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: Open - label assessment of levofloxacin for the treatment of acute bacterial sinusitis in adults .
### Output Text:
Open - label assessment of <span class="chemical">levofloxacin</span> for the treatment of acute bacterial sinusitis in adults . 