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: Clarithromycin is the most documented cytochrome P450 3A4 ( CYP3A4 ) inhibitor to cause an adverse interaction with simvastatin .
### Output Text:
<span class="chemical">Clarithromycin</span> is the most documented cytochrome P450 3A4 ( CYP3A4 ) inhibitor to cause an adverse interaction with <span class="chemical">simvastatin</span> . 