**4.1 CNS toxicity in monkey**

In a Cynomolgus monkey toleration study at the 100 mg/kg/day dose (repeat daily oral dosing), test article-related clinical signs observed in the male monkey were characterized by vomiting, ptosis, decreased activity, prostration, tremors, convulsion and ataxia. A slight safety margin was identified (approximately 7-fold); however, this margin was not large enough to confidently advance this drug candidate into longer GLP safety studies in monkey.

#### **4.2 Role of toxicokinetics in monkey CNS toxicity**

Unfortunately, the brains of these monkeys were not sampled after the monkey toleration study. However, plasma and brain exposures in the mouse were known for this drug candidate. Mice express similar membrane proteins (e.g., Pgp and BCRP) in their blood brain barrier compared to Cynomolgus monkeys (Ito et al., 2011); therefore, we hypothesized that brain penetration of this drug candidate in mouse may approximate the respective brain penetration in monkey.

The brain to plasma ratio of this drug candidate was large (i.e., 22) in mouse; furthermore, drug was retained in the mouse brain compared to plasma (Figure 3). These results suggested that the drug candidate was preferentially distributed to the brain with a large accumulation potential. This large accumulation potential suggested that the safety margin (established in the monkey toleration study) might decrease with the increased duration of the safety studies, further compromising the developability of this lead candidate.
