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 moderate - dose group received 5 mg / kg / day PDN released from a subcutaneous implant .
### Output Text:
The moderate - dose group received 5 mg / kg / day <span class="chemical">PDN</span> released from a subcutaneous implant . 