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: One , two or three doses of mesna were replaced with dexamethasone alone or with dexamethasone plus mesna .
### Output Text:
One , two or three doses of <span class="chemical">mesna</span> were replaced with <span class="chemical">dexamethasone</span> alone or with <span class="chemical">dexamethasone</span> plus <span class="chemical">mesna</span> . 