**6. Conclusion**

For the determination of a safety margin, drug and metabolites concentrations are sampled in plasma, which is the most practical and widely accepted way of assessing this risk. However, most safety issues are not observed in the plasma but in the organs and/or tissues. Assumptions about concentrations of drug and metabolites in tissues from extrapolation with plasma may result in an inaccurate assessment of target organ exposure to drug and metabolites. Therefore, plasma is sometimes not a good surrogate for tissue levels of drug and its metabolites, especially for the assessment of risk for some types of organ-specific toxicity.

Knowledge of toxicokinetics of an organ-specific toxicity can potentially assist in identifying a backup drug candidate that has a markedly lower potential for this organ-specific toxicity. Therefore, a hypothetical plan may be generated where focusing on tissue burden of the drug and its metabolites may actually ensure that a backup does not produce the same toxicity. For example, identifying a backup drug candidate with limited tissue distribution to the tissue where organ-specific toxicity was observed (e.g., testicular toxicity) markedly reduced the potential of these backups to cause these toxicities; furthermore, development of a backup drug candidate that is a substrate for efflux transporters which limit its distribution to the CNS (e.g., Pgp) can reduce the potential for this backup to cause CNS toxicity.

In the future, innovative models such as 1) noninvasive in vivo measurements such as sampling excreta (e.g., urine, feces, bile, and semen), 2) in vitro systems, such as primary in vitro human cell models (hepatocyte sandwich-cultured model), 3) humanized mice, and 4) PBPK models, will provide more insight into the concentration of drug and metabolites in human organs and/or tissues. Therefore, these innovations will provide a more thorough risk assessment of safety which will include safety margins from exposure of drug and its metabolites in the tissues (in addition to plasma) where organ specific toxicity is observed.

#### **7. Acknowledgment**

470 Toxicity and Drug Testing

the respective mouse tissue. For example, injection of cryopreserved human hepatocytes through a small, left flank incision into the inferior splenic pole in a SCID mouse created a mouse with humanized liver that was replaced by more than 80% of human hepatocytes (Okumura et al., 2007). In this chimeric mice model, cefmetazole (CMZ) excretions in urine and feces were 81.0 and 5.9% of the dose, respectively; however, excretions in urine and feces in control SCID mice were 23.7 and 59.4% of the dose, respectively (Okumura et al., 2007). Because CMZ is mainly excreted in urine in humans, the excretory profile in chimeric mice was demonstrated to be similar to humans. Interestingly in the chimeric mice, the hepatic mRNA expression of human drug transporters (e.g., MDR1, BSEP, MRP2, BCRP, OCT1, and OATP1B1/1B3) were detectable; whereas, the hepatic mRNA expression of mouse drug transporters in the chimeric mice was significantly lower than in the control SCID mice (Okumura et al., 2007). In conclusion, chimeric mice exhibited a humanized profile of drug excretion, suggesting that this chimeric mouse line would be a useful animal model to predict human ADME. Most studies have focused on humanized liver models; however, the potential for humanization of other organs and/or tissues in the mouse is evident in the near future. These new potential models will markedly improve the ability to

estimate drug and metabolite concentrations in human organs and/or tissues.

For the determination of a safety margin, drug and metabolites concentrations are sampled in plasma, which is the most practical and widely accepted way of assessing this risk. However, most safety issues are not observed in the plasma but in the organs and/or tissues. Assumptions about concentrations of drug and metabolites in tissues from extrapolation with plasma may result in an inaccurate assessment of target organ exposure to drug and metabolites. Therefore, plasma is sometimes not a good surrogate for tissue levels of drug and its metabolites, especially for the assessment of risk for some types of

Knowledge of toxicokinetics of an organ-specific toxicity can potentially assist in identifying a backup drug candidate that has a markedly lower potential for this organ-specific toxicity. Therefore, a hypothetical plan may be generated where focusing on tissue burden of the drug and its metabolites may actually ensure that a backup does not produce the same toxicity. For example, identifying a backup drug candidate with limited tissue distribution to the tissue where organ-specific toxicity was observed (e.g., testicular toxicity) markedly reduced the potential of these backups to cause these toxicities; furthermore, development of a backup drug candidate that is a substrate for efflux transporters which limit its distribution to the CNS (e.g., Pgp) can reduce the potential for this backup to cause CNS

In the future, innovative models such as 1) noninvasive in vivo measurements such as sampling excreta (e.g., urine, feces, bile, and semen), 2) in vitro systems, such as primary in vitro human cell models (hepatocyte sandwich-cultured model), 3) humanized mice, and 4) PBPK models, will provide more insight into the concentration of drug and metabolites in human organs and/or tissues. Therefore, these innovations will provide a more thorough risk assessment of safety which will include safety margins from exposure of drug and its metabolites in the tissues (in addition to plasma) where organ specific

**6. Conclusion** 

organ-specific toxicity.

toxicity.

toxicity is observed.

I would like to thank 1) Rita Geerts, Wenying Jian, Rick Edom, and David La for their contribution towards the rat testicular toxicity section; 2) Gregory Reich and Freddy Schoetens for their contribution towards the dog liver toxicity section; 3) David La for his contribution towards the monkey CNS toxicity section; and 4) Rob Thurmond, David Evans, Sandra Snook, Jan de Jong, and David La for reviewing this chapter.

#### **8. References**


**20** 

*India* 

**Environmental Toxicants Induced** 

**Identification and Mechanism of Action** 

*"Several observations on poor trends in Male Reproductive Health have been reported during the last Decades. These difficult trends include the increasing prevalence of Testicular Cancer, Low and possibly declining Semen Quality, high and possibly rising frequencies of Cryptorchidism* 

*(Undescended Testis) and malformation of the Penis (Hypospadias) as well as a increasing demand* 

The phrase 'endocrine disruption' has seemingly become inextricably linked with terms like 'environmental oestrogens' and 'falling sperm counts'. While these connections aid understanding about these issues, they represent a simplified view of the field of endocrine disruption. There is currently no strong data to suggest that environmental endocrine disrupters (EDCs) are responsible for the observed disintegration in human male reproductive health, but there are secular trends to suggest that it is declining. There is, however, very good evidence that lifestyle factors (e.g. smoking, alcohol consumption and/or use of cosmetics) can have an impact on fertility (Sharpe & Franks 2002; Sharpe & Irvine, 2004). Similarly, the notion that all EDCs act by mimicking oestrogen (environmental oestrogens) is too simplistic. The current literature illustrates that EDCs can act as oestrogens, anti-oestrogens, anti-androgens, steroidogenic enzyme inhibitors and can also act via interaction with the thyroid hormones and their receptors, or within the brain and the hypothalamo–pituitary axis, as well as the immune system (Fisher, 2004; Jana et al., 2006; 2010a). Reports of declining sperm counts over the past 50 years and other disturbing trends alerted scientists to the possibility that exposure to chemicals in the environment may damage male reproductive health (Carlsen et al., 1992). Testicular cancer, the most common malignancy in men 15-44 years of age, has increased markedly in incidence in this century in virtually all countries studied. The incidence of hypospadias, a developmental malformation of the male urethra, appears to be increasing worldwide. Cryptorchidism (undescended testicle), another developmental defect, may have increased in some human populations and appears to be increasing in wildlife (Toppari et al., 1996; Fisher, 2004, Sharpe, 2010). The causes of these trends have not been identified and relevant toxicological data about male reproductive effects of environmental toxicants are limited. Recent research efforts have

**1. Introduction** 

**Male Reproductive Disorders:** 

Kuladip Jana and Parimal C. Sen

*Division of Molecular Medicine,* 

 *Bose Institute, Kolkata,* 

*for Assisted Reproduction".* 

