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: For previously treated advanced breast cancer , there is no standard second - line therapy .
### Output Text:
For previously treated advanced breast cancer , there is no standard second - line therapy . 