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: Intradermal injection of mu , but not delta or kappa opioid - agonists , however , produced dose - dependent inhibition of prostaglandin E2 - induced hyperalgesia .
### Output Text:
Intradermal injection of mu , but not delta or kappa opioid - agonists , however , produced dose - dependent inhibition of <span class="chemical">prostaglandin E2</span> - induced hyperalgesia . 