk for reproductive disorders. Arch Toxicol 93: 253-272.

Appendix 1

List of MIEs in this AOP

Event: 26: Antagonism, Androgen receptor

Short Name: Antagonism, Androgen receptor

AOPs Including This Key Event

Stressors

Biological Context

Cell term

Evidence for Perturbation by Stressor

Overview for Molecular Initiating Event

A large number of drugs and chemicals have been shown to antagonise the AR using various AR reporter gene assays. The AR is specifically targeted in AR-sensitive cancers, for example the use of the anti-androgenic drug flutamide in treating prostate cancer (Alapi & Fischer, 2006). Flutamide has also been used in several rodent in vivo studies showing anti-androgenic effects (feminization of male offspring) evident by e.g. short anogenital distance (AGD) in males (Foster & Harris, 2005; Hass et al, 2007; Kita et al, 2016). QSAR models can predict AR antagonism for a wide range of chemicals, many of which have shown in vitro antagonistic potential (Vinggaard et al, 2008).

Triticonazole

Using hAR-EcoScreen Assay, triticonazole showed a LOEC for antagonisms of 0.2 uM and an IC50 of 0.3 (±0.01) uM (Draskau et al, 2019)

 

Flusilazole

Using hAR-EcoScreen Assay, flusilazole showed a LOEC for antagonisms of 0.8 uM and an IC50 of 2.8 (±0.1) uM (Draskau et al, 2019).►

Epoxiconazole

Using transiently AR-transfected CHO cells, epoxiconazole showed a LOEC of 1.6 uM and an IC50 of 10 uM (Kjærstad et al, 2010)

Prochloraz

Using transiently AR-transfected CHO cells, prochloraz showed a LOEC of 6.3 uM and an IC50 of 13 uM (Kjærstad et al, 2010)

Propiconazole

Using transiently AR-transfected CHO cells, propiconazole showed a LOEC of 12.5 uM and an IC50 of 18 uM (Kjærstad et al, 2010)

Tebuconazole

Using transiently AR-transfected CHO cells, tebuconazole showed a LOEC of 3.1 uM and an IC50 of 8.1 uM (Kjærstad et al, 2010)

Flutamide

Using the AR-CALUX reporter assay in antagonism mode, flutamide showed an IC50 of 1.3 uM (Sonneveld et al, 2005).

Cyproterone acetate

Using the AR-CALUX reporter assay in antagonism mode, cyproterone acetate showed an IC50 of 7.1 nM (Sonneveld et al, 2005).

Vinclozolin

Using the AR-CALUX reporter assay in antagonism mode, vinclozolin showed an IC50of 1.0 uM (Sonneveld et al, 2005).

Domain of Applicability

Taxonomic Applicability Life Stage Applicability Sex Applicability

Both the DNA-binding and ligand-binding domains of the AR are highly evolutionary conserved, whereas the transactivation domain show more divergence which may affect AR-mediated gene regulation across species (Davey & Grossmann, 2016). Despite certain inter-species differences, AR function mediated through gene expression is highly conserved, with mutations studies from both humans and rodents showing strong correlation for AR-dependent development and function (Walters et al, 2010). Likewise in fish, androgens are important for development of sexual characteristics (Ogino et al., 2014, 2023). One difference that must be mentioned is that in teleost fish, 11-ketotestosterone is the main androgen in addition to testosterone and DHT and that most teleosts have two ar ohnologs, ara and arb, with arb functioning in a similar manner to the AR in other vertebrates (Ogino et al., 2023).

This KE is applicable for both sexes, across developmental stages into adulthood, in numerous cells and tissues and across vertebrate taxa

Key Event Description

The androgen receptor (AR) and its function

The AR is a ligand-activated transcription factor belonging to the steroid hormone nuclear receptor family (Davey & Grossmann, 2016). The AR has three domains: the N-terminal domain, the DNA-binding domain and the ligand-binding domain, with the latter being most evolutionary conserved. Testosterone (T) and the more biologically active dihydrotestosterone (DHT) are endogenous ligands for the AR (MacLean et al, 1993; MacLeod et al, 2010; Schwartz et al, 2019). In teleost fishes, 11-ketotestosterone is the second main ligand (Schuppe et al, 2020). Human AR mutations and mouse knock-out models have established a pivotal role for the AR in masculinization and spermatogenesis (Walters et al, 2010). Apart from the essential role for AR in male reproductive development and function (Walters et al, 2010), the AR is also expressed in many other tissues and organs such as bone, muscles, ovaries, and the immune system (Rana et al, 2014). 

AR antagonism as Key Event

The main function of the AR is to activate gene transcription in cells. Canonical signaling occurs by ligands (androgens) binding to AR in the cytoplasm which results in translocation to the cell nucleus, receptor dimerization and binding to specific regulatory DNA sequences (Heemers & Tindall, 2007). The gene targets regulated by AR activation depends on cell/tissue type and what stage of development activation occur, and is, for instance, dependent on available co-factors. Apart from the canonical signaling pathway, AR can also initiate cytoplasmic signaling pathways with other functions than the nuclear pathway, for instance rapid change in cell function by ion transport changes (Heinlein & Chang, 2002) and association with Src kinase to activate MAPK/ERK signaling and activation of the PI3K/Akt pathway (Leung & Sadar, 2017). 

How it is Measured or Detected

AR antagonism can be measured in vitro by transient or stable transactivation assays to evaluate nuclear receptor activation. There is already a validated assay for AR (ant)agonism adopted by the OECD, Test No. 458: Stably Transfected Human Androgen Receptor Transcriptional Activation Assay for Detection of Androgenic Agonist and Antagonist Activity of Chemicals (OECD, 2016). The stably transfected AR-EcoScreen^TM cells (Satoh et al, 2004) should be used for the assay and are freely available from the Japanese Collection of Research Bioresources (JCRB) Cell Bank under reference number JCRB1328.

Other assays include the AR-CALUX reporter gene assay that is derived from human U2-OS cells stably transfected with the human AR and an AR responsive reporter gene (Sonneveld et al, 2004; van der Burg et al, 2010), various transiently transfected reporter cell lines (Körner et al, 2004), and more.

The recently developed AR dimerization assay provides an assay with an improved ability to measure potential stressor-mediated disruption of dimerization/activation (Lee et al, 2021).

The Rapid Androgen Disruption Activity Reporter (RADAR) assay included in OECD test guideline no. 251 detects AR antagonism in vivo in fish (OECD 2022).

References

Davey RA, Grossmann M (2016) Androgen Receptor Structure, Function and Biology: From Bench to Bedside. Clin Biochem Rev 37: 3-15

Heemers HV, Tindall DJ (2007) Androgen receptor (AR) coregulators: a diversity of functions converging on and regulating the AR transcriptional complex. Endocr Rev 28: 778-808

Heinlein CA, Chang C (2002) The roles of androgen receptors and androgen-binding proteins in nongenomic androgen actions. Mol Endocrinol 16: 2181-2187

Körner W, Vinggaard AM, Térouanne B, Ma R, Wieloch C, Schlumpf M, Sultan C, Soto AM (2004) Interlaboratory comparison of four in vitro assays for assessing androgenic and antiandrogenic activity of environmental chemicals. Environ Health Perspect 112: 695-702

Lee SH, Hong KY, Seo H, Lee HS, Park Y (2021) Mechanistic insight into human androgen receptor-mediated endocrine-disrupting potentials by a stable bioluminescence resonance energy transfer-based dimerization assay. Chem Biol Interact 349: 109655

Leung, J. K., & Sadar, M. D. (2017). Non-Genomic Actions of the Androgen Receptor in Prostate Cancer. Frontiers in Endocrinology, 8. https://doi.org/10.3389/fendo.2017.00002

MacLean HE, Chu S, Warne GL, Zajac JD (1993) Related individuals with different androgen receptor gene deletions. J Clin Invest 91: 1123-1128

MacLeod DJ, Sharpe RM, Welsh M, Fisken M, Scott HM, Hutchison GR, Drake AJ, van den Driesche S (2010) Androgen action in the masculinization programming window and development of male reproductive organs. Int J Androl 33: 279-287

OECD. (2016) Test No. 458: Stably Transfected Human Androgen Receptor Transcriptional Activation Assay for Detection of Androgenic Agonist and Antagonist Activity of Chemicals. OECD Guidelines for the Testing of Chemicals, Section 4, Paris.

OECD (2022). Test No. 251: Rapid Androgen Disruption Activity Reporter (RADAR) assay. Paris: OECD Publishing doi:10.1787/da264d82-en.

Ogino, Y., Ansai, S., Watanabe, E., Yasugi, M., Katayama, Y., Sakamoto, H., et al. (2023). Evolutionary differentiation of androgen receptor is responsible for sexual characteristic development in a teleost fish. Nat. Commun. 2023 141 14, 1–16. doi:10.1038/s41467-023-37026-6.

Ogino, Y., Hirakawa, I., Inohaya, K., Sumiya, E., Miyagawa, S., Denslow, N., et al. (2014). Bmp7 and Lef1 Are the Downstream Effectors of Androgen Signaling in Androgen-Induced Sex Characteristics Development in Medaka. Endocrinology 155, 449–462. doi:10.1210/EN.2013-1507.

Rana K, davey RA, Zajac JD (2014) Human androgen deficiency: insights gained from androgen receptor knockout mouse models. Asian J Androl 16: 169-177

Satoh K, Ohyama K, Aoki N, Iida M, Nagai F (2004) Study on anti-androgenic effects of bisphenol a diglycidyl ether (BADGE), bisphenol F diglycidyl ether (BFDGE) and their derivatives using cells stably transfected with human androgen receptor, AR-EcoScreen. Food Chem Toxicol 42: 983-993

Schuppe, E. R., Miles, M. C., and Fuxjager, M. J. (2020). Evolution of the androgen receptor: Perspectives from human health to dancing birds. Mol. Cell. Endocrinol. 499, 110577. doi:10.1016/J.MCE.2019.110577 

Schwartz CL, Christiansen S, Vinggaard AM, Axelstad M, Hass U, Svingen T (2019) Anogenital distance as a toxicological or clinical marker for fetal androgen action and risk for reproductive disorders. Arch Toxicol 93: 253-272

Sonneveld E, Jansen HJ, Riteco JA, Brouwer A, van der Burg B (2005) Development of androgen- and estrogen-responsive bioassays, members of a panel of human cell line-based highly selective steroid-responsive bioassays. Toxicol Sci 83: 136-148

van der Burg B, Winter R, Man HY, Vangenechten C, Berckmans P, Weimer M, Witters H, van der Linden S (2010) Optimization and prevalidation of the in vitro AR CALUX method to test androgenic and antiandrogenic activity of compounds. Reprod Toxicol 30: 18-24

Walters KA, Simanainen U, Handelsman DJ (2010) Molecular insights into androgen actions in male and female reproductive function from androgen receptor knockout models. Hum Reprod Update 16: 543-558

List of Key Events in the AOP

Event: 1614: Decrease, androgen receptor activation

Short Name: Decrease, AR activation

AOPs Including This Key Event

Biological Context

Domain of Applicability

Taxonomic Applicability Life Stage Applicability Sex Applicability

This KE is considered broadly applicable across vertebrate taxa as all vertebrate animals express the AR in numerous cells and tissues where it regulates gene transcription required for developmental processes and functions.

Key Event Description

This KE refers to decreased activation of the androgen receptor (AR) as occurring in complex biological systems such as tissues and organs in vivo. It is thus considered distinct from KEs describing either blocking of AR or decreased androgen synthesis.

The AR is a nuclear transcription factor with canonical AR activation regulated by the binding of the androgens such as testosterone or dihydrotestosterone (DHT). Thus, AR activity can be decreased by reduced levels of steroidal ligands (testosterone, DHT) or the presence of compounds interfering with ligand binding to the receptor (Davey & Grossmann, 2016; Gao et al., 2005).

In the inactive state, AR is sequestered in the cytoplasm of cells by molecular chaperones. In the classical (genomic) AR signaling pathway, AR activation causes dissociation of the chaperones, AR dimerization and translocation to the nucleus to modulate gene expression. AR binds to the androgen response element (Davey & Grossmann, 2016; Gao et al., 2005). AR does not, however, act alone in regulating gene transcription, but together with other co-factors that may differ between cells and tissues and life stages. In this way, the functional consequence of AR activation is cell- and tissue-dependent.

Ligand-bound AR may also associate with cytoplasmic and membrane-bound proteins to initiate cytoplasmic signaling pathways with other functions than the nuclear pathway. Non-genomic AR signaling includes association with Src kinase to activate MAPK/ERK signaling and activation of the PI3K/Akt pathway. Decreased AR activity may therefore be a decrease in the genomic and/or non-genomic AR signaling pathways (Leung & Sadar, 2017).

How it is Measured or Detected

This KE specifically focuses on decreased in vivo activation, with most methods that can be used to measure AR activity carried out in vitro. They provide indirect information about the KE and are described in lower tier MIE/KEs (see MIE/KE-26 for AR antagonism, KE-1690 for decreased T levels and KE-1613 for decreased dihydrotestosterone levels). In this way, this KE is a placeholder for tissue-specific responses to AR activation or inactivation that will depend on the adverse outcome (AO) for which it is included.

It should be mentioned that the Rapid Androgen Disruption Activity Reporter (RADAR) assay included in OECD test guideline no. 251 detects AR antagonism in vivo in fish (OECD 2022).

References

Davey, R. A., & Grossmann, M. (2016). Androgen Receptor Structure, Function and Biology: From Bench to Bedside. The Clinical Biochemist. Reviews, 37(1), 3–15.

Gao, W., Bohl, C. E., & Dalton, J. T. (2005). Chemistry and structural biology of androgen receptor. Chemical Reviews, 105(9), 3352–3370. https://doi.org/10.1021/cr020456u

Hutson, J. M. (1985). A biphasic model for the hormonal control of testicular descent. The Lancet, 24, 419–421. https://doi.org/https://doi.org/10.1016/S0140-6736(85)92739-4

Kaftanovskaya, E. M., Huang, Z., Barbara, A. M., de Gendt, K., Verhoeven, G., Gorlov, I. P., & Agoulnik, A. I. (2012). Cryptorchidism in mice with an androgen receptor ablation in gubernaculum testis. Molecular Endocrinology, 26(4), 598–607. https://doi.org/10.1210/me.2011-1283

Lee, S. H., Hong, K. Y., Seo, H., Lee, H. S., & Park, Y. (2021). Mechanistic insight into human androgen receptor-mediated endocrine-disrupting potentials by a stable bioluminescence resonance energy transfer-based dimerization assay. Chemico-Biological Interactions, 349. https://doi.org/10.1016/j.cbi.2021.109655

Leung, J. K., & Sadar, M. D. (2017). Non-Genomic Actions of the Androgen Receptor in Prostate Cancer. Frontiers in Endocrinology, 8. https://doi.org/10.3389/fendo.2017.00002

OECD (2022). Test No. 251: Rapid Androgen Disruption Activity Reporter (RADAR) assay. Paris: OECD Publishing doi:10.1787/da264d82-en.

Pang, T. P. S., Clarke, M. v., Ghasem-Zadeh, A., Lee, N. K. L., Davey, R. A., & MacLean, H. E. (2012). A physiological role for androgen actions in the absence of androgen receptor DNA binding activity. Molecular and Cellular Endocrinology, 348(1), 189–197. https://doi.org/10.1016/j.mce.2011.08.017

Event: 1687: decrease, transcription of genes by AR

Short Name: decrease, transcription of genes by AR

AOPs Including This Key Event

Biological Context

List of Adverse Outcomes in this AOP

Event: 1688: anogenital distance (AGD), decreased

Short Name: AGD, decreased

Key Event Component

AOPs Including This Key Event

Stressors

Biological Context

Organ term

Evidence for Perturbation by Stressor

Butylparaben

Butylparaben has been shown to cause decreased male AGD in rats following intrauterine exposure to 500 and 1000 mg/kg bw/day (Boberg et al, 2016; Zhang et al, 2014). A separate study using 600 mg/kg bw/day did not see an effect on male AGD (Boberg et al, 2008).

p,p'-DDE

p,p,DDE has been shown to cause decreased male AGD in rats following intrauterine exposure to 100-200 mg/kg bw/day (Loeffler & Peterson, 1999; Wolf et al, 1999).

Bis(2-ethylhexyl) phthalate

DEHP has been shown to cause decreased male AGD in rats following intrauterine exposure to 300-1500 mg/kg bw/day (Christiansen et al, 2010; Gray et al, 2000; Howdeshell et al, 2007; Jarfelt et al, 2005; Kita et al, 2016; Li et al, 2013; Lin et al, 2009; Moore et al, 2001; Nardelli et al, 2017; Saillenfait et al, 2009; Wolf et al, 1999).

Dexamethasone

Dexamethasone has been shown to cause decreased male AGD in rats following intrauterine exposure to 0.1 mg/kg bw/day (Van den Driesche et al, 2012).

Fenitrothion

Fenitrothion has been shown to cause decreased male AGD in rats following intrauterine exposure to 25 mg/kg bw/day (Turner et al, 2002).

Finasteride

Finasteride has been shown to cause decreased male AGD in rats following intrauterine exposure to 100 mg/kg bw/day (Bowman et al, 2003).

Flutamide

Flutamide has been shown to cause decreased male AGD in rats following intrauterine exposure to doses between 16-100 mg/kg bw/day (Foster & Harris, 2005; Hass et al, 2007; Kita et al, 2016; McIntyre et al, 2001; Mylchreest et al, 1999; Scott et al, 2007; Welsh et al, 2007).

Ketoconazole

Ketoconazole has been shown to cause decreased male AGD in rats following intrauterine exposure to 50 mg/kg bw/day in one study (Taxvig et al, 2008), but no effect in another study using same dose (Wolf et al, 1999).

Linuron

Linuron has been shown to cause decreased male AGD in rats following intrauterine exposure to 50-100 mg/kg bw/day (Hotchkiss et al, 2004; McIntyre et al, 2002; Wolf et al, 1999).

Prochloraz

Prochloraz has been shown to cause decreased male AGD in rats following intrauterine exposure to 150-250 mg/kg bw/day (Laier et al, 2006; Noriega et al, 2005).

Procymidone

Procymidone has been shown to cause decreased male AGD in rats following intrauterine exposure to doses between 50-150 mg/kg bw/day (Hass et al, 2012; Hass et al, 2007; Wolf et al, 1999).

Triticonazole

Triticonazole has been shown to cause decreased male AGD in rats following intrauterine exposure to 150 and 450 mg/kg bw/day (Draskau et al, 2019).

Vinclozolin

Vinclozolin has been shown to cause decreased male AGD in rats following intrauterine exposure to doses between 50-200 mg/kg bw/day (Christiansen et al, 2009; Gray et al, 1994; Hass et al, 2007; Matsuura et al, 2005; Ostby et al, 1999; Schneider et al, 2011; Wolf et al, 2004).

di-n-hexyl phthalate

DnHP has been shown to cause decreased male AGD in rats following intrauterine exposure to 500-750 mg/kg bw/day (Saillenfait et al, 2009a; Saillenfait et al, 2009b).

Dicyclohexyl phthalate

DCHP has been shown to cause decreased male AGD in rats following intrauterine exposure to 350-750 mg/kg bw/day (Aydoğan Ahbab & Barlas, 2015; Hoshino et al, 2005; Saillenfait et al, 2009a).

butyl benzyl phthalate

BBP has been shown to cause decreased male AGD in rats following intrauterine exposure to 500-1000 mg/kg bw/day (Ema & Miyawaki, 2002; Gray et al, 2000; Hotchkiss et al, 2004; Nagao et al, 2000; Tyl et al, 2004).

monobenzyl phthalate

MBeP has been shown to cause decreased male AGD in rats following intrauterine exposure to 375 mg/kg bw/day (Ema et al, 2003).

di-n-heptyl phthalate

DHPP has been shown to cause decreased male AGD in rats following intrauterine exposure to 1000 mg/kg bw/day (Saillenfait et al, 2011).

Domain of Applicability

Taxonomic Applicability Life Stage Applicability Sex Applicability

A short AGD in male offspring is a marker of insufficient androgen action during critical fetal developmental stages (Schwartz et al, 2019; Welsh et al, 2008). A short AGD is thus a sign of undervirilization, which is also associated with a series of male reproductive disorders, including genital malformations and infertility in humans (Juul et al, 2014; Skakkebaek et al, 2001).

There are numerous human epidemiological studies showing associations with intrauterine exposure to anti-androgenic chemicals and short AGD in newborn boys alongside other reproductive disorders (Schwartz et al, 2019). This underscores the human relevance of this AO. However, in reproductive toxicity studies and chemical risk assessment, rodents (rats and mice) are what is tested on. The list of chemicals inducing short male AGD in male rat offspring is extensive, as evidenced by the ‘stressor’ list and reviewed by (Schwartz et al, 2019).

Key Event Description

The anogenital distance (AGD) refers to the distance between anus and the external genitalia. In rodents and humans, the male AGD is approximately twice the length as the female AGD (Salazar-Martinez et al, 2004; Schwartz et al, 2019). This sexual dimorphisms is a consequence of sex hormone-dependent development of secondary sexual characteristics (Schwartz et al, 2019). In males, it is believed that androgens (primarily DHT) activate AR-positive cells in non-myotic cells in the fetal perineum region to initiate differentiation of the perineal levator ani and bulbocavernosus (LABC) muscle complex (Ipulan et al, 2014). This AR-dependent process occurs within a critical window of development, around gestational days 15-18 in rats (MacLeod et al, 2010). In females, the absence of DHT prevents this masculinization effect from occurring.

The involvement of androgens in masculinization of the male fetus, including the perineum, has been known for a very long time (Jost, 1953), and AGD has historically been used to, for instance, sex newborn kittens. It is now well established that the AGD in newborns is a proxy readout for the intrauterine sex hormone milieu the fetus was developing. Too low androgen levels in XY fetuses makes the male AGD shorter, whereas excess (ectopic) androgen levels in XX fetuses makes the female AGD longer, in humans and rodents (Schwartz et al, 2019).

How it is Measured or Detected

The AGD is a morphometric measurement carried out by trained technicians (rodents) or medical staff (humans).

In rodent studies AGD is assessed as the distance between the genital papilla and the anus, and measured using a stereomicroscope with a micrometer eyepiece. The AGD index (AGDi) is often calculated by dividing AGD by the cube root of the body weight.  It is important in statistical analysis to use litter as the statistical unit. This is done when more than one pup from each litter is examined. Statistical analyses is adjusted using litter as an independent, random and nested factor. AGD are analysed using body weight as covariate as recommended in Guidance Document 151 (OECD, 2013).

 

Regulatory Significance of the AO

In regulatory toxicology, the AGD is mandatory inclusions in OECD test guidelines used to test for developmental and reproductive toxicity of chemicals. Guidelines include ‘TG 443 extended one-generation study’, ‘TG 421/422 reproductive toxicity screening studies’ and ‘TG 414 developmental toxicity study’.

References

Aydoğan Ahbab M, Barlas N (2015) Influence of in utero di-n-hexyl phthalate and dicyclohexyl phthalate on fetal testicular development in rats. Toxicol Lett 233: 125-137

Boberg J, Axelstad M, Svingen T, Mandrup K, Christiansen S, Vinggaard AM, Hass U (2016) Multiple endocrine disrupting effects in rats perinatally exposed to butylparaben. Toxicol Sci 152: 244-256

Boberg J, Metzdorff S, Wortziger R, Axelstad M, Brokken L, Vinggaard AM, Dalgaard M, Nellemann C (2008) Impact of diisobutyl phthalate and other PPAR agonists on steroidogenesis and plasma insulin and leptin levels in fetal rats. Toxicology 250: 75-81

Bowman CJ, Barlow NJ, Turner KJ, Wallace DG, Foster PM (2003) Effects of in utero exposure to finasteride on androgen-dependent reproductive development in the male rat. Toxicol Sci 74: 393-406

Christiansen S, Boberg J, Axelstad M, Dalgaard M, Vinggaard AM, Metzdorff SB, Hass U (2010) Low-dose perinatal exposure to di(2-ethylhexyl) phthalate induces anti-androgenic effects in male rats. Reprod Toxicol 30: 313-321

Christiansen S, Scholze M, Dalgaard M, Vinggaard AM, Axelstad M, Kortenkamp A, Hass U (2009) Synergistic disruption of external male sex organ development by a mixture of four antiandrogens. Environ Health Perspect 117: 1839-1846

Draskau MK, Boberg J, Taxvig C, Pedersen M, Frandsen HL, Christiansen S, Svingen T (2019) In vitro and in vivo endocrine disrupting effects of the azole fungicides triticonazole and flusilazole. Environ Pollut 255: 113309

Ema M, Miyawaki E (2002) Effects on development of the reproductive system in male offspring of rats given butyl benzyl phthalate during late pregnancy. Reprod Toxicol 16: 71-76

Ema M, Miyawaki E, Hirose A, Kamata E (2003) Decreased anogenital distance and increased incidence of undescended testes in fetuses of rats given monobenzyl phthalate, a major metabolite of butyl benzyl phthalate. Reprod Toxicol 17: 407-412

Foster PM, Harris MW (2005) Changes in androgen-mediated reproductive development in male rat offspring following exposure to a single oral dose of flutamide at different gestational ages. Toxicol Sci 85: 1024-1032

Gray LE, Jr., Ostby J, Furr J, Price M, Veeramachaneni DN, Parks L (2000) Perinatal exposure to the phthalates DEHP, BBP, and DINP, but not DEP, DMP, or DOTP, alters sexual differentiation of the male rat. Toxicol Sci 58: 350-365

Gray LEJ, Ostby JS, Kelce WR (1994) Developmental effects of an environmental antiandrogen: the fungicide vinclozolin alters sex differentiation of the male rat. Toxicol Appl Pharmacol 129: 46-52

Hass U, Boberg J, Christiansen S, Jacobsen PR, Vinggaard AM, Taxvig C, Poulsen ME, Herrmann SS, Jensen BH, Petersen A, Clemmensen LH, Axelstad M (2012) Adverse effects on sexual development in rat offspring after low dose exposure to a mixture of endocrine disrupting pesticides. Reprod Toxicol 34: 261-274

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Appendix 2

List of Key Event Relationships in the AOP