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: Cardiac toxicity observed in association with high - dose cyclophosphamide - based chemotherapy for metastatic breast cancer .
### Output Text:
Cardiac toxicity observed in association with high - dose <span class="chemical">cyclophosphamide</span> - based chemotherapy for metastatic breast cancer . 