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 differential effects of bupivacaine and lidocaine on prostaglandin E2 release , cyclooxygenase gene expression and pain in a clinical pain model .
### Output Text:
The differential effects of <span class="chemical">bupivacaine</span> and <span class="chemical">lidocaine</span> on <span class="chemical">prostaglandin E2</span> release , cyclooxygenase gene expression and pain in a clinical pain model . 