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: The ST interval significantly widened from 15 . 0 + / - 1 . 5 to 56 . 8 + / - 11 . 8 ms in week 10 ( 7 weekly doses of 4 mg / kg doxorubicin given i . v . plus 3 weeks of observation ) .
### Output Text:
The ST interval significantly widened from 15 . 0 + / - 1 . 5 to 56 . 8 + / - 11 . 8 ms in week 10 ( 7 weekly doses of 4 mg / kg <span class="chemical">doxorubicin</span> given i . v . plus 3 weeks of observation ) . 