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: Of the 19 evaluable patients with prior doxorubicin exposure , 11 ( 58 % ) responded compared with nine of 15 ( 60 % ) without prior doxorubicin .
### Output Text:
Of the 19 evaluable patients with prior <span class="chemical">doxorubicin</span> exposure , 11 ( 58 % ) responded compared with nine of 15 ( 60 % ) without prior <span class="chemical">doxorubicin</span> . 