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: Because such a compensatory mechanism most likely occurs to reduce injury to the brain from cytotoxic compounds , the present data substantiate the concept that MRP2 performs a protective role in the BBB .
### Output Text:
Because such a compensatory mechanism most likely occurs to reduce injury to the brain from cytotoxic compounds , the present data substantiate the concept that MRP2 performs a protective role in the BBB . 