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: CONCLUSIONS : These results suggest that bupivacaine stimulates COX - 2 gene expression after tissue injury , which is associated with higher PGE2 production and pain after the local anesthetic effect dissipates .
### Output Text:
CONCLUSIONS : These results suggest that <span class="chemical">bupivacaine</span> stimulates COX - 2 gene expression after tissue injury , which is associated with higher <span class="chemical">PGE2</span> production and pain after the local anesthetic effect dissipates . 