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: Often , chemotherapy by doxorubicin ( Adriamycin ) is limited due to life threatening cardiotoxicity in patients during and posttherapy .
### Output Text:
Often , chemotherapy by <span class="chemical">doxorubicin</span> ( <span class="chemical">Adriamycin</span> ) is limited due to life threatening cardiotoxicity in patients during and posttherapy . 