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: Conversely , prostaglandin E2 - induced hyperalgesia was blocked by AH6809 ( greater than or equal to 500 ng ) but not by the substance P antagonist .
### Output Text:
Conversely , <span class="chemical">prostaglandin E2</span> - induced hyperalgesia was blocked by <span class="chemical">AH6809</span> ( greater than or equal to 500 ng ) but not by the substance P antagonist . 