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: Several cytopathic mechanisms have been suggested to mediate the dose - limiting cumulative and irreversible cardiomyopathy caused by doxorubicin .
### Output Text:
Several cytopathic mechanisms have been suggested to mediate the dose - limiting cumulative and irreversible cardiomyopathy caused by <span class="chemical">doxorubicin</span> . 