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: To develop a more sensitive echocardiographic screening test for cardiac damage due to doxorubicin , a cohort study was performed using dobutamine infusion to differentiate asymptomatic long - term survivors of childhood cancer treated with doxorubicin from healthy control subjects .
### Output Text:
To develop a more sensitive echocardiographic screening test for cardiac damage due to <span class="chemical">doxorubicin</span> , a cohort study was performed using <span class="chemical">dobutamine</span> infusion to differentiate asymptomatic long - term survivors of childhood cancer treated with <span class="chemical">doxorubicin</span> from healthy control subjects . 