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: However , the replacement of last two doses of mesna with saline or all of the mesna doses with dexamethasone did not prevent HC .
### Output Text:
However , the replacement of last two doses of <span class="chemical">mesna</span> with saline or all of the <span class="chemical">mesna</span> doses with <span class="chemical">dexamethasone</span> did not prevent HC . 