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 objective of this investigation was to test the hypothesis that carvedilol , a nonselective beta - adrenergic receptor antagonist with potent antioxidant properties , protects against the cardiac and hepatic mitochondrial bioenergetic dysfunction associated with subchronic doxorubicin toxicity .
### Output Text:
The objective of this investigation was to test the hypothesis that <span class="chemical">carvedilol</span> , a nonselective beta - adrenergic receptor antagonist with potent antioxidant properties , protects against the cardiac and hepatic mitochondrial bioenergetic dysfunction associated with subchronic <span class="chemical">doxorubicin</span> toxicity . 