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: In the high - dose group , implants containing PDN equivalent to 60 mg / kg / day were applied .
### Output Text:
In the high - dose group , implants containing <span class="chemical">PDN</span> equivalent to 60 mg / kg / day were applied . 