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 interaction is due to inhibition of CYP3A4 - mediated simvastatin clearance .
### Output Text:
The interaction is due to inhibition of CYP3A4 - mediated <span class="chemical">simvastatin</span> clearance . 