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 hyperalgesic effect of prostaglandin D2 was blocked by simultaneous injection of a substance P antagonist ( greater than or equal to 100 ng ) but not by AH6809 , a prostanoid EP1 - receptor antagonist .
### Output Text:
The hyperalgesic effect of <span class="chemical">prostaglandin D2</span> was blocked by simultaneous injection of a substance P antagonist ( greater than or equal to 100 ng ) but not by <span class="chemical">AH6809</span> , a prostanoid EP1 - receptor antagonist . 