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: No correlation could be demonstrated between cumulative dose of doxorubicin and cardiac status , except for heart rate variability .
### Output Text:
No correlation could be demonstrated between cumulative dose of <span class="chemical">doxorubicin</span> and cardiac status , except for heart rate variability . 