In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: METHODS : Using in vivo microdialysis , we compared acute exposure ( 450 mg / kg ) to an identical sub - chronic exposure ( 150 mg / kg per day for 3 days ) , followed by 1 - or 3 - day washout .
### Output Text:
METHODS : Using in vivo microdialysis , we compared acute exposure ( 450 mg / kg ) to an identical sub - chronic exposure ( 150 mg / kg per day for 3 days ) , followed by 1 - or 3 - day washout . 