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: However , the bupivacaine / placebo group reported significantly more pain at 24 h and PGE2 levels during the first 4 h were significantly higher than the other three treatment groups .
### Output Text:
However , the <span class="chemical">bupivacaine</span> / placebo group reported significantly more pain at 24 h and <span class="chemical">PGE2</span> levels during the first 4 h were significantly higher than the other three treatment groups . 