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: Comparison of developmental toxicity of selective and non - selective cyclooxygenase - 2 inhibitors in CRL : ( WI ) WUBR Wistar rats - - DFU and piroxicam study .
### Output Text:
Comparison of developmental toxicity of selective and non - selective cyclooxygenase - 2 inhibitors in CRL : ( WI ) WUBR Wistar rats - - <span class="chemical">DFU</span> and <span class="chemical">piroxicam</span> study . 