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: METHODS : Rats were treated with intravenous doxorubicin ( 1 mg kg ( - 1 ) week ( - 1 ) ) for 7 weeks and were sacrificed either 1 week ( ' short - term ' ) or 30 weeks ( ' long - term ' ) following the last dose .
### Output Text:
METHODS : Rats were treated with intravenous <span class="chemical">doxorubicin</span> ( 1 mg kg ( - 1 ) week ( - 1 ) ) for 7 weeks and were sacrificed either 1 week ( ' short - term ' ) or 30 weeks ( ' long - term ' ) following the last dose . 