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: Their combined cardiac toxicity may be due to independent effects of each drug ; however , they may also be due to cocaethylene ( CE ) , a cocaine metabolite formed only in the presence of ethanol .
### Output Text:
Their combined cardiac toxicity may be due to independent effects of each drug ; however , they may also be due to <span class="chemical">cocaethylene</span> ( <span class="chemical">CE</span> ) , a <span class="chemical">cocaine</span> metabolite formed only in the presence of <span class="chemical">ethanol</span> . 