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: Prostaglandin E2 showed a hyperalgesic effect at doses of 1 pg to 10 ng / mouse , but the effect lasted shorter ( 3 - 30 min ) than that of prostaglandin D2 .
### Output Text:
<span class="chemical">Prostaglandin E2</span> showed a hyperalgesic effect at doses of 1 <span class="chemical">pg</span> to 10 ng / mouse , but the effect lasted shorter ( 3 - 30 min ) than that of <span class="chemical">prostaglandin D2</span> . 