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: BACKGROUND : Sirolimus ( formerly rapamycin ) is an immunosuppressive agent that interferes with T - cell activation .
### Output Text:
BACKGROUND : <span class="chemical">Sirolimus</span> ( formerly <span class="chemical">rapamycin</span> ) is an immunosuppressive agent that interferes with T - cell activation . 