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: By using this strategy to study the involvement of MRP2 in brain access of antiepileptic drugs ( AEDs ) , we recently reported that phenytoin is a substrate for MRP2 in the BBB .
### Output Text:
By using this strategy to study the involvement of MRP2 in brain access of antiepileptic drugs ( AEDs ) , we recently reported that <span class="chemical">phenytoin</span> is a substrate for MRP2 in the BBB . 