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: Twin preterm neonates with cardiac toxicity related to lopinavir / ritonavir therapy .
### Output Text:
Twin preterm neonates with cardiac toxicity related to <span class="chemical">lopinavir / ritonavir</span> therapy . 