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: We have previously demonstrated that ritonavir treatment increases atherosclerotic lesion formation in male mice to a greater extent than in female mice .
### Output Text:
We have previously demonstrated that <span class="chemical">ritonavir</span> treatment increases atherosclerotic lesion formation in male mice to a greater extent than in female mice . 