**5. Future use of safety margins in tissues**

In the future, more thorough risk assessment of safety will include safety margins from exposure of drug and its metabolites in the tissues (in addition to plasma) where organ specific toxicity is observed. The challenge in this endeavor is the assessment of drug exposure in human tissues since these tissues cannot be easily sampled from most human volunteers. Therefore, creative sampling methods must be applied. For example, noninvasive in vivo measurements such as sampling excreta (urine, feces, bile, and semen) will be useful. Furthermore, in vitro systems such as the hepatocyte sandwich-culture model (Chandra & Brouwer, 2004), and humanized mice (Jiang et al., 2011), combined with physiologically based pharmacokinetic (PBPK) modelling (Kusuhara & Sugiyama, 2010) can also replace the need for direct sampling of human tissues.

Fig. 4. Dose Normalized CNS Concentration-Time Profile of Drug Candidate in Mouse Brain

0 10 20 30 40 50

Lead

Backup (Pgp substrate)

Time (hr)

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 where the prior lead drug candidate demonstrated this toxicity in animal

In the future, more thorough risk assessment of safety will include safety margins from exposure of drug and its metabolites in the tissues (in addition to plasma) where organ specific toxicity is observed. The challenge in this endeavor is the assessment of drug exposure in human tissues since these tissues cannot be easily sampled from most human volunteers. Therefore, creative sampling methods must be applied. For example, noninvasive in vivo measurements such as sampling excreta (urine, feces, bile, and semen) will be useful. Furthermore, in vitro systems such as the hepatocyte sandwich-culture model (Chandra & Brouwer, 2004), and humanized mice (Jiang et al., 2011), combined with physiologically based pharmacokinetic (PBPK) modelling (Kusuhara & Sugiyama, 2010) can

Fasted CD1 mice (n=54) were administered a single oral dose of 20 mg/kg of the lead or 5 mg/kg of the backup drug candidate. For the lead drug candidate, brains were collected at 0.25, 0.5, 1, 2, 4, 6, 8, 24, and 48 hours post dose from three mice at each time point. For the backup drug candidate, brains were collected at 0.5, 1, 4, 6, 24, and 48 hours post dose from three mice at each time point. Bioanalysis of brains for the lead and backup drug candidate

and Plasma after a Single Oral Dose (20 mg/kg for lead and 5 mg/kg for backup)

was performed.

1

10

100

**Dose normalized CNS Concentration**

1000

**4.4 Conclusion** 

safety studies.

**5. Future use of safety margins in tissues** 

also replace the need for direct sampling of human tissues.
